The Well-Functioning Steam Trap

Steam Traps Defined

For those who are not familiar with the steam trap, it is an automatic valve that, when operating properly, opens to purge a steam system of condensate and closes with the presence of steam.

There are three types of steam traps:

• Density operated, which includes the inverted-bucket (IB) and the float and thermostatic (FT) types
• Thermostatic (TH), specifically, the balance-pressure thermostatic (BPT) type
• Thermodynamic (TD), either the disc or impulse type

Trap Characteristics

When these traps fail, they usually fail closed. However, the ball float may become damaged and sink down, failing in the open position. The thermostatic element may also fail and cause a fail-open condition.

Thermostatic traps have, as the main operating element, a metallic corrugated bellows that is filled with an alcohol mixture that has a boiling point lower than that of water. The bellows will contract when in contact with condensate and expand when steam is present. Should a heavy condensate load occur, such as in start-up, the bellows will remain in a contracted state, allowing condensate to flow continuously. As steam builds up, the bellows will close. Therefore, there will be moments when this trap will act as a “continuous flow” type while at other times it will act intermittently as it opens and closes to condensate and steam, or it may remain totally closed.

These traps adjust automatically to variations of steam pressure but may be damaged in the presence of water-hammer. They can fail open should the bellows become damaged or due to particulates in the valve hole that will prevent adequate closing. Sometimes the trap becomes plugged and will fail closed.

Inverted bucket traps have a “bucket” that rises or falls as steam and/or condensate enters the trap body. When steam is in the body, the bucket rises, closing a valve. As condensate enters, the bucket sinks down, opening a valve and allowing the condensate to drain.

Inverted bucket traps are ideally suited for water-hammer conditions but may be subject to freezing in low temperature climates if not insulated. When an inverted bucket trap fails, it fails open due to the bucket losing its prime and sinking, or from impurities in the system that may prevent the valve from closing.

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While most traps operate with backpressure, they’ll do so only at a percentage of their rating, affecting everything down the line of the failed trap. Thermodynamic traps have a disc that rises and falls depending on the variations in pressure between steam and condensate. Steam will tend to keep the disc down or closed. As condensate builds up it reduces the pressure in the upper chamber and allows the disc to move up for condensate discharge.

This trap is a good general-type trap where steam pressures remain constant. It can handle superheat and water-hammer but is not recommended for process, since it has a tendency to air-bind and does not handle pressure fluctuations well.

A thermodynamic trap usually fails open. There are other conditions that may indicate steam wastage, such as “motor boating,” in which the disc begins to wear and fluctuates rapidly, allowing steam to leak through.

No matter which types of steam traps are in use, testing them is critical because failed traps negatively impact the entire steam system. They can waste valuable energy, adversely affect production, increase maintenance costs and create safety issues.

Why Do Steam Traps Fail?

Three general conditions can adversely affect traps, thus leading to premature failure:

• Dirt, the leading cause of failure, results in leaking, blowing or plugged traps.
• Pressure surges due to sudden steam-valve openings, improper piping or trap misapplications, or water-hammer can all damage the internal components of a steam trap.
• Over-sizing can cause traps to lose their prime, experience rapid cycling or wire-drawing. Wiredrawing can occur when condensate is not successfully eliminated from a steam system. Little beads of water in a steam line can eventually cut any small orifices through which the steam normally passes. Wire-drawing will eventually cut enough of the metal in the valve seat of a steam trap and prevents adequate closure of the steam trap, producing leakage in the system.

Know Your System

At start-up any steam trap will be open for a longer period of time depending on the volume of condensate produced by the equipment. You may need to return to the equipment at a later time to allow for the system to settle down before you attempt a re-test.

Each type of steam trap has its own operating characteristics. While the operation of an IB trap may sound different from a TD trap, all traps that are working properly must open and close.

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Testing Steam Traps

There are two ways to test steam traps:

• Temperature. A dated method of testing the performance of a steam trap is to use temperature to determine if the trap is partially or fully plugged. However, using the temperature method to test for a failed-open trap has limited use. In many instances the temperature of condensate and flash steam on the downstream side of a properly functioning trap will be at or close to 212° F. This is exactly the same as the temperature of condensate and live steam on the downstream side of a malfunctioning trap.
• Sound. When using an ultrasound instrument to test steam traps, research the type of trap. Instead of referring to the category of trap such as density operating, thermodynamic, etc., use an acoustic reference that relates to how the trap operates. This can be broken down into two types: on-off and continuous flow.

The on–off trap will hold steam, open to discharge condensate, and close. These types can be thermodynamic, thermostatic, inverted bucket and bi-metallic.

The continuous flow trap, such as a float and thermostatic, will utilize a ball float that moves up and down on a bed of condensate, producing a modulating, continuous flow. Test it at two points: upstream and downstream. Using a stethoscope or contact module attached to the instrument, touch upstream of the trap and adjust the instrument’s sensitivity to bring the meter intensity indicator toward midline. Then touch downstream and listen to the trap.

If it is an on-off trap, listen for the hold-discharge-hold pattern. A blowing trap will produce a steady, continuous flow. If the trap is cold and quiet, it can mean the trap is either not in service or plugged. Continuous traps should present a continuous, modulating flow. If testing a float and thermostatic type, test at both the downstream side of the trap and at the discharge location of the thermostatic element. Normally the thermostatic element will be closed and quiet.

Conclusion

The two most important reasons for maintaining an efficient steam system are obvious: Wasted energy is wasted money, and a well-maintained system is one that is safe for personnel.

Good energy management is like creating an excellent product. It ensures an edge over the competition and may ultimately determine your company’s profitability.

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Bruce Gorelick is the founder of Enercheck Systems, Inc. (www.enerchecksystems.com). During the past 30 years, he has tested tens of thousands of steam traps and steam system components. Alan Bandes is the vice president of marketing for UE Systems (www.uesystems.com).

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Good steam trap ­maintenance ­practice includes:

• Blowing down upstream trap strainers to keep the system clean.
• Draining seasonally used equipment of stagnant condensate to prevent system corrosion.
• Inspecting steam trap performance at least once a year.