December 2, 2017

No. 41 – Testing Steam Trap Stations

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Best Practice No. 41

Successfully Testing Steam Traps with High Frequency Ultrasound



When using high frequency ultrasound; the main question is where the sensitivity should be set to conduct the test.  Using a high sensitivity, then all the steam traps will test failed and using a low sensitivity will indicate all steam traps are operating properly.

The solution is using the field proven comparison method will provide an accurate test on each steam trap. The comparison method uses three or more test points on the steam trap station.  Two of the test points are the sensitivity base line settings on the ultrasound unit and the third is for the for testing the steam trap. The comparison method allows the steam trap station assessment person to establish a base ultrasound reading to filter out any competing ultrasounds that can be generated upstream or downstream of the steam trap.  Without using the comparison method of testing, it is very difficult to determine the steam trap performance due to the fact of not knowing the correct sensitivity setting. Each steam trap will be in slightly different installations and situations in the steam system, so the comparison method proves an accurate method on setting the ultrasound sensitivity.

1. Sensitivity Settings for a Digital Ultrasound

Figure 1. Three Points for Baselines and Testing

With the stethoscope module, contact at each point on the steam trap station per Figure No. 1.  The steam and condensate line should have baseline test points that are between 6 to 10” (estimated and will vary depending on the piping) upstream/downstream of the steam trap that is being tested. More baseline test points can be taken for baseline, but at least two need to be done for each steam trap location.

2. Setting Up and Using the High Frequency Ultrasound Unit

The ultrasound unit needs to be set at 25 kHz to provide the highest clarity for high frequency ultrasound that is generated by steam or condensate passing through an orifice in steam trap.

  1. Pull the trigger to turn on the ultrasound unit.  If the instrument is within sensitivity range, the decibel (dB) indicator (A in Figure 11) will blink.

Figure 2: Dial Readings on the Ultrasound Unit

  1. The decibel reading should be set to 20 dB.
  2. The kHz (frequency) indicator must be steady and not blink (B in Figure 11). If the kHz is blinking, then it is in the adjustment mode for frequency. Adjust it to the correct frequency level and push in the sensitivity knob to go back to the sensitivity adjustment-setting mode.
  3. Once in the sensitivity mode, turn the sensitivity control dial clockwise to increase the sensitivity and counterclockwise to decrease the sensitivity.
  4. The sensitivity control dial increases or decreases the sensitivity of the instrument simultaneously with the sound level in the headphones.

NOTE: The instrument needs to be in range for accurate testing.

     6. If the sensitivity is too low, a blinking arrow pointing to the right will appear, and no numeric decibel level will be visible in the display panel. If this occurs, increase the sensitivity until the arrow disappears (in low-level sound environments, the arrow will blink continuously, and it will not be possible to achieve a dB indication until a higher intensity level is sensed).

     7. If the sensitivity is too high, a blinking arrow pointing to the left will appear, and no numeric decibel value will be visible on the display panel. Reduce the sensitivity until the arrow disappears and the numeric decibel value is shown.

3. Comparison Testing: Testing Methods And Results

3.1. Proper Operation (PO)

Test Point 1: 32 dB

Test Point 2: 34 Db

Test Point 3: 34 dB   

If a steam trap is operating properly, the will not be a higher ultrasound level at test point 3 (than test point 1 or 2).

Figure 3. Ultrasound Levels at Test Point 3 Are Equal or Less Than Test Points 1 or 2

Note:  Test Point 3 will be equal or less that test point 1 and test point 2. The high frequency ultrasound readings, in addition to a temperature measurement that is appropriate for the system pressure, will indicate a properly operating steam trap.

Note:  If the ultrasound reading at Test Point 3 is higher than Test Point 1 and Test Point 2; then wait for 45 seconds to ensure the steam trap was not in a cycle mode.  During the cycle mode the ultrasound reading at Test Point 3 will be higher during the cycle. Which is the proper operation of a steam trap with on/off discharge cycle.

3.2. Float and Thermostatic Steam Traps – Four Test Points

The float and thermostatic steam trap has two orifices; one orifice for discharging the condensate and the other orifice for the air vent mechanism that discharges air and non-condensable gases.

The four test points:

  1. Upstream of the steam trap station
  2. Downstream of the steam trap station
  3. At the discharge side of the steam trap condensate orifice
  4. At the discharge side of the steam trap air vent

Note:  The ultrasonic level at “T4” should be equal or less than “T3”; then the air vent is in proper operation.  If, “T4” is higher that “T3” than the air vent mechanism has failed.

Figure 4. Float and Thermostatic Steam Trap: Four Test Points

3.3. Blowing or Completely Failed Steam Trap (BLW)

Test Point 1: 25 dB

Test Point 2: 32 Db

Test Point 3: 64 dB   

A significant increase (greater than two times the base level reading) in the decibel level at test point 3; indicates that the steam trap is failed open and allowing steam to pass. Continue to monitor the steam trap at Test Point 3 to see whether the steam trap cycles according to its design.

Figure 5. Failed Steam Trap

3.4. Steam Leakage

Test Point 1:  25 dB

Test Point 2: 32 dB

Test Point 3: 49 dB  

Test Point 4: 40 dB

An increase in the decibel level at Test Point 3 indicates leaking steam through the steam trap.

Figure 6. Steam Tap Leaking Stream

Again, if this increase is observed, take additional time at Test point 3 to determine whether the steam trap is in the middle of a discharge cycle. If the decibel level at Test Point 3 does not return to the baseline value established at Test Point 1, then the steam trap is leaking steam.

3.5. Competing Downstream Ultrasounds

Test Point 1:  22 dB

Test Point 2: 42 dB

Test Point 3: 28 dB  

Test Point 4: 28 dB

Test Point 5: 52 dB

Figure 7: Checking all steam components

The above readings show that ultrasound is being produced downstream of the steam trap. Perform further testing; testing at the other components in the steam trap station (test point 5); it is determined the high ultrasound levels are being generated by the check valve in the system. If the decibel value is higher at this location (test point 5), then there are competing ultrasounds in the system.  If the value is lower (test point 5), then conduct further examination of the piping and steam trap to determine the source of the higher ultrasound. Check valves can be a source of additional ultrasound in the piping system.

4. Sound Characteristics Using Ultrasound

While using ultrasound listening devices, the tester should be made aware of a few distinct sounds that he or she may hear while testing steam traps:

  • Crackling: This sound signature is generated by condensate flowing through a steam trap with flash steam occurring after the discharge orifice of the steam trap.
  • Whistling: A whistling sound is a characteristic of steam passing through a steam trap orifice.