How to test a steam trap: ultrasound and temperature

A steam trap is tested on a live system in two ways. Temperature (a pyrometer) finds cold, that is blocked, traps. Ultrasound compares the signal upstream of the trap and immediately downstream: a continuous and noticeably louder signal downstream means steam is blowing through. You need to know the trap type in advance, because healthy traps of different types sound different.

What a faulty trap costs you

A steam trap discharges condensate and non-condensable gases and holds back steam. It can fail in two ways:

Condition	What happens
Healthy	Discharges condensate, holds back steam
Passing steam (failed open)	Live steam escapes into the condensate return line
Blocked (failed closed)	Condensate is not removed, it backs up and the trap cools down

According to SDT, both failure modes waste energy and heat, and poor-quality steam reaching the point of use gradually damages equipment.

According to the U.S. Department of Energy (DOE), in systems that have not been maintained for 3 to 5 years, 15–30% of traps may have failed. With a regular testing programme, leaking traps should account for less than 5% of the population. Trap types and their failures are covered in Steam Trap Diagnostics: Trap Types, Failure Signs, and How to Detect Them.

Four ways to test

DOE lists four basic ways to test steam traps: temperature, sound, visual and electronic. Each answers its own question:

Method	What it shows	Limitation
Temperature (pyrometer, thermocouple)	A cold trap: blocked or out of service	Cannot tell steam blow-through from normal condensate discharge: steam and condensate can be at the same temperature
Ultrasound (contact sensor)	Flow pattern: discharge cycles or continuous steam blow-through	You need to know the trap type; adjacent traps and condensate load affect the signal level
Visual (sight glass, open discharge)	Steam in the discharge	Requires a sight glass or open discharge
Electronic (sensor in the trap)	Continuous state of the medium in the trap	Requires sensors to be installed

Spirax Sarco warns that a trained operator gets good results with an ultrasonic tester, while a novice can easily take a healthy trap for a failed one. That is why temperature and ultrasound are used together in practice, and the signal is read with the trap type in mind.

How to test a steam trap: the procedure

The procedure, based on SDT material:

1. Make sure steam is reaching the trap. Take an ultrasonic reading with the contact sensor on the steam line some distance upstream of the trap: this confirms flow and gives you the system's background signal level.
2. Measure the trap temperature. If condensate is backing up at the inlet, the trap is colder than it should be. SDT advises allowing for heat loss through the wall: the outside temperature is about 10% lower than inside. SDT's example: in a 100 psi (about 6.9 bar) system, steam inside is at 338 °F (170 °C), and the outside of the pipe will show about 300 °F (about 149 °C).
3. Take a reading immediately downstream of the trap. On a healthy trap the downstream signal is no higher than upstream, and discharge is heard as a cyclic rise in dBµV.
4. Assess the signal. If the downstream signal is noticeably louder than upstream, the trap is discharging. If the discharge is continuous and loud, the trap is passing steam.
5. Record the result against the trap number and schedule the repair: high-pressure traps passing steam first.

SDT's short rule: a trap that is hot and quiet is working. A detailed step-by-step method with field photos is in How to Survey Steam Traps with SDT Ultrasound: Step-by-Step Method.

What a healthy trap sounds like by type

Trap type	Healthy (according to TLV)	Passing steam	Blocked
Thermodynamic (disc)	Cycles: discharge for a few seconds, then a longer pause	No cycles, continuous loud signal	No flow signal, trap colder than normal
Float	Continuous discharge, flow depends on the condensate load	Continuous signal noticeably louder than upstream	No flow signal, trap colder than normal

TLV points out another pitfall: a trap with no discharge and a lower temperature is easy to confuse with one that is simply out of service. Before concluding that a trap is blocked, make sure the section is under steam.

How much steam a leaking trap loses

DOE gives an estimate of steam loss through a trap orifice discharging to atmosphere (Boiler Efficiency Institute data, discharge coefficient 0.72). The values below are in kg/h, converted from pounds per hour; diameters and pressures are converted to mm and bar.

Orifice diameter	1.0 bar (15 psig)	6.9 bar (100 psig)	10.3 bar (150 psig)	20.7 bar (300 psig)
0.8 mm (1/32")	0.4	1.5	2.2	—
1.6 mm (1/16")	1.5	6.0	8.6	16.4
3.2 mm (1/8")	6.2	23.9	34.4	65.8
4.8 mm (3/16")	13.9	54.0	77.1	147.9
6.4 mm (1/4")	24.8	95.7	137.4	262.6
9.5 mm (3/8")	55.8	215.5	309.3	591.0

Example using the DOE method: a trap with a 3.2 mm orifice on a 10.3 bar line is stuck open. Loss: 34.4 kg/h × 8,760 h = about 301 t of steam per year. To convert this into money, multiply by the cost of a tonne of steam at your plant. A calculation for a power plant survey is in Steam Trap Testing at a Power Plant: TRAPChecker + SDT270.

How often to test

DOE recommendations on testing intervals:

Steam pressure	Interval
150 psig and above (from 10.3 bar)	Weekly to monthly
30–150 psig (2.1–10.3 bar)	Monthly to quarterly
Below 30 psig (up to 2.1 bar)	Annually

For the most important traps, DOE suggests considering online monitoring. This option is described in Vigilant: Permanent Ultrasonic Monitoring of Steam Traps and Valves.

Which instrument to choose

• Steam traps only, surveys by operations staff: TRAPChecker. RS2 Needle contact sensor, about 2 minutes per trap, tells normal operation, steam blow-through and blockage apart by sound and temperature.
• Steam traps plus air leaks, bearings and electrical inspection, routes and trends: SDT340 with a built-in IR thermometer.

The same ultrasonic instrument finds compressed air leaks: see How to find a compressed air leak: 5 methods.

KEG TRK supplies SDT instruments for steam trap testing in Kazakhstan. Price on request.

Sources

• U.S. Department of Energy. Steam Tip Sheet #1: Inspect and Repair Steam Traps.
• SDT Ultrasound Solutions. Conducting Steam Trap Surveys.
• TLV. A Guide to Steam Trap Testing.
• TLV. How Disc Traps Work.
• TLV. Free Float Steam Trap: JX/JHX series.
• Spirax Sarco. Testing and Maintenance of Steam Traps.