Excess pressure valve · Diaphragm and bellows
S1 excess pressure valve
Up to DN150
Self-operated, single-seat excess pressure valve with a diaphragm actuator and a stem sealed by a stainless steel bellows. It limits the inlet pressure to the set value, from 0.5 to 15 barg, on steam, compressed air, gases, and liquids, from DN15 to DN150.
- SizesDN15 to DN150
- Pressure ratingPN25 – PN40 Class 150 – 300
- Set pressure range0.5 to 15 barg
- Temperatureup to 220 °C depending on diaphragm
- Kv3.5 to 240 m³/h
- FluidsSteam, liquids, air, and gases
Questions about set pressure or sizing? Call 933 720 888 or email valfonta@valfonta.com.
- In-house design and manufacturing in Badalona
- ATEX II 2 GD marking for zones 1 and 21
- Stem sealed by a double-layer stainless steel bellows
- Genuine spare parts by serial number
What the S1 is used for
The S1 is a self-operated excess pressure valve that needs no auxiliary power. It stays closed while the inlet pressure is below the set value and opens proportionally when the pressure exceeds it, so the upstream pressure does not rise above the set value.
It goes up to DN150 and a Kv of 240 m³/h. The valve stem is sealed by a double-layer stainless steel bellows with an anti-rotation system, and the diaphragm is fabric-reinforced. Learn more about self-regulating excess pressure valves.
- SteamUp to 220 °C with EPDM+PTFE diaphragm and condensate pot.
- Compressed air and inert gasesLimits line pressure to the set value.
- Water, oil, and other liquidsProportional relief of excess pressure.
- Large-diameter linesFrom DN15 to DN150, with Kv up to 240 m³/h.
- ATEX zonesCategory II 2 GD for zones 1, 2, 21, and 22.
S1 technical specifications
| Type | Self-operated, single-seat globe valve with diaphragm actuator |
|---|---|
| Sizes | DN15 to DN150 |
| Connections | EN PN16 to PN40 flanges · ANSI 150 / 300 flanges · BSP or NPT thread from ½″ to 2″ · Weld ends on request |
| Pressure rating | PN25 – PN40 · ANSI Class 150 – Class 300 |
| Set pressure range | 0.5 to 15 barg, depending on actuator and springs |
| Maximum differential pressure | 16 bar (DN15–DN50) · 12 bar (DN65–DN80) · 10 bar (DN100) |
| Kv coefficient | From 3.5 m³/h (DN15) to 240 m³/h (DN150). Reduced Kv or Cv on request |
| Temperature depending on diaphragm | NBR: −20 to 80 °C · EPDM: −40 to 125 °C · EPDM + PTFE: 125 to 220 °C |
| Temperature depending on seat | NBR: 80 °C · EPDM or FPM: 150 °C · PTFE + graphite: 220 °C |
| Body material | A216 WCB carbon steel or A351 CF3M stainless steel. On request: RG10 bronze, duplex, or super duplex |
| Internals | AISI 316L stainless steel. Double-layer stainless steel bellows |
| Pressure sensing point | External 10 × 1 mm control line. On request, direct sensing in the body for set pressures above 1 bar |
| Condensate tank | Required for steam and for liquids above 125 °C |
| ATEX | II 2 GD c IIC Tx / c IIIC TxºC · certificate LOM 14.034 U |
| Pressure Equipment Directive | PED 2014/68/EU: the nameplate indicates whether CE marking is required |
Dimensions and weight
| DN15 | DN20 | DN25 | DN32 | DN40 | DN50 | DN65 | DN80 | DN100 | DN125 | DN150 | |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Kv (m³/h) | 3,5 | 5 | 9 | 13,5 | 22 | 32 | 57 | 82 | 115 | 190 | 240 |
| Length A (EN) | 130 | 150 | 160 | 180 | 200 | 230 | 290 | 310 | 350 | 400 | 480 |
| Length A (ANSI 150) | – | – | 184 | – | 222 | 254 | 276 | 298,5 | 352,5 | – | 451 |
| Length A (ANSI 300) | – | – | 197 | – | 235 | 267 | 292 | 317,5 | – | – | – |
| Height L | 440 | 445 | 450 | 455 | 463 | 475 | 560 | 560 | 575 | 600 | 640 |
| Weight (kg) | 20 | 22 | 24 | 28 | 32 | 35 | 52 | 57 | 68 | 85 | 105 |
Dimensions in millimeters. DN150 EN: 450 mm on request. Dimensioned drawings in the datasheet.
Set pressure ranges and actuator diameter (mm)
| Range (barg) | DN15 to DN50 | DN65 to DN100 | DN125 and DN150 |
|---|---|---|---|
| 0,5 – 1,5 | 295 | 295 | 350 |
| 1 – 3 | 255 | 255 | 295 |
| 2 – 5 | 230 | 230 | 255 |
| 4 – 8 | 175 | 175 | 230 |
| 7 – 12 | – | 175 | 175 |
| 7 – 15 | 175 | On request | On request |
Exploded view and materials (22 parts)
| No. | Part | Material |
|---|---|---|
| 1 | Body | A216 WCB (1.0619) · A351 CF3M (1.4408) · RG10 bronze |
| 3, 39 | Seat holders | 316L stainless (1.4404) |
| 6, 51 | Covers | 316L stainless; no. 6 also in steel 1.1191 |
| 8 | Bellows guide | Steel 1.0570 or 1.1191, or 316L stainless |
| 9, 22 | O-rings | Viton |
| 10 | Springs | Spring steel 55Si7 (1.0904) |
| 11 | Ball bearing | Steel 100Cr6 (1.3505) |
| 12, 34 | Spring supports | Steel 1.1191 |
| 13 | Adjusting nut | Steel 8.8 |
| 14, 23, 24, 31, 32 | Plate, diaphragm plate and stem, column, and bellows nut | Steel 1.1191 |
| 7, 15, 16, 17, 27, 50 | Screws and nuts | Steel 8.8 (screws 7 and 50 also in A2 stainless) |
| 18 | Fitting | Brass |
| 19 | Diaphragm | EPDM or EPDM + PTFE |
| 20, 26 | Upper and lower actuator | Steel sheet 1.0335 with epoxy paint or AISI 316 stainless |
| 21, 29 | Diaphragm screw and adjusting stem | AISI 304 stainless (1.4301) |
| 25, 25b, 42 | Screw and nuts | A2 stainless |
| 28 | Anti-rotation system | Steel 1.1191 |
| 30 | Bearing guide | AISI 304L stainless (1.4307) |
| 33 | Bellows | Stainless steel |
| 35, 52 | Gaskets | Metal-reinforced graphite |
| 40 | Seat | Graphite-filled PTFE (others on request) |
| 45 | Seat | 316L stainless (1.4404) |
How the S1 works
The S1 works on a balance of forces between the adjusting springs and the inlet pressure acting on the diaphragm.
- Set pressure. The springs (K) compress the diaphragm (H) with the force set on the adjusting nut (F).
- Valve closed. Below the set value, the springs hold the plug against the seat.
- Proportional opening. The inlet pressure reaches the diaphragm through the control line (L). If it exceeds the set value, the valve opens in proportion to the pressure rise.
- Closing. When the pressure falls back below the set value, the springs close the valve.
- Stem sealing. The double-layer bellows (E) seals the stem; an anti-rotation system protects it.
An excess pressure valve is not a safety valve: if you need overpressure protection, also install one of our safety valves. See the difference between a safety valve and a relief valve.
S1 installation
The manual recommends installing the S1 with isolation valves on both sides, a strainer, inlet and outlet pressure gauges, and, if needed, a condensate pot. It is best installed on a bypass with its own isolation valves, rejoining the main pipe downstream of the pressure sensing point.
Installation and start-up
- Clean the pipe and install a strainer upstream of the valve to protect the seat.
- Follow the arrow on the body and install the valve in a horizontal pipe (with a slight slope if condensate is present).
- Mounting position: the standard position, with the actuator below the body, is suitable for any fluid above 0 °C; with the actuator above, only for gases and liquids up to 80 °C. Other positions are not permitted.
- Support the pipe, not the valve or the actuator: place supports close to the flanges.
- Connect the control port to the inlet pipe, at least 1 m away (or 10 × DN), using 10 × 1 mm tubing. If the pressure fluctuates, add a needle valve.
- Condensate tank for steam or liquids above 125 °C: at the highest point, with a slight slope toward the pipe, and filled until it overflows.
- Start up slowly: with the needle valve open and the system warm, open the isolation valves gradually, inlet first.
- Adjust the set pressure with the adjusting nut (Valfonta supplies the valve preset). Compressing the spring (turning clockwise) raises the set pressure. Check it on the inlet pressure gauge.
Installation must be carried out by specialized personnel, without exceeding the design pressure and temperature; in ATEX zones, ensure electrical conductivity with the installation. See also our guide on relief valve set pressure.
S1 technical documentation
S1 datasheet (in English)PDF · 6 pages · features, exploded view, dimensions, and set pressure ranges
To order spare parts, give the serial number (S/N) shown on the nameplate. Order certificates: see our quality certificates.
S1 vs. other excess pressure valves
The range covers DN15 to DN150 and set pressures from 0.01 to 20 barg.
| Model | DN | PN | Set pressure (barg) | Max. T (°C) | Steam | Standard body |
|---|---|---|---|---|---|---|
| PRV53 | 15 – 25 | 25 | 0,5 – 15 | 150 | No | CF3M stainless |
| PRV54 | 15 – 25 | 25 | 0,5 – 12 | 210 | Yes | CF3M stainless |
| PRV55 | 15 – 25 | 25 | 0,01 – 8 | 150 | Yes | CF3M stainless |
| S1 | 15 – 150 | 40 | 0,5 – 15 | 220 | Yes | Carbon steel or CF3M stainless |
| S2 | 15 – 100 | 40 | 0,02 – 10 | 180 | Yes | Carbon steel or CF3M stainless |
| S3 | 15 – 150 | 40 | 0,5 – 20 | 220 | Yes | Carbon steel or CF3M stainless |
| S4 | 15 – 100 | 40 | 0,5 – 20 | 220 | Yes | Carbon steel or CF3M stainless |
Data from the Valfonta summary catalog (2025); S2 set pressure from its datasheet. All accept water, oil, air, and non-flammable gases.
Frequently asked questions about the S1
Can the S1 be used as a safety valve?
No. It must be used only as a pressure relief valve. If the installation needs overpressure protection, also install a safety device, such as our safety valves.
Can it be used with steam?
Yes, up to 220 °C with an EPDM + PTFE diaphragm. A condensate pot must be installed to protect the diaphragm, and it must be filled until it overflows before start-up.
How do you adjust the set pressure?
With the adjusting nut and a standard tool: turning clockwise compresses the spring and raises the set pressure. Valfonta supplies the valve preset to the requested value to prevent premature diaphragm failure.
Can it be installed in an ATEX zone?
Yes. It carries ATEX II 2 GD category (certificate LOM 14.034 U) for zones 1 and 2 (flammable gases, vapors, and liquids) and 21 and 22 (combustible dust), according to the temperature class that corresponds to the fluid temperature.
What maintenance does it need?
Periodically check the gaskets and wear parts; the frequency depends on the operating conditions. This can be done with the valve installed, depressurized, and isolated, and only with genuine spare parts.
Other excess pressure valve models
S3 relief valveSpring-loaded, up to DN150 and 20 barg set pressure.
S2 excess pressure valveDiaphragm with internal sensing, DN15 to DN100.
PRV53 excess pressure valveCF3M stainless, DN15 to DN25, set pressure from 0.5 to 15 barg.- PRV54 excess pressure valveCF3M stainless, DN15 to DN25, steam up to 210 °C.
PRV55 excess pressure valveSet pressures from 0.01 barg, DN15 to DN25.
Request a quote for the S1
Give us your installation details and we will recommend the right size, Kv, actuator, and diaphragm for your set pressure.
- Fluid and operating temperature
- Set pressure and normal operating pressure
- Flow rate the valve must discharge
- Size, body material, and connection type
You can also call 933 720 888.
