Installation of thermal relief valve

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. 

A recommended safe practice is to install hydrostatic pressure relief valves in all pipelines wherever the possibility exists that liquid fluorocarbons may become trapped (such as between two valves) to prevent piping rupture through thermal expansion of the fluorocarbon. 

Isolation Circumvention Where the fluid is the same on each side of the isolation means, and no contamination will result, this is the optimum choice for thermal relief release. Consideration of the possible backpressure onto the thermal relief valve, rendering the valve ineffective, should be considered as part of the review for the installation. 

Consideration should be given to the effects of thermal expansion of liquids and pressure-relief valves installed unless ... 

The ASME code requires every pressure vessel that can be blocked in to have a relief valve to alleviate pressure build up due to thermal expan sion of trapped gases or liquids. In addition, the American Petroleum Institute Recommended Practice (API RP) 14C, Analysis, Design, Installation and Testing of Basic Surface Safety Systems on Offshore Production Platforms, recommends that relief valves be installed at vari ous locations in the production system and API RP 520, Design and Installation of Pressure Relieving Systems in Refineries, recommends various conditions for sizing relief valves. 

Typical solutions to thermal expansion deficiencies include locking open of oudet block valves on the cold side of exchangers, reliance on shutdown procedures to prevent the scenario, draining and venting out of service equipment or installing a relief device... 

But it must be emphasized again, that bursting disks and safety valves should always represent the last resort only. Instead, one should work towards a synthesis development which takes care of a significant distance between true reaction data under desired conditions and those limit values which correspond to the individual designs of relief systems. Such designs in the sense of safety barriers support the attempt to allow the performance of thermally inherent safer processes only. This way relief systems could be reduced in their purpose to what they actually should be a formally required installation to a vessel, which should never have to prove its efficiency. 

For pairs of pumps, an emergency isolation valve should be installed on the suction of each pump so that the valves can be tested during normal operations. Thermal pressure relief valves may be needed for low or ambient temperature... 

It is quite common that a liquid-carrying line can be isolated between two isolation valves. The isolated liquid is under thermal expansion due to heat gain from high ambient temperature. Small thermal expansion can be adequate to increase the pressme beyond the design pressure limit of the pipe. The exact calculation of thermal expansion and contingency is complex and is often not required. The volume released is small and a nominal 20 x 25 NB relief valve is installed to protect the system. However, the thermal expansion can be established mathematically as follows ... 

More Pressure 7. Isolation valve is closed in error whilst pump running. 8. Thermal expansion in the isolation valve section (fire). Lines subject to full delivery pressure. Possible line fracture or flange leakage. Covered by b). f) Perhaps worthwhile installing a pressure gauge upstream of the delivery pump. g) Provide thermal expansion relief in the valved section. 

More Pressure 8. All of 5, 6, and/or 7. 9. Thermal expansion in isolation valve section (fire). 10. PCV fails shut or isolation valve shut in error. As for 5, 6, and 7. Line fracture or flange leakage. As for 3. Covered by b) and c). f) Install thermal expansion relief in isolation valve section. Covered by b) and c). ... 

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