Back to Basics
- Published on Tuesday, 05 February 2013 15:54
- Written by Terry Acers
The Industrial Revolution of the late 1800s brought about widespread use of steam as a means of generating power, performing work and delivering heat to industrial process systems. While an effective and efficient means of providing heat and power, steam use brought several challenges, one of which is the effective removal of condensate to ensure thermal efficiency and prevent mechanical damage inside of piping, turbines and process equipment.
To meet this challenge, the steam trap was born. Today, as in its early days, the steam trap is the device primarily responsible for automatic discharge of condensate from steam systems. Effective use of these traps can ensure that maximum thermal efficiency is maintained at the same time that mechanical damage to equipment is avoided. Ineffective use of steam traps, however, can lead to accidents, reduced process capabilities and significant energy losses throughout a plant.
Steam traps are automatic valves that differentiate between steam and condensate. Their primary function is to discharge condensate from collection points in distribution piping and process equipment, and then close tightly on steam to prevent unnecessary energy loss. As the steam space in most systems is full of air at ambient temperatures, a secondary function is to vent air during startup.
STEAM TRAP APPLICATIONS
Steam trap applications can be understood best by dividing them into three broad categories: distribution drainage, forced heat process and steam tracing. By understanding the operating characteristics of each of these applications, proper steam trap selection and sizing can be performed.
Distribution drainage refers to intermediate drain points between a boiler and equipment that uses steam within a plant (Figure 1). These drain points are known by many names, including drip legs, steam mains, manifold drains and risers.
For saturated systems, steam will always be condensing as it flows through the piping, resulting in condensate collecting at low points in the system. These low points must be drained to prevent condensate from building up and eventually joining the steam flow, which would cause waterhammer, a very dangerous condition resulting from condensate traveling at high velocities within a steam system. Waterhammer occurs when the high-density water meets a restriction in the piping—such as a control valve, an elbow or tee—creating violent pressure shocks. Many of the accidents involving steam systems result from piping or valve failure due to waterhammer. Effective drainage of distribution lines will ensure that only dry steam reaches critical points.
Distribution drains require steam traps to vent large amounts of air at startup, discharging high initial condensate loads resulting because steam piping is at ambient temperatures followed by low, hot-running condensate loads. After startup, steam pressures tend to be stable until a system shuts down.
Forced Heat Process
Forced heat process applications encompass most of the heating and process applications in industry, including air coils, unit heaters, shell and tube heat exchangers (Figure 2), absorption chillers, platen presses and autoclaves. These applications are characterized by elevating the rate of heat transfer to above that of simple convection by forcing product across one side of a heat transfer surface with steam occupying the other side. Forced heat process equipment has large internal steam spaces that are full of air at startup and must be purged for heat transfer rates to reach their maximum. After the initial warm up, condensate loads can fluctuate rapidly as the forces driving the heat transfer rate will change due to product temperatures and flow rate. Steam supply is typically regulated by modulating control valves, resulting in changes to the differential steam pressure the trap is exposed to throughout the heating cycle.
Forced heat process applications require steam traps with large air-venting capacity, the ability to handle fluctuating condensate loads and the ability to handle fluctuating differential steam pressures.
Steam tracing is a method of providing freeze protection and maintaining temperature inside product distribution piping, instrumentation and storage vessels (Figure 3). Steam tracing consists of small-diameter tubing or piping, installed against the exterior surface of a product pipeline or vessel with insulation encapsulating both pipes. The tracing line is connected to a steam source at one end and a steam trap at the other. The heat from the steam tracer is transferred into the product pipeline, maintaining its temperature and/or preventing freezing. As steam temperature is determined by pressure, achieving the desired tracing temperature is simply a matter of controlling the steam supply pressure.
Most steam tracing applications require low-condensate capacity at relatively stable saturated steam pressures. However, steam tracing on high-temperature process piping can result in superheated steam being produced and reaching the trap, a factor that should be taken into account when selecting the proper steam trap.
TYPES OF STEAM TRAPS
Steam traps manufactured today fall into three major categories based on operating principles: mechanical, thermostatic and thermodynamic. These operating principles tend to determine the applications for which the steam trap is most appropriate.
Mechanical Steam Traps
Mechanical steam traps operate because of the difference in density between a liquid and gas. The most common types of mechanical steam traps in use today are float & thermostatic (Figure 4) and inverted bucket (Figure 5).
Float steam traps consist of a cast iron or steel body containing a sealed float connected to a valve mechanism. When condensate enters the trap body, the float becomes buoyant, opening the valve and releasing the condensate to the discharge side of the trap. When all condensate has been drained and the trap body fills with steam, the float keeps the outlet valve closed, preventing the loss of live steam. Since the float mechanism responds in the same way to air as it does to steam, by keeping the valve closed, a separate thermostatic element is often added to the steam trap to control the discharge of non-condensable gases. These thermostatic elements consist of either liquid-filled bellows or bimetallic plates. With the addition of such elements, the trap becomes a float & thermostatic (F&T).
F&T steam traps are most appropriate for low-pressure distribution drainage applications as well as forced heat process applications at low to medium pressure. By discharging condensate at steam temperatures, these devices ensure efficient drainage of process equipment. However, the thermostatic element tends to make them susceptible to damage by superheated steam as well as waterhammer. F&T traps can fail in both the open and closed position. Failure in the closed position can lead to significant damage to process equipment from freezing.
Inverted bucket steam traps consist of a cast iron or steel body containing a bucket, open on the bottom, connected to a lever valve located in the top of the trap (Figure 5). When condensate is present, the bucket sinks to the bottom, opening the lever valve and allowing free discharge of condensate. When steam or air enters the trap, the bucket becomes buoyant, floating upwards and closing the lever valve. Air is vented continuously through a small hole in the top of the bucket. However, when all air has been purged from the system, steam is continually lost through the air vent hole and periodically discharged through the lever valve as the bucket loses buoyancy.
Inverted bucket traps can be installed on both distribution drainage and forced heat process applications. These devices discharge condensate at steam temperature, making them well suited to draining process equipment. However, the limited air-venting capacity requires that a secondary air vent be installed in parallel on equipment that has large air-venting requirements or frequent start-up and shut-down cycles. Inverted bucket traps should not be installed on superheated steam applications because the excess heat will cause the internal water prime to be lost, resulting in the frequent and premature failure of these valves.