There are several codes and standards that call for hydrostatic relief valves to be installed in all segments of pipe that can be isolated. For example, the following two OSHA standards state the following…
1910.111(b)(9)(ix) A hydrostatic relief valve shall be installed between each pair of valves in the liquid ammonia piping or hose where liquid may be trapped so as to relieve into the atmosphere at a safe location.
1910.110(b)(10)(xii) A hydrostatic relief valve shall be installed between each pair of shut-off valves on liquefied petroleum gas liquid piping so as to relieve into a safe atmosphere. The start-to-discharge pressure setting of such relief valves shall not be in excess of 500 p.s.i.g. The minimum setting on relief valves installed in piping connected to other than DOT containers shall not be lower than 140 percent of the container relief valve setting and in piping connected to DOT containers not lower than 400 p.s.i.g. The start-to-discharge pressure setting of such a relief valve, if installed on the discharge side of a pump, shall be greater than the maximum pressure permitted by the recirculation device in the system.
Even as basic as these requirement appear, there are still many processes not utilizing hydrostatic relief protection for one reason or another. The design behind requiring these relief valves is to minimize the amount of hazardous materials emitted from a “blocked in scenario” where a segment of pipe has been isolated by the closing of two valves (either manual or automated valves). When these two valves (or maybe even a valve and blind/blank) the liquid product is trapped and if exposed to heat (even ambient heat) the materials will expand and possible cause catastrophic failure of the pipe which then would release 100% of its contents. A hydrostatic relief valve is much like a “pop-off” valve that will relieve the pressure from the expanding liquid, thus containing the vast majority of the hazardous material within the pipe.
There seems to be some confusion in that many believe the valve will fail before the piping and this just is not the case. Yes, the seals/packing/gaskets on segments of piping will be strained, but these devices are NOT intended nor designed to relief pressures. In fact I have seen fist hand what the lack of hydrostats can lead to making matters much worse. With hydrostats we can CONTROL the location where we relief the hazardous material to; without them, we may find our hazardous material in places we wish it was NOT!!!!
We also come across business that claim their “qualified operators” have beeb trained not to isolate the high vapor pressure materials between two closed valves. As I pointed out in one of my postings last year (Hydrostatic Relief Devices vs. Qualified Operator, which is required?) OSHA does not permit the use of trained personnel in lieu of hydrostatic relief valves. One such reason is “Emergency Shutdown Systems”! We always think of these “safety systems” as doing no harm and all good! However, imagine what happens at a truck rack when a driver/operator activates the emergency shutdown system because of an upset or leak. What happens when he/she hits that mushroom button? Most systems disengage pumps and CLOSE VALVES. If a system closes multiple valves we MAY have a problem; if the system closes two valves and the ESD was activated because of a fire at the truck rack we will certainly have a problem.
We have to consider the design our of systems before we brush off the use of hydrostatic relief valves. Even though we are proud of our operators and their abilities, they may do exactly what we have trained them to do and their actions are what will place the process at unnecessary risks!