Selecting Steam Traps

•Frequent start-up/shutdown of steam

•Vacuum breakers in system

Some applications require that the supply steam valve be opened and closed regularly. When systems are shut down, the steam remaining in the piping to the trap condenses, leaving large amounts of condensate.

Sometimes large amounts of non-condensables are present as well. When the steam condenses, a vacuum is formed. Air can be drawn into the system through leaks in gaskets or equipment, or through vacuum breakers.

•Localized flooding

Supply drip traps and rotary dryer drain traps are often required to handle slugs of condensate. Slugs are accumulations that originate quite suddenly and rise to block, or nearly block, the steam line in a confined area. Delayed handling of slugs will cause complete flooding of the line and may lead to water hammer.

•Elevated return lines

•High back pressure

A trap works best when the maximum differential pressure is applied across it (the maximum allowable pressure is found at its inlet and a minimum pressure at its outlet). When condensate lines must be elevated, or if there is a high pressure discharge into the return line, the outlet pressure applied to the trap (back pressure) rises. While the reduced differential pressure will necessarily lower the capacity of any trap, some traps may operate incorrectly because of a significant increase in back pressure.

•Wide differential pressure range

•Low differential ONLY reduces capacity

•Low condensing rate

•Modulated pressure

Many times traps are subjected to condensate load far below the maximum they are designed to handle. These "light loads" are especially common on main drips and where steam pressure to the heat exchanger is raised and lowered to meet a specific temperature demand. Where light loads are present, the trap must be able to sense the small increase in the condensate level and drain it effectively, yet be able to handle the maximum design load.

•Carryover of contaminants

•Flaking and scaling of pipes and equipment

Scale on boiler tubes and steam piping often flakes off and is conveyed through the steam system, where it adds to the general collection of carryover and residue collectively called dirt. Dirt is especially disruptive where there are small orifices in the trap because they can be blocked shut. It is also possible for valves to be blocked open by accumulations of dirt. If the trap design is unable to handle dirt, a strainer should be placed upstream of the trap.

•Valve near top of trap

•Abrupt, intermittent discharge

•Air trapped in system as by vacuum breakers

•Modulating or intermittent steam pressure

Applications where the trap is located above the drip point, called syphon drainage, and some that use modulating steam pressure may experience a great deal of air trapped in the steam space. Often, there is little steam pressure to push it through the trap. In these applications, the steam trap must be very effective at venting air.

•Operating element detects air

•Separate discharge for air and condensate

•Condensate retention

•Acceleration of condensate

•No closed elements

•Robust construction

•Condensate retention

•Sub-freezing environment

•Condensate retention

•Non-condensable retention

•Syphon drainage

•Large pressure drop across equipment

When condensate must be lifted from heat exchangers to the trap (syphon drainage), the pressure is reduced between the drain point and the trap inlet. A portion of the condensate being drained will flash back into steam because of this pressure reduction. Most traps are unable to distinguish flash steam from live steam, so they close, impeding drainage. In order to handle flash steam a steam trap, such as the differential condensate controller, provides a secondary discharge that can be manually metered to create sufficient velocity to draw off the flash steam and non-condensables. Energy-conscious designers pipe the secondary discharge to flash tanks and then to other operations.

•Separate discharge orifice for flash/air

•Provide for cascaded operation

•Localized condensate retention

•Localized non-condensable retention

In some systems condensate or non-condensables can pool in areas where steam flow is restricted. For example, condensate will often collect along the lower tubes of large horizontal coils. Air tends to collect high on the same coils, opposite the inlet.

•Abrupt, intermittent discharge

•Continuous venting of air/flash

•Maximize investment

•Minimize maintenance

•Robust construction

•Design of valve/linkage