Steam trap testing, pt. 2

January 30th, 2015

In the first installation of this two-part webinar, Tristian McCallion of Swagelok discussed the importance for manufacturing plant managers to conduct regular, consistent steam trap testing.

He began by naming the three main purposes of a steam trap: to remove condensate as soon as it forms, to prevent live steam loss and to remove air from the system. Steam traps accomplish these objectives by venting gases, discharging condensate and trapping steam. That way, steam traps prevent plants from wasting energy, losing efficiency, dropping production rates and creating unnecessary safety hazards.

McCallion then touched upon a few ways to go about steam trap inspection and various predictive maintenance tools to use. In part two, he will go deeper into techniques and examples of how best to setup and carry out a reliable steam trap testing program.

Steam trap varieties and their testing methods

When testing steam traps, there are two modes of operation to recognize. The first and most common is on-off, which opens only to discharge condensate and then closes. The second variety is continuous flow, in which condensate is discharged continuously.

At the onset of testing, there is a basic order of operations that will allow the team to easily determine the source of the problem. First, check that the steam trap is operating. If not, check to make sure all the valves are open. Once that has been confirmed, take the temperature. If the trap is cold or well below operating temperature, that indicates a whole or partial blockage. If it’s hot, then ultrasound leak detection is the next step.

The procedure used with ultrasonic testing depends on the trap’s mode of operation. Additionally, the closer the ultrasonic probe can get to the discharge orifice of the trap, the clearer the ultrasonic signature will be.

The comparison method
It’s a good idea to get a baseline idea of what the steam sounds like at different points of the trap. That helps the operator determine where in the line the malfunction occurs – at the orifice itself, upstream or downstream.

Take an ultrasonic reading upstream from the orifice first. In most cases, it should be relatively quiet – about 20 percent of scale. At the orifice itself, the noise will likely by much louder, up to 90 or 100 percent. Moving downstream, if the noise is reduced down to 30 percent or so, the operator can safely say the trap is failed in the open position.

However, if more noise is generated downstream, then the user should continue further down the line to find the source of the noise and try to isolate that area. Then he or she can return to the orifice and determine whether there is indeed a failure there or if something further downstream was generating a false signal.

The types of traps
Within those two categories, there are numerous traps that feature different operations and require different techniques for testing.

  • Inverted bucket trap: This on-off trap has an upside-down bucket inside that fills up with condensate. As the pressure mounts, the bucket lifts and the steam is released, causing a fairly loud hissing sound. They usually cycle about two or three times per minute.
  • Float and thermostatic traps: These are the only continuous flow traps that McCallion typically tests. As the name implies, they consist of two elements: a ball float and a thermostat. The thermostatic element is only used to remove air during startup and should remain closed otherwise. During operation, as condensate enters the trap, it pools at the bottom. The ball element begins to float and opens the valve as it moves higher, releasing condensate. The ultrasound operator can first listen at the bottom of the trap for a valve failure and, if there is no sign of malfunction, listen to the top of the valve where the thermostatic element is usually located.
  • Thermostatic traps: These traps rely on thermostats to sense temperature and react based on the difference between steam and condensate. When closed, they should be silent – hissing indicates leakage.
  • Thermodynamic/disc traps: Disc traps operate using the same principle that generates lift in airplanes – based on the pressure above and below a disc inside the trap. As steam enters the trap, the velocity below the disc is higher than above, so the disc drops and closes the cycle. As steam in the upper cavity of the trap condenses, the volume decreases and the pressure above the disc overcomes the pressure below, causing the condensate to discharge. These traps should not cycle more than four times per minute and they will fail in the open position.

It’s important to note that good steam traps should last six years with consistent testing, while industry standards say 10 percent of traps will fail on an annual basis. Without a trap testing program, 50 percent failure annually is not unheard of. Consider that 45 percent of all fuel burned by the U.S. is used to create steam, and that a proper testing program can improve steam savings by 20 percent.

High pressure traps should be inspected every month, while all other traps every three to six months. Additionally, operators should document all the results and try to examine the entire installation, rather than looking at individual steam traps alone.  Click here for Part 1.

Suggested Ultraprobe instruments for steam trap testing: Ultraprobe 3000; Ultraprobe 9000; Ultraprobe 10,000; Ultraprobe 15,000

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