Thermal relief overpressure protection

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. 

Thermal stability testing showed that the reaction mixture could decompose exothermically with self heating occurring on the plant scale from 145°C (the boiling point of the mixture is about 160°C). Decomposition of the reactant mass would lead to a runaway reaction with the generation of a toxic and irritant gas. This would be vented safely, as far as protecting the reactor from overpressure is concerned, by the emergency relief vent, but would cause a serious toxic and corrosive aerosol emission. 

In North America, the eductor pipe inside the vessel has an excess-flow valve at the top, immediately below the manhole cover. This valve closes the eductor pipe when the rate of liquid flow exceeds a set rate [2], [24]. North American tank cars have a spring-loaded. safety relief valve, which protects the vessel against overpressure in case of external heat. The tanks have thermal insulation. In Europe thermal insulation and safety relief valves are not used or recommended. 

Though Equation 4.16 can be used, it is often difficult to establish the relief contingency. However, because the contingency is small, a relief valve of 20 x 25 is used to protect a pipeline against thermal overpressure. 

---