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Steam Traps

How They Work

To help determine which steam trap is best for your application it is helpful to understand how each style of steam trap works.

Thermostatic Radiator Traps

The thermostatic element in a radiator trap responds to temperature to open or close.

  1. At start-up when air and condensate are cooler, the thermostatic element (diaphragm) contracts pulling the valve head off of the valve seat.
  2. The trap then opens and discharges air and cool condensate.
  3. As the condensate gets hotter the element expands driving the valve head into the valve seat closing the trap.
  4. The trap will stay closed until the condensate cools enough to contract the element and open the trap.

Applications: These traps are recommended for radiators and convectors.

Float & Thermostatic Traps

Float & Thermostatic steam traps contain a sealed stainless steel thermostatic air vent and stainless steel ball float.

  1. The thermostatic air vent is open at start-up to discharge large volumes of air to the condensate return. As steam enters the trap body the air vent closes.
  2. The float is closed at start-up and stays in the closed position while steam is in the trap body.
  3. When the steam condenses, the hot condensate lifts the float moving the valve head off the seat opening the trap to discharge condensate.
  4. As condensate discharges steam enters the trap body, the float falls and drives the valve head into the valve seat closing the trap.

Applications: Designed to continuously discharge condensate in modulating conditions. Perfect for heat exchangers, air handling coils and steam main drip stations.

Open Float & Thermostatic Traps

Designed to provide continuous air venting and condensate drainage using an open float, fail-safe design.

  1. Condensate fills the trap until it overflows into float. When the weight of the condensate overcomes the buoyancy of the float, the float begins to drop independent of the float valve head.
  2. The float continues to drop until the collar at the bottom of the valve stem engages the internal stop. At this impact point, the float falls to the bottom of the trap snapping the valve open.
  3. Condensate travels up the discharge tube, through the orifice and out the outlet port. The float will remain at the bottom with valve fully open so long as there is sufficient condensate entering the trap.
  4. As the discharge drains the float, buoyancy returns and the float begins to rise. The valve head is snapped closed into the valve seat by the velocity of the discharging condensate.

Applications: These traps are ideal for low to medium pressure main drip applications as well as heat exchangers, air handling coils and other process applications.

Inverted Bucket Traps

Inverted Bucket Traps must be manually primed at start-up to create the water seal around the inverted bucket which allows the trap to operate.

  1. At start-up the trap is open and air and condensate enters the trap body.
  2. Air is discharged through a small vent on the top of the inverted bucket while condensate fills the trap body and is discharged through the valve seat located on the top of the trap body.
  3. When steam enters the trap body it collects in the inverted bucket. The buoyancy of the steam raises the inverted bucket which pushes the valve head into the valve seat, closing the trap.
  4. When the steam condenses the bucket is no longer buoyant causing the bucket to drop, opening the valve seat and allowing condensate to discharge to the condensate return line.

Applications: These traps are ideal for use as main drip traps up to 250psi and on some steam equipment where minimal air venting capability is acceptable.

Thermodynamic Traps

  1. At start-up, air and condensate under pressure raise the disc off of the valve seat opening the trap allowing discharge into the condensate return line.
  2. Hot condensate flashes to steam as it goes through the trap body. The velocity of the flash steam creates a lower pressure area under the disc causing the disc to seat. The pressure of the flash steam in the cap keeps the disc on the valve seat, closing the trap.
  3. The trap remains closed until the flash steam condenses, allowing system pressure to raise the disc off of the valve seat.

Applications: These traps are ideal for use as high pressure main distribution line traps from 75-600psi.

Selecting the Proper Steam Trap

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Main Drip Applications

A main drip trap should be used every 100-150 feet of straight piping run. Traps should be used at each change of piping elevation and at risers as well as in front of expansion loops. The condensate load in a typical main drip application is small. It is unusual for main drip steam traps to be larger than 3⁄4" (20mm).


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Process Applications

Process Applications include: heat exchangers, air handling coils, pre or reheat coils, unit heaters, cooking kettles and absorption chillers. You Need to Know the Following to Properly Size a Process Trap:


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Pressure Reducing Applications


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Want more information?

For more information please see our Steam traps Products product guide.

Click here to get the product guide.

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