Pressure Drop Correlation with Fluid Loss 6

[ziptron]

Hi Everyone,

I have an underground water main that is quite old. It is an 8 inch diameter ductile iron pipe and is approximately 200 ft long. Wanting to see if it is still fit for service, we pressurized it to 150 PSI and left it over night. Next day, we lost 25 PSI. So 25 PSI loss over 24 hours.

Probably too big of a pressure drop to attribute to temperature fluctuations right? .. so Clearly there is a leak in there somewhere.

Are there any rules of thumb to use to see how much water we are actually losing?

Thank in advance,

 

[cvg]

you could have had leakage past a valve, you can't confirm the pipe is leaking. that much pressure loss could be just a very small leak. measure the amount of water added back in to increase back to 150 psi.

 

[BigInch]

Pressure drop with liquid usually happens with a very small volume. There are often other unknowns, such as air content, pipe restraint, and temperature changes, that will give significant errors to theoretical calculations. You might be able to correlate to temperature drop, but only if you are sure that there is no air left inside.

Best method at this point would be to pump in more water and see how much volume it takes to raise the pressure to 150 again.

Rule of thumb is to first measure what you pump into and correlate that to the calculated inside volume of pipe. At least that way you are less likely to contain a lot of air.

 

[ziptron]

Ahh.. great idea regarding measuring how much is required to raise pressure back up. I realized that if this was a leak (still unsure) it would be a small one but I had no idea how to figure out how small. The water meter I have access to is tiny, though as you say, I can simply measure the small amount of water required to pressurize this back up.

So you're still thinking that 25 psi could be related to temperature drop?

Any good way to account for temperature changes?

 

[LittleInch]

Try this.

http://www.eng-tips.com/faqs.cfm?fid=1339

Probably 1 or 2 degrees C

If its ductile Iron is this push fit pipe? It might just have expanded a mm or two?

a water main that is this tight is pretty good. Many would lose all the pressure in a few minutes

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ziptron]

Thanks! That fits well with my gut feeling as well.

 

[bimr]

Here is a rule of thumb:

http://www.nwpipe.com/wp-content/up...entitious-and-Metallic-Pressure-Pipe-2010.pdf

Q: What is the purpose of the “testing allowance” in ANSI/AWWA C600?

A: ANSI/AWWA C600 “Installation of Ductile-Iron Water Mains and Their Appurtenances” requires that newly installed Ductile Iron water mains be hydrostatically tested at not less than 1.25 times the working pressure at the highest point along the test section and not less than 1.5 times the working pressure at the lowest point of testing.

After the air has been expelled and the valve or valves segregating the part of the system under test have been closed, pressure is then normally applied with a hand pump, gasoline-powered pump, or fire department pumping equipment for large lines. After the main has been brought up to test pressure, it is held at least two hours and the make-up water measured with a displacement meter or by pumping the water from a vessel of known volume. The make-up water is called the “testing allowance,” and the allowable amount is a function of length of pipe tested, nominal diameter of the pipe, and the average test pressure. The hydrostatic pressure test helps to identify damaged or defective pipe, fittings, joints, valves, or hydrants, and also the security of the thrust restraint system.

The “testing allowance” is not a “leakage allowance.” Properly installed Ductile Iron Pipelines with properly assembled joints are bottle-tight and do not leak. The “testing allowance” is, however, a practical measure used to maintain the pressure, which might actually drop because of factors other than leakage, including trapped air, absorption of water by the cement lining, extension of restrained joints and other small pipe-soil movements, temperature variations during testing, etc.
(Issue: Spring/Summer 2002)

http://www.dipra.org/ductile-iron-pipe-resources/frequently-asked-questions/installation

 

[rconner]

All good information/responses (particularly the tried and true approach of BigInch). If you repeat the pressurization one or more times, and if the pressure loss behavior is identical or gets worse, you may very well have a leak, as opposed to the other possibilities in hydrostatic testing mentioned. [I would just add you may not actually have to (try to) hold the pressure over many hours of for full testing term; instead, with a watch or time on cell phone record the pressure vs time held -- if it loses at the same rate of presssure loss vs time both times, you may very well have a leak. If not and the apparent loss rate is improving, you may wish to keep pumping or testing until the system stabilizes satisfactorily for a proper longer term test.]

 

[georgeverghese]

If this pipe were completely liquid filled, even a small leak would have resulted in total pressure collapse if the temperature reference point is the same.
A 25psi drop on a 150psi line tells me youve still got some air in this pipe.

 

[ziptron]

Thanks everyone for your speedy and helpful replies. I literally had to give everyone in this thread a "great post" star because I couldn't decide who helped more .

Thanks again!

 

[BigInch]

Thanks for all the stars.
Now get all that air out of there and see if it holds.

 

[JStephen]

I was thinking that they made hydrotest pumps with meters specifically for applications like this. On a long run of municipal water pipe, they're allowed some small leakage rate, so the procedure is to pressure it up and measure the rate. Check around if you can rent such a pump. Although based on that pressure drop, it may simply read "zero".