Blowoff Valve on Water Main
Blowoff Valve on Water Main
(OP)
I am looking for the proper type of valve to install to bleed pressure off water mains when they have to be taken down for maintenance. The specific instance is a 72" main with a 6" tee and valve piped to open air to release water to drop pressure to zero. The problem I see is with a large volume of water (big pipe at 100psi) moving past a valve as it's opened, it's likely to have some damage to the gate. I see that RW gate valves specifically say 'throttling' is not an intended use. Any suggestions on a readily available municipal water works valve for this application?





RE: Blowoff Valve on Water Main
Alternatively the tortuous path CCI type boiler let down valve could be used but they are mega bucks.
RE: Blowoff Valve on Water Main
RE: Blowoff Valve on Water Main
RE: Blowoff Valve on Water Main
RE: Blowoff Valve on Water Main
RE: Blowoff Valve on Water Main
the 72" water main is full of water which is only marginally compressible. Isolate the water main, open your vent valve on the 6" Tee, and you'll lose one or 2 teaspoons of water as the water main vents to atmospheric pressure. If you want to DRAIN the water main, that will take longer. You'll have to open a vent valve and a drain valve, and provide someplace for the water to go. None of this has to be done with a throttling valve, and the most throttling that will occur is isolating the 72" main slowly enoufh to avoid water hammer.
Personally I hate gate valves for almost ANY application, because they almost invariably have metal-to-metal seating surfaces which are so likely to leak. I'll take a ball valve or a butterfly valve or a plug valve anytime for isolation if the valves are rated for the process conditions. On the 6" line, the Ball and Plug valves might be a it pricey, but the Butterfly will be economical.
RE: Blowoff Valve on Water Main
One other thing, using a plug valve will restrict water flow and slow the bleed off process due to the the fact that the valve only gives you an opening about 80% of the diameter of the pipe where as the gate valve is a "fully open port" Also most plug valves are used in sewer service and may or may not meet the requirements of NSF 61 so you may want to keep an eye out for that too.
Can the section of line in question be isolated on side from the supply side first to bleed most of the pressure off prior to opening the drain valve?
RE: Blowoff Valve on Water Main
BTW, metal to metal are disc valves and they leak a little but they are OK for this application, aside from the leakage, which makes it bad as a valve. the gate valve in question is a resilient wedge valve.
From what I've seen here, a plug valve will be my choice on the next installation and when we repair the current RW valve. The additional cost is not an issue as this application is not common in our system. The repair will be a nightmare, but the new plug valve should prevent that from reoccurring.
Thanks to everyone for the advice and help. It has been most useful.
RE: Blowoff Valve on Water Main
RE: Blowoff Valve on Water Main