Picking the Right Steam Trap for Your Application

There is no such thing as a bad steam trap but frequently steam traps are miss applied. It is important to understand the application you are trying to trap to properly going about doing so. Before I get into my example there are two characteristics of a steam system that are very important. The first is that steam traps do not rid the system of condensate. They simply trap the steam in a piece of equipment and provide an exit point for condensate, air, and other non-condensables. The motive force that physically forces the condensate to move is the upstream pressure. The upstream pressure must be greater than the downstream pressure after the steam trap. If it is not, condensate will not flow and will likely back up into the process. The second characteristic is that steam traps are to be sized or selected based off of the process' condensing rate or how much steam it will consume (typically in lbs/hr). Too many times a steam trap is selected based off a pipe connection size. Selecting a trap for piping convenience leaves the system vulnerable to water hammer, thermal shock, and poor process performance.

A common application is one of a steam coil. A steam coil is merely a fluid to air heat exchanger that uses steam to heat a flow of air to a desired temperature. The steam flow is controlled by a modulating valve or a "control valve". A temperature sensor downstream of the steam coil monitors the air temperature and adjusts the steam valve to reach or maintain the desired temperature. When steam flow is modulating, so is the pressure. On a very cold design day, it is likely that valve is fully open supplying adequate upstream pressure and condensate removal is not a problem. On a non-design Spring or Fall day, issues may surface where there is only a need for a small amount of steam in the coil to achieve the desired temperature. If the control valve only provides a small amount of steam, the only place for the condensate to go is the steam trap.

In this situation, the proper type of steam trap would be a float & thermostatic type. It will continuously drain condensate from the coil and provide a large body to hold the condensate in those moments we don't have enough upstream pressure to remove the condensate. Next I would use a 2:1 safety factor at 1/2 psig meaning the steam trap would have an orifice size that can handle twice the actual condensing rate of the coil with only 1/2 psig of pressure.

The 1/2 psig isn't an arbitrary number that was picked out of the air. Standard piping practices recommend a drip or dirt leg on the outlet of a coil of 12"-18". At minimum, a 12"drip leg provides 1/2 psig of upstream pressure. Majority of condensate systems are gravity systems or at "0" psig meaning we have achieved a 1/2 psig differential and condensate is draining.

Performing the appropriate due diligence before hand will provide the appropriate steam trap aiding in efficient performance and long service life of the coil. Remember there's no bad steam trap just misapplied traps. For more information or if there's anything I can do for you, please let me know.