Now that we have reviewed the operating principles, let’s consider the application. Table 1 gives an overview of the various operating characteristics that each trap exhibits and can help to select the best trap design for each application.
Table 1. A wide range of 1st, 2nd and 3rd generation choices can be considered for each application. (Click to enlarge.)
Equipment capital costs, installation and piping requirements, steam trap size and capacity, and ease of maintenance are a few of the factors that influence steam trap selection. The relative cost of the trap designs are shown on the chart. It is typically recommended to include a strainer and check valve (if backpressure is present) with each trap whenever possible. Traps whose design includes these items as standard or as an integral option will reduce installation cost. Traps with replaceable seats can extend the life expectancy with little additional cost. Selecting a trap with more moving parts can increase maintenance cost. There are additional factors that can influence trap selection.
The sizing safety factor of some traps (F & T and bucket) is large because the trap has a fixed orifice whose size is determined based on a small pressure range. Cold start up loads can be 10 times the hot loads present at saturated steam temperature. The larger trap size needed for startup can make initial costs much higher. Piping a manual bypass around the trap can reduce the size required, but the initial installation of extra components is higher; operating the bypass adds to the stack of headaches you normally endure during a start-up.
Dynamic operating conditions
Systems that are frequently shut down can accumulate large amounts of water which must be drained through the trap when steam is turned on. The inverted bucket traps shouldn’t be used in systems that are frequently in and out of service. Inverted bucket traps must be primed (filled with water) before they are put into service. If this procedure isn’t followed, the float mechanism will fail and the trap will pass live steam (fail open). Bimetallic traps don’t respond quickly to changes in load or pressure because the bimetal is slow to react to variations in temperature. Float-and-thermostatic traps are best suited for applications that are frequently in and out of service and require immediate discharge of varying loads of condensate.
Where a steam trap must operate can affect its performance. As a rule of thumb, don’t use inverted bucket, F & T or free-float traps at extremely cold temperatures. Residual condensate inside the trap may freeze (and expand) and damage the steam trap body. Disc traps shouldn’t be used in systems that pass a lot of dirt, air or are frequently in and out of service. Avoid using disc traps in systems that are exposed to rain or snow, as these conditions cool the steam above the disc too quickly, and as a result, the trap parts prematurely wear. In inverted bucket traps, the critical components and the orifice are located at the top of the trap. For this reason, the inverted-bucket traps are best suited for applications with excessive chemical usage, pipeline scale, or where rust or dirt is present in the steam line.
Traps used in dirty environments need to have dirt and scale flushed out to prevent them lodging in the trap’s seat. Traps that offer integral blowdown valves reduce installation and maintenance costs and will extend trap life. Blowdown valves are typically attached to strainers, so for traps without a strainer option the blowdown valves and strainer must be installed in the piping separately. Many plants don’t have a steam trap repair and maintenance program. A majority of plants do not know how may traps they have or where they are located. Steam systems that don’t have trap maintenance, over a period of three to five years often find 15% to 30% of their traps have failed. While some trap designs need less attention than others, no trap can be expected to operate forever without any maintenance.
Bellows, capsule and bimetallic traps are natural air vents and can be installed in any position. The bellows trap's smooth modulated discharge allows for better performance in process, sterilization and sanitary applications where temperature consistency is critical. F&T traps offer the benefits of both mechanical and thermostatic technologies. The orifice is positioned low on the trap body, creating a liquid seal and improved steam retention. F&T and inverted bucket traps must be mounted in the horizontal position to operate properly.
Efficient steam management
The placement of steam traps in strategic locations throughout the distribution system can be just as important as selecting the right design. Improperly located or no steam trap installed in a system can produce water-logging or water-hammer that could cause significant damage to the components connected to the steam system. Without effective trapping of the condensate produced when steam gives up its latent heat, there is no system control. The following guide for locating steam traps should avoid serious operating problems:
- All changes in elevation — at the lowest point;
- All changes in direction;
- Before any shutoff valve in the system;
- After all installed equipment that uses steam, e.g., a control valve;
- All changes in line size (before reductions and after expansions);
- The end of all main headers; and
- Every 150 to 200 ft. of steam main.
Make the best choice
An understanding of how steam traps operate will help you make the right selection. Appreciating the conditions under which the trap must work can take you even farther in your quest for the perfect steam trap.
Steve Frank is a project engineer at Richards Industries, Inc., in Cincinnati, Ohio; E-mail him at firstname.lastname@example.org.