Inverted Bucket Trap

Inverted Bucket Steam Trap Operating Principles

The inverted submerged bucket steam trap is a mechanical trap that operates on the principle of the difference in density between steam and water.

How It Works

		Step 1 Steam, condensate and non-condensables enter the trap through an inlet tube centered beneath the bucket. Condensate flows down and around the bottom of the bucket, rising in the body of the trap until it completely encloses the bucket. At this point the trap is considered primed.
		Step 2 Steam collects under the bucket, displacing the condensate, until the bucket is buoyed by the lower density of the steam. The trap's valve is pushed toward the seat by the rising bucket until the pressure differential across the seat snaps the valve closed.
		Step 3 A vent in the top of the bucket is sized so that the amount of steam flowing through it makes up for the radiation loss of the trap. Any air under the bucket flows through the vent as well. The steam and air collect in a chamber at the top of the bucket.
		Step 4 Steam trapped in the steam space of the heat exchanger gives up its heat, condenses, and is drained to the trap. As the steam under the bucket is replaced by the condensate from the heat exchanger, the bucket loses its buoyancy and sinks, pulling the valve from the seat. Condensate passes through the relatively small space under the bucket at an increased velocity, picking up any dirt that has fallen from the condensate and carrying it out of the valve. Any air that has collected in the space above the bucket is discharged ahead of the condensate.

Modulating Operation

The inverted bucket trap can also operate in "modulating" mode. This partially opening, instantly closing operation occurs quickly enough that the output flow from the trap appears to be continuous.

This operation is used during very low-load periods when fluid dynamics in the discharge orifice cause the trap to behave differently. When the valve begins to open, a small flow of condensate starts through the cracked valve. The pressure drop across the valve, combined with the small opening, causes the condensate to flash into steam and to increase the velocity of the steam through the orifice. This, in turn, increases pressure drop. The net result is an increase in the forces acting to close the valve.

When condensate flow into the trap is sufficiently large (relative to the instantaneous flow through the discharge port), the water level under the bucket rises rapidly. This counters the increased closing force at the discharge orifice, and the valve is pulled free of the seat. However, if condensate flow in the trap is quite low, the level under the bucket may not increase rapidly enough to counter the increased closing force, and the valve is pulled shut.

Armstrong IB Trap Features

	1.	The bucket vent provides continuous and automatic air venting. For situations with heavy start-up air loads, large or thermic vents can be substituted.
	2.	The "free-floating" linkage, self-lapping valve, and seat and bucket are made of stainless steel. Steam does not reach the water-sealed discharge valve. There is virtually no live steam loss through the valve. Purging action breaks up films and speeds them through the system. Continuous draining does not back up condensate.
	3.	The IB trap is available with a number of material choices, connection choices, and sizes. IB typically fails open. Back pressure diminishes capacity.
	4.	Resistant to multiple occurrences of water hammer and freezing damage.
	5.	The open bucket is resistant to damage from water hammer and freezing. The relatively small passage under the bucket ensures that small dirt particles will be picked up and carried out of the trap.

	Download Product Specification Sheets for IB Traps
	To see a synopsis for the video "Anatomy of the IB" click here.
	To see a synopsis for the video "A Matter of Principle" click here.