rural distribution system pressure problem 2

[douginpc]

I’m experiencing a low pressure issue in a new rural system recently installed. The new expansion includes a standpipe. The pressure at the new standpipe location is reported as 5psi. The new standpipe height will be approximately 118 ft therefore requiring 51psi to fill. 5psi isn’t good.

An existing tower is located about 5 miles away. The elevation difference between the two sites varies slightly but the overflow elevations are set the same. All elevations have been re-verified by our surveyor and there isn’t a bust. The new system has no connections made since the storage tower isn’t constructed yet. Since there isn’t any water usage, no friction head loss, the pressure at the new tower site should reflect the static column in the existing tower. Give or take a little but it should be close.

The district isn’t reporting any abnormal meter reading (ie, no breaks). All valves are open. They can force water to the new area by shutting some valves and running an existing pump station. That’s fine for flushing and filling but it doesn’t answer the problem at hand.

The client is being very cool about this. He can’t figure it out either. We’re all confused. Now they’re wanting to install a small pump station into the new system to push water and boost pressure. I’m hesitant until we figure out the root of the problem. I’ve beat this thing to death and don’t understand what’s going on. Am I wrong on something here?? Don’t be shy, let me have it if I am.

Thanks,
Doug

 

[bimr]

You need to verify some things.

1. What is the ground elevation at the existing tower and at the proposed tower.

2. What is the overflow at each point in the first question.

3. Check the pressure gauge to make sure that it is working properly.

4. Does the existing storage tank have an altitude valve.

5. If you are still unclear on the reason for the pressure discrepancy, start at the beginning of the water main and note the pressure and ground elevations as you go to the location of the proposed standpipe.

 

[douginpc]

1. What is the ground elevation at the existing tower and at the proposed tower.

Ans - They vary slightly but the overflows of both tanks are the same. The ground elevations have been verified by our surveyor to make sure we didn't have a bust.

2. What is the overflow at each point in the first question.

Ans - We used the same overflow elevation for both towers to prevent an overflow condition. The new tower will be shorter since the ground elevation is slightly higher at that site.

3. Check the pressure gauge to make sure that it is working properly.

Ans - Multiple guages were used to ensure consistancy. The pressure at the existing tower has been visually verified to the water height in the tower. In other words, the existing tower is full.

4. Does the existing storage tank have an altitude valve.

Ans - No

5. If you are still unclear on the reason for the pressure discrepancy, start at the beginning of the water main and note the pressure and ground elevations as you go to the location of the proposed standpipe.

Ans - We've done that and all the recorded pressures correspond correctly to the elevation difference. It doesn't get goofy until you get to the new tower site.

 

[redbridge]

It has to be a pressure reducer, partially closed valve or something in the line. I would also do some flow test to compare actual hgl and calculated hgl. I have seen new water lines which had 2x4s, whiskey bottles, partially closed valves and dead animals in the line which caused crazy conditions like you are experiencing. We have removed hydrants and flushed lines to get these things out. You are right in finding out what is wrong before installing a pump because it wont solve your problem if something is in the line or if a valve is partially closed. The point it starts to get goofy is the problem. With what little you have told me I would guess that something is in the line or a valve is partially closed. Let me know what the problem is. Good Luck!

 

[douginpc]

Thanks for the replies.

There is no water usage on this system yet until we resolve the pressure issue. It is connected to an existing tower, all the valves are open, so the pressures should reflect the static head of the existing tower. That's the problem, it's not even close.

An obstruction in the line or partially open valve would reduce the ability to flow water (additional headloss). With zero flow the static pressure should equalize as long as the water can get around any possible obstruction.

Thanks

 

[semo]

Could there be high points in the line that have trapped air? Air in the line can give funny and false pressures because ist is a compressible fluid and an air locked line will drastically limit the flow of water.

Make sure air or anything else as mentioned above is out of the line.

I don't know the layout or profile of the line; but, you may have to make several taps with corp stops to get the air out.

 

[cvg]

I think SEMO hit it on the head. Go to your high points, open the hydrant and flush the system. Get all the air out. Note that this should be SOP whenever a new line is put in service.

 

[douginpc]

good advise but flushing has been done already. it's possible that some air remains in the lines but there is no water usage in the system.

if i'm taking the point correctly, air binding would reduce pipe section and result in low flow/pressure conditions while water is moving.

in this case water isn't moving. am i wrong on this, the static pressure should equalize even if some air is present? are there weird circumstances where this might not be the case?

thanks

 

[cvg]

air pocket could cause low pressure at the other tank, regardless of any flow. It would also reduce the amount of flow. It doesn't just reduce the section, it completely blocks the pipe near the high point so there is no hydrostatic link between the two ends of the pipe. Flushing has to be done for the purpose of removing the air from the high point. Hopefully you have a hydrant at the major high points. If not, you should. Open these to blow off the air.

 

[semo]

Here is an article that includes some explanation of air lock. It is a pretty good explanation of what can happen if the air is not purged from the system.

http://www.agf.gov.bc.ca/resmgmt/publist/500Series/590304-5.pdf

If the line was flushed previously, then there must have been flow through it. As the water sets in the line, dissolved air can escape and build at the high points. If all of the air was not purged and this happens, a substantial amount can accumulate and cause the air lock.

Gorman Rupp use to have a demonstration of air lock with a pump providing the water. If air lock is the problem, a small pump may not overcome the forces and you will be in the same place you are now.

If your hydraulic design calculations (including your HGL not dropping below the line elevation) show the line to be able to provide flow, you need to find your problem and add more complexity to the system with a pump.

 

[semo]

Here is another site with info on air valves. It is the one I was looking for when I crossed the other site.

http://www.valmatic.com/technicalpapers.jsp

 

[bimr]

Ans - We've done that and all the recorded pressures correspond correctly to the elevation difference. It doesn't get goofy until you get to the new tower site.

What distance is it from the last good location?

Then it is obvious that you have a blockage or closed valve close to the new site. Do you hydrants or blowoffs along the route?

Have you reviewed this with the installation Contractor and your resident engineer for the project?

I agree with the previous posters that air in the lines will cause weird things to happen. However, the weird things happen when you have two phase flow, not static conditions.

As a last resort, if you can't find the blockage, perhaps you can pig the line. Some of the pigs have transmitters so you can determine the location where the pig will stop.

http://www.girardind.com/index.htm

 

[atook]

Is it that possible there is pressure reducing/sustaining valve along the route somewhere? A valve of that nature would keep pressure reduced even in a static condition.

Barring that, I'd have to say there's a bust in the elevations, I can't make any sense of it!

 

[Yellowtetra]

Doug,

Where are you measuring the low pressure? A hydrant connection or somewhere else? Make sure the place you are installing the gauge is not clogged.

How close to the new standpipe location is the nearest pressure reading that makes sense?

Finally, if everything was working right, what would the pressure be at the new standpipe location?



Dan Barr, PE
Burgess and Niple

 

[kchayfie]

A really goofy suggestion that may be completely off the wall - have you checked all your valves were put in properly? We once had a case where the actuators were put on wrong and they were reporting the valve as open when it was closed, and then trying to drive the valve even further closed when we tried to "close" it to get the valve open. Any chance on your system?

 

[ottomann]

Did you ever figure this out. Huge application of Murphy's Law no dought

 

[douginpc]

We beat the hydraulics to death and air lock was the only logical explanation. We advised them to fill the new tower with an existing pump station (pump around the air locked areas rather than relying strictly on the static head). Once a full tower was achieved we recommended they flush and bleed the lines. The head from the full tower should have been sufficient to push any remaining air out.

They didn't/wouldn't do it. The system operator used the situation as an excuse to justify the installation of a new pump station to the system. He didn't need it but he used the low pressure as an excuse to his water board. They decided they needed it, we told them they didn't. We agreed to design it to keep the peace and they basically told us to get screwed on any additonal fees (funny how they tell you that after you do the work).

They filled the new tower and the whole system now works beautifully.