
A steam trap, a device with industrial applications, is an automatic valve designed to remove air and condensate from steam lines while trapping steam inside. Steam traps may operate using thermostatic traps, which sense the different temperatures of fluids entering the steam lines. Density traps sense the different densities of fluids entering the steam lines and thermodynamic traps sense the different velocities of fluids entering the lines. Thermostatic, density and thermodynamic are the basic operating principles of steam traps.
Expansion Traps
An expansion trap is a thermostatic trap that includes an internal filling which expands and contracts when the temperature is changed. This expanding and contracting actuates the valve. The filling does not vaporize during the process. Expansion traps may include wax elements or petroleum-based elements. Wax elements expand when they are heated, but remain congealed when they are cool. Petroleum-based elements also expand when they are heated, but contract when they are cool.
Balanced Pressure Traps
A balanced pressure trap is a thermostatic trap whose elements have a filling that is a mix of mineral spirits and water. The mixture either becomes vaporized or condensed when the temperature is near-to-steam, and that sets the valve in motion. A balanced pressure trap may use drum or barrel elements; spiral bellows elements; edge-welded elements; encapsulated bellows elements; or encapsulated diaphragm elements. Drum or barrel elements have a cylindrical shape and soldered ends. These elements are usually constructed of copper. The spiral bellows elements feature convoluted tubes and are usually constructed of either copper or the nickel alloy known as Monel. Edge-welded elements have stainless steel plates linked by alternating outer and inner welds. Encapsulated bellows elements are kept inside a shell of stainless steel; the elements themselves are made of stainless steel plates. Encapsulated diaphragm elements are also enclosed in a stainless steel shell. The elements may be constructed using diaphragms of either stainless steel or a Hastelloy (another nickel alloy) equivalent.
Bi-Metal Traps
Bi-metal traps, the third thermostatic type, include elements that are composed of two strips of different kinds of metal bonded together. Any change in the temperature will cause deflection in one direction or the other, which will then actuate the valve. The element designs for bi-metal traps include circular plate elements, irregular plate elements, cantilever strip elements and single strip elements. Circular plate elements are made up of circular discs that are stacked surrounding the stem of the valve. The metal of each disc is placed so that it faces the same metal on the next disc. When the temperature changes, deflection occurs on the discs to open and close the valve. Irregular plate elements include plates of identical irregular shapes that are stacked around the stem of the valve, also with like metals facing each other. When the temperature changes, the opposed plates deflect and the valve is actuated. Cantilever strip elements contain rectangular shapes that are identical. Bi-metal strips are stacked on top of one another and attached to a mounting point at the end of the trap; in this arrangement, unlike metals face each other. The strips surround the stem of the valve on the opposite end of the mounting point. Temperature changes cause stack deflection, which opens and closes the valve. Single strip elements include one bi-metal strip that is bent in a "C" shape. Each end of the "C" surrounds the stem of the valve. When the temperature changes, this causes deflection in the strip, which actuates the valve.
Float Traps
Float traps are density traps that include lever floats and free floats. Lever floats include an oblong or spherical float that is attached to a lever and opens the valve. Free floats include a spherically shaped float that uses its own buoyancy to open the valve. Since density traps don't deal well with large volumes of air, a bi-metal or balanced pressure thermostatic element may be added to both float types.
Bucket Traps
Bucket traps are density traps and come in two forms. Inverted buckets have a cylinder-shaped part that opens downward; air or steam in the bucket helps operate the valve. Open buckets' chamber faces upward and uses the float's buoyancy to keep the valve shut. The valve is opened when the bucket is filled with condensate and the buoyancy is removed. As with float traps, a dual system may be used. An inverted bucket and thermostatic trap contains a separate bi-metal thermostatic element, while an open bucket and thermostatic trap has a separate balanced pressure element.
Thermodynamic Traps
Thermodynamic traps react so that condensate, which moves relatively slowly, is released while faster-moving flash steam is contained. The disc version closes completely. The valve opens when heat is lost and the control chamber steam condenses above the disc.
Thermodynamic piston traps, though much like disc traps, include a piston with a pilot hole that constantly leaks a combination of air and steam or condensate, which opens the valve.
Thermodynamic disc and thermostatic traps are usually needed for large amounts of air; they operate the same way as thermodynamic discs, only they contain a separate bi-metal thermostatic element.
Combination Traps
Float-actuated piston traps include a float as a pilot valve. The float releases steam pressure that loads a piston and opens the valve. Thermostatic-actuated piston traps' pilot valve features a thermostatic balanced pressure element to release steam and load the piston.
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