Thermal relief isolation valves

Themtal. Thermal relief is needed in a vessel or piping run that is liquid-packed and can be isolated, for example pig launchers and meter provers. Liquid is subject to thermal expansion if it is heated. It is also incompressible. The thermal expansion due to heating by the sun from a nighttime temperature of 80°F to a sun-heated temperature of 120 F can be enough to rupture piping or a vessel. The required capacity of thermal relief valves is very small. 

There are three main methods to dispose of releases from thermal relief valves. Discharge around a block valve (isolation circumvention) is widely used in most situations, however where this is not practical or economical disposal to a sewer or release to surface runoff can be specified in certain cases. 

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 possibility of backpressure onto the thermal relief valve, rendering the valve ineffective, should be evaluated before an analysis is finished. 

Thermal expansion on the isolation valve section (fire). Possible line fracture or flange leakage. g) Provide thermal expansion relief in the valved section. 

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). ... 

When a pipe or vessel is totally filled with a liquid which can be blocked in, for instance, by closing two isolation valves, the liquid in the pipe or pressure vessel can expand very slowly due to heat gain by the sun or an uncontrolled heating system. This will result in tremendous internal hydraulic forces inside the pipe or pressure vessel, as the liquid is non-compressible and needs to be evacuated. This section of pipe then needs thermal relief (Figure 2.9). 

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 ... 

Smaller relief valves are often used in isolated parts of the system where a check valve or directional control valve prevents pressure from being relieved through the main system relief valve or where pressures must be relieved at a specific set point lower than the main system pressure. These small relief valves are also used to relieve pressures caused by thermal expansion of fluids. 

Other Start-up Shut down Relief system Power/service failure Corrosion/erosion Materials of construction T oxicity/asphyxia Maintenance (provision of isolation, etc.) Double valves (trapped liquids) Valve access Provision for testing instruments, trips, alarms Fire Static electricity Noise Thermal radiation Ionising radiation Sampling Spares Anything else ... 

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