What is the difference between MST21 and MST21H models?

The MST21 is the standard capacity model available in ¼", ⅜", and ½" sizes, designed for typical instrument tracing loads. The MST21H is a high capacity version available in ½", ¾", and 1" sizes for applications requiring greater condensate handling capacity.

How do I select the correct capsule fill for my application?

Choose STD (standard) fill for most tracing applications—it operates at approximately 10°C below saturation. Use SUB fill (22°C subcool) when maximum condensate subcooling is required or in critical tracing where water hammer must be avoided. Select NTS fill (4°C subcool) when rapid condensate discharge with minimal subcooling is needed.

Can the MST21 be installed in a horizontal pipeline?

Yes, but the trap should be preceded by a short drop-leg to ensure proper condensate collection. For optimal self-draining performance, install the trap vertically with the inlet at the top and outlet at the bottom.

What spares should I stock for maintenance?

Spirax Sarco offers an internals set (parts 3, 4, 5, 6, 7) that includes the capsule, spring, spacer plate, strainer screen, and gasket. This allows complete refurbishment without replacing the body or cap.

Is this steam trap suitable for superheated steam applications?

The MST21 should not be operated beyond its rated PMO of 21 bar g at 235°C. Operating in the superheated region above the steam saturation curve may damage the thermostatic capsule. For superheated steam lines, consult PC Engineering for appropriate trap selection.

SPIRAX SARCO · STEAM TRAPS

Spirax Sarco MST21 Stainless Steel Balanced Pressure Thermostatic Steam Trap

Name: Spirax Sarco MST21 Stainless Steel Balanced Pressure Thermostatic Steam Trap
Brand: Spirax Sarco

Spirax Sarco MST21 stainless steel thermostatic steam trap for instrument tracing. Balanced pressure design, PN25 rated. Buy from PC Engineering India.

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Overview

The Spirax Sarco MST21 is a maintainable balanced pressure thermostatic steam trap engineered for relatively small condensate loads, typically found in instrument tracing applications. Constructed entirely from stainless steel, this trap offers excellent corrosion resistance and long service life in demanding industrial environments.

The MST21 operates using a thermostatic capsule that responds to temperature changes near steam saturation. Standard capsules operate at approximately 10°C below steam saturation temperature, with optional sub-cooled (22°C below) and near-to-steam (4°C below) fill options available for specific process requirements. The trap is designed for easy field maintenance, allowing quick replacement of internal components without removing the body from the line.

Available in two variants—MST21 for standard tracing loads and MST21H for higher capacity applications—this steam trap meets the requirements of the European Pressure Equipment Directive 97/23/EC and is suitable for a wide range of industrial tracing and small-load condensate drainage duties.

Key Features

All stainless steel construction — Body (303 SS), cap (420 SS), internals, and gasket for superior corrosion resistance
Balanced pressure thermostatic operation — Reliable condensate discharge with minimal steam loss
Multiple capsule fill options — STD (10°C subcool), SUB (22°C subcool), NTS (4°C subcool) to match application needs
Maintainable design — Internals set available as spare for in-line servicing
Compact and lightweight — Suitable for tight installations on instrument tracing lines
Built-in strainer screen — 0.8 mm perforations protect capsule from debris
Self-draining orientation — Vertical installation ensures complete condensate evacuation

Specifications

Parameter	Value
Body Design Conditions	PN25
Maximum Allowable Pressure (PMA)	25 bar g @ 300°C
Maximum Allowable Temperature (TMA)	400°C @ 23 bar g
Maximum Operating Pressure (PMO)	21 bar g @ 235°C
Minimum Operating Temperature	0°C
Cold Hydraulic Test Pressure	38 bar g
Sizes — MST21	¼", ⅜", ½"
Sizes — MST21H	½", ¾", 1"
Connections	Screwed BSP (EN ISO 228-1) or NPT
Body Material	Austenitic Stainless Steel 303 (ASTM A743 CF16Fa)
Cap Material	Stainless Steel 420 (AISI 420)
Strainer Material	Stainless Steel 316L (ASTM A240)
Weight Range	0.34 kg (¼") to 0.48 kg (½" to 1")
Compliance	European Pressure Equipment Directive 97/23/EC
Size	A	B	C	D	Weight (kg)
¼"	50.5	8.5	10	45	0.34
½"	61.0	12.5	14	45	0.48
⅜"	68.0	14.0	14	45	0.48
¾"	68.0	14.0	14	45	0.48
1"	75.0	14.0	14	45	0.48

Applications

The MST21 thermostatic steam trap is designed for condensate removal in applications with small to moderate loads:

- Instrument tracing lines — Protecting instrumentation from freezing or maintaining viscosity in sensing lines - Sample coolers and analysers — Ensuring proper condensate drainage in analytical equipment - Small process equipment — Draining jacketed vessels, small heat exchangers, and tank heating coils - HVAC terminal units — Suitable for small radiators and unit heaters - Pharmaceutical and food processing — Stainless steel construction meets hygiene requirements

Industries: Refineries, petrochemical plants, pharmaceutical manufacturing, food & beverage processing, chemical plants, and power generation facilities.

Frequently Asked Questions

Get a Quote

Contact PC Engineering for pricing, availability, and technical support for the Spirax Sarco MST21 Stainless Steel Thermostatic Steam Trap. Call us or fill in our inquiry form — our engineers will respond within 24 hours.

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