Mechanical traps operate well against backpressure. The principal limitation of this class of traps is its ability to survive severe water hammer. They should be installed with the buoyant object allowed to rise or fall vertically with the water level inside the trap to open or close the valve. They generally are available in a wide variety of sizes and capacities. Most models are offered with a choice of orifice sizes to operate over a specified pressure range.

Combination pump and float traps.

Many process or heating applications experience a damaging phenomenon known as "stall." A standard mechanical trap design cannot operate and overcome a negative pressure differential. For these applications, a fully synchronized pump and float trap combination, commonly referred to as a "pump/trap," will maintain complete condensate drainage of the system or equipment application.

When the steam pressure in the equipment is less than the return-line pressure, condensate collects in the pump/trap chamber and is forced out through the trap mechanism when the chamber is pressurized. When the pressure in the equipment is greater than the pressure in the return line, the trap section functions as an ordinary trap, and the pump chamber remains almost empty. In this arrangement, the trap mechanism must be a double-seated float type. A high instantaneous flow rate is needed to allow continuous condensate discharge through the trap without constraining the total pump discharge. Ideally, the trap portion should be contained in the pump body to simplify troubleshooting and sizing.

Thermodynamic designs

These traps operate on the principle that changes in static and dynamic pressures caused by changes in fluid velocity through the trap will open or close the valve. The greater the velocity of the fluid, the greater its dynamic pressure and the lower its static pressure. Generally speaking, vapor travels at a higher velocity than liquid, and these traps operate by this difference.

Thermodynamic disc traps

typically incorporate only one moving part ," the valve disc. When the steam system is down, the disc rests on the seat. Air and cold condensate flowing into the trap create a low-velocity static pressure, which lifts the disc off the seat and allows fluids to be discharged.

As the condensate approaches steam temperature, some of the condensate flashes to steam. The higher-velocity flash steam has a lower static pressure; therefore, it lowers the pressure under the disc. Flash steam then enters the control chamber, increasing the pressure on top of the disc. The reduced pressure under the disc and the increased pressure in the control chamber combine to snap the disc shut.

Radiation loss from the top of the trap above the control chamber gradually condenses the steam and reduces the chamber pressure. This allows the inlet pressure to force the disc open, repeating the cycle.

Most disc traps typically do not handle air well, particularly during startup. Air will have the same effect as steam ," to lower the pressure under the disc and increase the pressure above the disc, snapping it shut. Because air is incondensable, the pressure remains above the disc, air-binding the trap. A rough finish usually is applied to the lower side of the disc and the seat lands to create an air "bleed." Unfortunately, this rough finish also detracts from maximum steam sealing..

A combination thermodynamic disc and thermostatic trap design incorporates a bimetal ring thermostatic element that contracts in the presence of air or cold condensate to lift the disc off the seat. This design eliminates the need for a rough finish on the valve disc/seat surfaces, and a mirror-finish seating surface promotes a better seal for greater energy efficiency and life. The disc trap's service life can be further extended with the use of an external air jacket, which slows down the rate of radiation from the control chamber, allowing flash pressure to keep the disc closed longer. Longer closure means a lower cycle rate, reducing disc and seat wear and prolonging trap life.

Thermostatic designs

These traps operate on the principle that a falling temperature will open the valve, and a rising temperature will close it. Valve movement is governed by a thermostatic element. The temperature at this element must fall a certain number of degrees below saturation temperature before condensate can be discharged. In certain designs, the element's controlling temperature is constant; in others, it varies with system pressure.

Balanced pressure traps

are equipped with filled thermal elements. The filling is typically an alcohol/water mixture with a boiling point lower than that of pure water. Two common varieties of balanced pressure traps are available ," a state-of-the-art, fourth-generation sealed capsule and an older-style, second- or third-generation bellows trap.

Figure 3. Thermodynamic Disc and Thermostatic Trap

The sealed-capsule design has a housing constructed entirely of stainless steel, and the valve is fixed to one of several stainless steel or Hastelloy-equivalent diaphragms that overlay each other to create a durable ," yet flexible ," diaphragm.

At startup, the alcohol/water mixture in the element is in the liquid state, and the capsule is contracted to allow air and cold condensate to pass through the trap. As the condensate temperature increases, heat is transmitted rapidly through the capsule housing to the mixture. The condensate begins to vaporize as the temperature in the trap approaches saturation temperature.

The considerable increase in volume generated by the vaporization of liquid raises the vapor pressure inside the capsule. At some point, the internal pressure exerted on the diaphragms becomes greater than the system pressure around it, causing the diaphragms to extend in the direction of the valve and moving the valve closer to the seat. Before live steam reaches the trap, enough vaporization pressure builds up inside the capsule element to close the valve completely.