27.06 What are the causes of piping overpressure and the appropriate protections?

The most common cause of over-pressure in piping is caused by thermal expansion, an increase in temperature of a pipe section that is blocked between 2 closed valves. The situation occurs because relatively cool fuel may be drawn from a tank into a higher temperature piping system, or an exterior piping system may increase in temperature as the ambient temperature increases.

Thermal expansion of fluid will cause a pressure increase that exceeds the pressure rating of pipe and valves in the fuel system. It must be relieved through safety relief valves installed in the piping system. Relief valves should be installed in any piping section that could be blocked at both ends by closed valves. The safety relief valves should discharge to a return flow pipe or dedicated discharge pipe that is open to the fuel supply tank.

Some fuel system valves are available with internal pressure relief devices, however in general external relief devices are used.