Afterthoughts of Heating a Liquid-full Pipeline

You can easily block in liquid-full chemical process piping with no vapor space for expansion. Many fluids, including liquid ammonia, liquid chlorine, liquid propane, liquid vinyl chloride, and benzene, if improperly isolated, can develop hundreds of pounds per square inch (many atmospheres) of pressure with fluctuation in nighttime and daytime temperatures. The liquid tries to expand as a result of its physical properties, but it is prevented by 

Operations supervisors should provide procedures to ensure proper isolation of flammable, toxic, or environmentally sensitive fluids in pipelines. Typically these procedures must be backed up with the proper overpressure device. If the trapped fluid is highly flammable, has a high toxicity, or is otherwise very noxious it is not a candidate for a standard rupture disc or safety relief valve, which is routed to the atmosphere. Those highly hazardous materials could be protected with standard rupture disc or safety valve if the discharge is routed to a surge tank, flare, scrubber, or other safe place. 

In the previous chapter, we have discussed the behavior of water if it is heated in a confined liquid-full piping starting at 70° F (21 ° C) and atmospheric pressure (0 psi or 0 kPa)  

The Hazard ofWater in Refinery Process Systems [6] illustrates the benefits of a vapor space with increasing temperature of water. If water is confined in a piping system with a vapor space, and then heated, the pressure rises more slowly until it becomes too small due to compression or disappears due to the solubility of air in water. If a simple water system 

The benefits of vapor space are very dramatic. The examples of the water heated in a confined system without vapor space exhibit dangerously high pressures—high enough to rupture almost any equipment not protected with a pressure-relief device. 

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