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.
Thermostatic steam traps
Thermostatic steam traps operate through a temperature difference between steam and condensate. While steam and condensate can exist at the same saturated temperature, the thermostatic trap takes advantage of the tendency of condensate to rapidly lose temperature when it collects in dead legs off of the main flow path of the steam line.
Thermostatic steam traps employ two different mechanisms for sensing condensate temperature: liquid-filled bellows and bimetallic plates. Bellows-type steam traps (Figure 6) use hollow metallic bellows filled with a liquid that has a slightly lower boiling point than pure water. When the system temperature reaches the flash point of this liquid, the bellows expands, closing the valve before steam reaches the trap. As the temperature drops, the bellows contracts, opening the valve and allowing condensate to be purged.
Bellows traps tend to be suitable for low- to medium-pressure distribution drains and tracing. They can only withstand a small amount of superheat in the steam. Failure can be in either the open or closed position.
Bimetallic steam traps (Figure 7) employ a bimetallic plate or stack of plates to sense condensate temperature and actuate the discharge valve. Bimetals consist of a cohesive plate made from two metals having a different coefficient of expansion. When exposed to temperature changes, the plate bends or deflects in a predictable way.
Bimetallic designs are suitable for distribution drainage and steam tracing at low, medium and high pressures. The inherent durability of bimetallic plates make them ideal for superheated steam applications as well as use on saturated systems where waterhammer may occur. Although sub-cooling requirements vary from one bimetallic design to another, bimetallic steam traps are not generally thought to be the best choice on forced heat process applications where condensate discharge at steam temperature is required. Because the valve resides on the downstream side of the orifice, bimetallic steam traps incorporate a built-in check valve and tend to fail in the open position.
By discharging condensate at temperatures below that of steam, thermostatic steam traps typically maintain higher operational efficiencies than steam traps of other designs.
Thermodynamic steam traps
Thermodynamic steam traps are cycling devices that operate on the difference in flow velocity between liquid and gas. The most popular style of this type is the thermodynamic disc trap (Figure 8) or TD trap. TD traps consist of a cast or forged-steel body and cover with a free floating disc resting on the top of both the inlet and outlet orifices (Figure 9).
Upon startup, the differential pressure across the trap pushes the disc off the seating surfaces and flow begins. When the condensate is evacuated from the system, steam flows through the trap, increasing the flow velocity (Figure 10), creating a low-pressure area below the disc, pulling it onto the seating surface. As the disc seats, the greater surface area of the disc stops the flow.
The steam trapped above the disc keeps the disc seated (Figure 11). As radiant heat loss from the cover and body of the TD trap causes the steam trapped above the disc to condense, the disc lifts off of the seat and flow commences, causing the cycle of discharge and shutoff to continue (Figure 12).
Despite some manufacturers’ assertions that flash steam is responsible for the closing of the TD trap, testing demonstrates that live steam loss is necessary for the cycling action of the TD trap to occur. This steam loss has been found to be, on average, 0.032 pounds of steam per cycle. At current energy rates, and a cycle rate of 6 times per minute, the steam loss necessary to operate a TD trap is more than $1,000 per year.
TD traps are widely used on distribution drains and steam tracing for low-, medium- and high-pressure systems. Their compact size and relatively low price point make them attractive to plants that have large populations of steam traps. Failure mode of the TD trap is in the open position.
As with all mechanical equipment, steam traps are subject to wear and failure at any point in their life cycle. However, steam traps present a much larger potential to produce energy losses than many other types of valves. Many steam traps fail in the open position, resulting in large and unrecoverable losses of steam. Consider that a steam trap with a quarter-inch orifice installed on a 100 psig system can lose up to $15,000 per year if not repaired. This can be a very expensive prospect in most plants where steam trap populations can sometimes number in the tens of thousands. Also, failure rates can typically reach 10% annually, underscoring the need for a comprehensive testing and repair program for traps. Many companies provide steam trap testing and diagnostic services. Facilities that do not currently conduct an annual steam trap audit or have a testing and repair program, should contact their steam trap vendors, who can point in the right direction for getting trap failures under control.
The commercial landscape of steam traps around the world is populated with many choices. Because of the varied requirements of different applications in industrial plants, regional preferences, or sales and marketing history, plant engineers today are faced with a wealth of options for meeting their condensate drainage challenges. The success or failure to choose the right steam trap depends on the ability of the design-maker, who must meet the requirements of specific applications and match those requirements to the capabilities of different steam trap technologies.