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Steam Traps  
The steam trap is an essential part of any steam system. It is the important link between good steam and condensate management, retaining steam within the process for maximum utilisation of heat, but releasing condensate and incondensable gases at the appropriate time.

Although it is tempting to look at steam traps in isolation, it is their effect on the steam system as a whole that is often not appreciated. The following questions become important:

Does the plant come quickly up to temperature or is it slow to respond, and its performance less than it should be?

Is the system trouble free, or does inadequate steam trapping permit waterhammer,corrosion and leakage, and high maintenance costs?

Does the design of the system have a negative effect on the life and efficiency of the steam traps?

It is often true that if an inappropriate steam trap is selected for a particular application, no ill effects are noticed. Sometimes, steam traps are even shut-off completely without any apparent problems, for example on a steam main, where incomplete drainage of condensate from one drain point often means that the remainder is simply carried on to the next. This could well be a problem if the next drain point is blocked or has been shut-off too!

The observant engineer may recognise that wear and tear of control valves, leakage and reduced plant output, can all be remedied by paying proper attention to steam trapping. It is natural for any mechanism to suffer from wear, and steam traps are no exception. When steam traps fail open, a certain amount of steam can be passed into the condensate system, although it is often a smaller quantity than might be expected. Fortunately, rapid means of detecting and rectifying such failures are now available to the steam user.

No steam system is complete without that crucial component 'the steam trap' (or 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. To suit these varied conditions there are many different types, each having their own advantages and disadvantages. Experience shows that steam traps work most efficiently when their characteristics are matched to that 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 extremes of temperature or even waterhammer. 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 can not possibly be the correct choice for all applications.

How steam traps operate  
There are three basic types of steam trap 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.

Type of Steam Traps  




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