API · ASME · DIN · EN standard industrial valves
Pilot-Operated Safety Valves
Pilot-operated safety valves pair a main valve with an external pilot: the pilot senses line pressure and controls the main valve, giving full lift at set pressure with short blowdown while staying dynamically tight below it — so operating pressure can run close to set pressure without simmering or media loss. Typical class coverage runs DN25-DN200 (1" x 2" to 8" x 10" inlet x outlet) at ANSI Class 150-600 / PN16-PN100 in WCB, CF8 or CF8M bodies; the set-pressure window is confirmed against required capacity and pilot selection at RFQ. Pilots are pop-action or modulating and field-selectable; modulating types tolerate higher superimposed back pressure. Flanged (RF) ends. The pilot architecture is what buys the margins: with a dome area larger than the seat area the main valve stays tight to within 5% of set pressure (98% seat tightness) where spring valves already simmer, pop-action pilots open at about 1% overpressure and reseat at 97% (adjustable to 85%, blowdown -3% to -15%), modulating pilots track demand and reach full lift at 110% of set, and back pressure up to 70% - exceptionally 95% - is compensated with a backflow preventer. The governing failure mode is the pilot sensing line: particle-laden or crystallizing media plug its ~10 mm bore (keep the valve to clean service, mind coking and polymerization), and a rapid startup ramp can unbalance the dome and fake a lift. Installation keeps the remote pickup at 3/8-inch equivalent area up to 100 feet with no block valve in the supply line, and the field test connection allows in-service functional testing. ASME VIII UG-126(b) requires the pilot self-actuated and the main valve fail-open at full capacity. Selection turns on the operating-to-set-pressure ratio, back pressure, media and pilot action.
Part of the Safety Valves family
Last updated: 2026-11
Family specifications
| pressure class | ANSI Class 150-600 / PN16-PN100 |
|---|---|
| size range | DN25-DN200 (1in x 2in - 8in x 10in inlet x outlet) |
| body materials | WCB, CF8, CF8M |
| end connections | flange (RF) |
| temperature range | -162 to 320°C |
| pilot type | pop-action or modulating; field-selectable |
| design note | main valve + external pilot; full-lift at set pressure with short blowdown |
| operating pressure ratio vs spring limit | Spring-loaded valves struggle to close completely above roughly 90% of set pressure; the pilot-operated main valve closes with a dome area larger than the seat area so closing force exceeds opening force, letting system operating pressure run within 5% of set pressure without increased seat leakage - the standard choice for high operating-to-set ratios |
| pop action reseat and adjustable blowdown | Pop-action pilot: opens at about 1% overpressure, closes at 97% of set pressure adjustable down to 85%, with blowdown adjustable between -3% and -15% above the API standard; modulated-closure pilots hold blowdown to max 7% |
| modulating vs pop pilot principle | Pop-action fully empties the main-valve dome for snap full lift; in modulate action the dome is only partially emptied, with the extent of emptying determined by the inlet pressure, so the main valve modulates with the relief demand |
| modulating partial opening capacity control | Modulating diaphragm-pilot execution: the valve opens further as process pressure rises and reaches full lift at 110% of set (High Gain option by 105%); the pilot is the nonflowing type - with the main valve relieving at steady pressure, no process fluid flows through the pilot |
| back pressure tolerance | Pilot actuation is differential, so back-pressure tolerance is high: pilot-operated safety valves can consistently compensate 70% back pressure and in certain cases run at a 95% back-pressure ratio; operability stays unaffected up to 70% of set pressure provided a backflow preventer is used whenever back pressure can exceed inlet pressure |
| seat tightness high operating ratio | Pilot-operated SRVs can achieve seat tightness up to 98% of set pressure for both the main valve and the pilot valve - the basis for running close to set pressure without simmer |
| posv definition | A pilot-operated safety relief valve is a pressure relief valve in which the major relieving device (main valve) is combined with and controlled by a self-actuated auxiliary pressure relief valve called a pilot valve |
| asme ug 126b pilot requirements | ASME Code Sec. VIII UG-126(b) permits pilot-operated pressure relief valves provided the pilot is self-actuated and the main valve opens automatically at not over set pressure and discharges full rated capacity even if some essential part of the pilot fails - the fail-open requirement |
| pilot blowdown short adjustment | The pilot portion of some pilot-operated valves can be set at blowdowns as short as 2% - far tighter than direct-spring pop valves |
| sensing line blockage failure mode | The pilot sensing (pressure-tap) line is the governing failure mode: media containing particles or prone to crystallization can plug the roughly ten-millimeter line, after which the safety valve can no longer perform its intended function; pilot-operated valves are supplied with filters and generally recommended for relatively clean service |
| dirty service limits | Pilot-operated valves should not be employed in abrasive or dirty service, where coking, polymerization or corrosion of wetted pilot parts can occur, or where the lading fluid can freeze or condense at ambient temperature; for severe dirty service where interconnecting tubing can clog, a pilot-operated SRV might not be the best fit |
| rapid pressure ramp false lift | During plant start-up with an extremely rapid system-pressure ramp rate, the dome area may not equalize with the same inlet pressure, resulting in insufficient closing force - a pilot-operated-specific false-lift mode |
| soft seal and temperature limits pilot | Pilot soft seals (O-rings) bound the application envelope: at temperatures of 200 °C and above the safety valve manufacturer should be consulted |
| sensing line remote pickup sizing rules | Installation rule for remote pressure pickup: piping must have the equivalent flow area of 3/8 in tubing for lengths up to 100 feet, longer runs require factory consultation; a block valve in the remote pilot supply line is not recommended and, if used, must be opened before pressurizing the main valve |
| field test connection in service | A field test connection allows operators to functionally test the pilot-operated SRV while it remains in service - a maintenance advantage over direct spring valves that must be pulled for bench testing |
| oem maintenance resources posv toolbox | OEM maintenance reference: the valve manufacturer publishes a dedicated maintenance toolbox for pilot-operated types 811 (pop action) and 821 (modulating) with repair guidance to the manufacturer quality standards |
| set pressure range example diaphragm pilot | OEM example envelope (pilot-operated relief valve with diaphragm pilot): set pressure range 15 psig to 740 psig; reseat pressure defined as the supply pressure where the dome pressure increases to 75% of supply pressure - an industry-reference anchor, not a tenant supply claim |
| pon1 orifice area range | effective orifice areas of 0.128 through 45.664 inches |
| pon1 inlet size range | valve inlet sizes 1-inch through 8-inch |
| pon1 set pressure range | set pressures from 15 to 3,705 psig (1 to 260 barg) |
| pon1 continuous temperature range | Continuous service temperatures from -50°F to +450°F |
| pon1 nonflowing pilot principle | with the non-flowing pilot provides highly reliable system overpressure protection |
| pon1 dirty service pilot design | The pilot is a non-flowing design minimizing the entry of dirt and the accumulation of hydrates |
| pon1 pilot supply filtration | Inline tubing external filter is also available |
| pon1 vertical pilot mounting | Consistent set pressure and blowdown are assured compared to horizontally mounted pilots that are erratic |
| pon1 orifice nozzle replacement | The orifice size may be changed by replacing a single component |
| pon1 external blowdown adjustment | Adjustable blowdown is external, reducing time and cost-consuming expenditures of valve removal |
| pon1 replaceable soft seat | With a replaceable soft seat, expensive parts and time-consuming lapping of metal seats are no longer necessary |
| pon1 asme capacity verification | capacity is assured by an independent third party, the ASME National Board Laboratory in Columbus, Ohio |
| pon1 operating ratio capability | The valve can be set within 5% of system operating pressure allowing the process to be maintained at close tolerance without valve leakage |
| pon1 typical applications | for Gas Compressors, Pressure Vessels, Cold Box Storage, Meter Runs, Gas Plants, Pipelines, and Transmission Stations |
| pon2 low pressure vacuum protection | provide safe, dependable, and accurate low pressure and/or vacuum protection |
| pon2 api2000 sealing | Seals in accordance with API 2000 requirements for Pilot Operated Relief Valves |
| pon2 rated flow overpressure | Rated flow at 10% overpressure provides the ability to operate closer to the tank MAWP, increasing the operating range of the process |
| pon2 seat action flexibility | Flexibility in terms of film or o-ring seat and snap or modulating action allows product customization to specific application requirements |
| pon2 vacuum breaker variant | The Model 1662A incorporates a vacuum breaker. |
| pon2 size range | Sizes: 2” through 12” (50-300mm) |
| pon2 pressure settings | Pressure Settings: from 7.0* InWC to 15 psig |
| pon2 vacuum settings | Vacuum Settings: 7.0 InWC to 12 psig |
| pon2 standard body materials | Standard Body Materials: Carbon Steel (WCB/CS), Stainless Steel (CF8M/316), Aluminum (356) |
| pon2 supply media temperature range | Supply Media Temperature Range: -320° F to 300° F |
| pon2 drilling classes | 150# ANSI, PN10, PN16, JIS drilling classes available |
| pon2 pilot accessories | Pilot exhaust piped to discharge header, Field test connection, Manual blow down, Conical film seat pallet, Remote sense pickup, Pilot supply filter |
| pon3 size range | Size 1/2” to 20” |
| pon3 pressure rating | Pressure Rating 150# x 150# to 2500# x 300# |
| pon3 end connections | End Connection Flange End, Screwed End, Tri-Clover End |
| pon3 seat options | Seat Metal Seat / Soft Seat |
| pon3 orifice standard | Orifice As per API 520 |
| pon3 blowdown and overpressure | Blow Down <10% of Set Pressure, Over Pressure >10% of Set Pressure |
| pon3 body materials | Material Carbon steel, Stainless steel, Duplex-SS, nickel-molybdenum alloy, Alloy-20, nickel-copper alloy |
| pon3 pilot sensing design | Design - High Pressure, Semi- Nozzle/ Full- Nozzle, Piston Sensing Pilot |
| pon3 type options | Type Conventional, Balanced Bellow, Balance Piston, Pilot Operated, PTFE Lined |
| pon4 seat design options | Soft seat design: Provides repeatable bubble-tight performance before and after each relief cycle, Metal-to-metal seat design: Provides pilot valve performance in high temperature service |
| pon4 balanced design principle | Balanced design: Lift unaffected by back pressure; no expensive and fragile bellows required to balance against high back pressure |
| pon4 full lift dot maop capability | Full lift at set pressure: No overpressure required for full lift; allows D.O.T. installation to be set higher than Maximum Allowable Operating Pressure (MAOP) when pop action is used |
| pon4 replaceable seats and seals | Replaceable soft seats and seals: All seats and seals are renewable easily and quickly; no expensive, time-consuming seat lapping required |
| pon4 api526 center to face dimensions | API 526 direct spring center-to-face dimensions available for Series 200, 400, 500 and 800 pilots eliminating the need for piping modifications during upgrades |
| pon4 pop action cryogenic benefit | Pop action available: No main valve throttling, which helps prevent freeze-ups in cryogenic or refrigerant type services |
| pon4 modulating action benefits | Modulating action available: Significantly cut product loss per relief situation; reduce environmental pollution; avoid oversizing consequences; not as sensitive to inlet pressure losses as pop action |
Scope & source notes
| back pressure capability tension | Back-pressure capability tension: the page spec states superimposed back pressure up to ~50% of set, while OEM documents state 70% consistent compensation (LESER, 95% in certain cases; Crosby 70% with backflow preventer); the ~50% figure should be owner-reviewed against per-configuration OEM limits. |
|---|---|
| pilot closure figures | Generic vendor pages describe pilot closure near 90% of set pressure; OEM pop-action figures are 97% (adjustable to 85%, blowdown -3% to -15%) - the OEM figures govern; generic-website numbers were excluded from this page. |
| set pressure envelope | Emerson Series 400 shows a 15-740 psig (about 1-51 barg) set-pressure envelope, so the page 0.1-10 MPa range is a tenant selection, not an industry ceiling; temperature envelope (-162 to 320 °C) sits against soft-seal limits where the manufacturer must be consulted at 200 °C and above - quote per configuration. |
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 pilot-operated safety valves?
The published size range is DN25-DN200 (1in x 2in - 8in x 10in inlet x outlet), and the published pressure ratings are ANSI Class 150-600 / PN16-PN100. Availability of a specific size and rating is confirmed at quotation.
What materials are pilot-operated safety valves available in?
The published specification lists body materials: WCB, CF8, CF8M. Material availability for a specific size and rating is confirmed at quotation.
What working temperature range do pilot-operated safety valves cover?
The published working temperature range is -162 to 320°C. For a medium at the edge of the range, confirm the suitability of the selected seat and body materials at quotation.
What end connections do pilot-operated safety valves support?
The published specification lists flange (RF) as the available end connections. The connection for a specific size is confirmed at quotation.
What should a quotation inquiry for pilot-operated safety 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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