Steam Trap Types and Survey Methodology in Industrial Laundry Plants
A steam trap that fails open passes live steam continuously into the condensate return or drainage, wasting energy that the boiler has already consumed producing it. In a laundry plant with 30 to 80 traps serving washer-extractor heating coils, ironer chests, steam pipe drip legs, and tracing lines, a failure rate of 15 to 20 percent — typical in plants without a systematic survey programme — can represent a significant fraction of total boiler output being discharged without doing useful work. Trap selection, condition assessment, and periodic survey are straightforward maintenance activities with a clearly calculable payback.
Published June 30, 2026 — Stalwart Engineering Technical NotesSteam trap operating principles and failure modes
All steam traps operate on the same functional requirement: pass condensate and air when they are present, hold live steam. The difference between trap types lies in the physical mechanism that distinguishes condensate from steam. There are three principal mechanisms used in laundry plant applications:
| Trap type | Operating principle | Typical laundry application | Failure mode |
|---|---|---|---|
| Float and thermostatic (F&T) | A ball float rises and falls with condensate level, opening a discharge valve proportionally; a thermostatic element discharges air on start-up | Washer-extractor steam coils, dryer heating coils, ironer steam chests — any application with high and variable condensate loads | Fails open (seat erosion or float puncture) or fails closed (float waterlogged or seat clogged) |
| Thermodynamic disc | A disc lifts on condensate flash, closes rapidly when live steam velocity creates low pressure above the disc | Steam main drip legs, tracer lines, distribution headers at moderate to high pressures | Fails open (disc or seat wear) or chatters rapidly at low loads — accelerates seat wear |
| Bimetallic (thermostatic) | A bimetallic element flexes with temperature; opens below steam saturation temperature, holds closed at steam temperature | Tracer lines, pipe drip legs at lower pressures; not suitable for high condensate loads | Fails open or fails closed as bimetallic element fatigues |
For the high-condensate-load applications typical in laundry plants — particularly washer-extractor heating coils that handle large volumes of condensate rapidly during the heating phase of each programme — the float and thermostatic trap is the preferred choice. Its ability to discharge condensate continuously at load proportional to the float position makes it better suited than thermodynamic disc traps, which are cyclic by nature and can create water hammer in high-condensate-load applications.
Consequences of trap failure in a laundry plant
A failed-open trap on a washer-extractor heating coil passes live steam at the coil operating pressure directly to the condensate return header. The energy loss from a single 1/2-inch trap failed open at 7 bar can reach 50 to 120 kg of steam per hour depending on the pressure differential and orifice condition — equivalent to 35 to 85 kWh of thermal energy per hour lost. At a boiler fuel cost of Rs 4 per kWh of steam output, this represents Rs 140 to 340 per hour lost from a single failed trap running undetected through a production shift.
Beyond direct energy loss, a failed-open trap on a heating coil also reduces the temperature gradient driving heat transfer into the wash load, potentially extending the heating phase of the wash programme and reducing throughput. A trap that fails closed on the same coil results in condensate flooding the coil, reducing the effective steam heating surface and causing waterlogging that can lead to water hammer as the condensate slug is intermittently displaced by incoming steam.
How to conduct a steam trap survey
A trap survey requires an ultrasonic listening device or an infrared thermometer — or both — and should be conducted while the plant is in normal production. The procedure for each trap is as follows:
- Record the trap location and number on a site plan that maps every trap in the system. A laundry plant with 50 traps that has never been mapped is very likely to have traps that are unknown to the maintenance team, some of which may have been failed open for years.
- Check upstream and downstream temperatures with a contact thermometer or IR gun. The upstream (inlet) side should be at the saturation temperature corresponding to operating pressure. The downstream side should be at or slightly below saturation if condensate is being discharged; a downstream temperature equal to upstream temperature is a strong indicator of a failed-open trap passing live steam.
- Listen with the ultrasonic device. A correctly operating F&T trap has a continuous soft flow sound as condensate discharges. A thermodynamic disc trap has a regular cycling click. Continuous high-pitched hiss on a thermodynamic trap indicates failed-open operation. Silence on an F&T trap during a period of known condensate production indicates a failed-closed condition.
- Record the condition assessment as: operating correctly, suspected failed open, suspected failed closed, or requires further investigation.
- Calculate the estimated loss for each suspected failed-open trap using steam loss tables referenced to operating pressure and orifice size, and total the losses across the system.
Survey frequency and replacement standards
A comprehensive trap survey in a laundry plant should be conducted at minimum annually, with the results used to prioritise replacements on a cost-versus-failure-rate basis. Plants operating boilers at higher pressures (above 8 bar) typically see thermodynamic disc traps fail more frequently due to accelerated seat wear at higher differential pressures; annual survey of these traps is appropriate. F&T traps on washer-extractor coils should be inspected at the same interval, with the inlet strainer cleaned at every six-month maintenance visit since strainer blockage is the most common cause of F&T trap failure in plants with scale-forming water.
When a trap is found failed open and replaced, the root cause of failure should be recorded: seat erosion (most common, indicating the trap has reached end of service life and the specified replacement interval should be shortened), dirt or scale contamination (indicating strainer maintenance is inadequate), or mechanical failure of the float or bimetallic element (which may indicate a manufacturing quality issue if the trap has not reached its expected service life). This root cause data, accumulated across several survey cycles, provides the evidence base for rationalising trap specifications and replacement intervals rather than treating all failures as unpredictable events.