Steam traps > Balanced pressure steam traps
Spirax Sarco MST21 Stainless Steel Balanced Pressure Steam Trap
The Spirax Sarco MST21 is a maintainable stainless steel balanced pressure thermostatic steam trap for tracing lines and small condensate discharge duties. It uses a balanced pressure bellows mechanism that adjusts the discharge temperature automatically in response to steam pressure changes and provides effective air venting during start-up. The standard MST21 covers tracing line duties while the MST21H high capacity variant is available for heavier applications.
Quick facts
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.
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:
Please specify the bellows type when ordering.
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.
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:
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.
| Document | Reference | Language | Download |
|---|---|---|---|
| MST21 Stainless Steel Balanced Pressure Thermostatic Steam Trap | TI-P125-08-EN | English | Download |
MST21 selection usually involves confirming the tracing line layout, bellows fill type, installation orientation and maintenance approach at the same time.
Return to the balanced pressure steam traps category page to compare MST21 with BPC32, BPC32Y, BPS32, BPT13, BTM7 and other thermostatic trap routes.
When the duty involves clean steam, pharmaceutical or food-grade requirements, review the BT6-B and BTM7 models in the clean steam range.
Continue reading steam trap knowledge pages to understand how balanced pressure traps work and how to select the right model for your application.
