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TECHNICAL
Under pressure
by
Richard Wiederhold , Advanced Valves
Pressure Control Valves ( PCVs ), also called pressure reducing valves , are designed to limit the municipal supply water pressure and thereby protect household plumbing , especially hot water cylinders and geysers from bursting or other damage .
PCVs are critical components for controlling and safeguarding the flow of fluids in various environments . These valves mechanically adjust to changes in inlet pressure , ensuring that the output remains stable and within safe parameters .
PCVs are used in a variety of systems , from domestic water pipes to industrial networks . They are designed to withstand tough environments and ensure optimal performance . PCVs function as guards , monitoring and responding to the dynamics of fluid pressure . This is achieved through a sophisticated mechanism where the valve senses the pressure downstream and adjusts its position accordingly , either reducing or allowing more fluid to pass through , based on the need .
This operation is crucial in environments where precise pressure control is necessary , such as in water systems , gas pipelines and other industrial processes . A PCV mechanically senses the downstream pressure and shuts closed when a factory-set pressure is reached . The valve will open again when that pressure is relieved , for example , when a tap is opened .
A PCV requires precision engineering from its initial design to its manufacture and assembly to produce consistent and precise operations . This self-regulating mechanism ensures the downstream pressure remains stable , preventing risks associated with high municipal pressure and the subsequent over-pressurisation of plumbing fittings especially hot water cylinders and geysers .
It is essential that the appropriate PCV is installed with the rated pressure of the hot water cylinder / geyser , that is a 400kpa PCV with a 400 kPa geyser . For the plumbing industry a PCV requirement is laid down in SANS 10254 .
A cutaway of a pressure control valve .
How does it work ? The valve consists of a spring and diaphragm , which act as a balancing mechanism . It also has a washer seal and seat . When the outlet pressure drops below the set point , the spring pressure overcomes the outlet pressure and opens the valve . When the outlet pressure increases , the pressure under the diaphragm overcomes the spring pressure and closes the valve .
Back pressure controls whether the valve opens or closes . When the outlet or downstream pressure approaches the set point , the internal pilot passages allow water to flow from the outlet chamber to act on the diaphragm , causing the valve to close . When the downstream pressure is lower than the set point , the pilot passages drain water from the diaphragm , causing the valve to open . PA
Image by Advanced Valves www . plumbingafrica . co . za @ plumbingonline @ plumbingonline @ PlumbingAfricaOnline March 2025 Volume 31 I Number 01