TD42 thermodynamic steam trap for high-pressure steam systems
Spirax Sarco TD42 is a maintainable thermodynamic steam trap for removing condensate from steam systems while retaining steam under the documented operating conditions. Its stainless steel body, screwed connections and integral strainer make it a practical choice where a serviceable trap is required and the installation team needs access to the strainer and seating components for inspection or cleaning.
As a thermodynamic steam trap, the TD42 is intended for a defined engineering duty rather than for unrestricted use in every fluid service. The official installation instructions identify steam, air and condensate or water in Group 2 of the Pressure Equipment Directive as intended fluid groups. If another fluid is being considered, material compatibility, pressure, temperature and the consequences of a malfunction must be checked with the manufacturer before selection.
What the TD42 is designed to do
A thermodynamic steam trap responds to the different pressure and temperature behaviour of condensate and steam. In operation, the TD42 discharges condensate with a blast-type action at a few degrees below steam saturation temperature. This working principle is why the outlet arrangement, discharge safety and backpressure checks belong in the selection review. That discharge characteristic is important when planning the outlet: the discharge point must be safe, and the surrounding pipework and personnel must be suitable for the noise, movement and hot fluid associated with the release.
The trap is suited to steam mains drainage and other condensate-removal duties that fall within its pressure, temperature, differential-pressure and backpressure limits. It is not a substitute for a control valve, a pressure-reducing device or a safety device. The upstream system still needs appropriate isolation, protection and condensate drainage design. A correctly selected trap is only one part of a safe and effective steam system.
For engineers reviewing the wider selection, the steam trap selection guide provides the broader context. The TD42 page remains focused on the single model and on the conditions that must be checked before ordering.
Serviceable construction with integral filtration
The TD42 combines the trap body and strainer in one assembly. The integral screen provides a filtration stage before the seating area, while the removable cap and strainer cap provide access for planned service. This arrangement is useful during commissioning and maintenance because the screen can be withdrawn, cleaned or replaced without treating the whole trap as a disposable unit. It should not be confused with separate steam strainers and filters installed elsewhere in a line; the TD42 screen is part of the trap assembly.
Official material data identify stainless steel for the principal pressure-containing and working parts: ASTM A743 Grade CA 40 for the body, AISI 416 for the cap and strainer cap, BS 1449 420 S45 for the disc, ASTM A240 316L for the strainer screen, and BS 1449 304 S16 for the strainer cap gasket. These material designations should be retained in the procurement record and checked against the actual fluid and system environment; they are not a licence to ignore corrosion, contamination or temperature compatibility.
An aluminium insulating cover is available as an optional extra. It is intended to reduce the influence of excessive heat loss from wind, rain or low outside temperatures. An integral BDV1 or BDV2 blowdown valve can also be fitted to the strainer cap, or the cap can be prepared with the documented 3/8 inch threaded connection for a suitable plugged arrangement. Options should be confirmed at the time of order because they affect the installed configuration and the available maintenance space.
Pressure, temperature and backpressure envelope
The TD42 has a PN63 body design condition. Its maximum allowable pressure is 63 bar g at 100 °C, while the maximum allowable and operating temperature is 400 °C at 42 bar g. The recommended maximum operating pressure is 42 bar g. These values are conditional limits, not a general permission to operate at any pressure and temperature combination: the pressure/temperature relationship and the shaded prohibited region in the official chart must be observed together.
The documented minimum allowable and operating temperature is 0 °C. The technical information advises consultation for lower operating temperatures. The minimum operating differential pressure for satisfactory operation is 0.25 bar g. Differential pressure is the pressure available to drive condensate through the trap, so it must be checked at the actual operating point, including start-up and low-load conditions.
Backpressure is equally important. The maximum operating backpressure must not exceed 80% of the upstream pressure under any operating condition; beyond that relationship the trap may not shut off. This check must include the return header, other traps discharging into the return system, lift, control-valve effects and any downstream restrictions. The technical data also state that the product is safe for use under full vacuum conditions and give a designed maximum cold hydraulic test pressure of 95 bar g. A hydraulic test limit must not be confused with an allowable operating pressure.
Choosing a Spirax Sarco TD42 steam trap: size and duty
For the TD42 scope covered here, the documented nominal sizes are 3/8 inch, 1/2 inch and 3/4 inch. Connections are screwed BSP T Rp (ISO 7-1) or NPT. Select the size from the actual condensate load and available differential pressure, then confirm that the chosen thread standard matches the piping, fittings and local practice. This is the core of steam trap sizing: nominal connection size alone does not establish capacity or suitability.
Before the order is released, record the normal and maximum inlet pressure, start-up pressure, operating and maximum temperature, minimum differential pressure, expected backpressure, condensate load, fluid service, connection standard and discharge arrangement. Also check whether the optional insulation or blowdown arrangement is needed, whether a downstream non-return valve is required, and whether there is enough clearance to remove the cap and strainer screen. A Spirax Sarco TD42 selection should therefore be reviewed against the complete operating case before the connection options are treated as final.
TD42 steam trap installation and system integration
The TD42 is designed for installation with the capsule in a horizontal plane and the cover at the top. The installation instructions preferably call for a small drop leg before the trap. Isolation valves upstream and downstream allow the trap to be safely inspected, cleaned or replaced. Open those valves slowly until normal operating conditions are reached, then check for leaks and correct operation.
When condensate discharges into a closed return line, a downstream non-return valve is recommended where return flow or backpressure could occur; review a suitable steam check valve for that return-line requirement. When the discharge is to atmosphere, it must terminate in a safe location; the installation instructions warn that the discharged fluid may be at 100 °C. A steam diffuser is recommended for atmospheric discharge, and a sight glass or Spiratec system can be considered where the operating condition needs to be tested. A sight glass should be positioned at least 1 m downstream of a blast-action trap.
The discharge behaviour and the horizontal orientation should be included in the piping review, not left to the final installation stage. Do not support the trap by forcing pipework into alignment, and account for external stresses induced by the connected system. The installer remains responsible for reducing those stresses and for providing safe access, lighting, lifting arrangements and protective equipment.
Maintenance access and ordering checks
For TD42 steam trap maintenance, service can be completed with the trap in the pipeline after the supply and return lines have been isolated, pressure has been safely vented to atmosphere and the product has been allowed to cool. The body and cap joint faces should be clean during reassembly. The instructions recommend using genuine replacement parts, new gaskets where required, suitable tools and appropriate protective equipment. After service, open the isolation valves slowly and check for leaks.
Allow clearance for the screen, cap and optional cover withdrawal distances shown in the dimension drawing. Do not use Stillsons or a similar wrench on the cap, because distortion can result. The disc should be replaced when servicing the seating assembly; if the body seating faces are lapped, the total metal removed must not exceed 0.25 mm. Correct tightening torque and the specified high-temperature anti-seize or molybdenum disulphide grease are part of the maintenance procedure, not optional finishing details.
Certification or inspection requirements must be stated at the time of order. The manufacturer’s Typical Test Report is available for the product when requested. An accurate enquiry should therefore include the model, nominal size, BSP or NPT connection, operating duty, optional extras, certification requirement and discharge arrangement. This information lets the final configuration be checked against the TD42 technical and installation limits before it enters service.
TD42 selection summary
Maintainable thermodynamic steam trap with an integral strainer and screwed body connections.
Target sizes of 3/8 inch, 1/2 inch and 3/4 inch with BSP T Rp (ISO 7-1) or NPT connections.
PN63 body design condition, 42 bar g recommended maximum operating pressure and 400 °C at 42 bar g maximum operating temperature.
Minimum satisfactory operating differential pressure of 0.25 bar g and maximum backpressure of 80% of upstream pressure.
Horizontal capsule installation with the cover at the top, isolation for service and non-return protection where the return system requires it.
Spirax Sarco TD42 should be specified against the complete operating case, not against a catalogue number alone. The technical tables and installation instructions that follow preserve the detailed limits, dimensions, materials, safety conditions and service sequence needed to complete that check. For a Spirax Sarco TD42 enquiry, retaining the operating data and connection standard with the model reference makes the final review more reliable.
