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Features and operating principle of a positive displacement pump
Positive displacement pumps are hydrostatic machines. They operate with a positive transfer and should not work against a closed system.
All rotary pumps are designed after the same principle. Two rotors are arranged on parallel shafts and driven by an external synchronous gear box.
The rotors rotate in opposite directions to each other. Small radial and axial clearances assure that they have no contact with each other, or the pump body. The rotors are designed to form a barrier between the suction and pressure side of the pump in any position. The sealing is only maintained by narrow gaps. There are no additional seals or valves.
The increasing cavity between the rotors on the suction side is filled with the product. The product is displaced in a circumferential direction and discharged on the pressure side as the cavity between the rotors is collapsing. This generates a constant flow from the suction to the discharge side of the pump.
|Suction and filling of the upper chamber and simultaneous displacement of the product present in the lower chamber||Displacement of the product present in the upper chamber and simultaneous suction and filling of the lower chamber||Discharge of the product present in the upper chamber and simultaneous displacement of the product present in the lower chamber||Displacement of the product present in the upper chamber and simultaneous discharge of the product present in the lower chamber|
Fig. 1: operating scheme
Rotary pumps ensure a gentle fluid transfer with minimum stress or damage to the product.
(only for water)
With positive displacement pumps the flow rate Q is linear dependent on the pump speed n.
On a test stand the flow rate is determined for various speeds and total head. In order to allow a comparison between the various pump designs and types, these tests are always carried out with water.
Once the flow rate Q and the total head H have been determined, a pump speed n that corresponds to this operating point will result from the diagram.
Fig. 2: Performance
The positive displacement pump is usually operated with a fixed speed drive. The flow rate is constant.
The flow rate can be adjusted to the various operating conditions by changing the pump speed.
The viscosity of the product must be always taken into consideration for the design and selection of the pump type.
Fluids with higher viscosities require more time to enter the displacement chamber. In those cases the pump speed must be adjusted accordingly to avoid cavitation which reduces the volumetric efficiency and increases the wear. A pump operating with cavitation creates a considerable noise level.
Regardless of the low clearance between the rotor and the pump body, a slip from the pressure side back to the suction side will be generated when water like products are transferred.
In case of circumferential piston pumps the slip stops at a product viscosity of about 200 mPa s and at about 500 mPa s in the case of rotary lobe pumps.