What could cause the pressure in a wet pipe fire sprinkler systen to rise from a normal of 75psi to 350psi? The pressure immediately drops back to normal when a drain valve is even slightly cracked.
Trapped air, maybe?
Peace,
mangeorge
Where are you measuring this (what part of the system)?
I suppose water hammer could cause this, briefly. If there aren’t any backflow valves on the thing.
There are a couple guages near the foam check valve, near the tank.
There’s another guage at the valve where the system joins the utility water, and it indicates the same pressure. And the pressure staus high until we open the drain valve.
Municipal water? No pumps in the building involved?
What’s a “guage”?
If you shut off the input to the system, say with a check valve, the water can expand and cause a huge rise in pressure which a small leak will promptly alleviate.
Any system which can be closed off like that should have a pressure relief valve. Like a water heater does.
Yes, city water. It’s for a pilot plant inside a single story building. There are there wet glass bulb sprinklers, and when one of these releases it causes the foam to flow to two foam nozzles by activating the foam valve.
It’s a dyslexic gauge.
Water expands at ambient (50 - 80 degrees F) temperatures?
Are you serious? How do you think a thermometer works?
We get this problem periodically on some of our systems in the summer months when it gets really hot inside.We had to switch to a different type sprinkler head because the pressure would get high enough to blow one off once in a while.
Is there anything like:
pressure boost pumps that may be triggering when not needed?
Air pumps to fill the lines with air (wet dry system) that could be generating excess pressure.
IF its only a single story building that would rule out head pressure from a stuck/malfunctioning check valve.
The liquid in a common glass thermometer is usually alcohol. And the amount of liquid in the bulb is huge compared to what’s in the tube.
I’d guess that it is caused by a jockey pump.
I’ve never had that happen, but if it does in this application we’re going to have one hell of a mess.
We’ve had techs from two sprinkler companies, and they all say they’ve never seen any system go up this much. Maybe a few psi, but not 460%.
There are no pumps in the system.
It seems to me that if the problem were temperature related, the pressure would return to normal when things cooled down. Unless there’s a check valve.
I’ll check tomorrow and see if I can dig up a report to see what they did in our case. It still happens once in a while but it’s not such a big deal if the sprinkler heads can take it. It’s a VERY big deal otherwise.
They did make some changes to keep it from happening as much though. Once in the past two years that I can recall.
Wow! I live a sheltered life. I’ve been at this job for 25 years, and have never had a sprinkler head fail. Our weather here is pretty mild.
We have had a couple function correctly. Grudging kudos from the tennants. 
I’m looking forward to hearing about your problem.
I am not sure I follow you. Are you saying alcohol expands with heat but water doesn’t? All common materials expand with heat, some more than other.
What does that have to do with anything? The liquid in a thermometer has empty space into which it can expand. If you have a length of steel pipe full of water the water has nowhere to expand but into the steel and the pressure grows very rapidly.
It’s not nice o play dumb in GQ.
mangeorge, my previous comment was wrong.
I asked a friend who installs sprinklers and he said high temperatures (as **Hong Kong Fooey **mentions) can cause pressure increases. The company my friend works for often puts pop-off valves in their installations.
That’s what they’re doing with ours. So when the system cools, the pressure should go negative.