How It Works: Steam Traps

The industry defines a steam trap as a:

“Self-contained valve which automatically drains the condensate from a steam containing enclosure while remaining tight to live steam, or if necessary, allowing steam to flow at a controlled or adjusted rate. Most steam traps will also pass non-condensable gases while remaining tight to live steam.” – ANSI/FCI 69-1-1989

This type of valve has been in use since the early 1800s, following the Industrial Revolution, as steam become more widely used as a heating medium instead of a motive source (to power trains, pumps, etc.). There are three basic criteria for a valve to be considered a steam trap:

  1. Automatically discharges condensate
  2. Does not leak steam
  3. Can also discharge non-condensable gases, e.g. air

Steam traps are used in heating applications where latent heat is created and conveyed to a specific product (e.g., heating crude oil and make it less viscous). Once the latent heat/energy has been transferred to the product from the steam, the steam condenses and forms condensate. If this condensate is not removed effectively from the process, the efficiency of the system will suffer.

A steam trap works to remove condensate and non-condensables, without removing the steam.

The use of standard valves for this purpose, i.e. manually throttling them to allow only the condensate to escape has been tried, but this method is time consuming and inefficient. The manual valves have to be constantly adjusted to take account of varying steam loads (and the external environment), consequently the risk of losing steam is much higher.

There are three basic types of steam traps to consider.

Mechanical Traps

Mechanical traps sense the difference in density between steam and condensate. Condensate from this type of trap is continually discharged, leaving none to hinder the process. These types of traps are the most common utilized today in systems that require large discharge capacities, which includes the majority of process applications.

The bucket trap uses an inverted bucket as a float device, and a linkage connects the bucket to the valve head. When steam or air enters the bucket, it gains buoyancy and closes the valve. Condensate causes the bucket to lose buoyancy and sink, opening the valve and allowing the removal of the condensate.

The floating ball trap is a similarly simple mechanical trap. The weight of the ball, acting through a lever, keeps the valve closed when there is no condensate. As the condensate enters the trap, it raises (floats) the ball and opens the valve. When the condensate has been discharged, the ball drops back down and closes the valve. This type of trap is unable to discharge air, so a thermostatic air vent is installed inside the body of the valve for this purpose.

The Cameron DOUGLAS ITALIA portfolio offers both inverted bucket and floating ball traps.

Thermostatic Traps

Thermostatic traps utilize a temperature sensing element to determine when to discharge the condensate. Because of this, all thermostatically operated stem traps will cause condensate to back up in the system. In addition, they handle air and non-condensables extremely well because the valves are open on start-up when the system is relatively cool. Types of traps included in this category are balanced pressure and bimetallic.

  • Balanced pressure traps operate by balancing the steam pressure and the internal pressure of the thermostatic element (e.g., a bellows), which is partially filled with a volatile liquid whose saturation temperature is slightly lower than that of water. At start up, the trap is wide open, discharging condensate and non-condensables. As the temperature approaches that of the steam, vaporization of the volatile liquid creates a pressure differential, causing the bellows to expand and close the valve against its seat. As the condensate cools, the volatile liquid condenses and lowers the internal pressure on the bellows, causing it to retract and open the valve, permitting the condensate discharge cycle to continue. Examples of this include the DOUGLAS ITALIA Model TZ and Model TJ Traps which can be used in steam tracing applications.
  • Bimetallic trap operation is based on the characteristic that two dissimilar metals have different expansion rates. When the bimetallic element is heated the different expansion rates of the metals causes it to deflect or bend, which in turn provides movement to close a valve. Examples of this type of trap include the DOUGLAS ITALIA Model BB and Model BV Traps which can be used in steam tracing and superheated steam main applications.

Thermodynamic Traps

Thermodynamic traps use the difference in velocity between steam and condensate to operate. Condensate entering the steam trap body moves relatively slowly compared to steam and is freely discharged through a valve (a free floating disk). When steam reaches the underside of the disc its velocity is much higher than that of condensate, creating a pressure drop which closes the valve head. The valve stays shut until the steam pressure above the disc drops, allowing the valve to open and the discharge cycle to repeat.

The Cameron DOUGLAS ITALIA Model DA is an example of this type of trap, which can be used in steam mains and steam tracing applications.

Applications

Steam traps can be used anywhere there is a steam system and a need to discharge condensate. A few applications within the oil and gas and industrial markets include:

Oil & Gas Applications

  • Bulk storage tanks
  • Pressure reducing valve stations
  • Steam mains
  • Steam tracing
  • Process heaters (e.g., heat exchangers, reboilers)

Industrial Applications

  • Industrial dryers (multi-bank pipe dryers, rotating cylinders)
  • Laundries (garment presses, dry cleaning machines)
  • Space heating
  • Steam mains
  • Steaming ovens
  • Bulk storage tanks
  • Process equipment (boiling pans, retorts, digesters, coppers, reboilers, evaporators, vulcanisers)
  • Space heating (heat exchangers, radiant panels, unit heaters, air heater batteries, overhead pipe coils)
  • Process vats
  • Autoclaves