Every time the high cost of energy comes up, someone mentions steam traps. The engineers in the group then lapse into jargon which leaves the non-engineers baffled and wishing they were elsewhere.

The price of oil is presently at a ten-year high so as we go into this winter efficient steam trapping is something which even the uninitiated can no longer ignore. The Government introduces its carbon tax in April 2001, only six months away. So let us face up to the basic questions.

What is a steam trap?

Most laundry and drycleaning equipment is heated by feeding high-pressure steam to a heating element where it condenses and in changing from vapour to liquid releases about 90% of its heat energy.

To keep the heater battery working efficiently, the liquid condensate must be removed as quickly as possible without letting any steam out. The device which achieves this is called a steam trap and the best ones allow the condensate to flow straight through but do not allow any steam leakage.

Most steam contains some air and if this is not released periodically it will build up in a calender bed or heater battery and effectively bring heat transfer to a stop. Air is released via air vents on the horns of a calender bed and via every steam trap in the laundry or drycleaning plant.

What types of steam trap?

The commonest steam trap is a thermo-dynamic or TD type which has a small disc that forms the valve against the end of the condensate drainpipe, the disc being held in place by a slug of low-pressure steam held under a six-sided cap.

That is why a TD trap usually has a hexagonal cap that looks like a large bolt head. As the cap cools by exposure to the air the low-pressure steam within the cap condenses and drops its pressure and the disc opens to release the condensate. Once the condensate and air have passed through the high-pressure steam following repressurises the cap and closes it.

So the trap opens regularly every twenty seconds or so and releases everything there to be released. It then closes again until the cap cools, even though there may be considerable quantities of condensate forming. This means that it is possible to flood a heater battery or calender bed with an undersized TD trap.

Otherwise it is a cheap and cheerful device widely used and much maligned. Its critics have generally found that if traps are not regularly maintained, it is usually the TD traps which start to leak first and when they leak they allow large quantities of steam to pass, increasing fuel bills.

They leak because the discs wear and a good seal can no longer be obtained. The valve will first chatter open and close like a machine gun and finally jam wide-open. One launderer actually worked with an 80psi back pressure in his condensate main before he found the problem.

Thermostatic traps

A thermostatic trap depends on a bi-metallic strip or expansion bellows reacting to steam pressure and temperature to seal the orifice drain in the base of the trap.

As condensate flows into the trap and starts to cool the element, so the valve opens and the condensate is blasted away to drain.

When high-pressure steam hits the element it warms it up and so closes the orifice and the cycle is repeated. This trap has the same disadvantage as the TD trap in that it relies on cooling of the element to operate. Nevertheless, a great many laundries rely extensively on this inexpensive type of trap.

Float traps

A float trap has a large housing into which condensate flows to form a liquid level. There is a ball-cock type device in the liquid so as the level rises the ball-cock can open the valve in the base of the trap and allow the liquid to flow away to drain.

As the level drops so the valve closes and a perfect seal is maintained. This type of trap does not vent air very well through the liquid channel so it is usually equipped with a separate thermostatic air vent. Many types of float trap are also equipped with a steam lock release so that a slug of steam getting trapped in a rapid flow of condensate does not cause water-locking of the system. The big advantage with a float trap is that it discharges condensate at the same rate at which it is formed so it minimises any risk of flooding of the beds and loss of calender performance, providing it is correctly sized.

Inverted bucket traps

An inverted bucket trap operates in much the same way as a float trap but it relies on routine filling and emptying of the inverted bucket rather than on a ball-cock arrangement.

  It has the same advantage as a float trap in that it discharges condensate at the same rate as it is formed, so again it minimises water logging. It is often more resistant to water hammer than a float trap (which can suffer from a collapsed float) and so it is sometimes used in areas where water and pressure surges are more common. It used to be the favourite trap for a hard-working ironer although modern float traps do the job just as well.

Orifice traps

The range of traps described from thermodynamic through to inverted bucket are progressively more complicated mechanically.

The orifice trap is a carefully engineered hole with no moving parts and theoretically nothing to go wrong.

  A well sized orifice trap works on the principle that liquid condensate is much denser than steam vapour so if the orifice is sized to discharge the correct amount of liquid at the maximum continuous rating for the equipment there should be a steady flow of liquid condensate through it, no liquid back-up and no steam leakage.

  In practice laundry equipment does not work at its maximum continuous rating all the time and launderers also want rapid warm up so most orifice traps pass condensate for most of the time and leak a little bit of live steam for the rest, especially when the machine is idling.

Why do steam traps leak?

A thermodynamic trap usually passes a puff of steam at the end of its discharge cycle whilst the head is being repressurised and so does the thermostatic trap as the bellows or bi-metallic strip are expanding to close the drain hole. Both of these traps will leak a lot more as the tightness of the seal erodes and this leakage will get progressively worse because the leaking steam will quickly accelerate further damage.

The thermostatic trap is particularly prone to bits of debris lodging beneath the valve; the thermodynamic trap far less so.

The float trap usually leaks slightly because the air vent wears and eventually jams wide open. If a steam lock release is fitted, which is itself a continuous leak, this needs to be accurately adjusted and readjusted regularly otherwise the efficiency of this trap will fall below that of a TD. The worst leaks on a float trap follow collapsing of the float which usually damages the discharge mechanism permanently.

The inverted bucket trap leaks continuously through a small bleed hole in the top of the bucket designed to provide continuous air venting. This is non-adjustable so it has to be sized for the worst situations. Otherwise the bucket trap gives very good service until the main mechanism collapses and the bucket falls over at an angle and steam leaks through continuously.

The simple orifice trap is designed to leak continuously and this will only increase as the orifice itself becomes eroded. A trap from a reputable manufacturer will be made of the appropriate grade of hardened steel so this should not be a major problem. Partial blockage to debris in the pipe getting through the filter is probably more likely.

How much do trap leaks cost?

On the average laundry or drycleaning plant, if the steam traps have not been checked and repaired during the last twelve to eighteen months then steam consumption is probably 10-15% higher than it would be in a well maintained system. This translates into a fuel account 10-15% higher than it should be.

Theoretically it is possible to double the fuel account with half a dozen poorly maintained traps but in general even a badly maintained laundry will probably only use 30-40% more fuel from trap leakage than a well maintained one.

For a laundry spending say £70,000 per annum on fuel the excess cost is probably around £20,000 per year which many laundries seem quite prepared to accept.

How can traps be monitored?

TD traps probably pose the greatest single cause of steam loss in a laundry and these can be checked on a regular basis for major failure using a wooden screwdriver.

The engineer places the wooden handle in the ear and the metal tip on the trap and listens to the trap opening cleanly every 20 seconds or so, the whoosh of escaping condensate and the click as the trap closes and goes into relative silence for a further 20 seconds. A machine gunning trap, a trap jammed open and a trap which is starting to wear and opening every few seconds shows up very clearly.

It is possible to buy digital thermometers very cheaply now with two probes for measuring temperature difference. Upstream and downstream temperatures either side of the trap usually exceed 15-20oC whereas for a leaking trap they become virtually the same.

Some launderers stick temperature sensitive self-adhesive labels downstream of each trap and can watch the colour of these change when they start to leak.

It is possible to purchase ultrasonic leak detectors which listen to the whistle of the escaping steam through a trap and display this on a needle scale. This type of device can also be used to set the steam lock release in a float trap and monitor thermostat leakage or bucket hole leakage (in an inverted bucket trap).

One trap supplier provides automatic steam trap detection equipment but this needs a detection head to be fitted before each trap for routine interrogation (which can be carried out by an unskilled person using a plug-in indicator).

Which is the economical trap?

The most efficient launderers and drycleaners use the most appropriate steam trap for the duty and then maintain this to get the best economy.

TD traps and thermostatic traps are fine for line drainage, water knock-out pots, space heaters and non-critical applications. Hard working tumble dryers, ironers and tunnel finishers usually need a float trap or inverted bucket trap. In theory an orifice type trap can be used in any application but obviously they are going to give the best economy for plant which is in constant use at maximum output rather than for low throughput or intermittent use. Launderers are not good at turning off idle plant.

So the solution to steam trap losses lies not so much in selection of the trap type but in regular checking and maintaining of the traps fitted. If you have not had a good look at yours for a few months then the savings available should do a little to alleviate the financial pain this winter. Good hunting.