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Air-Locking in Steam Traps

Air Locking impedes continuous evacuation of condensate and hence impedes heat transfer.

Air will always fill a steam systems and equipments during periods of shut down and air and incondensable gases will be carried along with steam when the system is put to operation. While steam can be evacuated from the steam mains with the help of air vents, some of this air in the equipment coils may enter the steam traps. This phenomenon is called as air locking

Thus the condensate in the coils is not able to reach the traps until the accumulated air is discharged by the trap. During this period the condensate in the heating coils will accumulate, impeding heat transfer. Such instances are typically observed in plants that have batch operations or on off operations.

Needless to say, this air must be removed from the traps as quickly as possible. If the trap installed does not have a good air venting capability, special arrangements for the same should be made and this is especially true for inverted bucket traps. Inverted bucket traps because of their small vent on the bucket release the air very slowly and the pressure is only provided by the difference in the level of water inside and outside of the trap.

All traps of the thermostatic type are fully open when cold and allow air to be discharged freely both on start up as well as during normal operation. For float traps an inbuilt thermostatic air vent should be installed. This allows smooth evacuation of air thus ensuring smooth discharge of condensate.

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