General Charateristics of Rotary Pumps
Screw pumps have self priming property and therefore do not require any special arrangements for suction. Due to self priming property these pumps are able to pump liquid as well as gases without any loss of suction.
All types of rotary pumps continue to force liquid into the system regardless of the opposition to the transfer.Due to this reason, additional protection to these pumps is required or else the pump will continue to build pressure and finally lead to rupture or damage to the pump. This happens due to the building up of excessive pressure in the pump. To avoid this relief valves are fitted in the pumps. These relief valves are capable of bypassing entire throughput of the pump. the relief valves should operate only for short interval of time otherwise they would lead to increase in liquid and pump temperature.
Rotary pumps are generally used for high viscosity oils such as lube oil and heavy fuel oil.
For smooth pumping of these high viscosity oils,they are required to be preheated. Pre-heating can be provided within the pump by a steam coil or the oil can be heated before it enters the pump.
Rotary pumps are highly sensitive pumps and therefore special care is needed in order to prevent any abrasive material from entering the pump or it can damage the helices and clearances in the pump.
Advantages of Screw Pump
- Screw pumps allow a wide range of flows and pressures
- They can also accommodate a wide range of liquids and viscosities
- Screw pumps have high speed capability and this allows the freedom of driver selection
- They have low internal velocities
- All the screw pumps are Self-priming which allows them to have good suction characteristics
- Due to the close arrangement of rotating parts, a high tolerance for entrained air and other gases is produced.
Disadvantages of Screw Pump
Cost of manufacturing is high because of close tolerances and running clearances
Any changes in the viscosity of the fluid results in high fluctuations in the performance.
A screw pump with high pressure capability will require high pumping elements which increases the overall size of the pump.
Lobe pump also runs on the same principle as that of other rotary pumps. Lobe Pump is so named because of the rounded shape of its rotors’ radial surfaces which permit the rotors to remain continuously in contact with each other when the rotation is going on. Lobe pump can be of two types – single or multiple lobe pumps. Both these types, carry fluid between their rotor lobes in the same way as a gear pump does.Though both gear and lobe pumps are almost similar in construction, there are few differences.
Difference Between Gear and Lobe Pump
In gear pumps, the locus of the gear contacts always moves along the tooth surfaces but jumps discontinuously from tooth to tooth as the gears mesh and un-mesh with each other.This helps the gears in moving eachother.
In lobe pumps, the lobes are incapable of driving each other but the contact seal locus makes a continuous movement from one radial surface to another. This is the main difference between the gear and lobe pumps.
Pump Handbook- 4th Edition By Tata McGraw Hill
Marine Auxiliary Machinery, 7th Edition, 1995 by McGeorge