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Steam trap

A steam trap is a device used to discharge condensate and non condensable gases while not permitting the escape of live steam. Nearly all steam traps are nothing more than automatic valves. They open, close or modulate automatically.

The three important functions of steam traps are: 1) To discharge condensate as soon as it is formed. 2) Not to allow steam to escape. 3) To be capable of discharging air and other incondensible gases.

Basic Operation

The earliest and simplest form of steam trap is the orifice trap. It consists of simply a disc or short solid pipe nipple with a small hole drilled through it installed at the lowest point of the equipment. Since steam condensate will collect at the lowest point and this hot liquid is about 1200 times smaller in volume and denser than live steam, condensate is effectively removed and steam is blocked.

The three important functions of steam traps are: 1) To discharge condensate as soon as it is formed. 2) Not to allow steam to escape. 3) To be capable of discharging air and other incondensible gases.

Issues

The problem with orifice traps is the fact that they do not compensate for varying loads and pressures. If the condensate load increases, liquid will back up in the equipment. If the load is light, then there is little condensate present and live steam will escape through the orifice. Orifice traps will not handle or remove non-condensable gases successfully. These basic inabilities have spawned a multitude of steam trap designs and configurations to meet a whole spectrum of applications. No single steam trap design is ideal for each and every application. This makes understanding each design's abilities and limitations important in selecting the right trap for the right job.

Types

Steam traps can be split into three major types:

  1. Mechanical traps. They have a float that rises and falls in relation to condensate level and this usually has a mechanical linkage attached that opens and closes the valve. Mechanical traps operate in direct relationship to condensate levels present in the body of the steam trap. Inverted bucket and float traps are examples of mechanical traps.
  2. Temperature traps. They have a valve that is driven on / off the seat by either expansion / contraction caused by temperature change. They differ from mechanical traps in that their design requires them to hold back some condensate waiting for it to cool sufficiently to allow the valve to open. In most circumstances this is not desirable as condensate needs to be removed as soon as it is formed. Thermostatic traps and bimetallic traps are examples of temperature operated traps.
  3. Thermodynamic (TD) traps. Thermodynamic traps work on the difference in dynamic response to velocity change in flow of compressible and incompressible fluids. As steam enters, static pressure above the disk forces the disk against the valve seat. The static pressure over a large area overcomes the high inlet pressure of the steam. As the steam starts to condense, the pressure against the disk lessens and the trap cycles. This essentially makes a TD trap a "time cycle" device: it will open even if there is only steam present, this can cause premature wear. If non condensable gas is trapped on top of the disc, it can cause the trap to be locked shut.

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