Steam Trapping

Why Steam Traps?

A Steam Trap is simply a device that lets out water or condensate but holds back steam, in other words, it is an automatic condensate valve.

We have already learned about the importance of clearing the condensate out of the steam lines and the process equipment heating surfaces quickly - as soon as it forms. The Steam Trap does this by functioning in any one of several different methods which we shall describe a little later.

Water steam pipeline

But right here, before we get involved in a detailed discussion of Steam Traps and their operation, we should cover another very important Problem - the elimination of air and gas that are present in the lines along with the condensate.

AIR may enter the boiler system either through the boiler make-up water, or it may be sucked back into the system due to the condensate of the steam when the apparatus is shut down.

Steam industrial kettle

Other non-condesible gases such as Carbon Dioxide (CO2) and occasionally Carbon Monoxide (CO) may also enter the system due to minerals in the boiler feed water and the chemical action taking place during the boiling process.

These gases together with the air also tend to form layers that act as insulating blankets in a somewhat similar way to that described in case of condensate, but the air and gas also, if we are to maintain high temperatures and efficiency in our equipment.

Inside Pipeline

So we see that a steam trap must serve three major functions:

  1. It must let out condensate but hold back the steam to save it for useful work.
  2. It must eliminate air and gas quickly, particularly on start-up after apparatus has been shut down for a period.
  3. It must accomplish removal of condensate, air, and gas by responding promptly to changing conditions in the line.

In other words, a steam trap must open up quickly when condensate or air and gas come to it, but close off just as quickly when steam reaches the trap. 

Condensate in pipeline

The steam trap may do this by either:

  1. Responding quickly to temperature changes in the line, or, in other words, must be able to differentiate between hot steam and slightly cooler condensate; or
  2. It must be able to differentiate between liquid (water and vapour - steam).

In the first instance the trap opens and closes due to temperature changes ahead of it, in the second case it operates due to change of phase of the liquid, water to vapour.

Different Types of Steam Traps.

There are many different types of Steam Traps, but we will concern ourselves only with the heavy duty industrial types. They are divided into three general classes:

1. Mechanical Steam Traps

This class operates on change of phase of the fluid coming to the trap, i.e. whether water or steam - they open to water or condensate - but close on steam.

2. Thermostatic Steam Traps

Actuated by the temperature of the liquid flowing to the trap - open on cool condensate - close near steam temperature.

3. Thermodynamic Steam Traps

Operate by utilisation of the differences in thermodynamic energy available from steam and hot condensate. 

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