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Calculation of Constant backpressure, superimposed pressure and variab

Timewrap (Chemical) (OP)
24 Feb 09 15:31
Dear Experts,

I'm designing a system consists of at least 15 PSV's where most of them designed for fire case. All the PSV's are conventional spring loaded types, the flare header is purged with nitrogen with a contant pressure of 0.1 psig.

How to calculate constant backpressure, variable backpressure and superimposed backpressure.

Thanks in advance,

JAlton (Mechanical)
24 Feb 09 22:16
Superimposed Backpressure is present at the outlet of the PRV prior to the PRV Lifting and is a result of another source.  Superimposed Backpressure may be variable or constant. If it is constant, a Cold Differential Test Pressure may be applied to the Set Pressure of the PRV.  The constant superimposed backpressure is subtracted from the Set Pressure and the PRV is tested at a lower pressure and the Cold Differential Test Pressure (CDTP) plus the Constant Backpressure will result in a PRV which opens at the required Set Pressure in the System.  Variable Backpressure by definition is difficult to calculate, but may be compensated for easily by installing a bellows in the PRV which isolates the backside of the PRV Disc from the effects of backpressure and thereby eliminates the variability of the set pressure and the need for a CDTP calculation.  Install a bellows in the PRV and forget the issue of variable backpressure.    


greenche (Chemical)
26 Feb 09 17:06
While using a balanced bellows RV as JAlton suggests can allow the RV to handle higher variable backpressures, I respectfully disagree that you can simply install a bellows and "forget the issue."  Even with a balanced bellows RV it is still possible for the variable backpressure to be greater than 30% of the RV set pressure, which begins to reduce the relieving capacity (although not as much for the fire case) and most manufacturers and industry standards do not recommend operating >50%. You will not know if you are in these areas of operation without a flare system analysis to determine variable backpressures on the system of RVs.  Variable backpressure is difficult to calculate, which is why there are dedicated software applications that do it.  If you do not have access to one, engineering firms often contract for this type of work.
Timewrap (Chemical) (OP)
27 Feb 09 15:19
JAlton and greenche, Thanks for your replies.

I want to bring to your attention that this safety system is related to a petochemical plant where there are no significant or large Hydrocarbon inventories are nearby. Hence it is safe to assume that there is no pool fire and fire is restricted to one location only. In this scenario, there will be only one PSV going up in the event of fire. The system design will be based on the biggest PSV.  

Does this makes it easier to calculate the superimposed backpressure?. Can I assume that the N2 presure of 0.1 psig is the constant backpresure on all the PSV's?

Thanks in advance,

JAlton (Mechanical)
27 Feb 09 17:56
Greenche is correct regarding capacity restriction.  I apologize for the "forget the issue" comment.  My only point was that the Set Pressure issue is no longer a problem from the aspect of variability due to back pressure changes.

Greenche is correct about the effect of back pressure on capacity.  High Back Pressure does indeed restrict the relieving capacity of the PRV.



Timewrap (Chemical) (OP)
2 Mar 09 11:07
Hi JALton and greenche,

My intitial question was how to calculate the different types of backpresures when designing the flare system. Your answers seems to throw some light on the issue but the initial questions still remain answered. This is a new system, I do not have aceess to Flarenet, I have to specify backpressures to the PSV vendor for correct sizing.

In your opinion what are the baic data in terms of backpressure a vendor requires for a PSV design. This is assuming that this is a new sysrtem, the only data you have is PSV sizing scenarios and rated capacities of each valves. How to approximate diff. BP's during initial design.

Thanks in advance,

greenche (Chemical)
4 Mar 09 18:37
If you want to look at the scenario where only one valve is relieving, calculate backpressure by calculating the pressure drop from the RV outlet through the flare tip.  Base your flow rate on RV capacity at the overpressure for the scenario you are evaluating.  If backpressure is high enough that it impacts the RV capacity, you will have to iterate.
Timewrap (Chemical) (OP)
5 Mar 09 14:30

If there are two or more valves with different set pressure relieving into the same header. These valves are all sized for fire case and the header sizing is based on the fire case as well.

How would you recommend to calculate the back pressure on  each valve?

Is the flare header sizing based on 10 % of the lowest set pressure valve if all the valves are conventional spring loaded??

Thanks in advance,

greenche (Chemical)
9 Mar 09 16:57
If RV "A" is relieving due to fire and you want to know the backpressure on non-flowing RV "B", it is the pressure in the main flare line due to the flow from RV "A" where the tailpipe from RV "B" ties into the main flare line. To generalize, look at the pressure in the main flare line due to the flowing RV where each of the non-flowing RVs tie in to determine the backpressure on that non-flowing RV.

If you are talking about calculating backpressure with multiple RVs relieving, see my first post in this thread.

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