Bellows Steam Traps are thermostatic steam traps actuated by temperature sensitive devices, responding to changes in condensate temperature.
Thermostatic steam traps respond to changes in temperature and therefore discriminate very well between steam and cooler non-condensable gases. They can rapidly purge air from a system, especially on a cold start-up, and can be installed in various positions. Most frequently, actuation is by means of a bimetallic element or a bellows-like capsule filled with a vaporizing liquid.
Bimetallic actuated devices are characterized by their high resistance to damage from freeze-ups, water hammer and superheat. They are relatively small in size and lend themselves to high pressure designs. The condensate discharge temperature, however, does not follow the saturation curve very well, and the bimetallic elements are subject to corrosion with some reduction in closing force over time.
Bellows actuated steam traps, on the other hand, discharge condensate at a temperature which follows the saturation curve. The weak point is the bellows itself which can be damaged by superheat, water hammer or freeze-ups.
Thermostatic traps respond slowly to changing conditions even though the cause is usually misunderstood. It is not the heat sensitive element that is slow to respond. Rather it is the heat energy in the condensate inside the trap, which is slow to dissipate, that causes the time delay. Insulating thermostatic traps reduces their responsiveness even more. Mounting the trap at the end of a cooling leg in an area where air can circulate improves responsiveness and is the basis for installation instructions recommending a cooling leg at least three feet in length.
Bimetallic steam traps utilize the sensible heat in the condensate in conjunction with line pressure to open and close a valve mechanism.
The valve and seat system is usually arranged to produce a "flow under the seat" condition. Supply pressure, simply put, tends to open the valve. The bimetallic elements are in the form of small discs and are arranged to produce a closing force with increasing temperature. This closing force is in opposition to the opening force created by the supply pressure. Some bimetallic steam traps use a single leaf element rather than the stacked disc elements shown in below figure.
This simple operating principle provides many desirable operating characteristics. For example, the number of degrees below steam temperature at which the trap will open can be varied so the trap provides either a "hot" or "cold" discharge. Also the normal failure mode (open or closed) can be changed.
The characteristics of the actuating system can be affected by the liquid fill and natural free length of the actuator. The principles can best be explained by considering a bellows, even though they apply equally well to single diaphragm capsules.
Modern bellows/diaphragm steam traps have been improved in design, construction and materials to minimize their inherent disadvantages. Today they play an important role in steam trap application.
Concepts Defined
Natural free length — length of the bellows assembly before it is sealed.
Assembled free length — length of the bellows assembly after it is sealed, in its cold (contracted) condition. In the most common arrangement, the bellows is located upstream of the valve and thus senses upstream conditions. Flow direction is over the seat tending to close the valve. During cold start-up, the bellows is contracted, allowing condensate and air to be discharged. As the temperature of the flowing medium rises, the bellows also gets hot, the liquid inside it vaporizes and expands (strokes) the bellows to close the valve. Failure of this type of trap generally refers to the rupture of the bellows. After such a rupture, the bellows will return to its natural free length which can be designed so that the trap will be in either an open or closed condition.
Fail open design — This definition implies that the natural free length must contract the bellows away from the seat. To make this arrangement functional, the bellows must be filled with a liquid having a boiling point lower that that of water, because for the bellows to expand, the internal pressure must be higher than the external steam pressure. Low boiling point liquids, such as alcohols or ether, are frequently used in bellows but have the disadvantage that their saturation curve does not exactly correspond to that of steam. As a result steam traps having such a bellows will discharge condensate having different, levels of subcooling over a wide pressure range.
Fail closed design — This definition implies that the bellows remain expanded upon rupture. This can be accomplished by evacuating the bellows initially to obtain a contracted assembled free length. During normal operation when the bellows is hot, the pressure inside the bellows will approach the steam supply pressure, causing it to expand. Evacuated bellows are usually filled with water. The inherent advantage is that the condensate discharge temperature of traps having such a bellows will closely follow the steam saturation curve.
Prior to selecting a steam trap for your application, review steam traps selection with its advantages versus disadvantages and additional steam trap types: Disc Steam Traps, Piston Steam Traps, Lever Steam Traps, Closed Float Steam Traps, Inverted Bucket Steam Traps, Open Bucket Steam Traps, Bimetallic Steam Traps, Liquid or Solid Expansion Steam Traps (Wax Capsule Steam Trap), and Orifice Steam Traps.
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