Purpose

All steam traps have a set purpose for being used in any steam system mainly:


• Provide a pressure drop across steam user equipment to allow entering steam to condense and release its latent heat of evaporation.
  
  
• Allow liquid condensate to be drained from the heat user system.
  
  
• Remove none-condensable gasses from the steam cycle.
  
  
• When installed and sized properly can prevent water hammer.

Figure 1. Inverted Bucket Steam Trap
Source: Yarway Corporation

The most successful mechanical trap in the above steam systems is the inverted bucket trap. This trap uses a bucket type devise turned upside down inside the trap body that is forced up to the top of the trap when steam enters at the bottom of the trap body. This action, through levers attached to the bucket, closes the outlet orifice thus stopping flow through the trap.

As condensate enters through the bottom of the trap the volume of the pressure medium changes allowing the trap to lose buoyancy and the buckets drops to the bottom causing the orifice valve to open. A pressure differential now exists between the trap body and the discharge line forcing the condensate out of the trap through the orifice valve. When the condensate is evacuated from the trap body, steam entering at the bottom will cause the bucket to raise to the top again closing the orifice valve completing one cycle.

Mechanical Trap Disadvantages

• Mechanical traps operate in an on off mode, this can cause temperature and pressure variations that may affect product quality, and if not properly sized can cause water hammer.
  
  
• Inverted bucket traps allow some steam to blow-by each operating cycle.
  
  
• Mechanical traps have moving parts and small diameter orifices located inside the trap body that render the trap very inefficient when they fail.
  
  
• Repair kits should be installed in bucket traps every 18 months to 2 years, the longer these traps operate the more wear on internal parts which causes more steam blow-by and or complete trap failure.
  
  
• Traps either fail open or closed resulting in poor product quality before the condition is discovered.
  
  
• As many as as 20% of these traps may be mal-functioning at any one time.
  
  
• Trap kits replacement and a good preventive maintenance program can be very expensive.
  
  
• Every mechanical trap must be carefully sized for pressure differential and the amount of condensate to be returned otherwise they are very inefficient.

Orifice-Type “Traps”

Orifice-type devices have been used in place of conventional steam traps in some systems to simplify the condensate return system and attempt to increase reliability. These are simple devices that permit condensate (and some steam) to pass them in a continuous manner. They do not remove non-condensable gases. When sized correctly, and when the condensate load is relatively constant, most of the fluid passing through the orifice to the return lines will be condensate. However, orifices must be sized for the maximum condensate load and they will pass steam to the return lines when the condensate load falls below the maximum. In systems in which the condensate load varies, the use of orifice-type traps will result in lowered energy efficiency. This type of system has the advantage of providing continuous-flow operation, but does not block all steam passage and its performance varies with heat load.

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