Mechanical Steam Trap

There are several basic types of traps in this class, however, they all have one principle in common - all are operated by response to the difference in density between steam and water or condensate.

Float Trap

Inside Ball float

This is one of the earliest types of steam traps. It consists essentially of a closed housing (A), usually cast iron, within which is a ball float.

In the simplest form of the ball float trap, the float (B) is attached to the end of a rod (C). The opposite end of the rod is attached to a discharge valve (D). When condensate fills the body of the trap the float rises, gradually opening the discharge valve. This trap is seldom used today without a thermostatic or bi-metallic plate to control discharge.

Inverted Bucket Steam Trap

Inverted Bucket Trap

A newer and more commonly used mechanical steam trap is the inverted bucket steam trap. The top of the bucket is attached to a valve rod (B), which permits the discharge valve (C) to open and close as the bucket falls and raises. When the bucket is at rest it hangs downward with the valve open. 

Condensate enters the trap from the passage (D), in the side of the trap and then upward under the open end of the bucket, or if the trap has a bottom inlet the condensate will enter directly under the bottom of the bucket.

Steam traps

In operation, as long as condensate is flowing to the trap, the bucket stays down and flow continues out the orifice and discharge passage (E). Now when steam reaches the trap it fills the bucket which floats and rises to close the valve.

The steam in the bucket slowly condenses and also bleeds off through the small vent (F) in the top of the bucket, thus the bucket loses its buoyancy and finally sinks, opening the valve again to discharge more condensate.

While the vent (F) (about 1/16'' in diameter in a 1/2'' trap) allows some of the steam in the bucket to bubble up into the body of the trap and condense, another important purpose of the vent is to permit passage of the air and non-condensable gases. If it were not for this vent, the bucket would soon be filled with air which forces the valve to close, and makes the trap inoperative.

Because there is frequently a considerable amount of air to be eliminated from the lines on start-up, it is necessary to get rid of this air promptly in order to obtain quick heat-up of the apparatus.

Flow of air through the vent is limited as it is due only to the buoyancy of the air in the water, therefore if large amounts of air must be eliminated some means must be found to increase the venting capacity. It is not possible merely to make the vent larger for then so much steam would bleed through that there would be serious steam loss everytime the valve opened.

If enlarged beyond a certain point, the vent would then allow steam would bleed through so rapidly the bucket would never rise to close the valve, and this would cause an even greater steam loss. Some manufacturers of these traps therefore provide, as a substitute for the regular type, a bucket with a second or auxiliary air vent (G) for handling large volumes of air and gas. This auxiliary vent is much larger than the regular vent and is provided with a disc valve controlled by a thermostatic bimetallic Strip (H).

When the trap is cold the bimetallic strip bends downward opening the auxiliary vent valve wide. This provides quicker air elimination during the start-up period.

As warmer condensate and air enter the bucket, the bimetallic strip gradually bends toward the closed position. This, of course, reduces the capacity of the auxiliary air vent, however, the air is always cooler than the stream, so as long as any air is present the auxiliary vent closes completely leaving only the fixed vent open.

Where this auxiliary vent valve is needed it is of course most important that it be kept in good working order, for if it is not, the trap will either be very sluggish and will waste steam.

In as much as the mechanical traps as a class have cast iron bodies (except for the higher ranges of steam pressures where steel is used) that are always at least partially full of water, care must be taken to prevent freezing in outdoor insulations in cold weather areas. Should the trap freeze up, not only would there be danger from backing up condensate in the line ahead of the trap, but the trap could be damaged.

When a mechanical trap is properly sized there usually is not much danger from freezing but if the trap is oversized it may not discharge frequently enough to prevent the water in the trap body or reservoir from freezing.

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