Over Sizing Pressure Relief Valve? 2

[jessevi]

Hey,

I know ASME BPVC says the following under Safety Relief Valves...

The pressure setting shall be less than or equal to the maximum allowable working pressure

What are the concerns, failure modes, and risks of over sizing the safety relief valve by 5%? 10%? 20%? Can anyone provide some cited instances where over sizing has failed to prevent a catastrophic event?

Thanks

 

[JStephen]

I'm unclear on the question.
Are you anticipating that something will blow up because the valve was oversized by 10%? Or because it was oversized by ONLY 10%, and more would have been better?
Or are you confusing oversize with overpressure there?
I would think that the kinds of calculations involved in sizing a safety relief valve are highly approximate at best, if that helps any, so whether it's oversized by 10% or 20% wouldn't be a big difference.

 

[RVAmeche]

Code says relief devices must be sized to prevent overpressure from reaching 110% the MAWP for typical cases, 121% for external fire cases.

Is your question what's the downside to providing a very oversized relief valve?
Downsides include: Significant cost increase, larger flange (could affect nozzle layout), significant weight increase, larger relief valve forces, much larger piping (piping has to be designed to relief capacity of the relief valve installed, not only the required rate for the vessel).

In short dramatically oversizing a relief valve just makes everything a lot more expensive and doesn't improve safety.

 

[racookpe1978]

With JStephen above: To be clear, do you mean "oversizing" the PRV by "making it larger than required to release more than the calculated volume needed to reduce the pressure to safer levels"?

Or do mean "oversizing" as "set the release pressure higher than the MAWP by 5%, 10%, or 20%'?

 

[gerhardl]

The correct answer is already given in the posts above.

1. The set pressure for the PRV must be within allowed range for the vessel. A set pressure above this is not allowed. If at pressure test the PRV reliefs first above allowed pressure, it must be adjusted down accordingly.

2. The size of the pressure relief valve must be large enough to relieve within the pressure allowance for the vessels. Consequences for oversizing largeness ref. RVAmeche.


Any tampering with the rules: The party responsible take the juridical consequnces, eg. pays the bill for any damage and injury if anything fails.

 

[georgeverghese]

The API states that RVs' may cycle rapidly ( resulting in high piping reaction and nozzle loads) if the actual relief load is less than 25% of rated relief load. In actual practice, there would be many operational relief cases when this is true, since the rated relief load is the worst case relief load coincident with some low probability failure.

Industry practice on the use of relief valves in staggered configuration is mainly to avoid or minimise maintenance on large RVs' if they were to be triggered. So a smaller orifice RV set at a lower pressure (which is less cumbersome to dismantle and maintain) acts as a "sacrificial" device to handle the majority of actual operational relief events. If you ask me I would contemplate going for a staggered RV arrangement only if all of the following are applicable:
a)There are operational (non fire) relief events at this RV
b)Relief valve size, for a single RV configuration option with SP=MAWP, is bigger than say 4inch inlet
c)The worst case relief load results in some significant acoustic vibration concerns in the downstream disposal piping that needs to be addressed.

Also note that the probability of a large relief load is greater only if there is no reliable instrumented high pressure trip safeguard or if the process safety time assigned to the trip loop( PST = time taken for pressure to rise from PSHH to RV setpoint) is inadequate to prevent RV lifting.

Note that ISO rules have changed in Europe (and companies adopting ISO guidance)re setting for RVs' in staggered arrangement - the SP of the larger of the RVs' is to be no higher than MAWP. Previously, the larger of the non fire case RVs' in a staggered configuration could be set at up to 105% of MAWP(with 10% overpressure allowance to get to allocated relief load) - dont know if there have been similar recent changes to these rules in US practice.

There have been instances where RV chatter (due to insufficient lift)generated forces have mangled up connected piping, though I cannot cite any specific cases. None I recall have resulted in a loss of containment.