High Pressure Safety Valves on boilers in NYC

[rainman13]

I understand that 2 valves are needed when the heating surface area is over 500 ft, I also understand that the two valves are set to pop at different pressures intentially...say one ate 165psi and the other at say 185...but I don't understand why they wouldn't simply both pop at pressures very nearly the same. Is there a simple reason why for this? Thanks.

 

[gerhardl]

Any safety pressure release device has a given (statistical and/or tested) repetitive accuracy. Ergo, if set at a certain pressure it will pop at the preset pressure, with the given accuracy (plus or minus within, say a few percent, of the set pressure).

The rest is pure logic:

How far apart the cracking pressure for the two valves are, is depending on the set pressure and the accuracy respectively of the two valves.

At a rising pressure high eneough to reach past the first and activate the second valve, the time between the two pops is depending on how fast the pressure arizes.

One set lower than the other: the normal control logic. One high alarm to warn and possibly control alone. One 'high, high' to increase capacity and give a higher warning level.

 

[rainman13]

Gerhardl

Thanks for the insight, it is much appreciated.

 

[CRG]

The two valves have different set points to reduce chatter. Google "PSV chatter" and I am sure that you will find a lot of good information.

http://webwormcpt.blogspot.com/2008/08/psv-chaterring-is-destructivethe-ways.html

 

[rainman13]

CRG...good point, I had not considered that. I appreciate the insight.