API · ASME · DIN · EN standard industrial valves
Pressure Relief / Sustaining Valves
Pressure relief / sustaining valves guard upstream pressure: a normally-closed pilot senses the inlet line, the main valve stays closed while upstream pressure is below the pilot setting, and above the setting the pilot modulates the diaphragm chamber so the valve opens to spill surplus flow and hold a minimum upstream (sustaining / backpressure) level. The same pilot-controlled body serves pure relief, backpressure and unloading duty, and in a line between an upper zone and a lower heavy-demand area it stops the lower zone "robbing" the upper one. Response is surge-aware — opens fast, closes gradually — and settings are changed on the pilot within discrete spring ranges. Benchmark coverage is a 500X-class hydraulic diaphragm valve, DN20 to DN800, PN1.0/1.6/2.5, ductile-iron body with diaphragm actuation and pilot control; water service to about 82 degC for standard elastomers.
Part of the Pressure Reducing Valves family
Last updated: 2026-10
Family specifications
| size range | DN20-DN800 |
|---|---|
| pressure class | PN1.0, PN1.6, PN2.5 |
| outlet pressure range | 0 to 75 psi, 20 to 105 psi, 20 to 200 psi, 100 to 300 psi |
| body materials | Cast Iron, Ductile Iron, Carbon Steel, Stainless Steel |
| end connections | Flanged; per-size D/D1/D2 and bolt pattern tabulated for PN1.0, PN1.6 and PN2.5 |
| model designation | 500X pressure relief valve (water control valve line) |
| dimension tables | Face-to-face (L), face-to-centre (A/A1), height (H/H1) and flange dimensions tabulated per DN from 20 to 800 |
| diaphragm materials | EPDM, NBR |
| disc materials | Cast Iron, Ductile Iron, Carbon Steel, Stainless Steel |
| pilot valve materials | Stainless Steel, Brass |
| operation | Hydraulically operated, pilot-controlled, modulating |
| operating principle | A hydraulically operated, pilot-controlled modulating valve that maintains constant upstream (inlet) pressure within close limits; actuated by line pressure through a pilot control system. |
| functions supported | The same pilot-controlled body serves pressure relief, pressure sustaining, back pressure, or unloading functions in a bypass system. |
| zone protection | Installed between an upper zone and a lower area of heavy demand, the valve maintains desired upstream pressure to prevent "robbing" of the upper zone. |
| surge response | The valve opens fast to maintain steady line pressure but closes gradually to prevent surges; operation is fully automatic and pressure settings are easily changed. |
| fluid temperature limit | Water service to 180 degF (approx. 82 degC) for the standard elastomer configuration. |
| vfd pump duty | Complements VFD pump control: controls the maximum system pressure when VFD-driven pumps run at minimum speed. |
| pilot setpoint behavior | The normally-closed pilot senses upstream pressure through a connection at the valve inlet; valve and pilot remain closed until inlet pressure exceeds the pilot setting, and the pilot modulates the diaphragm chamber pressure to hold the set-point. |
| low pressure failsafe | If upstream pressure falls below the set-point, the valve closes or modulates so the set-point is maintained; the valve ensures minimum upstream pressure for critical use. |
Ordering & documentation
Documents
Ask for the documentation set with the inquiry — typical items (material certificates, test reports to the listed inspection standard, dimensional drawings) are confirmed and supplied with the quotation.
Quotation
Send the medium, temperature, pressure, size and connection for a quotation-ready inquiry. Lead time, MOQ and price are confirmed at quotation, together with the documents to be supplied with the order.
Sample & supply
Supply terms, order quantities and lead time are agreed per order — state the destination country and required schedule with the inquiry.
Storage & shipping
Store valves with seats and seals protected from UV, ozone and deformation, and keep end-face protectors on until installation; storage limits and shipping arrangements for the selected materials are confirmed at quotation.
Frequently asked questions
What sizes and pressure ratings are available for pressure relief / sustaining valves?
The published size range is DN20-DN800, and the published pressure ratings are PN1.0, PN1.6, PN2.5. Availability of a specific size and rating is confirmed at quotation.
What materials are pressure relief / sustaining valves available in?
The published specification lists body materials: Cast Iron, Ductile Iron, Carbon Steel, Stainless Steel. Material availability for a specific size and rating is confirmed at quotation.
What end connections do pressure relief / sustaining valves support?
The published specification lists Flanged; per-size D/D1/D2 and bolt pattern tabulated for PN1.0, PN1.6 and PN2.5 as the available end connections. The connection for a specific size is confirmed at quotation.
What should a quotation inquiry for pressure relief / sustaining valves include?
The governing standard and class, nominal size, body and seat materials, the service medium with temperature, and the destination — anything missing is confirmed during quotation.
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Model
Pressure Relief and Sustaining Valve
The pressure relief and sustaining valve is the pilot-controlled execution of the pressure reducing family that guards upstream pressure: it stays closed while
View specifications →Model
500X Pressure Relief Valves
The 500X pressure relief valve is the hydraulically operated, diaphragm-actuated execution of the water-control relief family: a pilot senses upstream pressure
View specifications →Request a quote for pressure relief / sustaining valves
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