Rankin Chemical Technologies, Inc.
engineering · consulting · process design

Pressure Relief Valve Sizing Example

Consider the following example:

A 7,800 gallon vertical process reactor with an MAWP of 150 psig has a set of internal heating coils in which the control valve supplying the heating fluid fails in the open position; the vessel vent valve is closed.  Under these conditions, the vessel will heat up, causing pressure to rise until it reaches the relief valve set point.  Preliminary sizing of this scenario reveals the following, as summarized below:

PRD Sizing Overview - Example

From the above analysis assuming all-vapor flow, a required relief valve area of 2.706 in2 is indicated.  Therefore, a 3x4 PSV having an "L" orifice is recommended.  During relief, choked flow will occur in the tailpipe, but total backpressure at the PRD outlet is within acceptable limits due to the balanced bellows design.

DIERS technology was employed to check for the presence of two-phase flow.  Because the chemical system is foamy in nature, the bubbly vessel flow model was chosen.  Using this approach, two-phase flow was strongly indicated.

Some questions immediately came to mind:

  • How long will two-phase flow be present during relief venting?
  • Will vessel overpressure exceed ASME limits during the relief event?
  • How much liquid will be vented along with the vapor? Will containment of liquid be an issue?

To properly answer these questions, a time-dependent dynamic analysis of the two-phase relief event is required, which is presented here.

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