Steam Trap Failure in a High-Pressure Drip Leg

Thermodynamic traps operate on the principle of a disc that opens and closes based on the velocity and pressure difference between the inlet and outlet. They are sensitive to dirt, backpressure, and superheat. Premature failure in this service could be due to: (1) Dirt or scale holding the disc off its seat; the strainer was cleaned six months ago, but fine particles may have passed through or the strainer may be damaged. (2) Backpressure from the flash tank: if the flash tank pressure rises above the trap's allowable backpressure (typically 50% of inlet pressure for thermodynamic traps), the trap may not close fully, causing live steam loss. The flash tank at 50 psig is well below the 600-psig inlet, so backpressure is not the primary issue unless the flash tank vent is blocked. (3) Superheat: at 750°F, the steam is highly superheated; thermodynamic traps can suffer from increased wear and may not cycle properly because the disc relies on flash steam formation for closing. (4) Water hammer or thermal cycling: the drip leg may experience slugs of condensate that slam the disc, causing damage. (5) Incorrect sizing: if the trap is too small, it may be held open continuously, eroding the seat. Evidence: the constant plume and whistle indicate the trap is stuck open. The discolored body suggests overheating, possibly from steam blow-through or from the trap being undersized and constantly passing high-temperature steam. The trap age (18 months) is short for this service, pointing to a fundamental application issue rather than normal wear. The fact that the strainer was cleaned recently suggests that dirt may not be the cause, but the strainer may be passing fines. The high-pitched whistle could also indicate that the trap is passing a mixture of steam and condensate at high velocity, eroding the disc and seat. A thermodynamic trap in superheated service should be installed with a check valve and a strainer, and the disc should be inspected for wire drawing.

Installing a bypass around the trap would allow replacement while the header remains in service, but it carries significant risks. The piping is schedule 80 chrome-moly (likely P22 or similar), which requires preheat and post-weld heat treatment to avoid cracking. Welding on a live 600-psig, 750°F header is a hot work hazard and requires a permit, fire watch, and possibly a shutdown if the weld procedure cannot be qualified for the operating conditions. The bypass itself must be designed for the same pressure and temperature, with a valve that can isolate the trap. However, a bypass valve that is left partially open or leaks can cause live steam to blow through to the flash tank, wasting energy and creating a hazard. The bypass should be a globe valve rated for the service, and the piping should be supported to handle thermal expansion. A safer alternative: use a temporary clamp-on or bolt-on trap (e.g., a trap with a isolation valve) if the existing piping has a flanged connection. If the trap is welded, consider using a online leak sealing technique or a temporary trap installed downstream of a isolation valve that can be operated with a chain wrench from a safe distance. However, the best approach is to schedule a short shutdown of the header during a low-demand period, isolate the drip leg, depressurize, and replace the trap with a more suitable type (e.g., an orifice trap or a thermodynamic trap with a larger disc and a strainer). If a shutdown is absolutely impossible, the bypass installation must be engineered: use a welded bypass with a high-pressure ball valve, perform the weld with the header at reduced pressure (if allowed by code) and with continuous temperature monitoring, and hydrotest the bypass before putting it in service. The engineer should also consider that the trap may have failed due to superheat; a better solution is to use a trap designed for superheated service, such as a piston-type or an orifice plate, and to ensure the drip leg is properly sized.