Water Main Pressure

[djcbgn]

this is a quick one....

If I have a section of water main that has hydrant on it. The hydrant has a pressure guage that reads 60 psi (static). There are valves on both ends of the water main, which are then closed to isolate this section. What happens to the pressre at the hydrant when the valves are closed? Does the pressure go to zero?

Thanks.

 

[cvg]

1st scenario - valves are 100% water tight, no leaks - pressure stays at 60

2nd scenario - valves are still watertight, but small amount of leakage somewhere between the valves and the hydrant - pressure drops

3rd scenario - valves do not hold perfectly - pressure stays at 60

 

[bimr]

If there are service connections on the water main, the pressure would drop.

If there is an elevation difference between the manholes, the static pressure would also be influenced by the elevation difference.

 

[djcbgn]

I am actually trying to isolate this section of pipe to determine if there are leaks on the line or the service lines. The service connections will be shut off at the meter.

 

[cvg]

good luck, however monitoring the pressure may not work to detect the leaks if the two shutoff valves aren't absolutely 100% watertight.

 

[bimr]

It would be more practical if you did acoustic leak detection:

http://www.sewerin.com/us/about-us.html

 

[cvg]

bimr has the right idea. by shutting off the main and the services, you should be able to reduce the flow to a small trickle and you might be able to detect and locate the leak, if the lines are not too deep. I have seen this work to detect a pinhole leak in buried copper pipe in a residential bathroom, below a concrete slab, below the sink / cabinet. Contractor did it twice and found two separate leaks.

 

[coloeng]

To determine if the valves are leaking by, isolate the line, drop the pressure to about 20 psi and then place a rod attached to a listening device on each valve nut to hear if any water is going through the valves. You can also use a valve key on the valve nut and place your ear on the end of the handle section of the valve key. You should be able to hear if there is any water passing through the valve.

If the valves don't leak by you can then either isolate the line and turn-off any services attached to it. Do a pressure test of the line up to about 100 psi and then monitor the pressure over a couple of hours to see if there is a drop in pressure. If you don't feel comfortable increasing the pressure just monitor to see if it drops below 60 psi. If all the services are turned off, any drop would indicate a leak.

Make sure that you don't have a leak where you connect the pressure gauge. If you find that you have a leak, if it is a major leak, you can then use the acoustic leak detection which should be able to pinpoint the leak.