A boiler safety valve is a spring-loaded valve on the shell that opens by itself when steam pressure reaches the set pressure and reseats when pressure falls back. It holds the vessel below its design pressure with nobody present. The pair on the Cochran we surveyed read 180 °C.
Boilers/Cochran/Rear and steam set/Mountings/This page
Dead weight. A weight holds the disc down. Found on older and low-pressure vessels, rarely on a modern package boiler.
Spring loaded. A spring holds the disc down and the set pressure is the spring compression. The ordinary fitting on a shell steam boiler.
High lift. Escaping steam lifts the disc further, so the valve passes more for the same seat diameter.
Improved high lift. A second lifting surface takes the disc further again, which is what lets a smaller valve discharge the full evaporation of the boiler.
The two valves on the machine we surveyed are spring loaded, on a common pad on the shell top.
Set pressure comes from the design of the vessel and is recorded on its documentation. Setting and testing is work for a competent person. In the United Kingdom the examination regime around it is the Pressure Systems Safety Regulations 2000, with the written scheme of examination naming the valve as a protective device. The shell boiler standard is BS EN 12953.
We did not set, test or lift the valves on the machine we surveyed. What we measured is the outside of the bodies while the boiler was running.
Where a shell boiler needs a second safety valve is decided by its output and by the standard it was built to. The machine we surveyed carries two. We have not verified the threshold that put them there and we quote none.
| Safety valve | Pressure relief valve | |
|---|---|---|
| Medium | Compressible: steam, gas | Incompressible: water, oil |
| Opening | Pops fully open at set pressure | Opens progressively as pressure rises |
| Purpose | Protects the vessel from overpressure | Limits pressure in a liquid circuit |
| On this machine | Two, on the shell top, 180 °C | Outside the surveyed views |
| Element | Temp °C | Area m² | Bare W | Panelled W | Surface °C |
|---|---|---|---|---|---|
| Safety valves ×2 | 180 | 0.68 | 1,963 | 99 | 39 |
| Total | 0.68 | 1,963 | 99 | ≤45 |
Temperatures are FLIR readings taken on a running boiler. Areas come from the CAD model of the same machine, uplifted for bolts and irregular geometry. Losses are ISO 12241:2022 steady state. The panelled column is capped at the 45 °C touch-safe design target, assessed to EN ISO 13732-1.
Set pressure comes from the design of the vessel and is recorded on its documentation. The work below is for a competent person under the Pressure Systems Safety Regulations 2000; the valve is named in the written scheme of examination as a protective device, and the shell boiler standard is BS EN 12953.
Inzonex did not set, lift or test the valves on the machine surveyed. What we measured is the outside of the two bodies while the boiler was running: 180 °C over 0.68 m², 1,963 W bare. A removable module leaves the easing gear, the cap and the discharge line clear, so none of the steps above changes once the valve is panelled.
A spring-loaded valve on the shell that opens by itself at set pressure and reseats when pressure falls, holding the vessel below its design pressure.
Dead weight, spring loaded, high lift and improved high lift. The two on the machine we surveyed are spring loaded.
A competent person, to the set pressure recorded for the vessel. In the UK the examination regime is the Pressure Systems Safety Regulations 2000 and the shell boiler standard is BS EN 12953.
The machine we surveyed carries two on a common pad. Whether a second is required depends on the output of the boiler and the standard it was built to; we have not verified the threshold and quote none.
A safety valve works on steam or gas and pops fully open at set pressure. A pressure relief valve works on liquid and opens progressively as pressure rises.
The pair we surveyed lose 1,963 W over 0.68 m² at 180 °C. Panelled they shed 99 W with the outside at 39 °C, and both keep their lift.
The surveyed view this page came from, with every element on it.Open
→/ 02The other five mountings measured on the same view, and where the split between mountings and accessories falls.Open
→/ 03The other mounting operated on a cycle, measured at 180 °C across two flanges.Open
→| Survey | FLIR S62 Pro, emissivity 0.90, reflected 25 °C, 3 m, full radiometric correction. UK commercial boiler house. |
| Geometry | Rebuilt in CAD from the surveyed machine. Area allowances are stated on the page they are used on and are allowances, not measured areas. |
| Calculation | ISO 12241 steady state. Heat loss is calculated from the measured surface temperature and the modelled area — it is not metered. Full method, basis and limits. |
| Regulation | The Pressure Systems Safety Regulations 2000 · HSE L122, Safety of pressure systems · HSE INDG261, Pressure systems |
Where surface loss sits inside the indirect method, and what 49.8 kW is against the fuel input.Open
→/ 02What has to be opened, how often, and who signs it off under PSSR 2000.Open
→/ 03Which machine this is, and why the losing surfaces differ between the two.Open
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