When performing pressure relief analysis, the blocked system outlet is one of the most common overpressure scenarios requiring consideration.  A blocked system outlet can occur on almost any piece of equipment.  A blocked outlet may occur upon inadvertent closure of a single manual valve or misoperation of an automatic control valve located on the outlet of the particular equipment item or further downstream of the system.  In addition to valve closure, other mechanisms also exist for blockage, such as plugging with hydrates or the shutdown of downstream equipment such as a pump.

Overpressure may be caused by continued feed flow and/or heat input to the system in the event of a blocked outlet.  According to ASME Section VIII Section UG 127, relief protection is required when closure of equipment outlet can result in a pressure higher than the system’s maximum allowable working pressure (MAWP).  Pressure build-up in a system, resulting from a blocked outlet, is usually due to vaporization within the system as a result of heat input or a higher pressure source directly feeding the system.  The following flowchart provides general guidance in determining upstream limiting pressure of various feed and heat sources for use in the blocked outlet overpressure credibility analysis.  Once the scenario is determined to be credible, an appropriate calculation methodology must be used to quantify the resulting required relief rate. 

A higher resolution copy of the above diagram can be downloaded using the link below:

Blocked Outlet Limiting Upstream Pressure (82 KB)