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.
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