Relieving Pressure when set is below MAWP

[MechBRR]

From my reading of API 520 for fire contingency, it seems that regardless of relief valve set pressure, the value for P1 (relieving pressure) used in the calculations is MAWP + allowable overpressure + 14.7 psia. I'm using Table 5 as the example here. MAWP is 100 psig, set is 90 psig, allowable (21% above MAWP) is 31 psi making the relieving pressure 135.7 psia. In a previous discussion, thread798-166100, pleckner explains, "...The PSV will pop open if the internal vessel pressure reaches this set pressure. However, the full relieving flow from the vessel will only occurr when the internal pressure reaches 21% of this set pressure. ASME allows a vessel in a fire to over pressure 21% above design or MAWP (Maximum Allowable Working Pressure)."

As I've cruised over other examples through various vendor literature, that subtelty of the overpressure being based on MAWP can get lost and overpressure can be based on set pressure. Okay, if that is done, full relieving capacity is obtained somewhere before 121% of MAWP is reached, right? Further, my relieving temperature should be based on vapor pressure equal to relieving pressure, right? Well, if I'm using overpressure based on 121% of MAWP and my set pressure is significantly lower than MAWP, say half, for example, physically my relieving temperature is really somewhere below that temperature used in the calculation. How does this effect the reality of what is happening as the valve is relieving?

It seems that using a lower value for P1, and the associated relieving temperature, just gets you to full relieving capacity faster and oversizes your valve. Any additional though to help me get my mind around this?

Thanks for the help. What a great forum.
MechBRR

 

[skearse]

I would agree with most of that assessment-typically, the "penalty" for using a lower P1 is simply having to install and maintain a larger valve-but regardless of set pressure, the fire case is 121% of MAWP-because it's the vessel that's being protected. However, I would ask why the valve has a set pressure lower than the MAWP? I've seen it done typically to either (a) protect downstream equipment with a lower MAWP, or (b) prevent things like runaway reactions.

As far as the action of the valve when relieving, is it a modulating or "pop" valve?

 

[MechBRR]

Thanks for the comment skearse. To answer your questions...The set is lower than MAWP to minimize spares. The capacity required is comparable to a similar vessel with a lower MAWP. It is "pop" valve.

 

[Longinthetooth]

I am not an expert, but arn't overpressures in API520 calculated from a gauge rather than an absolute basis?

 

[zdas04]

I've queried several "experts" on this and they are unanimous that all calculations must be done in local gauge pressure. The reasons for this seem to lost in the mists of time, but everyone I've talked to is in agreement.

David

 

[chemjock]

Another reason for the set pressure being lower than MAWP is that when a vessel is new the RV set pressure is typically set at design pressure (maximum operating pressure plus a margin). MAWP is calculated later when the vessel wall thickness calculations are done. This may oversize the valve initially but allows the possibility of revamping to a higher capacity in the future.

 

[skearse]

@zdas-I'm confused by your statement-do you mean that you use the local atmospheric pressure to caclculate the absolute pressure? Or are you saying that the overpressure is calculated based on 10% (or 21% or 16%) of gauge pressure and is then converted to absolute? API 520 sizing calculations are done using absolute pressures. The only exception (that I am aware of) is calculation of the capactiy correction backpressure curve for bellows valves.

 

[djack77494]

Note all pressure relief devices have the same characteristics, yet the codes/standards attempt to cover the widest possible range of situations. Thus a modulating relief valve is only beginning to open at its setpoint. THerefore, its capacity will necessarily be much lower than the requirement until there is some additional buildup of pressure (i.e. overpressure). That is why the relieving capacity must be related to some value of overpressure.