ASME SEC 1 PSVs set point

[nasim123]

Dears
We have 3 PSVs in our Boiler and the boiler operating pressure is 40Kg/cm2g. Can you please inform us, at what pressure we need to set each safety valve. As per code how much gap we need to keep on each psv?

Presently it is

1st PSV - 45Kg/cm2g,
2nd PSV - 49Kg/cm2g
3rd PSV - 49Kg/cm2g

Is it correct? Your valuable advice pls.

Thanks

 

[The Obturator]

nasim123
You state ASME I, so presumably you have installed a suitably qualified SV with a "V" stamp. What I do not know from your question is whether these valves are installed separately or in parallel/series/tandem. Hence, is your question "gap" relative to each valves setting tolerance or the staggering of set pressures around a nominal setting of 40 Kg/cm2 ?

Per ASME I rules the following applies where P = set pressure
Valve to be fully open at 3 % Overpressure (P x 1.03)
(up to 6 % is allowed in the case of multiple valve installation - caution staggered settings required).

Blowdown (closing) 2 to 4 % (% of set pressure below the set pressure)
(P x 0.98) to (P x 0.96)

At 40 Kg/cm2 the set pressure tolerance is +/- 10 PSI (+/- 0.70 Kg/cm2)

No temperature correction is applicable since the SV would normally have been set on steam.

Above all, the set pressure should never be greater than the protected vessels maximum allowable allowable pressure MAWP.

Hope that helps.

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.

 

[nasim123]

Dear avalveman
Thanks for your feedback. "V" stamped ASME Section 1 Safety Vales are installed in our boiler separately.(One Drum having 2 SVs with different set point 45 & 49 KGc/cm2g - 1st floor). 3rd PSVs is located in the ground floor with 49Kg/cm2g set pressure. During in-place test(on line popping test) we need to test all the three psvs.

My question is, if the operating pressure is 40Kg/cm2g, on what basis we can decide 1st PSV set pressure, 2nd PSV set pressure & 3rd PSV set pressure. I want to make sure these psvs set points are correct.( I am not asking the set pressure tolerance during the test).

We have 3 boilers in our plant. In two boilers, psv set point is 45 & 49 Kg/cm2g. However on the third boiler, psv set points are 49, 55 & 56 Kg/c2mg(1st SV, 2nd SV & 3rd SV set points).

All the three boilers are operating @ 40Kg/cm2g. However in boiler set pressure is totally different from the other two boilers.

So I need to know, on what basis we can decide each pzvs set pressure? We are thinking to revise the set pressure of 2 Boilers SVs.

Thanks

 

[The Obturator]

nasim123 Regarding your further comments. I am still rather not that clear what and how many SV's are actually in series per vessel. What I can advise you, is that the first SV should be set at MAWP to give 3 % overpressure, any additional in parallel needs to have their set pressure staggered to provide 6 % overpressure above MAWP. The range of the lowest to the highest SV should not exceed 10 % of the highest SV setting. I would also be recommending you to contact the supplier since you may be exceeding the present valves spring range and/or calculated/capacity by changing pressure.

A good publication to answer questions such as yours can be found within the Pentair (Crosby) Engineering Hanbook which you can find here

http://tycoflowcontrol-na.com/valves/Images/PVCMC-0296-US.pdf

refer to page 3.3

Per ISO, only the term Safety Valve is used for all overpressure eventualities regardless of design.

 

[nasim123]

Thanks Avalveman for your support