Thermostatic Steam Trap

There are several different types of thermostatic traps but all work on the same basic principle - they respond to temperature changes in the line - opening to cool condensate - closing on Steam. 

The most commonly used type of thermostatic trap is the heavy duty BELLOWS TYPE. The operating element consists of a corrugated bellows (A) mounted within a housing, usually of cast iron. 

Inside thermostatic trap

At the Bottom of the bellows is mounted a valve (B) which closes the orifice (C) when the bellow expands.

Usually the bellows is filled with a liquid, such as alcohol and water, which has a boiling point below that of water.

This means that when condensate approaching the steam temperature comes to the trap, the liquid inside the bellows vaporises building up a pressure inside, which causes the bellows to expand and close the outlet valve.

The valve remains closed until radiation of heat from the body of the trap, and cooling of the condensate within the trap and in the line ahead of the trap, allows the vapour within the bellows to slowly condensate. The bellows then contract opening the valve wide again.

Time required to condensate the vapour within the bellows depends on a number of factors, such as the size and material of trap body, length of pipe between trap and apparatus it is draining, and the temperature of air surrounding the trap. Obviously the cooler the location, the faster the trap will be able to cycle or respond to temperature changes in the line ahead of it.

As the valve is wide open when the trap is cool, this type of trap provides quick start-up of the equipment and excellent air handling ability. To obtain best performance from this type of trap the bellows is usually designed so that it will close as near steam temperature as possible.

This type of trap is freeze - proof if mounted to drain properly when the system is shut down. The bellows trap does have some limitations in application however. As the bellows is fairly sensitive to water hammer, care should be taken to avoid any dips in the line ahead of the trap, or sudden pressure changes which might permit slugs of water to strike and damage the bellows.

Water hammer

Some designs are provided with a baffle surrounding the bellows to cushion the blow from water hammer and minimize the possibility of damage to the bellows element.

In most designs the upper pressure limit for which this type of trap is recommended is 300 PSI. Best performance is usually obtained when the trap body is un-insulated and a cooling leg of several feet of un-insulated pipe is provided ahead of the trap.

Recently developed bellows type thermostatic trap designs include modifications intended to make the bellows more resistant to water hammer and overexpansion, as well as to make the trap more responsive to changing load and pressure conditions. These traps are also equipped with a ductile iron body and cover for use at operating pressures up to 600 PSI.

Dual range thermostatic traps

Another recent development in bellows-type traps is based on a dual capacity concept by which two traps are enclosed within one body. The bellows operates a pilot valve and is fully protected within a cage unit. The cage is free to open and close, and to provide high flow rates when needed.

The bellows is protected against water hammer and sudden pressure drops, both of which can result in bellows damage. The small pilot valve provides nearly continuous drainage and elements wire drawing of seats common in larger trap flows on small loads. The cage is free to respond quickly to the presence of high loads.

At a start up both pilot valve and main valve are open for high capacity discharge or air and condensate. Both valves remain fully open as the condensate temperature increases.

As the system reaches temperature and the rate of condensate formation decreases, the main valve closes.  Condensate continues to discharge through the pilot valve.

Condensate in thermostatic

As condensate at steam temperature enters the trap, the bellows closes the pilot valve. Both valves remain closed against the steam. As sub-cooling occurs, the bellows contracts, opening the pilot valve, and condensate flows at a low rate.

As fluid temperature increases or decreases, the bellows responds accordingly, moving the pilot valve toward the closed position with increasing temperature and moving the valve toward to open position with decreasing temperature. When the bellows is fully contracted, the pilot valve is wide open, the main valve lifts.

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