Conventional RV outlet losses above 10% 1

[jamesbanda]

We have located an installed valve (following a relief valve) audit which exceeds 10% for the outlet losses.

Are there some guidlines on what would be an acceptable limit above 10% limit outlet losses for a conventional valve. I understand if this was a new installation we MUST correct this and use ballanced bellows. but this is an older plant, which the orginal cal using an old method was just under 10% but using a more modern method now exceeds 10%. We found this updating the calcs to API521 so we considered all credible over pressure cases not just fire as was done previously (a long time ago)..

 

[don1980]

The only exception is the API 520 provision allowing the outlet losses to be as high as the percent overpressure. That's applicable when the set pressure is less than the MAWP, or in cases in which fire is the only credible scenario.

 

[StoneCold]

I think we need more information.
What does the PRV protect?
How big is it?
What is the relief scenario causing the problem?
What is the set pressure?
What is the MAWP of the item it is protecting?
What does the relief piping look like? Size? Where does it go?

If you can answer some of those things, others might be able to help you more.

Rgards
Stonecold

 

[TD2K]

Have you talked to the PSV vendor to see what they say about this particular valve?

The built-up backpressure (which includes any variable backpressure) must be less than 10% is because greater backpressures can cause valve instability but I don't know if vendors have any specific information for their valves for additional backpressure.

By considering all credible overpressure cases you are evaluating the line losses at the PSV's capacity for each case? For the case which fail, what is the required relieving load relative to the PSV capacity?

 

[jamesbanda]

What does the PRV protect?
Process equipment (column)
How big is it? (40 te/hr rated capacity)
What is the relief scenario causing the problem?
Valved in operation
What is the set pressure?
75 psig.
What is the MAWP of the item it is protecting?
90 psig.
What does the relief piping look like? Size? Where does it go?
direct to atmosphere, 6 inch

 

[Latexman]

Per don1980, you should be allowed 32% overpressure (90*1.1/75), thus outlet losses can be 32% too. Problem solved?

Good luck,
Latexman

 

[don1980]

When the allowable overpressure exceeds 10%, the built-up backpressure can also exceed 10%, provided that the built-up backpressure doesn't exceed the allowable overpressure.

For this specific case the peak allowable pressure is 99 psig. With the PRV set at 75 psig, the allowable overpressure is 32%. Therefore, the allowable built-up backpressure is 32% for this specific case.

 

[Jim111]

How to check the contengency of operating PSV?

 

[Latexman]

Start a new post please. Don't pile on. Makes future searchs very difficult.

Good luck,
Latexman

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[sheiko]

Max. allowable backpressure = 1.21*MAWP - Pset for fire case
Max. allowable backpressure = 1.10*MAWP - Pset for non-fire cases

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