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Tuesday, May 5, 2009

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Previous post "Several Criteria and Constraints for Flare Network - Process" has discussed several major important criteria and constraints related flare system. In particular, back pressure is critical to performance of pressure relief valve (PRV) from capacity and stability aspect. Back pressure at PRV is typically back calculated from flare tip, main header, sub-header and finally tail pipe. Isothermal equation may be used for flare pipe pressure drop calculation for conservatism.

AFSA or FLARENET is commonly used for flare network modeling. In calculating back pressure at the PRV, flare tip pressure drop is required. In some flare system, i.e. Refinery flare system, a water seal is provided at the bottom of stack to minimum air ingress into main flare system. This water seal (subject to design) may induce a rather fix pressure drop to the back pressure estimation, in particular to very low pressure system i.e. sour gas flare.

How shall engineer create a constant pressure drop in AFSA / FLARENET model ?

One of the way is to use the "Flow Bleed" component. "Flow Bleed" is normally use for fix flow splitter with a fix pressure drop. In this case, introduce a "Flow Bleed" component with following setting

  • Offtake Multiplier set to zero (0)
  • Offtake Offset set to zero (0)
  • Pressure drop set to intended pressure drop

Above image shown the use of "Flow Bleed" as fix pressure drop device. A fix pressure drop of 0.1 bar has been introduced in the flare network.

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posted by Webworm, 11:06 PM


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