Steam traps are automatic valves that release condensate from a steam space while preventing the loss of live steam. They also remove air and noncondensables from the steam space. Steam traps can be roughly separated into three groupings, thermostatic, mechanical, and kinematic; each of which contains a number of specific styles.

Thermostatic Steam Traps

Thermostatic steam traps open and close with the expansion and contraction of a temperature-sensitive element. These traps operate on the difference in temperature between steam, the cooler condensate, and air. This classification includes balanced traps, bimetallic traps, and pinch traps.  Balanced pressure steam traps open and close via the expansion and contraction of a temperature-sensitive element that responds to the lower temperatures created by the condensate and noncondensable gases in the trap.  The operating unit within the trap, a pressure-balanced disc or bellows, is filled with a liquid having a saturation temperature slightly below that of water.  Bimetallic traps have a metal element with the proper coefficient of expansion for the application. Pinch traps use self-regulating action. The pinching modulator begins to automatically close off flow as the wax around the modulator expands with the passage of hot condensate.  As the condensate builds up and cools the wax around the modulator contracts allowing the orifice to open and create flow.

Mechanical Steam Traps

Mechanical steam traps are buoyancy operated based on the difference in density between steam and the condensate.  These include float and thermostatic (F&T) traps, bucket traps, and variable orifice traps. F&T traps are used for draining steam equipment and as drip traps.  As the name implies this trap is a combination of two separate devices, a float trap and a thermostatic vent.  It consists of a chamber with a discharge valve at the bottom.  The valve is actuated by a float and lever.  The float is usually spherical in shape to resist the pressure of the steam.  The thermostatic vent remains open until the trap heats up, then it allows a large amount of air to pass.  A variation of this trap has been called a variable orifice trap.  Bucket traps are built around floating buckets that have their open side facing downward.  Steam entering these traps is fed into the bucket causing it to float to the top of the surrounding pool of condensate and close the discharge valve.  When only condensate enters the trap, the steam in the bucket condenses and the bucket sinks, opening the discharge valve. Variable orifice traps are a combination of two separate devices, a float trap and a thermostatic vent.  They consist of a chamber with a discharge orifice at the bottom.  The float responds to the amount of condensate and rises and lowers.  When it rises it opens the entry to the orifice and allows condensate drainage.

Kinematic Steam Traps

Kinematic steam traps operate based on the different flow characteristics of steam and the condensate.  This classification includes disc traps, piston or impulse traps, and orifice traps. Disc steam traps are used as drip traps and for low steam loads, such as steam tracing lines.  Disc traps consist of a flat disc resting on a smaller circular seat.  The disc is enclosed in a chamber above the seat where it moves around freely.  Condensate or steam enters the trap through the center of the seat, flows over the seat, and discharges through ports located under the perimeter of the disc.  When condensate is present, it lifts the disc and exits.  When steam enters the trap, the trap snaps shut. Piston or impulse traps open and close the valve port based on pressure. Orifice traps are simply holes between the steam main and the condensate system. Orifice traps are sized such that they release as much condensate as enters from the load.  There are no mechanical parts to fail.  These steam traps can only be used in applications with a constant load. If the load decreases, the liquid seal is lost and steam passes through the orifice. If the load increases, the orifice will be undersized and the condensate will back up into the equipment.