Trapped and losing steam

July 18, 2000
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Steam traps are essential components of any steam heating system.

During system operation, traps separate steam from the condensate and remove condensate properly from the system. During start-up, the traps help extract unwanted air from the system.

Although steam trap models are fully tested and highly reliable, any steam trap will eventually fail after long operation. Some of the problems might occur in the trap itself. System complications can also impair the function of the steam trap.

The most common trap problems include:

  • Worn seat and pins. After years of service, these components can wear and allow live steam to blow through.

  • Worn or bent linkages. These can cause misalignment of the pin and seat, which will allow live steam to blow through.

  • Leaking float. A leaking float will fill with condensate and lose its buoyancy. This will cause a float and thermostatic (F&T) style steam trap to fail in the closed position.

  • Loss of fill in the thermostatic elements. This usually causes the element to fail in an open position and blow live steam.

    Steam trap malfunctions and failures will result from either system operation or system design problems. These conditions include:

  • Excessive dirt. Dirt can plug strainers, plug internal passageways in the trap, or lead to deposits on the trap seat to prevent tight closings.

    With inverted bucket traps, dirt can plug the vent hole in the top of the inverted bucket. This allows air to be trapped in the top of the bucket and close the trap. Strainers should be used ahead of the trap and maintained to reduce dirt being carried into the trap.

  • Water hammer. This condition is created by high-temperature condensate flashing in the return line and the ensuing formation of steam bubbles. When the bubbles implode, the resulting force is known as water hammer. Water hammer can be severe enough to destroy trap floats and thermostats.

    Usually water hammer occurs at points of pressure change, lifts in the return line, or at traps that discharge into pressurized return lines. The situation can be corrected by installing check valves after traps that have lifts in the return line or discharge into pressurized lines. (See Figure 1.)

  • Poor air venting also can lead to steam trap malfunctions. Normally air is vented from the steam’s space into a gravity return line by the steam trap. However, traps discharging into wet return lines, pressurized return lines, or those that have a lift in the return line may vent poorly.

    In these situations, a solution may be separate air vents that vent the air to gravity returns or atmospheric conditions.

    Make a trap testing plan

    If a trap doesn’t seem to be operating properly, how can you diagnose and correct the real problem? A number of steps are involved, but a testing plan is a good place to start.

    As part of routine system maintenance, it is a good idea to check steam traps system-wide on a regular basis. Here is a step-by-step plan:

    1. Start by creating a plan showing all buildings and sites to be inspected.

    2. Mark all steam traps with a number on a drawing or schematic.

    3. Attach a metal tag with the corresponding number on all traps to identify them permanently.

    4. In making a record of all the traps, include information about each trap’s manufacturer, type, model number, orifice size, and application.

    5. Next, make sure the system steam is on.

    6. Note the steam supply pressure and check to see that the steam traps’ seat pressure ratings are correct.

    7. Verify that installed traps actually are recommended for the types of applications they are carrying out.

    Now, testing can begin.

    Let the tests begin!

    First, identify the design style on which your steam trap operates: thermostatic, float and thermostatic, bucket/inverted bucket, or thermo disc traps.

    For thermostatic traps:

    •  Using a digital thermometer, the temperature at the trap inlet (Figure 2) should be subcooled 10° to 30°F below the temperature ahead of the trap. A cold trap may indicate that it has failed closed. Confirm that steam is turned on.

    •  Listen to the trap cycle using an automotive-type stethoscope. A low-pitched whistle indicates a trap blowing live steam.

      For float and thermostatic traps:

    •  Check the unit’s test valve; the trap should be draining condensate with some flash steam. A failed open trap will blow live steam. A failed, closed trap will be cold.

    •  Test using a stethoscope; you should hear a wet gurgling sound as the trap drains condensate. Again, a pitched whistle indicates live steam is blowing.

    •  Confirm with a digital thermometer. Temperature at the trap inlet should be at or near the steam temperature ahead of the trap. If the trap is operating properly, temperatures downstream of the trap should drop off. Compare the readings with other traps in similar applications.

      Bucket and inverted bucket traps:

    •  Follow the same test procedures for float and thermostatic traps. Additionally, when using a test valve, you should be able to see the trap cycle. If you use a stethoscope, you should be able to hear the trap open and close.

      Thermo disc traps:

    •  Listen using either a stethoscope or your ear held against a screwdriver held against the trap. You should be able to hear the trap cycle open and close. With most disc traps, this occurs every 20 sec or less.

    Rapid cycling or machine gunning indicates a worn seat and wasted steam. A cold trap means the trap is plugged or has failed closed.

    Written by the staff of ITT Industries’ Little Red Schoolhouse, a training facility in Morton Grove, IL. This article appeared in “Tech Talk,” a publication of the company’s Fluid Handling Unit. For further information, call 847-966-3700.

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