Pressure relief valves
Pressure relief valves( PRVs) are designed to protect boilers, pressure vessels and piping and are considered the“ last line of defence” within industrial processes.
However, an industrial process must operate in such a way that there is the highest possible productivity and in accordance with its design. In order to achieve this productivity, process operators in many companies end up working with the operating pressure very close to the valve opening pressure, causing leaks through its sealing surfaces.
Complying with operating pressure limits in boilers and pressure vessels
By Artur Cardozo Mathias
Chemical process piping
Maximum recommended operating pressure
The operating pressure is the normal gauge pressure to which a pressure vessel or boiler is subjected constantly and without any considerable variation. The user’ s process engineer is the professional responsible for determining the maximum operating pressure of the vessel or boiler that will be purchased. These equipments are normally designed to have a Maximum Allowable Working Pressure( MAWP) that provides a margin above the maximum expected operating pressure to avoid any undesirable operation of the pressure relief device( s) caused by the smallest fluctuations in that pressure. Therefore, vessels and boilers are designed to operate with the maximum possible efficiency and at an operating pressure very close to their MAWP. In order for the PRV to have its complete operating cycle( opening, overpressure and closing), and still remain closed and sealing, the normal operating pressure of the protected equipment( vessel or boiler) must not be above 90 % of the valve opening pressure, if the fluid is compressible( gas or vapour), or 80 % if the fluid is liquid, for valves with metallic sealing surfaces. For those with resilient seats and operating with gases and vapours, that operating margin could be as low as 5 % below its opening pressure, however, the PRV closing pressure will occur at approximately 7 % below its opening pressure, therefore, it is not recommended to operate with such a low margin since when the valve opens it will not close again, unless the operating pressure of the vessel is reduced. Pilot operated PRVs can operate up to 3 % below their opening pressure. A value of 90 % of the opening pressure for springloaded valves with metal seats or 95 % for resilient seats and pilot operated valves, is common in selecting the maximum operating pressure. The ASME Section VIII – Division 1( Edition 07 / 2025) code, in Appendix M10( c)-( Non-Mandatory), recommends the following values as minimum differential pressure between the vessel operating pressure and the valve opening pressure:
• 5 psi( 0.34 bar) for opening pressures up to 70 psig( 4.82 barg);
• 10 % below the opening pressure for pressures above 70 psig( 4.82 barg) and up to 1000 psig( 68.9 barg);
• 7 % for opening pressures above 1000 psig( 68.9 barg).
It is due to the values mentioned above that the sealing surfaces of the valve disc and nozzle must be lapped with extreme flatness and a“ mirror finish” to ensure maximum tightness with low pressure differential and forces. If the operator wishes to reduce the valve opening pressure, the recommended minimum operating margin should not be reduced to avoid leaks. Note: A reduction in the valve opening pressure causes an increase in the fluid volume, if the fluid is compressible, so the valve may be undersized for that new pressure. Before reducing that pressure, the valve should be resized to
46 Valve World August 2025 www. valve-world. net