As pressure gap exists between fluid inside the pump and air outside the pump, fluid may leak along the clearance between shaft and pump shell, thus, a seal device is needed, i.e. shaft seal. 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: Packing Seal, Expeller Seal, Mechanical Seal.
Packing seal is a common kind of shaft seal, which feeds shaft seal water (as pressure water) into the packing to avoid slurries in the pump from leaking. As the multi-stage slurry pump in series could not apply expeller seal, packing seal is a better way. Packing seal can be classified into Full Flow Packing Seal and Low Flow Packing Seal.
Packing seal needs shaft seal water, and enough water pressure and flow. Pressure of shaft seal water should be outlet pressure of the pump plus 35kpa. The water flow is determined by the type of bearing assembly and bracket.
|Bracket type||NA||NB||NC HC||ND HD||NE HR||NF HF||G, HS HST, HT||HG HTU||HU|
|Water flow (m3/h)||0.54||0.9||1.26||1.98||2.7||4.32||5.76||7.56||7.56|
Expeller seal is an impeller force of reversed centrifugal force, preventing slurry leakage. Expeller seal can be used on the single-stage pump when the positive pressure of pump inlet is no more than 10% of pump outlet pressure or on the first-stage pump in the pump series. Expeller seal consists of an expeller, an expelling vane, a fixed guide vane and a stop seal device. Expeller is a small centrifugal pump impeller that only operates while the pump is rotating and has no seal effect when the pump is stationary. The expeller acts as a turbine to reduce the pressure of the slurry attempting to escape around the back of the impeller. Expelling vane is used to reduce the pressure of pump chamber attempting to prevent particles from passing into the seal device, while the fixed guide vane is to eliminate rotation of liquid. As the expeller only works when the pump is running, a stop seal device is essential to avoid liquid leakage when pump is stationary.
*Note: the standard seal structure applied in ZGB and ZJ series slurry pumps is a changed kind of expeller seal, which is called combined expeller and packing seal. It is actually a mixed seal structure of expeller seal and low flow packing seal.
Mechanical seals are not widely used in slurry applications but their uses 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. Mechanical is classified into Single-face Mechanic Seal and Double-face Mechanic Seal. The use of mechanical seals in slurry applications requires extreme care and attention due to the limited reliability. Seal costs are relatively high and require substantial justification to warrant their use.
|FPS||Full Flow Packing Seal (normally used)|
|LPS||Low Flow Packing Seal|
|SMS||Single-face Mechanic Seal|
|DMS||Double-face Mechanic Seal|