Generally pumps are classified in two categories according to the technology being used for the transfer of materials into Centrifugal pumps & Positive Displacement (PD) pumps. While centrifugal pumps are most commonly found but positive displacement pumps serve special purpose especially for liquids which are difficult to handle. The PD pumps have cavity on the suction side and as this cavity expands liquid flows into the cavity and then it is discharged as the cavity collapses. Hence when the pump moves the fluid moves and displaces the fluid ahead of it and so only a constant volume is displaced at a time. The positive displacement pumps have constant flow no matter what the discharge pressure. PD pumps have advantage over centrifugal pumps for viscous application as when viscosity increases the flow drops in centrifugal pumps but it remains same in case of PD pumps. The efficiency of the pumps also increases with increasing pressure.
PD pumps require pressure relief valve as a safety precaution which can be installed externally or internally as if the discharge valve is closed there can be pressure built-up as the pump keeps running and this might damage the line.
The positive displacement pumps are further classified into two types according to the movement of the pump
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Rotary Pumps: In this type, the parts within the pump casing rotate to enable the movement of liquid. For example in gear pump the rotor and idler gear rotates to transfer liquid inside the pump. The common types of rotary pumps are
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Gear Pumps
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Lobe Pumps
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Vane Pumps
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Twin Screw Pumps
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Progressive Cavity Pumps (Single Screw Pump)
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Reciprocating Pumps: In reciprocating pumps, the liquid is transferred due to back and forth movement i.e. the reciprocating movement of the pump. The types of reciprocating pumps are
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Plunger Pumps
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Diaphragm Pumps`
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