Overpressure from thermal expansion

Bleed off any excess pressure resulting from thermal expansion of the fluid. A small temperature rise can overpressure liquid-filled equipment. 

In cases where vessels are provided with PR valves for protection against overpressure from fire exposure or an operating failure contingency, additional thermal expansion protection is not required. 

The cold water feed can be provided by direct coimec-tion to the mains (in which case all components must be able to withstand mains pressure) or from a feeder tank (which is in an elevated position and filled from the mains through a float valve similar to the water closet-cistern float valve). For mains pressure systems most utilities or water supply authorities require the installation of a nonreturn valve. Whereas low pressure (feeder tank) systems are open to the atmosphere through a vent pipe, mains pressure systems must be protected against overpressure (due to thermal expansion of water as heated) by a pressure relief valve, as well as against collapse (implosion), which could be caused by a suction effect (vacuum formation) in case of an aceidental water discharge, by an air intake (or breather) valve. 

For liquid full equipment, such as filter, exposed to fire, four stages of relief need to be studied The stage is the liquid relief due to thermal expansion from absorbing the heat fiom fire at 21% overpressure. The 2" stage... 

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