The thermostatic trap is one of the smallest and least expensive designs, It consists of a case with an outlet orifice. Attached to the valve is a heat sensitive bimetallic or bellows mechanism. Relatively cool condensate and gasses pass through the bellows area, When hotter steam comes in contact with the element, it expands and moves the valve into the orifice to stop flow. When the bellows mechanism cools, it contracts to allow condensate flow. The advantage of this trap is that it can be used in areas subject to freezing temperatures. Since the orifice is always open when cool, the case can drain continuously.
The last type of steam trap is thermodynamic. The smallest available, this trap is considered to be the most rugged when properly sized. Usually, the thermodynamic trap is machined from solid bar stock It is constructed with a disc covering an orifice. This disc covers the inlet as well as the much larger outlet orifice. Above the disc is an expansion chamber.
As air and cool condensate enter the trap, the resulting pressure raises the disc and fills the expansion chamber. The raised disc opens the valve and allows immediate discharge. Air and condensate mix with steam in the expansion chamber and with the aid of the Bernouilli effect force the disc to drop back on the seat closing the valve. As steam in the upper chamber condenses and the pressure drops, the disc raises and the cycle is repeated.

Testing trap operation
There are three general methods for checking steam trap operation: visual, sound and temperature measurement. The visual method is perhaps the easiest and best. Live steam at the discharge indicates a failed trap. When the discharge begins to back up into the system, the trap has failed.
The second method is based on sound analysis. When a trap functions properly, it cycles. And using a sound device, one can physically hear it. Sound measurement devices vary in sophistication from ultrasonic testing equipment to a simple, handmade steel welding rod or large screwdriver. During normal operation, a mechanical-type steam trap will make a continuous cycling noise. When it fails, a whistling sound of steam blowing through at a high velocity can be heard, Non-cycling of a mechanical-type trap indicates that it is not functioning properly.
There is also a temperature range associated with any given trap. For example, a trap with 100 psi at the inlet and 0 psi at the outlet will maintain a temperature of 338° F at the inlet, Water at 0 psi is only 212°F. The higher the pressure, the higher the temperature. A temperature measurement on the pipeline immediately upstream and downstream of the trap can indicate a problem.
If the steam trap is operating properly, both the upstream and downstream pipe surface temperatures will be within a specific range. At 50 psi, for instance, steam temperature should be 298°F to 283°F. A downstream temperature that exceeds a predetermined range can indicate a failed system by showing that the hotter live steam is blowing through the trap.

Proper maintenance
Steam trap maintenance consists of simple steps such as checking for proper operation, cleaning the trap and replacing worn or damaged parts. When a steam trap malfunctions, overall system efficiency decreases and escaping steam produces energy loss.
When evaluating a steam trap, one of the first considerations is its length in service, The rule of thumb is that if a steam trap is more than three years old, it could require some type of maintenance. This does not imply that a trap will automatically fail after three years, but that it requires regular inspection.
In all steam traps, except the thermodynamic type, the seat and valve area is most subject to wear, In general, this area will begin to degrade after three years ser- vice, The degree of degradation depends on factors like pressure and type of water used. Far example, a trap will wear more at 600 psi than at 100 psi.
Under normal operating conditions, the valve and seat area is subjected to hot high-pressure water flow. Water is corrosive and causes surface erosion. With no maintenance, eventually the valve will to close properly and the trap will fail to seal and will waste live steam.
The presence of condensate, air or CO, in a heat exchanger reduces efficiency; thus, these elements should be removed from the system as soon as they accumulate late. Condensate standing in steam mains and lines is conducive to corrosion and may also cause water hammer.
When a trap fails to discharge water, it backs up into the system and can cause downtime. A regular maintenance program can prevent this. But if a steam trap does fail, two alternatives must be evaluated: is it cheaper to prepare or replace a failed trap? Two factors impact the ultimate decision, First is initial cost of the steam trap; and second, the cost to replace a failed component including the cost of labor.
Virtually all steam trap parts are available for replacement, In fact, it would be possible to strip a steam trap down tn its housing and rebuild it with renewable parts. The question is whether il can be justified economically. For example, if a $2000 trap needs a minor component, such as a lever, it would definitely pay to repair the trap instead of replacing it.
If the bellows mechanism on a thermostatic trap needs to be replaced, on the other hand, the situation is different. Bellows are relatively expensive. On a $100 trap, for example, replacement bellows can cost $40. And, when the cost of labor is factored in, the rebuilt trap could be more expensive than a new one. Some manufacturers fabricate traps that cannot be repaired, They are completely sealed units which are not designed for disassembly. This is done with the assumption that the cost to repair the unit would be greater than the cost of a replacement.
A rule of thumb regarding renewable parts is that thermostatic and thermodynamic traps are the most costly to repair. Under normal conditions, bucket and float traps are the least costly when it comes to repair.
In the final analysis each trap must be considered as an integral part of an overall system. And depending on that system, as well as the comparative cost of repair or replacement, the plant engineer must decide which is more economically feasible.