PROCESS STEAM TRAPS

 PROCESS STEAM TRAPS

The steam trap is an essential part of any steam system. It is the important link between steam use and condensate return, which holds back steam but releases condensate, as well as air and other non condensable gasses.

Only the discerning engineer will recognise that wear and tear of control valves, leakage and reduced plant output, can all be easily remedied by paying proper attention to steam trapping.

Any mechanism will suffer from wear, and steam traps are no exception to this rule. When steam traps fail open, a certain amount of steam can be passed into the condensate system, although it is often less in quantity than might be expected. Fortunately, rapid means of detecting such failures are now available to the steam user.

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 fairly stable condition is reached. The two extremes of variable condensate formation are generally known as the ‘start-up load’ and the ‘running load’. Frequent reference will be made to these terms as we consider the most efficient methods of condensate removal.

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 which cannot be tolerated under any circumstances. There is clearly a need for some means of discharging condensate without allowing steam to escape.

AUTOMATIC VALVES

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 trap which are currently available.

STEAM TRAP APPLICATIONS

It has already been mentioned that there is no such thing as a ‘universal’ steam trap which is suitable for all applications. For this reason, we must familiarize ourselves with each of the main steam trap groups and learn how best to take advantage of the merits of each type.

Air in a steam plant 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 has to 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 actually prone to ‘air binding’ - a condition in which the trap remains closed when it should be opening to release condensate. It is for this reason that frequent reference will be made to air, as well as to condensate, during our detailed examination of the main steam trap groups.

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 requirements that need to be satisfied also, like:-

• Air Venting 

• Condensate Removal

• Thermal Efficiency

• Reliability 

• Corrosion 

• Waterhammer

• Dirt 

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

STEAM TRAP TYPES

Thermostatic Group

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.

Mechanical Group

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.