(69c) Pump BLEVE Protection Initiator Determination

If a liquid-filled centrifugal pump is run while isolated, the mechanical energy input to the contained fluid results in a temperature increase. This increase in temperature results in thermal expansion, but with no vapor space in which to expand, the pressure increases significantly. The increasing temperature also increases the vapor pressure, but since the pressure increases more rapidly than the vapor pressure, boiling is suppressed. If the pump is allowed to continue to run, with no leakage, including pump seal failure, the internal pressure and temperature will eventually increase to a point where the pump or connected piping will yield and rupture. This rupture at high pressure can result in shrapnel which can harm both personnel in the area as well as nearby process equipment, potentially leading to additional safety hazards. With the pressure dropping to atmospheric pressure almost instantaneously, the previously contained liquid exits at high velocity and boils. If that contained liquid is flammable, it generally finds an ignition source, which may be from the rupture of the metallic pump casing itself, and ignites, resulting in an explosion. This is commonly referred to as a pump BLEVE. This situation is also very hazardous for non-flammable liquids, such as water or caustic, even without the ignition of the escaping liquid.

Safety reviews and associated LOPA’s or other risk quantification methods frequently conclude that SIL-rated protection is required for these centrifugal pumps. Once required protection level is assigned, the question becomes one of selecting which process variable to use as an initiator. Determination of appropriate initiator(s) requires evaluation of the properties of the contained liquid as well as pressure/temperature ratings of the pump and piping. Calculation of the Process Safety Time is also connected with the initiator selection, and requires knowledge of the power input requirements. This paper describes a methodology for determination of appropriate process variables for use as initiators and the related Process Safety Times.