Steam Trap Operation & Selection

Steam traps remove condensate from steam piping so only vapor continues through the system. As steam travels through distribution lines, it cools on pipe walls and forms hot distilled water. If not removed, condensate fills piping, blocks steam flow, reduces heat transfer, and can cause water hammer. This guide explains steam trap operation, trap types, selection criteria, testing methods, and common failure modes.

 

1. Steam Trap Operation

As steam moves through a plant, it condenses on pipe walls and collects at low points. Drop legs with collection pockets are installed to gather this condensate. At the bottom of each drop leg is a steam trap. The trap discharges condensate until steam reaches it, at which point it closes and waits for more condensate to accumulate.

 

Orifice Plate Trap

 

The simplest trap is an orifice plate—a small 1–1.5 mm hole in a 3 mm metal plate placed between flanges. It forces condensate to back up and discharge slowly. If condensate cools below 212°F (100°C), it drains as liquid. If hotter, it flashes into steam when entering a lower-pressure pipe, expanding and choking the orifice until the cycle repeats.

 

2. Steam Trap Types

Steam traps operate using one of three principles:

• Thermostatic traps: Sense temperature differences.
  • Fluid-filled bellows expand as temperature rises and close a valve.
  • Bi-metallic strips flex to move a plug on and off its seat.
• Mechanical traps: Sense density differences between liquid and vapor.
  • Float traps: A rising float opens the valve to discharge condensate.
  • Bucket traps: An inverted bucket drops as condensate enters, opening the valve.
• Thermodynamic traps: Use dynamic pressure differences. High-velocity steam creates low inlet pressure and higher outlet pressure, forcing the valve shut.

 

3. Steam Trap Selection

Selecting the correct trap requires knowing:

• Operating pressure and temperature
• Condensate load
• Valve orifice size
• Condensate temperature
• Pressure differential across the trap
• Presence of air or non-condensable gases
• System cleanliness
• Cost and expected service life

 

4. Is the Steam Trap Working?

Steam traps must be checked every six months. Traps may stop working due to:

• Dirt in valve orifices (always install upstream strainers)
• Steam and air locks (pipes must fall steeply to the trap; vent gases at high points)
• High back pressure from blockages or undersized piping
• Water hammer or freezing damage
• Group trapping causing unequal pressures in multiple legs

 

Testing Methods

• Sight glass: Installed 1 meter downstream to observe ebb and flow.
• Bleed point: Allows manual observation of discharge behavior.
• Acoustic sensors: Detect internal valve noises.
• Pyrometers: Measure upstream pipe wall temperature. A working trap shows temperature cycling as condensate cools and steam returns.

 

Tip for Float Traps

If condensate flow is too rapid to observe valve cycling, partially close the upstream isolation valve to reduce flow temporarily. Reopen fully after testing.

 

Adapted from original work by Mike Sondalini, Maintenance Engineer.