Function and Operation of Steam Traps
Steam Trap classification
Steam traps are classified as mechanical traps, thermostatic traps and thermodynamic traps based on the basic operating principle of the valves. In addition, each type of steam traps can be further divided into subtypes in table 2.2, depending on how each basic operating principle is embodied.
Table 2.2 Steam traps classification
|Classification||Operating principle||Main types|
|Mechanical steam traps||Difference in specific gravity between steam and condensate||・Inverted Bucket
|Thermostatic steam traps||Difference in temperature between steam and condensate||・Temperature Control
|Thermodynamic steam traps||Flow velocity difference between steam and condensate
Thermal dynamics and hydrodynamic characteristic difference
Mechanical steam traps
Mechanical steam traps use the difference in specific gravity between steam and condensate, to operate a valve with the buoyancy of a float.
Mechanical traps consist of two types, one operates with sealed spherical float, and another operates with bucket that is an open float. The former is called the float type, and the latter is called the bucket type. There are two types of bucket type, upward bucket type and downward bucket type. But nowadays most of them are the downward bucket type.
As will be described in the next section, ball float steam traps have the mechanism in which a float rises and open the valve when condensate fills the body. Bucket type traps have a mechanism that opens the valve when the bucket sinks due to the inflow of condensate.
Ball float steam traps have two types. One is that transmits buoyancy of the float to the valve via a lever (lever float type), and the other has no lever and the float itself acts as a valve at the same time (lever-free type).
Thermostatic steam traps
Thermostatic steam traps use the difference in temperature between steam and condensate, and operate the valve by expansion and contraction of a thermosensitive body.
The thermosensitive body consists of metallic capsule containing special liquid mixing with water and alcohol, bimetal, or wax. Condensate is at saturated temperature when it occurs, and following that the temperature decreases by heat loss. The set temperature and decreasing temperature against saturated temperature when opening the valve differs depending on the design of the steam trap. However, because of the limited response capability of thermal sensors, it is difficult to operate them at temperatures very close to saturated temperature.
Temperature control steam traps can set and change the operating temperature freely in advance, which marks them apart from other thermostatic steam traps.
Thermodynamic steam traps
Thermodynamic steam traps use the characteristics of hydrodynamics and thermodynamics based on the difference in flow rate between steam and condensate to operate the valve. The most typical trap type is the disc type, in which a space called a transformer chamber is provided on the upper side of a disk-shaped metal plate (disk), and fluid pressure is received on the opposite side of the disk. When discharging condensate, it fills the pressure chamber with a part of condensate and flash steam, and when the incoming pressure become higher than the pressure received when steam starts to pass the lower side of disc, the valve is closed. When the steam in the pressure chamber is condensed and the pressure become lower than the received pressure at the underside of disc, the valve is opened again.
The disc type of steam traps has simple structure, with the only moving part being the the disc. It is compact, has no limit on install direction, and is inexpensive. For this reason, it is the most widely used type among steam traps.
However, disadvantages are that the timing of valve opening and closing may be highly affected by weather and temperature and it makes a loud operating noise.
Moreover, the fact that the steam trap discharges a small quantity of steam just before the valve is closed can be considered a disadvantage in terms of energy saving.
The other type of thermodynamic steam traps is the impulse system which creates flow inside the steam traps by leaking a minute amount of steam, thus opening the valve by the impulse force of condensate density. It may be convenient to view the orifice type as a steam trap, but it consists of only an osculum and a path called labyrinth. Thus, it is merely a device based on steam traps without the function of “steam trapping”.