weir

CHAPTER 1 INTRODUCTION 1.1 Background In present scenario, the supply of electricity in Nepal lags way behind the actual demand of the nation. So, an ambitious plan to harness 10000 MW of electric potential in period of 10 years has been proposed by the government. In order to achieve the plan so proposed, expertise at high level and core competence is required among local manufacturers and technical experts. Such requirements have brought technological expertise in the country to their toes to develop excellence in their field. Therefore, Kathmandu University came up with an idea to fulfil the bridge of technological expertise in hydro turbines sector with establishment of Turbine Testing Laboratory (TTL), which is currently in construction phase within the University premises, (Turbine Testing Laboratory, 2009). Kathmandu University has intended to establish a turbine testing laboratory for the purpose of education, research and development in close cooperation with indigenous industries. To meet this formidable task, Kathmandu University has entered into a technical cooperation with Norwegian University for Science and Technology (NTNU)/Hydropower Lab and NORAD as major sponsor. The Turbine Testing Laboratory is aimed to come in operation by mid 2011, (Turbine Testing Laboratory, 2009). 1.2 Purpose, Project Goals and Success Criteria 1.2.1 Purpose The purposes of establishing a Turbine Testing Laboratory at Kathmandu Industry are listed below: Purpose 1:build competence and knowledge within the hydropower sector of Nepal 

Teaching/learning facility



Industrial courses



Staff training for the industry

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Purpose 2: motivate research 

Development of efficient turbines able to withstand sand erosion.



Development of turbine and pump technology



Maintenance of turbines

Purpose 3: provide a meeting place for the industry and university 

for research and student projects for the industry



open doors for collaborative research with national and international universities and research institutions,

1.2.2 Success Criteria The project goals and long time success criteria of establishment of Turbine Testing Laboratory at Kathmandu University are:  KU’s own planning and design capacity is enhanced to the extent that a similar future project can be implemented in a 50% shorter time period with at least the same quality  The five year goal after the completion of the project are: 

Testing and certification of at least 3 numbers of mini or micro turbines produced in Nepal for internal or external parties



At least 5 students at Master level use the TTL for their research.



At least 1 student actively uses TTL for his/her PhD work.



Undertake a minimum of 3 industrial courses or professional courses for researchers.



Publish a minimum of 3 articles in internationally recognized technical magazines. (Turbine Testing Laboratory, 2009)

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1.3 Organization Project Steering Committee Chairman Registrar Dean SOE, Reps from CED, Acc Dept, Proj Man Appoint management Approve plans and budget

External advisors NTNU Prof Dalhaug, Brandåstrø

Rep Advise on technical matters includ design Assist in testing and commissioning Training

Project Management Proj Man T Skeie Assist Proj Man R Shrestha

Implement project according to plans and budget

Design team

Accounting Dept

Civil/structural: CED Bajracharya

Fin Man K Baral

Electro-mech: D Bista, R Skrestha Account reports

Hydro-mech: R.Shrestha

Design of structures and equipment

Civil contractor Access road Landscaping Building Electrical wiring

Electro- Mech. Contractor

Hydro-Mech. Contractor

Pumps

Pipes System

Frequency Controllers

Weir

Figure 1.1 Organizational charts, TTL, KU Control Panel 3

EOT Crane

CHAPTER 2 TECHNICAL SPECIFICATION OF THE COMPANY 2.1 Lab specification 

30 metres open system head



150 metres closed system head

 

⁄ maximum flow 300 kW maximum testing capacity



capacity lower ( lab) reservoir



capcity upper reservoir



5000 kg EOT crane capacity

2.2 Block diagram of Turbine Testing Lab

Figure 2.1: Block diagram of Turbine Testing Lab

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2.3 Schematic diagram of Turbine Testing Lab

Figure 2.3: Schematic diagram of Turbine Testing Lab In figure 2.3, the numbers 1 to 10 represent the multi turn valve with electric actuator and     

a = Pump a b = Pump b c = upper reservoir d = lower (lab) reservoir j = drain valve

2.4 Standarization of Tests Turbine testing lab will follow International Electrotechnical Comission (IEC-60193) standard for conducting model tests. 2.5 On going research and development activities 

Two PhD and four masters students are currently working for the new design of Francis Turbine.



Fault analysis of 12 MW pelton runner of Khimti Hydropower plant is under progress.



Patnership with ESAP/RRE for suitable projects is under discussion.



Extension of Renewable Nepal project for establsihing local hydropower manufacturing company is under planning phase.

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CHAPTER 3 TRAINING DETAILS 3.1 Training Methodology 3.1.1 Introduction 

Introduction to the Turbine Testing lab (TTL)



Purpose of lab

3.1.2 Work Assigned 

Literature review



Design review



Field survey and documentation



Design and fabrication of head measurement prototype



Test of the prototype

3.1.3 Recommendation for selection 3.1.4 Observation    

Observation of work in progress to build the lab Observation of drawings of the lab Materials and Equipments used in the lab Management of the Technical personnels

3.1.5 Work Accomplished 

Literature review  Weirs  Head measurement mechanism  Flow Meters and calibration procedure



Design review  Weir installed in the lab



Field survey and documentation



Design and fabrication of head measurement prototype



Test of the prototype



Recommendation for selection

3.1.6 Documentation

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CHAPTER 4 WORK ACCOMPLISHED DETAILS 4.1 LITERATURE REVIEW ON WEIR 4.1.1 Background The weir is one of the oldest structures used to measure the flow of water in open channels. Several rating equations were developed for standard rectangular contracted weirs by different investigators. Generally, the data of each investigator are within +1.5 to +2.5 percent with respect to their individual equations, but comparisons of the various equations differ as much as several percent (King and Brater, 1976; Ackers et al., 1978). In the past, user organizations selected an equation, called it standard, and specified construction requirements and limitations of use. However, Kindsvater and Carter (1959) developed an improved method for computing rates of flow through rectangular, thin-plate weirs. Their method also applies to fully side suppressed, partially contracted, and fully contracted rectangular weirs. Kulin and Compton (1975) discuss the method and equation for rating fully contracted V-notch weirs with any angle between 25 degrees and 100 degrees. This method also rates partially contracted 90-degree, V notch weirs. The Kindsvater approach accounts for velocity of approach effects and the accompanying variation of discharge coefficient caused by changes of effective width and head. This method is preferred for calibrating or rating rectangular and triangular weirs. Also, this method will correct for excess approach velocity in standard weirs. Thus, this newer approach will accurately recalibrate some of the older weirs that are no longer operating as standard, as well as some that never were standard. 4.1.2 Definition of Weirs A measuring weir is simply an overflow structure built perpendicular to an open channel axis to measure the rate of flow of water. Inspecting and checking the critical parts of weir structures for degradation and improper operation are easy. A properly built and operated weir of a given shape has a unique depth of water at the measuring station in the upstream pool for each discharge. Thus, weirs can be rated with respect to an upstream head relative to the crest elevation versus discharge, and equations or tables which 7

apply to the particular shape and size weir can be generated. The crest overflow shape governs how the discharge varies with head measurement. 4.1.3 Principle The discharge over thin plate weirs is a function of the head on the weir, the size and the shape of the discharge area, and an experimentally determined co-efficient which takes into account the head on the weir, the geometrical properties of the weir and approach channel and the dynamical properties of the water. 4.1.4 Installation 4.1.4.1 Selection of site This type of weir to be used for discharge measurement is determined in part by nature of the proposed measuring site. Under some conditions of design and use, weirs shall be located in rectangular flumes or in weirs boxes which stimulate flow conditions in rectangular flumes. Under other conditions, weirs may be located in natural channels as well as flumes or weirs boxes with no significant difference in measurement accuracy. Specific site-related requirements of the installation are described in 6.3 4.1.4.2 Installation conditions 4.1.4.2.1 General Weir discharge is critically influenced by the physical characteristics of the weir and the weir channel. Thin-plate weirs are especially dependent on installation features which control the velocity distribution conformance with standard specifications. 4.1.4.2.2 Weir Thin plate weirs shall be vertical and perpendicular to the walls of the channel. The intersection of the weir plate with the walls and floor of the channel shall be watertight and firm, and the weir shall be capable of withstanding the maximum flow without distortion or damage. Stated practical limits associated with different discharge formulae such as minimum width, minimum weir height, minimum head, and maximum values of h/p and b/B (where h is the measured head, p is the height of crest relative to floor, b is the measured width of the notch and

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B is the width of the approach channel), are factors which influence both the selection of weir type and the installation. 4.1.4.2.3 Approach Channel For the purpose of this international standard the approach channel is that portion of the weir channel which extends upstream from the weir a distance not less than ten times the width of the nappe at maximum head. If the weir is located in a weir box, the length of the box shall be equal to the specified length of the approach channel. The flow in the approach channel shall be uniform and steady, with the velocity distribution approximating that in a channel of sufficient length to develop normal (resistance- controlled) flow in smooth, straight channels. Figure 1 shows measured normal velocity distributions in rectangular channels, upstream from the influence of a weir. Baffles and flow straighteners can be used to stimulate normal velocity distribution, but their location with respect to the weir shall be not less than the minimum length prescribed for the approach channel. The influence of approach-channel velocity distribution on weir flow increases as h/p and b/B increase in magnitude. If a weir installation unavoidably results in a velocity distribution which is appreciably uniform, the possibility of error in calculated discharge should be checked by means of an alternative discharge-measuring method for a representative range of discharges. 4.1.4.2.4 Downstream channel The shape and size of the channel downstream from the weir is of no significance, but the level of the water in the downstream channel shall be a sufficient vertical distance below the crest to ensure free, fully ventilated discharges, Free( no submerged) discharge is ensured when the discharge is independent of the downstream water level. Fully ventilated discharge is ensured when the air pressure on the lower surface of the nappe is fully atmospheric. 4.1.4.2.5 Measurement of head 4.1.4.2.5.1 Head measuring devices In order to obtain discharge measurement accuracies specified for the standard weirs, the head on the weir shall be measured with a laboratory-grade hook gauge, point gauge, manometer, or other gauge of equivalent accuracy. For a continuous record of head variations, precise float

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gauges and servo operated point gauges can be used. Staff and tape gauges can be used when less accurate measurements are acceptable. 4.1.4.2.5.2 Stilling Well For the exceptional case where surface velocities and disturbances in the approach channel are negligible, the headwater level can be measured directly (for example, by means of a point gauge mounted over the headwater surface). Generally, however, to avoid water-level variations caused by waves, turbulence or vibration, the headwater level should be measured in stilling well. Stilling well is connected to the approach channel by means of a suitable conduit, equipped if necessary with throttle valve to damp oscillations. At the channel end of the conduit, the connection is made to floor or wall piezometers or a static tube located at the head measurement section. 4.1.4.2.5.3 Head Measurement section The head measurement section shall be located a sufficient distance upstream from the weir to avoid the region of surface drawdown caused by the formation of the nappe. On the other hand it shall be sufficiently close to the weir that the energy loss between the head measurement section and the weir is negligible. For the weirs included in this International standard the location of the head measurement section will be satisfactory if it is at a distance equal to 4 t0 5 times the maximum head (4 to 5 h max) upstream from the water. If high velocities occur in the approach channel or if water-surface disturbances or irregularities occur at the head-measurement section because of high values of h/p or b/B, it may be necessary to install several pressure intakes to ensure that the head measured in stilling well is the average of the heads at the several measurement points. 4.1.4.2.5.4 Head-gauge Zero Accuracy of head measurement is critically dependent upon the determination of the head-gauge datum or gauge zero, which is defined as the gauge reading corresponding to the level of the weir crest( rectangular weirs) or the level of the vertex of the notch (triangular -notch weirs). When necessary, the gauge zero shall be checked. Numerous acceptable methods of determining the gauge zero are in use. Typical methods are described in subsequent clauses dealing specifically with rectangular and triangular weirs. 10

Because of surface tension, the gauge zero cannot be determined with sufficient accuracy by reading the head gauge with the water in the approach channel drawn down to the apparent crest (or notch) level. 4.1.4.2.5.5 Maintenance Maintenance of the weir and the weir channel is necessary to ensure accurate measurements. The approach channel shall be kept free of silt, vegetation and obstructions which might have deleterious effects on the flow conditions specified for the standard installation. The downstream channel shall be kept free of obstructions which might cause submergence or inhibit full ventilation of the nappe under all conditions of flow. The weir plate shall be kept clean and firmly secured. In the process of cleaning, care shall be taken to a avoid damage to the crest or notch, particularly the upstream edges and surfaces. Construction specifications for these most sensitive features should be reviewed before maintenance is under taken. Head- measurement piezometers, connecting conduits and the stilling well shall be cleaned and checked foe leakage. The hook or point gauge, manometer, float or other instruments used to measure the head shall be checked periodically to ensure accuracy. 4.1.5 Types of weir 4.1.5.1 Rectangular thin plate weirs 4.1.5.1.1 Types The rectangular thin plate weir is a general classification in which the rectangular-notch weir is the basic form and the full-width weir is a limiting case. A diagrammatic illustration of the basic weir form is shown in figure 2 with intermediate values of b/B and h/p. When b/B is 1.0 that is when the width of the weir (b) id equal to the width of the channel at the weir section (B), the weir is of full-width type (also referred to as a “suppressed” weir, because its nappe lacks side contractions. 4.1.5.1.2 Specifications for the standard weir The basic weir form consists of a rectangular notch in a vertical, thin plate. The plate shall be plane and rigid and perpendicular to the walls and the floor of the approach channel. The 11

upstream face of the plate shall be smooth (in the vicinity of the notch it shall be equivalent in surface finish to that of rolled sheet-metal). The vertical bisector of the notch shall be equidistant from the two walls of the channel. The crest surface of the notch shall be horizontal, plane surface, which shall form a sharp edge at its intersection with the upstream face of the weir plate. For the limiting case of the full-width weir, the crest of the weir shall extend to the walls of the channel, which in the vicinity of the crest shall be plane and smooth. To ensure that the upstream edges of the crest and the sides of the notch are sharp, they shall be machined or filled, perpendicular to the upstream face of the weir plate, free of burrs or scratches and untouched by abrasive cloth or paper. The downstream edges of the notch shall be chamfered if the plate is thicker than the maximum allowable width of the notch surface. The surface of the chamfer shall make an angle of not less than Pi/4 radians (45 degree) with the crest and side surfaces of the notch. The weir plate in the vicinity of the notch preferably shall be made of corrosion-resistant metal; but if it is not, all specified smooth surfaces and sharp edges shall be kept coated with a thin, protective film( for example, oil, wax, silicone) applied with a soft cloth. 4.1.5.1.3 Specifications for installations The specifications stated in 6.3 shall apply. In general, the weir shall be located In straight , horizontal, rectangular, approach channel if possible. However, if the effective opening of the notch is so small in comparison with the area of the upstream channel that the approach velocity is negligible; the shape of the channel is not significant. In any case, the flow in the approach channel shall be uniform and steady. If the width of the weir is equal to the width of the channel at the weir section (i.e. a full-width weir), the sides of the channel upstream from the plane of the weir shall be vertical, plane, parallel and smooth (equivalent in surface finish to that of rolled sheet metal). The sides of the channel above the crest of a full-width weir shall extend at least 0.3 h max downstream from the plane of the weir. Fully ventilated discharge shall be ensured as specified in 6.3.4. The approach channel floor shall be smooth, flat and horizontal when the height of the crest relative to the floor (p) is small and /or h/p is large. For rectangular weirs, the floor should be

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smooth, flat and horizontal, particularly when p is less than 0.1 m and/or h max /p is greater than 1. Additional conditions are specified in connection with the recommended discharge formulae. 4.1.5.1.4 Specifications for head measurement 4.1.5.1.4.1 Determination of gauge zero The head gauge datum or gauge zero shall be determined with great care, and it shall be checked when necessary. A typical, acceptable method of determining the gauge zero for rectangular weirs is described as follows a) Still water in the approach channel is drawn to a level below the weir crest. b) A temporary hook gauge is mounted over the approach channel, a short distance upstream from the weir crest c) A precise machinists’ level is placed with its axis horizontal, with one end lying on the weir crest and the other end on the point of the temporary hook gauge (the gauge having been adjusted to hold the level in the position). The reading of the temporary gauge is recorded; d) The temporary hook gauge is lowered to the water surface in the approach channel and its reading is recorded. The permanent gauge is adjusted to read the level in the stilling well, and this reading is recorded; e) The computed difference between the two readings of the temporary gauge is added to the reading of the permanent gauge. The sum is the gauge zero for the permanent gauge. 4.1.5.1.5 Discharge formulae- general Recommended discharge formulae for rectangular thin-plate weirs are presented in two categories: a) Formulae for the basic weir form ( all values of b/B); b) Formulae for full width weirs(b/B = 1.0) Common symbols used in the formulae are defined as follows: Q is the volume rate of flow, in cubic meters per second; C is the coefficient of discharge (non-dimensional); g is the acceleration due to gravity, in meters per second squared; 13

b is the measured width of the notch, in meters; B is the width of the approach channel in meters h is the measured height , in meters; p is the height of the crest relative to the floor, in meters. (Note: Additional symbols are defined following their first occurrence in a formula) a) Formulae for the basic weir form (all values of b/B) 4.5.1.5.1 Kindsvater- Carter formulae The Kindsvater-Carter formula for the basic weir form is …………………………………………………………………………… (1)

In which is the coefficient of discharge; is the effective width; is the effective head; Evaluation of

,

AND

Figure 4 shows experimentally determined values of values of b/B. Values of

as a function of h/p for representation

for immediate values of b/B can be determined by interpolation.

The coefficient of discharge

has been determined by experiment as a function of two

variables from the formula …………………………………………………………………... ……. (2) The effective width and head are defined by the equations ………………………………………………...................................…….. (3) …………………………………………………………………………….. (4)

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In which

and

are experimentally determined quantities in meters, which compensate for

the combined effects of viscosity and surface tension. Figure 5 shows values of

, which have been experimentally determined as a function of b/B

Experiments have shown that

can be taken to have a constant value of 0.001 m for weirs

constructed in strict conformance with recommended specifications. Formulae For For specific values of b/B the relationship between

and h/p has been shown by experiment

(see figure 4) to be of the linear form,

Thus for the values of b/B sown on figure 4 formulae for

can be written as follows:

………………………………………………….. (5) …………………………………………………... (6) ………………………………………… ……….. (7) ………………………………………… ……….. (8) …………………………………………………... (9) ………………………………………………… .(10) ………………………………………… …….. (11) …………………………………………… ……. (12) (Note: For intermediate values of b/B, formulae for interpolation)

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can be determined satisfactorily by

Practical limitations on h/p, h, b and p Practical limits are placed on h/p because head-measurement difficulties and errors result from surges and waves which occur in the approach channel at larger values for h/p. Limits are placed on h to avoid “clinging nappe” phenomenon which occurs at very low heads. Limits are placed on b because of uncertainties regarding the combined effects of viscosity and surface tension represented by the quantity

at very small values of b. Limits are placed on p and B-b to avoid

the instabilities which result from eddies that from in the corners between the channel boundaries and the weir when values of p and B-b are small. For conservative practice, limitations applicable to use of the Kindsvater- Carter formula are: a) h/p shall not be greater than 2.5; b) h shall be not less than 0.03m; c) b shall be not less than 0.15m; d) p shall be not less than 0.10m; e) either (B-b)/2 = 0 (full width weir) or (B-b)/2 shall not be less than 0.10m ( concentrated weir) 4.1.5.1.5.2 SIA formula The SIA formula for the basic weir form is: ………………………………………………………………………. (13) In which ……………. (14)

Practical limitations applicable to the use of the SIA formula are a) h/p shall be not greater than 1.0; b) b/B shall be not less than 0.3; c) h shall be not less than 0.025 B/b and not greater than 0.80m; d) p shall be not less than 0.30m For full width weirs equation (14) reduces to 16

……………………………………………. (15) Formulae for full width weirs (b/B = 1.0) In addition to formulae (5) and (15), which represent the limiting case of b/B = 1.0 in the Kindsvater-Carter and SIA formulae for weirs of the basic form, the following formulae are recommended for b/B = 1.0 only. 4.1.5.1.5.3 Rehbock Formula (1929) The Rehbock formula in the form proposed in 1929 is of the effective head variety: …………………………………………………………………. ..(16) In which ………………………………………………………………….. (17) ……………………………………………………………………….. (18) Practical limitations applicable to the use of the Rehbock formula are: a) h/p shall be not greater than 1.0; b) h shall be between 0.03 and 0.75m; c) b shall be not less than 0.30m; d) p shall be not less than 0.10m; 4.1.5.1.5.4 IMFT Formula The IMFT formula for full-width weir is: ………………………………………………………………(19) In which ……………………………………………………………(20)

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In which

is the average velocity in the approach channel,

, in which

is the area

of the flow at the head-measurement section. Because

is a function of Q, it must be computed by successive approximations.

Practical limitations applicable to the use of the IMFT formula are: a) h/p shall be not greater than 2.5; b) h shall be not less than 0.03m; c) b shall be not less than 0.20m; d) p shall be not less than 0.10m; 4.1.5.1.6 Accuracy of discharge coefficient-rectangular weirs The accuracy of discharge measurements made with a rectangular thin-plate weir depends primarily on the accuracy of the head and width measurements and on the applicability of the discharge formula and coefficients used. If great care is exercised in meeting the construction, installation, and operational conditions specified in this international standard uncertainties (at 95% confidence level) attributable to the coefficients of discharge will be not greater than 1.5% for values of h/p less than 1.0, not greater than 2% for values of h/p between 1.0 and 1.5 and not greater than 3% for values of h/p between 1.5 and 2.5. The specified uncertainties are applicable only if the additional restrictions on values of h, b, p, h/p and (B-b)/2 given in 9.6 and 9.7 are applied. The combination of all uncertainties which contribute significantly to the uncertainty of discharge measurements is treated in clause 11. Examples of estimated uncertainties in measured discharge are given in clause 12. 4.1.5.2 Triangular Notch thin plate weir 4.1.5.2.1 Specifications for the standard weir The triangular-notch thin plate weir consists of a V-shaped notch in a vertical, thin plate. A diagrammatic illustration of the triangular-notch weir is shown in figure-6. The weir plate shall be plane and rigid and perpendicular to the walls and floor of the channel. The upstream face of the plate shall be smooth (in the vicinity of the notch it shall be equivalent in surface finish to that of rolled sheet-metal).

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The bisector of the notch shall be vertical and equivalent from the two walls of the channel. The surfaces of the notch shall be plane surfaces, which shall form sharp edges at their intersection with the upstream face of the weir plate. The width of the notch surfaces, measured perpendicular to the face of the plate, shall be between 1 to 2 mm. To ensure that the upstream edges of the notch are sharp, they shall be machined or filed, perpendicular to the upstream face of the plate, free of blurs or scratches and untouched by abrasive cloth or paper. The downstream edges of the notch shall be chamfered if the weir plate is thicker than the maximum allowable width of the notch surface. The surface of the chamfer shall make an angle of not less than π/4 radians (45) with the surfaces of the notch preferably shall be made of corrosion-resistant metal; but if it is not, all specified smooth surfaces shall be kept coated with a a thin protective film ( for example oil, wax, silicone) applied with a soft cloth. 4.1.5.2.2 Specifications for the installation The specification stated in 6.2 shall apply. In general, the weir shall be located in a straight, horizontal, rectangular channel if possible. However, if the effective opening of the notch is so small in comparison with the area of the upstream channel that the approach velocity is negligible; the shape of the channel is not significant. In any case, the flow in the approach channel shall be uniform and steady, as specified in 6.3.3. If the top width of the nappe at maximum head is large in comparison with the width of the channel, the channel walls shall be straight, vertical and parallel. If the height of the vertex relative to the level of the floor is small in comparison with the maximum head , the channel floor shall be smooth, flat and horizontal. In general the approach channel should be smooth, straight and rectangular when B/b max is less than 3 and/or h max/p is greater than 1. Additional conditions are specified in connection with the recommended discharge formulae. 4.1.5.2.3 Specifications for head measurement 4.1.5.2.3.1 Determination of notch angle Precise head measurement for triangular-notch weirs require that the notch angle (angle included between sides of the notch) measured accurately. One of several satisfactory methods is described as follows: 19

a) Two true disks of different, micro metered diameters are placed in the notch with their edges tangent to the sides of the notch. b) The vertical distance between the centers for two corresponding edges) of the two disks is measured with a micrometer caliper. c) The notch angle α is twice the angle whose sine is equal to the difference between the radii of the disks divided by the distance between the centers of the disks. 4.1.5.2.3.2 Determination of gauge zero The head-gauge datum or gauge zero shall be determined with great care, it shall be checked when necessary. A typical acceptable method of determining the gauge zero for triangular-notch weirs is described as follows: a) Still water in the approach channel is drawn to a level below the vertex of the notch. b) A temporary hook gauge is mounted over the approach channel, with its point a short distance upstream from the vertex of the notch. c) A true cylinder of known (micro-metered diameter is placed with its axis horizontal, with one end resting in the notch and the other end balanced on the point of the temporary hook gauge. A machinists’ level is placed on the top of the cylinder precisely horizontal. The reading of the temporary gauge is recorded. d) The temporary hook gauge is lowered to the water surface in the approach channel and the reading is recorded. The permanent gauge is adjusted to read the level in the stilling well, and this reading is recorded. e) The distance (Y) from the bottom of the cylinder to the vertex of the notch is computed with the known value of notch angle (α) and the radius (r) of the cylinder i.e.

This distance is then subtracted from the reading recorded in c), the result being the reading of the temporary gauge at the vertex of the notch. f) The difference between the computed reading in e) and the reading of the temporary in d) is added to the reading of the permanent gauge in d). The sum is the gauge zero for the permanent gauge. An advantage of this method is that it refers the gauge zero to the geometrical vertex which is defined by the sides of the notch. 20

4.1.5.2.4 Discharge Formulae- General Recommended discharge formulae for triangular notch thin-plate weirs are presented in two categories: a) Formula for all notch between π/9 and 5π/9 radians (20° and 100°); b) Formulae for specific notch angles (fully contracted weirs Common symbols used in the formulae are defined as follows: Q is the volume rate of flow, in cubic meters per second; C is the coefficient of discharge (non-dimensional); g is the acceleration due to gravity, in meters per second squared; α is the notch angle, i.e., the angle included between the sides of the notch, in degrees; h is the measured head in metres (Note: Additional special symbols are defined following their first occurrence in a formula) Formula for notch angles between π/9 and 5π/9 radians (20° and 100°) The Kindsvater Shen formula for triangular notch weir is: ……………………………………….. (21) in which is the coefficient of discharge is the effective head The coefficient of discharge

has been determined by experiment as a function of three

variables (see figure 7) ………………………………………………….. (22) In which p is the height of the vertex of the notch with respect to the floor of the approach channel B is the width of the approach channel 21

is defined by the equation, ……………………………………………………… (23) In which

is an experimentally determined quantity, in .meters, which compensates for the

combined effects of viscosity and surface tension. Evaluation of

and

For triangular weirs with notch angle α equal to π/2 radians (90°), figure 7 shows experimentally determined values of

for a wide range of values of h/p and p/B. For α = π/2 radians (90°),

has been shown to have a constant value of 0.00085 for a corresponding range of values of h/p and p/B. For notch angles other than π/2 radians (90°), experimental data are insufficient to define

as a

function of h/p and p/B. However, for weir notches which are small relative to the area of the approach channel, the velocity of approach is negligible and the effects of h/p and p/b are also negligible. For this condition (the so-called “fully-contracted” condition), figure 8 shows experimentally determined values of

as a function alone. Corresponding values of

are

shown in figure 9. Practical limitations on α, h/p, p/B, h and p For reasons related to hazards of measurement-error and lack of experimental data, the following practical limits are applicable to the use of the kindsvater-Shen formula: a) α shall be between π/9 and 5π/9 radians ( 20° and 100°); b) h/p shall be limited to the range shown on figure 7 for α = π/2 radians (90°); h/p shall be not greater than 0.35 for other values of α; c) p/B shall be limited to the range shown on figure 7 for α = π/2 radians (90°); p/B shall be between 0.10 and 1.5 for other values of α. d) h shall be not less than 0.06m e) p shall be not less than 0.09m.

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Formula for specific notch angles (fully contracted-weir) BSI formula for three related angles This formula is for notch angles which have a special geometric relationship to each other; a) tangent α/2 =1 (α = π/2 radians or 90°); b) tangent α/2 = 0.50 (α = 0.9273 radian or 53° 8′); c) tangent α/2 = 0.25 (α = 0.4899 radian or 28° 4′); The BSI discharge formula is: …………………………………………………………………(24) And the experimentally determined values of C and Q for the condition of “full contraction” are shown in tables 1, 2 and 3. Practical limitations applicable to the use of this formula are: a) h/p shall be not greater than 0.4; b) h/B shall be not greater than 0.2; c) h shall be between 0.05 and 0.38m; d) p shall be not less than 0.45m; e) B shall be not less than 1.0m; Accuracy of discharge coefficients- Triangular-notch weirs The accuracy of discharge measurements made with a triangular-notch thin-plate weir depends primarily on the accuracy of the head and notch-angle measurements and on the applicability of the discharge formula and coefficients used.

If great care is exercised in meeting the

construction, installation, and operational conditions specified in this international standard, uncertainties (at 95% confidence level) attributable to the coefficients of discharge will be not greater than 1.0%.

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4.1.6 Criteria for the selection of standard weirs and flumes The essential criteria for selection from among the standard weirs and flumes are given below: 4.1.6.1 Available difference in water levels Thin-plate weirs and free over-falls require a sufficient difference between upstream and downstream water levels which will ensure free, fully ventilated flow under conditions of maximum discharge. Broad crested weirs may be used with relatively smaller differences in water level; triangularprofile weirs and standing wave flumes may be used with even smaller differences in water level. For all types of weirs and flumes included in this international standard the discharge should be free or independent of the downstream water level. 4.1.6.2 Accuracy of measurement The accuracy in a single determination, of discharge depends upon the estimation of the component uncertainties involved but approximate range of uncertainties for the weirs and flumes (at 95% confidence levels) are as follows 

Rectangular thin-plate weirs (full width and notch): 1 to 4%



Triangular notch weirs (notch angles between π/9 and 5π/9 radians or 20° to 100°): 1 to 2%.



Broad crested weirs: 3 to 5%



Triangular-profile weirs: 2 to 5%



Standing-wave flumes: 2 to 5%



Free over-fall: 5 to 10%

Deviations from the standard construction, installation or use may result in larger measurement errors. The larger figures given above are recommended conservative values for use under conditions of strict conformance with standard specifications. The smallest values can be obtained only for weirs under vigorous control, such as may be built and installed in wellequipped laboratories. Under field conditions, thin plate weirs are specially subject to errors caused by natural hazards.

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4.1.6.3 Dimensions and shape of open channel Rectangular full-width weirs and notch weirs (both rectangular and triangular), of large size relative to the size of the approach channel, should be located in vertical-walled level-floored rectangular channels, or in weir boxes of rectangular channels, or in weir boxes of rectangular cross-section for a distance extending upstream not less than 10 times the width of the nappe at maximum head. For thin plate weirs of small size relative to the size of the approach channel, especially if the velocity of approach is negligible, the size and shape of the channel is of no importance. Broad crested weirs are best used in rectangular channels, but they can be used with good accuracy in non-rectangular channels if a smooth, rectangular approach channel extends upstream from the weir a distance not than twice the maximum head. Flumes can be used in channels of any shape if flow conditions in the approach channel are reasonably uniform and steady. For weirs and flumes of all types the size and shape of the downstream channel are of no significance except that they permit free, fully ventilated flow under all conditions of use. 4.1.6.4 Flow conditions in the approach channel For weirs of all types, flow in the approach channel shall be sub-critical, uniform and steady. Ideally, especially for relatively high velocities of approach, the velocity distribution should approximate that in a channel of sufficient length to develop normal (resistance-controlled) flow in straight, smooth channels. For relatively low velocities of approach and for flumes, flow conditions in the channel are of less importance. In short channels and weir boxes, baffles and flow-straighteners may be used to stimulate normal velocity distribution. Care should be taken to ensure that erosion and/or deposition upstream of the weir or flume do not significantly alter the velocity of approach or velocity distribution to the measurement structure. Sub-critical flow is ensured when

25

In which is the average velocity in the approach channel on meters per second; g is the acceleration due to gravity in meters per second squared; A is the cross-sectional area of the channel, in square meters; is the width of the channel at the water surface, in meters. 4.1.6.5 Flow with sediment load For flows with suspended load, the use of thin-plate weirs should be avoided because the crest edge may be damaged or worn by the suspended materials. On streams with bed load, use of measurement structures which significantly reduce the stream velocity is not recommended as it may result in changing deposition-scour dependent on flow regime. Flumes will generally perform than weirs on streams with sediment load. 4.1.6.6 Flow with floating debris Broad crested weirs, triangular profile weirs, standing wave flumes and free overfall structures will normally pass floating debris more effectively than thin-plate weirs. The use of the triangular notch (V-notch) weir in particular should be avoided unless a debris trap is installed upstream. 4.1.6.7 Magnitude of discharge to be measured For reasons related to accuracy and construction, thin plate weirs are best used for the measurement of relatively small discharges. Broad-crested weirs, triangular-profile weirs and flumes are best used for large discharges. 4.1.6.8 Range of discharge to be measured For best overall accuracy over a wide range of small discharges, a triangular-notch (v-notch) weir should be used in preference to a rectangular-notch or rectangular full-width weir. For a wide range of larger discharges, a trapezoidal-throat or U-throat flumes should be used in preference to a broad-crested weir, free over-fall, rectangular-throated flume or triangular-profile weir.

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4.1.6.9 Construction Thin plate weirs must be constructed with precision tools under machine shop conditions. Flumes, broad crested weirs, triangular-profile weirs and free over-falls can be constructed satisfactorily in the field. In all cases, great care must be exercised in making the structures conform to standard specifications. Broad crested weirs, triangular weirs, free over-falls and flumes are inherently stronger and more easily maintained under conditions of high heads in large channels. 4.2 Literature review on Weir head measurement mechanism 4.2.1 Measurement of Head (Introduction) Selecting the proper water measurement device for a particular site or situation is not an easy task. Many site-specific factors and variables must be considered and weighed. In addition, each system has unique operational requirements and concerns. Reliable estimates on future demands of the proposed system and knowledge of the immediate measurement needs are beneficial. Factors influencing the accuracy of a single flow-rate measurement were considered and the importance of accurate measurement of upstream sill referenced head was discussed in this chapter. In fact, the measurement of head is so important that the success or failure of the measuring structure often depends almost entirely upon the effectiveness of the gage, sensor, or recorder used. When we use the term sill-referenced head, we mean that the head is measured with respect to the invert of the control section of the structure-i.e., the section at which the flow passes through critical depth. This control section is located in the flume throat at a distance of about L/3 from the downstream edge of the sill. In the direction of flow, the top of the sill (weir crest or invert of flume throat) must be truly level. If minor undulations in the elevation of the sill occur along its length, we recommend that the level at the control section be used as the sill-reference level rather than taking the average along the length of the sill. If the sill is intended to be horizontal in the direction perpendicular to the flow, then the average level across the width of the sill at the control section should be used as the sill-reference level. The gauging or head-measurement station should be located sufficiently far upstream from the structure to avoid the area of water surface drawdown, yet it should be close enough for the 27

energy loss between the gauging station and the structure to be negligible. This means it will be located at a distance between two and three times

from the leading edge of the sill or at

from the beginning of the converging transition, whichever is greater (Figure 7.1) If only occasional flow measurements are required, the water level at the gauging station can be measured by a vertical or an inclined gage installed in the approach channel. If continuous flow records are needed, or if the flow rate is to be transmitted electronically to a distant location, a water level transducer and/or an automatic recorder will be needed. Regardless of the type of head-measurement device used, it should be located to one side of the approach channel to minimize its interference with the flow approaching the structure.

Figure 4.1 Schematic diagram showing general terminology and location of gaging station and Control. 4.2.2 Types of Head Measuring Devices 4.2.2.1 Gages When continuous measurements of flow rate are not needed, or in channels where the fluctuation of flow is gradual, periodic readings on a calibrated physical gage may be satisfactory. Depending on the type of flume and the required accuracy of the head reading (see previous section), a point gage, dipstick, or staff gage may be used.

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4.2.2.2 Point gage A point gage is the most accurate head-measurement instrument (error of 0.1 mm). Its use is normally restricted to research facilities. The point gage is always used in combination with a stilling well. The point gage consists of a pointed, graduated rod suspended above the water surface and raised or lowered in relation to a fixed measurement scale, often including a vernier scale to increase measurement accuracy. The rod is lowered until the point just touches the water surface, and the vertical position of the point is then read from the vemier scale. 4.2.2.3 Dipstick A dip stick is essentially a stick or rod that is calibrated to indicate level. It is inserted into the water in the stilling well until the end of the stick rests on a base corresponding to the exact sillreference level of the structure. Usually the bottom of the tank is used to ensure that the dip stick is inserted to the correct depth. Reading the scale on the dip stick indicates the level measurement. The stilling well used in combination with a dipstick should have a sufficiently large diameter so that the stick does not raise the water level upon insertion. Even then, the stick should be inserted slowly until it rests on its reference point. A dipstick can supply very accurate information on head (error of 0.001 m). Most portable RBC flumes use a hardwood dipstick that is directly marked in flow rate units. A lead line acts in the same way as a dip stick. A steel measuring tape with a weight attached, the lead line can be used in most places that the dip stick can. Since the lead line can be rolled up into a smaller, compact unit, it is often easier to handle than a dip stick. 4.2.2.4 Sight Glasses and Gage Glasses The sight glass is an important method for visually determining level. The sight glass is a transparent tube of glass or plastic mounted outside the vessel and connected to the vessel with pipes. The liquid level in the sight glass matches the level of liquid in the process tank. In process systems that contain a liquid under high pressure a reflex sight glass is used. This device is armored, to permit it to tolerate higher temperatures and higher pressures. Gage glasses are typically glass covered ports in a vessel that make it possible to observe the level of the substance in the vessel. Many gage glasses will have a scale mounted on the tank that allows the level to be read. 29

4.2.2.5 Staff Gage A staff gage should be placed in such a manner that the water level can be read from the canal bank and so that its surface can be cleaned by the observer. For earthen channels, the gage can be mounted vertically on a support structure placed in the flowing stream. The support structure should not interfere with the flow of water through the flume throat or over the weir crest, and it should not catch floating debris. For concrete-lined canals, the gage can be mounted directly on the canal wall. For sloping canal walls, the length indicated on the gage will be greater than the corresponding vertical water depth. The relative slope lengths versus vertical lengths for the most commonly used side slopes are shown in Figure 4.2 Within an irrigation project, it is convenient to mark the gages of structures in Liter/s, m3/s, ft3/s, or other units of discharge rather than in head units. Once the gage has been mounted and checked, this eliminates the possibility of using the wrong rating tables. Direct read-out gages can also be used on movable weirs. With the software one can calculate a basic rating table showing discharge versus head (one discharge value in each line of the table). The software will also provide the vertical gage marking distances for a direct reading gage. In contrast, the ditch rider’s rating table can be printed with either the vertical gage marking distances or the distances along a sloped canal bank, allowing the use of a standard linear gage installed on a slope. The wall gage module of the software also can provide the gage dimensions and produce wall gages for mounting directly against the slope of the approach channel. For this gage, the marks need not be more than about 3 or 4 cm apart, since interpolation between marks will give reasonable accuracy. For example, on the gage shown, there is a 2.5 cm difference in elevation (4.5 cm along the sloped wall gage) between 2.20 and 2.40 m3/s. Interpolation between these marks by eye is relatively easy. With experience, an observer can easily read the discharge to within ±4% of the true discharge.

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Figure 4.2 Multipliers for layout of an inclined page

Figure 4.3 The inclined gage is mounted against the right canal bank. The gage is labeled with discharge units. Most permanent gages are constructed from paltes of enameled steel, cast aluminum, or polyester. Baked enamel steel gages with linear scales are available from commercial sources. These gages will last for a very long time. Gages marked in discharge units can be customordered in large quantities, but are considerably more expensive. Spray enamel paints with UV protection can also be used to make gages on steel. These are not as durable as the baked enamel, but are considerably less expensive. Gages in discharge units can also be made by dtamping

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aluminum but stock with a hammer, chisel, or metal-stamping discs. These gages require periodic cleaning, so they must be accessible. 4.2.2.6 Automatic recorders and water level sensors Automatic water-level recorders create a permanent a record of the variation of water surface elevations as a function of time. A sensor converts the water level into physical motion and/or an electrical signal that can be recorded on paper, magnetic tape, or other electronic form by the recorder (data logger). Automatic recorder systems have several advantages over ordinary gages: 

In channels with daily fluctuations of flow, continuous records provide the most accurate means of determining the daily average and total flow.



The entire hydrograph is recorded with the maximum and minimum water levels as a function of time. This provides data on the reaction time of the channel system to upstream changes in flow.



Observations can be made at remote places where observers are not available, or in locations that are not accessible under all conditions.

A number of meteorological instrument manufacturers produce a variety of commercially sensors and recorder. In some cases, the sensor and recorder systems are integrated together into a single system, while in other cases the sensor and recorder are separate devices. The type of sensor chosen for the site can have important ramifications for the design of the structure and appurtenances. Some of the most common types of sensors and their important characteristics are described below. 4.2.2.7 Submerged Pressure Transducers Pressure transducers convert the hydrostatic pressure of water at a given depth into an electrical signal that can be recorded. Transducers are available in many different configurations that exploit a variety of properties of different materials or devices to accomplish this conversion. Submerged pressure transducers can be suspended in a stilling well or installed in a protective pipe which is perforated to admit water. The transducer is fastened in place, submerged below the minimum expected water level. To produce an output of gage pressure (i.e., referenced to ambient atmospheric pressure), the transducer is vented to the atmosphere via a vent tube integrated into the cable carrying the electronic output signal of the transducer. The free end of 32

the vent tube should terminate in the instrument enclosure, and a desiccant should be used to prevent water vapor entry into the vent tube, as this can lead to corrosion of the transducer and errors in its output. A flexible bladder can be used as a desiccant replacement, provided that expansion and contraction of the bladder does not change the pressure (i.e., the pressure inside and outside the bladder must be the same) Advantages of pressure transducers are  The relative simplicity of installation, since a stilling well is not required, and their accuracy, which can range from ±1.0 to ±0.1 percent of the maximum range that can be measured by the transducer.  Accuracy and cost are generally proportional. Disadvantages include 

The need to maintain the desiccant .pack associated with the vent tube and the requirement to protect the transducer from freezing or remove it from service during the winter.



The most significant calibration issue for pressure transducers is avoiding drift of their output at zero pressure, since this can lead to relatively large percentage errors in flow rate at minimum discharge conditions.

 4.2.2.8

Fouling of the opening to the transducer can also be a problem. Pressure Bulb

This instrument consists of a flexible bulb that is placed in a perforated metal container for protection and connected by an air tube to a mechanical pressure gage and recorder or to a pressure transducer with an electronic output. The container and flexible bulb are fixed in place below the minimum water level to be recorded. Any change in water level changes the pressure inside the system and thus is recorded. Advantages of this recorder are 

The container and bulb do not require a stilling well and the distance between the bulb and the recorder may be up to 50 m (1 75 ft).Hence, the installation of the system is simple and relatively cheap while the recorder can be placed at a suitable location.

33

The major disadvantage of the pressure bulb is 

The error in the recorded water level is generally ±2% of the maximum range that can be measured by the recorder. If this range, for example, is 1.0 m, the error in recorded head is ±0.02 m for all heads. As a result, at the minimum flow condition the measured flow rate can be rather inaccurate. Also, system leaks can cause operational failure.

Despite these disadvantages, the pressure bulb is very suitable for relatively temporary installations and sites at which the greatest accuracy is not necessary. A regular calibration between the staff-gage reading and the recorded water level is required for this type of instrument to maintain sufficient accuracy. 4.2.2.9 Bubblers This instrument consists of a tube that is usually fastened with its open end at least 0.05 m below the lowest water level to be recorded. The tube is connected to a supply of air from a cylinder of compressed air or a small compressor and to a pressure gage or a pressure transducer plus a recorder. Air flows very slowly from the open end of the tube, and the pressure required to overcome the head of water above the end of the pipe is measured and recorded. The method by which the pressure is measured and recorded may be similar to that of the pressure bulb or may involve recent electronic devices. The advantages and disadvantages are somewhat similar to the pressure transducer system already described, except that the transducer is not submerged, so it need not be removed in freezing weather and there is less scaling and fouling of the transducer. Not submerging the transducer has proven to dramatically improve the reliability of bubbler systems compared to submerged pressure sensors. Relatively long transmitting distances can be achieved with the bubbler system. Installations of 300 m have been used. On these long lines, it is best to use two small 3-mm inside-diameter tubes for economy and accuracy. One tube carries the bubble air supply from the source to the bubble outlet at 3 to 5 bubbles per second. The second tube is attached as a branch line as near as practical to the bubble outlet, preferably within 2 to 5 m. This second tube then senses the bubble pressure at the desired distance. Because there is essentially no flow in the sensing line after stabilization, there are no appreciable friction losses. On the source line, even 3 to 5 bubbles per second cause a significant pressure drop in several hundred meters, and thus the source line cannot also serve as the sensing line at long distances. Thermal gradients and pressure change in 34

the sensing line may become significant only if large vertical distances are encountered. Figure 7.3 illustrates the schematic arrangement of a remote bubble gage. Transmission distance is limited primarily by the allowable response time needed for a change in flow to be detected. The gage becomes more sluggish with increasing sensing line length because larger volumes of air must be moved to achieve a new stable pressure reading. For 300 m of 3-mm line (inside diameter), stability is usually reached in several seconds, depending on the volume sensing requirements of the pressure sensing gage. For example, a large-bore manometer requires more volume shift than a small pressure gage, but the manometer may be more sensitive.

Figure 4.4 Schematic for a remote recording bubbler system that is not sensitive to transmission

distances up to 300 m.

Self-contained bubbler systems have been developed in recent years that integrate a small pressure compressor and optional pressure tank, the transducer, and associated electronics into a low-power unit that can easily function continuously on solar power (Figure 4.5). Another variation on the bubbler concept is the double-bubbler, in which bubbles are delivered alternately through two different tubes that terminate a fixed vertical distance apart in the water column. If the same transducer is used to sense the atmospheric pressure and the pressure in each tube, one can compensate for changes in the transducer calibration, producing a more accurate

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measurement. Accuracy of commercially available bubbler systems has improved in recent years and can now be on the order of ±0.003 m (±0.001 ft). 4.2.2.10 Ultrasonic level sensors Ultrasonic level sensors are mounted above the water surface and determine the position of the free surface by measuring the transit time of an acoustic pulse that travels from the sensor down to the water surface and is reflected back up to the sensor. To achieve useful accuracy, the sensor must be temperature compensated, since the speed of sound in air varies with temperature. Ultrasonic level sensors can be installed with or without a stilling well; a stilling well is preferred because it reduces waves on the water surface that can reduce the measurement accuracy. Details of the particular sensor should be considered when designing the stilling well, as the acoustic signal transmitted by the sensor radiates out in a cone pattern. The signal thus may be reflected back up to the sensor off the walls of the stilling well (especially if the walls are rough), causing the sensor to measure this distance rather than the distance to the water surface. Conversely, installing the sensor directly above a relatively small diameter smooth-walled pipe that extends down into the water works well with some sensors. This is because the acoustic signal is not reflected back up the pipe due to the flat angle of incidence of the acoustic signal with the pipe wall.

Figure 4.5 A self-contained bubbler water level sensor. (Courtesy Digital Control Corporation, Largo, Florida, USA)

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Advantages of ultrasonic level sensors are 

The relative ease of installation and the fact that they do not physically contact the water surface, making them a good choice for sites with pollutes or corrosive waters.

Disadvantages are 

They have only moderate accuracy



Even when temperature-compensated, are affected by temperature gradients that may exist in the air space between the sensor and the water surface. Temperature gradients can be extreme in many sites, especially in stilling wells located in the daytime sun where the top of the stilling well or instrument enclosure can reach temperatures of 60°C or higher.



They also require periodic maintenance to ensure a clear path between the sensor and the water surface; spider webs beneath the sensor have been known to cause erroneous measurements.

4.2.2.11 Electrical sensors 4.2.2.11.1 Capacitance A capacitor consists of two plates separated from each other by an insulating material called a dielectric. In applications involving capacitance measuring devices, one side of the process container acts as one plate and an immersion electrode is used as the other. The dielectric is either air or the material in the vessel. The dielectric varies with the level in the vessel. This variation produces a change in capacitance that is proportional to level. Thus, level values are inferred from the measurement of changes in capacitance, which result from changes in the level. Capacitance type level measurement devices offer many advantages. 

Simple in design, they contain no moving parts and require minimal maintenance.



The availability of corrosive resistant probes is also an advantage.

Capacitive level measurement devices have these limitations 

Measurement is subject to error caused by temperature changes affecting the dielectric constant of the material.



If the probes should become coated with a conductive material, errors in measurement may occur. 37

4.2.2.11.2 Conductivity A material's ability to conduct electric current can also be used to detect level. This method is typically used for point measurement of liquid interfaces of relatively high conductivity. Conductivity applications are usually limited to alarm devices and on/off control systems. A common arrangement is two electrodes positioned at the top in a tank. One extends to a minimum level and the other is positioned so that its lower edge is at the maximum level. The tank is grounded and functions as the common or third electrode. Usually, a stilling well is provided to ensure that the interface is not disturbed and to prevent false measurement. The advantages of conductivity method include  Low cost and simple design  As well as the fact that there are no moving parts in contact with the process material. These advantages make this type of system an effective method of detecting and indicating level for many water-based materials. There are limitations to the conductivity method which as follows: 

The first is process substance must be conductive.



Second, only point detection measurements can be obtained.



The possibility of sparking also makes this method prohibitive for explosive or flammable process substances.

4.2.2.11.3 Resistance Resistance type level detectors use the electrical relationship between resistance and current flow to accurately measure level. The most common design uses a probe consisting of two conductive strips. One strip has a gold-plated steel base; the other is an elongated wire resistor. The strips are connected at the bottom to form a complete electrical circuit. The upper ends of the strips are connected to a low voltage power supply. The probe is enclosed in a flexible plastic sheath which isolates the strips from the process material. As the level of the process material rises, the hydrostatic pressure forces the resistance strips together up to the interface. This action shorts the circuit below the interface level, and total resistance is reduced proportionately. Resistance sensing devices can be used for liquid-gas interfaces and for slurries or solids. As with the other

38

electrical level sensors discussed, resistance-type level detectors require relatively little maintenance. 4.2.2.12 Float-operated recorder Float-operated recorders have been one of the most commonly used instruments for measuring water level because of their relatively low cost, good accuracy (

, and wide

availability. The instrument consists of a float of sufficiently large diameter, which is attached to a tape or cable that passes around the float wheel of a recorder and then to a counter weight The float rises and falls with the water level, and its movement rotates the float wheel and thus is recorded. To function properly, the float must be located in standing water. Thus, a stilling well is required on all field installations (see Section 4.6). Care should be taken to ensure that when the float is rising its counterweight does not lodge on top of the float but keeps well above it or passes the float. If a high degree of accuracy is required, the counterweight should not be permitted to become submerged over part of the operating range since this will change the submergence of the float and thus affect the recorded water level. This systematic error may be prevented by 

Locating the counterweight inside a separate watertight and water-free pipe.



Mounting two different-sized drums on the axle of the recorder. The larger diameter drum serves to coil up the float wire and the small diameter drum coils up the Counterweight wire, yielding reduced movement of the counterweight relative to the float. The drums require a spiral groove for coiling up several turns of wire, otherwise there is an error due to coiling of cable on top of itself. Tapes cannot be used with this method.



Extending the stilling-well pipe to such a height that the counterweight does not touch the float wheel at low stage nor the water surface at the maximum expected stage.

Most of the earlier recorders relied on the friction drive of a cable on the float wheel of the recorder. To improve the accuracy of the head measurements, we recommend that a recorder be equipped with a calibrated float tape that passes over the float wheel. The float and counterweight should be attached to the ends of the tape by ring connectors. If the recorder is not equipped with a tape index pointer, one should be attached either to the shelter-house floor or to 39

the instrument case. The purpose of the calibrated tape and the index pointer is to enable the observer to easily check the registered water level against the actual water level in the float well and also against the water level shown on the independently placed staff gage. As such, the tape and index pointer provide an immediate check on whether the recorder mechanism, the float system, and the inlet pipe or slots are functioning properly. 4.2.3 Selection Criteria The main factors which influence the selection of a measuring device include: 4.2.3.1 Accuracy The target or desired accuracy of the measurement system is an important consideration in measurement method selection. Most water measurement devices can produce accuracies of +5 percent. Some devices are capable of +1 percent under laboratory settings. However, in the field, maintaining such accuracies usually requires considerable expense or effort (e.g., special construction, recalibration, maintenance, etc.). Selecting a device that is not appropriate for the site conditions can result in a nonstandard installation of reduced accuracy, sometimes greater than +10 percent. Accuracies are usually reported for the primary measurement method or device. However, many methods rely on a secondary measurement, which typically adds error to the overall measurement. For example, the primary calibration for a weir is the relationship between head and discharge; this relationship typically contains a small error. However, the head must be measured, which potentially introduces additional error. Chapters 3 and 8 contain discussion and examples concerning the influence of secondary devices on accuracy. 4.2.3.2 Cost The cost of the measurement method includes the cost of the device itself, the installation, secondary devices, and operation and maintenance. Measurement methods vary widely in their cost and in their serviceable life span. Measurement methods are often selected based on the initial cost of the primary device with insufficient regard for the additional costs associated with providing the desired records of flows over an extended period of time.

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4.2.3.3 Legal Constraints Governmental or administrative water board requirements may dictate types of accepted water measurement devices or methods. Water measurement devices which become a standard in one geographic area are often not accepted as a standard in another area. In this sense, the term "standard" does not necessarily signify accuracy or broad legal acceptance. Many water districts require certain water measurement devices used within the district to conform to their standard for the purpose of simplifying operation and maintenance. 4.2.3.4 Flow Range Many measurement methods have a limited range of flow conditions for which they are applicable. This range is usually related to the need for certain prescribed flow conditions which are assumed in the development of calibrations. Large errors in measurement can occur when the flow is outside this range. For example, using a bucket and stopwatch for large flows that engulf the bucket is not very accurate. Similarly, sharp-edged devices typically do not give good results with large flows, which are better measured with large flumes or broad-crested weirs. In some cases, secondary devices can limit the practical range of flow rates. For example, with devices requiring a head measurement, the accuracy of the head measurement can limit the measurement of low flow rates. For some devices, accuracy is based on percent of the full-scale value. Then, at low values, the resulting accuracy is much lower, limiting the usefulness of such measurements. Generally, the device should be selected to cover the range desired. In fact, choosing a device that can handle a larger than necessary flow rate, could result in elimination of measurement capability at lower flow rates, and vice versa. For practical reasons, it may be reasonable to establish different accuracy requirements for high and low flows. Examples in chapter 3 discuss some of these problems in more detail. 4.2.3.5 Head Loss Most water measurement devices require a drop in head. On existing irrigation projects, such additional head may not be available, especially in areas with relatively flat topography. On new projects, incorporating additional head loss into the design can usually be accomplished at reasonable cost. However, a tradeoff usually exists between the cost of the device and the amount of head loss. For example, acoustic flow-meters are expensive and require little head

41

loss; sharp-crested weirs are inexpensive but require a relatively large head loss. The head loss required for a particular measurement device usually varies over the range of discharges. In some cases, head used in measuring flow can reduce the capacity of the channel at that point. 4.2.3.6 Adaptability to Site Conditions The selection of a measurement device must consider the site of the proposed measurement. Several potential sites may be available for a given measurement; the selection of a device depends upon the exact site chosen. For example, discharge in a canal system can be measured within a reach of the channel or at a structure such as a culvert or check structure. A different device would typically be selected for each site. The device selected should not alter site hydraulic conditions so as to interfere with normal operation and maintenance. Also, the shape of the flow cross section will likely favor some devices over others. For example, the Parshall flume size selection process described in chapter 5 might result in a flume wider than the existing channel, adding substantial cost to the installation, whereas a long-throated flume might fit within the existing channel prism. 4.2.3.7 Adaptability to Variable Operating Conditions Most water delivery systems have a varying range of flows and conditions. The selected device must also be able to measure over the range of operating conditions encountered (e.g., variations in upstream and downstream head). Devices like weirs or flumes should be avoided if downstream water levels can, under some conditions, submerge the device. In addition, the information provided by the measuring device needs to be useful for the operators performing their duties. Devices that are difficult and time consuming to operate are less apt to be used and are more likely to be used incorrectly. In some cases, water measurement and water level or flow control need to be accomplished at the same site. A few devices are available for accomplishing both (e.g., constant-head orifice, vertically movable weirs, and Neyrpic flow module) (Bos, 1989). However, separate measurement and control devices are typically linked for this purpose. Special care is needed to assure that devices are compatible and, when used as a system, achieve both functions.

42

4.2.3.8 Type of Measurements and Records Needed An accurate measure of instantaneous flow rate is useful for system operators in setting and verifying flow rate. However, because flow rates change over time, a single, instantaneous reading may not accurately reflect the total volume of water delivered. Where accounting for water volume is desired, a method of accumulated individual flow measurements is needed. Where flows are steady, daily measurements may be sufficient to infer total volume. Most deliveries, however, require more frequent measurements. Totaliza-tion is essential where water users take water on demand. Totalizers and automatic recording devices are available for many measurement devices. For large structures, the cost for water-level sensing and recording hardware is small relative to the structure cost; but for small structures, these hardware costs do not change and thus become a major part of the measurement cost (often more costly than the structure itself). Many water measurement methods are suitable for making temporary measurements (flow surveys) or performing occasional verification checks of other devices. The method chosen for such a measurement might be quite different from that chosen for continuous monitoring. Although many of these flow survey methods are discussed in this manual, this chapter focuses on methods for permanent installations. 4.2.3.9 Operating Requirements Some measurement methods require manual labor to obtain a measurement. Current metering requires a trained staff with specialized equipment. Pen-and-ink style water-stage recorders need operators to change paper, add ink, and verify proper functioning. Manual recording of flows may require forms to be filled out and data to be accumulated for accounting purposes. Devices with manometers require special care and attention to assure correct differential head readings. Automated devices such as ultrasonic flow-meters and other systems that use transducers and electronics require operator training to set up, adjust, and troubleshoot problems. Setting gate controlled flow rates by simple canal level references or by current metering commonly requires several hours of waiting between gate changes for the downstream canal to fill and stabilize. However, flumes and weirs serve to quickly reach measured flow rate without waiting for the downstream canals to fill to stable conditions. The requirements of the operating personnel in

43

using the devices and techniques for their desired purposes can be easily overlooked and must be considered in meter selection. 4.2.3.10 Ability to Pass Sediment and Debris Canal systems often carry a significant amount of sediment in the water. Removal of all suspended solids from the water is usually prohibitively expensive. Thus, some sediment will likely be deposited anywhere the velocities are reduced, which typically occurs near flow measuring structures. Whether this sediment causes a problem depends on the specific structure and the volume of sediment in the water. In some cases, this problem simply requires routine maintenance to remove accumulated sediment; in others, the accumulation can make the flow measurement inaccurate or the device inoperative. Sediment deposits can affect approach conditions and increase approach velocity in front of weirs, flumes, and orifices. Floating and suspended debris such as aquatic plants, washed out bank plants, and debris such as fallen tree leaves and twigs can plug some flow measurement devices and cause significant flow measurement problems. Many of the measurement devices which are successfully used in closed conduits (e.g., orifices, propeller meters, etc.) are not usable in culverts or inverted siphons because of debris in the water. Attempting to remove this debris at the entrance to culverts is an additional maintenance problem. 4.2.3.11 Device Environment Any measurement device with moving parts or sensors is subject to failure if it is not compatible with the site environment. Achieving proper operation and longevity of devices is an important selection factor. Very cold weather can shrink moving and fixed parts differentially and solidify oil and grease. Water can freeze around parts and plug pressure ports and passageways. Acidity and alkalinity in water can corrode metal parts. Water contaminants such as waste solvents can damage lubricants, protective coatings, and plastic parts. Mineral encrustation and biological growths can impair moving parts and plug pressure transmitting ports. Sediment can abrade parts or consolidate tightly in bearing and runner spaces in devices such as propeller meters. Measurement of wastewater and high sediment transport flow may preclude the use of devices that require pressure taps, intrusive sensors, or depend upon clear transmission of sound through the flow. Water measurement devices that depend on electronic devices and transducers must

44

have appropriate protective housings for harsh environments. Improper protection against the site environment can cause equipment failure or loss of accuracy. 4.2.3.12 Maintenance Requirements The type and amount of maintenance varies widely with different measurement methods. For example, current metering requires periodic maintenance of the current meter itself and maintenance of the meter site to assure that is has a known cross section and velocity distribution. When the flow carries sediment or debris, most weirs, flumes, and orifices require periodic cleaning of the approach channel. Electronic sensors need occasional maintenance to assure that they are performing properly. Regular maintenance programs are recommended to ensure prolonged measurement quality for all types of devices. 4.2.3.13 Construction and Installation Requirements In addition to installation costs, the difficulty of installation and the need to retrofit parts of the existing conveyance system can complicate the selection of water measurement devices. Clearly, devices which can be easily retrofit into the existing canal system are much preferred because they generally require less down time, and unforeseen problems can be avoided. 4.2.3.14 Device Standardization and Calibration A standard water measurement device infers a documented history of performance based on theory, controlled calibration, and use. A truly standard device has been fully described, accurately calibrated, correctly constructed, properly installed, and sufficiently maintained to fulfill the original installation requirements and flow condition limitations. Discharge equations and tables for standard devices should provide accurate calibration. Maintaining a standard device usually only involves a visual check and measurement of a few specified items or dimensions to ensure that the measuring device has not departed from the standard. Many standard devices have a long history of use and calibration and, thus, are potentially more reliable. Commercial availability of a device does not necessarily guarantee that it satisfies the requirements of a standard device. When measuring devices are fabricated onsite or are poorly installed, small deviations from the specified dimensions can occur. These deviations may or may not affect the calibration. The difficulty is that unless an as-built calibration is performed, the degree to which these errors 45

affect the accuracy of the measurements is unknown. All too frequently, design deviations are made under the misconception that current metering can be used to provide an accurate field calibration. In practice, calibration by current metering to within +2 percent is difficult to attain. An adequate calibration for free-flow conditions requires many current meter measurements at several discharges. Changing and maintaining a constant discharge is often difficult under field conditions. 4.2.3.15 Field Verification, Troubleshooting, and Repair After construction or installation of a device, some verification of the calibration is generally recommended. Usually, the methods used to verify a permanent device (e.g., current metering) are less accurate than the device itself. However, this verification simply serves as a check against gross errors in construction or calibration. For some devices, errors occur as components wear and the calibration slowly drifts away from the original. Other devices have components that simply failCthat is, you get the correct reading or no reading at all. The latter is clearly preferred. However, for many devices, occa-sional checking is required to assure that they are still performing as intended. Selection of devices may depend on how they fail and how easy it is to verify that they are performing properly. 4.2.3.16 User Acceptance of New Methods Selection of a water measurement method must also consider the past history of the practice at the site. When improved water measurement methods are needed, proposing changes that build on established practice are generally easier to institute than radical changes. It can be beneficial to select a new method that allows conversion to take place in stages to provide educational examples and demonstrations of the new devices and procedures. 4.2.3.17 Vandalism Potential Instrumentation located near public access is a prime target for vandalism. Where vandalism is a problem, measurement devices with less instrumentation, or instrumentation that can be easily protected, are preferred. When needed, instrumentation can be placed in a buried vault to minimize visibility.

46

4.2.3.18 Impact on Environment During water measurement device selection, consideration must be given to potential environmental impacts. Water measurement devices vary greatly in the amount of disruption to existing conditions needed to install, meet standard upstream and downstream conditions, operate, and maintain. For example, installing a weir or flume constricts the channel, slows upstream flow, and accelerates flow within the structure. These changes in the flow conditions can alter local channel erosion, local flooding, public safety, local aquatic habitat, and fish movement up and down the channel. These factors may alter the cost and selection of a measurement device. 4.2.4 Calibrating water level sensors Periodic calibration of water level sensors is one of the tedious tasks necessary for successful flow measurement using flumes and weirs. Sensors can be calibrated in an office or laboratory environment prior to first installation, but once installed; quick field calibration is much preferred over removal and recalibration of the sensor in the laboratory. The following calibration procedure for submerged water level sensors such as pressure transducers and bubblers has proven to be accurate and straight forward, thereby promoting frequent recalibration of sensors in the field. The procedure calibrates the sensor only, and does not ensure that the sensor is properly set with respect to the sill-reference level of the structure. Procedures for zeroing sensors to the sill-reference level are given in Section 4.9. The procedure for calibrating submerged water level sensors is as follows: 

Install the sensor by suspending from a fixed point, for example by fastening the cable of a submerged pressure transducer to a cap that fits over the top of the transducer’s protective pipe.



Read the output from the sensor when it is located in its permanent measurement position.



Raise the sensor by a known vertical height by inserting a precision spacer between the top of the sensor pipe and the cap attached to the sensor cable. Read the output from the sensor in this position and use with the previous reading to compute the slope of the water level versus sensor output relation, called sensor or transducer gain (see Figure 4.7)

47



Raise the sensor out of the water and read the output to determine the sensor output at zero pressure.

An alternative is to have a rack on which the pressure transducer is mounted. The rack is removable and has two positions that are a fixed, known distance apart. On position is at the operating level and the second position is used to determine transducer gain. To calibrate the sensor, a reading is taken with the rack in each position and with the sensor out of the water.

Figure 4.6 A set of precision spacers used to calibrate a submerged pressure transducers

4.3 LITERATURE REVIEW ON FLOW METERS 4.3.1 Background For the measurement of flow, different types of flow meters have been introduced into the market. But most of these flow meters do not give the flow as they were rated by the manufacturers. All those flow meters have some errors which induce significant effect in the actual measurement processes. So, in this chapter we will be discussing about the different types of flow meters, the reasons for conducting the calibration processes and the correction factor. 4.3.2 Units of measurement Both gas and liquid flow can be measured in volumetric or mass flow rates, such as liters per second or kilograms per second. These measurements can be converted between one another if the material's density is known. The density for a liquid is almost independent of the liquid 48

conditions; however, this is not the case for gas, the density of which depends greatly upon pressure, temperature and to a lesser extent, the gas composition. When gases or liquids are transferred for their energy content, such as the sale of natural gas, the flow rate may also be expressed in terms of energy flow, such as GJ/hour or BTU/day. The energy flow rate is the volume flow rate multiplied by the energy content per unit volume or mass flow rate multiplied by the energy content per unit mass. Where accurate energy comes to the time of the legit flow rate is desired, most flow meters will be used to calculate the volume or mass flow rate which is then adjusted to the energy flow rate by the use of a flow computer. In engineering contexts, the volumetric flow rate is usually given the symbol Q, and the mass flow rate

.

4.3.3 Types of flow meters 4.3.3.1 Variable area meter The variable area (VA) meter, also commonly called a rotameter, consists of a tapered tube, typically made of glass, with a float inside that is pushed up by fluid flow and pulled down by gravity. As flow rate increases, greater viscous and pressure forces on the float cause it to rise until it becomes stationary at a location in the tube that is wide enough for the forces to balance. Floats are made in many different shapes, with spheres and spherical ellipses being the most common. Some are designed to spin visibly in the fluid stream to aid the user in determining whether the float is stuck or not. Rotameters are available for a wide range of liquids but are most commonly used with water or air. They can be made to reliably measure flow down to 1% accuracy. 4.3.3.2 Turbine flow meter The turbine flow meter (better described as an axial turbine) translates the mechanical action of the turbine rotating in the liquid flow around an axis into a user-readable rate of flow (gpm, lpm, etc.). The turbine tends to have all the flow traveling around it. The turbine wheel is set in the path of a fluid stream. The flowing fluid impinges on the turbine blades, imparting a force to the blade surface and setting the rotor in motion. When a steady rotation speed has been reached, the speed is proportional to fluid velocity.

49

Turbine flow meters are used for the measurement of natural gas and liquid flow. Turbine meters are less accurate than displacement and jet meters at low flow rates, but the measuring element does not occupy or severely restrict the entire path of flow. The flow direction is generally straight through the meter, allowing for higher flow rates and less pressure loss than displacement-type meters. They are the meter of choice for large commercial users, fire protection, and as master meters for the water distribution system. Strainers are generally required to be installed in front of the meter to protect the measuring element from gravel or other debris that could enter the water distribution system. Turbine meters are generally available for 1-1/2" to 12" or higher pipe sizes. Turbine meter bodies are commonly made of bronze, cast Iron, or ductile iron. Internal turbine elements can be plastic or non-corrosive metal alloys. they are accurate in normal working conditions to 0.2l/s however are affect greatly with dog mix interference. Fire meters are a specialized type of turbine meter with approvals for the high flow rates required in fire protection. They are often approved by Underwriters Laboratories (UL) or Factory Mutual (FM) for use in fire protection. Fire hydrant meters are a specialized type of portable turbine meter that are attached to a fire hydrant to measure water out of the hydrant. The meters are normally made of aluminum to be light weight, and are usually 3" capacity. Utilities often require them for measurement of water used in construction, pool filling, or where a permanent meter is not yet installed 4.3.3.3 Vortex flow meters Another method of flow measurement involves placing a bluff body (called a shedder bar) in the path of the fluid. As the fluid passes this bar, disturbances in the flow called vortices are created. The vortices trail behind the cylinder, alternatively from each side of the bluff body. This vortex trail is called the Von Kármán vortex street after von Kármán's 1912 mathematical description of the phenomenon. The frequency at which these vortices alternate sides is essentially proportional to the flow rate of the fluid. Inside, atop, or downstream of the shedder bar is a sensor for measuring the frequency of the vortex shedding. This sensor is often a piezoelectric crystal, which produces a small, but measurable, voltage pulse every time a vortex is created. Since the frequency of such a voltage pulse is also proportional to the fluid velocity, a volumetric flow rate is calculated using the cross sectional area of the flow meter. The frequency is measured and the 50

flow rate is calculated by the flow meter electronics using the equation f = SV / L where f is the frequency of the vortices, L the characteristic length of the bluff body, V is the velocity of the flow over the bluff body, and S is the Strouhal number, which is essentially a constant for a given body shape within its operating limits.

Figure 4.7 Vortex flow meter An obstruction in a fluid flow creates vortices in a downstream flow. Every obstruction has a critical fluid flow speed at which vortex shedding occurs. Vortex shedding is the instance where alternating low pressure zones are generated in the downstream. These alternating low pressure zones cause the obstruction to move towards the low pressure zone. With sensors gauging the vortices the strength of the flow can be measured. 4.3.3.4 Ultrasonic (Doppler, transit time) flow meters Ultrasonic flow meters measure the difference of the transit time of ultrasonic pulses propagating in and against flow direction. This time difference is a measure for the average velocity of the fluid along the path of the ultrasonic beam. By using the absolute transit times both the averaged fluid velocity and the speed of sound can be calculated. Using the two transit times tup and tdown and the distance between receiving and transmitting transducers L and the inclination angle α one can write the equations:

And

51

Where v is the average velocity of the fluid along the sound path and c is the speed of sound. Ultrasonic flow meters are also used for the measurement of natural gas flow. One can also calculate the expected speed of sound for a given sample of gas; this can be compared to the speed of sound empirically measured by an ultrasonic flow meter and for the purposes of monitoring the quality of the flow meter's measurements. A drop in quality is an indication that the meter needs servicing. Recently, Ultrasonic flow meters are also being used for measurement of LNG flow. 4.3.3.5 Coriolis Flow meter Direct mass measurement sets Coriolis flow meters apart from other technologies. Mass measurement is not sensitive to changes in pressure, temperature, viscosity and density. With the ability to measure liquids, slurries and gases, Coriolis flow meters are universal meters. Coriolis Mass Flow meter uses the Coriolis effect to measure the amount of mass moving through the element. The fluid to be measured runs through a U-shaped tube that is caused to vibrate in an angular harmonic oscillation. Due to the Coriolis forces, the tubes will deform and an additional vibration component will be added to the oscillation. This additional component causes a phase shift on some places of the tubes which can be measured with sensors. The Coriolis flow meters are in general very accurate, better than ±0.1% with a turndown rate more than 100:1. The Coriolis meter can also be used to measure the fluids density. 4.3.3.6 Open Channel Flow meters A common method of measuring flow through an open channel is to measure the height of the liquid as it passes over an obstruction as a flume or weir in the channel.

Figure 4.8 Open channel flow meters 52

Common used is Sharp-Crested Weir, V-Notch Weir, Cipolletti weir, Rectangular-Notch Weir, the Parshall Flume or Venturi Flume. 4.3.3.7 Positive Displacement Flow meter The positive displacement flow meter measures process fluid flow by precision-fitted rotors as flow measuring elements. Known and fixed volumes are displaced between the rotors. The rotations of the rotors are proportional to the volume of the fluid being displaced. The number of rotations of the rotor is counted by an integral electronic pulse transmitter and converted to volume and flow rate. The positive displacement rotor construction can be done in several ways: 

Reciprocating piston meters are of single and multiple-piston types.



Oval-gear meters have two rotating, oval-shaped gears with synchronized, close fitting teeth. A fixed quantity of liquid passes through the meter for each revolution. Shaft rotation can be monitored to obtain specific flow rates.



Nutating disk meters have moveable disks mounted on a concentric sphere located in spherical side-walled chambers. The pressure of the liquid passing through the measuring chamber causes the disk to rock in a circulating path without rotating about its own axis. It is the only moving part in the measuring chamber.



Rotary vane meters consist of equally divided, rotating impellers, two or more compartments, inside the meter's housings. The impellers are in continuous contact with the casing. A fixed volume of liquid is swept to the meter's outlet from each compartment as the impeller rotates. The revolutions of the impeller are counted and registered in volumetric units.

The positive displacement flow meter may be used for all relatively nonabrasive fluids such as heating oil, lubrication oil, polymer additives, animal and vegetable fat, printing ink, Freon, and many more. Accuracy may be up to 0.1% of full rate with a Turn Down of 70:1 or more.

53

4.3.3.8 Electromagnetic Flow meter An electromagnetic flow meter operates on Faraday's law of electromagnetic induction that states that a voltage will be induced when a conductor moves through a magnetic field. The liquid serves as the conductor and the magnetic field is created by energized coils outside the flow tube. The voltage produced is directly proportional to the flow rate. Two electrodes mounted in the pipe wall detect the voltage which is measured by a secondary element. Electromagnetic flow meters can measure difficult and corrosive liquids and slurries, and they can measure flow in both directions with equal accuracy. Electromagnetic flow meters have relatively high power consumption and can only be used for electrical conductive fluids as water.

Figure 4.9 Industrial magnetic flow meter 4.3.3.9 Calorimetric Flow meter The calorimetric principle for fluid flow measurement is based on two temperature sensors in close contact with the fluid but thermal insulated from each other.

Figure 4.10 Calorific meter One of the two sensors is constantly heated and the cooling effect of the flowing fluid is used to monitor the flow rate. In a stationary (no flow) fluid condition there is a constant temperature 54

difference between the two temperature sensors. When the fluid flow increases, heat energy is drawn from the heated sensor and the temperature difference between the sensors are reduced. The

reduction

is

proportional

to

the

flow

rate

of

the

fluid.

Response times will vary due the thermal conductivity of the fluid. In general lower thermal conductivity requires higher velocity for proper measurement. The calorimetric flow meter can achieve relatively high accuracy at low flow rates 4.3.3.10 Differential pressure flow meters Differential pressure flow meters (in most cases) employ the Bernoulli equation that describes the relationship between pressure and velocity of a flow. These devices guide the flow into a section with different cross section areas (different pipe diameters) that causes variations in flow velocity and pressure. By measuring the changes in pressure, the flow velocity can then be calculated. Many types of differential pressure flow meters are used in the industry: 

Orifice Plate: A flat plate with an opening is inserted into the pipe and placed perpendicular to the flow stream. As the flowing fluid passes through the orifice plate, the restricted cross section area causes an increase in velocity and decrease in pressure. The pressure difference before and after the oriffice plate is used to calculate the flow velocity. A calculator for the orifice plate flow meters can be found in the fluid mechanics section.

Figure 4.11 Orifice Plate

55



Venturi Tube A section of tube forms a relatively long passage with smooth entry and exit. A Venturi tube is connected to the existing pipe, first narrowing down in diameter then opening up back to the original pipe diameter. The changes in cross section area cause changes in velocity and pressure of the flow. A calculator for the venturi tube flow meters can be found in the fluid mechanics section.

Figure 4.12 Venturi Tube 

Nozzle A nozzle with a smooth guided entry and a sharp exit is placed in the pipe to change the flow field and create a pressure drop that is used to calculate the flow velocity.

Figure 4.13 Nozzle 56



Segmental Wedge A wedge-shaped segment is inserted perpendicularly into one side of the pipe while the other side remains unrestricted. The change in cross section area of the flow path creates pressure drops used to calculate flow velocities.

Figure 4.14 Segmental wedge 

V-Cone: A cone shaped obstructing element that serves as the cross section modifier is placed at the center of the pipe for calculating flow velocities by measuring the pressure differential.

Figure 4.15 V-Cones

57



Pitot Tube A probe with an open tip (Pitot tube) is inserted into the flow field. The tip is the stationary (zero velocity) point of the flow. Its pressure, compared to the static pressure, is used to calculate the flow velocity. Pitot tubes can measure flow velocity at the point of measurement.

Figure 4.16 Pitot Tube 

Averaging Pitot Tube Similar to Pitot tubes but with multiple openings, averaging Pitot tubes take the flow profile into consideration to provide better overall accuracy in pipe flows.

Figure 4.17 Averaging Pitot Tube 

Elbow When a liquid flows through an elbow, the centrifugal forces cause a pressure difference between the outer and inner sides of the elbow. This difference in pressure is used to 58

calculate the flow velocity. The pressure difference generated by an elbow flow meter is smaller than that by other pressure differential flow meters, but the upside is an elbow flow meter has less obstruction to the flow.

Figure 4.18 Elbow 

Dall Tube: A combination of Venturi tube and orifice plate, it features the same tapering intake portion of a venturi tube but has a 'shoulder' similar to the orifice plate's exit part to create a sharp pressure drop. It is usually used in applications with larger flow rates.

Figure 4.19 Dall Tube

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4.3.4 Flow meter Calibration Calibration of the measuring instrument is the process in which the readings obtained from the instrument are compared with the sub-standards in the laboratory at several points along the scale of the instrument. As per the results obtained from the readings obtained of the instrument and the sub-standards, the curve is plotted. If the instrument is accurate there will be matching of the scales of the instrument and the sub-standard. If there is deviation of the measured value from the instrument against the standard value, the instrument is calibrated to give the correct values. All the new instruments have to be calibrated against some standard in the very beginning. For the new instrument the scale is marked as per the sub-standards available in the laboratories, which are meant especially for this purpose. After continuous use of the instrument for long periods of time, sometimes it loses its calibration or the scale gets distorted, in such cases the instrument can be calibrated again if it is in good reusable condition. Even if the instruments in the factory are working in the good condition, it is always advisable to calibrate them from time-to-time to avoid wrong readings of highly critical parameters. This is very important especially in the companies where very high precision jobs are manufactured with high accuracy. One dictionary defines "to calibrate" as "to standardize (as a measuring instrument) by determining the deviation from a standard so as to ascertain the proper correction factors." There are two key elements to this definition: 

Determining the deviation from a standard,



Ascertaining the proper correction factors

Deviation from a standard To measure deviation, it is first vitally important to determine the actual flow rate. In flow meter calibration, this is normally done by one of two methods: using a master meter, or weighing the flow to get a gravimetric reading of mass flow. A master meter is a flow meter that has been calibrated to a very high degree of accuracy. Types of flow meters used as master meters include turbine meters, positive displacement meters, Venturi meters, and Coriolis meters. The second method involves gravimetric weighing of the amount of fluid (liquid or gas) that actually flows through the meter into or out of a container during the calibration procedure. This 60

is normally done using a weigh scale that has a very high degree of accuracy. The gravimetric method is generally regarded as the most accurate way to measure the actual amount of flow regarded as the most accurate way to measure the actual amount of flow. Correction Factors The second component of calibration is determining the proper correction factors. It is easy to think of flow meter calibration in terms of adjusting a bathroom scale, where we simply use a knob to zero the scale when there is no weight on it. Once this adjustment is made, the scale is "calibrated" and we may then assume that it reads correctly. But you may notice that you always seem to weigh more on your own scale than you do at your health club. Both scales may be zeroadjusted, yet one may consistently give higher weight readings than the other. There are some parallels to flow meter calibration, since getting a flow meter to read zero flow under no-flow conditions may be part of the calibration process, and is not always as simple as it sounds. There is often no simple hardware adjustment to make the flow meter start reading correctly. Instead, the deviation from the correct reading is recorded at a variety of flow rates. The data points are plotted, comparing the flow meter output to the actual flow rate as determined by the master meter or weigh scale. In many cases, the data reflecting flow meter performance at a range of flow rates is made available to the end user. This data may also be used to create a compensating formula so the user can determine the flow rate within a specified range of accuracy. This compensating formula represents a simple correction formula for a scale. But usually it will be more complex, since flow meter performance often varies at different flow rates. For example, many flow meters have a more difficult time accurately measuring low flow rates than they do higher flow rates.

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Table 4.3.1 Reasons to Calibrate Different Types of Flow meters Type

Technology

Reasons

Differential

The flow path is obstructed with a primary Orifice plates, cones, and venturi’s wear,

Pressure

element and the difference in pressure is pilot tubes dirty; transmitter failure measured before and after the primary element

Magnetic

The voltage generated by electrically conductive Electrode coating, liner damage, electronic material passing through a magnetic field is failure. measured to calculate flow rate.

Coriolis

Fluid passes through a vibrating tube. The Wear and coating of the flow tubes; vibration amplitude is proportional to mass flow.

electronic failure.

Open

Some use liquid levels at flumes or weirs; other Accumulated

debris;

level

transmitter

channel

calculates flow from velocity, depth, and level calibration; electronic failure of transmitter and or diameter data.

Positive

- Fluid is captured in a known volume and

displacement

Corrosion, dirty lquids and abrasion change

released-flow rate is calculated by counting the volumes.

volume;

bearing

wear

degrades

accuracy; solids can cause plugging; gear service affects calibration.

Thermal loss

Heat loss or temperature rise is measured when Sensor wear or failure, leaks the fluid passes over a heated wire thermostat or over a heater

Turbine

Fluid

passes

over

a

rotor-flow

rate

proportional to rotor speed

is Bearings dirty or affected by chemicals, rotors

wear,

bearing

service

affects

calibration, electronic failure Ultrasonic

Fluid speed measured by the time it takes an Calibrate before placing in service, changes ultrasonic wave to travel a specified distance 62

in fluid sonic properties, electronic failure

Variable area

Flow rate proportional to the height of a float in Plugging material buildup, metering tube a metering tube

Vortex

failure

A shedder bar generates vortices proportional to Vibration, expansion due to temperature the flow rates

variations , flow meter off-center in pipe, electronic failure

63

4.4 Literature review on Buoyancy and Floatation 4.4.1 Introduction A body immersed partially or fully in a fluid experiences a vertical upward force. This vertical upward force on a floating or submerged body is known as buoyant force and its magnitude can be determined by the Archimedes’ principle of buoyancy. The tendency of a submerged body to rise in a fluid because of the upward fluid pressure which opposes the downward force of gravity is known as buoyancy. 4.4.2 Archimedes’ Principle of buoyancy It states that when a body is totally or partially immersed in a fluid, it is buoyed up (or lifted up) by a force which equals the weight of the fluid displaced by the body. Upward or buoyant force = Weight of the fluid displaced As buoyant force is vertical and is equal to the weight of fluid displaced, therefore it acts through the centre of gravity of the displaced fluid. The point of application of the buoyant force is known as centre of buoyancy. 4.4.3 Floatation A body immersed in any fluid is acted upon by two forces namely the weight of the body W acting vertically downward and the buoyant force

acting vertically upward.

If

; the gravity force i.e. weight will cause the body to sink

If

; then body will remain in equilibrium at any level

If

; the buoyant force

acting vertically upward shall move the body upward. The body

rising up shall come to rest only when a sufficient portion of the volume is still submerged to produce a buoyant force equal to its own weight. Then the body is said to be floating and this phenomenon is known as floatation.

64

4.4.4 Stability or conditions of Equilibrium By stability of any submerged or floating body is meant the tendency of the body, either to return or to go away from its original position, when slightly disturbed. Therefore a body in static equilibrium may or may not restore to its original position. A submerged or floating body can have three possible conditions of stability: Stable, neutral or unstable. 4.4.4.1 Stable Condition The equilibrium of a body is said to be stable if any change from its position, however small, is accompanied by the introduction of forces or moments tending to return it to its original position. 4.4.4.2 Neutral Condition The equilibrium of a body is said to be neutral when by giving a small displacement the body takes a new position. In this position neither a stabilizing nor an overturning couple exists an thus the body remains at rest in its new position. 4.4.4.3 Unstable condition The equilibrium of a body is said to be unstable if any change from its position, however small, is accompanied by the introduction of forces or moments tending to increase the displacement of body still further. a) Stability of submerged bodies in horizontal and vertical displacement When a submerged body is displaced horizontally or vertically neither the magnitude nor the location of the buoyant force is changed, therefore the equilibrium of the submerged body is not affected at all. In other words, the body remains in stable condition of equilibrium when displaced horizontally or vertically. b) Stability of floating bodies in horizontal and vertical displacementIf a floating body is displaced vertically upward, the volume of displaced fluid and consequently the buoyant force is decreased with the result a downward unbalanced force comes into action and it brings back the body to its original equilibrium position. Therefore a floating body, on vertical displacement, comes back to its original position and remains in stable equilibrium. 65

In case of horizontal displacement, neither the magnitude nor the location of buoyant force is changed; therefore the horizontal displacements do not affect the equilibrium of floating bodies. 4.4.5 Buoyancy Equations 1) Buoyant force, Where

………………………………………………………… (1)

is displaced Volume of the fluid is Specific weight of fluid

2) Weight of Solid,

………………………………………………………….(2)

Where V is the volume of the solid is the density of the solid 3) Static Equilibrium in the vertical direction: ……………………………………………………………………………. (3) Thus in any object that is floating, at static equilibrium, we get

»

……………………………………………………………………. (4)

4.5 Field Survey The facility established is as a site for conducting different tests for the hydraulic machineries. The tests include testing of turbines by manufacturing the prototypes in the lab and calibration of pumps and different flow meters. As defined by the ISO, weirs are one of the precise devices for measuring discharge. If the weirs are accurate enough then, they can be used to calibrate the flow meters and pumps as well. With the errors in hand, suitable correction factors can be implied to the flow meters for yielding better performance at the field. For this purpose, in the lab, different kinds of weirs are installed. The weirs, as discussed in the earlier chapters are of different kinds and have their specific predefined range and site for measurement. Broadly, the weirs are of triangular, rectangular and trapezoidal types. The widely 66

accepted and used weirs at the sites are mainly triangular and rectangular for they have better accuracy and manufacturability. So the specifications of the weirs, installed at the lab, are as follows: 1. Triangular V-notch I.

Measured width of the notch in meters,

b = 0.710

Width of the approach channel in meters,

B = 0.940

III.

Measured height in meters,

h = (to be measured)

IV.

Height of the crest relative to the floor in meters p = 0.0325

II.

V.

α = 50

Notch angle in degrees

Figure 4.5.1 Triangular V-notch Weir

67

2. Triangular V-notch I.

Measured width of the notch in meters,

b = 0.515

Width of the approach channel in meters,

B = 0.585

III.

Measured height in meters,

h = (to be measured)

IV.

Height of the crest relative to the floor in meters p = 0.0355

II.

V.

Notch angle

α = 40

Figure 4.5.2 Triangular V-notch weir

68

3. Rectangular I.

Measured width of the notch in meters,

b = 0.544

Width of the approach channel in meters,

B = 0.8

III.

Measured height in meters,

h = (to be measured)

IV.

Height of the crest relative to the floor in meters p = 0.288

II.

Figure 4.5.3 Rectangular Weir For the measurement of discharge, different formulas are available for these weirs. Depending upon the accuracy required the specific formulas can be used. But one of the parameters for the measurement is unknown i.e. the head. For the measurement of the head, hollow plastic pipes are installed along the weirs. Thus, for the measurement of the head different devices can be used 69

like float method and most accurate ones like ultrasonic, conductance method etc. Depending upon the specification of the site and level of precision required, suitable one must be selected. Thus, with all the parameters known the discharge measurement with high precision can be conducted. Limitations 

The thickness of the weir plate along notch should be 2mm as defined by the ISO standards. But at TTL lab the weirs are not as per the standard which might be the primary source of uncertainty.



Similarly, the approach channels of the weir are not long enough for water to settle which might produce uneven water level and may produce difficulties in measuring head accurately.

4.6 Design and fabrication of the prototype 4.6.1 Selection of the head measurement mechanism For the measurement of head, in the weirs, a proper mechanism must be selected. As discussed in the early chapters, that different types of head measurement devices are available. So among these devices, one has to be selected considering the work space, cost, accuracy required and many other different factors. For the prototype design and fabrication, in this study we have selected the simple but effective i.e. float mechanism. 4.6.2 Design and Fabrication of the prototype 4.6.2.1 Components The major components of the float head measurement mechanism are as follows: 4.6.2.1.1 Frame: The frame is a basic platform that supports the bucket and all other mechanisms for the measurement. The frame is basically fabricated with L shaped rods made of mild steel. The dimension of the frame is 457 * 305 * 127.

70

Figure 4.6.1 Frame of the mechanism 4.6.2.1.2 Vertical scale for pointer The stainless steel scale stands vertically upward, attached with nut and bolt on to the frame. The Pointer tip points the level of water on to the scale. The dimension of the scale is 600 *10*4.

Figure 4.6.2 Vertical Scale for the pointer 71

4.6.2.1.3 Vertical Scale for Level pipe The stainless steel scale is also mounted vertically upward on to the frame. And the level pipe is attached on to the scale thus the level of water can be seen on the scale co-incident to the level pipe. The Dimension of the scale is 400*10*4.

Figure 4.6.3 Vertical Scale for level pipe 4.6.2.1.4 Reservoir The reservoir here is simply a plastic bucket. It acts as a device for storing the water for level measurement. It is provided with holes on the either sides for level measurement and water filling.

Figure 4.6.4 Reservior

72

4.6.2.1.5 Glass Syringe set The glass syringe is basically used here as a friction less sliding mechanism. The contact between the glass surfaces coated with special material provides a very smooth motion for the measurement of the level.

Figure 4.6.5 Glass syringe(Left) and Glass piston( Right) 4.6.2.1.6 Glass Syringe holder This device is simply a circular disc with a pipe welded through the centre. The pipe is bored on the sides to clamp the syringe at the centre of the disc and prevent any sort of movement. The holder is made of up of mild steel.

Figure 4.6.6 Glass Syringe holder 73

4.6.2.1.7 Clampers The rectangular bars are so constructed that they clamp the syringe holder at the center of the bucket. The clampers are provided with slot features to attach it on to the walls of the bucket. There are three numbers of these kinds of clampers to provide maximum support to the holder and flexibility as well.

Figure 4.6.7 Clamper 4.6.2.1.8 Funnel The funnel is basically a device to pour water into the reservoir thus raising the water level for measurement. The funnel is made of up plastic. The funnel is connected to the reservoir at the bottom with level pipe.

Figure 4.6.8 Funnel with level pipe 74

4.6.2.1.9 Funnel holder The funnel holder is simply an extension of the clamp for holding funnel vertically upwards preventing spillage of water. The holder is made up of mild steel and permanently welded on to the clamper.

Figure 4.6.9 Funnel holder 4.6.2.1.10 Pointer set (Stand and Tip) and joint The pointer set is device for indicating level of the water in the reservoir by making vertical movement with changing level of water. The pointer set is permanently attached to the syringe piston with m-seal. The pointer stand is made up of aluminum whereas the tip is made up of mild steel. Thus, the tip is attached to the stand with m-seal. The joint acts as an extension of the stand for easy assembly of the stand and the piston.

Figure 4.6.10 Pointer set (stand and tip) 4.6.2.1.11 Float The float is the most essential component for the measurement mechanism. It’s a rectangular plate, with density far less than water, hence having a good ability to float. The float carries all

75

the weight of glass piston, pointer stand and tip and provides a buoyancy force to keep it over the water surface. The float is made up of thermo coal.

Figure 4.6.11 Float 4.6.2.2 Calculation of the Buoyancy Force Volume of the glass piston, Volume of pointer, Volume of Tip, Volume of the float Density of Glass

= 2600 kg/m3

Density of Aluminum

= 2700 kg/m3

Density of Thermo-coal

= 1.64 kg/m3

Density of mild steel

=7800kg/m3

Volume of displaced water = Density of water At static equilibrium along the vertical direction,

76

Then, the equation becomes (

)

Putting the value in the equation, we get

i.e. 0.059 N Thus, the upward force of 0.059N continuously pushes the indicator as the level rises. 4.7 Testing of the Prototype 4.7.1 Features of the Prototype The features of the prototype designed to measure the head using the float mechanism are as follows: 

The design is not so complicated and requires very less space for installation.



The prototype is provided with the pointer option for indication of the head in the reservoir and also a level pipe is mounted into the reservoir for cross checking of the head indicated by the pointer.



The prototype has an accuracy of ± 1 mm.



The friction less glass syringe, for sliding of the pointer, provides a very smooth motion to the pointer. Thus, making it easier for the indication of the level in the scale.



The steel clamps firmly hold the syringe holder at the centre of the reservoir, thus keeping the pointer on the scale avoiding the parallax error.



The use of thermo-coal for the float provides an efficient buoyancy force resulting in the smooth operation of the mechanism.

4.7.2 Limitations of the Prototype During the testing of the prototype in the lab, different limitations were encountered they are as follows: 77



The range of the mechanism is very less, only able to measure a head of 10cm, which might not be suitable for the lab operations.



The friction less glass syringe literally stops working, when the surface come in contact with the water, so the syringe’s surfaces must be kept free of water.



The perpendicularity at the joints of the pointer and syringe piston is not maintained so well which causes non smooth motion of the pointer.



The scale reading is very difficult, which operates in the principle of difference that may lead errors in the measurement.



The mechanism works smoothly only from a certain scale reading i.e. 10 cm in the main scale, due to erosion of the frictionless coating on the inner glass surface.

4.7.3 Recommendations The design of the prototype after fabrication and testing in the lab, showed the above mentioned limitations. So, if the mechanism is to be used in the field for the measurement then the following points must be considered and improvised in the existing design. 

To improve the range of the mechanism, a glass syringe of higher capacity can be used.



To obtain a smooth frictionless operation, brand new syringes must be preferred



The surfaces of glass syringe must be protected from the water otherwise the friction inside the syringe increases and the mechanism stops working. For efficient operation the glass syringe must be uninstalled and cleaned with soft piece of cloth before every operation.



The perpendicularity at the joints must be maintained as far as possible. The use of try squares can be a feasible option but if any advanced devices are available, then they can be used as well.



To improve the accuracy of the measurement, more advanced measuring devices can be used with better least counts like vernier caliper, micrometer screw gauge to simple steel scale.

78



To operate the mechanism smoothly, the level of water in the reservoir must be maintained to the minimum level of “11 cm” reading in the main scale.



The glass syringes are very fragile so the soft materials must be inserted in between for clamping inside the holder with bolts.



The area of the float can be increased further more to improve the buoyancy force and for smooth operation of the mechanism.



A drain valve can also be installed in the reservoir for draining out the water. But the valve must be placed such that the minimum level of water is maintained in the reservoir.

4.7.4 Conclusion Thus, we can conclude that the float mechanism is a very simple and effective mechanism for head measurement if designed well enough. After the fabrication and testing we have encountered different problems and limitations in the design but the working of the mechanism was a success. So if the device is to be used for the field operations, then the design must be improvised by following the recommendations mentioned above. The above mentioned sectors of improvements can results in an efficient head measuring device, if committed upon.

79

CHAPTER 5 SELECTION From the conclusion drawn from the above chapter shows the sensitivity of the level/head measurement device is quite low as per required for the standard of the lab. For measuring the discharge available, with the above mentioned instrument i.e. level indicator, the so obtained data for head will be less accurate and uncertainty will be too high for any consideration to be made. So, more advanced method for head measurement should be selected. In the sight, the most widely used and precise instrument for the head measurement is ultrasonic level meter. This instrument uses sonar for the measurement and operated in the appropriate range can give results with very less errors. Different types of ULM are available in the market but as the operation in the lab is concerned, UlM-70 will be the best one. The further discussion on the ULM-70 is as follows 5.1 ULM-70- Introduction The ULM ultrasonic level meters are compact measurement devices including an electroacoustic converter, central processor unit and display module. Using the electro-acoustic converter, the level meters transmit the sequence of ultrasonic pulses that spread towards the surface level. The converter recuperates reflected acoustic waves that are subsequently processed in the electronic module. The intelligent evaluation block filters out interfering signals, compares the cleaned received signal with the false reflection map(i.e. from mixers, ladders, reinforcement etc) and selects a suitable reflection (echo). Based on the period during which the individual pulses spread towards the surface level and back and based on the measured temperature in the tank, the instant distance to the surface level is calculated. According to the level height, the level meter output is set and the measured value is displayed on the display. The level meters are suited to level measurement of various liquid materials, sewerage waters, and mash and paste materials, suspensions in closed or open vessels, sumps, reservoirs and open channels. In case the level of bulk solid materials is measured, the measurement range is reduced. We recommend consulting the use with the manufacturer. 5.2 Features of Variants a) ULM-70-02-I  Measuring range from 0.15m to 2m  Plastic PVDF transmitter, mechanical connection with thread G 1ʺ b) ULM-7-06-I  Measuring range from 0.25m to 6m 80

 Plastic PVDF transmitter, mechanical connection with thread G 1 1/2ʺ c) ULM-70-10-I  Measuring range from 0.4m to 10m  Plastic PVDF transmitter, mechanical connection with HDFE polyethylene flange (version “N”) or aluminum alloy flange (version “Xi”) d) ULM-70-20-I  Measuring range from 0.5m to 20m  Plastic PVDF transmitter, mechanical connection with aluminum alloy flange Table 5.3 Technical Specifications of ULM-70

Measuring range

ULM-7002-I

ULM-70-06-I

0.2-2m

0.25-6m

ULM-70-10-I N

Supply Voltage

ULM-70-20-I

Xi 0.4-10m

18-36V DC

0.5-20m

18-30V DC

Output

4 to 20 mA ( limit value 3.9 to 20.5 mA), HART

Resolution

< 1mm

Accuracy(within the total Range)

0.15 %

Temperature Error

Max. 0.04% /K

Beamwidth (-3dB) Ambient Temperature

10°

14°

10°

-30 to +70° C

12° -30 to +60°C

Short-time temperature stress resistance

+90°c/1 hour

Max. Operation Overpressure (on transmission surface)

0.1 MPa

Sensitivity

3 steps (low- medium- high)

81

Damping

0-99 sec

Measuring period

1-4 sec

Delay between supply power rise time and first measurement

30 sec

Additional technical data (only for variant Xi)- Max. internal values

Ui=30V DC Ii=132 mA Pi =0.99W Ci =370nF Li = 0.9mH

Failure Indication(echo loss, level in dead zone, internal failure)

3.75 mA; 22mA; last measured value

Protection class

IP67

Mechanical Connection

Adjustable in modes;

Screwing with thread G 1ʺ

Recommended cable

Screwing with thread G 1 1/2ʺ

HPDE flange

PVC 2* 0.75

Aluminum alloy flange

( 3* 0.5

Aluminum alloy flange

)

Current output load Resistance (U= 24V DC) Weight

  

0.3 kg

0.4 kg

0.7 kg

1.2 kg

3.1kg

In case the level of bulk-solid materials is measured, the measurement range is reduced. Allowing temperature range in the zone 0: -20°C to +60°, Allowed pressure range in the zone 0: 80 to 110 kPa. Including 250R resistor in case of hart connection. 82

Table 5.4 Area Classification of ULM-70 ULM-70N-I

Performance for non-explosive areas

ULM-70Xi-02-I

Explosive proof -suitable for explosive areas (combustible gases or vapours) II 1/2 G Ex ia IIB T5 with isolating repeater (IRU-420) the whole level meter-zone 1, front head part- zone 0

ULM-70Xi-06-I ULM-70Xi-10-I

Explosive proof- suitable for explosive areas (combustible gases or vapours) II 1/2G Ex ia IIB T5 with isolating repeater (IRU-420) the whole level meter-zone 1, front head part-zone 0

ULM-70Xi-20-1

Explosive proof-suitable for explosive areas (combustible gases or vapours) II2G Ex ia IIA T5 with isolating repeater (IRU-420) the whole level meterzone 1

5.5 Installation 

Level meter is installed into the upper lid of the tank (vessel), using a fixing nut or a flange.



If installed in an open channel (sumps, reservoirs, etc), install the level meter as closest as you can to the maximum level expected.



The front of the level meter must be vertically to be measured level.



Foam on the level absorbs the acoustic wave reflection which might cause malfunction of the level meter. If possible select the location where the foaming is as low as possible.



Protect the level meter against direct sunlight.

5.6 Mounting Recommendation 5.6.1 Electrical Connection The ultrasonic level meter is designed to be connected to supply unit or to controller through a cable with the outer diameter of 6/8 mm (recommended cross section of cores 0.5-0.75 ) by means of bolted clips placed under display module. Connect the plus pole (+U) to the terminal “+”, the minus pole to 0V to the terminal “-” and the shielding to the terminal “╧” ( only for shielded cables). With regard to possible occurrence of electrostatic charge on non-conductive parts of the level meter for explosive areas (Xi – version) must be grounded with ground terminal 83

The power can be a stabilized voltage supply unit of 18 + 36V DC (30V DC for Xi version) that is included in evaluation or display unit. In case of strong electromagnetic interference (EMI), parallel supply cable with power lines, or when the cable length exceeds 30m, we recommend you to use a shielded cable. Always disconnect the supply voltage before connecting the level meter. 5.6.2 Level Meter Setting Set the level meter using 3 buttons placed on the display module. All settings are accessible in the ULM-70 set-up mode access. For detailed information read at the instruction manual. Button OK   

Set-up mode access Confirmation of selected item in the menu Saving of set-up data

Button UP AND DOWN  

Move in the menu Change of values

Button ESC  

Cancelling of carried out changes Shift one level up

5.6.3 Range of applications 

For continuous non-contact level measurement of liquids (water solutions, sewerage water, etc), mash and paste materials (sediments, sticks, resins etc.) in closed or open vessels, sumps, reservoirs and open channels.



In case the level of bulk solid materials is measured, the measurement range is reduced. We recommend consulting the use with the manufacturer.

84

5.6.4 Order code ULM-70

-

Output type: I-current Maximum range: 02-0.2 to 2m 06- 0.25 to 6m 10- 0.4 to 10m 20-0.5 to 20m Performance: N- normal usable in nonexplosive areas Xi- Explosion Proof – suitable for explosive areas

85

CHAPTER 6 CONCLUSION Through this research we have seen different methods of head measurement. All of those techniques have their utility and significance in their own field. But as we discuss, for TTL accuracy and consistency are the major concerns in the head measurement mechanism, as it is to be established as the site for excellence. So, in the conclusion of this research, for head measurement, ultrasonic level meter will the most suitable one considering the site specifications and accuracy required.

86

BIBLIOGRAPHY 1. http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/, Extracted on July 17, 2011 2. Bos M.G. (ed.), Discharge Measurement Structures, 3rd edition, International Institute for Land Reclamation and Improvement, Publication 20, Wageningen, The Netherlands, 1989. 3. http://en.wikipedia.org/wiki/Flow_measurement, Extracted on July 16, 2011. 4. http://kolmetz.com/pdf/EDG/ENGINEERING%20DESIGN%20GUIDELINE%20instrument%20Rev%203.pdf, Extracted on July 27, 2011. 5. http://www.earthwardconsulting.com/library/600R01043Complete.pdf, Extracted on July 18, 2011. 6. http://www2.alterra.wur.nl/Internet/webdocs/ilri-publicaties/publicaties/Pub58/pub58h4.1.pdf, Extracted on July 28, 2011. 7. http://www.bickfordscience.com/04-Density_and_Buoyancy/02-buoyancy.html, Extracted on July 31, 2011. 8. http://www.tc.gc.ca/eng/marinesafety/tp-tp5579-chap-3-188.htm, Extracted on July 31, 2011. 9. Robert L. Mott, Applied Fluid Mechanics, 6th Edition. 10. Jesse Yoder, Flow-meter Calibration: How, Why, and Where, August 01, 2008. 11. Ronald V. Giles, Jack B. Evett, PhD, Cheng LIV, Fluid Mechanics and Hydraulics, 3rd Edition, Tata MCGRAW-HILL (Edition), Page No.58-70. 12. J.F Douglas, J.M. Gasiorek, J.A. Swaffield, Fluid Mechanics, 3rd Edition, Funded by the British Government. 13. Dr. Jagdish Lal, Fluid Mechanics and Hydraulics ,9th Edition, Metropolitan Book Co. Private Ltd, Page No. 142-152 14. International Standard ISO 1438/1,Water flow measurement in open channels using weirs and venture flumes-Part 1: Thin- plate weirs, 1st Edition,1980. 15. http://www.usbr.gov/pmts/hydraulics_lab/pubs/wmm/chap07_03.html Extracted on July 15, 2011 16. http://www.engineeringtoolbox.com/weirs-flow-rate-d_592.html Extracted on July 15, 2011 87

BILL OF MATERIALS TABLE: FOR FABRICATION OF HEAD MEASUREMENT MECHANISM AT TTL S.

Component

N

Component

Dimensions (mm)

Amount

Rate ( Rs per)

L

kg

Pcs Kg

Total

Remarks

code

o.

B

H

D

M

Pc

1

Frame

FR01

457 305 127 -

2.4

1

100 -

-

240

B

2

Vertical rod for

VR01

600 12

4

-

0.4

1

100 -

-

40

B

VR02

400 12

4

-

0.2

1

100 -

-

20

B

-

pointer 3

Vertical rod for level pipe

4

Steel Scales

SS01

300 -

-

-

2

-

-

30

60

B

5

Reservoir

R01

-

350 290 -

1

-

-

-

-

A

6

Glass Syringe

GSS01

140

-

22

1

80

0.05 1

A

Set 7

Glass Syringe

GSH01

40

100 -

-

A

Holder 8

Funnel

FU02

1

20

20

B

9

Funnel Holder

FH01

1

30

30

B

10 Holder clamps

HC01

120 18

11 Pointer Set

PS01

250

5

12 Pointer Joint

PJ01

50

6

13 Level pipes

LP 01

500

8

6

0.4

88

3

A

0.02 1

B B

-

1

-

-

40

20

B

14 Level pipes

LP02

15 M Seal

MS 01

16 Fitting charge

FC 01

500

6

-

1

-

-

20

10

B

1

50

50

B

2

50

100

M

Grand Total

Rs. 590

(Note: B- Bought, A- Available in the lab, M-Miscellaneous, kg- Kilogram, m-meter, pc-piece)

APPENDIX II LOG SHEET

KATHMANDU UNIVERSITY 89

TURBINE TESTING LAB TIME-SHEET of WORK of SHORT TERM EMPLOYEE Employment Type: Full-Time Daily Basis; Normal Working Hours Name

: Aatma Ram Kayastha

Position

: BE Internship Students

Primary Place of Work

: Kathmandu University, TTL, Dhulikhel, Nepal

Work Unit

: No. of days worked against no. of days supposed to work

Required Work Units

: All working days of a month period excluding weekly, public, and other holidays of KU in that period

Time Sheet Period

: July and August

Summary of Work in the Period : Weir Head Measurement Mechanism Date & Day

Location

Major Activity of the Day

15

Friday

TTL

16

Saturday

TTL

17

Sunday

TTL

Literature Review on Weirs , ISO 1438/1

18

Monday

TTL

Literature Collection on Head Measurement Mechanism

19

Tuesday

TTL

Literature Review on Head Measurement Mechanism

20

Wednesday

TTL

Literature Collection on Head Measurement Mechanism

21

Thursday

TTL

Literature Review on Head Measurement Mechanism

22

Friday

TTL

Further Literature Review and Design Review

23

Saturday

TTL

24

Sunday

TTL

Reviewed ULM-70 and Surveyed the installation of weir

25

Monday

TTL

Literature Review on Calibration and on site

Remarks

Work identification, preparation of work schedule Holiday

Holiday

90

measurement 26

Tuesday

TTL

Literature Review on Types of Flow Meters

27

Wednesday

TTL

Design Review of Float method for head measurement

28

Thursday

TTL

Model preparation in solid works for weir at TTL

29

Friday

TTL

Literature Review on buoyancy (Archimedes’ principle)

30

Saturday

TTL

31

Sunday

TTL

Design of the Prototype

1

Monday

TTL

Design of the Prototype

2

Tuesday

Workshop Fabrication of the prototype

3

Wednesday

Workshop Fabrication of the prototype

4

Thursday

Workshop Fabrication of the prototype

5

Friday

6

Saturday

TTL

7

Sunday

TTL

Testing of the Prototype

8

Monday

TTL

Further improvisation made in the design to mitigate the

Holiday

Workshop Further improvisation on the design and fabrication Holiday

problems encountered 9

Tuesday

TTL

Drawn different conclusions from the design

10

Wednesday

TTL

Report Preparation

11

Thursday

TTL

Report Preparation

Summary: Total no. of days for internship: 25 days

91

Total no. of holidays in the month: 4 days (weekly and public holidays) Total no. of days supposed to work: 21 days Total no. of days worked: 21 Total no. of days on leave: 0

Signature of Staff with Date

Signature of Employer with Date

92

APPENDIX III GANTT CHART July, 2011 S.N

Task Name

Start

Finish

August, 2011

Duration 15

1

Prepare plans and schedule

15-072011

16-072011

1

2

Literature Review for Weir, Head Measurement, Flow meters and their Calibration

17-072011

27-072011

10

3

Design Review for Weir and Field Survey

21-072011

28-072011

7

5

Select an appropriate one for the site

28-072011

29-072011

1

6

Prototype Designs and Fabrication

29-072011

03-082011

4

7

Test of the prototypes

03-072011

05-082011

2

8

Document the results and draw conclusion for recommendation

05-082011

06-082011

1

9

Documentation

15-082011

08-082011

23

10

Prepare For Presentation

08-082011

09-082011

1

93

17

19

21

23

25

27

29

31

1

3

5

7

9

11

94