PSVs: Allowable overpresure vs accumulation 1

[MichD]

Does anyone know of any situation where it would be required (or beneficial) for process reasons or otherwise, to have a pressure safety valve set at significantly less than MAWP, and hence allow overpressure to exceed 10% (for single, non fire case valve).

To allow overpressures >10% when set pressure < MAWP is clearly not prohibited in API520, and allows for a smaller orifice area due to increased driving force during relief. Yet it does not seem logical to do this when the vessel could potentially see 10% accumulation anyway if the dictating relieving scenario occurs.

Assuming that associated pipework/fittings are also rated to vessel MAWP and are hence not the weak link, has anyone encountered a situation such as this? Or, is it more common for designers to simply design for 10% overpressure regardless of whether set pressure = MAWP?

Thanks...

 

[Latexman]

I've seen it done quite a bit on runaway reaction scenarios. Opening the relief early while temperatures are lower gives you more relieving time before the reaction builds up to the 10% overpresssure point.


Good luck,
Latexman

 

[EGT01]

MichD,

I'm not sure I totally agree with your statement
"allows for a smaller orifice area due to increased driving force during relief".

In general, relief valves in vapor service will achieve full lift by the time they reach 10% overpressure so the required area will be no different for a given upstream relieving pressure regardless of the combination of set pressure and overpressure. You should get the same calculated required area whether you have
set = 60 and overpressure = 10% as you would with
set = 55 and overpressure = 20%, the upstream relieving pressure is 66.

The only times I'm aware that this may not hold true is for some relief valves used in liquid service are not certified to 10% overpressure and require 25% overpressure for full lift. Also, conservation vent style relief devices for storage tanks typically do not achieve full lift until reaching 100% overpressure. The calculations for these devices will include a factor based on %overpressure.

Off hand, I can't think of another "safety" reason other than the one Latexman noted. However, if you have a process reason for limiting the pressure, you shouldn't be using your "safety" valve as a process control element, that's a job for a control valve. Though I have known some to use relief valves as a quick acting pressure control, they were not designated as a "safety" device.

 

[MichD]

EGT01;
I agree with the first half of your statement, that "relief valves in vapour service will achieve full lift by the time they reach 10% overpressure" (single, non fire case valve). However later on you say that you should get the same calculated required area whether you have set = 60 and overpressure = 10% as you would with set = 55 and overpressure = 20%. I agree that this would give you the same required orifice area with P1 = 66, however in this 2nd example you cite, you no longer have 10% overpressure - you have 20% overpressure. This was my point.

If you have any reason for limiting vessel pressure to below the MAWP as indicated by a lower set pressure - then it seems logical that you would therefore want to limit the OVERPRESSURE to 10% regardless of the fact that you would be ALLOWED to account for overpressure >10% as indicated by code (because higher relieving pressure the smaller the required orifice area). In otherwords, using your figures, set = 30 and overpressure = 10%, i.e relieving pressure = 33 as opposed to set 30 with much higher overpressure....

 

[EGT01]

MichD,

It is always difficult to talk about these subjects in general terms. Each case can be different and requires a thorough analysis of the system to determine the limiting factors and thus set pressure plus overpressure allowed for relief system design. In that respect, there is no simple answer to "is it more common for designers to simply design for 10% overpressure regardless of whether set pressure = MAWP?"

As you have noted in your last post, if there is a reason to limit pressure to below vessel MAWP then that must mean something else in your system, other than vessel MAWP, is the limiting factor. Then you must size the protecting relief system according to that limiting factor's allowable pressures regardless of what the vessel allows. You must also design the relief system in accordance with the characteristics of the relief device to be used.

Using 10% overpressure for relief system design is reasonable for systems following API520 allowances which reflect ASME Sec 8 allowances. Typically, those systems would use a relief device that had been certified at 10% overpressure. However, as a word of caution, don't become fixated with 10%. Not all equipment is designed with the type of allowances you see in ASME Sec 8. Don't assume that 10% accumulation (per ASME Sec 8, single valve, non-fire case) is acceptable for all equipment and that all relief valves are certified to 10% overpressure. For a vessel which allows no accumulation, you would want set pressure less than MAWP such that during relief, set plus overpressure is less than MAWP.

As another example of when you might see set pressure less than equipment MAWP, you may find that a relief system is designed according to what is defined as the design pressure for the protected system. Any equipment to be installed in the protected system will be purchased according to that required design pressure. What you will mostly likely find is that the MAWP for the equipment received may be noticably greater than the required design pressure. However, there is the potential that some equipment MAWP may not be greater than the system design pressure. Hence the relief system is designed at conditions in accordance with the limiting component which should not be less than the design pressure. The use of 10% overpressure for this case would reflect ASME Sec 8 type allowances.

 

[MJCronin]

MichD,

Why does the Code required PSV protecting the vessel have to be set at a lower pressure ? How about an additional relief device or some type of control valve set at the desired lower pressure ? I believe that there is nothing in the current codes or standards to prevent control of process pressure at a point lower than the vessel MAWP

Operators of municipal and industrial boilers have for many years included an PORV control valve to relieve pressure below the setpoint of the boiler safeties. The control valve, of course is selected for this high pressure drop service.

I believe you might be causing trouble with any type of valve/vessel certification or inspection in the future.

Stick with the valve set at the MAWP, and add another device...

My opinion only


MJC