Re.: PSV sizing

[Deremis]

Hi all

I need to check if the PSV's on a plant are adequately sized for an increase in the feed rate to a unit.

Can anyone tell me if there is any material that will help in this regard. I have looked at the API guidelines but I just want to add that I'm pretty new at this and any help would be greatly appreciated. I don't want to not consider certain occurrences but I don't want to be too conservative either.

Thank you.

 

[22466]

Hi Deremis

If you are looking for a pressure relief valve sizing and calculations you can refer ASME sce VIII div 1

 

[MikeClay]

The first place to look is the process safety information package. The calculations for the existing relief valves should be there. The cases considered for each valve need to be reviewed to determine if they are still valid.

The next step is to look at the expansion process hazards analysis to determine if any other scenarios have been identified.

Finally, look over API 521 to see if any of potential scenarios might apply.

 

[CHD01]

Most of the information you will need can be found in ASME Section VIII, Divsion I, API 520 Parts I and II, API 521, and API 2000. All a reasonble price for major companies and even some individuals. Other information can be obtained from major relief valve manufacturers (they also have free software sizing and selection programs that you may download). Try a web search for companies like: FARRIS, CONSOLIDATED and CROSBY for relief valves; look at GROTH, PROTECTOSEAL and SHAND&JURS and WHESSOE VAREC for vents; and BS&B and ZOOK for rupture disks. Be aware that any tubular ended valves are different than conventional valves in that extremely high overpressures may often be required for the required relieving rate; usually these types of valves are only used in pilot plant type operations (look for NUPRO) - they should not be used in the CPI industry for major plant processes unless the designer is highly qualified and experienced in sizing and installing relief devices.

Finally, recognize that sometimes there are additional regulations to follow depending on the type of fluid involved (gas and liquid) and the type of equipment involved (mobile quipment for over-the-road transportation for example.

The more you learn, the less you are certain of.

 

[Buchi]

Hi Deremis,

i have been through a similar responsibility, and the first thing you want to do is to look at the existing PSV datasheets and see what the controlling cases were. You can waive away a lot of PSVs by mere looking at the basis on which they were sized before vs the modification to the existing plant. here is what i mean, if a PSV A was sized for fire, the only thing that can change its size is a change in the size of the vessel it is protecting(which directly affects the wetted surface area) and the composition (which directly affects the latent heat of vaporization) if a negligible change was made to these two identified parameters, you can be sure that the PSV's size will not be affected appreciable so you can tick it as OK.

Also check out for changes in flow rate, where blocked discharge is the controlling case.

and to be very honest with you, if by engineering judgement, you can also tell if a PSV will definitely be affected by modifications so you have to re-size such PSV.

good luck.

Buchi

Buchi

 

[CHD01]

Buchi,

Just a few cautionary comments:

1. If the rv calculation was done prior to 2000, enough has changed that calcs should be redone in my opinion. Even the fire sizing basis from our standards have changed if you go far enough back in time - I recommend new calcs if older than 5 years. Actually many code changes have been made over the years - for example, we used to size based on required relief rate; but now size based on maximum flowrate for the valve purchased with only a very few exceptions.
2. Determining if the orifice is large enough is only 1/2 the battle, plus if you do not know how to determine if 2-phase flow exists you won't get that right.
3. Rest of the battle is verifying the installation protects the vessel as designed, which requires accurate pressure drop analysis in and out of the relief valve, considering flashing and 2-phase flow if applicable, and whether the relief valve is subjec to chatter; not to mention determine the required vent height for dispersion of hazardous gases, etc.
4. Bottom line is this is not a subject to be lightly treated, while a review of previous calcs and spec sheets is helpful it is best used to help define what needs to be addressed as first priorities - not whether to analyze it or not.


The more you learn, the less you are certain of.

 

[Buchi]

Hi CHD01,

I agree with you on the need for caution on a safety issue like PSV. however, i my approach does not advocating non-adherance relevant and updated standards.

hey! but don't make it sound dramatic, such changes in the standards cannot make that tremendious amount of difference in the sizes of a PSV if the paramaters as i have identified in my earlier example are held constant. afterall, in the end, you will still have to select a PSV from the vendors catalogue. with PSV calculation, you have to justify such extra details which glaringly will not make a measurable difference, hence engineering judgement and experience is inevitable.



Buchi