Control Devices ST25 — 1/4" NPT ASME Soft-Seat Safety Relief Valve
The Control Devices ST25 is an ASME-rated soft-seat safety relief valve with a 1/4-inch NPT connection. The ST series from Control Devices is a well-established product line for compressed air, refrigeration, and light industrial pressure vessel applications. This valve is distributed through Cast Air as part of their accessory and replacement parts offering.
At $12.71, this is a low-cost component that is also one of the most operationally and legally critical items on any pressurized air system. A safety relief valve that weeps at normal operating pressure — or worse, one that fails to open at overpressure — is not a minor issue. Replace weeping valves when they show any sign of leakage below the set point, and always match the set pressure to your vessel's MAWP (Maximum Allowable Working Pressure).
Soft Seat vs. Metal Seat Design
The ST series uses a soft seat — a resilient seat material rather than metal-to-metal contact. Soft seats provide:
- Tighter initial seal: The resilient material conforms to minor seat surface irregularities, providing a more consistent seal at lower pressure differentials
- Better reseat performance: After actuating, the soft seat is more likely to reseat cleanly and stop flowing at or near the set pressure
- Reduced weeping tendency: Less prone to the minor leakage that can occur with metal-to-metal valves when operating near the set pressure
For small-port applications on auxiliary vessels, carry tanks, and secondary pressure lines, the soft-seat design is typically preferred where the valve is exposed to varying conditions.
Key Features
- ASME Code Compliant: Manufactured to ASME standards for pressure-relieving safety devices — required for code-compliant installations on pressurized vessels
- Soft-Seat Design: Resilient seat material for tighter seal and better reseat performance vs. metal-seat alternatives
- 1/4-Inch NPT: Smaller port size — appropriate for small compressor tanks, carry tanks, secondary pressure vessels, and auxiliary pressure lines
- ST Series: Control Devices' established product line with decades of field use in compressed air and refrigeration applications
- Automatic Operation: Opens at set pressure, reseats when pressure drops — no manual operation required for relief function
- Test Lever: Allows periodic manual verification of valve operation (test sparingly — repeated manual actuation wears the seat)
Technical Specifications
- Part Number: ST25
- Connection Size: 1/4 inch NPT
- Seat Type: Soft seat
- Rating: ASME code compliant
- Brand: Control Devices (distributed by Cast Air)
- Price: $12.71
Important: Safety relief valves are rated at a specific set pressure — the pressure at which the valve opens. Confirm the required set pressure for your application and order the ST25 at the correct set point for your vessel's MAWP. The set pressure is stamped on the valve body. Call 866-217-0063 for help with pressure selection.
ST25 vs. SF50 — Choosing the Right Safety Valve
- ST25 (this product) — 1/4" NPT, soft seat, ASME rated. For small tanks, secondary vessels, auxiliary pressure lines, carry tanks ($12.71)
- SF50 — 1/2" NPT, standard seat, ASME rated, 50–250 PSI. For main compressor tank safety valve ports on Cast Air industrial-series units ($32.19)
When to Replace the Safety Relief Valve
- Valve is weeping or leaking at pressures below the set point — the seat is not sealing correctly and the valve must be replaced
- Valve has been manually actuated repeatedly — multiple manual lifts damage the seat material and cause weeping
- External corrosion or mineral scale is visible on the valve body or vent port
- Valve is more than 5 years old, particularly in humid environments where internal components are exposed to condensate
- Tank has been reconfigured for a different 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 confirm the correct set pressure and verify the ST25 fits your tank port before you order
- Full manufacturer warranty
Call 866-217-0063 for quick answers and help!
