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[mahmooddalvi09]

Hii all, 

 

I have a query related to flare header and sub-header sizing in FLARENET.

 

In our case, the governing scenario for flare header sizing is a power failure, which results in a flow of approximately 4000 tonnes/hr. This scenario also governs the maximum calculated backpressure among all evaluated cases.

 

In FlareNet, I sized the flare header by specifying a backpressure of 1.5 barg at the flare stack inlet (as per the design basis). FlareNet then back-calculated the pressure drops, and I obtained a backpressure of 5.0 barg at the PSV outlet flanges and 3.0 barg at the KOD inlet. I have considered 3.0 barg as the maximum superimposed backpressure for all PSVs in the flare network.
 

The purge gas used is Nitrogen, and the minimum backpressure maintained within the flare system is 0.2 barg. The flare stack package is in vendor's scope.
 

My queries are as follows:

1. What is the correct approach or industry practice for determining the maximum superimposed backpressure? Should it be 5.0 barg (at the PSV outlet flange for the power failure case) or 3.0 barg (at the KOD inlet)? Since all global overpressure scenarios are independent, I believe 5.0 barg should not be applied as the maximum superimposed backpressure for all PSVs.
    

2. I have sized the flare header for the governing flow based on constraints of momentum, Mach number, and noise. For other global and operating contingencies (which govern specific PSVs), should I size the tail pipes and respective sub-headers by specifying 0.2 barg at the flare stack inlet? I assume this is not necessary, as the relieving fluid pressure will dissipate within the flare header and the sweeping gas will handle it.
    

3. For non-governing PSVs, I have sized the tails and sub-headers based solely on constraints of momentum, Mach number, and noise. Could the client request tail pipe and sub-header sizing based on maintaining 0.2 barg at the flare stack inlet? My understanding is that doing so would unnecessarily restrict the allowable pressure drop across the network, which in turn could lead to oversized sub-headers and tail pipes.

    

Your guidance will be highly appreciated.
 

Edited by mahmooddalvi09, Yesterday, 05:39 AM.