Description
Product Overview
Spirax Sarco MST21 Internals Set is a dedicated replacement parts package designed for MST21 and MST21H balanced pressure thermostatic steam traps. This internal component assembly renews operating integrity and shut-off capability in steam tracing lines and light process equipment without requiring removal of the trap body from the piping.
Each set consolidates the active working elements that govern condensate discharge and system sealing. The kit comprises the thermostatic capsule, conical return spring, internal spacer plate, 0.8 mm perforated cylindrical strainer screen, and body gasket.
Key Features
- All-stainless steel wetted components including 316L strainer screen, 304 spacer plate, and 302 spring
- Factory matched thermostatic capsule available in STD (10 °C sub-cooling), SUB (22 °C sub-cooling), and NTS (4 °C sub-cooling) options
- Integral 0.8 mm perforated strainer screen protects internal seating surfaces from pipeline debris
- Direct drop-in assembly facilitates fast in-line servicing without disturbing pipeline connections
- Standard replacement gasket incorporates a stainless steel support ring
Technical Specifications
| Parameter | Specification |
|---|---|
| Kit Designation | Internals set for MST21 |
| Included Parts (Fig. 2) | Capsule (3), Spring (4), Spacer plate (5), Strainer screen (6), Gasket (7) |
| Non-included Trap Parts | Body (1), Cap (2) |
| Capsule Fill Options | STD (approx. 10 °C below saturation), SUB (approx. 22 °C below saturation), NTS (approx. 4 °C below saturation) |
| Strainer Screen Perforation | 0.8 mm diameter holes |
| Capsule Material | Stainless steel |
| Spring Material | Stainless steel BS 2056 302 S26 |
| Spacer Plate Material | Stainless steel BS 1449 304 S15 |
| Strainer Screen Material | Stainless steel ASTM A240 316L |
| Cover Gasket Material | Stainless steel BS 1449 304 S15 with support ring |
| Compatible Host Traps | MST21 (1/4”, 3/8”, 1/2”), MST21H (1/2”, 3/4”, 1”) |
| Host Maximum Operating Pressure (PMO) | 21 bar g @ 235 °C (455 °F) |
| Host Maximum Operating Temperature (TMO) | 235 °C @ 21 bar g (305 psi g) |
Applications
- Routine maintenance and overhaul of Spirax Sarco MST21 steam traps on instrument tracing lines
- In-line refurbishment of MST21 and MST21H steam traps discharging condensate on critical tracing and sampling systems
- Capsule replacement for thermal tuning between near-to-steam (NTS) and sub-cooled (SUB) operating conditions
Replacement Procedure
Replacing the internals of an MST21 steam trap follows standard maintenance steps:
- Isolate the steam trap from both the supply line and return line, allowing internal pressure to safely normalize to atmospheric conditions.
- Allow the trap body to cool to ambient temperature before handling.
- Unscrew the cap (item 2) using an appropriately sized spanner.
- Lift out the strainer screen (item 6), conical spring (item 4), capsule (item 3), and spacer plate (item 5).
- Clean all internal joint and seating faces thoroughly.
- Insert new internals in the exact reverse sequence: spacer plate (5), capsule (3), conical spring (4), and strainer screen (6). Ensure the conical spring is installed with its narrow end in contact with the capsule.
- Fit a new cover gasket (item 7). Handle carefully, as the thin stainless steel support ring can cause injury if mishandled.
- Apply anti-seize compound to the cap threads, reinstall the cap (2), and torque to the recommended tightening value.
Recommended Cap Tightening Torques
| Connection Size | Spanner Size (A/F) | Tightening Torque (N m) | Tightening Torque (lbf ft) |
|---|---|---|---|
| 1/4” | 22 mm | 100 - 110 | 73.5 - 80.9 |
| 3/8” | 32 mm | 100 - 110 | 73.5 - 80.9 |
| 1/2” | 32 mm | 100 - 110 | 73.5 - 80.9 |
| 3/4” | 36 mm | 100 - 110 | 73.5 - 80.9 |
| 1” | 41 mm | 100 - 110 | 73.5 - 80.9 |
Ordering Information
Specify the exact description “Internals set for MST21”, state the nominal connection size and trap model (MST21 or MST21H), and define the desired capsule fill type:
- STD: operation approximately 10 °C (18 °F) below steam saturation temperature.
- SUB: sub-cooled operation approximately 22 °C (39.6 °F) below steam saturation temperature.
- NTS: near-to-steam operation approximately 4 °C (7.2 °F) below steam saturation temperature.
Example order specification: 1 - Internals set for a 1/4” Spirax Sarco MST21 balanced pressure thermostatic steam trap having an ‘STD’ fill capsule.