Pump types--part 3 (Special rotodynamic pumps)

It’s possible to construct rotary machines which use the kinetic energy of all liquids but which don’t obey the conventional mechanics of the centrifugal pump. Impellers don’t look like conventional pump impellers. Kinetic energy can be transferred to the liquid in increments, by the use of many vanes, or all the energy can be achieved by a single stage. These pumps occupy a position between rotodynamic pumps and rotary positive displacement pumps; their theory of operation having some of the characteristics of both types.

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. These pumps have therefore good self-priming characteristics. High heads are possible with a single-stage but efficiency is lower than other pump styles.

__4.1 Peripheral pumps

Fig. 57 Typical tractor driven centrifugal pump with built-in gears

Fig. 58 Example of performance curve for a peripheral pump

Peripheral pumps are also called regenerative pumps. In principle, liquid within the casing is separated into elements by means of rotating vanes but the radial clearance permits a large percentage of recirculation. Energy is successively imparted to the liquid by contact with each vane in a manner which is reminiscent of rotodynamic pumps. When pumping, energy is transferred to each enclosed element of liquid by the action of the passing vanes. The increase in head for a given outer diameter and speed is 5 to 10 times that of an equivalent rotodynamic pump. There are usually many vanes mounted on the periphery of the rotating disc.

In practice the vanes are formed by making cut-outs in the periphery of the disc and supply energy during almost a complete revolution. The H-Q characteristic of peripheral pumps is characteristically steep, Fig. 58, which is ideally suitable for automatic operation on conjunction with an accumulator. Also the NPSHr characteristic is steep. This type of pump is not usually permitted to operate at very low flow rates. Because of the pumps steep H-Q characteristic the power demand increases with reduced flow rate and increased differential head. This can cause overloading of electric drive motors designed to meet the demand at the design operating point.

Reduction of flow rates to low 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 integral by-pass valves are available. However, operation under integral by-pass control should be carefully monitored to avoid excessive temperature rises. If prolonged operation at low flows is necessary, a by-pass which recycles to the suction source not the pump suction, is preferred. Pump efficiencies will generally be below 40%. A longitudinal section through a single-stage peripheral pump is shown in Fig. 59. A transverse section through the stage is shown in Fig. 60. Peripheral pumps are used extensively for domestic and commercial clean water applications. Many pumps are used for marine applications: bilge, wash-down, engine seawater coupling, fuel oil transfer. Versions for direct engine mounting are readily available. Pumps in brass, bronze and stainless steel provide flows up to 70 m3/h and heads to 70m in popular sizes.

Fig. 61 shows an end-suction multi-stage peripheral pump.

This pump us unusual in that the first stage is a centrifugal impeller.

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Fig. 59 Section through a single-stage close coupled peripheral pump; Intake Outlet Pump disq, Channel, Pump disc with cut-outs

Fig. 60 Peripheral pump stage

Fig. 61 A hybrid peripheral pump

Fig. 62 Performance curves for a Pitot tube pump

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__4.2 Pitot tube pumps

Pitot tubes are also called, erroneously, jet pumps. If a small close-coupled pump is driven by an electric motor and does not require motive fluid input, compressed air, steam, high pressure water then, it’s not a jet pump. Pitot tube pumps are unusual in that the casing rotates and not the impeller. Within the rotating casing of a Pitot tube pump the pumped liquid is accelerated to a high tangential velocity. The velocity depends upon the shape of the blades in the walls of the casing and the effects of frictional forces. This velocity is captured by a stationary tangential Pitot tube which delivers a flow at increased pressure, Fig. 62.

Assuming that the tangential velocity of the liquid at the level of the Pitot tube is equal to the peripheral velocity of the casing then the theoretical pressure rise is:

Normally pressure is reduced as the result of flow losses and due to rotational slip between the liquid and casing. The design of the Pitot tube and blades is of critical importance with regard to losses. The stationary Pitot tube is sealed inside the rotating casing by a mechanical seal. As the casing may be rotated at speeds up to 10000 rpm the mechanical seal selection is very important. For small flows and high pressures, low specific speed centrifugal pumps exhibit low efficiency due to the large impeller friction losses. The Pitot tube pump is more efficient in this respect. Maximum efficiencies of 50 to 60 % are possible.

The performance curves for a Pitot tube pump are similar to those for a centrifugal pump, see Fig. 62. Curves can be unstable and minimum flow requirements must be observed.

Absorbed power increases as flow increases. Safety valves, as a protection against overloading or overpressure, are not necessary and the pump can be regulated by throttling.

The advantages of the Pitot tube are:

--Large differential head in a single stage

--Reasonable efficiency for small flows

--Non-lubricating liquids can be pumped

The Pitot tube pump cannot compete with the centrifugal pump for normal flows. Pitot tube pumps are not suitable for contaminated, abrasive or viscous liquids. Some manufacturers see their pumps as being in direct competition with piston and plunger pumps. The Pitot tube pump is not a direct competitor because it still has a rotodynamic style characteristic while being able to achieve quite high pressures in a single stage. The velocities involved are much greater that the 2 m/s from a reciprocating pump. Pitot tube pumps can achieve about 820 m or 80 bar based on water at flows up to 105 m3/h. Small close-coupled pumps, see Fig. 3 are used for domestic water boosting applications for private sources originating from wells.

__4.3 Disc pumps

The disc pump is very similar in construction to the end-suction centrifugal pump however the impeller does not have vanes like a centrifugal pump impeller. Two rotating parallel discs are used to create viscous drag which generates the forces to move the liquid. The pump characteristics are very similar to centrifugal pumps. The disc pump can handle small and large solids, stringy solids and is not so susceptible to viscosity degradation as a centrifugal pump of the same size. The pump is classified as non-clogging. Impeller wear in solids handling applications is greatly reduced because solids don’t impinge on rotating surfaces. The disc pump can also handle quite large volumes of entrained gas without losing prime.

Disc pumps have proved useful in the paper industry. Dense pulp, up to 18% dry solids, has been handled in existing installations. The disc pump can also be built to 3A-Sanitary requirements. These pumps can handle flows to over 2000 m^3/h at heads up to 300 m.

__4.4 Pumps as power-recovery turbines

Rotodynamic pumps can run in reverse as turbines. This fact is long-established and is one of the reasons why non-return valves are fitted in pump discharge lines. Pumps and their drivers have suffered damage as a result of turbining due to process upsets or non-return valve failures. The power available from pumps running in reverse can be captured and harnessed for useful work.

Serious interest in power recovery turbines stems from the oil crisis of the 70s and the escalating cost of energy. Some widely used processes are ideal for considering power recovery. In oil refining, the hydrocracker process operates at over 100 barg and gas scrubbing at various pressures over 50 barg. In drinking water production by reverse osmosis from seawater, the process operates at 70 barg. In these cases the process only degrades the pressure slightly leaving a large amount of pressure energy to be throttled or recovered.

When considering power recovery from high pressure process streams there are three basic choices for installation:

--Use the power as the sole power supply to drive a pump or other machine

--Use the power as the sole power supply to drive a generator

--Use the power to assist in driving a pump or other machine

Standard mixed-flow and centrifugal pumps are popular for these applications; both types are in effect fixed geometry Francis turbines. The Francis turbine is an inward flow radial turbine and, depending upon the head to be converted, can be mixed-flow or truly radial. Power generation turbine manufacturers, in general, don’t build small machines thus making standard pumps an ideal option. The slight loss of efficiency, 2 or 3%, is more than offset by the economic benefits of purchasing a standard piece of equipment.

If the power recovered is used as the sole power supply it must be remembered that the turbine won’t produce power until the flow reaches 40% of design. Also, speed control may be necessary. If the recovered power is used to assist in driving then the speed control of the prime driver should suffice. Assisted driving is probably the most common application because of its simplicity but it’s not the most efficient utilization of either energy stream.

Consider a pump driven by a squirrel-cage motor which has two drive shafts, a double extended motor. The extra drive shaft is coupled, via an automatic clutch, to the turbine. The pump is started in the normal manner. If there is no flow through the turbine it remains stationary. The pump process conditions can be adjusted and allowed to stabilize. If during this period flow, and some pressure, appear at the turbine inlet the turbine will start to spin. As the turbine design conditions are approached the turbine speed will increase until it tries to run faster than the motor. As the turbine tries to overspeed, the clutch locks and power is transmitted to the motor. As the turbine transmits power into the motor, the motor speed increases slightly, unloads by the amount of power supplied by the turbine and the motor power consumption is reduced. If the turbine liquid supply fails the turbine will reduce in speed and when the motor attempts to drive the turbine the clutch will disengage leaving the motor to supply all the power.

In typical applications, such as hydrocracking and reverse osmosis, the pump and turbine are the same pump type.

Hydrocrackers use radially-split, barrel machines. Depending upon the size, reverse osmosis systems use multi-stage segmental or axially-split machines. It’s possible to purchase multi-stage segmental machines with the pump and turbine in the same casing; precautions must be taken to prevent the turbine running dry.

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