Qatar Storm Drainage Manual
March 21, 2017 | Author: Balaji Naik | Category: N/A
Short Description
Storm water Drainage...
Description
State of Qatar - Public Works Authority Drainage Affairs
Contents Contents .................................................................................................................................i 1
Drainage Systems Design ........................................................................................1 1.1
1.1.1
Flood Standards and Acceptability................................................................................2
1.1.2
Run-off and Recharge of Groundwater.........................................................................3
1.1.3
Multi-purpose Use of Attenuation Storage Areas..........................................................4
1.2
Standards .....................................................................................................................5
1.3
Sources of Information..................................................................................................5
1.4
Catchments...................................................................................................................6
1.5
1.6
1.4.1
Catchment Boundary Definition. ...................................................................................6
1.4.2
Catchment Characteristics ............................................................................................6
Design Storms (Rainfall Intensity & Rainfall Depth)......................................................6 1.5.1
Introduction....................................................................................................................6
1.5.2
Rainfall Data Availability................................................................................................6
1.5.3
Historic Design Rainfall Parameters .............................................................................8
Run-off Estimation ......................................................................................................14 1.6.1
Urban Run-Off .............................................................................................................14
1.6.2
Non-Urban Run-off......................................................................................................14
1.6.3
Runoff Characteristics of Qatar...................................................................................14
1.6.4
Estimation of Runoff....................................................................................................15
1.7
Ground Permeabilities ................................................................................................16
1.8
Groundwater Levels and Quality.................................................................................16
1.9
Hydraulic Analysis Processes.....................................................................................16
1.10
1.11
1.12
Volume 3
Policies and Environmental Controls ............................................................................1
1.9.1
Models (physical and mathematical)...........................................................................17
1.9.2
Formulae .....................................................................................................................19
1.9.3
Prescribed Software....................................................................................................20
General Design Considerations..................................................................................21 1.10.1
Gullies .........................................................................................................................21
1.10.2
Pipeline systems and Outfalls .....................................................................................21
1.10.3
Pumping Stations (policy for surface water and groundwater discharge)...................22
1.10.4
Attenuation Areas and Detention Ponds.....................................................................23
Pipelines .....................................................................................................................24 1.11.1
Minimum Pipe Sizes and Gradients ............................................................................24
1.11.2
Minimum and Maximum Flow Velocities.....................................................................25
1.11.3
Pipeline Materials........................................................................................................25
1.11.4
Pipe Bedding Calculations for Narrow and Wide Trench Conditions..........................26
1.11.5
Manhole Positioning....................................................................................................28
1.11.6
Manholes and Access Chambers ...............................................................................29
1.11.7
Reinstatement and Back-filling....................................................................................29
Soakaways..................................................................................................................30 1.12.1
Standard Soakaways ..................................................................................................30
1.12.2
Borehole Soakaways ..................................................................................................31
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1.12.3
1.13
1.14
2
1.13.1
Ponds/Depressions .....................................................................................................31
1.13.2
Tanks...........................................................................................................................32
Groundwater Control...................................................................................................36 1.14.1
Groundwater Levels ....................................................................................................36
1.14.2
Ground Water Drains ..................................................................................................36
Pumping Stations....................................................................................................38 2.1
Standards ...................................................................................................................38
2.2
Hydraulic Design.........................................................................................................38
2.3
2.2.1
Hydraulic Principles.....................................................................................................38
2.2.2
Pump Arrangements ...................................................................................................40
Rising Main Design.....................................................................................................40 2.3.1
Rising Main Diameters ................................................................................................40
2.3.2
Twin Rising Mains .......................................................................................................40
2.3.3
Economic Analysis ......................................................................................................40
2.3.4
Rising Main Alignment ................................................................................................41
2.4
Maximum and Minimum Velocities .............................................................................41
2.5
Pipe Materials .............................................................................................................41
2.6
Thrust Blocks ..............................................................................................................41
2.7
Air Valves and Washout Facilities...............................................................................42
2.8
Page ii
Soakaway Trenches....................................................................................................31
Storage Facilities ........................................................................................................31
2.7.1
Air Valves ....................................................................................................................42
2.7.2
Vented Non-return Valves...........................................................................................42
2.7.3
Wash – Outs................................................................................................................42
2.7.4
Isolating Valves ...........................................................................................................42
Flow Meters ................................................................................................................42 2.8.1
Application and Selection............................................................................................42
2.8.2
Magnetic Flowmeters ..................................................................................................42
2.9
Surge Protection Measures ........................................................................................44
2.10
Screens.......................................................................................................................45
2.11
Pumping Stations – Selection.....................................................................................46
2.12
Pumps and Motors......................................................................................................50
2.13
Sump Design ..............................................................................................................50
2.14
Suction/Delivery Pipework, Isolation...........................................................................52
2.15
Pumping System Characteristics................................................................................52
2.16
Pump Pumps and Over-pumping Facilities.................................................................55
2.17
Power Calculations including Standby Generation.....................................................55 2.17.1
Introduction..................................................................................................................55
2.17.2
Load Type ...................................................................................................................55
2.17.3
Site condition...............................................................................................................55
2.17.4
Generator set operation and control ..........................................................................56
2.17.5
Type of installation ......................................................................................................56
2.17.6
Type of control panel...................................................................................................56
2.17.7
Ventilation system .......................................................................................................56 Volume 3
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State of Qatar - Public Works Authority Drainage Affairs
2.17.8
Fuel system.................................................................................................................56
2.17.9
Starting method ...........................................................................................................57
2.17.10 Service facility .............................................................................................................57 2.17.11 Generator set sizing ....................................................................................................57
2.18
Switch Gear and Control Panels.................................................................................58 2.18.1
Type–tested and partially type tested assemblies (TTA and PTTA)...........................58
2.18.2
Total connected load...................................................................................................58
2.18.3
Short circuit level.........................................................................................................58
2.18.4
Type of co-ordination ..................................................................................................59
2.18.5
Form of internal separation .........................................................................................59
2.18.6
Bus Bar rating..............................................................................................................60
2.18.7
Type of starter .............................................................................................................60
2.18.8
Protection device.........................................................................................................61
2.18.9
Interlocking facility.......................................................................................................62
2.18.10 Accessibility.................................................................................................................63 2.18.11 Cable entry ..................................................................................................................63
2.19
PLC’s SCADA/Telemetry............................................................................................63 2.19.1
PLC .............................................................................................................................63
2.19.2
RTU .............................................................................................................................64
2.19.3
SCADA and Telemetry Systems .................................................................................64
2.20
Lighting .......................................................................................................................65
2.21
Maintenance Access...................................................................................................68
2.22
Gantry Cranes and Lifting Facilities............................................................................69
2.23
Ventilation, Odour Control and Air Conditioning .........................................................70
2.20.1
2.24
3
Light Fitting Selection Criteria .....................................................................................65
2.23.1
Ventilation....................................................................................................................70
2.23.2
Odour Control..............................................................................................................71
2.23.3
Air Conditioning ...........................................................................................................71
Structural Design ........................................................................................................72 2.24.1
Substructures ..............................................................................................................72
2.24.2
Superstructures ...........................................................................................................79
2.25
Site Boundary Wall/Fence ..........................................................................................86
2.26
Site Facilities...............................................................................................................86
Documentation ........................................................................................................87 3.1
General .........................................................................Error! Bookmark not defined.
3.2
Guidance on Environmental Impact Statements.........................................................87
3.3
Building Permit............................................................................................................87
4
Health and Safety ....................................................................................................88
5
References...............................................................................................................89
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State of Qatar - Public Works Authority Drainage Affairs
1
Drainage Systems Design
As stated in Volume 2 - Foul Sewerage, the drainage system in Qatar, for managing surface water (mostly stormwater runoff) and groundwater, is separate to that for managing foul sewage. The surface water drainage system serving development in Qatar is a mixture of highway and road drainage, and dedicated surface water drainage systems. The groundwater control system is the responsibility of the DA, and is usually combined with the surface water system, so that the combined flows are directed to common attenuation tanks, pumping stations and outfalls. These shared arrangements will minimise land use and environmental impacts of separate systems. The shared system will also operate all year round with groundwater flows, thus maximising reliability to deal with the much larger flows from the infrequent rainfall events. Highway and road drainage is the responsibility of the Roads Department, and is to be designed in accordance with the Qatar Highway Design Manual (QHDM)i. The Qatar Highway Design Manual requires that “the highway engineer must carefully consider adjacent development and its discharge points and characteristics in order to accurately assess the total catchment that may be contributing to the highway drainage system under design.” Highway drainage is provided for all urban roads, collecting all rainfall within the catchment area, and disposing of it within the highway limits or to a designated outfall point. Thus the highway drainage system should cater for both carriageway drainage and drainage of adjacent developments discharging to the road. Carriageway drainage is achieved by longitudinal and transverse gradients of the road surface to direct storm runoff flows to gullies located in the edge channel or gutter. The gullies are then linked to the stormwater disposal system. Due to the flat topography and limited stormwater drainage system, the road gullies often discharge to an adjacent soakaway or infiltration trench. Most developments discharge their stormwater to
adjacent roads as surface runoff. There are thus relatively few pipeline systems discharging stormwater from developments to dedicated stormwater systems. The topography of Doha is relatively flat but undulating, and thus catchment boundaries and natural drainage routes are often poorly defined. Recent extensive development has caused flooding to become more problematic, especially in the Greater Doha area, due to: •
Increased roofed and paved areas producing greater and quicker surface water runoff flows;
•
Reduced permeable areas for surface waters to soak into the ground;
•
Interference with natural flood paths by urban development and road construction;
•
No provision within the roads services hierarchy for surface or groundwater drainage systems;
•
Development becoming increasingly distant from natural drainage outlets on the coast;
•
Greater public awareness of flooding;
•
Rising groundwater table, reducing the rate of surface waters soaking into the ground.
The drainage system is designed to address such flooding problems by managing both surface runoff and groundwater flows. The existing Doha surface water and groundwater control systems comprise individual schemes to address particular flooding problems.
1.1
The difficulty in draining catchments that have no natural outlet to the sea or to low-lying inland areas is recognised. The advantages of controlling surface runoff at source are also accepted. The policy principles for design of surface water and groundwater control systems are: •
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Policies and Environmental Controls
Surface water and groundwater systems should use common facilities where possible;
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State of Qatar - Public Works Authority Drainage Affairs
•
Where stormwater discharges above ground level, such as from buildings, runoff control systems (i.e. source control) should be installed;
•
Runoff control systems should be installed at source to regulate discharge to the public infrastructure drainage systems;
1.1.1
Flood Standards and Acceptability
Flood Return Periods
•
Where development is likely to be slow, soakaway systems and / or use of retention areas should be used as an interim solution.
The levels of flood protection required by the DA are shown in Table 1.1.1 below.
•
Positive drainage systems should be provided to drain flows to the sea or other approved discharge areas;
Table 1.1.1 - Levels of Flood Protection Required for Various Areas in Qatar
•
Where a SW system is planned or already exists, the permissible peak flow from the new sub-catchment into the SW system will be determined by DA. If the calculated peak flow exceeds this figure, the difference must be catered for by a combination of attenuation tanks and soakaways;
Event
Area
I in 2 Years Storm
Parks, playgrounds, natural areas and minor roads
1 in 5 Years Storm
Low cost housing, major roads
1 in 10 Years Storm
Government, institutional and other official development, technically sensitive property, basements, power equipment, etc High cost housing
1 in 25 Years Storm
High prestige or ceremonial developments
•
Rate of runoff should be attenuated by the use of short-term flooding of roads, storage areas or tanks;
•
Soakaways to drain surface waters may be required to attenuate runoff to positive drainage systems or retention areas;
Acceptable Highway Flood Standards
•
Flood plains and routes are to be identified and kept clear of development to facilitate runoff of surface waters;
The guidelines for flood standards proposed by the DA are shown in Table 1.1.2 below.
•
Positive drainage systems, using pipes and culverts should be constructed where possible in carriageways in accordance with the agreed services hierarchy. The designer should note that there is currently no allowance for positive drainage systems within the road hierarchy and therefore the location of all drains must be agreed with the DA.
Much work is being carried out to manage the surface water and groundwater regimes in Qatar. However the urgent need for a thorough Master Plan Review is evident. This Review would bring focus to the ongoing drainage activities and allow future development, road construction and drainage infrastructure works to progress with confidence.
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Table 1.1.2 – Guidelines for Flood Standards on Qatar Roadways Road
Acceptable flooding
Small Local Roads
Flood depth of 0.15m maximum depth and duration of 2 hours
Main Local Roads
Flood depth of 0.15m maximum depth and duration of 1 hour
Major Roads
Flood depth of 0.10m maximum and duration of 30 minutes
Primary Routes
Flood depth of 0.10m maximum and duration of 10 minutes
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Surcharge • Drains should not be surcharged under groundwater flows, but may be surcharged under periodic flows from surface water runoff. Building Levels Finished floor level in all buildings shall be constructed a minimum of 150mm above adjacent road levels.
1.1.2
1.1.2.1
Run-off and Recharge of Groundwater Surface Water Control
There are few districts served by truly separate systems (i.e. a foul sewer and storm drain in each road way serving all properties). The Wadi Musherib system was constructed to deal with a major flooding area within C-Ring Road. Engineered drainage schemes have been designed to cater for 2, 5 and 10-year storms, depending on the areas and importance of the buildings and facilities to be protected. For storms of greater severity, it is normal practice to retain storage areas (“flood plains”) to retain or convey the flood flows. Flood plains are areas that would not be damaged on inconvenienced by flooding, such as car parks and recreational areas. The limit of the flood plain is defined by the contour of the maximum level which the floodwaters would be expected to reach during the specified storm. The enclosed catchments (being those without outlet to the sea or suitable low-lying inland areas) are the most problematic to drain, and therefore it is essential that both the volume and rate of storm runoff be controlled to minimise storage and pumping requirements downstream.
These maps will include basic information on the main drainage routes, and overland flow routes for each catchment. The intention is that these maps will be used by various government departments (i.e. Roads, Planning) to control development in “flood plain” areas and across natural drainage routes.
1.1.2.2
Primary Flood Plain areas which are subject to an increasing magnitude and frequency of flooding as urban development takes place in the upper catchment areas;
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Groundwater Control
Groundwater levels in many parts of Doha have risen markedly in recent years, causing deterioration of buildings and buried services. The high water levels have restricted the performance of septic tanks and soakaways, and have caused subsidence of surfaces. In inland areas, the flat and undulating topography, combined with highly impervious rock strata,results in perched water tables Such areas are without efficient drainage routes, and hence susceptible to significant rises in groundwater levels during heavy rainfall. Urban development has also increased flows soaking into the ground, due to septic tanks, water supply leakage and irrigation. The result has been significant rises in groundwater levels, due to limited permeability of the ground. Groundwater levels in Doha have been studied since the early 1980’s. The main conclusions are: •
Groundwater levels have been rising due to recharge as a result of leakage;
•
Ground conditions and permeability are highly variable even within very localised areas;
•
Reductions in groundwater levels have been seen where sewerage systems are installed, due to closure of septic tanks and infiltration into the sewers;
•
Most groundwater levels rise by between 1.0m and 1.5m during a wet period when monthly rainfall exceeds 30mm. Level rise
The DA is preparing detailed maps to define “flood plain” areas in two categories: •
Secondary Flood Plain areas which can be in-filled or have drainage systems installed to avoid flooding problems, provided the storm runoff can be transferred (i.e. drain) to lower lying areas.
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State of Qatar - Public Works Authority Drainage Affairs
reduces to between 0.5m and 0.6m nearer the coast. Groundwater drainage systems have used permeable drains, as were successfully used for the Wadi Musherib scheme. These are installed in the same trench as the deeper surface water drain, both systems discharging to common manholes, attenuation tanks and pumping stations.
1.1.2.3
Re-Use of Groundwater
Water is a scarce and expensive commodity in the Middle East, and therefore every opportunity for its re-use should be exploited. Irrigation Currently in Qatar, water supply for irrigation is supplied by both fresh (drinking) water and treated sewage effluent (TSE). Fresh water is used to irrigate public areas, such as parks, due to concerns about possible infection from TSE. TSE is used elsewhere, such as for irrigating planting along highways, etc. As landscaping works spread, the demand for irrigation water is increasing, and therefore groundwater presents a possible source for highway irrigation. It is unlikely that groundwater would be of satisfactory quality for irrigation of public areas. To re-use groundwater for irrigation would require: •
Groundwater of suitable quality for planting, in view of the potential pollutants of groundwater from salts and chemicals derived from soils, and from septic tanks;
•
Infrastructure systems to treat as necessary, and transfer groundwater to the irrigation system.
Re-Use for Flushing Another destination for groundwater could be for flushing toilets, vehicle washing, etc. As with irrigation re-use, the groundwater would need to be of adequate quality to be safe for inadvertent exposure to humans. The necessary infrastructure would also need to be in place, such as dual storage and flushing systems in commercial and domestic premises. Such dual systems are used in other parts of the world, e.g. Hong Kong, where sea water is used for flushing toilets. Ongoing Arrangements
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Re-use of groundwater depends on water quality, and the feasibility of groundwater being treated to the required quality for reuse. Information on groundwater quality is limited, so a sampling and analysis regime should be set-up to analyse groundwater sources. The main potential sampling points would be the attenuation tanks and pumping stations on the surface water/groundwater control system, and boreholes within or near the urban area. Assuming that an adequate supply of groundwater, of suitable quality can be made available, then a feasibility study should follow to assess the practicality of transferring groundwater into any potential re-use systems. The study should include cost-benefit analysis, comparing costs of re-use arrangements, with costs of expanding current arrangements. The DA will produce (in co-ordination with SCENR and other interested parties) the reuse regulations for surface and groundwater. The existing GW quality analysis results, the outcoming results of the comprehensive GW sampling programme, as well as the reuse regulations will provide a database. The feasibility study will then assess the practicality of transferring groundwater into any potential re-use systems.
1.1.3
Multi-purpose Use of Attenuation Storage Areas
Positive SW systems are designed to collect and discharge rainfall as quickly as possible from the areas on which it falls. This is not always the most appropriate means of disposal because of the high runoff peaks that can be generated. Sustainable drainage systems are being introduced in some countries, including facilities for the attenuation of surface runoff by the use of attenuation basins, and in some cases detention areas. These facilities can provide an opportunity for enhancing the environment by the creation of wildlife friendly habitat. This is particularly the case with detention ponds, which can be permanent features of the landscape. Where it is only required to provide attenuation during high rainfall events, this can be achieved by Volume 3
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State of Qatar - Public Works Authority Drainage Affairs
using recreational areas such as football pitches or parkland areas. These areas are allowed to flood on those occasions when the rate of rainfall requires it. Such areas must be generally below the elevation level of the surrounding developed areas. Surface water is collected in a conventional system and discharged to the attenuation basin, from which it is allowed to drain at a reduced rate, commensurate with the ability of the receiving water course to accept the flows without damage. Where ground conditions allow, infiltration may occur which helps to reduce the impact on the receiving waters. Constructed wetlands (CWs) are defined as engineered or constructed wetlands that utilise natural processes involving wetland vegetation, soils, and their associated microbiological features to assist, at least partially, in treating an effluent or other water source. The degree of wildlife habitat provided by CWs varies broadly. At one end of the spectrum are those systems that are intended only to provide temporary storage for an effluent such as TSE, and provide little or no wildlife habitat. At the other end are those systems that are intended to provide water reuse, wildlife habitat, and public use.
1.2
Standards
The following standards are of interest to designers in SW and foul sewerage systems. This list is by no means exhaustive, but is intended as an easy initial reference. (References are also included at the end of this volume). Volume 1, Section 1.5, also contains the complete list of references for all volumes of this manual. •
BS EN 752ii – Drain and sewer systems outside buildings (supersedes BS 8005iii, which is withdrawn, and part of BS 8301iv). Part 1: 1996
Generalities and Definitions
Part 2: 1997
Performance Requirements
Part 3: 1997
Planning
Part 4: 1998
Hydraulic Design and Environmental Considerations
Part 5: 1998
Rehabilitation
Volume 3
SW Drainage
Part 6: 1998
Pumping Installations
Part 7: 1998
Maintenance and Operations
•
BS EN 598: 1995 – Ductile iron pipes, fittings, accessories and their joints for sewerage applications – Requirements and test methodsv.
•
BS EN 1610: 1998 – Construction and testing of drains and sewersvi.
•
Sewers for Adoption – 5th Edition (WRc)vii.
•
BS EN124: 1994 Gully tops and manhole tops for vehicular and pedestrian areas – Design requirements, type testing, marking, quality controlviii.
1.3
Sources of Information
The following publications are of interest to designers in SW and foul sewerage systems. This list is by no means exhaustive, but is intended as an easy initial reference. (References are also included at the end of this volume). Volume 1, Section 1.5 also contains the complete list of references for all manuals. •
Department of the Environment National Water Council Standing Technical Committee Reports, 1981, Design and analysis of urban storm drainage - The Wallingford Procedure, National Water Council UK.
•
State of Kuwait Ministry of Planning & Hyder Consulting, 2001, Kuwait Stormwater Masterplan Hydrological Aspects - Final Report. Cardiff, (AU00109/D1/015), Hyder Consulting.
•
Highways Agency, 2002, DMRB Volume 4 Section 2 Part 5 (HA 104/02) – Geotechnics and Drainage. Chamber pots and gully tops for road drainage and services: Installation and maintenance, London, Highways Agency.
•
Water Research Council, 1997, Sewerage Detention Tanks – A Design Guide, UK, WRC.
•
Sea Outfalls – construction, inspection and repair – CIRIA.
•
Construction Industry Research and Information Association, 1996, Report R159:
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Sea Outfalls – construction, inspection and repair, London, CIRIA. •
Building Research Establishment, 1991, Soakaway Design, BRE Digest 365, BRE Watford UK.
•
HR Wallingford DC Watkins, 1991, Report SR271 -The hydraulic design and performance of soakaways, Wallingford UK.
•
Construction Industry Research and Information Association, 1996, Infiltration Drainage – Manual of Good Practice, London UK, CIRIA.
•
Chartered Institution of Water and Environmental Management, 1996, Research and Development in Methods of Soakaway design, UK, CIWEM.
•
HR Wallingford and DIH Barr, 2000, Tables for the Hydraulic Design of Pipes, Sewers and Channels, 7th Edition, Trowbridge, Wiltshire, UK Redwood Books.
•
Ministry of Municipal Affairs and Agriculture, 1997, Qatar Highway Design Manual, January 1997, Qatar, MMAA.
•
Construction Industry Research and Information Association, 1996, Design of sewers to control sediment problems, Report 141, London CIRIA.
•
Clay Pipe Development Association Limited, 1998, Design and construction of drainage and sewerage systems using vitrified clay pipes, Bucks, UK, CPDA.
•
Construction Industry Research and Information Association, 1998, Report 177, Dry Weather Flows in Sewers, London, CIRIA.
•
Water Research Council, equations, UK, WRC.
•
Bazaraa, A.S., Ahmed, S., 1991. Rainfall Characterization in an Arid Area, Engineering Journal of Qatar University, Vol. 4, pp35-50.
1.4
1994,
Velocity
Catchments 1.4.1
Catchment Boundary Definition
The boundaries of each catchment can be defined either by survey or by reference to contour maps. The boundary will be defined such that any rain that falls within it will be directed to a point of discharge under gravity. It should be noted that catchment boundaries are not always readily definable from Page 6
larger scale maps, and may often have changed significantly due earthmoving operations. It is therefore essential that sufficient topographical survey is carried out to verify the catchment boundary during the design process. Once the drain layout has been defined approximately, the main catchment can be divided into sub-catchments draining to each pipe, or group of pipes in the area. Sub-catchments may also be defined, for convenience, to represent areas with different runoff characteristics (see section 1.4.2 below). Catchment and sub-catchment areas can be measured using a planimeter from paper plans or, preferably, using electronic methods where the catchment is represented on a GIS or other electronic format. Some computer based simulation software, for example InfoWorks, is capable of importing catchment data directly into the model from GIS records.
1.4.2
Catchment Characteristics
Catchment characteristics are the various types of development and surface within the catchment. The different characteristics represent their potential to generate surface water runoff to be managed by the drainage system.
1.5
Design Storms (Rainfall Intensity & Rainfall Depth) 1.5.1 Introduction
This Section provides information about the availability of rainfall records in Qatar and their use in the development of design rainfall parameters such as intensity-duration-frequency curves, storm duration and storm profiles.
1.5.2
Rainfall Data Availability
There is a scarcity of rainfall data in Qatar. The only operating long-term rain gauge in Qatar is Volume 3
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State of Qatar - Public Works Authority Drainage Affairs
located at Doha International Airport (Latitude 25 15’N and Longitude 51 34’E)ix. The rain gauge is operated and maintained by the Department of Civil Aviation and Meteorology, of the Ministry of Communication and Transport, State of Qatar. It is a seven-day disk chart recorder that has been in operation since November 1976. Prior to that, a storage gauge recorded daily rainfall from about 1962.
The tipping buckets record every 0.1mm of rain to the nearest second and can provide hyetographs for even very short duration events. Remote logger downloads take place in Doha using telemetry, however manual downloads are also possible.
Table 1.5.1 summarises the type of processed rainfall data available from the Doha International Airport rain gauge.
The following table summarises the period in which sizeable rainfall events were recorded by the logger rain gauges.
Table 1.5.1 - Rainfall data availability from the Doha International Airport Rain Gauge
Table 1.5.3 - Rainfall event data availability from logger rain gauges around Doha.
Rain Gauge and Location
Doha International Station Latitude 25 15’N Longitude 51 34’E
Data Type
Data Length
Daily rainfall depths
1962-2000
Rainfall event data
Nov 1976, Feb 1988 & Mar 1995
Monthly rainfall totals
1962-2000
Monthly 24 hr 1962-2000 maximum rainfall No. of rain days per month
SW3
Al Dana (PS SW 3)
R02759
SW5
Abu Hamour (PS SW 5)
R02758
Rain Gauge
Data Type
Period of Rainfall Events
SW1 Wadi Musheirib
Event Logger
Apr 2002
SW2 Luqta
Event Logger
Mar 2002, Apr 2002, Nov 2002 & Dec 2002
SW3 Al Dana
Event Logger
Mar 2002 & Apr 2002
SW5 Abu Hamour
Event Logger
Apr 2002
1962-2000
A number of other long-term gauges have been referenced in several sources but, during the compilation of this Manual, data could only be obtained for the Doha Airport station.
Initial data from these gauges highlights the highly localised nature of rainfall in Qatar. Although the rain gauges are only 5km apart, the first thirteen events recorded were specific to a single gauge.
In a Qatar University report by Bazaraa and Ahmed (1991)x, reference is made to a second climatological station installed in 1978 at Doha Port, within 4km of Doha International Airport Station. A comparison of rainfall data (1979-1989) at these two sites is outlined in Section 3 of Volume 1- Meteorology.
As previously stated, there is a distinct lack of long-term rainfall data in Qatar. The accuracy of rainfall analysis relies directly on having an adequate amount of rainfall data, which is currently not the case in Qatar. Proposals exist within the DA to install further rain gauges and collect additional rainfall data over the next ten years. These proposals include establishing a further five permanent rainfall gauges around Doha, in areas of new development, such as:
Since March 2002, the DA has had four tippingbucket logger rain gauges (log–able SEBA) situated in pumping station compounds around Doha. Details of the logger rain gauges are also shown in Table 1.5.2. Table 1.5.2 - Details of logger rain gauges in Doha Reference
Location
Logger ID
SW1
Wadi Musheirib (PS 1)
R02756
SW2
Luqta (PS SW 2)
R02757
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a
Wakrah PS W1
b
Doha Industrial area STW – Inlet Pumping Station
c
PS 32
d
Wajbah PS
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State of Qatar - Public Works Authority Drainage Affairs
e
Duhail Army Camp – PS 12
n=
0.787 (fitting parameter)
f
West Bay Lagoon
d=
10 (fitting parameter)
One intention for the proposed rain gauges will be to enable real-time control on flood attenuation ponds in Qatar.
1.5.3 1.5.3.1
Historic Design Rainfall Parameters Intensity-DurationFrequency
The scarcity of reliable long-term rainfall data in Qatar has hampered the development of reliable design rainfall criteria. To date, IDF (intensityduration-frequency) curves have only been developed using records from the recording rain gauge at Doha International Airport. A thirteen-year series (1977-1989) was used by researchers from Qatar University for this purpose, but it was recognised that the limited number of storm events rendered even this data set inadequate. Although statistical procedures for data processing and analysis could not be applied rigorously, a method was adopted whereby an IDF relationship was developed. This was considered to compare well with previous research for Qatar and neighbouring Bahrain. The IDF relationship in common use for surface water drainage projects in Qatar is given by equation 1.5.1 below:
I=
CTr
The IDF curves based on the work by Qatar University are shown in .Figure 1.5.1. A tabulated form of the IDF relationship is provided in Table 1.5.4., outlining rainfall intensities (mm/hr) for varying storm durations (5 mins to 24 hrs) and return periods (2 to 100 years). The IDF relationship provides the average intensity of rainfall during a storm event with a specified duration and frequency of occurrence (return period). This can be taken forward to design runoff calculations. Alternatively, the IDF relationship can be used to estimate the return period of a recorded event, given the total rainfall depth and its duration. Rainfall data collected in Doha has been used to develop the IDF relationship. There is insufficient data elsewhere to determine if it is equally valid in all areas of the State. Although storms are generally highly localised in nature, this does not preclude the IDF relationship from being similar across Qatar. Within the next ten years or so, the DA will review the rainfall data collected from various gauges around Greater Doha and an exercise will be undertaken to update the IDF curves presented in this manual.
m
(t + d )n Equation 1.5.1
Where: I=
Rainfall intensity (mm/hr)
C=
410 (fitting parameter)
Tr =
Return Period (years)
t=
storm duration (minutes)
m=
0.206 (fitting parameter)
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Table 1.5.4 - Intensity-Frequency-Duration (IDF) values recommended for use throughout Qatar Return Period (years)
Duration (mins) 2
5
10
25
50
100
5
56.1
67.8
78.2
94.4
108.9
125.7
10
44.8
54.1
62.4
75.3
86.9
100.2
15
37.6
45.4
52.3
63.2
72.9
84.1
20
32.5
39.3
45.3
54.7
63.1
72.8
30
25.9
31.3
36.1
43.6
50.3
58.1
45
20.2
24.4
28.1
34.0
39.2
45.2
60
16.7
20.2
23.3
28.1
32.4
37.4
2hrs
10.3
12.4
14.3
17.3
19.9
23.0
3hrs
7.6
9.2
10.6
12.8
14.8
17.0
6hrs
4.5
5.4
6.3
7.6
8.7
10.1
12hrs
2.6
3.2
3.7
4.4
5.1
5.9
24hrs
1.5
1.9
2.1
2.6
3.0
3.4
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Ra infall "In tens it y-Fr equen cy-D uration" P rofiles (base d o n r esea rch by Q ata r U niver sity) 120 .00 2 -Y ear S torm 5 -Y ear S torm 10-Y ear S torm
100 .00
25-Y ear S torm Rainfall Intensity (mm/hr)
50-Y ear S torm 80 .00
60 .00
40 .00
20 .00
0 .00 0
20
40
60
80
T im e (m inu tes)
Figure 1.5.1 - Intensity-Duration-Frequency Relationship
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1.5.3.2
Selection of Design Storm Duration
A design storm duration and a specified design return period are required to determine a design rainfall intensity, i (mm/hr) from IDF relationships as discussed in Section 1.5.3.1. This section discusses the recommended processes for selecting a suitable design storm duration. For sewerage and drainage design it is common to take the design storm duration as the time of concentration of the catchment, tC. Time of concentration is defined as the interval in time from the beginning of the rainfall to the time when water from the furthest point in the catchment reaches the point under consideration. Time of concentration, tC, can be estimated by one of a number of formulae. Many formulae have been derived from catchments with well-defined drainage networks and do not necessarily lend themselves well to non-urban arid areas, where a combination of wadi and overland ‘sheet’ flow predominates. The following equation for tC is based on flow rate computations and has been recommended for use in Kuwaitxi. It is considered that it will also have reasonable applicability in Qatar in undeveloped (non-urban) areas:
t c = 526 ×
nL1 / 3 S 1 / 3i 2 / 3 Equation 1.5.2
Where: tC =
Having derived a time of concentration, the corresponding rainfall depth is used to define a storm profile. This estimate of storm duration should be considered as an initial estimate. Shorter and longer duration events should also be applied to the catchment until the duration giving rise to the highest peak flow has been identified. One alternative for determining time of concentration is outlined in the QHDM, which recommends the following equations. Manning’s equation, as shown in Equation 1.5.3 is initially used for the calculation of flow velocity.
R 2 / 3 S 1/ 2 V= n Equation 1.5.3 Where: V=
Mean velocity of flow (m/s)
n=
Manning’s coefficient of roughness
R=
Hydraulic Radius (m)
S=
Slope (percent)
The mean velocity, V, calculated from Manning’s Equation, is used to determine the time of concentration using the following equation:
Time of concentration (mins)
n=
Manning’s roughness coefficient
L=
Length of flow for furthest point (metres)
S=
Average slope (metres/metres)
i=
Average rainfall intensity (mm/hr)
In this method the rainfall intensity, i, is required. The calculation is therefore performed iteratively using values of i from the IDF curves for different durations for the selected return period, until the value calculated for tC equals the duration that corresponds to that of the rainfall intensity used to derive it. It is recommended that an appropriate ‘n’ value to use for
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undeveloped areas of Qatar lies between 0.020 and 0.035. This range has been established using a method for developing Manning’s ‘n’ values for floodplains (Arcement and Schneider, 1989)xii.
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tc =
L V Equation 1.5.4
Where: tc =
Time of concentration (seconds)
V=
Mean velocity of flow (m/s)
L=
Length of flow path from the point of consideration to the furthest catchment extremity (metres)
This approach is recommended for use when the rational method of runoff calculation is used, and
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is particularly appropriate for developed (urban) areas. However, it is recommended that in Qatar for non- urban areas, Equation 1.5.2 is used instead. When considering short duration storms, the rainfall intensity changes rapidly with only a small change in storm duration. This is exemplified in Figure 1.5.1. Therefore, it is crucial for small drainage areas that an accurate assessment of tC is undertaken.
1.5.3.3
Generally in Qatar, storms are of short duration (typically 0.5–2 hours) and catchments are small such that tC is usually of a similar magnitude.
As previously mentioned, there are insufficient records of local rainfall in Qatar, particularly hyetographs, with which to derive design storm profiles.
Where inter-catchment transfers are involved, pumped-storage schemes are usually designed to cater for the runoff from a storm duration of 24 hours. This takes into account the total rainfall from multiple events occurring in a day. In this case, the use of IDF curves and a storm duration equal to the time of concentration of the catchment is superseded by the use of a 24-hour rainfall depth derived by the DA. The following depths are currently recommended for the design of small storage systems (e.g. soakaways, storage tanks, etc.). For higher return periods, these depths are comparable with those derived by the IDF curves for 24-hour duration events: 2yr
25mm
5yr
45mm
10yr
55mm
25yr
65mm
The above values are expected to be updated by the DA periodically. For larger storage systems which have a significant outflow (e.g. detention ponds) it may be more appropriate to use rainfall depth values for durations of between 2 and 24 hours.
Design Storm Profiles
The IDF relationship provides an average intensity of rainfall for given storm durations and return periods. Where a hyetograph (distribution of rainfall over time) is required, a storm shape or profile is needed. The storm profiles are used to simulate a design storm over a catchment.
Such profiles have, however, previously been developed for the nearby State of Kuwait in the Kuwait Stormwater Masterplan (KSM)xiii. Kuwait shares a similar climate to Qatar, generally experiencing an annual rainfall of less than 100mm/year. Kuwait also experiences large interannual and regional rainfall fluctuations (refer to Section 3.6.2 of Volume 1 – Meteorology). A number of steps were taken in order to develop storm profiles for Kuwait. A total of 477 storms with depths greater than or equal to 5mm were available for analysis. The profiles of the available storms were analysed by grouping them into sets of varying storm durations including: 0-3 hours; 36 hours; 6-12 hours; longer than 12 hours. Each storm was examined to determine the proportion of rainfall falling with variation in time. The developed storm profiles are provided in Table 1.5.5 In the absence of any other data to suggest otherwise, it is recommended designers use the above storm data for Kuwait for the purposes of design in Qatar.
The choice of return-period depends on the design standard for the land use type concerned. However, for storms with a return period in excess of 25 years it is recognised and accepted that inundation will occur to some extent, and the focus changes from achieving a higher design standard to inundation management.
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Table 1.5.5 - Average storm profiles for varying duration, recommended for use in Qatar Duration
0-3 hr storms
3-6 hr storms
% duration
0
0.16
0.33
0.52
0.72
0.86
1
0
0.12
0.3
0.51
0.71
0.88
1
%rainfall
0
0.23
0.38
0.54
0.69
0.8
1
0
0.17
0.34
0.53
0.69
0.87
1
Duration
6-12hr storms
% duration
0
0.12
0.3
0.51
0.71
0.89
1
0
0.11
0.31
0.51
0.71
0.9
1
%rainfall
0
0.14
0.32
0.52
0.72
0.89
1
0
0.13
0.34
0.54
0.71
0.88
1
1.5.3.4
Areal Rainfall Reduction
12+ storms
Where relatively large catchments are being considered, with times of concentration in excess of three hours, lower intensity, longer duration rainfall events may become important in peak runoff generation. Single thunderstorms cover only a relatively small area. However, a large thunderstorm can be of the order of up to 20km diameter with the storm centre up to around 8km in diameter, and can last several hours. Such storms may sit entirely within large catchments, and in these cases the runoffgenerating part of the catchment is restricted to the area of the thunderstorm overlying it. While these events contain the highest intensities, catchments with much larger areas may generate more runoff from more widespread, lower intensity, frontal-type events. For this reason, the area of the catchment under consideration becomes important in estimating the depth of rainfall over the whole catchment from the IDF curves, rather than the area of the thunderstorm. Given the finite areal extent of thunderstorms, it is recommended that catchments with areas less than 50km2, that is, areas less than the centre of a large storm cell with a diameter of 8km, have no areal rainfall reduction applied to them. The centre of the storm cell should be considered to lie over the catchment, and rainfall depths taken from the IDF curves should be used directly. These rainfall depths can be considered as conservative. It should be noted here that, implicit in this recommendation, is the assumption that the IDFcurves are based on maximum rainfall intensities at the centre of thunderstorms.
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Catchments in the Greater Doha area are all smaller than 50km2, however, some developed catchments are linked with pumped-storage schemes to facilitate runoff disposal. For intercatchment transfer schemes where the total catchment area exceeds 50km2 , an areal reduction factor should be applied to the overall catchment rainfall. This is to account for the decrease in rainfall intensity with increasing distance from the centre of the storm. A method to account for this decrease is given by Equation 1.5.5xiv.
(
Pa = Pm 1 − 0.03 A
) Equation 1.5.5
Where: Pa =
Catchment average precipitation
Pm =
Catchment maximum precipitation from IDF curve (mm)
A=
Catchment area (km2)
The rainfall reduction described above is to account for the reduction in storm intensity when moving away from the centre of the storm (i.e. a storm-centred areal rainfall reduction rather than the more common statistical rainfall reduction usually referred to as Areal Reduction Factor ARF). It therefore only applies to thunderstormtype events. For design purposes, rainfall durations of up to three hours are considered to be associated with thunderstorm events, whereas those of longer duration are considered to be associated with frontal-type rainfall. For this reason, the rainfall
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State of Qatar - Public Works Authority Drainage Affairs
reduction should not be applied to rainfall in excess of three hours duration. For those catchments or inter-catchment transfer schemes with times of concentration in excess of three hours, it is recommended that two types of storm be applied to the catchment: 1
2
A three-hour storm with areal rainfall reduction from Equation 1.5.5 applied, using the storm profile for storms up to three hours. This event corresponds to a thunderstorm-type event; A storm with a duration corresponding to the correct time of concentration for the catchment, without any rainfall reduction applied. The storm profile for the relevant duration should be used. This event corresponds to the more widespread rainfall associated with frontal-type conditions.
Proposals exist within the DA to install further rain gauges and collect additional rainfall data over the next ten years. For inter-catchment transfer schemes, where the total catchment area exceeds 50km2, it is recommended that efforts are made to reassess ARFs based on the latest rainfall data available at the time of scheme design. Because of the lack of rainfall data, periodic reviews of the ARF’s shall be confirmed with DA.
1.6
Run-off Estimation 1.6.1
Urban Run-Off
Once the total catchment area has been defined, estimates must be made of the extent and type of surfaces that will drain into each part of the system. The percentage impermeability (PIMP) of each area is measured by defining impervious surfaces such as roofs, roads, paved areas, etc. This can be done from maps or from aerial photographs. Alternatively, the PIMP can be related approximately to the density of development. A dimensionless runoff coefficient, C, is defined that accounts for initial losses such as surface depression storage, and continuing losses such as infiltration. This coefficient is applied to PIMP and may be a typical value as defined in Table 1.6.1 or may be determined by careful examination of the catchment characteristics.
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Table 1.6.1 - Examples of Runoff Coefficients. Area description
Runoff coefft
Surface type
Runoff coefft
City Centre
0.70-0.95
Asphalt & concrete paving
0.70-0.95
Suburban business
0.50-0.70
Roofs
0.75-0.95
Industrial
0.50-0.90
Recreation areas
0.05-0.35
Residential
0.30-0.70
Open areas, gardens
0.05-0.30
Note: Weighted average coefficients are needed for areas of mixed land use.
1.6.2
Non-Urban Run-off
Run-off from undeveloped, non-urban areas takes place via overland ‘sheet’ flow and, less frequently, via wadis and incised drainage channels. Qatar’s runoff characteristics are discussed in the following sections, but, in general, due to the low but undulating topography and shallow land gradients, runoff coefficients are typically of the order of 5–10%.
1.6.3
Runoff Characteristics of Qatar
Qatar’s runoff characteristics are determined primarily by its aridity, its generally very low relief, and its mainly sandy soil surface. These three factors result in infrequent runoff. As a consequence, there are no perennial streams in Qatar. There are no flow gauging stations in the country, nor, it is believed, has there been any attempt to measure runoff from any non-urban area in Qatar. Consequently, there is no local knowledge nor are there any local data with which to estimate runoff characteristics for non-urban areas. Generally, land gradients throughout Qatar are gentle (typically 1:350–1:400 in Greater Doha). Runoff, when it does happen, therefore generally occurs as overland flow. Very little incision has taken place, although there are localised occasions where runoff has cut through underlying Volume 3
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sandstone to form a more definite channel. However, these occurrences are rare and generally do not continue for more than a few tens of metres before opening back onto sandy plains. Where gradients are sufficient (and certainly in excess of around 1%), wadi channels may be identified as slight depressions, usually no more than a few centimetres to a few tens of centimetres deep. The sand in these channels is usually of a lighter colour than that of the surrounding area, and is also much softer. Although much has been written on the nature of arid zone runoff from mountainous desert conditions, literature searches have revealed that there is almost no information on the runoff characteristics of sandy desert conditions.
1.6.4
Estimation of Runoff
Once the design storm has been defined, its effect on the catchment needs to be determined. Various hydrological processes happen that generally reduce the amount of rainfall that ends up as runoff from the catchment. These processes normally include interception by vegetation cover, satisfaction of soil moisture deficits and/or exceedence of infiltration capacities of soils in the catchment, and the filling of depressions (for overland flow). They are normally referred to as catchment losses.
1.6.4.1
Interception Losses
In the desert catchments of Qatar, vegetative cover is minimal due to over-grazing. Consequently, the vast majority of rain falls directly onto bare soil, and interception losses are negligible.
1.6.4.2
Infiltration Losses
Infiltration losses occur when rainfall hits the ground. They are the most important component in the estimation of the amount, and timing, of rainfall that produces runoff. The rate at which water enters the soil is known as the infiltration rate, and this reduces as the storm progresses and the soil becomes wetter. Surface runoff will only occur once the rate at which the rain falling on the surface exceeds the infiltration rate of the soil. The nature of the soil, and its infiltration
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capabilities, are therefore one of the most important components in the rainfall-runoff process. The maximum infiltration rates of sandy soils are very high, way in excess of the maximum intensity of the heaviest rainfall. If this were the whole situation, runoff would never occur from any of the sand-dominated soils of Qatar. However, research conducted in the arid regions of the eastern Mediterranean has shown that the impact of raindrops falling on bare sandy soils causes the formation of a surface crust or membranexv. The infiltration rate of this membrane is very low, relative to the original infiltration rate of the dry soil, and has been shown to form after a certain depth of rain has fallen. The rate at which the membrane forms is relatively independent of the soil composition, the main factors being the exposure of the soil to the direct impact of rain drops, and the occurrence of antecedent rainfall.
1.6.5
Depression Losses
Depression losses represent surface runoff that fills depressions before it can proceed to the catchment outlet. As such, it occurs after infiltration rates have been exceeded and overland flow has commenced. Typical figures for natural catchments are usually between 10-15mmxvi although they are sometimes assumed to be zero in large stormsxvii. Values of 2mm and 6mm have been used for pavements and turf at the plot scale, respectivelyxviii. Depression storage is used up exponentially (decaying) through the storm once rainfall exceeds the infiltration rate, according to Equation 1.6.1.
(
VS = S d 1 − e − Pe
Sd
) Equation 1.6.1
Where: VS =
Volume of water in depression storage (mm)
Sd =
Maximum depression storage capacity (mm)
Pe =
Precipitation in excess of interception and infiltration (mm) Page 15
State of Qatar - Public Works Authority Drainage Affairs
Once the volume and timing of excess rainfall, or runoff, has been determined, it needs to be routed down the catchment. As time progresses more and more of the catchment will contribute to the flow at the downstream end, until rain from the furthest point reaches the catchment outlet. The timing of this process, and the consequent build up of flow to derive the outflow hydrograph, is termed runoff routing. The nature of the desert catchments in Qatar is such that there are no permanent watercourses or natural conveyance channels. In some cases there are wadis formed in the sand plains, but these are more concentrations of overland flow than of formal channel flow. Given the character of runoff production from these sand-dominated catchments, with rainfallinduced surface crusting being the principle means, runoff occurs primarily as overland flow.
1.7
Ground Permeabilities
Indicative data on the bulk permeability of the ground are available from site investigations carried out for the studies listed in Volume 1, Section 1.4 of this manual. This data may be regarded as a starting point for estimation but studies indicate a wide range of values across comparatively small areas and variation with depth is similarly likely. As such, if permeability is a critical factor in the design process, actual site investigation data from permeability tests or pumping tests are essential. Data for greater Doha compiled by the DA indicate the following permeability values as being applicable (all m/s): •
Maximum: 10-4 - 10-5 10-6 -
10-7
•
Average: rock)
•
Minimum: 10-8 - 10-9
(upper loose sands) (fissured or weathered (rock)
These values are generally applicable to the upper 20m of the Simsima Limestone and must be treated with caution, as the database has not been systematically collected to represent the whole of the Greater Doha area.
1.8
Groundwater Levels and Quality
Most groundwater levels and quality data relevant to the drainage issue that are available are applicable to the Greater Doha area. A review of groundwater levels and quality conditions is provided in Section 4.2 of Volume 1. A summary of the most important conditions is as follows: Groundwater Levels •
The rock formation most relevant to this issue is the Simsima Limestone, which outcrops over most of Qatar;
•
In Greater Doha, shallowest groundwater levels (less than 2.5m depth) are found within 3km of the West Bay Lagoon area and in topographic depressions;
•
Elsewhere, levels are 2.5–5m depth or greater;
•
Urban development has been accompanied by a general rise in groundwater levels due to over-irrigation, leakage from water mains, use of soakaway drainage systems etc. This rise has been checked in some areas where drainage systems have been installed but is continuing in others;
•
A seasonal variation of 1–2m has been recorded in some areas;
•
This propensity for GW changes must be taken into account in the design of drainage systems.
Groundwater Quality The Electrical Conductivity of groundwater is a guide to its total dissolved solids (TDS) content; under natural conditions, EC values of 10,000– 20,000umohs may be expected but values of 5000-6000umohs are sometimes recorded indicating the effects of urban leakage.
1.9
Hydraulic Analysis Processes
The ‘Modified Rational Method’ for hydraulic analysis has the following aspects: •
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It depends on a thorough knowledge of the local rainfall characteristics; Volume 3
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•
It requires accurate IDF curves from which rainfall intensities can be deduced for different storm durations for the design return period;
•
It assumes that, for a given return period, longer storms have lower intensities and shorter storms have higher intensities;
•
It assumes that rain falls uniformly across the catchment;
•
Contributing impermeable areas are required for each pipe length;
•
A time of entry must be determined in order to avoid unrealistically high rainfall intensities;
•
Base flows from groundwater can be included in the design;
•
Iterative process for design;
•
Pipe sizes and gradients are adjusted to provide appropriate self-cleansing velocities;
•
Half pipe flow velocity is numerically equal to full pipe flow velocity;
11. Check Qp < Qf and vmax > vf > vmin.
•
The user must be aware of the limitations of this method of design;
•
The Modified Rational Method is suitable for catchments up to 150ha.
Design method The following procedure should be followed for the Modified Rational Method:
8.
Read rainfall intensity from the IDF curves for t = tc for design storm return period T.
9.
Estimate the cumulative contributing impervious area.
10. Calculate Qp from the formula:
Qp = 2.78CAi.i Equation 1.9.1 Where: Qp =
flow in drainage pipes
C=
runoff coefficient
AI =
contributing runoff area
i=
average rainfall intensity
12 Adjust pipe diameter and gradient as necessary within the physical constraints pertaining and return to step 5. Minimum Pipe Sizes and Gradients The following aspects need to be considered: •
Determine from table 1.1.1 and confirm acceptance with DA the design rainfall return period (T), pipe roughness (ks), time of entry (te) and runoff coefficient (C).
Pipes should be of sufficient size to carry maximum design flows at a depth D, i.e. at pipe full condition;
•
2.
Prepare a preliminary layout of drains, including tentative inlet locations.
Surface water drains require higher velocities than foul sewers for self-cleansing purposes because of the higher density of solid material to be transported;
•
Surface water drains should not be less than 300mm in diameter;
3.
Mark pipe numbers on plan in accordance with numbering convention.
•
Self-cleansing velocities increase with pipe size;
4.
Estimate impervious areas for each pipe.
•
5.
Assume approximate gradients and pipe diameters for each pipe.
Pipe sizes should not decrease downstream even when the calculations indicate that this would be hydraulically satisfactory;
•
Pipes should be designed to run parallel to the ground surface wherever possible.
1.
6.
Calculate pipe full velocity (vf) and pipe full capacity or discharge rate (Qf = πD2vf/4)
7.
Calculate time of concentration from time of entry and time of flow (tc = te + tf). For downstream pipes compare alternative feeder branches and select the branch resulting in the maximum tc.
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1.9.1
Models (physical and mathematical)
Modelling is the process of replicating the hydraulic performance of drainage, pumping and treatment systems by constructing models of the Page 17
State of Qatar - Public Works Authority Drainage Affairs
intended/existing installations. These models need to be verified before use to provide confidence that they adequately represent the actual performance of the system. The verified model is then used to test system performance under its proposed use. The model must be capable of modification to test various physical configurations and operating regimes for the installation, to produce the optimum solution for actual construction. Traditionally physical models were favoured, especially for coastal/estuary/river systems and complex pumping installations. In recent years mathematical models, have superseded physical models. Mathematical models are exploiting increased computer hardware and software capability, and are more efficient than physical models in time and effort.
1.9.1.1
Physical Models
Physical modelling consists of constructing a reduced scale, geometrically similar model of a proposed system, and operating the model to simulate fullscale flow conditions. Model tests can provide the designer with the assurance that the proposed scheme operates satisfactorily, or allows him to improve the flow conditions and achieve a better design. Changes in the model can be made by trial and error, and are usually based on the experience and intuitive understanding of the engineer conducting the tests. The amount of modification which can be undertaken on a physical model is limited, and therefore the initial model should be as accurate as possible. Factors to be considered in deciding on the need for physical models include: •
•
The similarity of the proposed scheme to existing satisfactory designs. As well as the designer’s own experience, much information is available from manufacturers’ published reports and design guides. However it should be recognised that most large scale and/or complex designs will be unique, and hence modelling will be needed; The size and cost of the proposed scheme. Bearing in mind that physical modelling can take many months with corresponding high costs, then designers of small schemes should seek to adopt standard and well-proven designs for
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small schemes. Large schemes, such as terminal pumping stations with multiple pumps and complex inlet arrangements would merit modelling. For general guidance, DA classify SW pumping station sizes as: Small
flows 1.0m3/s
As a rule, small pumping stations, will not require modelling, whereas large installations do. Medium sized installations will only require modelling if a new design philosophy is proposed, which hasn’t previously been adopted in Qatar. Physical models are still required where theory does not represent flow conditions in sufficient detail or readily cater for changes in boundary conditions (eg at entry to pumps) Where specially mentioned in the PSA, this will take precedence over the manual. All hydraulically significant details such as screens, penstocks, support channels and benching, should be included in the model. No components above maximum water level need be modelled. Model construction should be in durable and waterproof materials, with clear Perspex being the best for viewing purposes. Model size should be as large as costs allow. Scales can vary from perhaps 1:4 for very small sumps, up to 1:50 for large intakes to reservoirs or tanks. For sump models, 1:25 would be the smallest desirable scale. Physical testing could typically take between one and six months for construction, testing and reporting.
1.9.1.2
Mathematical Models
Mathematical models almost exclusively use computers and bespoke software to build and apply the model. Relevant computer modelling systems include sewerage, drainage and sewage treatment. Sewerage and drainage models use construction record data to build representations of the system as linked pipes and nodes, with specific modules for ancillaries such as pumping stations and overflows. Inflows from connected developments Volume 3
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and contributing areas are directed to the nodes, and a computerised hydraulic engine simulates the hydraulic performance of flows around the system.
problems, providing an alternative to testing or physical modelling, at an earlier stage in the design cycle.
The veracity of the model is established by verifying flows and depths predicted by the model against actual measurements taken by flow monitors temporarily installed at hydraulically significant points around the system. After the model has been verified, then simulations of future changes and system modifications are run to check the effect on the system and the effectiveness of proposed upgrading.
CFD provides information on flow characteristics such as pressure loss, flow distribution and mixing rates and complements traditional testing and experimentation. CFD is used for early conceptual studies of new designs, for detailed design and development, for scale-up, for troubleshooting and for system retrofitting.
Sensitivity analysis may be performed on the verified model by varying some of the input parameters to indicate their impact on the theoretical outcomes. This is used to determine more cost effective and / or efficient design options It should be noted that the rainfall characteristics in Qatar will not make it possible to verify models in accordance with common practice. The WRC Guide to Short Term Flow surveysxix recommends a minimum 5 week survey duration; however, surveys in Qatar should be planned to commence in October and may need to last up until April to capture a sufficient number of discrete rainfall events. Should these occur early in the survey, then it can be curtailed before the forecasted completion date, but conversely the survey may need to be extended for another rain season if insufficient rainfall occurs. Hydraulic models shall be constructed, verified (where possible) and reported in accordance with the Code of Practice for the Hydraulic Modelling of Sewer Systems, as published by the Waste Water Planning Users Group (WaPUG)xx. Models shall be retained electronically by the designer for a minimum period of 12 years from the date on which the last modifications for which the model was used have been commissioned and taken over by the DA. DA propose to model all of the trunk mains, and follow with infill models of local areas during the coming years. Computational Fluid Dynamics (CFD) software is a general-purpose tool for fluid engineering analysis. The software applies established hydraulic equations to solve energy and mass transfer for laminar and turbulent flows. The software is extremely versatile and can be tailored to address a wide range of fluid flow issues. It permits cost effective, detailed analysis of fluid flow Volume 3
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As computing power increases, CFD is being used for modelling of larger hydraulic structures, such as pumping station sumps, reservoirs and tanks.
1.9.2 1.9.2.1
Formulae The Colebrook-White Equation
The Colebrook-White equation allows calculation of velocity of flow in a gravity drain flowing full for any given gradient, diameter, and roughness coefficient, as follows;
k 2.51υ v = −2 (2 gDS ) log s + 3.7 D D (2 gDS )
Equation 1.5.1 Where
g = acceleration due to gravity, m2/s D = diameter, m S = slope or headloss per unit length
k s = roughness coefficient, mm
υ
= kinematic viscosity of water (m2/s).
Thus, for a 400 mm diameter pipe with
k s = 1.5 ,
and slope 1 in 157, flow temperature 15oC, the velocity will be 1.33 m/s Using the relationship : Q=Av Equation 1.5.2 Where: Q = flow in the pipe (m3/s)
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A = Cross-sectional area of flow V = velocity of flow This allows the pipe full discharge to be calculated where:
For detailed surface water modelling applications, the viscosity should be varied for a range of temperatures, but for routine applications a conservative approach will be to use the lower temperature of 150C.
A=πD2/4 Equation1.5.3 Thus, for the above pipe at full flow, the capacity will be 167 l/s A sample calculation sheet for sewers using the above formulae is included in Volume 1 Appendix 1
1.9.2.2 Manning’s Equation Manning’s equation is an empirical formula for uniform flow in open channels. Manning’s equation is:
Tables are available from hydraulic research giving values for a wide range of pipe sizes at a range of gradients for various values of ks.
v=(1/n)R2/3S0½
Tables 1.5.1 and 1.5.2 below give recommended values of ks and υ . Both are taken from the Wallingford design tablesxxi.
Where: n is Manning’s roughness coefficient, S0 is bed slope, R is the hydraulic radius (= A/P, where A is the cross-sectional area of flow and P is the wetted perimeter of fluid in the conduit).
Table 1.9.1 - Pipe Roughness ks Values Material ks Value (mm)
Equation 1.9.5
Normal
Poor
Concrete (Precast + O Rings)
0.15
0.6
Concrete (Steel Forms)
0.6
1.5
DI (PE Lined)
0.06
0.15
Channel Material
n range
GRP
0.06
-
Cement
0.010-0.015
VCP
0.06
0.15
Concrete
0.010-0.020
Brickwork
0.011-0.018
Further values can be obtained by direct reference to pages 32 to 33 of the Wallingford design tables. Caution should be exercised in the use of the Wallingford tables. It should be noted that the quality of pipes can vary considerably from one manufacturer to the next, and that condition of pipes can vary with time. Designers should avoid using the optimistic values quoted by some plastic pipe manufacturers, as these invariably refer to new pipes under laboratory conditions. Sewers for Adoptionxxii recommends a value of 0.6 for all new design, which allows for deterioration in pipe surface and normal wear. Table 1.9.2 - Kinematic Viscosity υ Values Temperature, 0C Viscosity, m2/s x 106
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15
1.141
25
0.897
35
0.727
Typical values of Manning’s n are given below. Table 1.9.3 - Typical Values of Manning’s n
Manning’s equation is only valid for rough turbulent and steady state flow conditions.
1.9.3
Prescribed Software
The DA approved software is: •
InfoWorks CS for modelling of sewerage and drainage systems;
•
Microsoft Access or Jet Access for asset databases;
•
STC25 for management of sewerage and drainage asset information;
•
MapInfo for GIS mapping.
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1.10
General Design Considerations
The requirements for drainage systems are essentially similar to those for sewerage systems in respect of layout arrangements and standards (e.g. avoidance of buildings constructed over drains/sewers). Flood routing and overland flow is a basic requirement for all surface drainage systems, to ensure that flows generated by storms in excess of the design event and drain capacity will not cause serious damage by flooding or erosion
1.10.1
Gullies
Gullies in Middle Eastern countries can be particularly problematic. For the majority of the year they are redundant and tend to fill with sand so that when the rain does arrive, the gullies do not function and flooding of the highway and adjacent areas can occur. Increased maintenance is one way of tackling this problem but can be expensive. Prevention of sand ingress is the best way to address the problem. Different types of road gullies can be used but all are prone to siltation. Preventing the ingress of sand into the gullies is one of the most important considerations during design. The different types of road gully are listed below: 1.
2.
Conventional gully pot with surface grating. This type of gully can be protected from sand ingress by the attachment of a flat sealing plate over the grating that would have to be removed for the rainy season. Side entry gully. This type of gully is set under the line of the kerb and water enters it by way of an opening in the face of the kerb. This type of gully could be protected by the installation of a vertical sealing plate to cover the gully opening.
3.
Combined side entry and horizontal grating. These gullies are a combination of 1 and 2 above and therefore would require both a horizontal sealing plate and a vertical sealing plate.
4.
Slotted kerb drainage. This system comprises a concrete kerb with and integral pipe cast within the kerb. Water enters the pipe by way of a
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longitudinal slot running the length of the kerb. At intervals along the length of the kerb line there are take-off points which connect to the surface water drain. As these systems are as long as the road, sealing them against sand ingress is difficult as it would mean sealing the entire length of the drain slot. Note that design and spacing of gullies is undertaken as part of roads projects and controlled by the RA, with spacing specified in QHDM. The road surface (including gullies) and soakaways are the responsibility of the RA, whereas the pipe network (positive drainage) is that of DA. Design of gullies is to QHDM, but note that spacing only caters for five year storms. The guidelines do not cater for greater periods, which may be required to prevent flooding in underpasses and other special areas. Table 1.1.2 refers The prescribed gully spacing in the QHDM is used as a basis for attenuating storm runoff from the carriageway surfaces to the SW system
1.10.2
Pipeline systems and Outfalls
System Layout The system layout shall comply with any overall Master Plan requirements, and be subject to manhole and chamber positioning requirements stated herein. Drains and culverts shall preferably be located within public roads and highways. There is no agreed services hierarchy, and the location must be to the approval of DA and RA. Control of Maintenance
Siltation
and
Access
for
Steps should be taken to prevent sediment and debris from entering the drainage system. This can be achieved by requiring developers to include sediment settlement and wheel washing facilities in their site facilities to minimise discharge of sediment laden flows to the public drainage system. Liaison with the RA is also needed on gully cleaning and highway resurfacing operations.
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Due to Health and Safety requirements access to the drainage system for cleaning should be at designated locations only. Pipelines should be designed (using self-cleansing velocities, low flow channels, etc) to direct sediment to these locations. Sediment clearance facilities should minimise the need for man entry by providing access for mechanical equipment, either by ramp for full machine entry or by large openings for bucket entry. Sediment clearance facilities should be located to minimise disruption to traffic and the public during cleaning operations. Inlet and Outfall Locations and Structures Most upstream inlets to the system will be from highway gullies and other paved areas. However, there will occasionally be a requirement to design intake structures to capture flow from wadis and attenuation areas. Hydraulic conditions at these locations will require careful consideration to prevent siltation and local scour, whilst passing the required design flow. Outfalls should be located in accordance with any overall Master Plan requirements, and in accordance with Development Plans. Outfall structures are likely to be large and visually intrusive constructions, and therefore liaison with the Planning Department will be required for their arrangement and finishes. Structural design will need to take account of aggressive conditions due to seawater and potentially polluted flows, as well as from traffic loading. Increased cover to reinforcement should be applied, with possible additional non-structural protective finishes. Coarse screens should be provided at all entry points accessible to the public. Such points would include large intakes and outfall pipes These screens have the dual purpose of preventing entry into the system and of retaining coarse materials. Screens should be constructed of stainless steel, with 75mm spaces between the bars. The screens should be top hinged and lockable for maintenance by the DA. Lockable bollards should also be provided to prevent entry or parking at large outfall structures. Tidal Influences and Sea Outfall Siltation Due to the low level and flat terrain of the coast of Qatar, it is to be expected that sea outfalls will be subject to tidal influences. Unless the foreshore is rocky, the outfalls will attract deposition of sand. Page 22
Outfall designs should recognise that it is impractical to remove such sand depositions. Allowance should therefore be made in the hydraulic design of the outfalls for the permanent presence of sand to a level consistent with that of the adjacent foreshore. Marine organism and seaweed will be attracted to sea outfall structures. However levels of such marine growth are generally not hydraulically significant. The DA require all standard outfall pipe inverts to be at least 0.9m QND and weir outfalls at least 1.35m QND (ie highest recorded sea level). This, together with the requirements for minimum cover and large pipe diameters, means that all outfalls will require special design, such as the use of pressure sewers, and the use of wide diffusion aprons in low-lying areas. Erosion protection should be provided at sandy outfalls where the beach or inland depression could be scoured. At beaches the outfall structure can alter the beach profile with time. Riprap or gabion surface protection should be provided to protect against scour. Submerged outfalls to offshore deep water may be required in exceptional cases
1.10.3
Pumping Stations (policy for surface water and groundwater discharge)
As explained in the Introduction, the choice of area for provision of surface water/groundwater systems has been the need to drain areas of flooding and high water table. This policy will continue, supported by the future Master Plans. The problem of intermittent operation of stormwater pumping stations, and accompanying reliability problems, has to some extent been addressed by combining stormwater and groundwater systems. Thus attenuation tanks and pumping stations are operating all year round, although at much lower flows than result from rainfall. Nevertheless, overall system planning should focus on achieving gravity flow throughout the
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network, thus minimising the need for pumping facilities. Preferred Types of Pumping Stations As with sewage pumping stations, the preference is for centrifugal pumps using wet well/dry well or submersible pumping station arrangements, depending on the flow rates involved. Septicity and odour are not usually of concern with surface and groundwater. Control facilities would therefore not be provided, unless there are specific problems such as polluted flows for septic tanks. Storage Tanks Due to the depth of the incoming drains and the large volumes to be stored such tanks are usually very large, deep and expensive installations. Hydraulic modelling to confirm that the most cost-effective combination of pumping and storage installation is being provided should therefore support their design. Tank configuration should discourage deposition of sediment with sloping floors and low-flow channels. Settling ponds or catchbasins should be provided immediately upstream of the wet well to prevent accumulation of sediment in the vicinity of the pump suctions, with consequent wear on the pumps. Inlet Screens Inlet screens or trash racks should be provided upstream of the pumping station. Screens should be of stainless steel construction and with 50mm spacing between the screen bars. Standby Power Generation Due to the likely intermittent operation of the storm pumps, it could be argued that the least reliable aspect of the pumping station is the pumping and control equipment, rather than the power supply. Standby power generation is therefore not required at stormwater/groundwater pumping stations. However arrangements for entry, location and connection of portable power generators is to be provided.
1.10.4
Attenuation Areas and Detention Ponds
These are also known as EFA’s (Environmental Flooding Areas) in Qatar. They originate in undeveloped areas from natural depressions. DA policy is to preserve such areas for incorporation into Volume 3
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the drainage master plans, but in recent years several of these areas have been built upon, exacerbating flooding problems. (see also section 1.13) The publications “Sustainable Urban Drainage Systems – Design Manual for England and Wales”xxiii together with “Sustainable Urban Drainage Systems – Best Practice Guide”xxiv published by DETR, CIRIA, provide detailed guidance on the design of detention structures for surface runoff. It is generally accepted that open areas that retain water should be designed with gentle side slopes, not exceeding 20% gradient, and should not result in a retained water depth exceeding one metre. These structures should be designed as either detention or retention structures, depending on the intended function. Detention basins are normally dry and are used to attenuate flows of surface runoff in times of rainfall. As such, these structures can have a dual use, for example sports pitches can be used as detention basins. The water in a detention basin will be lost by one or more processes: •
Firstly by discharge via a flow control device to a surface water drain or wadi;
•
Secondly, by evaporation (up to 2.0m/yr for Qatar);
•
Thirdly by soaking into the ground.
All these processes help to reduce the impact of the surface runoff discharge on the environment. These arrangements require very careful design that takes full account of any health and safety issues. The primary concern relates to the danger of drowning in the attenuation basin when it is full of water. For this reason, these basins are always designed with gently sloping sides and a maximum depth of 1m at the centre. It is possible to fence off water retaining features but this is not recommended as it makes the job of rescue more difficult in the event of an emergency and also interferes with maintenance. DA prefers the construction of permanent structures to prevent future alternative development. Please also refer to section 1.13.1 Where water is likely to be retained for prolonged periods, consideration should be given to Page 23
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problems that may arise from breeding mosquitoes. Siting The following are key considerations involved in the potential siting of a constructed wetland (CW). Opportunity for restoration of degraded or former wetlands: In general, CW’s should only be located in existing wetland areas if the source of water meets water quality standards; its use would result in net environmental benefit to the aquatic system; it would help restore the aquatic system to its historic natural condition. Water-depleted and effluent dependent systems: Constructed wetlands may provide particularly valuable ecological benefits in regions where water resources, especially wetlands, are limited due to climatic conditions, such as Qatar. Pre-treated effluent from wastewater treatment works might be the only source of water for wetlands and their dependent ecosystems. Note that SCENR has defined water quality standards and requirements for such initiatives. Other site selection factors: The suitability of a site for constructing a wetland may depend on the condition of one or more of the following factors – substrate, soil chemistry, hydrology/geomorphology, vegetation, presence of endangered species or critical habitat, wildlife, cultural/socio-economic impacts, the surrounding landscape, land use zoning, health and safety. Project proponents should carefully examine these factors, and consult with the appropriate agencies (SCENR, Agriculture and Water Resources Department, Planning Department) in determining the most appropriate site(s). The need for an EIA review, procedures or other requirements needed for final site location and characteristics, should be considered. Design Issues Any design should ensure that adverse impacts on waters or lands of Qatar be avoided. Potential adverse impacts to be considered during design include: disruption of the natural composition and diversity of plant and animal communities; alteration to existing hydrological regime, introduction or spread of noxious species; threats to groundwater resources. The margins of wetlands should be designed as natural transition zones. Where possible, the facility
Page 24
should be integrated with other natural resource features to provide wildlife corridors and open space. Where possible, the wetland design should provide habitats with a diversity of native species comparable to similar wetlands elsewhere in Qatar. The design should maximise vegetative species without increasing the proportion of weedy, non-indigenous, or invasive species at the expense of native species. The design should utilise ‘forebays’ for sediment collection/settling and ease of maintenance. Multiple cells should be considered for optimum management without disruption to the overall system (phased settlement, and better maintenance). The public’s perception of the CW should be considered during design. Where appropriate, public use should be encouraged, e.g. for environmental education and general amenity value. Further design guidance is provided in ‘Constructed wetlands treatment of municipal wastewater process design manual’, US EPA 625R-99-010xxv.
1.11
Pipelines 1.11.1
Minimum Pipe Sizes and Gradients
CIRIA Report R141 Design of Sewers to Control Sediment Problems 1996xxvi defines self-cleansing drains as follows. An efficient self-cleansing drain is one having a sediment-transporting capacity that is sufficient to maintain a balance between the amounts of deposition and erosion, with a time averaged depth of sediment deposit that minimises the combined costs of construction, operation and maintenance. Public surface water drains should be at least 300mm diameter and laid to a gradient of 1 in 60 or 1.67%. This gradient can be relaxed to 1 in 150 (0.67%) where significant surface areas are Volume 3
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connected to the head of the drain, but in this case the standard of workmanship during construction must be high. As drain sizes increase, so too do self-cleansing velocities with the result that very large surface water drains require velocities to exceed 2m/s to be selfcleansing. Such velocities in large diameter pipes pose a safety hazard and facilities, such as safety chains, must be provided to prevent operatives being washed away downstream in these drains.
1.11.2
Minimum Maximum Velocities
and Flow
CIRIAxxvi recommends that sewers should be designed to: 1.
transport a minimum concentration of fine particles in suspension.
2.
transport coarser granular material as bed load.
3.
erode cohesive particles for a deposited bed.
In order to minimise the maintenance requirements of any given length of surface water drain, it is normal to design the drain to be “self-cleansing” at design flow. This means that the drain is designed to achieve a velocity that will carry all solid deposited material along the pipe and not leave any materials deposited in the invert of the drain. Table 1.11.1 is based on the simplified CIRIAxxvi method of assessing self-cleansing velocities in drains. Surface water drains require generally higher self-cleansing velocities because of the higher particle densities. Table 1.11.1 - Approximate Self-Cleansing Velocities for Surface Water drains
Pipe size (mm)
Approximate selfcleansing velocity (m/sec)
900
0.88
1000
0.92
Where large diameter drains (over 1m diameter) are laid to steep gradients, very high flow velocities occur. For example, a 600mm diameter pipe laid to a gradient of 1 in 100 or 1.0%, will have a velocity of flow of around 2.45m/sec when flowing 450mm deep, or a 1000mm pipe laid to a similar gradient with a depth of flow of 750mm will have a discharge velocity of approaching 3.4m/sec. Such velocities may be considered unacceptable and the engineer may wish to implement energy dissipation measures. It should be emphasised that scour in pipes at these velocities is not a significant problem with modern materials. DA policy is to limit peak velocities to 2.5 m/s 3.0m/s in extreme cases, for SW drains. In small drains, less than 600mm diameter, it is not necessary to include measures to limit flow velocity. The use of backdrop manholes for this purpose is discouraged. However, backdrop manholes may be justified where there is a significant difference in level between a branch drain and the trunk drain it is to join. In this case, the economics may justify the construction of a backdrop to minimise excavation for the branch drain trench. The discharge from a backdrop into a manhole requires careful design to prevent flows from washing over the benching opposite the discharge. Backdrops for large diameter drains are complex structures, which may involve the creation of vortices to dissipate energy, and these require specialist design.
Pipe size (mm)
Approximate selfcleansing velocity (m/sec)
300
0.75
400
0.77
500
0.82
Clay (VC) Pipes
600
0.86
700
0.87
800
0.88
The preferred pipe materials for use in SW and GW gravity drainage pipelines to 1000mm nb shall be VC, due to its availability and good resistance
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1.11.3
Pipeline Materials
Please refer also to Volume 1 Section 4.3
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1.11.4
to aggressive groundwater conditions. Specification to be in accordance with QNBC Concrete Pipes Also widely used is concrete, especially for sizes >1000mm.nb Due consideration must be given to prevention of corrosion from aggressive groundwater conditions, and materials shall be specified in accordance with QNBS. Where ground water drains are flowing continuously, coatings shall be provided internally and externally. Where drains are for SW only, internal coatings may be omitted, but external are still required.
Pipes can be categorised into rigid, flexible and intermediate pipes as follows:
a
Rigid pipes support loads in the ground by virtue of resistance of the pipe wall as a ring in bending.
b
Flexible pipes rely on the horizontal thrust from the surrounding soil to enable them to resist vertical load without excessive deformation.
c
Intermediate pipes are those pipes which exhibit behaviour between those in (a) and (b). They are also called semi-rigid pipes.
Ductile Iron (DI) Pipes The preferred pipe materials for use in SW and GW pressure pipelines shall be DI. Due consideration must be given to prevention of corrosion from aggressive groundwater conditions, and materials shall be specified in accordance with QNBS. All DI pipes should be lined internally and externally against corrosion. Other Pipe Materials Where pipes are installed by trenchless techniques (See Vol 2, section 5), composite materials must be selected in conjunction with the system in use. High strength concrete jacking pipes may be slip-lined, typically with GRP or MDPE. The internal pipe becomes the watertight element, and the concrete provides the structural element. Pipeline Jointing Pipe joints must be selected and specified in accordance with the proposed conditions of use. Buried joints in concrete gravity pipe will generally be socket and spigot, with internal sealing gaskets. Jacking pipes will require a smooth external finish, and the internal GRP pipes will be formed with sleeve joints. Where MDPE pipes are inserted by slip-lining, they shall be butt fusion jointed in situ. DI joints should be socket and spigot for straight runs of buried pipe, but self-anchoring may also be used in critical areas. Where pipes pass through chambers and pumping stations, a combination of victaulic and flanged pipework will be necessary to facilitate removal of valves and other appurtenances. The ability of the joints to withstand static and dynamic thrust at such locations should be carefully considered. Page 26
Pipe Bedding Calculations for Narrow and Wide Trench Conditions
Concrete pipes and vitrified clay pipes are examples of rigid pipes while steel, ductile iron, UPVC, MDPE and HDPE pipes may be classified as flexible or intermediate pipes, depending on their wall thickness and stiffness of pipe material. The load on rigid pipes is concentrated at the top and bottom of the pipe, thus creating bending moments. Flexible pipes may change shape by deflection and transfer part of the vertical load into horizontal or radial thrusts, which are resisted by passive pressure of the surrounding soil. The load on flexible pipes is mainly compressive force, which is resisted by arch action rather than ring bending. The loads on buried gravity pipelines are as follows:
a
The first type comprises loading due to the fill in which the pipeline is buried, static and moving traffic loads superimposed on the surface of the fill, and water load in the pipeline.
b
The second type of load includes those loads due to relative movements of pipes and soil caused by seasonal ground water variations, ground subsidence, temperature change and differential settlement along the pipeline.
Loads of the first type should be considered in the design of both the longitudinal section and cross section of the pipeline. Provided the longitudinal support is continuous and of uniform quality, and the pipes are properly laid and jointed, it is
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sufficient to design for the cross section of the pipeline. In general, loads of the second type are not readily calculable and they only affect the longitudinal integrity of the pipeline. Differential settlement is of primary concern especially for pipelines to be laid in newly reclaimed areas. The effect of differential settlement can be catered for by using either flexible joints (which permit angular deflection and telescopic movement) or piled foundations (which are very expensive). If the pipeline is partly or wholly submerged, there is also a need to check the effect of flotation of the empty pipeline. The design criteria for the structural design of rigid pipes is the maximum load at which failure occurs, while those for flexible pipes are the maximum acceptable deformation and/or the buckling load. The approach for rigid pipes is not applicable to flexible pipes. For the structural design of flexible pipes, it is necessary to refer to relevant literature such as manufacturers’ catalogue and/or technical information on material properties and allowable deformations for different types of coatings, details of joints etc. Please refer to Volume 1 Appendix 7 for Pipe bedding Calculations
1.11.4.1 Bedding Design for Rigid Pipes The design procedures for rigid pipes are outlined as:
1.11.4.2 Narrow Trench Conditions When a pipe is laid in a relatively narrow trench in undisturbed ground and the backfill is properly compacted, the backfill will settle relative to the undisturbed ground and the weight of fill is jointly supported by the pipe and the shearing friction forces acting upwards along the trench walls. The load on the pipe would be less than the weight of the backfill on it and is considered under ‘narrow trench’ conditions.
1.11.4.3 Wide Trench Conditions When the pipe is laid on a firm surface and then covered with fill, the fill directly above the pipe yields less than the fill on the sides. Shearing friction forces acting downwards are set up, resulting in the vertical load transmitted to the pipe being in excess of that due to the weight of the fill directly above the fill. The load on the pipe will then be determined as in ‘wide trench’ condition.
1.11.4.4 Bedding Factors The strength of a precast concrete or vitrified clay pipe is given by the standard crushing test. When the pipe is installed under fill and supported on a bedding, the distribution of loads is different from that of the standard crushing test. The load required to produce failure of a pipe in the ground is higher than the load required to produce failure in the standard crushing test. The ratio of the maximum effective uniformly distributed load to the test load is known as the '‘bedding factor'’ which varies with the types of bedding materials under the pipe and depends to a considerable extent on the efficiency of their construction and on the degree of compaction of the side fill.
a
Determine the total design load due to:
•
the fill load, which is influenced by the conditions under which the pipe is installed, i.e. narrow or wide trench conditions;
•
the superimposed load which can be uniformly distributed, or concentrated traffic loads; and
•
the water load in the pipe.
b
Choose the type of bedding (whether granular, plain or reinforced concrete) on which the pipe will rest. Apply the appropriate bedding factor and determine the minimum ultimate strength of the pipe to take the total design load.
For design, it is required that the total external load on the pipe will not exceed the ultimate strength of the pipe multiplied by an appropriate bedding factor and divided by a factor of safety.
c
Select a pipe of appropriate grade or strength.
The design formula is as follows:
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1.11.4.5 Design Strength
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We ≤ where
Wt Fm Fs
We = total external load on pipe, Wt = ultimate strength of pipe, Fm = bedding factor,
Fs = design safety factor of 1.25 for ultimate strength of pipe
1.11.5
Manhole Positioning
The drainage system should be designed to facilitate flows by gravity in a branched arrangement of small local drains connected to larger district drains, connected to the major trunk drains. All public drains should be located in government owned lands, to permit access for construction and maintenance and to facilitate connection from private premises. Manholes, drains and culverts should be sited with due regard to public utility services. Drains and culverts in roads and highways should be located in accordance with the Standard Services Reservations Drawings as published by the Roads Department. The location of manholes in the drainage system is dictated by a number of factors: •
Spacing between manholes should not exceed 90mxxvii for non man–entry drains For man–entry pipes up to 1800mm diam, a spacing of up to 200mxxviii may be permitted. Greater spacings may only be provided in special cases, where due consideration is given to maintenance, and subject to DA approval;
•
Manholes should not be constructed close to kerb lines;
•
Manholes should be constructed at the head of each system, and at every change of diameter, direction and/or gradient;
•
A manhole should be constructed at every significant junction;
•
Manholes should not be constructed in locations on bends in highways, which may cause vehicles to skid;
•
Manholes should be accessible at all times;
•
Where a connecting branch joins a main drain on a junction, a manhole should be constructed
Page 28
within 10m of the junction on the connecting branch. Manholes and chambers will form the main points for access to the enclosed drainage system for operation and maintenance. They should therefore be located with adequate access for maintenance vehicles. Where new manholes are to be constructed in existing highways, close liaison is required with the Roads Department. Although the Standard Services Reservation Drawings (available from the DA) should be followed where possible, care must be taken to ensure that the locations of all existing utilities in the vicinity are known, and that the proposed manhole location will not interfere with such utilities. Manholes should not be located such that they would prevent access to utility equipment, especially in an emergency situation. Building over or near to a drain or culvert, and associated manholes and chambers will not be permitted. Building over drains or directly adjacent to them, causes major problems with access for maintenance and renewal of drainage assets. In extreme cases demolition of premises could be required. The land along the line of the drain or culvert for construction, and access for maintenance and replacement, is called the easement width. Where access to a drain or culvert is restricted on both sides, the easement width required is a minimum of 6m, being normally 3m either side of the centre line of the pipeline or culvert. Where the depth from finished ground level to invert exceeds 3m, or the drain diameter (or culvert width) exceeds 600mm, the easement widths required are the greater of two times the depth to the invert of the drain plus the pipe diameter (or culvert width), or 10 times the diameter of the drain (or culvert width). Foundations and basements of buildings adjacent to easements shall be designed to ensure that no building load is transferred to the drain or culvert. The nearest point of the building or basement must not fall within a 45-degree line of influence from the base of the trench. These requirements refer to permanent easements required in connection with pipe-laying and subsequent maintenance. They exclude Volume 3
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temporary storage areas, and the like, used during construction.
1.11.6
Manholes and Access Chambers
These installations are required within drainage systems for testing, inspection, maintenance, repair and removal of debris. Every drain length on the public system should be accessible without the need to enter premises or cross property boundaries. Manholes and chambers generally fall into two categories, being: •
Inspection Chambers - These structures are of shallow depth (less than 2m to pipe invert) and are intended for use on drainage systems within property boundaries and for the terminal manhole (MH 1) of the house connection. These chambers are generally used for inspection of drain pipelines and clearance of blockages.
•
Drainage System Manholes - These structures are of depth to suit the levels of the drain pipelines, and are the means of access into the public drainage system. The arrangement and dimensions of manholes depend on the diameter of the connecting drains and their depth to invert below finished ground level.
Elements of Design Manholes and chambers shall generally be constructed in accordance with the Standard Drawings contained in Volume 8. Minimum cover size should provide sufficient access to admit persons with normal hand tools and cleaning equipment, and to admit persons wearing breathing apparatus in emergencies. Maximum cover size should be limited by the weight which persons can safely lift. Access shafts should be sufficiently large for persons to go down to the drain in comfort (with breathing apparatus in emergencies) and yet be small enough for the nearness of the walls to give a sense of security.
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Where the invert of the manhole or chamber is more than 6m from the cover level, intermediate platforms shall be provided at regular intervals. Headroom between platforms should not be less than 2.1m nor greater than 6m. The platform should be fitted with handrailing and safety chains around the access opening to protect persons from falling. The location of openings in successive platforms shall be offset to prevent dangers of free-falling. Inverts and benching should be neatly formed. The ends of pipes should protrude a minimum length into the manholes. The channel inverts should be curved to that of the connecting pipes and carried up the full diameter of the pipes in flat vertical surfaces, matching the cross-sections, levels and gradients of their respective drains. The benching should be formed from plane surfaces, sloping gently towards the drains. Benching slopes should not be too steep to cause persons to slip into the drain, nor too flat to accumulate sediment. A suitable gradient for benching is 1 in 12.
1.11.7
Reinstatement and Back-filling
Reinstatement and back-filling of pipelines and around manholes and chambers has the potential of creating dangers to road users due to settlement. These dangers are caused by undulations in the road surface due to settlement of trench backfill and sudden level differences at manholes and chambers, due to settlement of the surrounding backfill. Minimising settlement relies on good quality backfilling, at the specified loading, and using material at optimum water content. Designers should therefore be aware of the need to adequately supervise all backfilling and reinstatement operations. They should also liase with the Roads Department regarding permanent reinstatement of carriageways. Further information on reinstatement and backfilling is contained in the Standard Drawings contained in Volume 8, and the Qatar Construction Specificationxxix.
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1.12
•
Methods of determining permeability are described in Volume I, Chapter 3;
•
The propensity for soakaways to become inefficient by becoming ‘clogged’ is also an issue. This may be due to silting up (which can occur early as the initial ‘slug’ of water laden with silt can be generated when groundwater is disturbed during construction work), and/or ‘smearing’ when the soakaway is drilled (i.e. the fissures are blocked off by the crushed drill cuttings). These problems may be minimised by jetting the system clean prior to its first use. All infiltration devices should be constructed late in the construction programme to minimise these effects;
•
Silting up, possibly incorporating an element of chemical encrustation, may also cause longer-term deterioration. It is often preferable to construct these devices last in the programme rather than early in the programme as would normally be expected for drainage features;
•
Ideally, soakaways should not be sited under or adjacent to buildings or roads because of the danger of induced subsidence due to the increased flow of ground water;
•
In rural areas, where groundwater wells are used as a source of water supply, a minimum protective distance should be provided between these wells and the location of soakaways to avoid cross-contamination. This distance must be agreed with the DA, and will depend upon ground investigation information at the site.
Soakaways
Policy Considerations Soakaway is the general term applied to structures that facilitate the detention and subsurface dispersion of stormwater. There is frequent reference in the literature to use of soakaways in Qatar (see Volume 1, Section 1.5) and the UK Building Research Establishment has provided a general design guide in BRE Digest 365 (BRE, September 1991)xxx. The DA itself has produced a drawing showing details of a standard soakaway chamber, which is reproduced in Volume 8. It may be noted that as groundwater levels rise generally in an area the ability of soakaways to function may be reduced. However soakaways are recognised as being an inexpensive method of dealing with small-sized storms, although the extent to which they attenuate storm waters is limited. General Design Issues The following issues require consideration when the use of soakaways is contemplated. The list is indicative only and site-specific geotechnical and hydrogeological investigation and interpretation is always required as the basis for successful design: •
Groundwater level is an issue because sufficient hydraulic gradient is needed between the water level in the soakaway and the groundwater level to cause downward and outward flow;
•
For the standard ring-type soakaway chamber of some 3m overall depth, it is recommended that in areas where groundwater levels are rising use minimum 4m below ground level as a precautionary measure;
•
The permeability of the ground has a significant influence on the ability of soakaways to function. In Qatar most permeability is attributable to subvertical jointing which is very variable over short distances and use of trench type soakaways may overcome this because the chances of encountering fissured zones is increased.
Generally, the following guidelines apply:
1.12.1
Standard Soakaways
Guidance on the design of soakaways has been given by CIRIA, and this indicates that the size of a soakaway excavation will be dependent on the results of the soakage tests. Three types of standard soakaway are in common use: •
Perforated manhole rings;
•
Rubble filled geotextile; Brick or block built chambers with open vertical joints.
•
Standard soakaway: permeability (k) > 1 x 105m/s required;
•
•
Soakaway trench: permeability (k) > 1 x 10-6m/s required;
The standard soakaway is typically 2.4m deep with a 0.5m clearance path for outflow. It is formed from a series of 1500mm or 1800mm diameter
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precast concrete rings in a granular surround and geotextile membrane with a pre-cast concrete cover. Storm water enters the chamber near the ground surface and the chamber provides local attenuation of storm water.
connect otherwise separate zones of higher permeability, thereby facilitating the dispersal of the stormwater. Also for a given stored volume, they have a larger internal surface area for dispersion of the water onto the ground.
Wherever soakaways are to be used, trench soakaways shall be preferred.
They can also be used as a conveyance system and converted to positive drainage systems by future interconnection
The DA Drawing - CAD REF SOAK.DWG dated July 1997) provides design details.
1.12.2
Borehole Soakaways
Borehole soakaways consist of a vertical borehole into which surface water is discharged via an effective head discharge structure. The head structure must be capable of intercepting all solid material that would otherwise tend to block the borehole. Borehole soakaways should only be used where the possibility of polluted water being discharged into them is absolutely minimal. This criterion is only likely to be met by discharges from roofs. Borehole depths are dependent on the nature of the sub-strata, and should only be drilled where the stratum drains freely such as fissured rock or gravels. Boreholes should be lined where the materials through which they are passing are non-cohesive, such as sands and gravels. Confirmatory SI shall be carried out to the approval of DA in all cases Boreholes should be sunk last in the programme of construction in order to avoid the possibility of contamination from site works. Borehole soakaways should never be constructed where the underlying aquifer is, or may in future be, used for the supply of drinking water. DA Drawing - CAD REF SOAK.DWG dated July 1997 provides details.
1.12.3
Infiltration trenches are normally constructed parallel to the edge of a carriageway or other liner feature to be drained. They generally comprise a trench, up to 1m deep and 500mm wide, filled with single sized stone. The stone may be surrounded with suitable fine-textured geotextile. Normally water is allowed to run off the edge of the carriageway directly into the infiltration trench. The water should not be allowed to flow across open ground before entering the trench as it may pick up soil and sand which, if deposited in the trench, will reduce its effective life. When constructed adjacent to carriageways, the top layer of stone may be bound together by the application of a bituminous spray, at application rates sufficient to bind the stone but insufficient to seal the top of the trench. This treatment is intended to reduce the risk of the stones being displaced and causing an accident should a vehicle inadvertently veer off the carriageway and onto the infiltration drain. The highway authority should be consulted on the size of single-sized stone that is acceptable adjacent to the carriageway.
Soakaway Trenches
The advantages of soakaway trenches have been recognised in Doha as a means of overcoming the problem of locally impermeable ground. Because the jointing that increases permeability is sub-vertical, a horizontal soakaway is likely to encounter and Volume 3
The design principles of trench-type soakaways are given in BRE digest 365xxx referred to earlier. Detailed design will need to take into account sitespecific conditions and the requirements of a detailed hydrogeological and geotechnical site investigation are given in Chapter 3 of Volume 1.Several different types of design are accepted in Qatar, eg culverts, plastic modules, and perforated pipes. Choice will be dependent upon local conditions
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1.13
Storage Facilities 1.13.1
Ponds/Depressions
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There are several types of storage which can be designed and the nomenclature is often used interchangeably.
•
Flow controls, hydraulic conditions, inlet and outlet structures;
•
Accessibility for operation and maintenance;
Storage facilities can be described as attenuation areas, detention basins, or retention ponds or tanks. All of these have similarities in that their purpose is to attenuate peak flows into the drainage system and store a percentage of flood water for a predetermined time. Ponds used for storage of floodwater in Qatar are commonly called EFA’s. This is a similar concept to detention basins used in other countries. Types of storage area which might be designed for Qatar are:
•
Operability as a storage facility in conjunction with other uses, e.g. how siltation will be dealt with in sports pitches and playgrounds.
•
EFA’s i.e. detention basins (see section 1.10.4), which will be formed in low lying areas and can be subjected to an acceptable level of surface flooding during rainfall;
•
Permanent wetlands/Constructed Wetlands (see section 1.10.4), are retention ponds which permanently contain water either naturally or by design, but accommodate flood peaks by varying water level during rainfall events;
•
Combined EFA’s with constructed storage tanks. These may benefit by enhancing the natural amenity of the EFA (normally arising from the original topography of the area) with engineered storage structures to provide a permanent land feature. This method of flood attenuation is preferred by DA as it optimises the use of land in Qatar;
•
Storage Tanks (see section 1.13.2 below).
There are no set procedures for designing such facilities, but major considerations which must be addressed by designers include the following: •
Soil conditions and geology;
•
Environmental factors (see section 1.10.4 above, and Volume 1);
•
Health and safety;
•
Land availability;
•
Required storage volume;
•
Detention period. This will typically be taken as 24 hrs for initial sizing, but precise determination of the detention period will depend upon the available reserve in the system and the storm size under consideration, all of which will be determined by a modelling exercise, and agreed with the DA;
•
Rates of evaporation;
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From the above considerations, it will be apparent that CW’s will not generally be viable in Qatar, as the health and safety requirement for a maximum depth of 1m is less than the rate of evaporation. This means that all such ponds are likely to dry out between the infrequent rainfall events. This leads towards provision of EFA’s as more practical. However, CW’s may be considered appropriate in certain areas, and at present there are two in operation (at Abu Nakla and Messaimeer Lake, both of which are associated with wastewater treatment). They may be included as elements of permanent landscaping, where appropriate measures will be required to control depth and retain water during dry periods. This may involve compartmentalising and use of pumping.
1.13.2
Tanks
Layouts Tank arrangements fall into two main categories, namely on-line and off-line, of which there are many further sub-classes. Figure 1.13.1 shows schematic layouts. On-line tanks are storages constructed along the route of the pipe in question, and share the same hydraulic gradient. On-line tanks (with perhaps the exception of emergency storage) always drain flows to the downstream drain by gravity. On-line tanks would normally be preferred to off-line from an operational point of view, but require certain hydraulic conditions to be satisfied in order to present a viable option. All storage tanks, are generally equipped with flow control devices on their outlet to limit peak flows from the tank, unless the flow control is provided by downstream constraints. Off-line tanks are constructed along a route separated from the main drain, and may return flows to the main drain by gravity or pumping, again depending upon the hydraulic conditions.
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There are many different possible arrangements for such tanks, each design being dependent upon required level of service and local topography. Materials and Construction Materials for tank construction may be concrete, GRP, plastic or coated steel. In-situ reinforced concrete is the most obvious choice for construction of specific designs, but certain applications will lend themselves to the use of proprietary products, e.g. large diameter pipes, precast concrete box-culverts and modular thin-walled plastic or GRP tanks with mass concrete surrounds. Designs using plastics should ensure adequate resistance to jetting pressures. All underground structures should have adequate resistance against uplift due to groundwater pressures. On-line Storage This is the simplest type of arrangement, and should be used wherever possible. Hydraulic conditions will determine the viability. The tank will need to operate within the hydraulic regime of the existing system – on-line tanks of any size will not be practical in very flat drains or culverts, due to the large surface area requirement. On-line tanks become more practical with increased gradient, but on extreme slopes, due consideration will need to be given to the greater pressures developed at the downstream ends, e.g. at pipe joints. In such cases, consideration may be given to the use of backdrops and cascades of tanks. An on-line tank will operate by surcharging as the flow approaches the predetermined pass-forward flow. This flow may be the capacity of the downstream drain or pumping station, as in Case 1, Figure 1.13.1, or a lesser value to prevent downstream flooding, as in Case 2, whereby a flow control is required to limit the pass-forward flow. In both of the above cases, care should be taken to ensure an adequate self-cleansing velocity, to prevent sediment build up. In large diameter tanks with low base-flows, this may be difficult. In such cases, a dry weather flow channel should be provided. It is recommended (Sewerage Detention Tanks – A Design Guide, WRc, 1997xxxi) that the longitudinal slope of the tank be kept to a minimum of 1:100 in on-line tanks, and that sidewall slopes into the centre channel are a minimum of 1:4. Care should be taken with benching in on-line and off-line tanks - this should be steel trowel finished with granolithic topping to prevent accumulation of solids. Volume 3
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Off-line Storage Off-line storage with gravity return is shown in Cases 3 to 6, Figure 1.13.1. This would typically be preferred where construction could proceed without the need for over-pumping, or insufficient length is available for on-line storage. The storage may be provided in a single tank, an over-sized pipe/box-culvert or groups of pipes. Care should be given to flow distribution at the upstream end, and the order of preference in filling. As the tank may not be 100% filled on a regular basis, selection of a preferential flow channel will reduce the need for desilting operations. Operational Issues Operation and maintenance of such underground structures present particular health and safety issues for access and maintenance. These aspects include: •
Blockage of flow control devices: access needs to be provided to safely enter the structure and for clearance tools and removal of debris. Where a blockage has resulted in water being retained for some time, clearing the blockage suddenly may have an unacceptable impact on downstream facilities, such as pumping stations and outfalls. Designs therefore need to consider facilities for gradual emptying or removal of flows;
•
Removal of sediment: access needs to be provided to safely enter the structure and for clearance tools and removal of debris;
•
Design to optimise removal of sediment: to minimise time and effort needed inside underground structures. Modifications to the structure of the tank to allow sediment to be removed from ground level; use of low friction coatings to discourage accumulation of sediment; modification of inlet design to increase scour; steepening of benching and installation of dry-weather flow channels to encourage self-cleansing; use of mechanical plant and flushing mechanisms to periodically remove sediments.
A checklist of typical design considerations is included in Table 1.13.1 below. The designer should note that this is an aide memoir, but not exclusive, as each application will have its own issues which require resolution.
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Figure 1.13.1 Alternative Tank Layouts Storage Tank
1. On-line Storage
Storage Tank
2. On-line Storage + Flow Control
Flow Control
3. Off-line Storage + Gravity Return Storage Tank
Flow Control
4. Off-line Storage + Screened Overflow + Gravity Return
Overflow & Screen
Storage Tank
Non-return Valve
Outfall
5. Off-line Storage + Screened overflow + Gravity Return
Flow Control Overflow & Screen
Storage Tank Outfall
Flume
6. Off-line Storage + Gravity Return + variable flow control
Flow Control Overflow & Screen
Storage Tank Outfall Flume
7. Off-line Storage + Pumped Return + screened overflow
Overflow & Screen
Storage Tank Outfall
Pump
Figure 1.13.1 – Alternative Storage Tank Layouts
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Table 1.13.1 - Storage Tank Design Checklist Consider maintenance & cleaning operations Consider the erection/removal of falsework in confined spaces during construction (use false soffits or pre-cast slabs for roof sections) Design benching to be self-cleansing Ensure sufficient access of adequate size are incorporated (NB can plant be removed once constructed) Consider type of covers (think about manual handling, and security of access) Incorporate a sufficient number of davit sockets What telemetry is required? On-line or Off-line tank? Are welfare facilities required? Is a gravity discharge achievable? Otherwise pumps will be required. Is a power supply needed? Is a water supply needed for washing down? Planning permission is required for all control kiosks and permanent accesses to the site Is a standby generator required? DA and RA Discharge consents for emergency overflow What is required in the way of control kiosks/buildings Ensure that access for a tanker is possible Place screens on inlet to tanks on off-line tanks Consider the type of screen required Design out any possible maintenance hazards Ensure adequate ventilation is achieved Is odour control required? Consider retention times of the tank How long does it take to empty the tank? Consider follow on storm events Provide a facility for overpumping of the tank Are overflows required? Provide penstocks on the tank inlets/outlets to enable flows to be diverted or isolated What return period is tank designed to (1 in x year)? Provide a penstock protected bypass pipe Is a flow control required on the tank outlet/bypass pipe? Reinstatement of area, consider future access requirements Does the site need to be purchased? HARAS complete? EIA complete ?
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1.14
Groundwater Control
1.14.1
Groundwater Levels
It is generally desirable to achieve certain critical groundwater levels to ensure successful operation of urban infrastructure. As a general guide, the following are recommended. Table 1.14.1 – Guideline Depths of Infrastructure and Minimum GW Levels Facility
Depth (MBGL)
Minimum Depth to ground water
Septic tanks and soakaways
Formation level 3.0-4.0
0.5 below formation level
Telephone cables and chambers
0.4-1.5
2.0
Power cables and chambers
0.4-1.5
2.0
Potable water system; pipes and chambers
0.9
1.4
1.0
Roads; formation level of base course
0.3-1.0
Buildings foundation level Buildings basements
1.5
This methodology makes the distinction between horizontal flow and radial flow patterns in the subsoil. Where flow of groundwater is horizontal, using parallel ditches reaching down to an impervious floor, the flow will be horizontal, and the Hooghoudt Equations can be used to determine flow quantity per unit area, as follows: 2
q = (8h/Lo ) (K1D1 +K2D2)
Where: q
= flow per unit surface area per expressed as m/d.
0.5 below formation level
K1
= hydraulic conductivity of the soil (flow region) above drain level (m/d).
1.0-1.5
0.5 below formation level
K2
4.0-4.5
0.5 below formation level
= hydraulic conductivity of the soil (flow region) below drain level (m/d) - for homogeneous soils, K1 = K2.
D1
= average depth of flow region above drain level, or average thickness of the soil layer through which the flow above the drains takes place.
D2
= average depth of pervious flow region below drain level, (depth to an impervious layer or depth of flow).
h
= hydraulic head, i.e. height of water table above drain level midway between drains (m).
Lo
= drain spacing (m).
Ground Water Drains
Horizontal groundwater drains present an opportunity to passively maintain groundwater levels at district level. This type of drainage should not be confused with building or foundation drainage, which is also commonly referred to as land drainage in Qatar. Because installing this form of drainage involves relatively deep excavation, it is disruptive to install in existing urban areas and is better suited to areas under development.
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A key reference for the design of appropriate systems is ‘Computing Drain Spacings’xviii. The work describes methods of computing drain spacings for a range of ground conditions. The output is the spacing required between drains set at different depths for specified values of recharge, depth to water table at the midpoint between drains and permeability. The calculation is steady state, so that Q(in) is equal to Q(out).
Equation 1.14.1
TSE system, pipes and chambers
1.14.2
The need for groundwater drains will be determined by the DA and included in the PSA. A key consideration is the required depth to the water table, as determined by the guidelines tabled in 1.14.1 above.
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In this case, a “ditch” can mean an open drainage channel or a French drain, containing pipe and porous drainage Media surround, acting as a cut-off drain. The pipe or channel is then designed as in section 1.9 above, to cater for the flow determined from the above equation. The above equation applies to horizontal flow only, which will cover most situations in Doha where there is an impervious layer overlain by a pervious upper layer. Where a combination of radial flow and horizontal is anticipated, the above equation may overestimate drain spacings, and the generalised Hooghoudt – Ernst equation should be used, which is: [L/Lo]3 + [8c/(π Lo)][ L/Lo]2 – [L/Lo]-B[8c/(π Lo)] = 0 Equation 1.14.2 Where: L = drain spacing based upon both radial and horizontal flow (m). Lo = drain spacing based upon horizontal flow only (m).
Equation 1.14.4 This equation holds where c/Lo < 0.3 and B < 0.1 (although the authors of the above publication state that it is not applicable for some uncommon situations, such as K = 0.25m/d, D > 5m). This equation provides a useful approach for approximate sizing of land drainage during the planning process. Work with this methodology shows it to be very sensitive to the value of permeability used so site investigation that includes permeability testing is essential for all projects incorporating land drainage. A common method of groundwater collection into the SW drainage system is via the twin-pipe system. This is preferred by DA and a typical trench cross-section detail is included in the Volume 8. Note that this method is not readily applicable to perched water table conditions.
c = D2ln(aD2/u), a radial resistance factor (m). a = geometry factor for radial flow, generally taken as 1 for simple situations where no more than one layer of differing permeabilities exists below drain level. u = wetted perimeter. B = K1D1/KD = flow above drain as a fraction of the total horizontal flow. KD = K1D1+ K2D2 Equation 1.14.3 Where: K = Equivalent value . D = overall depth. The above equations are normally solved by graphical methods, and examples of this approach are shown in the above referenced publicationxviii. A further simplified version of the Hooghoudt – Ernst equation is also presented in the above paper, as follows: L = Lo – c
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2
Pumping Stations
2.1
Standards
The standards to be used are listed in Section 1.2, and sources of information in Section 1.3 of this volume.
2.2
Hydraulic Design
The overall design philosophy of the pumping system needs to be a balanced design with due considerations of functional, environmental and economic aspects. For pumping systems in the vicinity of sensitive receivers, reliability of the system is of key concern. Particular attention should be paid to the following issues: •
Design flow;
•
Standby power supply or temporary storage;
•
Overflows and emergency bypass;
•
Twin rising mains;
•
Availability of QGEWC power supply;
•
Land area available and proximity to housing or public areas;
•
Access to the proposed site.
•
Duty/Assist pumps;
The pumping station will probably be pumping both groundwater (low flows all year round) and stormwater (high flows after rainfall). Therefore, the pumps and rising mains need to be sized for the range of flows resulting from these very different flow regimes. The pumping station will probably be serving an area of new development. It is likely that the initial flows to the station will be much smaller than those expected for the full design. Flows will increase in the following years to reach design capacity of the station. If the inflows are greatly below the pump output, the result will be excessive periods of inactivity of the station, with the potential for premature failure of equipment. Such infrequent operation of pumps will also result in retention of flows in the rising main and retention tank, corrosion and adverse environmental effects on the outfall system. Page 38
Consideration should therefore be given to the sizing and numbers of pumps to match the range of groundwater and stormwater flows, and the likely build-up of incoming flows. For GW flows, storage shall be designed at the pump station to enable pumping for periods of 30 – 60 minutes, followed by storage recharge periods of up to 3 or 4 hours.Pumps shall be designed primarily to meet surface water discharge conditions but be capable of meeting the more frequent demand for groundwater discharge. Pump operations shall be rotated to ensure that all pumps are activated at least for several days each month For SW flows, storage shall be determined to meet attenuation requirements. Generally a minimum of 2 to 3 pumps shall be required (ie 1 duty plus 1 or more assist). This approach ensures that failure of any pump will not reduce peak discharges by more than 33% Generally, no standby pumps will be required. Where possible, similar pumps should be installed, on duty and assist basis, with similar standby pump(s). The use of similar pumps will avoid any changes in pumping regime due to the rotation of duty pumps for operational reasons. Consideration should also be given to installing twin rising mains. One main only would be used in the early years of the scheme to achieve satisfactory maximum flow velocities and hence minimise siltation. When flows increase then the second main would be brought into use. Although not strictly required for the early years of a scheme, it would not be economic to construct one rising main and then construct the second within a short period, say five years. The additional costs and disruption of digging a second trench, together with operational and safety requirements of working adjacent to a “live” rising main, would be avoided.
2.2.1
Hydraulic Principles
A pumping system may consist of inlet piping, pumps, valves, outlet piping, fittings, open channels and/or rising mains. When a particular system is being analysed for the purpose of selecting a pump or pumps, the head losses Volume 3
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through these various components must be calculated. The station loss (i.e. the loss on the suction and delivery pipework from the sump to the common header) should also be considered. The frictional and minor head losses of these components are approximately proportional to the square of the velocity of flow through the system and are called the variable head.
level or the pressure above that level is lower than suction level or pressure. Fixed system heads are called static heads.
Friction losses should be determined using the Colebrook–White Formula.
A system head curve is a plot of total system head, variable plus fixed, for various flow rates. It may express the system head in metres and the flow rate in cubic metres per second. Procedures to plot a system-head curve are:
Losses in fittings at the station, and outside of it should be determined using the formula: δH = kv2/2g
Equation 2.2.1
Where δH denotes the fitting headloss (m), k is the loss coefficient, v the velocity (m/s) and g is the gravitational constant, 9.81m/s2. Indicative values of k are given in Table 2.2.1 below. Table 2.2.1 – Indicative Minor Loss Coefficients, k, for Various Fittings Fitting
Coefficient k
Standard 900 bend
0.75
Long Radius 900 bend
0.4
Standard 450 bend
0.3
Tee - line to branch
1.2
Tee – flow in line
0.35
Taper up
0.5
Sharp Entry
0.5
Bellmouth Entry
0.1
Sudden Exit
1.0
Non-return valve*
1.0
Gate Valve, fully open*
0.12
*Note that for valves it is advisable to obtain manufacturers data on headlosses. System head calculations would normally be carried out using valve open figures.
It is also necessary to determine the static head required to raise the liquid from suction level to a higher discharge level. The pressure at the discharge liquid surface may be higher than that at the suction liquid surface, a condition that requires more pumping head. These two heads are fixed system heads, as they do not vary with rate of flow. Fixed system heads can be negative, if the discharge Volume 3
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The Total Dynamic Head (TDH) for a system is the sum of the major and minor friction losses plus the static head. The duty point for a pump selection will be the required flow at the TDH.
1. Define the pumping system and its length; 2.
Calculate the fixed system head;
3.
Calculate the variable system head losses for several flow rates;
4.
Combine the fixed head and variable heads for several flow rates to obtain a curve of total system head versus flow rate.
The flow delivered by a centrifugal pump varies with system head. Pump manufacturers provide information on the performance of their pumps in the form of characteristic curves of head versus capacity, commonly known as pump curves. By superimposing the characteristic curve of a centrifugal pump on a system-head curve, the duty point of a pump can be determined. The curves will intersect at the flow rate of the pump, as this is the point at which the pump head is equal to the required system head for the same flow. The recommended values for coefficient of Colebrook–White Roughness Factor (Section 1.9.2 above) ks for use in rising mains are contained in Table 2.2.2 below. Note also the values indicated in Table 1.9.1, which refer to gravity sewers. Table 2.2.2 – Recommended Values of Colebrook-White Roughness Factors (ks) for use in Rising Mains Mean Velocity in m/s
ks (mm)
Up to 1.1m/s
0.3mm
Between 1.1m/s and 1.8m/s
0.15mm
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The discharge capacity for multiple pumps will not be simply the sum of the discharge capacity of individual pumps because the system-head curve for multiple pumps will be different from that of a single pump.
2.2.2
Pump Arrangements
The number of pumps to be installed depends on the station capacity and the range of flows. The maximum discharge rate from a pumping station, when all duty pumps and rising mains are in use should be slightly greater than or equal to the maximum design flow of the station. This will be less than the peak incoming flow due to storage attenuation. Pumps should be selected with headcapacity characteristics that correspond as closely as possible to the overall station requirements. Because of the infrequency with which theSW pumps operate in Qatar, the concept of “standby” pumps is no longer required by DA policy. . It is not desirable to have pumps of different sizes for operation and maintenance reasons, unless the flow ranges vary widely throughout the day. To cater for slow build-up of flow in the early years of operation, phased installation of pumps, or the use of a smaller diameter impeller should be considered.
2.3
Rising Main Design 2.3.1
Rising Main Diameters
The minimum diameter of pumping mains is controlled by the need to avoid blockage, and therefore should not be less than 100mm. Where surface water is effectively screened before pumping the minimum diameters should not be less than 80mm. The maximum and minimum diameters are sized to maintain flow velocities for all stages of pumping within the ranges specified in Section 2.4.
2.3.2
A thorough risk assessment should be carried out, which should include the likelihood of a main bursting, the consequence of failure, area affected, sensitive receivers affected and the feasibility of temporary diversion. A cost benefit analysis should include all tangible factors (such as cost of pipework, land cost, energy cost) and intangible factors (such as nuisance). Twin rising mains should be considered in the following circumstances: •
To accommodate a wide range of flow conditions, such that the velocity in the mains can be kept within acceptable limits. For instance, a pumping system serving a new development may have very low initial flows with a slow build-up of flow;
•
To provide continued operation for a major pumping system when one of the mains is damaged and where the failure of the system would have serious consequence;
•
To minimise adverse environmental impacts to sensitive areas;
•
To facilitate future inspection and maintenance of major pumping systems, while the normal surface or ground water flow can be maintained.
When twin mains are found to be preferred, it is advisable to use both mains as duty rather than one as duty and the other as standby from an economical and operational point of view. Should one of the duty mains be taken out of operation, the remaining one would still be able to deliver a higher quantity of flow at a higher velocity. The occurrence of overflow or bypass can be minimised or even eliminated.
Twin Rising Mains
The use of twin rising mains should be considered on a case by case basis. The main factors for consideration include the design elements, the risk assessment and cost benefit analysis.
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Considerations for the design elements comprise; the rate of build-up of flow, the range of flow conditions, the range of velocity in the mains, the availability of land for the twin mains and associated valve chambers as well as the complications in pump operations.
2.3.3
Economic Analysis
As the size of the rising mains increases, the velocity and the system head will decrease, with savings in the cost of pumping. The increase in Volume 3
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the capital cost of rising mains will be offset by the power cost of pumping. However, it is also important that the velocity in the mains should be within a suitable range to minimise the deposition of silt. Excessive hydraulic head losses are to be avoided.
2.5
The selection of a suitable size for the rising mains should be based on economic analysis of capital cost and recurrent cost of the pumping system, including the power cost. A trial and error approach should be adopted in order to arrive at the optimal solution while maintaining the velocity within acceptable limits.
2.6
Therefore, combinations of different sizes of rising mains and the system head should be evaluated, taking into account both the capital cost and the energy cost of pumping.
2.3.4
Rising Main Alignment
The alignment of the rising main should discourage surge in its flow conditions. Where possible the rising main should be laid with continuous uphill gradient, and with gentle curves in both horizontal and vertical planes. Otherwise, air-release valves should be provided at high points, and as the profile of the main dictates. Washouts should be installed at low points. The arrangement and locations of valves should be planned together with the alignment of the rising mains.
Pipe Materials
Materials for use in pumping stations will be Ductile Iron, as discussed in section 1.11.3.
Thrust Blocks
Thrust blocks are concrete blocks designed to prevent pipes from being moved by forces exerted within the pipe by the flow of water hitting bends, tapers, and closed or partially closed valves. In the design of pressurised pipelines, thrust blocks are essential on flexibly jointed pipelines, where any pipe movement would open up the joints in the line and cause water leakage. Thrust blocks are also necessary near valves where a flexible joint is located, to facilitate removal of the valve for maintenance purposes. The size of block is dependent upon the deflection of the flow, the size of the pipe and the head of water inside the pipe. Please also refer to the recommendations of pipe manufacturers. An example thrust block calculation spreadsheet in shown in Volume 1 Appendix 1 The following design assumptions are to be adopted: •
Thrusts developed due to changes in direction of pipeline, dead end or change in diameter should be considered. Force due to change in velocity head is assumed negligible unless there is a drastic change in pipe diameter;
Maximum and Minimum Velocities
•
Thrust blocks should be designed for the condition of no support being available from the backfill;
The maximum velocity should not exceed 2.5m/s, or 3m/s in extreme cases, governed by the concerns for the power cost. The desirable range of velocity should be 1m/s to 2m/s with due consideration given to the various combinations of number of duty pumps in operation. Velocities should be slightly higher in SW rising mains than foul, because of the higher density of the silt when compared to foul sewage.
•
The restraining effect of the ground behind blocks should be ignored on the basis that the ground might be disturbed by work on adjacent services. The block should be designed so that the total thrust in the pipeline is resisted by the self-weight of the block, or frictional resistance offered by the self-weight. Special foundations, such as raking piles, may be required in such circumstances;
Minimum velocities should be in accordance with table 1.11.1
•
For pipes with flexible joints such as DI pipes with socket and spigot joints, all the thrust is assumed to be taken up by the blocks.
Long flat lengths of rising main should be avoided, as should pumping downhill to the discharge point.
2.4
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2.7
Air Valves and Washout Facilities
These facilities are required to minimise the adverse effects of surge and to facilitate the operation and maintenance of the rising main.
2.7.1
Each air valve will operate independently and therefore several valves may be required along the pipeline if there are numerous rises and falls in the vertical profile of the rising main.
Vented Non-return Valves
An air valve combined with a vented non-return valve allows air to enter the pipeline freely on separation, but controls the expulsion of air as the column rejoins. This has the effect of creating an air buffer between the column interfaces, thus reducing the impact velocity of the rejoining column and the surge potential of the system.
2.7.3
Wash – Outs
The purpose of the washout system is to drain the rising main to allow maintenance. The washout should be installed at low points of the pipeline profile, and needs to be located carefully, taking into account that water will be discharged. For long rising mains with few low points, wash-outs should be strategically located at suitable intervals to reduce the time required for emptying the main in an emergency. The washout chamber should be provided with a sump so that the drained contents of the rising main may be tankered away, if a direct connection to a suitably sized drain is not available.
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Isolating Valves
For long rising mains, Isolating valves should be included to allow sections of the rising main to be isolated and emptied within a reasonable time. Inline sluice or gate valves are often used as isolating valves. The isolating valve installation may incorporate washout facilities.
Air Valves
Air-relief valves are installed at locations of minimum pressure. Air is sucked into the air-relief valve when pipeline internal pressure is below atmospheric. Upon subsequent pressure rise, the admitted air is then expelled. Air valves are usually installed at convex points where air could accumulate or need to be introduced, such as at local high points and at severe vertical bends (which should be avoided, if possible).
2.7.2
2.7.4
2.8
Flow Meters 2.8.1
Application and Selection
The variety of choices facing an engineer confronted with a flow measurement application is vast. For example, the positive displacement principle types include rotary piston, oval gear, sliding vane, and reciprocating piston. Each type has advantages and limitations and no one type combines all the features and all the advantages. Differential pressure meters have the advantage that they are the most familiar of any meter type. They are suitable for gas and liquid, viscous and corrosive fluids. Their usable flow range is limited and they require a separate transmitter in addition to the sensor. Some of the most important parameters for flowmeters are accuracy, flow range, and whether the medium is sewage or water. Meter selection should be done in two steps. The first step is to identify the meters that are technically capable of performing the required measurement and are available in acceptable materials of construction; then, to select the best choice from those available. Assess the need for special features such as reverse flow, pulsating flow, response time and so on.
2.8.2
Magnetic Flowmeters
It is not normally considered necessary to measure flows of surface water but if it is decided that this should be done, then magnetic-type flowmeters may be used. These devices use Faraday’s law of electromagnetic induction for making a flow measurement. That is when a conductor moves through a magnetic field of given field strength, a voltage level is produced in the conductor that is dependent on the relative velocity between the Volume 3
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conductor and the field. Faraday foresaw the practical application of the principle to flow measurement, because many liquids are adequate electrical conductors. So these meters measure the velocity of an electrically conductive liquid as it cuts the magnetic field produced across the metering tube. The principal advantages include no moving components, no pressure loss, and no wear and tear in components. Magnetic flowmeters offer the designer the best solution for pumped surface water flow. With nothing protruding into the flow of water, the chances of a blockage if installed correctly are non-existent. Magnetic flowmeters should always be installed with full pipe conditions. Care should be taken during design to provide sufficient straight run, up-stream and down-stream of the flowmeter in accordance with the manufacturers installation instructions. As a general guideline, 12 pipe diameters of straight pipe on the inlet, and 6 pipe diameters on the outlet, will ensure that the flowmeter is able to achieve the specified accuracy. If the amount of space available is restricted, then the minimum usually accepted by manufactures is inlet run > 5 pipe diameters, and outlet run > 3 pipe diameters. Refer to standard installation details Volume 8. The installation should allow for the future removal and replacement of the flowmeter. The following International and British Standards are a good source of information on flowmeter selection and installation. BS EN ISO 6817: 1997: Measurement of Conductive Liquid Flow in Closed Conduitsxxxii. BS 7405: 1991: Guide to Selection and Application of Flowmeters for the Measurement of Fluid Flow in Closed Conduitsxxxiii. Flowmeters should be pressure tested, calibrated by the manufacturer, and certified to a traceable international standard. As a minimum, the overall accuracy should be better than 0.5% of the flow range. The repeatability of the result should be within 0.2%.
•
Programmable in-built alarm relays for empty pipe, low and reverse flows;
•
In-built digital display for flow rate, total flow and alarms;
•
Transmitter enclosure shall be protected to IP67;
•
Calibration and programming kit.
The earthing rings should be included according to the individual manufacturer’s instructions. The sensor lining should be neoprene or an equivalent material of similar or improved properties, suitable for the application of pumped surface water. In below ground flowmeter chamber installations, the installed equipment should be submersible to the maximum chamber depth. Ultrasonic Flowmeters Ultrasonic meters are available in two forms: Doppler and transit-time. With Doppler meters, an ultrasonic pulse is beamed into the pipe and reflected by inclusions, such as air or dirt. This may render them unreliable if the water being pumped is very clear. The Doppler meter is frequently used as a “clamp on” device which can be fitted to existing pipelines. It detects the velocity only in a small region of the pipe cross section and as such its accuracy is not good. The single or multi-beam transit-time flowmeters project an ultrasonic beam right across the pipe at an acute angle, first with the flow and then in opposition to the flow direction. The difference in transit time is proportional to flow rate. This type of ultrasonic meter is considerably more expensive but offers better accuracy. Unlike the Doppler meter, it requires a relatively clean fluid. The main use of this type of flowmeter in pumped surface water flows is in retrospective installation, where the pumping main cannot be broken into for operational reasons. A clamp-on ultrasonic flowmeter can be used to give reasonably accurate flow measurement. For new installations, the lower cost of in-pipe ultrasonic flowmeters could make them a viable alternative to magnetic flowmeters for large diameter pipe installations.
In addition to the calibration certificate the flowmeter manufacturers should provide the following: •
Isolated 4-20mA dc and pulse outputs;
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2.9
Surge Protection Measures
Surge (or water hammer) is an oscillating pressure wave generated in a pipeline during changes in the flow conditions.
If the surge pressure approaches zero or the pipeline maximum pressure, a full surge analysis should be carried out. Surge Suppression Methods
•
pump starting;
Surge suppression could be achieved using one of -the following devices. The most appropriate device will depend on the individual circumstances of the installation:
•
pump stopping/power failure;
•
Flywheel;
•
valve action;
•
Pressure vessel with bladder;
•
improper operation of surge control devices.
•
Dip-tube surge vessel;
The most likely one of these is the sudden stopping of pumps caused by a power failure.
•
Surge tower.
There are four common causes of surge in a pipeline:
A surge analysis should usually be carried out unless the system is simple. An approximate calculation for a simple pipeline is: V
P=ax g
Equation 2.9.1 Where: P = Pressure change (m) a = pressure wave velocity (m/s) V = flow velocity change in 1 cycle (m/s) g = acceleration of gravity (9.81m/s2) The simple cycle time can be calculated with the formula: Cycle time = 2 x pipeline length Wave velocity Equation 2.9.2 Table 2.9.1 – Indicative Surge Wave Velocity Values for Selected Pipe Materials Pipe Material
Velocity (m/s)
Ductile Iron
1000 – 1400
Reinforced Concrete
1000 – 1200
Plastic
300 – 500
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Air valves should not be used as a method of surge control, but their operation under surge conditions should be carefully considered. Flywheels Flywheels absorb energy on start-up, slowing the rate of velocity change in the pipeline. In reverse, when the pump is stopping, the flywheel releases energy again, slowing the rate of velocity change. Together these two actions reduce the peak surge pressure. As the flywheel must be located on the drive shaft it is not suitable for submersible pumps or closecoupled pumps. However, they are simple devices for wet well/dry well pumps and are preferred where possible. If submersible pumps have been chosen, a larger pump running at a slower speed may have the effect of a flywheel. Because the flow continues through the pump after the stop signal, the effect on the stop and start levels should be carefully considered. Pressure Vessels Pressure vessels for surge suppression are tanks partially filled with a gas (air or nitrogen). Usually the liquid is contained in a bladder with gas on the outside to prevent the liquid absorbing the gas or coming into contact with the inside of the pressure vessel, and this is the preferred type. The bladder material should be carefully selected for use in the conditions experienced in Qatar.
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Refilling is usually from a high-pressure cylinder and care should be taken to avoid over pressurisation of the bladder. Bladders should not lose pressure in normal operation, but they can fail, leading to absorption of the gas into the liquid, and a drop in pressure. Vessels without a bladder are charged with air pressure from an air compressor, either manually or automatically. There is therefore additional machinery and an additional maintenance requirement. This type of surge vessel is not recommended. On pump start-up, liquid enters the vessel, compressing the gas until it equals the liquid pressure. When the pump stops, the gas pressure forces liquid back out into the pipe system, both actions slow the rate of pressure change, which reduces the peak surge pressure. To dampen oscillations, a non-return valve may be fitted to the surge vessel outlet pipe, to allow unrestricted flow into the pipeline, and a bypass around the NRV fitted with an orifice plate to restrict the flow back into the vessel. Dip Tube Surge Vessels A dip tube surge vessel is pressure vessel, the top portion forming a compression chamber limited by a dipping tube with a shut off float valve. This type of vessel is particularly appropriate for use on rising mains with flat profiles.
However, air valves, particularly if fitted with a vented non-return valve or in-flow check valve, may assist in surge control, and their operation must be carefully considered. Air valves require regular maintenance because if the air valve does not function correctly, large or negative surge pressures could result, with consequent damage to equipment or personnel. If air is allowed into the rising main on pump stop/trip through an air valve, the pump control system should be designed to prevent a restart until the transient pressures have stabilised. Control of the pumps is usually by start/stop level signals, but where surge on start-up may have a significant effect, the use of ‘soft’ starters should be considered.
2.10
Screen Selection Screens should generally be provided for pump protection, unless they are small circular submersible stations with small inlets and a design flow of less than 15–20l/s. Screens should incorporate the following features: •
Screen chambers should be separate from the wet wells;
•
Coarse screens should be fitted in the screen chambers at the inlet to pumping stations to protect the pumping equipment. They should remove coarse screenings greater than 100mm;
•
‘L’ shaped or coarse basket screens should be provided;
•
The screens should be set in guides with lifting facilities at ground level so they can be manually removed and cleaned;
•
Minimum of one duty and one standby screen.
Surge Towers A surge tower is a vertical tank or pipe fitted into the pipeline, open to atmosphere and the energy storage is by the static head of the liquid in the tower. Surge towers are only practical for systems with relatively low heads and surge pressures, but can pose an odour risk. Due to the design of a surge tower, there is no routine maintenance required to ensure the surge tower keeps operating correctly. It is unlikely that surge towers would be appropriate for use in Qatar. Air Valves Air valves are required on the pumping mains to release air, but they should not be used as a surge protection measure. Volume 3
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Screens
Screen Installation The manual duty and standby screen should be installed in the incoming channel, so that the standby screen can be lowered into position to protect the pumps while the duty screen is removed and cleaned.
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Screenings Handling Mechanically removed screenings should be placed in a container until removed from site.
2.11
Pumping Stations – Selection
Surface water pumping station type selection should be carefully considered for each scheme. In general, submersible pumping stations are generally selected for flows up to 100l/s and wet well/dry well stations for larger flows. However, each station should be treated on its own merits and the following considerations assessed:
Submersible pumping stations should incorporate the following features: •
Minimum of two duty pumps, that when working together meet the full flow requirement ( standby not required);
•
Non return and gate valve for each pump isolation;
•
Valves to be in a separate, easily accessible chamber adjacent to the pump sump;
•
Operation level controls (either air reaction bubbler, electrode or ultrasonic) as follows: - High level alarm; - Pump start;
•
Initial and final design flow;
- Pump stop;
•
Total head on the pumps;
•
Rising main profile and the requirements for surge protection (dry well pumps usually have a greater moment of inertia than submersibles);
- Low level pump protection, in addition to the method installed for pump control.
•
Requirement for variable speed drive (submersible motors are not always adequately rated for use with VSD);
•
Space available for pumping station (submersible stations usually require less space);
•
Proximity of housing or public areas (opening submersible pump wells may create odour nuisance).
An alternative to wet well submersibles and dry well pumps is the dry well submersible. These should normally be considered only where an existing dry well installation is being uprated and there is insufficient space to install a conventional dry well pump and motor. Particular attention should be paid to motor cooling and cabling if dry well submersibles are to be considered.
Ultrasonic level controls should be configured to hold the last measurement in the event of a lost echo and should be protected against accidental damage. Where the available pumps have unsuitable duties for the full range of flows, the use of variable speed drives should be considered. However, due to the additional heat generated in the motor, the approval of the pump manufacturer should be obtained before variable speed drives are used. Submersible Pump Sump Design The CIRIA guide ‘The hydraulic design of pump sumps and intakes’ by M. J. Prosserxxxiv should be referred to when designing pump sumps. Some pump manufacturers also provide guidance on the design of sumps for their pumps. Sump design should be in accordance with the following criteria: •
Sumps should be designed so that dimensions satisfy the requirements for minimum sump volume to ensure maximum rated pump starts per hour for motor and switchgear are not exceeded;
•
Sumps should be designed to provide a uniform steady flow of water into any pump without creating swirl or entraining air. Unsteady flow can lead to fluctuating loads, vibration, noise and premature failure. Swirl can affect the flow capacity, power and efficiency. It can also result in local vortices
Axial flow pumps should be considered for very high flows against low head situations and are therefore very suitable for surface water lifting stations. The designer should present three alternative pump suppliers for tender purposes. Submersible pumping stations
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that introduce air into the pump, also leading to fluctuating loads, vibration, noise and premature failure; •
•
Sumps should be designed to prevent the accumulation of sediment, scum and surface flotsam; Sump corners should be benched to 45o. Minimising the sump floor area and residual volume will increase the velocity into the pumps and improve scouring;
•
The use of flushing devices to improve scour in pump sumps should be considered;
•
The velocity in the pump riser pipe at the design duty should be as high as practicable to reduce the risk of solids deposition. However, the velocity should not normally exceed 2.5m/s to avoid significant headloss and risk of pipe erosion;
•
•
Wet/Dry Well Pumping Stations. Wet well/dry well pumping should incorporate the following features: •
Two sumps, normally with 2 duty and 1 standby pump for each sump for the ultimate flow;
•
Non return and 2 gate valves for each pump isolation;
•
Where possible, the discharge manifold should be below ground level to minimise additional pipework and friction losses;
•
Where wet well/dry well pumping stations are being uprated, dry well submersible pumps could be considered;
•
Operation level controls (either electrode or ultrasonic) as follows:
The water surface in the sump should be as free from waves and turbulence as possible to provide a strong and reliable echo for ultrasonic level controls;
- High level alarm; - Pump start; - Pump stop;
At the designed stop level there should still be sufficient water surface area without obstructions to provide a good echo return.
Submersible Pump Installation
- Low level pump protection in addition to the method installed for pump control. •
When submersible pumps are installed, the following should be considered: •
There should be sufficient space between them to prevent interaction between the pump suctions;
•
There should also be sufficient space for someone to stand beside each pump, should work be required in the sump;
•
Pump mounting stools and duckfoot bends should be securely bolted to the structural concrete of the sump, and not the benching;
•
Discharge non-return and isolating valves should be located outside the sump in a valve chamber;
•
Pump guide rails should rise close to the underside of the sump covers above the pumps;
•
The covers should have a clear opening large enough to allow the removal of the pump while on the guide rails;
•
Support points for the pump power cables and lifting chain should be provided under the pump covers, these should be easily accessible from the surface.
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Ultrasonic level controls should be configured to hold the last measurement in the event of a lost echo.
Where the available pumps have unsuitable duties for the full range of flows the use of variable speed drives should be considered. However due to the additional heat generated in the motor, the approval of the pump manufacturer should be obtained before variable speed drives are used. Wet Well Design The CIRIA guide ‘The hydraulic design of pump sumps and intakes’ by M. J. Prosserxxxiv should be referred to when designing wet wells, which should incorporate the following features: •
Wet wells should be designed to provide a uniform steady flow of water into any pump without creating swirl or entraining air. Unsteady flow can lead to fluctuating loads, vibration, noise and premature failure. Swirl can affect the flow capacity, power and efficiency, it can also result in local vortices that introduce air into the pump also leading to fluctuating loads, vibration, noise and premature failure;
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•
Careful thought should also be given to the shipping route for removing equipment;
•
Access to the dry well and machinery should be by staircase so that tools and equipment can be carried in and out safely;
•
Lifting arrangements for the pumps and valves (see also sections 2.21 and 2.22);
The use of flushing devices to improve scour in wet wells should be considered;
•
The water surface in the wet well should be as free from waves and turbulence as possible to provide a strong and reliable echo for ultrasonic level controls;
The dry well floor should slope gently towards the dividing wall and then to one side where a sump pump should be installed to keep the floor as dry as possible;
•
The sump pump should be installed in a small well, large enough to accommodate the pump and should discharge back through the wall into the wet well. Consideration should be given to the sump pump discharge to avoid backflow from the wet well to the dry well;
•
A high level alarm should be installed in the dry well to give a warning of flooding before damage to machinery occurs.
•
Wet wells should be designed to prevent the accumulation of sediment, scum and surface flotsam;
•
Wet well corners should be benched to 45o. Minimising the sump floor area and residual volume will increase the velocity into the pumps and improve scouring;
• •
•
At the designed stop level there should still be sufficient water surface area without obstructions to provide a good echo return;
•
Wet wells should be designed so that the dimensions satisfy the requirements for the minimum sump volume to avoid excessive pump starts;
•
•
The pump suction pipes should be installed through the wet/dry well dividing wall with a downward bend and bellmouth to position the pump suction as close to the sump floor as possible to assist in sediment removal; There should be sufficient space between the bellmouths to prevent interaction between the pump suctions.
Dry Well Design Dry well design should incorporate the following features: •
The pumps should be installed along the wet/dry well dividing wall with sufficient space between them to allow access for maintenance and repair;
•
The pump distance from the dividing wall will be set by the length of the protruding stub pipe, suction valve and pump inlet pipe;
•
Drive shafts should be supported from concrete beams spanning the dry well;
•
Consideration should also be given to access around the pumps and valves. Platforms and walkways should be installed to provide access to all equipment at a suitable level for safe operation, maintenance and repair;
•
The general floor level should be higher than the sump level to reduce the size of pump plinths and the need for access platforms;
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Pump Installation For the most compact arrangement, a closecoupled pump can be mounted horizontally with the discharge upward, however this results in the motor being low in the dry well and at risk from flooding. The most common arrangement is for a vertical pump shaft with the motor above. This will require a bend between the suction valve and the pump suction. The bend should be fitted with a handhole and valve to enable the pump to be drained prior to maintenance. Further bends may be required to direct the pump or manifold discharge upwards. Where space allows, installation of the discharge manifold at the pump level, with the discharge directly through the sidewall should be considered. Pipes should be sized to achieve sensible velocities and the risk of cavitation through insufficient NPSH should be considered when designing suction pipework. Pumps must be selected to ensure satisfactory operation when only one pump is in operation in a new rising main. Axial Flow Pumps For situations requiring high flows at low heads when transferring surface water from one level to another axial flow pumps should be considered. Axial flow pumps are usually mounted vertically and traditionally have been of the ‘trunk slung’ type Volume 3
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with the pump bowls in a wet sump and the motor mounted above in a dry room. The discharge pipework may be located either above or below the motor room floor. It is usual for the pump to have its own thrust bearing, mounted in the motor support stool, to support the weight of the rotating element and absorb the hydraulic thrust. Axial flow pumps are now also available as a submersible pump installed in a vertical discharge tube of nominal diameter. The vertical discharge tube may be a steel pipe or a concrete structure. This type of installation is not so suitable where the pumps have to deliver into a rising main. The operating parameters of axial flow pumps at all conditions of operations should be carefully considered, particularly the NPSH requirements. Sump Design The CIRIA guide ‘The hydraulic design of pump sumps and intakes’ by M. J. Prosserxxxiv should be referred to when designing wet wells. The relevant pump manufactures recommendations for sump configurations should be referred to when designing the sumps and intakes. More care is needed when designing a sump for an axial flow pump as this type of pump is more sensitive to inlet flow conditions: •
An anti swirl plate is often required to remove any swirl at the inlet to the pump;
•
The inlet, forebay or sump should be designed to provide a uniform steady flow of water into any pump without creating swirl or entraining air. This may require an anti swirl plate, a draft tube intake or formed suction intake to provide for smooth acceleration and turning as the flow enters the pump;
•
Unsteady flow can lead to fluctuating loads, vibration, noise and premature failure. Swirl can affect the flow capacity, power and efficiency. It can also result in local vortices that introduce air into the pump also leading to fluctuating loads, vibration, noise and premature failure;
•
As the inflow to the pumps is critical, multiple pump systems should be designed around the individual sump modules to ensure uniform inflow to each pump;
•
The water surface in the wet well should be as free from waves and turbulence as possible to provide a strong and reliable echo for ultrasonic level controls;
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•
At the designed stop level there should still be sufficient water surface area without obstructions to provide a good echo return.
Pump Installation A ‘trunk slung’ pump installation should incorporate the following features: •
The motor should be mounted on a support stool, which incorporates the pump thrust bearing;
•
Oil filled thrust bearings will normally require cooling. This may be filtered product water provided suitable controls & alarms are incorporated;
•
Shaft protection tubes should not be necessary if product cooled ‘cutless rubber’ pump sleeve bearings are utilised;
•
The distance between shaft couplings should be co-ordinated with the pump lifting arrangements;
•
Isolating and non-return valves will be required unless the pump delivery point is above TWL in the discharge channel;
•
Delivery pipework may be above or below the motor room floor. However, access for maintenance is likely to be easier if above floor level.
A submersible pump installation incorporate the following features:
should
•
Concrete shaft or steel tube to support the pump;
•
Suitable discharge arrangement. If free discharge above TWL occurs in the receiving channel, then no valves are required. If it is a submerged outflow, some type of non-return arrangement will be required to prevent backflow;
•
A sealed cover plate and sealed cable exit will be required if the top of the riser pipe is below delivery channel level.
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2.12
Pumps and Motors
Centrifugal Pumps These are the most common type of pumps for surface water and are available in a variety of forms. The pump operates by passing the liquid through a spinning impeller where energy is added to increase the pressure and velocity of the liquid. Submersible pumps are centrifugal pumps. Surface water pumps should preferably have an open type impeller with a minimum passage of 100mm. Impellers with smaller passages are at risk of blockage. Dry well centrifugal pumps should normally have a maximum running speed of 980rpm. Submersible pumps may run at up to 1450rpm (4 pole motor) but pumps operating at 2900rpm (2 pole motor) will suffer excessive wear and premature failure and should not be used.
Axial Flow Pumps Axial flow pumps can be submersible or trunk slung as detailed above. Pump Motors Pump motors should normally be fed from 415 volts, 50 hertz, 3-phase power supply. However for larger motors 690V or 3.3kV can be used. An explosion risk assessment study should be carried out to identify whether motors on submersible pumps should be certified for use in an explosive atmosphere. The risk is more likely to be from petroleum than methane. Because additional heat is generated in the motor when used with a variable speed drive, the approval of the pump manufacturer should be obtained before VSDs are used. For dry well and screw pumps where the motors are installed vertically or at a steep angle they should be specifically designed for that purpose with adequately rated end thrust bearings. Where flywheels are installed, the motor rating shall be suitably uprated.
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2.13
Sump Design
The CIRIA guide ‘The hydraulic design of pump sumps and intakes’ by M.J. Prosserxxxiv should be referred to when designing sumps or wet wells. Sumps should be designed to provide a uniform steady flow of water into any pump without creating swirl or entraining air. Unsteady flow can lead to fluctuating loads, vibration, noise and premature failure. Swirl can affect the flow capacity, power and efficiency. It can also result in local vortices that introduce air into the pump also leading to fluctuating loads, vibration, noise and premature failure. Sumps should also be designed to prevent the accumulation of sediment and surface scum. Most sumps and wet wells at standard pumping stations will probably be uniform in section and can be designed to avoid turbulent flows. For non-standard pumping stations, which may have high flows, multiple pumps or complex shapes, or where turbulent flows, vortices, swirl or air entrainment are more likely to occur, modelling should be considered. For pumping stations, a physical model built to scale can be very effective in identifying flow problems and in some cases modelling by computational fluid dynamics (CFD) methodology may have benefits.
Sump Volume Pump sumps should have a minimum sump volume calculated to ensure that in the worst flow conditions any pump installed does not exceed the maximum allowable starts per hour. The CIRIA guide ‘The hydraulic design of pump sumps and intakes’ by M. J. Prosserxxxiv should be referred to when designing sumps or wet wells. The minimum sump volume is the volume between the start and stop levels of the duty pump, and for a single pump, the worst case occurs when the inflow is exactly half of the pumping rate. To calculate the minimum sump volume for a specific pump the formula used in the above CIRIA guide is: Volume 3
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T = 4V/Qp Equation 2.13.1 Where:
The maximum allowable starts per hour should be as specified by the pump or motor manufacturer. In the absence of any specified figure the following are suitable guidance figures:
- T is the cycle time for the pump, e.g. if the recommended maximum starts per hour for a pump is 10, then the cycle time will be 6 minutes (60/10 = 6).
Less than 100kW:
15 starts/hour
>100kW - 200kW:
8 starts/hour
- V is the volume of sump between the start and stop levels in m3.
Stop/start levels for single and multiple pump operation
- Qp is the pumping rate in m3/minute.
The start and stop levels for single pump operation should be set within the maximum and minimum start/stop levels defined in the previous section provided that the minimum sump volume is attainable.
Therefore, if Qp is 1.2m3/min (20l/s) and the maximum number of starts is 10/hour, the volume required will be: V (m3) = 6 (min) x 1.2 (m3/min) / 4 V = 1.8m3 For 10 starts per hour this could also be expressed as V = 1.5 x Qp. The sump volume when multiple pumps are installed is calculated as for a single pump where the minimum sump volume is the capacity between the start and stop level for each pump. However, additional capacity is required to allow a vertical distance of 150mm between the start or stop levels of consecutive pumps.
The start level for each additional pump should be set a suitable height above the previous pump to prevent accidental pump starts caused by surface waves or level sensor errors. The stop level for each additional pump should be set the required distance below the start level to provide the minimum sump volume for that particular pump. The stop level will normally be just above the previous duty pump stop level. The effect of flywheels should be considered in determining stop/start levels because the flywheel increase the pump start-up and stop times.
Pump duty level Maximum and minimum start / stop levels. The minimum stop level should be the level at which the pump can be stopped and restarted without losing suction, or as specified by the pump manufacturer. To avoid turbulence at pumping stations, the lowest pump stop level is usually set at the invert of the incoming drains, the last section of which is laid to a steep fall to avoid the surface water drains being used as the sump.
The pump duty level for a single pump should be the midpoint between the pump start and stop levels. For multiple pump installations it should be the midpoint between the top water level (last duty pump start level) and the bottom water level (first duty pump stop level). Pumps should also operate within their performance curve at both top and bottom water levels, under single or multiple pump operation.
The minimum start level should be the required distance above the stop level to provide the minimum sump volume. Allowable pump starts per hour
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2.14
Suction/Delivery Pipework, Isolation
•
Pipework Only superior materials are acceptable for use in pumping station pipework. The pipework installation should incorporate the following features:
All flexible couplings should be restrained on both sides by securely fixed equipment, thrust blocks or tie straps across the coupling to prevent displacement of the coupling under pressure.
Valves Valves should incorporate the following features: •
Isolation valves should be of the doubleflanged wedge-gate type with a bolt-on bonnet. When fully open, the gate should be withdrawn completely from the flow. The valve should have an outside screw rising stem and the handwheel direction of operation should be clockwise to close. Station valves should have metal seats;
Each submersible pump should be installed with a discharge isolation valve to permit isolation of the pump from the discharge pipework for maintenance;
•
Valves greater than 350mm diameter should be fitted with actuators. Where installed in chambers, they could be fitted with non-rising stems to limit the headroom required;
•
Each pump should also be fitted with a nonreturn valve to prevent reverse flow back through the pump when stopped;
•
•
Valves should be positioned to permit the removal of each pump and non-return valve without draining either the wet sump or discharge manifold and allow the other pumps to continue operating normally;
Reflux valves should be of the double flanged, quick action, single door type, designed to minimise slam on closure by means of heavy doors weighted as necessary. The door hinge pin/shaft should extend through the side of the body and be fitted with an external lever to permit back flushing;
•
Suction isolating valves for dry well pumps should be bolted directly to a flanged pipe securely fixed through the sump wall;
•
•
Discharge isolation valves should be bolted directly to a flange on the discharge pipe or manifold;
Reflux valves should be provided with covers for cleaning and maintenance without the need to remove the valve from the pipeline. The covers should be large enough so that the flap can be removed and the valve can be cleaned;
•
•
Discharge non-return valves should be bolted directly to the discharge isolation valve, they should be installed in horizontal pipework with a short length of pipe and a flange adapter on the pump side to allow dismantling;
The non-return valves should have proximity switches to prevent dry running and allow a change of duty (standby on high level will then start);
•
All reflux valves should be installed in the horizontal plane;
•
Butterfly valves may be used if there is insufficient space for a standard gate valve, but their use should be avoided if possible for pump isolation purposes.
•
Sufficient bends and flange adapters to allow easy dismantling and removal of pumps, nonreturn valves or other major items of equipment;
•
Each dry well pump should be installed with suction and discharge isolation valves to permit isolation of the pump from the wet sump and discharge pipework for maintenance;
•
•
Where the pump delivery pipework joins the pumping station discharge manifold, the entry should be horizontal;
•
At the opposite end of the pumping station discharge manifold, a valved connection back to the sump should be provided for draining the discharge pipework, or for flushing the sump;
•
Consideration should be given to providing an isolating valve on the pumping main before it leaves the pumping station/chamber and before any over pumping connection to allow the pumping station to be fully isolated and the fixed pipework drained for repair;
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2.15
Pumping System Characteristics
Net Positive Suction Head (NPSH), vibration, cavitation and noise are characteristics that must be considered in pump station design.
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NPSH is used to check an installation for the risk of cavitation. NPSH required is the minimum total pressure head required in a pump at a particular flow/head duty. It is normally shown as a curve on the pump performance sheet. NPSH available is calculated as: NPSH = Pa – Vp + Hs – Fs Equation 2.15.1 Where: Pa = atmospheric pressure at liquid free surface Vp = vapour pressure of liquid
The pump duty point is where the pump performance curve and the system curve cross. It shows the flow that a particular pump will deliver through the pipework system at a particular total head at the pump duty level. In multiple-pump installations it is essential that the operating conditions of a single pump running are carefully checked to ensure that the pump will operate at maximum and minimum static heads satisfactorily and without risk of cavitation. The duty point should be used when considering the suitability of alternative pumps for a particular duty by comparing the efficiency and power requirements for each pump at the duty point.
Hs = height of supply liquid free surface above eye of pump impeller Fs = suction entry and friction losses Ref to Fig 2.15.1 Cavitation is the formation and collapse of vapour bubbles in a liquid. Vapour bubbles are formed when the static pressure at a point within a liquid falls below the pressure at which the liquid will vaporise. When the bubbles are subjected to a higher pressure they collapse, causing local shock waves. If this happens near a surface, erosion can occur. Cavitation will typically occur in the impeller of a centrifugal pump where it can cause noise and vibration as well as affecting the pump efficiency. If allowed to persist, it can lead to damage to the pump, or even breaking away of foundations. Pump duty point Each pump has a performance curve where the flow is plotted against head. Each pipework system has a friction curve where the friction head is plotted against flow. The system curve is obtained by adding the static head to the friction losses and plotting the total head against the flow. In order to avoid cavitation, the NPSH available should be at least 1m greater than the NPSH required by the selected pump at all operating conditions. When calculating NPSH available, absolute values for atmospheric and liquid vapour pressures are used.
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Head (m)
Pump Performance Curve (s ) Flow →
a = Flow Pump A b = Flow Pump B a + b = Total Flow Figure 2.15.1 – Characteristic Curve for Multiple Pumps
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2.16
Pump Pumps and Overpumping Facilities
Sump pumps should be provided for all dry wells and wet wells at pumping stations. For dry wells they should be used to remove any water that may collect at low level. For wet wells they should be used to empty the wet well prior to man entry. Over-pumping facilities should be provided where there is a single sump and access may be required for maintenance/repairs of pumps/screens/etc. A suction chamber should be provided before the pumping station with a penstock to isolate all flows into the pump sump. A connection into the pumping main should be provided for the over-pumping discharge. Consideration should be given to providing an isolating valve on the pumping main before the over pumping connection to allow the pumping station to be fully isolated and the fixed pipework drained for repair. Sump Pump Installations Sump pumps should incorporate the following features: •
Sump pumps should discharge to the wet well above the water level to prevent siphoning;
•
Discharge pipes should be fitted with a non-return valve and isolating valve in an easily accessible position;
•
The sump pump should be fitted with a discharge connection and guide rail to allow the pump to be easily removed from the sump for cleaning or unblocking;
•
Where a temporary sump pump is to be used a power supply point and discharge connection should be provided. Both should be located at high level in the dry well and easily accessible from the access walkways.
Sump pumps should be installed in a sump of sufficient dimensions for the proposed pump and allow a suitable level controller to operate within the sump, the minimum depth should be 500mm. The sump pumps should be sized for the possible leakage of glands and seals. A guide should be 0.5l/s for each leakage point, with a minimum of 5l/s. An assessment should also be made of any possible inflow from outside the dry well (i.e. rain and flooding).
2.17
2.17.1
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Introduction
A standby power generator set is not required in every case but is necessary in applications where the loss of the power supply could result in flooding of sensitive areas. In less sensitive areas plug in facilities for a mobile generator may be sufficient. The generator set configuration and sizing will vary from one application to another dependent on the load type, operation characteristics, site condition, and application requirements. The sizing and selection of the generator set should take into consideration the aspects raised in the following sub-sections.
2.17.2
Load Type
In some applications, the total connected load in the pumping station will need to be powered from the generator set in case of power failure, while in other locations only the essential load will need to be kept running (partial loads). The designer should consider the requirements according to the site characteristics and the proposed application to size the required generator set. The following points are to be investigated at the initial stage to select the type of generator that is required: •
Voltage level according to load voltage level (415v, 3.3kv, 6.6kv, 11kv);
•
Total generator connected load;
•
Individual load characteristics such as kilowatt rating, maximum allowable voltage dip by the motor manufacturer, starting method, sequence of operation;
•
Load type - inductive or capacitive;
•
Load profile.
2.17.3
Site condition
The site condition should also be examined and the following data collected and submitted to the generator set manufacturer to be considered in the sizing process: •
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Power Calculations including Standby Generation
ambient temperature;
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2.17.6
•
elevation above sea level;
•
humidity;
•
wind direction and dust contamination in air;
•
nearby residential consideration.
2.17.4
areas
for
sound
The control panel can be unit-mounted on the generator set unit, or remotely mounted inside the control room. level
Generator set operation and control
The generator set operation and control varies from application to application depending on the following points: •
Number of units to be controlled;
•
Manual or automatic synchronisation;
•
Manual or automatic start-up;
•
Manual or automatic changeover switch between main local authority incomer and main generator set incomer (control panel outgoing feeder).
2.17.5
Type of installation
Standby generator may be eather Portable or Permanent Depending on the size of the pumping station. Where a portable generator is provided plug in facilities must be provided at the central panel. For permanent standby generator the installation can be categorised in the following ways: •
•
Building installation: The unit will be installed inside a building suitable to accommodate all the units and their ancillaries. This type of installation is recommended in large or major pumping stations, or treatment plants; Weatherproof enclosure: The unit is mounted inside a weatherproof enclosure on a trailer suitable for transportation between different sites;
•
Soundproof enclosure: The unit is installed inside a soundproof enclosure, mounted on a trailer suitable for transportation and operation in residential areas;
•
Skid mounted unit: For temporary site work (e.g. construction site).
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Type of control panel
The control panel is used to operate and monitor the unit in case of power failure. The panels come with many options depending on the type of operation required, and the mode of operation e.g. one unit, two units, automatic start, or manual start.
2.17.7
Ventilation system
Unit ventilation and the cooling system is critical to the overall system performance and capability. The ventilation system is required to keep the surrounding atmosphere temperature as per the specified ambient temperature, to avoid any temperature rise due to heat generation from the engine. The ventilation system should be by means of forcing air out of the room using a fan installed at a level above the highest point of the generator e.g. roof-mounted or wall-mounted. The air will be delivered through air louvers mounted at the lowest permissible level to avoid sand ingress from the surrounding area, and at the same time to guarantee airflow across the generator set body. In addition to the room ventilation, the generator should have an engine-driven fan. This will draw air through sand trap louvers in the wall and over the alternator/engine, discharging the air through a setmounted radiator and wall-mounted outlet louvers.
2.17.8
Fuel system
The fuel system usually consists of a main storage tank, daily fuel tank, fuel transfer system, and fuel line between tanks and the generator set. •
Main storage tank. This will be required in applications where the fuel consumption at site is very high due to a large number of units installed, or due to the difficulty in providing daily supply of fuel to the site. In that case, the storage facility of the main storage tank should be sufficient for three days consumption. The bulk tanks should normally be mounted partially below ground level within bunds to enable the day tank to empty under gravity back to the bulk tank in the event of a fire;
•
Daily fuel tank. The daily fuel tank should be suitable for eight hours full load operation, and
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normally mounted on a stand beside the generator set to enable gravity feed to the engine; •
•
The fuel transfer system. A fuel transfer system is required between the main tank and daily tank to keep the daily tank full and ready for operation. The tank level should never fall below a minimum level. The system consists of transfer pumps, level sensor, control panel, valves (solenoid valves, actuated valves, hand operated valves) and flow meter to monitor the units consumption, as well as the delivery supply to the main tanks; A thermal ‘cut-off’ link must be mounted above the engine, arranged to close both a valve on the fuel line between the day tank and the engine, and also a dump valve to drain the day tank back to the bulk tank in the event of a fire.
2.17.9
Starting method
The generator starter method is usually one of the following methods: •
Air starting method. This type of starting is suitable for large generator sets requiring a high starting torque, especially medium and low speed engines (750RPM, 600RPM). This usually consists of: a)
Air operated starter unit (sized by the generator set manufacturer);
b)
Air tank vessel (suitable for six starts before refill);
c)
Electrically operated air compressor unit (capable of refilling the tank within 15 minutes);
d)
Diesel operated air compressor with the same capacity working as backup for the electrical air compressor;
e)
Air piping between air vessel and starter unit.
•
Electrical starting method. This type of starter is suitable for small loads, transportable and enclosed units, which work at high speeds (1500 RPM). The starting method consists of an electrically operated starter, battery, and charging alternator. A battery charger is required to keep the battery fully charged and ready for operation in cases where the unit is rarely operated. The battery type should be maintenance free for high reliability starting;
•
Starting aid. Some applications require immediate starting and load handling without any delay due to
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critical load type. To get the generator set ready for such an application the unit should be equipped with a jacket water heater to keep the engine warm and ready for load immediately after starting without any delay for warming the engine before applying the load.
2.17.10
Service facility
The generator set building should be equipped with an overhead crane capable of lifting the heaviest part likely to be encountered during maintenance of the generator set. The main inlet and outlet louvers and building shall be designed such that the complete generator set can be installed and removed through the louver openings. For container or enclosure units, a lifting facility should be provided for offloading and transporting the unit. The enclosure should be capable of having the side and roof dismantled and removed for ease of maintenance and parts replacement.
2.17.11
Generator set sizing
Generator sizing is best left to the manufacturers/suppliers. Details of the loads to be carried and applied along with the generator performance class, e.g 10 kW building supplies + 1 x 75kW motor + 1 x 45kW motor + 1 x 90 kW motor (largest starting load @ 675kVA), Governing Performance Class G3 to ISO 8525 Parts 1&5. Details of the motor starting type i.e D.O.L , star/delta, soft start or variable speed drive should be provided. The supplier should also be given the site conditions as the ambient temperature will affect the engine performance.
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2.18
Switch Gear and Control Panels
Low voltage switchgear and control panels form the link between the electrical load, such as; motors, lighting, actuator valves, air conditioning equipment and the power generation source (main authority supply, generator set). The design of the switchboard should take into consideration the points discussed in the following subsections.
2.18.1
Type–tested and partially type tested assemblies (TTA and PTTA)
According to BS EN60439-1xxxv the low voltage switchgear (assembly) and its component parts shall be made in a way that it can be safely assembled and connected. Assure that this configuration of assembly and its components are safely operated without any risk to the operator or equipment. Some of the risks that can affect the operation to be considered include:
IP degree of protection.
•
The partially type-tested assemblies (PTTA) are assemblies that contain both type-tested and non typetested arrangements (derived by calculation from the type-tested arrangements compliant with tests required for TTA).
2.18.2
Total connected load
The control panel sizing and design to cover the demand of the total load connected, including the standby load.
2.18.3
Short circuit level
The short circuit level calculation carried out according to the total connected load and power source from the local authority electricity network. The short circuit level is one of the most important criteria in switchboard design. Its’ importance arises from the need to protect the equipment with the correct protection device, suitable for the specific level of short circuit, so that no damage or harm can affect the equipment or human safety. Care must be taken in the design stage to control the fault level. If the total connected load is too high, the total connected load to the switchgear can be split into two or more assemblies to reduce the fault level.
1.
Direct and indirect contact with live parts.
2.
Temperature rise.
3.
Electrical Arc.
The short circuit level can be calculated according to the following steps.
4.
Overload.
Step-1 Determine the transformer full load amperes:
5.
Insulation failure.
6.
Mechanical failure.
To achieve a type-tested assembly (TTA) the following performance requirements should be verified:
I(fl)
=
KVAx100 0E (l-l) x 1.732
Equation 2.18.1
Where: I(fl)
= transformer full load
•
Temperature – rise limits;
KVA
= transformer capacity volt ampere
•
Dielectric properties;
E (l-l)
= line to line voltage
•
Short circuit withstand strength (main circuit);
•
Effectiveness of protective circuit;
•
Short circuit withstand strength of the protective circuit;
•
Clearance and creepage distance;
•
Mechanical operation test;
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Step-2 Find the transformer multiplier Multiplier
=
100 %Z (T)
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Equatio n 2.18.2
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Where: Z (T)
= transformer impedance
Step-3 Determine the transformer let through short circuit current I s.c
=
I (fl) x Multiplier
2.18.5
Equa tion 2.18.3 Where: I s.c
= transformer let through short circuit current
Table 2.18.1 shows some examples of expected and standard fault level. Table 2.18.1 – Example of Expected and Standard Fault Level Short circuit level
Type of application
16KA/1sec
Distribution board Amp)
35KA/1sec
Motor Control Centre (≤400 Amp)
50KA/1sec OR 50 KA/3sec
Motor Control Centre (≤2000 Amp)
80KA/1sec OR 80KA/3sec
Motor Control Centre (≤3000 Amp)
120KA/1sec OR 120KA/3sec
2.18.4
well as the electrical components. In the event of a short circuit, this type of co-ordination will ensure that the components are reusable after fault clearance. Type-1 co-ordination only guarantees personal and electrical installation safety, and the equipment may not be able to resume operation without repair or replacement of the affected part.
(≤250
Motor Control Centre (≤5000 Amp)
Type of co-ordination
Form of internal separation
The form of separation should be according to BS EN60439-1xxxv or suitable equivalent. The designer should consider Form-4 (see Figure 2.18.1) in all designs for high personal safety and equipment protection. In the case of multiple incomers and/or feeders, Form-4 should be considered for ease of maintenance without the need for interruption to other equipment as would be the case with Form-2. In case of multi-incomer and outgoing starters/feeders, Form-4 should be considered for ease of carrying out maintenance without interruption to other equipment, in case of isolation of certain feeders. The Type to be used can vary between Type-3 and Type-7 as shown in Figure 2.18.1, diagram (1and 2). According to the project requirements or budget limitations, Form-2, Type-2 (diagram-3, Figure 2.18.1) should be considered in some applications, such as unit mounted control panels (e.g. scrubber units, sludge drying beds) where the shutdown of the unit is mandatory to carry out maintenance on the unit.
Electrical component co-ordination according to IEC 974-1xxxvi, provides two types of protection. Manufacturers test components such as contactors and circuit breakers in unison to confirm what will happen under short circuit conditions. According to IEC 947-4-1, the co-ordination between the electrical components can be categorised into the following two types: Type – 1: co-ordination (personal safety only) Type – 2: co-ordination (personal/components safety) The designer, where possible, should select type-2 coordination to assure full protection of personal safety as
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Figure 2.18.1 – Form and Type of Internal Separation Form-4 type-3: Diagram-1 Bus bar
Function unit
Enclosure
Internal Separation
Terminal for external conductor
Cable gland
Form-4-Type-7: Diagram-2 Bus bar
Enclosure
Table 2.18.2 – Guideline Starter Methods for Motor Ratings (kW) Motor rating KW
Starting method
≤ 5kw
Direct online (DOL)
5 ≥ kW ≤25
Star delta (Y/D)
>25kw
Soft starter ( solid state drive) (S/S)
Function unit
Motor duty and application
Internal Separation
Terminal for external conductor
Bus bar
Enclosure
Function unit
Internal Separation
Terminal for external conductor
Cable gland
2.18.6
Bus Bar rating
The bus bar rating should be suitable to carry the total connected load. As mentioned previously, consider any future loads by increasing the size of the bus bars and also consider the suitability of extension at both ends.
2.18.7
The motor duty will vary according to its application. The following table gives examples of such duties. Table 2.18.3 – Example Motor Duties and Applications
Form –2 – Type-2: Diagram-3
Type of starter
The designer should consider the following points when choosing the starter type to be used. Motor size The motor size (kW) will determine if a standard starter can be used (direct on line DOL or start delta starter
Page 60
Y/D), or if a more advanced type of starter such as a soft starter is required. The main issue to consider is the starting current. The greater the (kW) rating, the greater the starting current required. A high starting current has an overall effect on the system stability and other equipment installed. The following ratings can be considered as general guidelines only. The designer should apply knowledge and experience to justify the starter method to be used.
Duty type
Application example
Continuous run at constant load and speed
Potable water
Short run at constant load and speed
Sewage pumping station
Continuous run at variable load and speed
Irrigation network
Intermittent periodic duty
Injection system
Motor Application The type of motor starter can also be selected according to the motor application as mentioned in Table 2.18.3, as a high number of starts per hour will cause even a small motor to overheat. An example of a suitable starter for each application is presented in Table 2.18.4. Table 2.18.4 - Example Starter Methods for Duty Types Duty type
Starter
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Continuous run at constant load and speed
DOL, Y/D, S/S
Short run at constant load and speed
DOL, Y/D S/S if sufficient cooling time between operations
Continuous run at variable load and speed
VSD
Intermittent periodic duty
D.C starter, DOL
Table 2.18.5 – Examples of Protection Required for Load Types Load type Main incomer feeder (local authority/ generator set)
Notes: DOL: direct online, Y/D: star/delta , s/S: soft starter, VSD: variable speed drive.
Voltage level Starter type can be varied according to the voltage level. In the medium voltage range (e.g. 3.3kv) the starting current will be very low when compared with a lower voltage (e.g. 415v). In this case, the use of a direct contact starter would be acceptable.
Valve actuator
Cost considerations The cost of the starter should also be considered when compared to the motor size and application. As an example, a soft starter could be used to reduce the starting current for a 10kW motor. Taking into account the cost of the soft starter and comparing it to the cost of the motor, the starter could cost more than the motor however. Star delta starters can for most applications be considered more economically viable than a soft starter, therefore balance the motor cost against soft starter cost.
2.18.8
Pump, grinder
Protection device
The designer should categorise all loads connected to the switchgear according to critical status in the process and effect on operator safety. Table 2.18.5 provides examples.
Instrument (level/ flow/ pressure) Building services (lighting/ sockets)
Note:
Type of protection Overload, short circuit, restricted earth fault, phase losses, phase reveres.
Protective device
Main MCCB or ACB
Overload, short circuit, earth leakage, phase losses, phase reveres, under voltage, motor stall, winding temperature.
1- conventional protection device (OLR), MCCB
Overload, short circuit, earth leakage.
Conventional protection device
Overload, short circuit, earth leakage
Conventional protection device
Overload, short circuit, earth leakage, phase losses , phase reverses.
ELCB OLR MCCB ACB
2- Electronic protection devices 3- motor manager protection unit
(OLR), ELCB
(OLR), ELCB Conventional protection device MCB, ELCB, Fuses,
= Earth leakage circuit breaker = Over load relay = Moulded case circuit breaker = Air circuit breaker
Type of protection 1.
Short circuit protection: This type of protection is required to protect the equipment against short circuit (with three phase, two phase or single phase), which can occur due to: insulation failure or damage, or by an incorrect switching operation. Short circuits are associated with electrical arcs and can therefore pose a fire risk.
2.
Overload protection: This type of protection is required to protect the equipment against overload current which is due to operational over current present for an excessive period of time. This over current will raise the motor
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winding or cable temperature above the permissible level and shorten the service life of the insulation. The task of overload protection is to allow normal operational overload current to flow, but to interrupt these currents before the permissible loading period is exceeded. 3.
Under/over voltage protection: This type of protection is required to protect the equipment against over/under voltage which is present due to main power supply instability (e.g. transformer tap changing/load fluctuating) or unstable supply from a standby generator (due to large load connected, faulty governor or voltage regulator). Operation with an under-voltage condition will draw more current from the supply, this over current will raise the motor winding or cable temperatures above the permissible level and shorten the service life of the insulation. The same will be the case with over-voltage which will effect the insulation of the motor or cable leading to insulation failure. This type of protection can be applied at the main incomers of the switchgear by a special relay to sense the voltage supply and trip the main incomers if the set limits are exceeded.
4.
Phase losses/phase reversal protection: This type of protection is required to protect the equipment against phase loss from the main supply, or phase reversal which can happen in the event of main supply reconnection or reconnection of the motor after maintenance. Operation with phase loss will raise the motor winding temperature due to an unbalanced current in the motor winding. In the case of phase reversal, the motor direction will be reversed, which will result in equipment damage or faulty operation (pump vibration, high sound levels etc). This type of protection can be applied at the main incomers of the switchgear or motor feeder by a special relay to sense the phase status (direction/availability) and trip the main incomers/feeder when a fault occurs.
5.
This type of protection can be applied at the switchgear outgoing feeders (motor / distribution board) by a special relay which senses the earth leakage current through a summation current transformer, the unbalanced current from the transformer will release a mechanism that will trip the breaker when a fault occurs.
Earth leakage protection: This type of protection is required to: protect the equipment and personnel in the event of indirect contact; give additional protection in the event of single phase direct contact; earth fault protection; and protection against fires resulting from earth fault leakage current.
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6.
Motor protection relay (electronic relay):
This type of protection is used to protect the motor against many faults that can affect the motor operation and safety. The actual protection type can be varied according to the motor application (critical/normal) and size (cost). The following types of protection can be achieved by a motor protection relay: •
Over / under current;
•
Phase loss/ unbalance/reversal;
•
Ground fault;
•
Locked rotor;
•
Motor stall.
This type of protection can be applied at the motor terminals. The fault signal from the relay will release a mechanism that will trip the breaker when a fault occurs. Fault indication will usually be displayed on a LCD screen or by indication LED’s.
2.18.9
Interlocking facility
An interlocking facility is required where more than one incomer is used in the switchgear required. Some examples are as follows: • Supply from two transformers/local authority supply; • Supply from two incomers - one from transformer/local authority supply, and one from standby generator(s) panel; • Supply from three incomers - two from transformers/local authority supply, and one from standby generator(s) panel. The interlock facility should guarantee the safety of operation by not allowing under any condition the connection of two different incomers to the same bus bar section (transformer/transformer) or (transformer /generator) or main bus bars with the bus coupler closed.
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2.18.10
Accessibility
The panel access for cable termination and maintenance can be arranged in the following format: •
Front access (suitable for installation area with limited space at the back of the MCC);
•
Back access (suitable for installation area with available space at the back of the MCC, minimum one metre);
•
Front/back access.
2.18.11
Cable entry
Cable entry to the MCC can be arranged in the following format: •
Bottom entry (suitable for MCC fixed at the top of cable/MCC trench);
•
Top entry (suitable for MCC with cables such as feeders and incomers installed at ground level or above the MCC top level). Top entry panels are not preferred and should only be used in special circumstances.
Cables should be sized and installed in accordance with the IEE (Electrical Wiring) Regulations and QGEWC Regulations, and de-rated in accordance with the Electrical Research Association Report No. 69-30xxxvii. Instrument, alarm, and control cables should be segregated from power cables. The designer should consider the following when selecting cable routes: •
Number, size and function of cables;
•
Access for installation and maintenance;
•
Interface with other equipment, e.g. cable routes should not prevent other equipment being removed for maintenance;
•
Risk of mechanical damage;
•
Means of support;
•
Effect of installation method on de-rating factors;
•
Hazardous area classification.
2.19
PLC’s SCADA/Telemetry 2.19.1
PLC
PLC stands for Programmable Logic Controller. The PLC is a microprocessor-based device which is
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programmed to perform certain controlling tasks. The PLC is the brain of the overall process. It can receive analogue and digital signals from the process devices, analyse them and send digital and analogue signals to control these devices or activate certain alarms. PLCs were originally used for controlling purposes. Almost all PLCs are now equipped with signal transmitters (i.e. include some RTU features) that are capable of transmitting data to the network operation centre. A redundant PLC system with hot standby configuration is highly recommended for critical applications where uninterrupted control is required. The power supply for the PLC system is usually 24Vdc or 110Vac. In case of power failure, the equipment should be backed up by a UPS system, which can supply the PLC with up to eight hours of power depending on the importance of the process. The modular type CPU (Central Processing Unit) in the PLC is capable of: solving application logic; storing the application program; storing numerical values related to the application processes and logic; and interfacing to the I/O systems. The PLC carries out PID control, which is a significant task. PID (Proportional-Integral-Derivative) control action allows the process control to accurately maintain a setpoint by adjusting the control outputs. For example, pump flowrate setpoint is maintained by the following: •
Proportioning Band: is the area around the setpoint where the controller is actually controlling the process. The output is at some level other than 100% or 0%. The band is generally centred around the setpoint (on single output controls), causing the output to be at 50% when the setpoint and the flow rate are equal;
•
Automatic Reset (Integral): corrects for any offset (between setpoint and process variable) automatically over time by shifting the proportioning band. Reset redefines the output requirements at the setpoint until the process variable (flowrate) and the setpoint are equal;
•
Rate (Derivative): shifts the proportioning band on a slope change of the process variable. Rate, in effect applies the ‘brakes’ in an attempt to prevent overshoot (or undershoot) on process upsets or start-up. Unlike Reset, Rate operates anywhere within the range of the instrument. Rate usually has an adjustable time constant and should be set much shorter than reset. The larger the time constant, the more effect Rate will have;
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•
Modulated Simplex I/O system: is the preferred solution for safe process since the duplex (redundant) I/O system is usually expensive, and the modulated simplex I/O configuration guarantees that any failure of a single I/O card will not cause the relevant I/O rack to fail. For instance, if a rack contains three I/O cards, which controls three pumps (two duty, one standby), the failure of one card will cause the whole pumping process to fail. In Modulated Simplex I/O systems however, it will cause the failure of one pump, which will be classed as the standby pump, and the other two pumps will continue run normally.
2.19.2
RTU
RTU stands for Remote Telemetry Unit. This unit delivers remote information back to network operation centres. Operations staff can access remote sites that have RTUs, via a web browser, SNMP (Simple Network Management Protocol) Manager, and XML (Extensible Markup Language). If an ethernet connection is not available, then the RTU's may be accessed via PSTN (Public Switched Telephone Network), normal dialup and even SMS (Short Message Service) messaging. Earlier generation RTUs were hardwired and supported limited functionality’s such as data transfer and alarming. The new generation RTUs are equipped with powerful processors that allow the RTU to control certain instruments and devices, and to receive/transmit analogue and digital I/O (input/output) signals. The microprocessor based RTU have a proven track record within the water and wastewater industry, a robust modular construction, and are constructed for ease of maintenance and repair. These are intelligent devices, capable of handling data collection, logging, report by exception, data retrieval and pump sequence control programs. RTU’s equipped with RS232/485 links are recommended for interconnection to standalone control systems, standard equipment packages and PLCs (Programmable Logic Controller). A dedicated serial port should be provided for connecting a hand-held programming unit or PC. The RTU software enables the RTU to process local input equipment information, before transmitting it to the master station to reduce transmission overheads. A report by exception operation is necessary for cost effective communication. The report is triggered by change of state of digital values, analogues reaching
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threshold values or varying by specified amounts. The RTU also reports when polled, and when the memory buffer is full.
2.19.3
SCADA and Telemetry Systems
Supervisory Control And Data Acquisition (SCADA) is an industrial measurement and control system consisting of a central host or master (usually called a master station, master terminal unit or MTU); one or more field data gathering and control units or remotes (RTU’s); and a collection of standard and/or custom software used to monitor and control remotely located field data elements. Contemporary SCADA systems exhibit predominantly open-loop control characteristics and utilise predominantly long distance communications, although some elements of closed-loop control and/or short distance communications may also be present. Systems similar to SCADA systems are routinely seen in factories and treatment plants. These are often referred to as Distributed Control Systems (DCS). They have similar functions to SCADA systems, but the field data gathering or control units are usually located within a more confined area. Communications may be via a local area network (LAN), and will normally be reliable and high speed. A DCS system usually employs significant amounts of closed loop control. SCADA systems on the other hand generally cover larger geographic areas, and rely on a variety of communication systems that are normally less reliable than a LAN. Closed loop control in this situation is less desirable. The main use of SCADA is to monitor and control plant or equipment. The control may be automatic, or initiated by operator commands. The data acquisition is accomplished by the RTU's scanning the field inputs connected to the RTU (it may be also called a PLC programmable logic controller). This is usually at a fast rate. The central host will scan the RTU's (usually at a slower rate). The data is processed to detect alarm conditions, and if an alarm is present, it will be displayed on special alarm lists. Data can be of three main types: •
Analogue data (i.e. real numbers) will be trended (i.e. placed in graphs);
•
Digital data (on/off) may have alarms attached to one state or the other;
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•
2.20.1
Pulse data (e.g. counting revolutions of a meter) is normally accumulated or counted.
The trending function can be a powerful diagnostic tool for use by the operators or maintenance personnel. The data stored and archived can be viewed over any period of historic time, which allows fault patterns, which would otherwise go unnoticed to be detected. For stormwater stations the data can be analysed to determine how the station coped with storms. Based on this data, modifications can be made to the operation of the station to improve its response during such incidents. The primary interface to the operator is a graphical display (mimic) which shows a representation of the plant or equipment in graphical form. Live data is shown as graphical shapes (foreground) over a static background. As the data changes in the field, the foreground is updated, e.g. a valve may be shown as open or closed. Analogue data can be shown either as a number, or graphically. The system may have many such displays, and the operator can select from the relevant ones at any time. A further function of the SCADA system is the production of maintenance data and management reports. For example, SCADA systems can be easily configured to produce maintenance requests for equipment that has run a set number of hours, or if its’ performance has been declining over time. If a standalone maintenance system is already in place, SCADA systems can feed information directly to the maintenance software. For managers, SCADA systems can produce detailed reports on subjects such as power or chemical usage. Combined with the trending facility that is also inherent within SCADA, and by inputting cost data, it can produce cost forecasts for a wide range of process consumables.
2.20
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Light fittings are selected according to the following criteria and application.
2.20.2
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Installation Location
The location of the light fittings to be designed has a large affect on the type of luminiare to be specified. Generally, the following categories can be considered:
1. Internal Lighting Internal lighting fittings are required in places such as: a.
Motor control centre rooms (MCC);
b.
Control and SCADA monitoring rooms;
c.
Substation (11kv & transformer);
d.
Pump rooms;
e.
Off-loading bay & walk ways;
f.
Kitchen and toilets;
g.
Administration offices;
h.
Machinery rooms (compressor, generator, chemical storage, and chemical dosing system room).
2. External lighting a.
Building (external wall mounted fittings);
b.
Internal road lighting (inside station boundary);
c.
Water storage tank lighting;
d.
External installed machinery (settlement tanks, inlet works aeration tanks);
e.
Pump wet wells and screen chambers.
2.20.3
Lighting
The designer should follow the guidelines and information given below to design a proper lighting system. The British standards specified within and the CIBSE lighting guidexxxviii should be considered during the design.
Light Fitting Selection Criteria
Environmental Conditions
In many industrial applications the environmental condition is hostile or hazardous as explained below. 1)
Hostile conditions - damage to light fittings can occur due to: a.
High ambient temperatures;
b.
Windy and vibrating environments;
c.
Corrosive atmosphere (hydrogen sulphide gases, high humidity);
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2)
d.
Wet atmosphere (water ingress);
Tank area
50
e.
Dusty atmosphere.
Building (external wall and door
70
Hazardous conditions - The operation of light fittings in certain environments can cause fire or explosion due to gas generation or fumes (methane, etc).
A risk assessment on the source of ignition and type of explosive atmospheres should be carried out using the methodology suggested in BS EN 1127-1xxxix for all potentially hazardous areas such as screen chambers and wet wells.
2.20.3.1 Luminance Level Required (Lux) The luminance level required varies from one area or application to another. The luminance level should generally be in accordance with the CIBSE lighting guidexxxviii. The relevant levels are replicated below for convenience in Table 2.20.1. Table 2.20.1 – Luminescence Levels for Various Service Areas Service area
Luminance level (lux)
Internal area (inside building) Motor control centre room
300
Control / SCADA room
500
11kv switchgear room
300
Transformer bay
150-200
Kitchen
150
Toilets
150
Store
200
Offloading bay / walkway
100-150
Pump house
150-200
Cable gallery
150-200
Administration offices Machinery room
300 150-200
External area (Inside station boundary) Internal Road lighting
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50- 100
entrance) External installed machinery
2.20.4
100
Type of Light Fitting
Light fitting types that can be used in different locations can be categorised as follows. 1. Fluorescent fitting - The fluorescent fitting is a combination of lamps and luminaries. The fittings are available with different lamp sizes (18w, 36w, 58w), arrangements (3x18w, 4x18w, 2x36w, 2x58w) and installation type (surface mounted, recessed mounted). This type of fitting is ideally suited to internal installation use. It can be used in most locations with some changes in the body material, IP rating and lamp wattages. 2. Flood lights - Flood lights are used mainly for external building area lighting such as tank areas, and machinery areas (grit removal, settling tank, aeration tanks etc). The lighting installation can be wall mounted on external buildings or post mounted in working machinery areas, or ground level mounted and directed to the tank walls in case of tank area lighting. The fittings should be a minimum of IP65; and the body should be suitable for the environment of the application (corrosion resistant, UV protected). 3. High bay lights - High bay lighting should be used in pump rooms when the bay heights are above six meters. The high bay lamps can provide lighting for maintenance purposes, in the case of regular inspections and access to the pump house. Side mounted (4-meter height) fluorescent fittings can be used due to the extended start-up time of high bay lamps. 4. Emergency lights - Emergency lights are used in case normal lighting fails or the power supply fails. They give light in emergency situations such as a fire, to provide escape-route sign lighting and emergency-exit sign lighting as per BS 5266xl. The type and installation of emergency lighting should consider the following points: •
Escape route signs shall be mounted above building exit doors at 2-2.5m above floor level;
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•
Escape route lighting such as Corridors, gangway and stairs shall have a horizontal luminance on the floor (centreline of escape route) of not less than 0.2lux;
•
Emergency lighting in large open areas such as open plan offices should have an average horizontal luminance for escape purposes of not less than 1.0lux;
•
Emergency lighting in Motor control centre rooms and operator control rooms (SCADA) should have an average horizontal luminance not less than 2.0lux.
Emergency light system There are two types of emergency light system: a.
Self-contained
b.
Centrally powered
Luminaire mode of operation There are two modes of operation as follows: •
Maintained: lamp used as normal when the building is occupied. The power supply is from the normal source directly or indirectly;
•
Non-maintained: lamp off as long as the normal power supply is available. The lamp will energise from the emergency power supply automatically in the event of normal power failure.
5. Roadway lighting -The design of roadway lighting should be according to BS 5489-1xli. For lighting required for pumping station roads, the selection of the suitable light fittings, post heights and post spacing will be according to the level of lux required. The light fitting body and canopy material should be suitable for the installation location and environmental conditions. Usually, three types of lamp are commonly used. These are; high-pressure sodium, metal halide, and highpressure mercury. The installation of the fitting on the column can be on the post top, bracket or side entry. 6. Bulk head - Bulk head light fittings are used at the entrance of the pumping station building (located on top of the door or at the side) as well as in substation entrance doors and gates. The fitting can be suitable for indoor or outdoor installation and should be IP65 with either a high pressure sodium or incandescent lamp type). 7. Lighting design calculation - The following formula is used to check the level of lux provided and adjust the number of fittings to be used. Professional software can be used for increased accuracy and speed of design. The following guide is given as an aid for the experienced lighting engineer and not as a learning guide for the novice engineer. The information required to populate the formulae can be found in manufacturer’s literature. Internal Lighting (Lumen Method) Formula:
Types of emergency lighting The following types of emergency lighting luminaire are commonly used: •
Self-contained separate (maintained/non-maintained);
•
Normal luminaires modified to contain a battery pack and conversion unit (maintained);
•
Normal luminaires fed from a central battery system with conversion unit (maintained);
•
Normal luminaires with a separate lamp for use with a battery pack, invertors, rechargeable unit (non-maintained);
•
•
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Normal luminaires fed from a central power source (maintained/ non-maintained).
SW Drainage
A
luminaire
Normal luminaires with a separate lamp for emergency use, fed from a central battery system (non-maintained)/(sustained luminaire);
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Es = F x n x N x UF x MF
Equation 2.20.1
Es
= Average illuminance (lux) of the plane
F
= Initial bare lamp lumens flux (lumens)
n
= Number of lamps per luminaire
N
= Number of luminairies
UF
= Utilizsation factor
MF
= Maintenance factor
A
= Area (m2)
Calculation procedure Calculate the room index (K), floor cavity index (CIf) and ceiling cavity index (CFc).
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(K) = (LxW)/(L+W)hm Equation 2.20.2
(CIf) OR (CFc) = L x W/(L+W)h Equation 2.20.3
The calculation for roadways can be carried according to BS 5489-3xli. Caution must be taken in lamp post foundation design to ensure that the wind effect on the post is fully considered. The flood light calculation can be carried out using the same formula applied for internal lighting calculation with slight modification: E=
Where:
N x L x BF x WLFxMF A
L
=
room length
W
=
room width
Hm
=
height of the luminaire plane above the
Where:
horizontal reference plane
E
= Illuminance required (lux)
depth of the cavity
L
= Lamp output per lumens (lm)
BF
= Beam factor number of lamps per luminaire
N
= Number of luminaries
WLF
= waste light factor (usually considered as 0.9)
MF
= maintenance factor
A
= area to be lighted (m2)
H
=
Equation 2.20.4
Calculate the effective reflectance (REx) of the ceiling, wall and floor cavity (from tables using above calculated (Cif). Determine the utilisation factor value (UF) using luminaire manufacturer data sheets; room index and effective reflectance (apply any correction factors). Determine the maintenance factor (MF): MF = LLMF x LSF x LMF x RSMF Equation 2.20.4 Where: LLMF
= lamp lumen maintenance factor
LSF
= lamp survival factor
LMF
= luminaire maintenance factor
RSMF
= room surface maintenance factor
Thus, the lighting design is determined as follows: •
Using the lumen method formula, calculate the number of luminairies required (N);
•
Determine the suitable layout;
•
Check if the (spacing / height) ratio of the layout is within the range according to UF;
•
Check that if the proposed layout is does not exceeding the maximum ratio limit;
•
Calculate the luminance that will be achieved by the final layout.
External and Roadway Lighting Calculation
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Light control: The control of the lighting system can be provided by the following means to control the operation of different lighting systems within the pumping station: •
One-way light switches can be used for controlling a lighting system in an area with a single access, for example at the main access door to the station;
•
Two-way light switches can be used for controlling a lighting system in an area with multiple access and egress points;
•
The automatic control of external lighting systems can be achieved by two main methods: a)
Photocell controller for automatic dusk till dawn control;
b)
Time clock operation for full control of when external lights are in operation.
2.21
Maintenance Access
Safe access should be provided to all equipment and local control panels at all times. Access walkways, platforms and stairs should be designed so that no dismantling is required for normal routine maintenance. Vertical access should be by staircase so that tools and equipment can be carried in
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and out safely. Ladder access should be restricted to infrequent visual inspection points. Access around equipment for operation should be installed at a level where all the controls can be reached and operated easily without excessive stretching or bending and where all indicators can be seen. Access around equipment for maintenance and repair should be installed at a level where all the maintenance points can be reached, dismantled and removed without excessive stretching or bending. Particular attention should be paid to lifting gear access and operation where heavy equipment is involved. Access below ground to dry wells should be by staircase so that tools and equipment can be carried in and out safely. Permanent access to wet wells and screen chambers should be provided, using stainless steel or GRP to just above TWL to allow for cleaning. The access arrangements should be designed such that an operator could be rescued from the sump with a safety harness and man-winch. When designing access to equipment, careful thought should be given to shipping routes for removing equipment to a suitable position for further work, or for removing from the pumping station completely. Exit routes for equipment should not be the same as for personnel access unless there is an alternative escape route. When the lifting gear has taken the weight of equipment and the equipment is released from its position, the clearance in the shipping route should be large enough for the equipment to pass through without rearrangement.
2.22
Gantry Cranes and Lifting Facilities
Permanent or temporary lifting facilities should be provided for equipment that can not be easily lifted. Consideration should be given to the weight, shape and position of the item to be lifted. As a guide lifting facilities should be provided for anything over 25kg. For long or heavy lifts, gantry cranes should be powered in all motions. Trolley cranes should generally be power lift with manual motion, but small units should be manual on all motions.
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Access must be provided to permanent lifting equipment, particularly gantry cranes, for maintenance as generally described in section 2.21. The following types of lifting equipment are available: •
Lifting Eye and Chain Block. Suitable for single straight lifts only inside a building or dry well. Not suitable for side forces, but may be used in conjunction with other suitable lifting eyes to swing a load sideways;
•
Davit, Socket and Chain Block. Suitable for most small single lifts i.e. submersible pumps up to 250kg. Above this, the davit becomes too heavy to be manhandled;
•
Runway Beam, Trolley and Chain Block. Suitable when there are a number of loads in a straight line, or where a single load must be moved sideways. For heavy loads or long lifts, the chain block and trolley should be electrically powered;
•
Overhead Gantry Crane. Suitable for installations where there are dispersed or heavy loads that must be moved in all directions;
•
Mobile Crane. Suitable for single heavy loads outdoors which must be moved in all directions i.e. large submersible pumps.
Submersible pumps should be fitted with stainless steel chains, with change-over rings every 1.0m, and the lifting equipment should be fitted with a change-over sling. Location of lifting equipment •
Lifting equipment should be provided adjacent to all heavy items that require lifting;
•
Lifting equipment should be positioned to provide a straight lift of the load and also be able to lower the load directly to a suitable setting down position;
•
Where lifting through openings in floors, the lifting gear should be positioned to allow a direct single lift up through all floors without moving the lifting point or rearranging the load.
Controls for Lifting Equipment •
Overhead electric cranes and chain blocks should be provided with a low voltage pendant control suspended from a glide track, independent of the lifting block. The pendant control should extend to within 500mm of the operating floor, but not touch the floor;
•
Electric chain blocks should be provided with a low voltage pendant control suspended from the block. The pendant control should extend to within
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500mm of the operating floor but not touch the floor; •
Hand operating chains should extend to within 500mm of the operating floor but not touch the floor;
down. All supply and exhaust ventilation louvers should shut automatically to compartmentalise the buildings and below ground chambers. This restricts the spread of the fire and smoke, and ensures effective use of automatic fire extinguishing systems.
•
Long travel drive chains should be located to avoid snagging, and allow the operator safe passage;
Other points to consider include:
•
With the load hook in its highest position, if a load chain touches the operating floor or any item of plant, a chain collection box should be fitted.
2.23
Ventilation, Odour Control and Air Conditioning 2.23.1
The air conditioning systems, ventilation fans and odour control equipment should be run simultaneously and ventilation fan louvers should shut, when the fan stops;
•
Louvers should be sized to keep the air velocity through them below 0.5m/s;
•
Air ducts should be designed to ensure the velocity through them does exceed 10m/s in occupied areas;
•
Materials should be selected to limit the corrosion effects of hydrogen sulphide (H2S).
Ventilation
Ventilation of pumping stations is required to prevent the accumulation of high levels of potentially hazardous chemicals, and ensure that working conditions meet health and safety requirements. UK occupational exposure limit (OEL) concentrations for hydrogen sulphide and other gases associated with septic conditions are given in section 1.6 of EH40/2002xlii. Typical ventilation rates for odour containment in pumping stations used in current operational practice in Doha are given in Table 2.23.1. Table 2.23.1 – Typical Ventilation Rates for Odour Control in Pumping Stations Air changes per hour Pumping station (no man access)
One for local covers
Pumping station working area (current practice)
20 during man access (initiated by light switch)
Dry wells (current practice)
12
Separate screen chamber
Passive ventilation through carbon filter (where there is no other route for odour escape)
12 for pumping stations extracted from close to the sump and process units
Ventilation systems should be designed so that in the event of a fire being detected in any area, all the air conditioning equipment and ventilation systems are shut
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•
Ventilation of Pump Rooms and Dry Wells Air supply should be provided by either two or three duty fans and one standby fan, depending on the size of the pump room. Exhaust air should be removed by either two or three duty fans and one standby fan, depending on the size of the pump room. The exhaust fans should have approximately 5% less flow capacity than the air supply fans to keep the building at a slight positive air pressure. This is to avoid drawing unfiltered dust laden air into the pump room which can drastically shorten the equipment life. Pump rooms and dry wells should typically have 12 air changes an hour for normal operation, increasing to 16 air changes an hour during man entry. The cable basement should be ventilated as part of the pump room ventilation system. Ventilation of Wet Areas - Pump Sumps & Screen Chambers) Wet areas should normally be ventilated by air extraction only, with a natural air supply to keep the wet area under slightly negative pressure and avoid releasing odours to the atmosphere. Exhaust air should be removed by duty/standby fans, depending on the size of the wet areas. Each fan should have a two-speed motor.
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During man entry, the additional air supply should be provided by the fans running at high speed. The fans should be sized so that with all fans running at high speed, the required air changes per hour for man entry are achieved. Ventilation rates should be designed to ensure a maximum of 3ppm of H2S in the wet areas. The system should be designed to achieve this with only one fan operating. Wet areas should typically have 12 air changes an hour for normal operation, increasing to 20 air changes an hour during man entry.
2.23.2
Odour Control
Air vented from pumping stations in most cases will not require odour treatment. However, the risk of sewage or contaminated surface/groundwater entering the system should be assessed.
The designer shall assess the potential for corrosion of A/C units, particularly from H2S, and ensure that they are appropriately designed and located. Air Conditioning of Electrical Switch Gear Rooms Electrical switchgear rooms should be completely isolated from the remainder of the building for the following reasons: •
The thermal loads are higher than elsewhere in the building;
•
In the event of a fire being detected the air conditioning should be switched off to allow the fire suppression equipment to operate effectively.
Two split AC units working independently (mechanically and electrically) of each other should be used to air condition the room, with air diffusers discharging horizontally towards the panels. Return air should be sucked back by the split unit, via receiving air diffusers located at evenly placed points between the supply air diffusers, and fixed to the ceiling.
The required air conditioning systems and ventilation capacities are shown in the tables below.
Each split AC units should be rated at 50% above the required capacity (i.e. 150% total), so that should one unit fail, the other unit will provide 75% of the required air conditioning capacity.
Table 2.23.2 - Air Conditioning (AC) Systems
The required thermal load should be calculated on the basis of peak conditions.
2.23.3
Air Conditioning
Location
Air Condition system
Electric Switch Gear
Dual Split AC unit system
Control Room
Split AC unit system
Table 2.23.3 - Ventilation Capacities Location
Ventilation (l/s) per person
Ventilation (l/s) per sq.m.
Approximate air changes per hour. *
Electric Switchgear Room
-
0.8
1
Control Room
10
1.3
2
Kitchen and Toilet
-
10
8
Note: Figures extracted from BS 5720, Table 1. *Depending on the dimensions of the rooms.
The required quantity of exhaust air should be removed from electrical switchgear rooms to atmosphere by a fan with an actuated louver. Air inlet should be by natural supply through a filtered and actuated louver. In the event of a fire, the electrically actuated louvers should be closed to seal electrical switchgear rooms during the use of any fire extinguishing system. Air Conditioning of Control Rooms, Kitchens and Toilets A single split AC unit should be provided for air conditioning the control room. No air conditioning should be provided for the kitchen or toilet. The kitchen and toilet areas should be air conditioned by exhausting part of the control room air through them. Exhaust air in the kitchen and toilet areas should be discharged outside the building. The fans should be run continuously for the following reasons: •
Volume 3
SW Drainage
1st Edition June 2005 - Copyright Ashghal
To provide the required air changes for the control room and kitchen;
Page 71
State of Qatar - Public Works Authority Drainage Affairs
•
To keep the toilet and kitchen area ventilated.
Equation 2.24.1
Air louvers should be fitted in the bottom of kitchen and toilet doors.
2.24
&
ρ bar =
0.67φ bar 2S max
Structural Design
General Design Requirements Unless local design standards dictate otherwise, in general, he design of concrete structures shall be in accordance with BS 8110-1 “Structural Use of Concrete”xliii and BS8007 “Design of Concrete Structures for Retaining Aqueous Liquids”xliv. Likewise, the design of steel structures shall be in accordance with BS5950-1 “Structural Use of Steelwork in Buildings”. Local standards shall govern if any conflict arises. All structures shall be designed based on a ‘limitstates’ philosophy. Unless required otherwise, all structures shall be designed for a minimum service life of 60 years. The designer shall prepare calculations for each design package, including as a minimum the following information: Description of the structure and design methodology adopted; All assumptions made for design (i.e. geotechnical parameters, loadings, etc); Standards, guidelines and specifications used for design;
Equation 2.24.2 Were: max
= allowable crack width (0.2mm maximum)
Smax
= likely crack spacing (mm)
R
= restraint factor (0
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