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COTENT
Some Design Constraints
Time:2012-9-26 9:11:10    View:115    Author:FAMOUSS PUMP

Shaft Sealing

The shaft seal is one of the most important mechanical elements in any centrifugal slurry pump and the correct type of seal must be carefully selected to suit each individual pump system. The three most commonly used seal types are as follows.

Centrifugal (or Dynamic) Seal

The centrifugal seal is a dynamic, dry seal that only operates while the pump is rotating and has no seal effect when the pump is stationary. A secondary seal maintains the liquid within the pump when it is stationary. The secondary seal can be either a rubber lip seal or a grease lubricated packing

The centrifugal seal consists of expelling vanes on the back of the impeller and an expeller which rotates in unison with the impeller located in a separate chamber behind the impeller.  The expeller acts as a turbine to reduce the pressure of the slurry attempting to escape around the back of the impeller. The expeller forms a pressure ring within the expeller chamber and prevents the slurry from passing into the secondary seal area.

Because of its effectiveness and simplicity, the centrifugal seal is the most common seal used in slurry applications, but it is limited by the pump inlet pressure and the pump speed, (rpm).  Performance data is available for centrifugal seal limitations for specifi c pump sizes generally

Gland Seal

The soft, packed gland seal is the second most commonly used seal in slurry applications. The gland seal comprises a number of soft packing rings, compressed against a chamber (stuffi ng box) and a protective wear sleeve which is fi tted to the pump shaft.  This type of seal requires continuous clean liquid lubrication and cooling between the rotating shaft sleeve and the compressed packing to prevent overheating due to the friction.The quality, quantity and pressure of this gland sealing water is of prime importance and must be carefully matched to the duty required. Note:  This gland seal arrangement can be provided as a low flow seal by substituting the stainless steel lantern restrictor with a close tolerance antern restrictor.  This will reduce the gland water consumption by at least half.

Mechanical Seal

Mechanical seals are not widely used in slurry applications but their use in special circumstances are increasing. The mechanical seal consists of a stationary and a rotating face pressed together under mechanical and hydraulic pressure to prevent leakage.

Alpha-grade silicon carbide or tungsten carbide are the most common materials used for manufacutre of these seal faces.

the use of mechanical seals in slurry applications requires extreme care and and attention due to the limited reliability common in this developing product. Seal costs are relatively high and require substantial justification to warrant their use.

Seal specialists are actively developing this type of seal expecting that a greater reliability and lower production costs will lead to an increase in use. Applications where a centrifugal seal cannot be used and where the addition of water cannot be tolerated, provide the most likely areas for the use of mechanical seals.

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