Pressure Relief Safety Question...

RobLC

We changed our pressure relief valve on our steam boiler with the same one that's been there for many, many years.  It is a Cobraco Industies, Inc,, 13500 Series.



Here is a warning on the tag of the relief valve, verbatim:



<span style="font-size:12pt"><strong>The minimum required operating pressure margin for this type of safety valve is 4 PSI.  UNDER NO CIRCUMSTANCES SHOULD THIS MARGIN BE LESS THAN 4 P.S.I.!  </strong></span>

<span style="font-size:12pt"><strong> </strong></span>

<strong> </strong>

<span style="font-size:12pt"><strong>Failure to maintain this margin may result in leakage past the seal and an accumulation of deposits on the seating surface.  Excessive deposits may prevent the safety valve from operating properly, and a dangerous pressure buildup and equipment rupture may result.</strong></span>



Low pressure is what we're looking for when operation a steam boiler, 1 to 2 psi, maybe in some cases 3 psi.



The question is:



If we only want 1 - 3 psi, why does the tag on the relief valve caution us that a minimum of 4 psi must be maintained to avoid problems?



Any pros or knowledgeable DIY'ers offer an explanation?



Thank you for your inputs and thoughts on this.



RobLC

Unknown

What they mean is,,,

Do not use a 15PSI relief valve if the normal system operating pressure is 13PSI.  ;-) 

Jamie Hall

Or another way to put it

is that the pressure relief valve setting must be at least 4 psi greater than the greatest normal operating pressure of your system.  Reason being is that if the pressure gets close to the relief pressure, the valve may 'lift' slightly from time to time, which leads to corrosion or 'wire-drawing' on the seat and valve itself, which may lead to either a leak (not good) or the valve failing to open when it should (very bad).

On the other hand, the pressure relief valve rating must be less (I forget the exact factor, but it's a good bit less) than the pressure test rating of the rest of the system -- particularly the boiler.  And you must have a pressure gauge on the system which reads to at least twice the pressure relief valve rating.

RobLC

Thank you, Dave and Jamie Hall...

Ok, I understand what you're saying, i.e., if your system's normal operating pressure runs at 14psi then you shouldn't use the 15psi relief valve.



We have a residential gas fired steam boiler that heats approximately 2800 sf.



The Nat Grid tech says the 15psi relief valve is ok for our boiler and we have the 30psi gauge on the front.



Thank you for taking the time to answer my question.



RobLC

Unknown

I think I can speak for Jamie too,,,,

your welcome Rob. ;-)

Glen

That 4 psi -

is the blow down range of that particular safety valve; e.g. a steam safety valve will typically open at its pressure setting and then will remain fully open until the blow down range has been met. So again - if your boiler is operating at 13 psig but the safety valve is rated at 15 - the safety valve will probably not close fully because of the 4 psig blow down range requirement. Steam safety valves are not designed the same as hydronic relief valves - which will relieve an over pressure only. Lots of good information on the attached website re safety valves and relief valves.

http://www.absa.ca/IBIndex/newsindex.aspx#PressureReliefDevices

RobLC

Thank you, Glen...

Very good, I understand better now.



We were concerned - - your explanation has given us piece of mind.



Thank you for the great resource on safety valves and relief valves.



Thank you for taking the time to answer my question.



RobLC