Setting a Pressure Relieve Valve

Tell us, what do you think it is?

trick question ?

"trick" wouldn't have been my choice of words.

to me its the point where it starts poping...

The set point is where the valve first opens. Full opening occurs when the "accumulation" pressure capacity reaches 110% of opening pressure; this is also the pressure where the valve's capacity is rated.

Once it pops the pressure can't go any higher. Therefore that is the setting. (as soon as it opens, the degree of opening doesn't matter).

That confused me..so in simple words considering follwing:

set pressure = 100 Psig

+3/-3 % = 103 / 97 Psig

so the valve can be start popping at 97 Psig. if it pops before 97 Psig the technician needs to adjust the setting. please explain if iam right or wrong.

Here our techicians said that valve could pop from -10% of the set point.

There's no need to adjust it unless you must have 100 psig in the vessel, eg for process reasons. If it pops too often you could increase it to 100 psig. +3/-3 % sounds like a reasonable tolerance, -10% seems high to me. When it has started to relieve, if the reason for over-pressure is cured, eg switch a pump off, it will likely not reseat until a pressure below set pressure is reached. This could be -10%, perhaps that is what your tech is thinking of.

As others have said, the pressure is allowed to rise to 110% of set pressure when relieving, and you need to tell the valve supplier the maximum flow it must handle so he can size it. In practice most valves are sized generously so the pressure rise is usually less than 10%.

I believe the vessel design pressure can equal the pressure relief valve set pressure, so the vessel design pressure can be exceeded in a relief situation (somebody will say if that's wrong). But for a bit of safety I prefer to have the relief valve set pressure above the max operating pressure by a margin, to avoid frequent popping, and vessel design pressure 110% of relief valve set pressure.

during the testing of safety spring relieve valves, according to EN 12266. one of the tests is ; seat must be leak tight at 90% of it's setting pressure during a certen amount of time (depending on it's set pressure).

what application is this safety relief valve? 30 lb boiler, 150 lb water heater, other?.

also thermal expansion will start to relieve excess pressure.

Butch you said that very well. That was the point I was trying to make. The pressure that it opens IS the set pressure. I see a lot of hydraulic systems solely regulated by pressure relief valve. The pumps always pump, and the relief valve unseats at SET pressure and seats again at a slightly lower pressure. I totally agree that if pressure continues to build with a relief unseated (open), that it is undersized with regard to flow. With regards to safety, the relief should be set below vessel or system pressure. It's job is to protect your system, much like a circuit breaker in electrical systems.