Castair SF50 — 1/2" NPT ASME Safety Relief Valve, 50–250 PSI
The Castair SF50 is an ASME-rated pressure safety relief valve with a 1/2-inch NPT connection and a pressure range of 50 to 250 PSI. A safety relief valve is not optional — it is a required component on every pressurized air receiver tank. This is the last line of defense if the pressure switch fails to cut the compressor off: the relief valve opens automatically when tank pressure exceeds the set point, expelling air until pressure drops back to safe operating levels, then reseats.
The ASME stamp means this valve has been manufactured and tested to the American Society of Mechanical Engineers pressure vessel code requirements for safety relief devices. On commercial and industrial compressor installations, an ASME-stamped valve is typically required for code compliance and insurance purposes. Do not substitute a non-ASME valve on a pressure vessel — and do not ignore a weeping or leaking relief valve. Either replace it or investigate why it is activating.
Why Safety Relief Valves Fail
Relief valves are designed to open and reseat repeatedly at their set pressure. But every time a valve lifts — whether from overpressure or from manual testing — the seat takes wear. Valves that have been manually pulled many times, valves that have accumulated corrosion or mineral deposits on the seat, or valves that are operating near their set pressure on every cycle will eventually begin to weep — leaking air at normal operating pressure rather than seating cleanly. A weeping valve needs to be replaced. Attempting to reseat it by manually pulling the test lever repeatedly makes it worse.
Key Features
- ASME Code Compliant: Manufactured and tested to ASME Section VIII Division 1 pressure-relieving safety device requirements — required for code-compliant commercial and industrial installations
- 1/2-Inch NPT Connection: Standard pipe thread — fits the majority of compressor tank safety valve ports in the 1/2-inch size class, including most Cast Air industrial-series tanks
- 50–250 PSI Operating Range: Covers the full operating range of reciprocating piston air compressors — order at the correct set pressure for your specific tank MAWP
- Automatic Pressure Relief: Opens at set pressure, relieves air until pressure drops, reseats automatically — no manual operation required for pressure relief function
- Test Lever: Manual test lever allows periodic verification that the valve will open — test annually per ASME guidelines (note: do not test excessively as this causes seat wear)
- Cast Air Compatible: Sized for Castair compressor tanks using 1/2-inch NPT safety valve ports
Technical Specifications
- Part Number: SF50
- Connection Size: 1/2 inch NPT
- Pressure Range: 50–250 PSI
- Rating: ASME Section VIII code compliant
- Manufacturer: Cast Air (Castair)
- Price: $32.19
Important: Safety relief valves must be set to a pressure at or below the tank's MAWP (Maximum Allowable Working Pressure). The set pressure is stamped on the valve body. Match the valve set pressure to your tank's rating. Call us if you need help determining the correct set pressure for your specific compressor tank.
SF50 vs. ST25 — Choosing the Right Safety Valve
- SF50 (this product) — 1/2" NPT, 50–250 PSI, ASME rated. Standard safety valve for Cast Air industrial-series compressor tanks ($32.19)
- ST25 — 1/4" NPT, soft-seat design, ASME rated. Smaller port valve for small tanks, secondary vessels, and auxiliary pressure lines ($12.71)
When to Replace Your Safety Relief Valve
- Valve is weeping or leaking at pressures below the set point — the seat is damaged and the valve is not seating cleanly
- Valve has been manually pulled more than a few times — manual actuation damages the seat, eventually causing it to weep
- Valve shows external corrosion or mineral buildup that may be affecting operation
- Valve is more than 5 years old and has never been replaced — particularly important in humid environments where internal components are exposed to moisture
- Compressor has been reassigned to a higher or lower pressure application and the existing valve set pressure no longer matches
Why Buy From Air Compressor Factory Direct?
- Authorized Cast Air dealer since 2016
- Free shipping on qualifying orders
- Free technical support at 866-217-0063 — we can help you confirm the correct set pressure and confirm the SF50 fits your tank before you order
- Full manufacturer warranty
Call 866-217-0063 for quick answers and help!
