What is a steam trap and what does a steam trap do?

 What is Steam Trap?

The steam trap is an essential part of any steam system. It is the divider between steam use and condensate return, which holds back steam but releases condensate, as well as air and other incondensable gases which are not useful for process heating.

What does a Steam Trap do?

When steam first enters the circuit from the boiler it will encounter the cold surfaces of the distribution pipework and process equipment. The temperature difference between the steam and metal walls will be greater during this initial warm-up period than at any other time. We know that the highest rate of heat transfer will occur when the temperature difference is greatest, and it is for this reason that during start-up steam consumption is at a peak.

As the steam system warms up, the gradual decrease in temperature difference brings about a corresponding decrease in the rate of steam condensation, until a stable condition is reached. The two extremes of variable condensate formation are generally known as the ‘start-up load’ and the ‘running load’. 

If an adequately sized hole is provided at the bottom of a piece of process equipment, any condensate which is formed will be free to drain away. The problem is that steam will also be allowed to escape, a waste of energy that cannot be tolerated under any circumstances. There is clearly a need for some means of discharging condensate without allowing steam to escape.

An automatic valve, which is somehow able to sense the difference between steam and condensate and react accordingly, is known as a ‘steam trap’ and its function is to discharge condensate without allowing live steam to escape. All steam traps are designed to do just that, but they do not all do it in the same manner.

If the conditions in every item of steam-heated plant were the same, it would be reasonable to use one type of steam trap for all applications. In practice, however, a steam trap, which is ideal for draining, for example, a steaming oven, could never be used successfully on a heater battery. Considerations such as these account for the many different types of steam traps that are currently available.

Application of Steam Traps.

It has already been mentioned that there is no such thing as a ‘universal’ steam trap that is suitable for all applications.  Air in the steam plants can cause poor heat transfer between the steam and the process surface as we have already discussed. It can also cause problems with the steam traps themselves. When steam is shut off, air will be drawn in to take up the space formerly occupied by steam, since this air must be removed from the system on start-up, it is a considerable bonus if the steam traps have a good air venting capability. While this is the case with certain traps, other types are prone to ‘air binding’, a condition in which the trap remains closed when it should be opening to release condensate. 

The need for Steam Traps.

The need for a trap to remove the condensate is the first concern of an Engineer or Designer, but along with this he has to look at other requirements that also need to be satisfied like

• Air Venting 
• Condensate Removal
• Thermal Efficiency
• Reliability 
• Corrosion 
• Water hammer
• Dirt 

The prime requirement however is the adequate removal of air and condensate. This requires a clear understanding of how traps operate.

Some widely used steam trap types: -

Thermostatic Traps

This type identifies steam and condensate by their temperature difference. This operates a thermostatic, valve-carrying element or capsule. The condensate must cool to below steam temperature before it can be released.

Float Traps or Bucket Traps

Traps of this type operate mechanically, sensing the difference in density between steam and condensate. The movement of a ‘float’ or a ‘bucket’ operates the valve.

Thermodynamic Group

These traps consist of a simple disc and body. They work on the basis that flash steam which is produced when hot condensate enters the chamber of the trap, reduces the pressure below the disc, thus closing the valve.