STEAM TRAP PURPOSE

STEAM TRAP

A steam trap is a device used to discharge condensates and non-condensable gases with a negligible consumption or loss of live steam. Steam traps are nothing more than automatic valves. They open, close or modulate automatically.

The three important functions of steam traps are:

Discharge condensate as soon as it is formed (unless it is desirable to use the sensible heat of the liquid condensate)
Have a negligible steam consumption (i.e. being energy efficient)
Have the capability of discharging air and other non-condensable gases.

TYPES OF STEAM TRAP:

Mechanical traps. These remove condensate through the use of the mechanical properties of steam vs condensate. Since liquid is denser than the steam it will travel to the bottom of the system. Mechanical traps will have a bucket or float that rises and falls in relation to condensate level and this usually has a mechanical linkage attached that opens and closes the valve. Mechanical traps operate in direct relationship to condensate levels present in the body of the steam trap. Inverted bucket and float traps are examples of mechanical traps. Float traps can have a mechanical linkage or can seal the trap through use of the float itself.

2. Thermostatic traps. These remove condensate through the temperature difference of steam vs the liquid phase. The valve is driven through expansionand contraction of an element that is exposed to the heat from steam or condensate. These traps take a temperature drop below the saturation curve to open and remove condensate. These traps can perform this by a filled element or bellows or bimetallic element.

3.Thermodynamic traps. These operate on the dynamic principals of steam vs condensate and the use of Bernoulli’s principle. When condensate released through an orifice the speed increases and a pressure drop occurs. This will flash steam to create higher pressure to close a valve (disc) or slow the discharge speed of the trap.

Considerations for steam trap selection

Air venting

At ‘start-up’, i.e. the beginning of the process, the heater space is filled with air, which unless displaced, will reduce heat transfer and increase the warm-up time. Start-up times increase and plant efficiency falls. It is preferable to purge air as quickly as possible before it has a chance to mix with the incoming steam. Should the air and steam be mixed together they can only be separated by condensing the steam to leave the air, which must then be vented to a safe place. Separate air vents may be required on larger or more awkward steam spaces, but in most cases air in the system is discharged through the steam traps. Here thermostatic traps have a clear advantage over some types of trap since they are fully open at start-up. Float traps with inbuilt thermostatic air vents are especially useful, while many thermodynamic traps are also quite capable of handling moderate amounts of air. However, the small hole in fixed orifice condensate outlets and the bleed hole in inverted bucket traps both vent air slowly. This could increase production times, warm-up times, and corrosion.

Condensate removal :

Having vented the air, the trap must then pass the condensate but not the steam. Leakage of steam at this point is inefficient and uneconomical. The steam trap has to allow condensate to pass whilst trapping the steam in the process. If good heat transfer is critical to the process, then condensate must be discharged immediately and at steam temperature. Waterlogging is one of the main causes of inefficient steam plant as a result of incorrect steam trap selection.

Plant performance:

When the basic requirements of removing air and condensate have been considered, attention may be turned to ‘plant performance’. Simply put, unless specifically designed to waterlog, for a heat exchanger to operate at its best performance, the steam space must be filled with clean dry steam. The type of steam trap will influence this. For instance, thermostatic traps retain condensate until cooled to below saturation temperature. Should this condensate remain in the steam space, it would reduce the heat transfer area and the heater performance. The discharge of condensate at the lowest possible temperature may seem very attractive, but generally most applications require condensate to be removed from the steam space at steam temperature. This needs a steam trap with different operating properties to the thermostatic type, and this usually means either a mechanical or thermodynamic type trap.

Before choosing a particular steam trap it is necessary to consider the needs of the process. This will usually decide the type of trap required. The way in which the process is connected to the steam and condensate system may then decide the type of trap preferred to do the best job under the circumstances. Once chosen, it is necessary to size the steam trap. This will be determined by the system conditions and such process parameters as:

Maximum steam and condensate pressures.

Operating steam and condensate pressures.

Temperatures and flowrates.

Whether the process is temperature controlled.

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