Steam traps come in many makes and models yet their different features and performance levels are not always considered in the selection process. This is because some plant personnel fall into the trap of thinking "a trap is a trap." Also, sometimes plant maintenance personnel base their decisions on the pipe connection size and trap availability in plant stores. Moreover, contractors or engineering constructors working on new construction projects often focus on cost or delivery factors.

Contrary to what some believe, all steam traps do not work the same. When choosing steam traps for a chemical facility, users should keep in mind that they have a choice between mechanical, thermodynamic and thermostatic designs.

Mechanical Designs

In mechanical traps, the movement of closed or open floating objects activates the valve. These designs work well against backpressure, but they do not hold up well against harsh water hammer. There are three types of mechanical traps (or combination pump/traps):

Float and thermostatic traps contain a sealed spherical or elliptical float that becomes afloat when the water level in the trap goes up. A temperature-detecting thermostatic element opens to allow air and other incondensables, which flow into the trap during operation, to escape.

Inverted bucket traps are old-fashioned density traps that feature an inverted cup-like cylinder that becomes afloat in an amount of liquid when exposed to the vapor pressure of a contained vapor within the float.

For process or heating applications that require more than a standard mechanical trap design because of a negative pressure differential, the most ideal mechanical trap is the combination pump and float trap. Such traps are fully synchronized, supporting complete condensate drainage of the system or equipment application.

Thermodynamic Designs

The operation of these traps hinges on the principle that static and dynamic pressure changes, a result of shifts in fluid velocity through the trap, will open or close the valve. Higher fluid velocity translates to greater fluid dynamic pressure and lower static pressure. In general, vapor moves at a higher velocity than liquid, and these traps work because of this difference.

Thermodynamic disc traps usually feature only one moving part—the valve disc. The disc lies on the seat when the steam system is not in operation. Air and cold condensate entering the trap produce a low-velocity static pressure, which elevates the disc off the seat and permits the release of fluids. Most disc traps tend to handle air poorly, often requiring a rough finish on the lower side of the disc and the seat lands to produce an air "bleed." However, this rough finish compromises steam sealing.

Thermostatic Designs

These traps' operation is based on the principle that a decreasing temperature will open the valve and that an increasing temperature will close it. A thermostatic element controls valve movement. The temperature at this element must drop a given number of degrees below saturation temperature before condensate can be released. In some designs, the element's guiding temperature is set, while in others, it changes with system pressure.

Balanced pressure traps and bimetallic traps are two examples of thermostatic designs. The former contains filled thermal elements, with the filling usually an alcohol/water mixture with a boiling point lower than that of just water. The latter, in comparison, features one or more strips or discs made up of two different metals with dissimilar thermal-expansion rates. Bimetallic traps are only minimally affected by backpressure. They excel in projects requiring high energy use or temperature control.

Other Criteria

Aside from trap features and performance levels, users should also examine a host of other factors in their decision-making process. For starters, they should look at application, design and operating pressure/temperature, required capacity with safety factor and piping orientation. Moreover, they should consider supplementing any trap that can handle backpressure with a sound check valve.

Source: Don't Get Steamed
Tracy Q. Snow
Chemical Processing, April 11, 2003
http://www.chemicalprocessing.com/Web_First/CP.nsf/ArticleID/CBOH-5LHK2M/