Shutting down the sprinkler system

[Tregg Smith]

An electrician and I were the 2nd shift maintenance crew in this factory-new on the job when a sprinkler head in the ceiling went off as a result of being too close to a newly installed heater. Panic and an irate foreman. Water building up on the floor. Well we'll just close the valve outside when we determine which one to close and that should stop the water as it bleeds off. It took us at least 20 minutes to locate the valve and close it. Water now several inches deep on the floor and the broken head still going full flow. I thought "it has to stop because the circuit has an open/water going out and because water doesn't compress it soon has to slow and quit. Nope! Water kept coming now for almost an hour. It was only when I opened the drain valve to let the pressure drain the water outside that the flow stopped from the sprinkler head. My question is why if water can't compress didn't the pressure quickly drop because of the broken and open sprinkler head? There must have been maybe a thousand feet of two inch pipe involved in that section of the system. Water was exiting at maybe a gallon in one or two seconds. And btw I turned off the jockey pump that keeps pressure on the system at about 60 lbs. and that did nothing.

 

[JaredJames]

Consider mains water.

You shut off the main valve to the house and turn on a tap. The smaller the opening you create with the tap, the longer it will take to relieve the pressure and the longer water keeps flowing.

The water doesn't have to compress to be pressurised. If the system pressure is high enough (and it's a large enough system) - with only one relatively small vent it will continue for a long time.

You have a sprinkler system with multiple heads, designed to be under pressure and supply and entire building in the even of a fire at an effective flow rate. You then remove all but one of those heads, keep the system under pressure (although not being boosted by a pump) and have it vent through that one head. With a large enough system (as you indicate) it's not stopping anytime soon.

What is today's lesson? Always have individual isolation valves for each head / section of the system.

EDIT: I'd also add that given the nature of the system, it may be specifically designed so that in the event of the pumps / power failing, if activated the sprinklers can provide the required flow rate under the held pressure - for obvious reasons.

 

[maimonides]

One rather obvious source of pressure is elasticity of the tubing. Is it PP or PE?

 

[russ_watters]

How about head? What floor were you on and how many floors are there?

Id also be surprised if there wasn't an isolation valve on the floor.

 

[Tregg Smith]

"The water doesn't have to compress to be pressurised. If the system pressure is high enough (and it's a large enough system) - with only one relatively small vent it will continue for a long time."

No head-all on the same floor. Iron pipe would expand but very little and the open on the system would I think quickly bring pressure to zero. Thanks for your inputs but I don't quite see how there could be pressure on the system with no auxillary pump trying to maintain it and access to city water pressure disconnected. It was going full flow for almost an hour until I opened the drain valve.

 

[Tregg Smith]

"You shut off the main valve to the house and turn on a tap. The smaller the opening you create with the tap, the longer it will take to relieve the pressure and the longer water keeps flowing."

I agree. Done this many times but even with a 12 inch city supply providing 60 lbs. pressure it still seems strange to me but that's what it did. Thanks for your reply.