Ball Float Trap
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.

Ball float steam trap	Ball float steam trap principle of operation

Open Bucket Float
The open bucket trap consists of a float (A) that is open at the top, a valve rod (D) extending through a discharge tube (E). The discharge valve (F) seats the orifice (G). Other parts are the fulcrum (C) and the body (B). In some designs the bucket surrounds the discharge tube, (see right picture), but operation is essentially the same.

When condensates enter the trap, it gradually fills up and the bucket floats, causing it to push the valve close. The condensate will finally spill into the bucket, and the bucket sinks due to the added weight and pulls the valve open. The pressure inside the trap will force the condensate through the discharge tube. As soon as the bucket has emptied it floats again and pushes the valve close.

Inverted Bucket Float
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). As long as condensate is present the bucket stays down, but as soon as steam enters the trap, the bucket floats and rises causing the valve to close.

Steam slowly condensates and also bleeds off through the small vent (F) at the top of the bucket. Another important purpose of the vent (F is to permit gases to escape. If it were not for the vent, the bucket would become filled with air and keep the valve closed all the time.