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Return to the balanced pressure steam traps category page to compare MST21 with BPC32, BPC32Y, BPS32, BPT13, BTM7 and other thermostatic trap routes.
Where MST21 sits in the balanced pressure trap range
MST21 is part of the Spirax Sarco balanced pressure steam traps family, targeting tracing lines and smaller condensate discharge duties. While the BPC32 range covers larger sizes and broader capacity needs, MST21 focuses on compact, maintainable tracing line drainage with full stainless steel construction.
The standard MST21 covers ¼”, ⅜” and ½” connections, while the MST21H high capacity variant extends to ½”, ¾” and 1” for heavier tracing or light process applications.
Working principle
The key working element is a bellows filled with a small amount of special liquid whose boiling point is lower than water. During cold start-up the bellows is contracted, the valve plug lifts off the seat and the valve is fully open, allowing free discharge of air.
When condensate flows through the trap, heat transfers to the liquid inside the bellows. The fill liquid boils before steam arrives, the bellows expands and the valve closes. As the body loses heat the temperature drops below the boiling point, the bellows contracts and the valve reopens, entering the next cycle.
Three bellows fill options control the discharge temperature:
- STD (standard): discharges condensate approximately 10 °C below saturated steam temperature
- SUB (sub-cooled): discharges approximately 22 °C below saturation
- NTS (near-to-steam): discharges approximately 4 °C below saturation
Please specify the bellows type when ordering.
Technical specifications and materials
The MST21 valve body design pressure is PN25. Maximum working pressure is 21 barg at 270 °C. Maximum design cold hydraulic test pressure is 38 barg.
Main component materials:
| No. | Component | Material |
|---|---|---|
| 1 | Valve body | Austenitic stainless steel ASTM A743 CF16Fa |
| 2 | Valve cap | Stainless steel 420A UGIMA AISI 420 |
| 3 | Bellows | Stainless steel |
| 4 | Spring | Stainless steel BS 2056 302 S26 |
| 5 | Baffle | Stainless steel BS 1449 304 S15 |
| 6 | Filter screen (0.8 mm) | Stainless steel ASTM A240 316L |
| 7 | Gasket | Stainless steel BS 1449 304S15 |
The product fully complies with the European Pressure Equipment Directive 97/23/EC.
Installation and selection checks
MST21 should be installed vertically with the bellows horizontal, inlet at the top and outlet at the bottom to ensure self-draining. If horizontal installation is needed, a drop leg before the trap is recommended.
Before ordering or replacing, confirm:
- Standard capacity (MST21) or high capacity (MST21H) is required
- Connection size: ¼”, ⅜”, ½”, ¾” or 1”
- Thread standard: NPT or BSP
- Bellows type: STD, SUB or NTS
- Whether fully stainless steel construction is needed for corrosion resistance
- Whether isolation valves are needed for future maintenance
Documentation coverage
MST21 technical information (TI-P125-08) covers operating range, dimensions, discharge capacity, materials and installation dimensions. The installation and maintenance guide (IM-P125-07) covers safety, installation, commissioning, operation, maintenance and spare parts replacement.
For tracing line maintenance and replacement enquiries, these are typically the first two documents to confirm model selection and spare parts.
Related routes
- Return to balanced pressure steam traps to compare MST21 with BPC32, BPS32, BPT13 and other product routes.
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- Explore steam trap knowledge to understand how balanced pressure traps work and how to select the right model.