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Pressure sustaining valves are back pressure valves, designed to maintain a pre-set back pressure or upstream pressure regardless of downstream demands or pressure changes. Back pressure sustaining valves protect upstream process systems from low pressure loss and lack of system pressures required to protect vital process systems. The pressure sustaining valve will stay open allowing flow to downstream equipment whilst the upstream inlet pressure is above the minimum pressure setting. If the inlet pressure or upstream pressure drops close to the minimum pressure setting the pressure sustaining valve will slowly close to maintain its set back pressure. When the upstream pressure increases above the minimum safe limit the pressure sustaining valve will slowly open allowing flow downstream.
Pressure sustaining valves can be manually adjusted within their set limits with the back pressure or inlet pressure being displayed by an external pressure gauge.
Back pressure control valves are available with a wide range of fully adjustable pressure ranges such as 1 to 5 bar, 4 to 10 bar, 8 to 13 bar, 0.2 to 1.5 bar, 0.2 to 4 bar and 3 to 10 bar, and all can be manually adjusted within these pressure ranges to suit each application, with custom pressure ranges being typically available upon request.
Pressure sustaining control valves or back pressure valves are available with either threaded 1/2", 3/4", 1", 11/2 or 2" BSP or NPT thread or flange DN15, DN20, DN25, DN40, DN50, DN65, DN80, DN100, DN125, DN150, DN200, DN250, DN300, DN350, DN400, DN450, DN500 up to DN1000 with PN16 or ANSI150Lb flange type.
Back pressure valves are commonly available with either stainless steel 304 or 316, bronze, cast iron or ductile iron bodies with epoxy coating. Most pressure sustaining valves are designed to control water pressures, the body materials to suit the process application. Most water processing systems will utilise a cost effective brass, cast or ductile iron control valve, however for food or chemical applications and where corrosive or anti bacterial cleaning agents are used stainless steel is the preferred material.
Like most valve products used in the processing industry today valves have evolved to meet the ever demanding application requirements for each application. The most popular variants are:
A direct acting pressure sustaining valve or back pressure valve is designed to maintain a stable minimum pre-set back pressure and does not reply on pressure differentials (pressure difference between inlet and outlet ports) to operate. The direct acting nature of the back pressure control valve means it is ideally suited to applications where flow is not guaranteed such as outlets to sinks, toilets, washing facilities or to other less demanding process equipment. These are typically very robust design and well able to maintain pressures up to about 25 bar (375 psi), depending on manufacturer or model chosen. Adjustment is typically from a protruding bolt or T handle from the valve base or head, which can be manually turned to either increase or reduce the sustained back pressure, which is typically displayed by an external pressure gauge mounted directly into the valve body.
A direct acting low pressure sustaining valve or back pressure valve is designed to maintain a more stable more sensitive back pressure by means of a larger pressure balance diaphragm. The direct acting functionality of this valve makes it ideally suited to low pressure systems where flow or pressure differentials are not guaranteed, such as part time processes, closed loop systems, taps, sinks and wash down facilities. These back pressure valves are quite robust in design and are well suited for sensitive back pressures from 0.2 bar (3psi) up to around (22 psi). Back pressure adjustment is typically via an external bolt, T handle or small hand wheel located either at the top or base of the main valve body, with the current back pressure displayed by an external pressure gauge installed into the valve body pressure cavity.
A pressure pilot actuated pressure sustaining or back pressure valve utilises the pressure differential (difference between inlet pressure and outlet pressure at the ports) for the hydraulic operation of the valve via an external pressure controller mounted to the main valve body. As these hydraulic servo pressure assisted sustaining valves are typically physically much larger in size (i.e. normally ranging from DN50 or 2"port), their size typically guarantees a pressure difference as they will be fitted to larger mainline process pipe system that is constantly under pressure. The advantages of this pilot design reduces costs as compared to large direct acting back pressure valves, but also as the external pressure controller can be removed, greatly aiding shipping, handling, installation and maintenance processes.
A lockable pressure adjustment screw, bolt or T handle should be visible from the controller, which can be turned in either direction to either increase or reduce the sustained back pressure, which should be displayed by an externally mounted pressure gauge commonly provided by most back pressure valve manufacturers and suppliers.
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