No steam system is complete without that crucial component: the steam trap (or simply “trap”). This is the most important link in the condensate loop because it connects steam usage with condensate return.
A steam trap quite literally purges condensate as well as air and other incondensable gases out of the system, allowing steam to reach its destination in as dry a state/condition as possible to perform its task efficiently and economically.
The quantity of condensate a steam trap has to deal with may vary considerably. It may have to discharge condensate at steam temperature (i.e. as soon as it forms in the steam space), or it may be required to discharge below steam temperature, giving up some of its sensible heat in the process.
The pressures at which steam traps can operate may be anywhere from vacuum to well over a hundred bar (1450 PSI). To suit these varied conditions there are many different types, each with their own advantages and disadvantages. Experience shows that steam traps work most efficiently when their characteristics are matched to those of the application. It is imperative that the correct trap is selected to carry out a given function under given conditions. At first sight it may not seem obvious what these conditions are. They may involve variations in operating pressure, heat load, or condensate pressure. Steam traps may be subjected to temperature extremes or even water hammer. They may need to be resistant to corrosion or dirt. Whatever the conditions, correct steam trap selection is important to system efficiency.
It will become clear that one type of steam trap cannot possibly be the correct choice for all applications.
There are three basic types of steam traps into which all variations fall; all three are classified by International Standard ISO 6704:1982.
Types of steam trap:
Thermostatic (operated by changes in fluid temperature) — The temperature of saturated steam is determined by its pressure. In the steam space, steam gives up its enthalpy of evaporation (heat), producing condensate at steam temperature. As a result of any further heat loss, the temperature of the condensate will fall. A thermostatic trap will pass condensate when this lower temperature is sensed. As steam reaches the trap, the temperature increases and the trap closes.
Mechanical (operated by changes in fluid density) — This range of steam traps operates by sensing the difference in density between steam and condensate. These steam traps include ‘ball float traps’ and ‘inverted bucket traps’. In the ‘ball float trap’, the ball rises in the presence of condensate, opening a valve which passes the denser condensate. With the ‘inverted bucket trap’, the inverted bucket floats when steam reaches the trap and rises to shut the valve. Both are essentially ‘mechanical’ in their method of operation.
Thermodynamic (operated by changes in fluid dynamics) — Thermodynamic steam traps rely partly on the formation of flash steam from condensate. This group includes thermodynamic, disc, impulse, and labyrinth steam traps.
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