Centrifugal pumps normally have to have some form of rotary seal at the point where the rotating shaft carrying an impeller passes out through a stationary casing.

The rotary seal can be in the form of a device known as a mechanical seal and mechanical seals require a certain environment in order to operate successfully.

For example, it is important to maintain sufficient pressure in the fluid surrounding the mechanical seal in order to avoid local boiling at the faces of the seal, since boiling can damage the seal and reduce life.Bellows Seal Valves Also, it is important to avoid suspended solids in the surrounding fluid since such solids can cause accelerated wear of the seal faces. Finally, prior to start-up, any gas needs to be vented from the immediate vicinity of the seal, i.e. in the so-called seal chamber.

Most existing designs of process pumps achieve maintenance of sufficient pressure in the surrounding fluid by provision of back-pressure clearances. Flushing flow of fluid is arranged to pass from a pump discharge of the impeller back to a suction side of the impeller via such a clearance. This creates higher pressure in and around the mechanical seal. The device to cause this to happen is often referred to as a throat bushing, which is fixed, the clearance being between the inner cylindrical face of the throat bushing and the cylindrical face of the rotating shaft.

A negative effect of known throat bushes is that while sufficient pressure is maintained in the surrounding fluid, suspended solids in the surrounding fluid are not removed and a separate vent hole must be provided to vent any gas prior to start-up.API Cast Steel Valves However, the cross-sectional area of the vent hole can be significant relative to the throat bush clearance. This reduces the back-pressure and also encourages wasteful extra leakage.

Any solids flowing into the seal chamber from the impeller discharge tend to get centrifuged to the walls. These solids find it difficult to escape through the throat bush clearance since they have to move against the centrifugal force field.REDUCING VALVES Accordingly, there is a tendency for solids to accumulate at the seal chamber walls but whatever solids do escape could damage the shaft in the vicinity of the clearance.

Finally, it is often necessary to dismantle the pump in order to drain the seal chamber fully.

The foregoing illustrates limitations known to exist in present centrifugal pumps. Thus, it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.

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2011-12-26

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