On a spring-loaded pressure relief valve the bonnet, or spring housing, encloses the spring stack, carries the adjusting screw and transmits the set-pressure load into the body. It also has to vent safely, guide the spring without buckling, and in balanced-bellows designs provide a sealed chamber. Casting gives the smooth internal spring bore and the external ribbing needed for stiffness without excess mass, while CNC machining establishes the spring seat, guide surfaces and thread that control set pressure. Ningbo Ruican casts these housings in CF8M, CF8, CF3M, duplex 2205, WCB, LCB/LCC and QT450-10, then machines and tests them to the purchaser’s specification.
Specification
| Parameter | Capability |
|---|---|
| Materials | CF8M / AISI 316, CF8 / 304, CF3M / 316L, duplex 2205, WCB carbon steel, LCB/LCC low-temperature carbon steel, ductile iron QT450-10 |
| Process | Silica-sol or water-glass investment casting; resin sand casting + CNC for large and high-pressure housings |
| Dimensional tolerance | ISO 8062 CT5–CT7; spring bore and seat faces to CT4–CT5 after machining |
| Size range | DN15–DN200 investment cast; DN200–DN400 resin sand cast |
| Pressure rating | Supplied to customer specification for ASME Class 150 / 300 / 600 / 900 / 1500 and PN10–PN250 designs |
| NDT | RT per ASTM E94 / E446, UT per ASTM A609, MT per ASTM E709, PT per ASTM E165 |
| Testing | Hydrostatic proof test of the pressure chamber, spring chamber leak test, dimensional verification of spring seat and guide |
| Heat treatment | Solution annealing for stainless and duplex; normalising or quench & temper for WCB and LCB/LCC |
| Surface finish | Shot blast, pickling and passivation; Ra 1.6 µm on spring guide bore, Ra 3.2 µm on sealing faces |
| Documentation | EN 10204 3.1 or 3.2, FAI and PPAP, PMI report, heat-to-part traceability |
Key features
- Smooth as-cast internal spring bore reduces wear on the spring OD and keeps the stack concentric.
- External stiffening ribs sized by FEA so the housing resists spring thrust without unnecessary mass.
- Spring seat, guide bore and adjusting screw thread machined in one setup for axial alignment.
- Cast-in vent bosses and drain ports located exactly where the purchaser’s drawing places them.
- Balanced-bellows variants supplied with a machined bonnet chamber and leak-test port.
- Low-temperature LCC housings Charpy impact tested to the purchaser’s specified temperature.
Applications
- Steam boiler and pressure vessel overpressure protection
- Refinery and petrochemical process relief and thermal relief duties
- Compressor, pump and pipeline surge protection skids
- Cryogenic and refrigerated storage tank relief valves in LCC
- Food, beverage and pharmaceutical process vessels in CF3M / 316L
Quality & compliance
Relief valve bonnets and spring housings are produced under an IATF 16949-certified quality system with full melt control, documented heat treatment and calibrated inspection. Each heat is released to EN 10204 3.1, with EN 10204 3.2 available when the purchaser nominates a third-party inspector. PMI verifies alloy chemistry on stainless and duplex production. Hydrostatic proof testing of the pressure chamber and leak testing of the spring chamber are performed per the purchaser’s procedure, and every dimension that affects set pressure is reported in the FAI. PPAP Level 3 supports series release. Components are supplied to customer specification with full traceability; we do not hold or claim an API monogram licence or a PED notified-body licence.
FAQ
Is the spring housing a pressure-retaining part?
On a conventional relief valve the bonnet encloses the spring but is not normally pressurised during relief, so it is a non-pressure-retaining part. On balanced-bellows and pilot-operated designs the bonnet chamber can see pressure, and we cast and test it as a pressure boundary. Please state the design intent on your drawing.
Can you hold the concentricity needed for stable set pressure?
Yes. The spring seat, guide bore and adjusting screw thread are machined in a single setup on CNC turning and milling centres, so concentricity is held within 0.05 mm TIR. Capability studies on these features are included in PPAP.
Do you offer balanced-bellows bonnet variants?
We do. These require a machined bonnet chamber, a leak-test port and a sealed upper spring seat. Send the bellows interface drawing and we will quote the housing and the matching seat geometry.
Which material do you recommend for sour or chloride service?
CF8M / 316 covers most general chemical duty. Duplex 2205 is preferred where chloride stress corrosion cracking or higher strength is a concern, and LCC for low temperature. Final selection rests with the valve designer and the end user.
Do you test the assembled valve?
No. We supply the cast and machined bonnet and housing as a component. Set-pressure verification, popping and blowdown testing belong to the valve assembler. We provide the dimensional, material and hydrostatic evidence for the part we supply.
What is the typical tooling investment?
Investment casting tooling for a bonnet in this size range is modest and is usually amortised over the first production order. We will quote tooling separately so you can see the unit cost impact at different volumes.
