TD42S2 Thermodynamic Steam Trap

TD42S2 thermodynamic steam trap for socket-weld steam service

Spirax Sarco TD42S2 is a maintainable thermodynamic steam trap with a forged steel body and socket weld ends. It is intended to remove condensate from steam, air or condensate/water service within the documented Group 2 application boundary, while giving the maintenance team access to the cap, disc and integral strainer screen. The page covers TD42S2 and its explicitly documented lower-capacity variant TD42S2LC. The separate low-temperature route referenced later in this page is not part of the TD42S2 product scope.

The socket-weld connection is important when a welded pipe installation is part of the design. The official connection range is ½ inch, ¾ inch and 1 inch, with each size also available in an LC lower-capacity designation. Ends are socket weld to ASME (ANSI) B 16.11 Schedule 80 or BS 3799 Class 3000 lb. The final order still needs to identify the nominal size, the required capacity version and the applicable connection standard; nominal pipe size alone does not select the correct trap duty.

What the TD42S2 is designed to do

A thermodynamic steam trap operates as a condensate-removal device at the boundary between steam and condensate. The installation instructions describe the TD42S2 discharge as a blast-type action at a few degrees below steam saturation temperature. That behaviour should be considered when planning the outlet, because an atmospheric discharge can release hot fluid and a closed return line can impose backpressure. The trap is one component in the drainage system and does not replace isolation, pressure protection or safe discharge arrangements.

During selection, compare the condensate load with the differential pressure actually available across the trap. The capacity chart distinguishes the ½ inch, ¾ inch and 1 inch curves from the lower-capacity ½ inch LC, ¾ inch LC and 1 inch LC curves. The manufacturer specifically directs users to select the LC version for mains drainage and tracing applications irrespective of pipe size. This makes the operating duty, rather than the pipe connection alone, the decisive selection input.

For a broader explanation of operating principles and steam-system selection, the steam trap selection guide provides the appropriate category-level context. The model page remains focused on the verified TD42S2 construction, limits, installation conditions and ordering checks.

Serviceable construction and integral straining

The TD42S2 combines a forged steel body with an internal stainless steel strainer screen. The screen is positioned before the seating area and can be withdrawn for cleaning or replacement during planned maintenance. The cap and strainer cap are removable with suitable tools, allowing the disc, seating face, screen and gasket to be inspected without treating the trap as a sealed disposable item. The detailed maintenance section preserves the required isolation, cooling, cleaning and tightening sequence.

The technical information identifies ASTM A105N steel for the body and strainer cap, AISI 416 stainless steel for the cap, BS 1449 420 S45 stainless steel for the disc, AISI 304 stainless steel for the strainer screen, and BS 1449 304 S16 stainless steel for the strainer cap gasket. These designations are useful for procurement and compatibility review, but they do not remove the need to check the actual fluid, temperature, contamination and corrosion conditions of the complete system.

The integral screen is part of the trap assembly, not a substitute for a separately selected upstream filter. Where the pipework needs additional dirt-holding capacity or protection for downstream equipment, review the relevant steam strainers and filters as a separate system component. Keep the distinction clear when preparing an enquiry or maintenance plan.

Pressure, temperature and differential-pressure envelope

The TD42S2 body design condition is ANSI/ASME 300. The maximum allowable pressure is 51 bar g at 38 °C, and the maximum allowable temperature is 427 °C at 28 bar g. For saturated steam service, the maximum operating pressure is 42 bar g; the maximum operating temperature is 400 °C at 34 bar g. These are conditional values from the pressure/temperature envelope, not independent ratings that can be combined without checking the official curve.

The documented minimum allowable and operating temperature is 0 °C. Satisfactory operation requires at least 0.25 bar g inlet pressure, and the maximum differential pressure is 42 bar. Operating backpressure should not exceed 80% of the inlet pressure. Include the return header, lift, restrictions and other discharges in that calculation. The product is documented as safe under full vacuum conditions, while the 76 bar g cold hydraulic test pressure is a test limit and must not be presented as an operating rating.

Steel exposed for a prolonged period above 425 °C may undergo conversion of its carbide phase to graphite. The official information describes use above that temperature as permissible but not recommended for prolonged service. The pressure/temperature graph and its prohibited regions therefore need to be reviewed together with the actual start-up, running and shutdown cases. For lower operating temperatures, consult the manufacturer; the separate TD42S3 is the documented low-temperature route down to -46 °C and is outside this page’s product scope.

Capacity and size selection

The TD42S2 capacity curves are provided against differential pressure and condensate load. Read the curve at the pressure difference available at the trap, including the start-up and low-load cases, and allow for return-line conditions. Do not convert the chart into one universal capacity number: the selected nominal size, the standard or LC capacity designation, the pressure drop and the required condensate duty must all agree. The technical chart is retained below so the selection can be made from the original curve relationship.

For mains drainage and tracing, use the LC designation irrespective of the pipe size, as stated by the technical sheet. Other duties should be checked against the full condensate load and the applicable ½ inch, ¾ inch or 1 inch curve. If the available differential pressure is close to the minimum or changes substantially between start-up and normal operation, record both cases for review rather than selecting from the nominal line size.

The dimensions section gives approximate A to H values, withdrawal distances for the cap, strainer screen and optional insulating cover, and the approximate weight for every documented size. These clearances should be reserved in the pipe layout. A trap that fits between the weld ends but cannot be opened safely is not a maintainable installation.

Insulation, certification and configuration choices

An aluminium insulating cover is available as an optional extra. It is intended to reduce the influence of excessive heat loss caused by low outside temperatures, wind or rain. The cover is a configuration option, not a correction for an incorrectly sized trap, and its withdrawal distance must be included in the maintenance access review. State the requirement at order placement so the supplied assembly and documentation match the installed arrangement.

The product complies with the requirements of the European Pressure Equipment Directive 2014/68/EU and is classified in the source installation instructions as category SEP. Certification to EN 10204 3.1 is available. Certification and inspection requirements must be stated when the order is placed; they should not be assumed from the connection size or from a later maintenance request.

For a Spirax Sarco TD42S2 enquiry, provide the size, TD42S2 or TD42S2LC designation, socket-weld standard, normal and maximum pressure, temperature, differential pressure, condensate load, expected return-line backpressure and discharge destination. Add the insulating-cover requirement and EN 10204 3.1 certification request where applicable. This information gives the final configuration review enough detail to distinguish the standard and lower-capacity duties.

Installation and system integration

The preferred installation position is the horizontal plane, with a small drop leg preceding the trap. The technical sheet also notes that the trap can operate in other orientations, but the actual flow direction and installation situation must be checked against the nameplate and the system layout. Install isolation valves so the trap can be safely serviced or replaced, and provide a practical method of checking operation, such as a sight glass or Spiratec system. A sight glass should be at least 1 m downstream of any blast-action trap.

When condensate discharges into a closed return system, a downstream non-return valve should be fitted to prevent return flow. Review the relevant steam check valve range when return-flow or backpressure protection is required. If the discharge is to atmosphere, route it to a safe place because the discharged fluid may be at 100 °C. A steam diffuser can be considered where the system requires controlled atmospheric discharge.

Open isolation valves slowly until normal operating conditions are reached, then check for leaks and correct operation. Welding into the pipeline requires a procedure consistent with the relevant national and international welding standards. The installer must also account for external pipe stresses, safe access, lighting, lifting arrangements, hazardous fluids, hot surfaces and protective clothing. These conditions belong in the design review rather than being left until commissioning.

Maintenance planning and ordering checks

Before maintenance, isolate the trap from both the supply and return lines, allow the pressure to normalise safely to atmosphere and allow the product to cool. The joint faces must be clean during reassembly. Use a suitable spanner or socket on the cap; the installation instructions specifically prohibit Stillsons and similar wrenches that can distort it. The disc should be replaced with a new one during seating service, and lapping of the body seating faces must not remove more than 0.25 mm of metal.

The integral strainer can be serviced by removing its cap, withdrawing the screen, cleaning or replacing it, and refitting the screen with clean gasket faces. The recommended thread lubricants and tightening torques are retained in the installation section below. After maintenance, open the isolation valves slowly and check for leaks. Use genuine replacement parts and preserve enough access to remove the screen, cap and optional insulating cover.

The available spare categories are the disc, strainer screen and gasket, strainer cap gasket and insulating cover. A spare-parts request should state the size and trap type. A complete Spirax Sarco TD42S2 selection should carry the operating case and the connection designation with the model reference, so the replacement, capacity and documentation requirements remain unambiguous.

TD42S2 selection summary

• Maintainable thermodynamic steam trap with a forged steel body, socket weld ends and an integral stainless steel strainer screen.
• ½ inch, ¾ inch and 1 inch socket-weld sizes, with standard and lower-capacity LC designations.
• ANSI/ASME 300 body design condition, 42 bar g maximum saturated-steam operating pressure and 400 °C at 34 bar g maximum operating temperature.
• Minimum 0.25 bar g inlet pressure for satisfactory operation, maximum 42 bar differential pressure and backpressure limited to 80% of inlet pressure.
• Preferred horizontal installation with a preceding drop leg, isolation for maintenance and safe planning for blast-type discharge.

Use the verified pressure/temperature envelope, capacity curve, size table and installation conditions together. A model number by itself does not establish suitability. For a final enquiry, retain the load, pressure, temperature, differential pressure, return-line backpressure, connection standard, LC choice, optional cover and certification requirement with the product reference.