Steam Traps Explained: Types, Selection, Testing and Common Failures

Steam traps are automatic valves that let condensate and air out of a steam system while holding the steam in. Every steam main, heater, coil and jacket needs one. When steam traps work, the plant heats quickly and evenly. When they fail, the plant either wastes live steam or floods its equipment with water.

Traps are small and cheap, so plants often ignore them. Yet failed traps are one of the most common causes of high fuel bills. This guide explains how they work, how to choose them and how to keep them healthy.

What do steam traps do?

Steam gives up its heat and turns back into water. That water is condensate. If it stays in the pipe or the heater, it blocks heat transfer and causes water hammer. So something must remove it.

A simple open drain would work, but it would also blow steam away. A trap solves this. It opens for condensate and air, and then it closes when steam arrives.

A good trap does three jobs:

• it discharges condensate as soon as it forms
• it vents air and other gases, especially at start-up
• it holds back live steam

The main types of steam traps

Traps fall into three families. Each one senses the difference between steam and condensate in a different way.

Mechanical traps

A ball float trap uses a float that rises with the water level and opens the valve. It discharges condensate continuously, at steam temperature. Therefore, it suits heat exchangers and process equipment where the load varies. Most float traps include a small thermostatic vent for air.

An inverted bucket trap uses an upturned bucket that sinks when it fills with water. It is robust and tolerates water hammer well. However, it vents air slowly and can lose its water seal on sudden pressure drops.

Thermodynamic traps

A disc trap has one moving part, a small steel disc. Flash steam closes the disc, and it opens again when the trap cools. It is compact, cheap and tough. So it is the usual choice for draining steam mains, even at high pressure. On the other hand, it is noisy and wastes a little steam each cycle. It also dislikes high back pressure.

Thermostatic traps

A balanced pressure trap contains a liquid-filled capsule. The capsule expands near steam temperature and closes the valve. It vents air very well. A bimetallic trap uses metal strips that bend with heat. Both types hold condensate back until it cools slightly. That suits tracing lines, but it can flood equipment that needs fast drainage.

How do you select steam traps for each job?

Start with the application, not the catalogue. The table gives a first guide.

After that, size the trap properly. You need three figures: the condensate load, the pressure before the trap and the back pressure after it. The difference between the two pressures drives the flow.

Do not size a trap by the pipe diameter. A trap that is too small will flood the equipment. One that is too large will wear quickly and may pass steam. Also apply a sensible safety factor for start-up, when the load is heaviest.

How should you install steam traps?

Even the right trap fails in the wrong installation. So follow these rules.

• Fit a drain pocket on mains. A full-size pocket at the bottom of the main collects condensate. A small tapping in the side of a pipe catches very little.
• Place drip points regularly. Fit them every 30 to 50 metres on a main, and before every rise, valve and end of line.
• Fit the trap below the equipment. Condensate must flow to the trap by gravity.
• Add a strainer. Dirt is the main enemy of any trap.
• Add isolation valves and a test point. Then people can check and service the trap without a shutdown.
• Give each heater its own trap. Group trapping lets one unit flood another.
• Check the flow arrow. A trap fitted backwards does not work.

Finally, think about where the condensate goes. Size the return line for the mix of water and flash steam, and avoid long rises after traps that cannot handle back pressure.

Why do steam traps fail?

All traps wear out. Still, some causes shorten their lives.

• Dirt and scale. Debris holds the valve open or blocks it shut.
• Water hammer. Slugs of water crush floats and damage capsules.
• Wear. Fast discharge erodes the seat over time.
• Corrosion. Acidic condensate attacks internal parts.
• Wrong size or type. An unsuitable trap cycles badly from day one.
• Freezing or flooding. Outdoor traps suffer when they hold water during shutdowns.

A trap fails in one of two ways. It either fails open or fails closed.

Failed open. The trap blows live steam into the return line or to drain. Fuel use rises, and the return system may overheat and pressurise.

Failed closed. The trap passes nothing. Condensate backs up, so the heater runs cold and output drops. In mains, the trapped water leads to water hammer, which can break pipes and fittings.

How do you test steam traps?

Testing is simple once you have a routine. Use more than one method where you can.

1. Look. Where a trap discharges to an open end, watch it. Flash steam is a lazy white cloud that appears with the condensate. Live steam is a fast, clear jet near the outlet.
2. Measure temperature. Use an infrared thermometer before and after the trap. A cold trap is blocked or isolated. However, temperature alone cannot prove a trap is passing steam.
3. Listen. An ultrasonic tester picks up the sound inside the trap. With practice, it gives the most reliable result. A disc trap clicks on and off, while a float trap gives a steady flow sound.
4. Use sight glasses or sensors. Some plants fit these on critical traps for a direct check.

Record every result. Then tag each trap with a number, and keep a register that shows its type, size, location and history.

How much do failed steam traps cost?

The loss depends on pressure and orifice size. Even so, the numbers are sobering. A single trap that fails open on a medium pressure line can waste many kilograms of steam every hour. Over a year of continuous running, that is many tonnes of steam and a great deal of fuel.

In a plant with no trap programme, surveys often find that 15 to 30 percent of traps have failed. Therefore, a first survey usually pays for itself within weeks. The US Department of Energy recommends regular trap testing as one of the best low-cost steam savings.

How often should you check steam traps?

Check frequency depends on pressure and duty. As a guide:

• test high pressure traps weekly to monthly
• test medium pressure traps every one to three months
• test low pressure traps at least once a year

In addition, survey the whole plant twice a year. After each survey, repair or replace the failed traps promptly. A list of failures in a drawer saves nothing.

What other good practice protects steam traps?

Traps work as part of a system. So look after the system as well.

First, warm up steam lines slowly, with drains open, to avoid water hammer. Second, keep the boiler water in good order, because carryover fills the mains with dirty water. Third, clean strainers on a schedule. Finally, return the condensate. It is hot, clean water, and it cuts fuel and chemical costs.

Good pipework design matters too. Mains should fall in the direction of flow, and branches should leave from the top of the pipe. We explain this in our guide to steam piping installation. For the wider picture on saving fuel, see our guide on how to improve boiler efficiency.

Frequently asked questions about steam traps

What is the most common type of steam trap?

On steam mains, the thermodynamic disc trap is the most common. On process heaters, the ball float trap is the usual choice.

How long do steam traps last?

Most last three to six years in normal service. Clean steam, good strainers and correct sizing extend that life. Dirt and water hammer shorten it.

How do I know if a steam trap is leaking?

A leaking trap is hot on both sides and gives a continuous rushing sound on an ultrasonic tester. At an open outlet, you will see a fast, clear jet of live steam.

Should steam traps be insulated?

Float and bucket traps can be insulated. However, do not insulate disc traps or most thermostatic traps, because they need to lose heat in order to operate.

Keep your traps working

Healthy steam traps mean dry steam, fast heating and lower fuel bills. So tag every trap, test on a schedule and fix failures quickly.

Maven Technical Services designs, installs and maintains steam and condensate systems for plants across Nigeria. We can survey your traps, replace failed units and correct poor installations. Speak to our engineers to arrange a survey, or view our steam work on the projects page.