Spirax Sarco MST21/MST21H are balanced-pressure thermostatic stainless steel steam traps, offering standard-capacity and high-capacity options, respectively. The bodies are made of austenitic stainless steel and support NPT, BSP, and EN228-1 threaded connections, with a maximum working pressure of 21 bar g. They are suitable for condensate drainage from instrumentation, small-bore process piping, and steam-tracing circuits; the SUB capsule enables discharge at approximately 22°C subcooling.
MST21 Application Photo
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Spirax Sarco MST21/MST21H Stainless Steel Balanced Pressure Thermostatic Steam Trap
Stainless steel balanced pressure thermostatic steam trap for condensate drainage from tracer lines, available in standard- and high-capacity versions.
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Models
MST21 and MST21H are both Spirax Sarco stainless steel pressure-balanced thermostatic steam traps. Their main differences are capacity type and connection size range. MST21 is the standard-capacity model, available with 1/4 in., 3/8 in., and 1/2 in. connections, making it suitable for standard-capacity and smaller connection size requirements. MST21H is the high-capacity model, available with 1/2 in., 3/4 in., and 1 in. connections, making it suitable for higher-capacity and larger connection size requirements. Both are suitable for operating conditions up to 21 bar g@270°C and can be fitted with STD, SUB, or NTS capsules according to the required discharge temperature. When selecting a model, first verify the actual condensate load, then determine the model based on connection size, operating pressure, operating temperature, and required subcooling.
MST21Standard-capacity pressure-balanced thermostatic steam trap
MST21 is the standard-capacity model in this series. It operates on the pressure-balanced thermostatic principle and is available with 1/4 in., 3/8 in., and 1/2 in. threaded connections, with a maximum operating pressure of 21 bar g@270°C. STD, SUB, or NTS capsules are available to discharge condensate at approximately 10°C, 22°C, or 4°C below saturated steam temperature, respectively.
MST21HHigh-capacity pressure-balanced thermostatic steam trap
MST21H is the high-capacity model in this series. It operates on the pressure-balanced thermostatic principle and is available with 1/2 in., 3/4 in., and 1 in. threaded connections, with a maximum operating pressure of 21 bar g@270°C. STD, SUB, or NTS capsules are available to discharge condensate at approximately 10°C, 22°C, or 4°C below saturated steam temperature, respectively.
Specifications
- Nominal Pressure
- PN25
- Maximum Allowable Pressure
- 25 bar g@300°C
- Maximum Allowable Temperature
- 400°C@23 bar g
- Maximum Operating Pressure
- 21 bar g@270°C
- Maximum Operating Temperature
- 270°C@21 bar g
- Minimum Operating Temperature
- 0°C
- Maximum Cold Hydraulic Test Pressure
- 38 barg
- Thread Connection Standards
- NPT, BSP, EN228-1
- Connection Sizes
- MST21: 1/4", 3/8", 1/2"; MST21H: 1/2", 3/4", 1"
- Trap Type
- Pressure-balanced thermostatic
- Body Material
- Austenitic stainless steel ASTM A743 CF16Fa
- Strainer Material
- Stainless steel ASTM A240 316L (0.8 mm aperture)
- Capsule Material
- Stainless steel
- Subcooling
- STD approx. 10°C/SUB approx. 22°C/NTS approx. 4°C (below saturated steam temperature)
- Capacity Version
- Standard capacity (MST21), high capacity (MST21H)
- Safety Certification
- European Pressure Equipment Directive 97/23/EC
Application scenarios
Steam Tracer Condensate Drainage for Instrumentation and Small-Bore Lines
Used in steam tracing circuits for instrumentation and small-bore process lines to discharge condensate generated during tracing. MST21 is designed for relatively low condensate drainage duties and is available with small-size threaded connections.
Condensate Drainage from Steam Tracer Lines
Used for condensate drainage from steam tracing circuits on process pipelines. MST21 is a balanced pressure thermostatic steam trap designed for relatively low condensate drainage duties, such as those found in tracer lines.
Subcooled Discharge in Tracing Systems Using SUB Capsules
Used in steam tracing systems where condensate is permitted to discharge after subcooling. When fitted with a SUB capsule, the trap discharges condensate at approximately 22°C below the saturated steam temperature.
Selection points
Standard- and high-capacity series
Available in the standard-capacity MST21 and high-capacity MST21H models, allowing selection according to actual condensate load.
Three subcooling capsule options
Available with STD, SUB, and NTS capsules, which discharge condensate at approximately 10°C, 22°C, and 4°C below saturated steam temperature, respectively, allowing selection based on the required subcooling.
Stainless steel body with integral strainer
The body, capsule, and strainer are all made of stainless steel. The integral strainer has a 0.8 mm mesh opening and is made of 316L stainless steel.
Connection sizes matched by model
MST21 is available with 1/4 in., 3/8 in., and 1/2 in. connections, while MST21H is available with 1/2 in., 3/4 in., and 1 in. connections.
Installation & maintenance notes
Verify operating pressure and temperature ranges
Before installation, verify that site steam system operating conditions do not exceed the model's maximum operating pressure of 21 bar g and maximum operating temperature of 270°C.
FAQ
What operating conditions is the MST21 suitable for? What is the difference between the MST21 and MST21H?
The MST21 is suitable for applications with relatively low condensate discharge requirements, such as tracing lines, and is the standard-capacity version. The MST21H is the high-capacity version for special applications. The specific configuration should be confirmed according to the actual model and application conditions.
How does the MST21 operate? What discharge temperature options are available for the capsules?
The MST21 is a pressure-balanced thermostatic steam trap. The STD capsule discharges condensate at approximately 10°C below saturated steam temperature; the SUB capsule discharges at approximately 22°C below saturated steam temperature; and the NTS capsule discharges at approximately 4°C below saturated steam temperature. The capsule type should be specified when ordering.
What sizes and connection options are available for the MST21 and MST21H?
The MST21 is available in 1/4", 3/8" and 1/2" sizes, with optional NPT or BSP threaded connections conforming to EN228-1. The MST21H is available in 1/2", 3/4" and 1" sizes, with optional NPT or BSP threaded connections conforming to EN228-1.
What are the maximum operating pressure and temperature of the MST21?
The maximum operating pressure is 21 bar g, and the maximum operating temperature is 270°C. The body design rating is PN25, with a maximum allowable pressure of 25 bar g@300°C and a maximum allowable temperature of 400°C@23 bar g.
What should be considered when installing the MST21 or ordering spare parts?
The trap should be installed vertically, with the capsule horizontal and the inlet at the top. If horizontal installation is required, a downpipe should preferably be installed upstream. Suitable isolation valves should be installed upstream and downstream of the trap. Available spare parts are internal components; when ordering, specify the trap size, model and capsule type.