By not letting steam escape, heated systems can more efficiently maintain correct process temperatures. Steam traps are automatic valves that sense the difference between steam, air, condensate, and noncondensable gases such as CO2. Steam traps vent air, condensate, and noncondensable gases from systems while “trapping” steam.
Inverted Bucket
Inverted bucket steam traps operate on the difference in density between steam and water.
(Copyright Armstrong International, Three Rivers, MI)
General Information
Inverted bucket steam traps are mechanical traps that contain an upside-down bucket or open float. The bucket is submerged when condensate is present. When steam enters the trap, the bucket floats, closing the vent and trapping the steam.
(Copyright Armstrong International, Three Rivers, MI)
Equipment Design
Steam (pink) entering under the inverted, submerged bucket causes it to float, closing the discharge valve. Condensate (blue) entering the trap causes the bucket to sink. When the bucket sinks, the valve opens to discharge the condensate. Air and noncondensable gases (yellow) continuously pass through a vent in the top of the bucket and accumulate at the top of the trap. The accumulated air and gases are discharged with the condensate.
Usage Examples
Inverted bucket steam traps are most typically used on intermittent steam applications with constant steam pressure and condensate return pressures, and are rated for pressures from 0 to 2500 psig. They are used as part of boiler systems, heaters, shell and tube heat exchangers, and jacketed vessels. They are usually the best choice when it is required to vent large amounts of carbon dioxide and air or when the dirt is being handled.
Inverted bucket traps are selected based on size and pressure rating, which can range from 0 to 2500 psig. The steam trap shown below to the right is rated for 600 psi and 4,400 lb/hour. The steam trap to the left is rated for 250 psi and 20,000 lb/hour.
(Copyright Armstrong International, Three Rivers, MI)
Advantages
- The longest service life of any steam trap.
- Excellent energy conservation.
- Excellent resistance to wear.
- Handles very light loads.
- Handles dirt.
Disadvantages
- When handling high pressures and capacities traps become large, expensive, and difficult for personnel to handle.
Float & Thermostatic
Float and thermostatic steam traps are mechanical traps that operate on both density and temperature principles.
(Copyright Flowserve GESTRA U.S., Louisville, KY)
General Information
Float and thermostatic traps continuously drain condensate. They consist of a ball float that opens and closes a primary valve that condensate drains from. A thermostatic element at the top of the trap operates a vent that discharges air and noncondensable gases.
(Copyright Flowserve Corporation, Irving, TX)
Equipment Design
When sufficient condensate (blue) enters the trap it lifts the ball float. The lifted float opens the primary valve, draining condensate. If all the condensate in the unit drains, the ball float will drop and the valve will close. A separate thermostatic element at the top of the trap opens a vent to discharge air and noncondensable gases (yellow) as soon as they cause a small temperature drop within the trap. When steam (pink) enters the trap, the thermostatic element senses the higher temperature and closes the vent.
Usage Examples
Float and thermostatic traps are most typically used for continuous operations with varying inlet pressures. Applications include shell and tube heat exchangers, unit heaters, air handlers, and chillers. These traps can remove gases such as carbon dioxide and air via the thermostatic vent but only as the trap cools below the steam saturation temperature. A safety factor of 2:1 is usually recommended for most float and thermostatic steam trap applications. Safety factors are based on application, the type of equipment being used, and the differential pressure. Steam trap size can then be determined using the safety factor, the condensate load, and the differential pressure.
The float and thermostatic steam trap pictured has a float trap with an additional pump mechanism to prevent water hammers. When the velocity of a liquid flowing in a pipe suddenly goes to zero due to a fast-closing valve, it creates a pressure wave within the pipe. The “pounding” of the pipe by this pressure wave is commonly known as a water hammer.
(Copyright Flowserve GESTRA U.S., Louisville, KY)
Advantages
- Handle very light loads
- Excellent service during startup
- Operate efficiently against back pressure
Disadvantages
- Handle dirt poorly
- Sensitivity of the float ball to damage by water hammer
Thermodynamic
Thermodynamic traps, also called disc traps, operate as a function of velocity.
(Copyright Armstrong International, Three Rivers, MI)
General Information
Thermodynamic traps contain only one moving part, a flat disc. The disc moves between a cap and seat, regulating the flow of condensate out of the trap. The movement of the disc depends upon pressure changes within the trap. Disc traps may also have a screen that filters out particles from the system. The filtration prevents particles from getting caught in the pressure chamber and jamming the disc.
(Copyright Armstrong International, Three Rivers, MI)
Equipment Design
On startup, cold condensate and airflow raise the disc and open the discharge port. When steam (pink) arrives there is a decrease in the pressure below the disc, lowering the disc and closing the discharge port. The discharge port remains closed as long as pressure is maintained above the disc. When hot condensate (blue) enters the steam trap, heat radiates out through the cap, diminishing the pressure over the disk and opening the trap to discharge the condensate.
Usage Examples
Thermodynamic disc traps are most commonly used for applications similar to that of inverted bucket traps but with lighter condensate loads. These traps are usually chosen for heat trace lines and smaller steam distribution lines. Their poor ability to remove gases can limit their use in many applications. The typical disc trap safety factor used for trace line applications is 2:1. The steam trap on the left has a maximum working pressure of 1,010 psi and capacities up to 1,800 lb/hr. The steam trap on the right has a maximum working pressure of 450 psi and capacities up to 800 lb/hr.
(Copyright Armstrong International, Three Rivers, MI)
Advantages
- Excellent corrosion resistance.
- Relatively small and compact for the amount of condensate they are capable of discharging.
- Can handle a wide range of pressures.
Disadvantages
- Poor energy conservation.
- Poor resistance to wear.
- Cannot handle very light loads.
- Cannot handle dirt.
- If pressure is not maintained above the disc, the trap cycles frequently waste steam and fail prematurely.
- Difficulty in discharging air and other noncondensable gases.
Thermostatic
Thermostatic steam traps operate on the difference in temperature between steam, condensate, and air.
(Copyright Flowserve Corporation, Irving, TX)
General Information
In thermostatic steam traps, valves open and close with the expansion and contraction of a temperature-sensitive element such as a bimetallic strip or a sealed bellows. The general operation of the temperature-sensitive element of thermostatic traps is shown below. On the left, the temperature-sensitive element is at a lower pressure than the condensate, making it contract and open the valve. When the temperature rises and reaches the saturation temperature, shown on the right, the temperature-sensitive element is fully expanded and closes the valve.
(Copyright Flowserve Corporation, Irving, TX)
Equipment Design
On startup, and in the presence of condensate, the thermostatic bimetallic discs are flat, with the valve wide open which allows condensate, air and noncondensable gases (shown in blue in the diagram) to be discharged. As the temperature rises the discs expand and deflect, which starts to close the valve. Once the saturation temperature is reached, the discs are completely expanded and the valve closes, trapping the steam (shown in red in the diagram). When the temperature drops again, the bimetallic discs contract and open the valve to allow condensate to flow through.
(Copyright Flowserve Corporation, Irving, TX)
Shown below is a schematic of a thermostatic steam trap that uses bimetallic disks.
(Copyright Flowserve Corporation, Irving, TX)
Usage Examples
Thermostatic steam traps such as the ones shown here are used in heating systems with light-to-moderately high condensate loads.
(Copyright Flowserve Corporation, Irving, TX)
Advantages
- Handles very light loads.
- Small, lightweight, compact.
- Thermal elements can be selected to operate within a range of steam temperatures.
- Bimetallic strip design is resistant to damage from water hammer.
Disadvantages
- Normally not recommended for extremely high condensate requirements.
- Must be set, generally at the plant, for a particular steam operating temperature.
- The sealed bellows design may be damaged by a relatively mild water hammer.
Acknowledgments
- Armstrong International, Inc., Three Rivers, MI
- Flowserve Corporation, Arlington, TX
- Flowserve GESTRA U.S., Louisville, KY
References
- Deba, Richard J. “Simplifying Steam Trap Selection.” Plant Engineering 48 (Jan 1994): 68-70. Print.
- Drake, Steve and Steve McNabb, J. O. Galloup Co., Battle Creek, MI, personal correspondence, May 2013.
- Fischer, David W. “Back to Basics – Steam Traps 101.” Plant Engineering 51 (June 1997): 82-86. Print.
- Fischer, David W. “Searching for Steam System Efficiency.” Plant Engineering. 50 (Dec 1996): 65. Print.
- Northcraft, Lionel George. Steam Trapping and Air Venting. London, New York: Hutchinson’s Scientific & Technical Publications, 1944. Print.
- Perry, Robert H. and Don W. Green. Perry’s Chemical Engineers’ Handbook. 7th ed. New York: McGraw-Hill, 1997: 6-44. Print.
- Picut, Richard “All Steam Traps Aren’t Equal.” Plant Engineering 50 (Sept 1996): 102-104. Print.
- Starbuck, R. M. and W. F. Colby. Modern Heating Illustrated. Hartford, CT: R. M. Starbuck & Sons, Inc., 1942: 91. Print.
- Stewart, Gordon. Modern Steam Traps: Their Construction and Working. New York: D. Van Nostrand Co., 1907. Print.
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