Bimetallic Steam Traps

Bimetallic 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.

The bimetallic steam traps are generally factory-adjusted so that at saturated steam conditions, the temperature created force of the bimetallic elements prevails, closing the valve and preventing loss of steam. As the temperature of the condensate cools, the line pressure becomes the dominant force, causing the valve to open and allowing the discharge of condensate. Backpressure in a closed return system provides an additional closing force resulting in a lower opening temperature than the same steam trap discharging to atmosphere. The discharge temperature, therefore, is affected by backpressure.

A design problem for bimetallic steam traps is created by the non-linearity of the saturation curve. Shaping and stacking techniques of the bimetallic elements have made it possible for these steam traps to have a discharge temperature that approximates the saturation curve. This has expanded the useful pressure range of bimetallic steam traps without adjustment.

The modern bimetallic steam traps have many technical and practical advantages. They were first used in Europe but they are widely used everywhere today.

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, Bellows Steam Traps, Liquid or Solid Expansion Steam Traps (Wax Capsule Steam Trap), and Orifice Steam Traps.