Drainage Design Manual ~ Vol 2 Foul Sewerage
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Qatar - Drainage Design Manual ~ Vol 2 Foul Sewerage...
Description
State of Qatar -Public Works Authority Drainage Affairs
CONTENTS 1
Sewerage Systems Design ....................................................................................... 1 1.1
Standards ..................................................................................................................... 2
1.2
Sources of Information .................................................................................................. 2
1.3
Estimation of Flows ....................................................................................................... 3 Domestic ....................................................................................................................... 5
1.3.2
Industrial ........................................................................................................................ 8
1.3.3
Commercial ................................................................................................................... 8
1.3.4
Institutions such as Schools, Health Centres, Hospitals and Mosques ........................ 9
1.3.5
Infiltration ....................................................................................................................... 9
1.4
Peaking Factors .......................................................................................................... 10
1.5
Hydraulic Design ......................................................................................................... 13
1.6
1.5.1
Formulae ..................................................................................................................... 13
1.5.2
Minimum Pipe Sizes and Gradients ............................................................................ 16
1.5.3
Minimum and Maximum Velocities.............................................................................. 16
Septicity in Sewage, Odour Control and Ventilation ................................................... 17 1.6.1
Explosion and Combustion Risk ................................................................................. 18
1.6.2
Corrosion ..................................................................................................................... 18
1.6.3
Impact on Subsequent Treatment Processes ............................................................. 18
1.6.4
Odours......................................................................................................................... 18
1.6.5
General Design Guidelines for Odour Control in Sewerage Systems ........................ 19
1.7
Pipeline Materials and Jointing ................................................................................... 24
1.8
Pipe Bedding Calculations for Narrow and Wide Trench Conditions .......................... 24 1.8.1
Bedding Design for Rigid Pipes .................................................................................. 25
1.8.2
Bedding Factors .......................................................................................................... 26
1.8.3
Design Strength........................................................................................................... 26
1.9
Manhole Positioning.................................................................................................... 27
1.10
House Connections..................................................................................................... 28
1.11
Construction Depths ................................................................................................... 28
1.12
Manholes, Chambers, Access Covers, and Ladders .................................................. 30 1.12.1
Inspection Chambers .................................................................................................. 30
1.12.2
Sewer System Manholes ............................................................................................ 30
1.12.3
Elements of Design ..................................................................................................... 30
1.13
Industrial Wastes ........................................................................................................ 31
1.14
Septic and Sewage Holding Tanks ............................................................................. 31 1.14.1
Design of Septic Tanks and Soakaways ..................................................................... 32
1.14.2
Sewage Holding Tanks ............................................................................................... 32
1.15
Oil and Grease Interceptors ........................................................................................ 32
1.16
Flow Attenuation Methods .......................................................................................... 32
1.17
Volume 2
1.3.1
1.16.1
Flow Controls .............................................................................................................. 33
1.16.2
Attenuation Storage Tanks and Sewers...................................................................... 33
Abandonment of Sewers............................................................................................. 39
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2
Pumping Stations ....................................................................................................39 2.1
Standards ................................................................................................................... 39
2.2
Hydraulic Design......................................................................................................... 39
2.3
2.2.1
Hydraulic Principles .................................................................................................... 40
2.2.2
Pump Arrangements ................................................................................................... 41
Rising Main Design ..................................................................................................... 42 2.3.1
Rising Main Diameters................................................................................................ 42
2.3.2
Twin Rising Mains....................................................................................................... 42
2.3.3
Economic Analysis...................................................................................................... 42
2.3.4
Rising Main Alignment ................................................................................................ 43
2.4
Maximum and Minimum Velocities ............................................................................. 43
2.5
Pipe Materials ............................................................................................................. 43
2.6
Thrust Blocks .............................................................................................................. 43
2.7
Air Valves and Washout Facilities .............................................................................. 44
2.8
2.7.1
Air Valves .................................................................................................................... 44
2.7.2
Vented Non-return Valves .......................................................................................... 44
2.7.3
Wash – Outs ............................................................................................................... 44
2.7.4
Isolating Valves........................................................................................................... 45
Flow Meters ................................................................................................................ 45 2.8.1
Application and Selection ........................................................................................... 45
2.8.2
Magnetic Flowmeters.................................................................................................. 45
2.8.3
Ultrasonic Flowmeters ................................................................................................ 46
2.9
Surge Protection Measures ........................................................................................ 46
2.10
Screens....................................................................................................................... 48
2.11
Pumping Station Selection.......................................................................................... 49
2.12
Pumps and Motors...................................................................................................... 52
2.13
Sump Design .............................................................................................................. 53
2.14
Suction/Delivery Pipework, and Valves ...................................................................... 55
2.15
Pumping System Characteristics ................................................................................ 56
2.16
Sump Pumps and Over-Pumping Facilities ................................................................ 59
2.17
Power Calculations including Standby Generation ..................................................... 59 2.17.1
Introduction ................................................................................................................. 59
2.17.2
Load Type ................................................................................................................... 59
2.17.3
Site condition .............................................................................................................. 60
2.17.4
Generator set operation and control .......................................................................... 60
2.17.5
Type of installation ...................................................................................................... 60
2.17.6
Type of Control Panel ................................................................................................. 60
2.17.7
Ventilation system....................................................................................................... 60
2.17.8
Fuel system ................................................................................................................ 60
2.17.9
Starting method .......................................................................................................... 61
2.17.10 Service facility ............................................................................................................. 61 2.17.11 Generator set sizing.................................................................................................... 61
2.18
Switch Gear and Control Panels ................................................................................. 65 2.18.1
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Type–tested and partially type tested assemblies (TTA and PTTA) .......................... 65
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State of Qatar -Public Works Authority Drainage Affairs
2.18.2
Total connected load ................................................................................................... 65
2.18.3
Short circuit level ......................................................................................................... 65
2.18.4
Type of co-ordination .................................................................................................. 66
2.18.5
Form of internal separation ......................................................................................... 66
2.18.6
Bus Bar rating.............................................................................................................. 67
2.18.7
Type of starter ............................................................................................................. 67
2.18.8
Protection device ......................................................................................................... 68
2.18.9
Interlocking facility ....................................................................................................... 70
2.18.10 Accessibility ................................................................................................................. 70 2.18.11 Cable entry .................................................................................................................. 70
2.19
PLC’s SCADA/Telemetry ............................................................................................ 70 2.19.1
PLC ............................................................................................................................. 70
2.19.2
RTU ............................................................................................................................. 71
2.19.3
SCADA and Telemetry Systems ................................................................................. 72
2.20
Lighting ....................................................................................................................... 73
2.21
Maintenance Access ................................................................................................... 77
2.22
Gantry Cranes and Lifting Facilities ............................................................................ 77
2.23
Ventilation, Odour Control and Air Conditioning ......................................................... 78
2.20.1
2.24
3
Light Fitting Selection Criteria ..................................................................................... 73
2.23.1
Ventilation.................................................................................................................... 78
2.23.2
Odour Control .............................................................................................................. 79
2.23.3
Air Conditioning ........................................................................................................... 80
Structural Design ........................................................................................................ 81 2.24.1
Substructures .............................................................................................................. 81
2.24.2
Superstructures ........................................................................................................... 90
2.25
Site Boundary Wall/Fence .......................................................................................... 97
2.26
Site Facilities ............................................................................................................... 97
Documentation ........................................................................................................ 98 3.1
Reference Standards .................................................................................................. 98
3.2
House Connection Survey .......................................................................................... 98
3.3
Building Permit ............................................................................................................ 98
4
Health and Safety .................................................................................................... 99
5
Trenchless Technologies ..................................................................................... 100 5.1
5.2
5.3
Volume 2
Alternative Techniques ............................................................................................. 100 5.1.1
Pipe jacking (Open/Close Face) ............................................................................... 100
5.1.2
Microtunnelling (Closed Face) .................................................................................. 102
5.1.3
Directional drilling ...................................................................................................... 104
Planning and Selection of Techniques...................................................................... 104 5.2.1
Initial Planning ........................................................................................................... 105
5.2.2
Selection Criteria ....................................................................................................... 110
5.2.3
Factors Affecting Choice Of Method ......................................................................... 110
Geotechnical Investigations ...................................................................................... 110
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5.4
5.5
5.6
5.3.1
Geological Strata Overview ...................................................................................... 110
5.3.2
Groundwater Regime................................................................................................ 110
5.3.3
Soil/Rock properties.................................................................................................. 111
5.3.4
Indicative Scope of Interpretative Reporting............................................................. 113
Design....................................................................................................................... 113 5.4.1
Feasibility Study........................................................................................................ 113
5.4.2
Pipe Design .............................................................................................................. 113
5.4.3
Shaft Design ............................................................................................................. 114
5.4.4
Ground Movements .................................................................................................. 115
Environmental Assessment ...................................................................................... 117 5.5.1
Vibration .................................................................................................................... 117
5.5.2
Noise ......................................................................................................................... 117
5.5.3
Dust........................................................................................................................... 118
Approvals – Procedures and Formats ...................................................................... 118 5.6.1
6
Page iv
Guidance for Design Check ...................................................................................... 118
5.7
Risk Assessment ...................................................................................................... 118
5.8
Trenchless Construction References........................................................................ 122
5.9
Trenchless Construction Glossary ............................................................................ 123
References .............................................................................................................125
Volume 2
Foul Sewerage
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State of Qatar -Public Works Authority Drainage Affairs
1
Sewerage Systems Design
This volume of the Manual covers the design of new and existing sewerage systems, detailing the design standards, parameters and approaches to be adopted. However, this information should not be regarded as prescriptive in all situations, as each design needs to be prepared, reviewed and approved by appropriately skilled and experienced staff, both within the designers’ and the Drainage Afairs (DA) organisations. The sewerage systems in Qatar are separate in that foul sewage, comprising domestic, commercial and industrial effluent is collected in a separate system to that which collects stormwater runoff and ground waters. The sewerage system for Qatar collects foul flow discharges from premises, located within the developed areas of its towns and cities, and directs the collected flows to the Sewage Treatment Works (STW). Sewage flows discharge, generally by gravity, into the sewerage system through house connections to the sewer pipelines and manholes outside the property boundary. This network of branch and trunk sewers directs flows by gravity to pumping stations, which pump flows to the STW. The flat topography of Qatar discourages long lengths of gravity sewer due to the resulting great depths of construction that would be required. The sewerage systems therefore include many pumping stations, with the result that sewage flows will often be pumped several times before arriving at the STW. The major sewerage systems and STWs are located in Doha, with similar systems in the smaller towns such as Al Khor. The Doha Catchments The Doha sewerage system is contained within three catchments, being the Doha West Catchment Area, the Doha South Catchment Area and the Industrial Area. The system in each catchment is similar, in that it comprises networks of sewers and
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manholes, directing flows to numerous pumping stations. The flow from each catchment is then pumped to either the Doha South or Doha West STW. The Doha South Catchment can be broadly defined as that part of Doha being southeast of the Salwa Road and east of the Industrial Area, along with the central business district within the B Ring Road. The Catchment extends southwards to include Abu Hamour, the Airport area and onwards as far as Wakrah, as well as including Wukair and areas to the north and east of the Abu Hamour area. The extent of the system and the considerable distances over which sewage is transferred across flat terrain, necessitate some 52 sewage pumping stations. The layout of the network results in foul sewage from certain locations being pumped through as many as six or seven pumping stations before reaching Doha South STW. Development in the catchment is of predominantly low to medium density, with higher densities in the central business district. In total, some 415km2 of land falls within the catchment that it is predicted will be sewered to Doha South STW. In broad terms, only one quarter of this area is presently developed. The Doha West Catchment comprises some 250km2 of western and northern Doha. The area also includes North Doha, Rural and Urban Rayyan and the Umm Slal Planning Areas of Qatar. The Catchment lands rise from sea level in the east, to some 35m above sea level in the west. The ground level at Doha West STW is about 45m above sea level. The sewerage network in the Doha West Catchment is served by a terminal pumping station (PS 32) at the south-west edge of the built-up area, from which sewage is delivered in two parallel rising mains to Doha West STW. Development in the Catchment is of low to medium density, with some areas completely undeveloped. The major future development area is located at the northern end of Doha Bay, where high-density residential and commercial development is planned. In order to minimise construction, operation and maintenance costs for pumping stations, new designs should use gravity for the movement of
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State of Qatar -Public Works Authority Drainage Affairs
sewage flows. However there are the following practical considerations: •
•
Depth of trench excavation should generally not exceed 5.0m, or 7.5m maximum in extreme cases, dictated by excavator access and pipeline strength, where possible. It is acknowledged that greater depths are often necessary in Doha, but these should be avoided because of the danger of deep excavations and the difficulty of achieving good compaction in the backfill;
1.1
The following standards are of interest to designers in surface water 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. •
Gradients should not be flatter than the minimum stated herein, to minimise siltation and septicity.
In theory a separate sewerage system should exhibit no increases in flows from rainfall. However, all systems suffer from infiltration to some extent due to faults and openings in the fabric of the system and illegal connections of stormwater collection systems. Many sewerage authorities deal with such flow increases by incorporating overflows which divert foul flows to watercourses at times of rainfall. However such arrangements are impractical for Qatar due to the lack of watercourses operating all year round, and the resulting unacceptable pollution which would result from discharge of foul flows to wadis with little or no flow. The extent of infiltration is not fully understood in Qatar, but knowledge will improve with ongoing studies and Drainage Area Plans. In the meantime the sewerage system should avoid the need for overflows, with any increased flows being contained within the sewerage system. The only overflows permitted are for emergency use only, and only to be located at pumping stations. These emergency overflows are only to operate on failure of pumps, through mechanical or electrical breakdown. Pumps are to be rated to pump all flows expected to be received at the station. All elements of the sewerage system, including pipelines, manholes, chambers, are to be located on publicly owned lands. Pumping stations and associated facilities shall be on DA owned land. Ideally, access for operation and maintenance of the sewerage system should also be located on publicly owned lands. If not, wayleave agreements should be in place to facilitate such access.
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Standards
BS EN 752 – Drain and sewer systems outside buildingsi. This supersedes BS 8005ii, which is withdrawn, and part of BS 8301iii. Part 1: 1996
Generalities and Definitions
Part 2: 1997 Part 3: 1997 Part 4: 1998
Performance Requirements Planning Hydraulic Design and Environmental Considerations
Part 5: 1998
Rehabilitation
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 methodsiv.
•
BS EN 1610: 1998 – Construction and testing of drains and sewersv.
•
Sewers for Adoption – 5th Edition (WRC)vi.
•
BS EN124: 1994 Gully tops and manhole tops for vehicular and pedestrian areas – Design requirements, type testing, marking, quality controlvii.
1.2
Sources of Information
The following publications are of interest to designers in surface water 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.
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State of Qatar -Public Works Authority Drainage Affairs
•
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.
•
Construction Industry Research and Information Association, 1996, Report R159: Sea Outfalls – construction, inspection and repair, London, CIRIA.
Practice Manual for England, Scotland, Wales, and Northern Ireland, London UK, CIRIA. •
Velocity equations for the hydraulic design of pipes – Wallingford Research.
•
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.
•
Report for the hydraulic design of pipes – Wallingford Research.
•
Building Research Establishment, 1991, Soakaway Design, BRE Digest 365, BRE Watford UK.
•
Construction Industry Research and Information Association, 1998, Report 177, Dry Weather Flows in Sewers, London, CIRIA.
•
HR Wallingford DC Watkins, 1991, Report SR271 -The hydraulic design and performance of soakaways, Wallingford UK.
•
Water Research Council, 1994, Velocity equations, UK, WRC.
•
Bazaraa, A.S., Ahmed, S., 1991. Rainfall Characterization in an Arid Area, Engineering Journal of Qatar University, Vol. 4, pp35-50.
•
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.
•
Construction Industry Research and Information Association, 2000, C522 Sustainable Urban Drainage Systems – Design Manual for England and Wales, London UK, CIRIA.
•
Construction Industry Research and Information Association, 2001, C523 Sustainable Urban Drainage Systems – Best
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1.3
Estimation of Flows
The flows in a foul sewerage system are made up of contributions from a number of different sources, including: domestic properties; commercial areas; industrial facilities; institutional contributions from hospitals, schools, etc.; groundwater infiltration; and surface run-off. The contributions to the system from each of these sources must be determined before the required hydraulic capacity of the sewerage can be established. Each of these contributions will follow a different diurnal pattern, with flows varying over a 24-hour period. The design of the system must take these fluctuations into account and be
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State of Qatar -Public Works Authority Drainage Affairs
capable of catering for the peak flows likely to be encountered in any 24- hour period. Diurnal flow patterns will be different on working days, from the patterns on rest days.
4. Identify any existing and proposed commercial establishments within the catchment, together with their working populations and diurnal variations. Section 1.3.3 provides detailed guidance on this.
The starting point for the design of foul sewerage should be the estimation of the average flow rate or the Dry Weather Flow (DWF). This is calculated from the following formula:
5. Identify any existing and proposed institutional establishments such as schools, health centres, hospitals and mosques that are within the catchment boundary. Determine the usage of these institutions and derive a diurnal flow pattern for them. Section 1.3.4 provides details of this process.
DWF = PG + I + E
Equation 1.3.1
DWF = dry weather flow (litres/day) P = population served G = average per capita domestic water consumption (l/hd/day) I = Infiltration (l/day) E = average industrial effluent discharged in 24 hours (l/day)
The process for establishing flow rates should follow the sequence set out below: 1. Define catchment and sub-catchment boundaries for the area under consideration. This should include all the properties and establishments that contribute to the system and may include future developments as well as existing. The catchment represents the entire upstream area contributing to a point or node in the sewerage system. Generally, catchments are taken to contribute to trunk sewers, while sub-catchments contribute to branches. Thus a catchment may comprise a number of subcatchments.
6. Determine infiltration rates into the sewerage system using the methods described in section 1.3.5. These may increase with time or it may be proposed to rehabilitate the system to reduce infiltration. 7. The flows that are likely to occur in the sewerage system can now be estimated. This is done by adding together the total daily contributing flows from each upstream source to any given point in the network. This is usually done sub-catchment by sub-catchment working down the trunk sewer. It can be done graphically and will establish the maximum likely flow that has to be catered for at the given location. The total daily flow from each contributing source is calculated and summed to give a total daily flow through a given point. This flow is then averaged for a 24-hour day to give an average Dry Weather Flow or DWF. The peak flow for design purposes in upper catchment areas can be taken as 6xDWFviii. From the peak flows the required pipe sizes can be determined. However, it should be noted that the peaking factor would decrease in downstream catchment areas (see section 1.4 for information on peaking factors). Hydraulic design is described in section 1.5.
2. Determine the numbers and types of dwellings within the catchment and from this, determine the existing and future contributing domestic population and hence the flows from that population to the network. Section 1.3.1 gives detailed guidance on this process. Establish the diurnal flow pattern for the domestic contribution. 3. Identify any existing and proposed industrial establishments in the catchment, together with their daily contributing flow and diurnal flow pattern. Section 1.3.2 gives guidance on this.
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Table 1.3.1 below gives the discharge rates that should be used for the design of foul sewerage systems. Discharges in the table below are averaged over 24 hours in the determination of DWF because the application of peaking factors allows for the diurnal profile.
Figure 1.3.1 -Typical chart showing diurnal variations in domestic sewage flows 2.50
2.00
Table 1.3.1 – Typical Daily Discharges in the ME Development Discharge Unit type l/day
Flow, l/s
1.50
1.00
0.50
0.00 00:00
04:00
08:00
12:00
16:00
20:00
170
Litres/head/day
Domestic low density high value properties
250
Litres/head/day
Average Infiltration
100
Litres/jhead/day
Infiltration range
0- 250
Litres/head/day
00:00
Hours
Where sewerage systems are very long and the time of flow from top to bottom is significant, peak flows will be heavily attenuated. This is because, for example, locally generated domestic flows in the lower parts of the catchment will have passed downstream by the time the flows arrives from the upper areas. This has the effect of smoothing out the peaks in flows.
1.3.1
Domestic
Domestic
Domestic flows form the largest proportion of flows in foul sewers. They derive from normal domestic appliances such as sinks, basins, toilets, showers, washing machines, baths, etc., and are dependent on the number of persons in a dwelling. In order to determine suitable domestic contributions to the sewerage system, it is necessary to make certain assumptions. For example, each property is assumed to house a certain number of persons, and this will vary from one type of property to another. The assumption is made that all properties of a given type will contain a given number of persons.
The figures in this table provide general guidance for the design of foul sewerage systems. The figure to be used for design purposes in Qatar where there is no better information is 270l/h/d, comprising 160 l/hd/day or sewage and 110 l/hd/day infiltration. Where the area to be served is low density palaces and villas consideration should be given to the use of 200 l/head/day. If the catchment is inland and the ground water table level is low then the infiltration allowance can be reduced or even eliminated. Design populations of the existing and proposed properties are based on the plots indicated on the Action Plans that can be obtained from the Land Information Centre and the occupancy levels given in Table 1.3.2. The number of discharge units per property is then allotted based on BS 8301, as shown in Table 1.3.2.
Butler and Daviesix suggest that between 75% and 85% of water used in a dwelling in the Middle East is returned to the sewerage system. Thus, if a property is metered, a good assessment of return to sewer flows can be obtained.
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State of Qatar -Public Works Authority Drainage Affairs
Table 1.3.2 – Indicative Occupancy Levels (from BS 8301) Plot Description
Occupancy Levels
Discharge
For plots less than 1225m2
6 people
14
For plots equal to and between 1225 and 2500m2
9 people
21
Small Palaces
15 people
35
Larger Palaces
25 people
58
For plots greater than 2500m2
Units
The dry weather flow is then obtained from Figure 1.3.2, which has been reproduced from BS 8310, Figure 2. Where no Action Plan plot or housing information is available, the future area can be assumed as developed at an average of the existing planned plot density.
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Flow (l/s)
100
10
1 1
10
100
1000
10000
Discharge Units
Figure 1.3.2 – Conversion of Discharge Units to Flow Rates
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State of Qatar -Public Works Authority Drainage Affairs
1.3.2
1.3.3
Industrial
Estimation of daily discharges from Industrial areas will be dependent on the type of industry occupying the area. The majority of industries in Qatar are “dry” industries such as warehousing and workshops. These will have lower consumption rates than “wet” industries such as concrete or paper manufacture. If possible, metered water consumption rates should be used in design but where these are not available or are impractical to use, the values in Table 1.3.3 can be applied. Table 1.3.3 – Design Allowance for Industrial Wastewater Generation Category Volume (l/s/ha) Conventional
Water - Saving
Lightx
2
.5
Mediumx
4
1.5
Heavy
8
2
Category
Volume
Slaughterhousexi
6600 l
Per tonne of produce
Drink Productionxi
8400 l
Per cubic metre of produce
Laundryxiv
1500 – 2100 l/d
Per machine
Tanneryxii
30 – 35 m3
Per tonne of produce
Tanneryxi
7600 l
Per tonne of produce
In the above table, light industry may be taken as “dry industries which generally handle materials and goods which do not include washdown facilities. Heavy industries will include factories with washdown facilities and using water in the unit processes. These figures are to be used only in initial land usage planning, and developers must obtain confirmation from end users before final design.
Commercial
Most significant developments include a degree of commercial activity and this should be included in the assessment of discharges to the foul system. This activity can range from a single small office or shop, up to major shopping, hotel or office complexes. Each development type needs to be assessed. Commercial activities include all those listed above and each may have its own characteristic discharge profile, which will inevitably be different from the standard domestic profile. Table 1.3.4 gives an indication of the likely discharges from various types of commercial activity. Table 1.3.4 – Typical flows from commercial premises Development type
Discharge l/day
Per
Commercial Centresxiii
50
Customer per 12 hour day
Airportxiv
11 - 19
Passenger
Hotelsxv
150-300
Bed
Restaurantsxvi
30-40
Customer
Social Clubsxvii
10 – 20
Customer
Cinemaxviii
10
Seat
Officesxix
750
100m2
Shopping Centresxx
400
100m2
Department Storexxi
2000
Per toilet
Recreationalxxii Centres
80
Customer per 6 hour day
Commercial premisesxxiii
300
100m2
Where possible, the above discharge rates should be checked using installed water supply meters for existing developments. Proposed developments
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State of Qatar -Public Works Authority Drainage Affairs
should be assessed using the figurers given in the table above. Diurnal profiles should be derived for each type of commercial development and applied to the daily discharge rate from the table.
1.3.4
Institutions such as Schools, Health Centres, Hospitals and Mosques
profiles by reading water meters at say, hourly intervals throughout the day. The resulting profile is then applied to the daily consumption.
1.3.5
Infiltration describes flows in the foul system, which are not legitimate discharges. Infiltration comprises two components: •
inflows from faulty manhole covers, crossconnections from storm and groundwater control systems, and tidal sources. Inflows can also come from the illegitimate practice of lifting manhole covers to drain surface water during and after storms;
•
infiltration of groundwater through displaced and open pipe joints, cracks, fractures and breaks in the fabric of the main sewers and lateral connections, manholes and chambers.
Table 1.3.5 contains typical values of discharges from various types of institutional premises. Table 1.3.5 – Typical Institutional Discharges Development Discharge Per type l/day Educational Centresxiii
70
Pupil per 8 hour day
Day schoolsxxiii
50 - 100
Pupil per 8 hour day
Residential schoolsxxiv
150-200
Pupil
Mosquexiii
100
Worshiper per 12 hour day
Sports Centrexxiii
10 – 30
visitor
Retirement Homexxiii
250
Bed
Nursing Homesxxiii
300 - 400
Bed
Assembly Hall
11 - 19
Guest
Prison
300 - 570
Inmate
Hospitalsxxiii
500-750
Bed
Each category of premises will have a different diurnal discharge profile, with day schools only contributing during the school day, and hospitals likely to contribute flows for much of the waking day. As with other types of development, metered water supply records should be consulted wherever possible to provide an indication of actual consumption figures. A suitable return to sewer factor should then be applied to the results. Sometimes, it may be possible to determine diurnal
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Infiltration
Infiltration causes reduced capacity for legitimate sewage flows, increased requirements for pumping and sewage treatment, and possible structural damage. Infiltration into foul sewerage systems can be problematic. It generally derives from groundwater entering the pipe network through: poor joints in the pipes; cracks or fractures; defects in manholes; or through private drainage connections. Infiltration generally occurs in areas with a high water table. In coastal areas, infiltration can be saline which can have a detrimental effect on sewage treatment processes and can cause corrosion of metalwork in manholes and pumping stations. It is normal to allow a figure of 10% of DWF for infiltration. Infiltration should be excluded from the calculation of flows using peaking factors. Thus for a peaking factor, Pf, peak design flow would be given by the equation:
Q = Pf (PG + E) + I Equation 1.3.2 Where: Q = Peak Design Flow (l/d) Pf = Peaking Factor P = Population
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State of Qatar -Public Works Authority Drainage Affairs
G = Daily per capita flow (l) E = Daily Industrial Flow (l) I = Daily infiltration flow (l) A sample calculation sheet for sewers using the above formula is included in Volume 1 Appendix 1 Where local conditions indicate that the figure of 10% DWF for infiltration is too low, then a higher figure may be adopted. However, this must be justified by supporting information, such as the analysis of flow survey results. At the time of drafting this manual, DA suggest that for G in the above formula, an overall figure of 270l/hd/day be used for all domestic flows. This will be revised when flow survey results become available . Conversely, where the water table is known to be well below the level of the sewerage system, the allowance for infiltration will be less significant locally. Infiltration is often associated with exfiltration, which is the leakage of foul flows due to faults and openings in the pipework, manholes and chambers. Exfiltration of foul flows results in contamination of the surrounding soils and possible pollution of groundwater. Since both infiltration and exfiltration involve flows passing through physical defects in the sewerage system fabric, they often occur together in conjunction with fluctuating groundwater levels. This continuing flow mechanism can result in erosion of the surrounds and foundations to pipes and manholes. In serious cases, failure of the asset or ground subsidence has resulted. The Sewer Rehabilitation Manual provides a detailed explanation of the factors involved in infiltration. Two CIRIA reportsxxv,xxvi describe various methods for estimating base-flow infiltration. Inflow of stormwater runoff is estimated from the area of development contributing to the flow monitor. Estimation of both components relies on detailed flow and rainfall monitoring, combined with hydraulic modelling to understand the relative contributions of the components in wet and dry weather. The Infiltration Reduction Procedure contained in the Sewerage Rehabilitation Manual should be
Page 10
followed, where infiltration is to be reduced. This is an iterative approach to successively focus on sources of excessive infiltration, and to ensure that reduction measures are cost-effective. It is very evident that removal, or more realistically, significant reduction of infiltration, is a timeconsuming and expensive process. It is far more cost-effective to avoid its occurrence in the first place. This can be done by strictly controlling the quality of new and renovated sewerage installations, and by ensuring that best quality materials and construction techniques are used, to provide a longlasting leak-free system. Such standards should be applied to both private and public sewerage. Property connections should also be correctly made, and abandoned sewers and septic tanks properly sealed.
1.4
Peaking Factors
As described in section 1.3, the rate of discharge of sewage from a given property to the sewerage system will vary during the day. The sewerage system must be able to cope with the highest flows likely to occur in the day. Different contributors to the system will have different discharge profiles. For example, shopping areas will generally only contribute flows during the periods when the shops are open, and then the flows will be in proportion to how busy the shops are through the day. Domestic properties generally show marked morning and evening peaks, which coincide with peak domestic activity. This suggests that foul sewers should be designed to cope with higher than the average, or dry weather flow (DWF), and a common way of designing systems is to cope with a flow of up to six times DWFviii. While this approach may be satisfactory for the smaller sewers at the head of the system, it will tend to over design the larger sewers and ignores the attenuation effects as the flows move downstream. At the head of a sewerage system, discharges tend to be pulsed, with individual pulses of flow being the discharge from individual appliances. As the pulses flow along the pipe system, the peaks tend to become attenuated and as the flows progress down the system, these pulses combine to form a more consistent flow. The peaking factor will depend on the upstream population and the distance the
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sewage has travelled. A number of different ways of determining the peak factor have been proposed which take account of the attenuation downstream with increasing population. There are several formulae for calculation of peaking factorix, for which the Babbit formula is most representative in Qatar The Babbit Formual (1952) is;
PF =
5 5
,
P
Where PF represents the peaking factor, and P is the population in thousands. However, the formula is not representative at low populations.
Therefore, the upper limit for peaking factors shall be taken as six for populations up to and including 500.. For populations over 500 the Babbit formula shall be used. The minimum value of peaking factor shall be 3. It is considered that values in excess of six, and below three, are unrealistic for conditions in Doha, but these figures may be revised after a detailed flow survey is carried out (see section 1.3 above). The variation of peaking factors with population is shown graphically in Figure 1.4.1, which follows
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State of Qatar -Public Works Authority Drainage Affairs
Peaking Factors Babbit
BSEN 752
7.00 F a c to r
6.00 5.00
Maximum value 6
4.00 3.00 Minimum value 3
2.00 1.00 0.00 Population 100
200
500
1000
2000
5000
10000
20000
Figure 1.4.1 Plot of Peaking Factors v Population
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1.5
Hydraulic Design
The hydraulic design of sewerage systems involves achieving a balance between pipe size, pipe gradient and pipe depth, such that self-cleansing velocities are achieved without surcharge, but with the most economical combination of size and depth. Wherever possible, pipe depths should be used that avoid the need for concrete bed and surround.
downstream in order to maintain minimum gradients (see section 1.5.1 below). Trunk sewer sections serving larger catchments are likely to become very deep (but see also section 1.11); •
General Principles The general principles of foul sewer design are as follows: •
Pipe size should not generally decrease downstream;
•
Sewers should be designed to convey peak flows without surcharge;
•
Sewers should achieve self-cleansing velocity at least once per day. Note that half-pipe velocity is numerically the same as full-pipe velocity.
•
•
To allow for ventilation of the system, maximum design depth of flow should not exceed 0.75 x pipe diameter or d/D ≤ 0.75. Where there is a chance of heavy construction plant tracking over new sewers laid during construction of a site, the minimum depth of cover should be measured from the formation level of the site above the sewers;
• Self-cleansing velocities increase with pipe size (see sections 1.5.1 and 1.5.2 below); •
•
•
At manholes, all pipes should be laid such that their soffits are at the same level. Pipes in manholes should not be laid with the inverts level, as this can promote the deposit of solids in minor branches leading to odour problems and blockages; Junctions should not enter a sewer at right angles but should enter at an angle of 45° to the direction of flow of the main sewer; Sewers should commence at minimum depth upstream and follow ground profiles if possible to minimise excavation. However, it is recognised that in Qatar, due to flat topography, depths will gradually increase
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Backdrop manholes should be used where there is a difference >600mm in level between a branch/rider sewer and the main sewer. Backdrops (see also section 1.12.2 below) should not be used to reduce gradients on main sewer lines.
Design Tools Hydraulic computer models are invaluable tools for understanding the performance of sewerage systems. Hydraulic models are of particular value for: •
Understanding the performance of the complete system, in particular attenuation of flows;
•
Understanding the flow regime of complex and interdependent systems, such as those with bifurcations and loops;
•
Understanding the flow characteristics of multiple pumping systems, as found in Doha;
•
Readily understanding the effects of changes in development on existing systems;
•
Simulating modifications to the construction and/or operation of the system.
Hydraulic computer models should use InfoWorks CS software, and be verified against flow and depth measurements carried out on the actual system.
1.5.1
Formulae
1.5.1.1 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;
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Table 1.5.1 - Pipe Roughness ks Values Material ks Value (mm)
k 2.51υ v = −2 (2 gDS ) log s + 3.7 D D (2 gDS )
Normal
Poor
Concrete (Precast + O Rings)
3.0
6.0
Concrete (Steel Forms)
3.0
6.0
DI (PE Lined)
0.6
1.5
GRP
0.6
1.5
VCP
1.5
3.0
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
Further values can be obtained by direct reference to Appendix 1 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. The figure to be used for design purposes shall be 1.5 in all cases
Table 1.5.2 - Kinematic Viscosity υ Values Temperature, 0C Viscosity, m2/s x 10-6
Using the relationship: Q=AV Equation 1.5.2 Where: Q = flow in the pipe (m3/s) A = Cross-sectional area of flow V = velocity of flow This allows the pipe full discharge to be calculated where: 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
15
1.141
25
0.897
35
0.727
For detailed sewage 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 graph for proportional velocity and discharge in part-full circular sections is reproduced in Figure 1.5.1. This illustrates the relationship between depth of flow, and velocity. It can be used for estimating the velocity of flow in partially full pipes, and should be used to check velocities for self cleansing velocities at low flow (see table 1.5.4)
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. Tables 1.5.1 and 1.5.2 below give recommended values of ks and υ . Both are taken for the Slimed sewers category from Wallingford design tablesxxvii.
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Figure 1.5.1 - Proportional Velocity and Discharge in Part-Full Circular Sections
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1.5.1.2 Manning’s Equation Manning’s equation is an empirical formula for uniform flow in open channels. Manning’s equation is: v=(1/n)R2/3S0½ Equation 1.5.4 Where: n is Manning’s roughness coefficient S0 is bed slope R is the hydraulic radius of the flow The equation may be useful in pumping station approach hannels and elements of sewage works. However, all pipe calculations must use Colebrook White
0.75m/sec can be assumed to be self-cleansing in pipes of 150mm diameter. As sewer sizes increase, so too do self-cleansing velocities, with the result that very large foul sewers require velocities to exceed 1.5m/sec to be selfcleansing. Such velocities in large diameter pipes pose a safety hazard and facilities must be provided to prevent operatives being washed downstream in these sewers.
1.5.3
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 from a deposited bed.
Typical values of Manning’s n are given below. Table 1.5.3 - Typical values of Manning’s n Channel Material n range Cement
0.010-0.015
Concrete
0.010-0.020
Brickwork
0.011-0.018
Manning’s equation is only valid for rough turbulent flow conditions.
1.5.2
Minimum Pipe Sizes and Gradients
CIRIA Report R141xxviii defines self-cleansing sewers as follows: “An efficient self-cleansing sewer 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”. Foul sewers should be at least 200mm 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 several dwellings are connected to the head of the sewer, and the standard of workmanship during construction is high. Peak flow velocities of
Page 16
Minimum and Maximum Velocities
In order to minimise the maintenance requirements of any given length of sewer, it is normal to design the sewer to be “self-cleansing”. This means that the sewer is designed to achieve a velocity at least once per day that will carry all solid deposited material along the pipe and not leave any materials deposited in the invert of the sewer. Table 1.5.4 is based on the simplified CIRIA method of assessing self-cleansing velocities in foul sewers. Surface water sewers require generally higher selfcleansing velocities because of the higher particle densities. Table 1.5.4 – Approximate Self-Cleansing Velocities for Foul Sewers Pipe size Approximate self- cleansing (mm) velocity (m/sec) 200–300
0.75
400
0.77
500
0.82
600
0.86
700*
0.87
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Pipe size (mm)
Approximate self- cleansing velocity (m/sec)
800
0.88
900*
0.88
1000
0.92
1200
1.03
*700 and 900 are non preferred sizes Where large diameter sewers (over 1.0m diameter) are laid to steep gradients, very high flow velocities occur. For example,; a 1000mm pipe laid to 1:100 gradient with a depth of flow of 750mm will have a discharge velocity approaching 3.4m/sec, which is unacceptable in foul sewers. The designer should implement energy dissipation measures in such cases. It should be emphasised that scour in pipes at these velocities is not a significant problem with modern materials, but if velocities become very high, odour emissions can be increased and noise can become a problem. As a general rule, it is preferable to aim to achieve self-cleansing velocity at least once per day. The designer should aim to achieve a velocity at the design flow (i.e. peak flow) of between selfcleansing and 2.0m/s, with 2.5m/s as an upper limit. In small sewers, 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 sewer and trunk sewer. In this case, the economics may justify the construction of a backdrop to minimise excavation for the branch sewer trench. The discharge from a backdrop into a manhole requires careful design to prevent flows from washing over the benching. Backdrops for large diameter sewers are complex structures that may involve the creation of vortices to dissipate energy, for which specialist design is required. These are often purpose-made in stainless steel. A typical example is included in the standard drawings, Volume 8.
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1.6
Septicity in Sewage, Odour Control and Ventilation
In rising mains and shallow gravity sewers, respiration of bacteria in wastewater and slimes present on submerged sewer walls rapidly depletes any dissolved oxygen or nitrates causing anaerobicity (septicity)xxix. One of the main impacts of septicity is the formation of sulphide by the bacterial reduction of inorganic sulphate present in the wastewater. Some of the sulphide will combine with metals in the sewage. The remainder will be present in ionised and unionised form, as below. S2- ⇔ HS- ⇔ H2S Only the un-ionised form is released to the atmosphere. The proportion of sulphide present in the un-ionised form is dependent upon the pH value of the sewage and is about 50% at a pH value of 7. For example, a liquid concentration of 2mg/l of sulphide at pH 7.0 would be in equilibrium with a gaseous H2S concentration of 300ppm (ml/m3). At a pH value of 8.0 this would decrease to about 60ppm. Septicity can have an impact on health and safety, corrosion, subsequent treatment processes and odours. Hydrogen sulphide is a toxic gas. WHO guidelines for dose-effect relationships for H2S are given in Table 1.6.1xxx. Table 1.6.1 - Health Impacts of Hydrogen Sulphide H2S Level Health Impact (ppm) 1000-2000
Immediate collapse with paralysis of respiration
530-1000
Strong central nervous system stimulation, followed by respiratory arrest
320-530
Risk of death
150-250
Loss of olfactory sense
50-100
Serious eye damage
10-20
Threshold for eye irritation
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State of Qatar -Public Works Authority Drainage Affairs
UK Occupational exposure limit (OEL) xxxi concentrations of hydrogen sulphide and other gases associated with septic conditions are given in Table 1.6.2. Table 1.6.2 - Exposure Limits for H2S and Other Gases Gas
Long term OEL (8-hour) (parts per million)
Short term OEL (15 minute) (parts per million)
Hydrogen sulphide
5
10
Methyl Mercaptan (methanethiol)
0.5
-
Ethylmercaptan (ethanethiol)
0.5
2
Ammonia
25
35
Methylamine
10
-
Ethylamine
10
-
Dimethylamine
10
-
1.6.1
The lower explosive limit for hydrogen sulphide is 40000ppm. This concentration is unlikely to be achieved under normal operation, and risk is therefore minimal. Table 1.6.3 - Flammable Gases in Sewers Lower explosive limit % v/v in air
Upper explosive limit % v/v in air
Carbon Monoxide
12.5
-
Hydrogen sulphide
4.0 (40000 ppm)
46
Petroleum
100 ppm
Methane
5.3
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1.6.2
Corrosion
Hydrogen sulphide is associated with the corrosion of concrete and mortar as the result of its bacterial conversion to sulphuric acid. High levels of hydrogen sulphide may develop below covers, with consequent impact on the structure of the tank or manhole as has been found at a number of sites. Metal work and electrical equipment is also vulnerable to H2S corrosion. Selection of construction materials for tanks, manholes, covers, and fittings should take into account the potential for corrosion.
Explosion and Combustion Risk
The WRC report ‘Enclosed Wastewater Treatment Plants’xxxii considers the potential risk of the development of flammable concentrations of gases arising in a STW. The possible gases considered are given below in Table 1.6.3.
Gas
Spontaneous combustion of sulphur around the edge of lifted manhole covers has been reported in Doha. In this instance, the reaction of hydrogen sulphide with iron oxide at the manhole cover has led to its catalytic oxidation to sulphur, which is spontaneously combustible. Operational procedures may be required to reduce this risk. Although these are beyond the scope of normal design functions, it is important that the designer is aware of such issues and to include mention of them in the design HARA’s.
15
1.6.3
Impact on Subsequent Treatment Processes
The discharge of septic sewage can increase the rate of development of sulphide in the primary sedimentation stage sewage and sludges. High levels of septicity have been associated with poor settleability of activated sludges. High levels of septicity or sulphates have been associated with poor gas yields from mesophilic anaerobic digestion processes.
1.6.4
Odours
The discharge of septic sewage can be a significant source of odours at the discharge point, whether to an intermediate pumping station or to the inlet of a STW. Threshold levels for various odours are listed in Table 1.6.4. The odour threshold level of hydrogen sulphide measured in a laboratory is about 0.5 parts per billion (ppb). The level above which odour problems
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can occur is typically ten times this value. Background H2S levels are often in the range 25ppb.
1.6.5
Table 1.6.4 – Odour Threshold Levels Gas
Odour threshold (parts per billion)
Hydrogen sulphide Methyl Mercaptan (methanethiol) Ethylmercaptan (ethanethiol) Ammonia
0.5 0.0014-18
General Design Guidelines for Odour Control in Sewerage Systems
The design of sewerage systems to reduce the development of septicity is the subject of a number of guidesxxxiii. Guidelines include:
Rising mains 0.02
•
Minimise lengths of pumping mains, and use lift pumps rather than long rising mains to minimise retention under anaerobic conditions( there is no satisfactory minimum length of rising main which can be quoted for design purposes. Even a retention time of 30 minutes is sufficient to develop anaerobic conditions. );
•
Minimise turbulence at the discharge point;
•
Discharge into the gravity sewerage system at low level to avoid turbulence and consequent release of odours;
•
Location of discharge point should NOT be immediately prior to hydraulic drops or sharp bends;
•
Manhole covers at discharge points may need to be sealed.
130-15300
Methylamine
0.9-53
Ethylamine
2400
Dimethylamine
23-80
Pumping stations
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•
Minimise turbulence at inlet to sump. Use submerged, rather than overflow weirs;
•
Use level detectors to minimise the volume of sump used under normal flow conditions;
•
Use frequent pumping regimes to minimise retention time in sump, and also spread odour load more thinly over the day;
•
Maximise benching to give self-cleansing conditions and ensure no accumulation of grit. Guidelines are given in BS 8301xxxiv;
•
Ensure any screenings or grit can be removed, or are washed back into main flow of sewage;
•
Active/passive odour control unit may be required depending on the sensitivity of the site, size of installation, and other factors such
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State of Qatar -Public Works Authority Drainage Affairs
as degree of septicity of sewage under normal flow conditions.
Gravity sewers •
Maintain self-cleansing velocities;
•
Avoid turbulent flow (including sharp bends and drops);
•
Minimise length of siphons (which will act as rising mains);
•
Ensure there is ventilation of the sewer (by provision of vents);
•
Design to ensure no accumulation of grit or debris.
and slimes causes localised septicity at points where turbulence is insufficient to remove such debris. An indicator of the likelihood of septicity in a gravity sewer is the ‘Z formula’ with the effect of different values of Z as indicated in Table 1.6.5.
Storage Tanks and Shafts •
Minimise turbulence of discharges to tanks and shafts (discharge at low level);
•
In sensitive areas (i.e. next to houses) odour control may be needed to treat displaced odours when levels rise.
Refer also to section 2.23 of this volume, and section 1.5 of volume 5 The formation of sulphide in rising mains and gravity sewers has been the subject of extensive studies xxxv, xxxiii. The concentration can be estimated from the following equationxxxvi:
Cs=K tCOD[(1+0.004D)/D]1.07(T-20) Equation 1.6.1 Where: Cs = concentration of sulphide (mg S/l) Kc = constant, usually taken to be 0.00152 t = anaerobic retention time (mins) D = diameter of rising main (cm) T = temperature of sewage (°C) COD = COD of sewage (mg/l) In gravity sewers, there is a balance between sulphide formation when flow is stagnant, and sulphide release and oxidation during turbulent flow. In practice, little sulphide should be formed in a wellventilated, self-cleansing, partially-filled gravity sewer used for domestic sewage. Where problems do occur, they are typically associated with sewers of shallow gradients where accumulation of grit, silt
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Z as calculated below is a dimensionless parameter that indicates the potential for sulphide generation.
Z=
3(EBOD) x P S0.5Q0.33 b Equation 1.6.2
Where: EBOD = 5 day BOD (mg/l) multiplied by a temperature factor 1.07 (T-20) T = sewage temperature (co) S = slope of sewer (m/100m) Q = wastewater flow (l/s) P = wetted pipe wall perimeter (m) b = surface width of the stream (m) Table 1.6.5 - Values of Z, Indicating Sulphide Generation Potential Value of Likely condition Z 200kW
- 8 starts/hour
Pipework Only superior materials are acceptable for use in pumping station pipework. The pipework installation should incorporate the following features: •
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;
•
Each submersible pump should be installed with a discharge isolation valve to permit isolation of the pump from the discharge pipework for maintenance;
•
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 well or discharge manifold, and allow the other pumps to continue operating normally;
•
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;
•
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;
Stop / start levels for single and multiple pump operation 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. 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 at 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 increases the pump start-up and stop times.
Pump duty level 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).
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Suction/Delivery Pipework, and Valves
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•
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 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; 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 for sewage should be of the double-flanged 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;
•
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 should not be used with sewage.
2.15
Pumping System Characteristics
NPSH, Vibration, Cavitation and Noise Net Positive Suction Head (NPSH) is used to check the pumping installation for the risk of cavitation. 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.
•
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;
NPSH 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.
•
Reflux valves for sewage 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;
NPSH = Pa – Vp + Hs – Fs
•
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;
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Equation 2.15.1 Where: Pa = atmospheric pressure at liquid free surface Vp = vapour pressure of liquid Hs = height of supply liquid free surface, above eye of pump impeller Fs = suction entry and friction losses
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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, absolute values for atmospheric and liquid vapour pressures are used.
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. 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.
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Characteristic curve for new pipe
Figure 2.15.1 – Characteristic Curve for Multiple Pumps
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2.16
Sump Pumps and Over-Pumping Facilities
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).
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.
2.17
Power Calculations including Standby Generation
Over-pumping facilities should be provided where there is a single sump and access may be required for repair 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 overpumping connection to allow the pumping station to be fully isolated and the fixed pipework drained for repair.
2.17.1
Introduction
A standby power generator set is essential in applications where the loss of the power supply can not be accepted due to critical loads. 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.
Sump Pump Installations Sump pumps should incorporate the following feature: •
Sump pumps should discharge to the wet well above the water level to prevent gas release;
•
Discharge pipes should be fitted with a nonreturn 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 a high level in the dry well, and be 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 300mm.
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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 capacative;
•
Load profile.
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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:
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).
•
ambient temperature;
•
elevation above sea level;
2.17.6
•
humidity;
•
wind direction and dust contamination in air;
The control panel can be unit mounted (on the generator set unit) or remotely mounted (inside the control room).
•
nearby residential areas for sound level consideration.
2.17.4
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 sets can be installed by different means according to the site requirement and unit size. The type of 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;
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Type of Control Panel
The control panel is used to operate and monitor the unit in case of power failure. Panels have many options depending on the type of operation required, and the mode of operation (one unit, two units, automatic start, manual start, etc).
2.17.7
Ventilation system
Unit ventilation and the cooling system are critical parts of 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 the means of forcing air out of the room using a fan installed at a level above the highest point in 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 surrounded 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 and engine, discharging the air through a set mounted 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:
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•
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 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);
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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 (1500RPM). 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 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
The following procedure can be used to size the generator set according to the available data from pump motors and other loads (e.g. lighting/other
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non-motor load) as well as the sequence of operation, and starting of the motor: 1)
Starting KVA (SKVA) calculation
-
Calculate lock rotor current (LRA) = for DOL x Full load current
-
Calculate the SKVA = (LRA V * 1.732)/1000
2)
Effective SKVA
Use Table 2.17.1 as a guideline for calculating the effective SKVA. Suppose that we have three motors, which will start and run in sequence (motor-1, motor-2 and motor3). Using the highest effective SKVA calculated and the required voltage DIP (10%, 20%, and 30%) as specified by the motor manufacturer, the generator set can be selected from the data sheet provided by the generator set manufacturer.
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Table 2.17.1 – Guide to Generator Set Sizing – Effective SKVA
Step
Motor 1
Motor 2
Motor 3
Comments
1
Motor load (KW)
A
B
C
2
Starting KVA (SKVA)
X
Y
Z
KW motor/ motor efficiency LRA*V* 1.732 /1000
3
Total motor load connected before the required motor start in sequence Total motor load connected after motors have been started in sequence
0
A
A+B
A
A+B
A+B+C
0 D
(A/(A+B))*100 E
((A+B)/(A+B+C))*100 F
X*D
Y*E
Z*F
Q
R
S
4
5 6
(Step3/Step4)*100 Using step-5 result, obtain compensation for motor already started from fig.2.17.1 Multiply (step-2xstep-6) Obtain the reduce voltage factor from fig.2.17.2 Effective SKVA ( Step-7 x Step-8)
7 8 9
From Fig. 2.17.1
From Table 2.17.2
1.4
1.3
1.2
1.1
1
0.9 multiplier 0.8
0.7
0.6
0.5 40
50
60
70
80
90
100
Figure 2.17.1 - Reduced voltage starting factor
Table 2.17.2 - Reduce voltage starting factor
Volume 2
Type
Multiply SKVA BY
Star/Delta Auto transformer 80% , 65%, 50% DOL Solid state
0.33 0.68, 0.46 , 0.29
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1.0 Estimate 300% of full load KVA
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Worked example: The following motors required a standby generator i. 90 kw 3-phase motor, soft starter , voltage dip 30% ii. iii.
70 kw 3-phase motor, star/delta ,voltage dip 30%
45 kw 3-phase motor , direct online (DOL), voltage dip 30% 1- Calculated Locked rotor current 90 kw motor = 6 x
90,000 √3 x 415x0.8
= 939 amp
75 kw motor = 6 x
75,000 √3 x 415x0.8
= 730 amp
45 kw motor = 6 x
45,000 √3 x 415x0.8
= 469amp
2- Calculated SKVA 90 kw motor = 939 x 415 X 1.732 1000
= 674.9
75 kw motor = 730 x 415 X 1.732 1000
= 524.7
90 kw motor = 469 x 415 X 1.732 1000
= 337.1
Table 2.17.3 – Generator Set Sizing – Worked Example Ste p 1 2
Motor load KW Starting KVA (SKVA)
Motor1
Motor 2
Motor 3
Comments
90 674.9
70 524.7
45 337.1
KW motor/ motor efficiency LRA*V*1.732/1000
3
Total motor load connected before the required motors start in sequence
0
90
160
4
Total motor load connected after motors have been started in sequence
90
160
205
(Step-3/step-4)*100
0
56.3
78
Using step-5 result obtain compensation for already start motor Multiply (step-2xstep-6) Obtain the reduce voltage factor Effective SKVA ( Step-7 xstep-8)
1
1.15
1.25
674.9
603.4
421.4
3 337.5
0.33 199.1
1 421.4
5 6 7 8 9
from Fig. 2.17.1
from Table 2.17.2 NB Motor 1 is a solid state starter
The selected generator will be sized for the highest effective SKVA @30% Voltage dip = 421.4KVA.
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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 sub-sections.
2.18.1
Type–tested and partially type tested assemblies (TTA and PTTA)
According to BS EN60439-1xliii 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:
•
Clearance and creepage distance;
•
Mechanical operation test;
•
IP degree of protection.
The partially type-tested assemblies (PTTA) are assemblies that contain both type-tested and non type-tested 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;
4.
Overload;
5.
Insulation failure;
The short circuit level can be calculated according to the following steps.
6.
Mechanical failure.
Step-1 Determine the transformer full load amperes:
To achieve a type-tested assembly (TTA) the following performance requirements should be verified: •
Temperature – rise limits;
•
Dielectric properties;
•
Short circuit withstand strength (main circuit);
•
Effectiveness of protective circuit;
•
Short circuit withstand strength of the protective circuit;
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I(fl)
=
KVAx100 0E (l-l) x 1.732
Equation
2.18.1 Where: I(fl)
= transformer full load
KVA
= transformer capacity volt ampere
E (l-l)
= line to line voltage
Step-2 Find the transformer multiplier
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I s.c
=
I (fl) x Multiplier
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Multiplier
=
100 %Z (T)
Equation 2.18.2 Where: Z (T)
= transformer impedance
Step-3 Determine the transformer let through short circuit current I s.c
=
I (fl) x Multiplier Equation 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 (≤250 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
Motor Control Centre (≤5000 Amp)
2.18.4
Type of co-ordination
Electrical component co-ordination according to IEC 97-4-1xliv, 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.
Type – 2: co-ordination (personal/components safety). The designer, where possible, should select type-2 co-ordination to assure full protection of personal safety as 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.
2.18.5
Form of internal separation
The form of separation should be according to BS EN60439-1xliii 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 (1& 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.
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);
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Figure 2.18.1 – Form and Type of Internal Separation
Form-4 type-3: Diagram-1
Bus bar
2.18.7 Function unit
Enclosure
Type of starter
The designer should consider the following points when choosing the starter type to be used. Motor size
Internal Separation
Terminal for external conductor
Cable gland
Form-4-Type-7: Diagram-2
Bus bar
Enclosure
Function unit
Internal Separation
Terminal for external conductor
Form –2 – Type-2: Diagram-3
Bus bar
Enclosure
The motor size (kW) will determine if a standard starter can be used (direct on line DOL or start delta starter 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. 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
Internal Separation
Terminal for external conductor
Cable gland
Motor duty and application 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
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.
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Duty type
Application example
Continuous run at constant load and speed
Potable water
Short run at constant load and
Sewage pumping station
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speed Continuous run at variable load and speed
Irrigation network
Intermittent periodic duty
Injection system
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
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
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.
Table 2.18.5 – Examples of Protection Required for Load Types
Duty type
Starter
Continuous run at constant load and speed
DOL, Y/D, S/S
Load type
Short run at constant load and speed
DOL, Y/D
Main incomer feeder
Continuous run at variable load and speed
VSD
Intermittent periodic duty
D.C starter, DOL
S/S if sufficient cooling time between operations
Notes: DOL: direct online, Y/D: star/delta , s/S: soft starter, VSD: variable speed drive
(local authority/ generator set)
Pump, grinder
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. 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
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Protection device
Valve actuator Instrument (level/ flow/ pressure) Building services (lighting/ sockets)
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.
2- Electronic protection devices 3- motor manager protection unit
(OLR), ELCB
(OLR), ELCB Conventional protection device MCB, ELCB, Fuses
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Note:
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.
ELCB = Earth leakage circuit breaker OLR = Over load relay MCCB = Moulded case circuit breaker ACB = 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 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.
Phase losses/phase reversal protection:
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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. 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.
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 undervoltage 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.
5.
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;
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•
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.
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:
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•
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. 69-30xlv. 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 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.
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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-IntegralDerivative) 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
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reset. The larger the time constant, the more effect Rate will have; •
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
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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 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
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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;
•
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
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that is also inherent within SCADA, and by inputting cost data, it can produce cost forecasts for a wide range of process consumables.
c.
Water storage tank lighting;
d.
External installed machinery (settlement tanks, inlet works aeration tanks);
2.20
e.
Pump wet wells and screen chambers.
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 guidexlvi should be considered during the design.
2.20.1.2 Environmental Conditions In many industrial applications the environmental condition is hostile or hazardous as explained below. 1)
2.20.1
Light Fitting Selection Criteria
Light fittings are selected according to the following criteria and application.
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);
d.
Wet atmosphere (water ingress);
e.
Dusty atmosphere.
2.20.1.1 Installation Location The location of the light fittings to be designed has a large affect on the type of luminaire 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)
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 11271xlvii for all potentially hazardous areas such as screen chambers and wet wells.
2.20.1.3 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 guidexlvi. The relevant levels are replicated below for convenience in Table 2.20.1.
2. External lighting a.
Building (external wall mounted fittings);
b.
Internal road lighting (inside station boundary);
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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
used in most locations with some changes in the body material, IP rating and lamp wattages. 2.
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.
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
50- 100
Tank area
50
Building (external wall and door
70
entrance) External installed machinery
100
1.
4.
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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 5266xlviii. 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;
•
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.
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
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.
2.20.1.4 Type of Light Fitting Light fitting types that can be used in different locations can be categorised as follows.
Flood lights
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Emergency light system There are two types of emergency light system: a. Self-contained; b. Centrally powered.
6.
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.
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, inverters, rechargeable unit (non-maintained);
•
5.
luminaire
Normal luminaires with a separate lamp for emergency use, fed from a central battery system (nonmaintained)/(sustained luminaire);
•
Normal luminaires fed from a central power source (maintained/ nonmaintained). Roadway lighting
The design of roadway lighting should be according to BS 5489-3xlix. 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
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conditions. Usually, three types of lamp are commonly used. These are; high-pressure sodium, metal halide, and high-pressure mercury. The installation of the fitting on the column can be on the post top, bracket or side entry. 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
Es = F x n x N x UF x MF A
Es
Equation 2.20.1 = Average illuminance (lux) of the plane
F
= Initial bare lamp lumens flux (lumens)
n
= Number of lamps per luminaire
N
= Number of luminairies
UF
= Utilisation 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 Where:
•
Calculate the luminance that will be achieved by the final layout.
External and Roadway Lighting Calculation The calculation for roadways can be carried according to BS 5489-3xlix. 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.
L
=
room length
W
=
room width
Hm
=
height of the luminaire plane above the
E
=
N x L x BF x WLFxMF A
horizontal reference plane Equation 2.20.4 H
=
depth of the cavity
Where:
Calculate the effective reflectance (REx) of the ceiling, wall and floor cavity (from tables using above calculated (CIx).
E
=
Illuminance required (lux)
L
=
Lamp output per lumens (lm)
Determine the utilisation factor value (UF) using luminaire manufacturer data sheets; room index and effective reflectance (apply any correction factors).
BF = luminaire
Beam factor number of lamps per
N
=
Number of luminaries
WLF as
=
waste light factor (usually considered
Determine the maintenance factor (MF) MF = LLMF x LSF x LMF x RSMF
0.9)
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;
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MF
=
maintenance factor
A
=
area to be lighted (m2)
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:
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a)
Photocell controller for automatic dusk till dawn control;
b)
Time clock operation for full control of when external lights are in operation.
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 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 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.
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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. 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 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.
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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 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;
•
Long travel drive chains should be located to avoid snagging, and allow the operator safe passage;
•
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
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) concentrationsl for hydrogen sulphide and other gases associated with septic conditions are given in section 1.6 of this manual. 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 One for local covers (no man access) 12 for pumping stations extracted from close to the sump and process units 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)
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 down. All supply and exhaust ventilation louvers should shut automatically to
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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. Other points to consider include: 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;
the size of the wet areas. Each fan should have a two-speed motor. 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.
Air ducts should be designed to ensure the velocity through them does exceed 10m/s in occupied areas;
Wet areas should typically have 12 air changes an hour for normal operation, increasing to 20 air changes an hour during man entry.
Materials should be selected to limit the corrosion effects of hydrogen sulphide (H2S).
2.23.2
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, the number and configuration depending on
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Odour Control
Air vented from pumping stations will in most cases require odour treatment. In most cases, a two bed (duty/standby) system using carbon regenerated using alkali (caustic soda or potash) is preferred. At larger pumping stations consideration may be given to pre-treatment of strong sources using catalytic iron filters. Further details of requirements are given in Volume 5 Section 1.5. Reference should also be made to Section 1.6 of this Volume Typical conditions to be considered in the design of the odour control unit are given in the table below. Table 2.23.2 – Conditions to be Considered in Odour Control Unit Design Sewage temperature 25 – 35oC Ambient temperature
0–50oC
Relative humidity
Up to 100%
Temperature of air vented from the sewerage system to an Odour Control Unit
Up to 30oC
Radiating temperature
surfaces
Hydrogen sulphide from below covers
85oC maximum 250ppm
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Hydrogen sulphide with workplace air
2.23.3
10ppm
Air Conditioning
The required air conditioning systems and ventilation capacities are shown in the tables below. Table 2.23.3 - Air Conditioning (AC) Systems Location
Air Condition system
Electric Switch Gear
Dual Split AC unit system
Control Room
Split AC unit system
•
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. 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. The required thermal load should be calculated on the basis of peak conditions. The required quantity of exhaust air should be removed from electrical switchgear rooms to atmosphere by a fan with an actuated louver.
Table 2.23.4 - Ventilation Capacities Location
Ventilation (l/s) per person
Ventilation (l/s) per sq.m.
Approximate air changes per hour. *
Air inlet should be by natural supply through a filtered and actuated louver.
Electric Switchgear Room
-
0.8
1
Control Room
10
1.3
2
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.
Kitchen and Toilet
-
10
8
Note: Figures extracted from BS 5720, Table 1. *Depending on the dimensions of the rooms.
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
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.
Electrical switchgear rooms should be completely isolated from the remainder of the building for the following reasons:
Exhaust air in the kitchen and toilet areas should be discharged outside the building. The fans should be run continuously for the following reasons:
•
•
To provide the required air changes for the control room and kitchen;
•
To keep the toilet and kitchen area ventilated.
The thermal loads are higher than elsewhere in the building;
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Air louvers should be fitted in the bottom of kitchen and toilet doors.
2.24
Structural Design
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”li and BS8007 “Design of Concrete Structures for Retaining Aqueous Liquids”lii. 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 ‘limit-states’ 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 geotechnical parameters, loadings, etc);
•
Standards, guidelines and specifications used for design;
•
Input and output from software where appropriate.
(i.e.
Calculation of the reinforcement requirements for control of early-age thermal cracking shall be in accordance with BS 8007lii. For the calculation of the likely maximum crack spacing and the reinforcement ratio the following formula shall be used:
ϖ max Rα (T1 + T2 ) Equation 2.24.1
Volume 2
Were: ωmax = allowable crack width (0.2mm maximum) Smax = likely crack spacing (mm) R = restraint factor (0
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