10 replies to this topic

[Wheristo]

Hello-

 

I have been confused by allowable Backpressure Criteria for a Conventional PSV. Quoted from API520..  “In a conventional PRV application, built-up backpressure should not exceed 10 % of the set pressure at 10 % allowable overpressure..”

 

Presented below an example:

A flare system superimposed backpressure is set at 10.5 psig (I know that this is pretty high but there is a reason behind it). A Conventional-type PSV-123 has a set pressure of 130 psig. Visual Flow simulation for a Power Failure design contingency gave a backpressure of 20.3 psig on the PSV outlet. I take this as “Total Backpressure”. Now my question, Is the 10% criteria for Conventional PSV should be based on..

1. ((20.3-10.5)/130)psig*100 = 7.5%

OR

1. (20.3/130)psig*100 = 16.16%

 

Visual Flow would give me ( as the result, but I thought that by the API definition stated above that it should have been (a). Appreciate expert clarification.

[fallah]

Wheristo,

 

Calculation of built-up back pressure based on: ((20.3-10.5)/130)psig*100 = 7.5% is correct and is within allowable rsange, because superimposed BP equal to 10.5 psig should has already been compensated through the PSV spring adjustment by PSV manufacturer based on submitted customer's data sheet.

[Bobby Strain]

You failed to clarify if 10.5 psig is constant backpressure, when nothing is relieving into the flare system. If so, then the relief valve spring is set accordingly. If, however you are assuming that during a plant-wide relief the backpressure at a particular valve due to other relieving sources is constant 10.5 psig, then you are wrong. You must consider whether the particular valve in question is required to relieve when there is a power failure. If the answer is yes, then the built-up backpressure is 20.3 psig.

 

Bobby

[Wheristo]

Gents-

Thanks for the replies. This is interesting but still I'm confused and not sure whether the statement made by the two of you are the same or conflicting.

 

 

Fallah,

Quoted your earlier statement;

Calculation of built-up back pressure based on: ((20.3-10.5)/130)psig*100 = 7.5% is correct and is within allowable rsange, because superimposed BP equal to 10.5 psig should has already been compensated through the PSV spring adjustment by PSV manufacturer ....

If the has already been compensated by the spring, then shouldn't the backpressure now be 20.3/130*100 = 16.16% instead?

 

 

Bobby,

The SIBP of 10.5 psig is constant. Yes the valve in question is required to relieve during the power failure. You said the built-up backpressure is 20.3 psig. Isn't this 20.3 psig the Total Backpressure?

I learned that Total Backpressure = Superimposed + Built-up

[Bobby Strain]

What is the normal pressure in the header with no relief? If it is near 0 psig, then 20.3 is the built-up backpressure. If it is 10.5 psig, then the built-up back pressure is 9.8 psig.

 

Bobby

[fallah]

Wheristo,

 

You did mention in your first post:

 

"A flare system superimposed backpressure is set at 10.5 psig...."; means before PSV opening the pressure in PSV tail pipe is 10.5 psiog.

 

Then you did mention in the same post:

 

"...for a power failure design contingency gave a backpressure of 20.3 psig on the PSV outlet."; means this is superimposed backpressure plus built-up one i.e. total backpressure.

 

Because the flare system superimposed backpressure is already set at 10.5 psig, built-up back pressure has to be the difference of total backpressure and superimposed one i.e.: 20.3-10.5=9.8 psig

 

Now 10% criteria is only applied on built-up backpressure i.e. ((20.3-10.5)/130)psig*100=7.5%

 

During PSV relief the total backpressure is 20.3 psig but part of this pressure (10.5 psig) is already considered in PSV spring adjustment; means even with having 10.5 psig, the PSV before to be opened is balanced such that opens at its set pressure (here 130 psig). After PSV opening the built-up backpressure to be added to superimposed one leading to total backpressure of 20.3 psig.  

 

Let's know where is the point causes you are to be confused...

[Wheristo]

Fallah/Bobby-

Thanks again for your replies. That improved my understanding. The point that confused me was the output result from Visual Flow & other software i.e. PPM that gave the % of backpressure output (for conventional) differently. See attached example based on the case I described earlier.

 

 Conv PSV Backpressure.docx   39.62KB   104 downloads

[fallah]

Wheristo,

 

It might you has already fixed  the superimposed backpressure equal to 0 barg (or not specified and software gone as per the default), then all backpressure automatically has been considered as built-up backpressure.... 

[Wheristo]

Hmmm.. not sure. If you can see the attached (overall view of the flare network in Visual Flow), in the simulation the pressure on the outlet (Flare) have been set to 10.5 psig.

 

 Flare1.PNG   248.76KB   15 downloads

[fallah]

Wheristo,

 

With zoom in the attached picture the data isn't so clear to be readable. Anyway i think the software output is affected by input setting and you should recheck the input...

[Wheristo]

Many thanks Fallah. Appreciate your inputs.