Product Category
Safety Valves & Pressure Relief Valves
Automatic overpressure protection for vessels, piping, and process systems. Spring-loaded, balanced bellows, and pilot-operated designs for steam, gas, and liquid service. API 520/526, ASME Section VIII, EN ISO 4126 certified.
Safety Valve vs Relief Valve
The terms are often used interchangeably but have distinct technical meanings under ASME and API standards:
- Safety valve: Operates with a full-open "pop" action — rapid opening to full lift on reaching set pressure. Used primarily for compressible fluids (steam, gas, vapour). The rapid full opening is essential to relieve large quantities of gas quickly.
- Relief valve: Opens proportionally to overpressure above the set point — gradual opening proportional to the excess pressure. Used primarily for incompressible liquids (hydraulic systems, liquid process lines) where proportional relief prevents excessive blowdown.
- Safety relief valve (SRV): Can function as either a safety valve or a relief valve depending on the media — dual-service design used for vessels handling both liquid and vapour.
Operating Principles
Spring-Loaded Safety Valves
A compression spring holds a disc against a nozzle seat. When process pressure acting on the disc area exceeds the spring force (set pressure), the valve opens — in a safety valve, the disc lifts to a fixed stop for full flow area, then reseats at blowdown pressure (typically 7–10% below set pressure). The most common design for steam, air, and gas service. Simple, reliable, no external power required.
Direct spring-loaded safety valves are sensitive to back pressure in the outlet (discharge) piping — back pressure reduces the effective set pressure. If variable back pressure exceeds 10% of set pressure, a balanced bellows design should be specified.
Balanced Bellows Safety Valves
A bellows element is installed between the disc and the spring housing. The bellows is vented to atmosphere — it cancels the effect of back pressure on the disc, so set pressure is not affected by variable built-up back pressure in the discharge piping. Required when:
- Variable back pressure exceeds 10% of set pressure (API 520 guideline)
- The media is corrosive and must be isolated from the spring and bonnet
- Low blowdown is required (the bellows allows consistent blowdown setting regardless of back pressure)
Brands: Emerson/Crosby 900 series, Flowserve/Farris 2600 series, Leser Type 441, Consolidated 1900 series.
Pilot-Operated Safety Valves (POSV)
The main valve disc is held closed by process pressure acting on its full bore area (dome-loaded). A small pilot valve monitors line pressure and controls a sense line — on reaching set pressure, the pilot vents the dome, and the process pressure under the disc lifts it open. Key advantages:
- Can be set to open at up to 98% of MAWP — much closer to operating pressure than spring-loaded types (typically set at 90% of MAWP to avoid simmer)
- Back pressure has no effect on set pressure
- Suitable for very large bore sizes where spring-loaded designs would be impractical
- Used in oil & gas, pipelines, storage tanks, and petrochemical process vessels
Brands: Emerson/Crosby JOS-E/JBS-E, Flowserve/Farris pilot-operated range, Consolidated pilot-operated, Curtiss-Wright/CIRCOR.
Sizing and Standards
- API 520 Part I: Sizing, selection, and installation of pressure-relieving devices — the primary sizing standard for refinery and chemical plant applications.
- API 520 Part II: Installation guidelines — inlet and outlet pressure drop, reactive forces, tailpipe sizing.
- API 526: Flanged steel pressure-relief valves — standard orifice designations (D through T), body dimensions, and materials. Specifying API 526 ensures interchangeability between manufacturers.
- ASME Section VIII Appendix 11: Rules for the design and inspection of pressure vessels — establishes required relieving capacity and MAWP for vessels.
- EN ISO 4126: European safety devices for protection against excessive pressure — Parts 1 (direct-acting), 2 (bursting discs), 4 (pilot-operated) are the main parts for safety valves.
- PED (Pressure Equipment Directive) 2014/68/EU: Mandatory CE marking for safety devices used in EU pressure systems.
API 526 Orifice Designations
API 526 defines standard orifice sizes (effective flow areas) identified by letter designations D through T. These ensure that a safety valve from Crosby, Farris, or Consolidated with the same orifice letter designation has the same flow area — critical for sparing and interchangeability:
| Orifice | Effective Area (in²) | Effective Area (cm²) |
|---|---|---|
| D | 0.110 | 0.71 |
| E | 0.196 | 1.26 |
| F | 0.307 | 1.98 |
| G | 0.503 | 3.24 |
| H | 0.785 | 5.06 |
| J | 1.287 | 8.30 |
| K | 1.838 | 11.86 |
| L | 2.853 | 18.41 |
| M | 3.600 | 23.23 |
| N | 4.340 | 27.99 |
| P | 6.380 | 41.16 |
| Q | 11.05 | 71.29 |
| R | 16.00 | 103.2 |
| T | 26.00 | 167.7 |
Safety Valve Brands We Supply
| Brand | Parent | Key Products | Specialty |
|---|---|---|---|
| Crosby | Emerson | JOS-E, JBS-E, HCI, HE series | World's leading safety valve brand. Full API 526 range, pilot-operated. |
| Farris Engineering | Flowserve | 2600, 2700, 2800 series | Balanced bellows, spring-loaded — API 526 interchangeable. |
| Consolidated | Baker Hughes | 1900, 2900, 3900 series | API 526, pilot-operated, steam service — long track record. |
| Leser | Leser GmbH (Germany) | Type 441, 526, 630 series | European market leader. Full EN ISO 4126 and API 526 range. |
| Spirax Sarco | Spirax-Sarco Engineering | SV600, SV620, SV640 series | Steam safety valves — low-pressure steam, condensate systems. |
| Weir Group | Weir Group plc | Sebim pilot-operated range | High-capacity pilot-operated safety valves for power and oil & gas. |
| CIRCOR / Petroval | CIRCOR International | Pilot-operated and conventional range | Process industry, petrochemical safety relief. |
Frequently Asked Questions
What information is needed to size a safety valve?
To size a safety valve per API 520, you need: relieving fluid (gas, vapour, liquid, or two-phase), molecular weight or fluid properties, relieving pressure (set pressure + allowable overpressure — typically 110% of set pressure for process vessels), relieving temperature, required relieving capacity (from contingency analysis — fire case, blocked outlet, etc.), back pressure (built-up and superimposed), and inlet/outlet flange ratings. From these inputs, the required effective area is calculated, and the appropriate API 526 orifice designation selected. Send us your process data sheet or relief valve data sheet and we will confirm the correct model.
What is the difference between set pressure and MAWP?
MAWP (Maximum Allowable Working Pressure) is the maximum pressure at which a pressure vessel or piping system is designed and certified to operate — established during design per ASME Section VIII or equivalent. Set pressure is the pressure at which the safety valve begins to open. For a single safety valve protecting a vessel, the set pressure must not exceed the MAWP. In practice, set pressure is often 5–10% below MAWP to provide a margin against simmer (partial opening at pressures close to set). For fire case relief valves (a second valve), the set pressure may be up to 110% of MAWP.
Do you supply safety valves with third-party certification?
Yes. We supply safety valves with ASME "UV" stamp (required for ASME Code pressure vessels in the USA and Canada), PED CE marking (required for European pressure equipment), and API 526 compliance documents. For nuclear applications, ASME N-stamp certified safety valves (Crosby, Consolidated, Velan) are available. All safety valves we supply come with manufacturer test certificates, nameplate data, and traceability documentation. For certified replacements on existing installations, provide the existing valve nameplate details and we will match the approved replacement.