3.5.6 Liquid ring pump
Liquid ring pump
Liquid ring pump occupy a position between Rotodynamic pumps and positive displacement pumps. Their method of operation having some of the characteristics of both types. In principle, fluid is separated by means of rotating vanes, as in the case of displacement pumps with rotating vanes (vane pumps), although with a large percentage of re-circulation. Energy is successively imparted to the liquid by contact with each vane in a manner which is reminiscent of turbine pumps. When pumping, energy is transferred to each enclosed element of fluid by the action of the passing vanes. The increase in pressure head for a given outer diameter and speed is 5 to 10 times that of an equivalent turbine pump.
The positive displacement feature makes it possible to pump air or gas provided that a certain quantity of liquid is available to act as a seal. Liquid ring pumps and peripheral pumps have therefore good self-priming characteristics.
The working principle of a liquid ring pump is illustrated in figure 3.56a.
The pump impeller is fitted with vanes A, which cause the liquid in the pump casing to rotate. The duct B in the wall of the pump casing has its greatest depth at point C, thus causing the volume of the pocket between two adjacent vanes to vary as the pump impeller rotates. The volume between two adjacent vanes is increased as the duct depth reaches a maximum at C. This causes a partial vacuum which sucks more liquid in through the suction port D. The depth of the duct decreases at E thus reducing the volume of the liquid pocket between two adjacent vanes. This causes a build up of pressure which forces the liquid out through the exhaust port F. The shape and depth of the duct relative to the intake and exhaust ports determines the evacuation capacity and, to a certain extent, the performance characteristics. The ducts or side channels can be replaced by equivalent channels located outside the periphery of the pump impeller.
The Q-H curve of liquid ring pumps is characteristically steep, figure 3.56b, which is ideally suitable for automatic operation in conjunction with a hydrophone. This type of pump is not usually permitted to operate at very small volume flow rates.
Figure 3.56b Example of performance curves for a large single stage liquid ring pump
Because of the liquid ring pump’s steep Q-H curve the power demand increases with reduced volume flow rate and increased counter-pressure, This can cause overloading of electric drive motors designed to meet the demand at the design operating point. Reduction of volume flow rates to small values should therefore be carried out by means of by-pass control i.e. by returning liquid from the delivery side to the suction side using a control valve connected in the return pipe. Special designs with built-in by-pass valves are available, figure 3.56c
Figure 3.56c Liquid ring pump with built-in by-pass valve.
Liquid ring pumps produced by different manufacturers are so similarly constructed that it is even possible to interchange some of their spare parts. The most usual type is illustrated in figure 3.56d and consists of an intermediate section and one impeller per stage together with two end covers and connections. The shaft runs in a liquid lubricated plain bearing and a greased ball bearing. The most common material is grey cast iron with bronze pump impellers. Special stainless steel types are available, primarily for use in the foodstuffs industry.
Liquid ring pumps are used in great numbers for domestic and agricultural water supply from wells and also as small high pressure pumps. Maximum delivery head for multi stage pumps is about 300 m. Disadvantages are wear, in the case of sand-contaminated water, and the relatively high noise level despite the low speeds. Efficiency is lower than for a centrifugal pump of the same performance. The big advantage of the liquid ring pump is that it is self priming if it contains liquid prior to starting, and also that it can pump liquids containing small quantities of entrained gas.