Eventually, the condensate in the trap cools as the result of radiation from the trap body, allowing the internal pressure in the element to subside. When the vapor pressure is reduced, the diaphragms relax and open the valve, allowing condensate to discharge again, and the cycle repeats.

The top and bottom casings, with the diaphragms sandwiched between them, are welded together to produce a strong, lightweight construction that is highly resistant to water hammer or superheat effects. The shape of the diaphragms should be contoured to match the shape of the bottom casing to give maximum protection from over-extension in the event the element is exposed to excessive temperatures. The use of more diaphragms and Hastelloy-equivalent materials instead of stainless steel maximizes diaphragm life.

The older-style bellows element is cylindrical and has a thin corrugated metal wall made of a highly conductive material for rapid heat transfer into and out of the bellows. The valve is attached to one end of the element, and the other end of the element is fixed to the trap housing. The operation of the bellows element is similar to that of the capsule design: An increase in internal vapor pressure will expand the bellows to close the valve, and a reduction in vapor pressure will contract the bellows and open the valve.

The balanced-pressure element automatically adjusts to changes in steam pressure because the associated change in temperature of the condensate generates the respective vapor pressure in the element necessary to close the trap. Unfortunately, the thin-walled design and proportionately large volume of the bellows-style element make it less effective against severe conditions than sealed-capsule designs. The bellows thermostatic traps are susceptible to damage from fatigue or water hammer, and exposure to superheated steam can lead to overexpansion of the element, causing permanent damage.

Bimetallic traps

incorporate one or more strips or discs consisting of two compatible ," yet dissimilar ," metals with different thermal-expansion rates bonded together. When the bimetal element is heated, the material with the higher thermal expansion coefficient expands faster than the other, causing the element to bend. When the element cools, it returns to its normal state.

By constraining the element within a trap body, the forces generated during heating and cooling can be used to open and close a valve. Bimetallic trap operation and subsequent life expectancy are largely based on whether the trap design uses downstream or upstream valve head construction.

Operation of the bimetallic trap with upstream valve head design is based on both the deflection of the bimetal element with changing temperature and the thermodynamic "suction" effects created by condensate discharge. The element is made up of a stack of opposed bimetal discs to provide maximum head travel. The trap's opening/closing temperature range can be changed by turning an external adjustment screw at the trap's top.

For this design, temperature changes at the element produce opening/closing forces. The effect of backpressure on this trap is minimal, so no significant opposing forces create stresses on the bimetals.

In downstream valve head designs, the valve is on the discharge side of the valve seat, connected to the bimetal element by a stem that passes through the orifice. When hot condensate enters the trap, the element bends away from the orifice, pulling the valve head toward the seat. At a particular temperature, the force exerted by the element is sufficient to close the valve.

Such bimetallic traps are designed to work at sub-cooled temperatures; therefore, an appreciable amount of condensate will be present ahead of the trap during operation, depending on operating conditions and the trap's set temperature. These traps provide excellent results on high-energy-use projects or in situations in which temperature control is needed.

Other selection considerations

Other aspects of trap selection also should be considered ," application, design and operating pressure/ temperature, required capacity with safety factor and piping orientation, to name a few. In addition, a good-quality check valve probably should be installed after any trap that has the potential for backpressure.

Snow is a senior engineer/training supervisor with TLV Corp., Charlotte, N.C. Contact him at (704) 597-9070.