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Pressure and Flow Measurement

I am a student in final year Civil Engg. at IT-BHU, Varanasi. I am analysing BHU water-supply system. I need help in the following two cases:

  1. I want to measure the pressure in taps in hostels.In hostels, we have two types of incoming supply lines: one goes directly to taps, the other goes to tanks on roof. I am interested in knowing how much pressure is there in directly supply line in the hostels. I want to know how this could be done with and without pressure-meters (without pressure-meter is preferred).
  2. In one of the pumping stations in BHU, underground water is pumped by using submersible multi-stage pump. Water is directly supplied to OHT(about 25m. height) and then water goes to main supply line. I want to measure the discharge of pump. It does not seem possible to install any flowmeter in the pipeline between pump and OHT. Any other alternate ideas?

Any suggestion or question is most welcome.

Thank You

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Comments

1. Pressure and Flow Measurement

Dear " TheWind",

If you do not want to use a pressure gauge, which is a much easier thing to accomplish, yo can simply attach a pipe and raise it vertically up . Best way along the staircase then simply measure the height till which the water rises in the pipe when the tap is opened, and multiply it by 0.434 to get the pressure equivalent in PSI.

The pressure will be less  if more than one tap is operational as it gets distributed between the taps.( need not be equally)

For measuring the discharge of the pump where it discharges in to the OHT, you could calculate the volume of water getting filled in the tank in a fixed interval of time,

 or

measure the inner dimensions of the tank and calculate the area in meters, measure the starting water level in the tank, and see how long it takes to raise the water level by 1 meter in the tank, ( 1 cubic meter is equivalent to 1000 liters), and back calculate the discharge per hour.

or use a fixed volume container ( bucket of 20 liters) and see how much time it takes to get filled, use a stopwatch for this and do this atleast 3 to 5 times and use the average of the value that you get.

However, I must say, if you are a third year student of Civil engineering, there is something wrong in what is being taught to students as basics.

All the best

Dr. Anil Lalwani

www.wellwaterworks.com

2. Thanks for the reply.   I am

Thanks for the reply.

I am sorry for late response here.

All the methods suggested by you have been either tried or thought by me. I am looking for forward for some method so that data could be measured repeatedly and not just one time. I would like to give some additional clarification with respect to measuring discharge.

In the method of measuring rise in water level in OHT, we have to shut the supply off since if the supply is on, the change in water level will depend on inflow-outflow. (I will talk to personnel concerned and tell you whether it is possible or not.) The second difficulty is that with rise in water level the head on which the pump is operating will change. Hence if I want to find the efficiency of pump, it doesn't help.

Suggestions are most welcome.

Any sort of clarifications will be provided.

I have decided to get a pressure-gauge to measure pressures at different points within BHU.

Dr.Lalwani, why do you think there is something wrong with what is being as basics?

 

Waiting for reply and comments.

Thanking You

TheWind

3. Flow measurement

Thanks for your reply!

I have decided to use pressure gauge for pressure measurement since it is faster, and repeatedly done at various locations within BHU.

For flow measurement using rise in water level in OHT, following problems are there:

  • Rise in water level in OHT depends on inflow-outflow. Outflow has to be restricted in order to relate rise to inflow. This will mean shutting off supply. I will have to talk to personnel concerned with this.
  • The second thing is that with change in water level, the head on which pump is operating will change. Hence, it would not be helpful to find the efficiency of the pump. But I think this is a minor effect. Any comments on the extent of change in discharge due to rise in water level in tank.

And apart from this, there are some more pumps with no OHT. Any idea how to measure their discharge?

 

Thaking you,

Malay Goel

Civil Engineering UG Student

IT-BHU

4. Thanks, still some more doubts.

Thank you for your comment. I still have the following doubts:

  • About Pressure Measurement: I have already tried your method(raising the pipe vertically up) although in a different manner. I measured the discharge of my nearest tap (using the same bucket-time thing). Measured its Internal Diameter. Hence found average velocity(Q/A). From this I found the total energy head, (V*V/2g, assuming datum at tap level and atmospheric pressure). It came out to be 2.38m. I believe that your method and this method would give same answer. I am just looking for quicker method. Now, I think to turn to pressure-gauges. Any suggestions or questions?
  • All the methods which you suggested for measuring discharge have crossed my mind. The one with bucket is not possible since there is no chance of making intrusion in the pipeline anywhere in the network. What everyone at the pumping station is suggesting is to measure discharge by Ultrasonic flowmeter, but it will be expensive

The method of measuring water-level. I have thought about this. The only problem is that there is one inlet pipe to the OHT (from pump) and one outlet to distribution mains. In order to perform the experiment, I would have to shut-off the outlet. And also, with increase in height in OHT, the head on which pump is operating (difference between groundwater level and tank level) will increase and it will influence the discharge of pump. How to take it into account?

Any suggestions or questions?

  • Why do you think there is something wrong in what is being taught?

 

Thanking You

TheWind

5. Dear TheWind, I feel

Dear TheWind,

I feel extremely sad that a final year student of Civil Engineering is asking on a public forum how to measure the flow or pressure in a pipeline.  It doesn’t reflect so much on you, as it does on your alma mater. I wonder why did you not look up Modi & Seth, or Ven Te Chow, the standard textbooks in India for graduation level hydraulics. In any case you first need to brush up your pipe flow hydraulics, because unless you do that, even if you are able to measure the pressure, what will you do with that measurement ?

Consider a supply tank at a height, connected to three taps at a lower level by three different lines, one is a 1” dia pipe, other is a 0.75” dia pipe, and third is also a 0.75” but with a valve in its middle and the valve is only marginally open. If you close all the taps and measure the pressure near the taps, the pressure in all three lines will be same, and equal to the height difference between the taps and water level in the tank. This is called static pressure.

Even though the pressure under static conditions is same in all three, yet you can guess that when you open the taps, the flow rate isn’t going to be same for all three. Why ? So, lesson #1 is – it is not the pressure alone that matters. In electrical analogy, take a 6V battery eliminator, measure the voltage across it, now connect a load and measure the voltage again. See any difference ?

In Modi & Seth, you will find an equation Hf = f (L/D) (V2/2g)

Ponder over what it means, particularly the  Hf and the significance of  (V2/2g).

Wish you all the best.

Chetan Pandit

6. Clarifications for Mr Chetan Pandit

Sir,

I think that I have not presented my problem in a good enough manner. I am once again trying to explain it here.

Water-supply system of BHU is about 35-40 years old. There are 14 submersible pumps. I want to measure the total water consumption in BHU. For this purpose I need to know the discharge from individual pumps. Now, the piping systems is totally underground except some valves and electric supply system just above the bore. And Electric and Water Supply Department,BHU is not allowing me to make any intrusions in the piping system for any flow meters like orifice meter, venturimeter etc. They are suggesting to use ultrasonic flow-meter, but these are quite expensive and I cant afford these. I am looking for some method to measure discharge in existing pipe system with minimum possible cutting of pipes etc. (I will get EWS,BHU to cut the pipes if I need to)

Once, I know the discharge I would like to measure the head delivered by the pumps so that I could analyse the performance of the pumps in terms of efficiency(electric meters are already installed for electrical power consumption) 

Referring to the problem which you gave above: The Three Tap problem

I would like to say that I am well-versed with what you say and the problem has been discussed in lectures. I am referring to Fox-McDonald here. I know the concept of pressure drops in pipe flow and why in certain situations booster pumps are required. The purpose of measuring pressure in taps is to find out the variation in pressure with location in BHU and with different combinations of pumps running. There are 14 pumps and 6 OHTs. I would like to analyse how the position of OHTs, pumps and tap outlets is affecting (1)the head in taps(outlets at hostels and colleges) and (2) the head to be delivered by the pump(hence affecting the efficiency of pump)

I would like to have your comments on the above as well as some doubts below:

  1. When analysing practical problems like the one I am involved in, I know Hf and hence can estimate the friction factor "f". How to interpret this friction factor, i.e., can it tell something about the roughness of the pipes, the distribution efficiency, the deterioration of pipes etc.? I am acquainted with Moody's Diagram and Darcy-weisbach equation; any other "field" equation will also be welcome.
  2. I measured head(dynamic) in one of the taps here and it was nearly 2.3m. Is it good enough? What minimum pressure should be there in a good "water-supply" system? I read US standards and it was 280 kPa but they have kept the fire flow in mind. We in India don't design water-supply for fire flow. Then what would be the criteria to determine this minimum value?

Thanking you

TheWind

7. Query of BHU student for pressure and discharge measurement.

For pressure measurement without pressure gauges.

This can be done by measuring the differential pressure method, considering 0 pressure at the flowing end of the tap. Measure the discharge by observing the time taken to fill a known volume container. Note the connection diameter and the length of the connection pipe from the main pipe, assume a certain c value depending upon the connection pipe age and condition, you can safely assume to be around "80". From the formula Hf=10.67*(Q in cumecs)^1.85/C^1.85/d^4.85*L should give you the required result in Kg/sqcm. the discharge end of the connection being 0 the resultant pressure would be the pressure of the main at distance L from the discharge pt., (hf =10.67*(Q/C)^1.85*L/D^4.87.

The pump discharge can be measured either by noting the tank volume that gets filled in a given time. ( provided the tank has a functional level indicator), or actually measuring the tank level difference ( if the tank top is accessible). 

Alternatively if there is facility of inserting pressure gauges at two points located at a convenient distance (to be measured and noted), the method narrated in the first para would be useful in quantifying the discharge of the pump. In this case the pressure differentials of the two observed point and the known length and assumed C could provide us with near approx. discharge data.

For further clarification please contact me on mcmehta49@yahoo.co.in

M.C.Mehta 

8. Pressure and discharge measurement as required by the BHU studen

Dear Dr Lalwani,

The pump discharge measurement at the head at which it is presently pumping needs to be considered, the bucket measurement at the ground level would provide the data at the ground level head of the pump, that would be not matching its operational head, which the student wishes to measure.If the OHT is not accessible (normally they are not) then the bucket measurement would fail to provide the required results.

Kindly requested to comment on this point.

with regards,

M.C.Mehta

9. Pressure and discharge measurement as required by the BHU studen

Dear Mr Mehta,

Actually speaking I really don't think any comment is really required, but as you have specifically asked me to comment I will...

Practically looking at any tank construction, there is always an access to all tanks that I have come across, agreed that in some of the older construction, the ladder to climb up is in a bad condition.

Unless, there is a major design flaw in the construction, or the concerned party has eliminated some part of the basic essential requirements to save on costs. (bad prat ice)

Measurement using a bucket can be done only for low discharges, for higher discharges the bucket would not be possible as the time would be in seconds and there is a higher possibility of personal error.

And as you are telling me that the Over head tank itself would be inaccessible, then there is no point in talking about measurement of discharge at that point at all.

In such cases one has to restrict the measurement at the UG tank only and assume that all the water is being pumped to the OH tank only.

There will have to be a continuous record of the draw down in the UG tank to get accurate values of discharge at OH tank, taking in to account the increasing head due to the draw down in the UG tank.

Well if this answers your curiosity,.

Regards

Anil Lalwani

10. >I think that I have not

>I think that I have not presented my problem in a good enough manner.

I fully agree. And after your second attempt, I am still not clear exactly what are you trying to investigate.

> . . . I want to measure the total water consumption in BHU. . . . so that I could analyse the performance of the pumps in terms of efficiency(electric meters are already installed for electrical power consumption) . . . . I would like to analyse how the position of OHTs, pumps and tap outlets is affecting (1)the head in taps and (2) the head to be delivered by the pump. . .

So, you want to investigate a) total water consumption, b) energy efficiency of submersible pumps, and  c) How the position of OHT, pumps, and tap outlets is affecting head in the taps, and head to be delivered by the pump. Is that OK ? Or any thing else ?

a) and b) are clear and we may be able to find a solution to your problem. But what do you mean by “analyse how the position of OHT, pumps, and taps is affecting. . . ” ?

> How to interpret this friction factor, i.e., can it tell something about the roughness of the pipes,

Actually, you can’t even estimate the f, far less interpret it. Because there can’t be one size of pipe all through and the total head loss is the sum of losses in each section of pipe, it will depend not only on the pipe roughness, but also on the number of bends joints and other such fixtures that cause head loss; there is bound to be a lot of sediment in the pipes that are 30 years old, which not only increases f, but also effectively reduces the D, etc. There is absolutely no way you can separate the impact of each of these to get a value of f.

> I measured head (dynamic) in one of the taps here and it was nearly 2.3m. Is it good enough? What minimum pressure should be there in a good "water-supply" system?

But why are you interested in the pressure at all ? The purpose of a higher pressure is to obtain a satisfactory flow rate. Pressure is relevant if you are planning to install something that needs a certain pressure and want to know if it will work. e.g. the single lever mixers which need about 40’ of head. But if a single lever mixer is already installed, where is the need to measure the pressure ? You open the tap and see whether you get adequate flow rate. If you do, then the pressure is “good enough”, else the pressure is less. What will you do with a pressure measurement ?

To explain with an analogy, different lighting devices – incandescent bulb, fluorescent, CFL, halogen, LCD etc. have different sensitivity to voltage (= pressure). If I am to buy a lighting device, then I may want to measure the voltage, to decide which type to buy. But if the device is already installed, then I only need to switch it on. If it gives a light as is expected of it, then the voltage is sufficient, else it isn’t.

I still think the main problem here is, you are not clear what are you trying to investigate and how. Methods of measurement come later.

Chetan Pandit

11. Another Attempt

  • So, you want to investigate a) total water consumption, b) energy efficiency of submersible pumps, and  c) How the position of OHT, pumps, and tap outlets is affecting head in the taps, and head to be delivered by the pump. Is that OK ? Or any thing else ?
  • a) and b) are clear and we may be able to find a solution to your problem. But what do you mean by “analyse how the position of OHT, pumps, and taps is affecting. . . ” ?

I believe that for a given configuration of pumps and load centres(hostels, labs etc.), different position of OHTs determine different HGLs in the distribution system. Hence, position of OHTs can be optimised so that the pumps operate under lesser head. Decrease in head(towards the design head) would result in lesser power consumption, more life of pumps etc. Although it may not be possible to change the current system, it will give an idea that how much cost of operation could have been saved by proper design of distribution network. Also, with the given configuration of pumps and OHTs and load centres, I would like to investigate the effect of different combinations of pumps running simultaneously. For example, the schedule of operation of pumps is purely on the basis of convenience rather than any technical background, I would like to determine what combinations should be run at different times so that pumps operate as much near their design parameters as possible.

  • Actually, you can’t even estimate the f, far less interpret it. Because there can’t be one size of pipe all through and the total head loss is the sum of losses in each section of pipe, it will depend not only on the pipe roughness, but also on the number of bends joints and other such fixtures that cause head loss; there is bound to be a lot of sediment in the pipes that are 30 years old, which not only increases f, but also effectively reduces the D, etc. There is absolutely no way you can separate the impact of each of these to get a value of f.

Thank you for this. I was thinking on the same line, but was hopeful of some solution. So, if I want to determine the condition of pipes (i.e. rusting, leakage, reduction in diameter) how should I proceed?

However, for the purpose outlined in the first answer, will not it be reasonable to determine equivalent "f" for different sections of the network e.g. from pipe to OHT, from OHT to main line, from main line to hostels etc? Please comment.

  • To explain with an analogy, different lighting devices – incandescent bulb, fluorescent, CFL, halogen, LCD etc. have different sensitivity to voltage (= pressure). If I am to buy a lighting device, then I may want to measure the voltage, to decide which type to buy. But if the device is already installed, then I only need to switch it on. If it gives a light as is expected of it, then the voltage is sufficient, else it isn’t.

In my case, everything is in operation. If the voltage is too high, the lighting device will burn up. This is not so in case of water-supply, I believe. Increased pressure will mean more swift discharge in taps, a boon for users. If there is possibility of increasing the pressure with some small changes, I think it is worth it. The changes I am talking about are in the first answer. I wanted a value of  "minimum pressure" for water supply in hostels and colleges since changing configuration of distribution components may increase or decrease the pressure being supplied now. I want to remind here that I am a student and this study is purely initiated by me, and the there is a very high chance that none of it will be implemented. I am doing this study to learn the different aspects of water-supply(including hydrology, hydraulics,drinking water quality, wastewater treatment), and I am finding it challenging and involving. I wish to be a water-supply engineer---- who deals with drinking water supply, irrigation, hydropower, river and lakes. And this is just a small step.

  • I still think the main problem here is, you are not clear what are you trying to investigate and how. Methods of measurement come later.

This discussion has helped me to clarify my own doubts. Also, I am leaning here how important it is to express clearly to others. Thank you for your patience.

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