Introduction to Pumps

Transcription

1 Introduction to Pumps 1

2 Introduction to Pumps 1.0 INTRODUCTION There are many different types of pump now available for use in pumped fluid systems. A knowledge of these pump types and their performance characteristics is extremely useful in selecting the most suitable pump for a particular application. 2.0 PUMP CHARACTERISTICS AND THEIR MEASUREMENT To understand the performance of a pump it is necessary to be familiar with the following terms: FLOW or Discharge or Delivery litres/minute DELIVERY HEAD ) metres SUCTION HEAD ) See diagram below metres TOTAL HEAD ) metres EFFICIENCY % SPEED DRIVING POWER rpm kilowatts The Driving Power is the output power delivered by the engine or electric motor driving the pump. Water Power is the energy given to the water by the pump; it has no significance except in the calculation of efficiency. The efficiency of a pump is never 100%. What you get out is always less than you put in. Efficiency Output Input Water Power Driving Power 100% The Head terms are best illustrated by the following installation, where water is drawn from a well and delivered to an overhead. tank (Figure 1). 2

3 Figure 1 Pump Terms The Suction Head, A, is the vertical height from the water source to the pump. The Delivery Head, B, is the vertical height from the pump to the tank, and the Total Static Head is A + B. On the suction side the pump has to overcome not only static suction head but also the friction head in the pipe, including the loss at fittings. This extra head is represented by x and for convenience is shown as the equivalent suction lift. Similarly there is a friction head, y, for the delivery side. With a very long delivery pipe this may represent the larger part of the delivery head, but can be kept at a minimum by increasing the diameter of the delivery pipe. The total Working Head (A + B + x + y) determines the load against which the pump works, and thus the general type of pump required and the power of the driving motor. To test the performance of a pump the characteristics listed above are measured under closely controlled conditions in the laboratory. The pump is run over its full operating range and the data from the tests are presented either in tabular or graphical form. Using these results, which are generally included in pump catalogues, a pump can be selected which will meet the requirements of any job and which will operate at the best efficiency. Examples of performance graphs for different types of pump are given in the sections which follow. 3

4 3.0 TYPES OF PUMP Of the pumps in use on farms, by far the greatest number belong to one of three main types, Reciprocating, Rotary and Centrifugal. There are other types, for example Jet pumps, Airlift pumps and Hydraulic Rams. Reciprocating Pumps Construction A typical reciprocating pump has a ram or piston working backward and forward within a cylinder or pump barrel. This motion is usually obtained from a crank revolving at constant speed, and a connecting rod. Automatic valves control the flow of liquid into and out of the cylinder. Such a piston pump may be single-acting, in which flow occurs only in alternate strokes, or double-acting where the flow occurs every stroke, and is thus more uniform. See Figures 2 and 3. Reciprocating pumps are characterised by the intermittent delivery of the liquid. Figure 2 Single-acting Piston Pump 4

5 Figure 3 Double-acting Piston Pump A deep-well reciprocating pump operates in the same way as described above, but the crank is at ground level above the well, and the cylinder is placed down the well at the lowest expected water level. A drop pipe carries the pumped water from the cylinder to the surface, and long rods transmit the reciprocating motion from crank to piston. See Figure 4. Figure 4 Figure 5 Deep-well reciprocating pump Diaphragm pump 5

6 The diaphragm pump, commonly used for pumping milk, is also a positive displacement reciprocating pump. It works in the same manner as the piston pump, but the piston is replaced with a flexible rubber diaphragm. This is shown in Figure 5. Performance The Reciprocating pump delivers a volume of liquid which changes in proportion to speed, but which is almost independent of delivery head. So for a particular pump speed, the head/discharge characteristic is nearly vertical: Slow Medium Fast TOTAL HEAD (metres) DISCHARGE, (l/min) Piston pumps are capable of pumping to high heads, 200 metres or more, and are quite widely used in water supply work for small flows, but because of their high cost they are not economical for low heads. They operate at low speeds, usually rpm, so a large speed reduction is necessary where a petrol engine or electric motor is used. Rotary Pumps Construction A rotary pump consists of two cams or gears which mesh together and rotate in opposite directions inside an oval casing. The rotating parts fit the casing closely, and the liquid trapped between them and the casing is forced through the pump as they rotate. A definite amount of water depending on the size and shape 6

7 of the gears, is passed with each revolution the gear pump is shown in Figure 6. Figure 6 Gear Pump Performance Rotary pumps have a head/discharge characteristic similar to reciprocating pumps but provide a steadier flow, do not require priming, and, having no valves, are of simpler construction. They have almost the same suction characteristics as reciprocating pumps, and will pump against high heads. They have not proved popular for water supply work as they wear badly if the water contains sand or grit. Their main use is in hydraulic systems and sprayers, or for the pumping of viscous fluids like molasses. Centrifugal Pumps Construction The rotating part of this type of pump is called the impeller. This impeller may be shaped either to force the water from the inlet at the centre of the impeller outwards, at a right angle to the pump axis this is called radial flow or to force the water in the direction of the pump axis axial flow. Axial flow machines are more correctly referred to as propeller pumps. The impeller, fitted with blades or vanes usually curved backwards towards the tips, is fixed to a spindle, and rotates rapidly inside a 7

8 casing. The impeller may be of the closed type where the vanes are mounted between discs, or of the open type consisting of a hub to which the vanes are attached. The open impeller does not have such a high efficiency as the closed type, but is less likely to become clogged and hence is better adapted to handling liquids containing solids. The casing of centrifugal pumps may be of the volute or turbine type. In the volute the casing gradually increases in cross-section, causing the velocity head of the water leaving the impeller to be changed into pressure head as the velocity is reduced towards the outlet. In the turbine, or diffuser type, a series of fixed blades has the same effect: this type is not used much in water supply or irrigation, except in deep well pumps, where the turbine casing shape lends itself to a cylindrical body. The difference in casing shape is shown in Figures 7 and 8. Figure 7 Figure 8 Volute Centrifugal Pump Turbine Centrifugal Pump Centrifugal pumps give their best performance at a certain rate of discharge and head, and their efficiency falls off when these conditions are varied. The head against which a centrifugal pump with a single impeller called a Single-Stage Pump will pump satisfactorily is usually not very great, less than 50 metres without undesirably high speeds. For higher heads a Multi-Stage Pump is used, with two or more impellers arranged in such a way that the discharge from one impeller enters the centre, or eye, or the next 8

9 impeller so that delivery head is increased in stages see Figure 9. This principle is used in many submersible pumps, where pump and motor are both placed below the water surface. Delivery to the surface is through a riser pipe on which the assembly is suspended. Figure 9 Multi-stage Centrifugal Pump Centrifugal pumps need to be primed before starting unless they are located below the source of water. The suction performance of centrifugal pumps is not very good, generally less than 5 metres, and they should be placed as near to the water as possible. Proper arrangement of the piping is essential if the pump is to operate at its greatest efficiency. Figure 10 shows a typical centrifugal pump installation. Figure 10 Typical Centrifugal Pump Installation 9

10 A propeller-type pump has only two or four blades, and thus has large unobstructed passages which permit the handling of water containing debris without clogging. On larger pumps the blades are adjustable for maximum efficiency. A typical installation is shown in Figure 11. Figures 11 Typical Propeller Pump Installation Performance For a centrifugal or propeller pump, the delivery, head and power input all increase as speed is increased. The delivery is proportional to speed; head is proportional to the square of the speed; power input is proportional to the cube of the speed. This means that if we double the speed, the delivery doubles, the head is increased 4 times, and the power input is increased 8 times. A typical performance curve for a centrifugal pump, as obtained in a laboratory test, would appear as Figure

11 H e a d (M e t r e s ) P o w e r P o w e r (k W ) E f f i c i e n c y (% ) E f f i c i e n c y H e a d D i s c h a r g e ( 1 / m i n ) Figure 12 Typical Performance Curve Centrifugal Pump Repeating the test over the complete speed range of the pump gives an overall picture of the operational characteristics of the pump. Unlike positive displacement pumps, the head developed by a centrifugal pump varies with the flow or discharge produced by the pump. In other words the pump can pump a lot of water at a lower head or a smaller amount of water at a higher head. The exact flow and head is determined by the interaction of the pump and pipe system (more about this next year). It must be noted also that there is a particular flow and head at which the pump operates most efficiently. 11

12 3.1 Summary of Pump Characteristics Advantages Reciprocating Positive action. Pumps Efficient over a wide range of delivery and head. Rotary Pumps Positive action. Occupy little space. Wide range of speed. Steady discharge. Centrifugal Pumps Simple design. Quiet operation. Steady discharge. Efficient for pumping large volumes. Suitable for direct connection to electric motor. May be either horizontal or vertical. Can be multi-staged. Propeller Pumps Generally as above. Usually vertical mounting. Pump very large quantities at low heads. Widely used for land drainage and flood control. Disadvantages Discharge pulsates. Subject to vibration. Sometimes noisy. Subject to abrasion. Likely to get noisy. 4.0 PUMP CONTROLS 4.1 Pressure Tanks Historically pressure tanks were the only means of controlling pumps. With the advent of other control systems, pressure tanks are losing favour but there are still large numbers in use today. Pressure tanks operate by pumping water into the tank and compressing the air trapped at the top of the tank. The pumping operation continues 12

13 until the air pressure reaches the preset level on the pressure switch and the pump cuts out. During the initial stages of water draw-off the air in the tank forces the water out of the tank into the pipe system. When the air pressure drops to a preset point the pressure switch starts the pump. The time between stopping and starting of the pump is known as the cycle time. Most people assume that the buffer storage capacity of the pressure tank is equal to the volume of the tank. This, in fact, is incorrect. The volume of water stored between cut in and cut out is controlled by Boyle s Law. This law states that the product of the initial pressure and volume is equal to the product of the final pressure and volume, i.e. P i V i = P f V f where i = initial f = final For example: Assuming that a 300 litre pressure tank is empty at atmospheric pressure, and that the cut in pressure is 5 bars absolute (i.e. not gauge pressure). Therefore: P i = 1 bar absolute V i = 300 litres of air P f = 5 bar absolute V f =? from P i V i = P f V f Vf = P V i i Pf = 1 x = 60 litres of air That is the air which did occupy 300 litres now occupies only 60 litres. So the volume of water in the tank is = litres. 13

14 = litres = 240 litres of water stored If the cut out pressure is set at 7 bar absolute, then: P i = 5 bar absolute V i = 60 litres of air P f = 7 bar absolute V f =? from P i V i = P f V f Vf = P V i i Pf 5 bar x 60 litres = 7 bar = 42.8 litres of air That is the air now occupies 42.8 litres, so the volume of water in the tank = = litres of air = litres The amount of water stored between cut in and cut out is the difference between these two numbers, i.e. volume of water stored = = 17.1 litres The volume stored is known as the Effective Water Storage (EWS) and in this case is considerably smaller than the volume of the tank. These calculations can be represented graphically (Figure 13). Note that precharging the cylinder with air significantly increases the EWS. 14

15 7 C u t o u t 6 5 C u t i n 4 C y l i n d e r p r e - c h a r g e d t o 3 b a r a b s o l u t e N o p r e c h a r g i n g. E W S E W S = 1 7 l i t r e s P r e s s u r e t a n k v o l u m e ( l i t r e s ) Figure 13 Pressure/Volume Relationship in a Pressure tank The disadvantage of pressure tanks system is that air at high pressure dissolves into the water with the result that over a period of time the water will fill the air space. This is known as water logging and reduces the EWS, decreases the cycle time and increases the cycle frequency. Rapid cycling (pump starting and stopping frequently) increases the wear and tear on the pump motor and can lead to overheating problems. 15

16 4.2 Aquacells The low EWS and water logging characteristic of the pressure tank led to the development of the aquacell (Figure 14). Since the aquacell only provides a means of operating the pressure switch, its size is kept to a minimum. The rubber bladder prevents water logging. The effect of a small EWS (3-5 litres) with large capacity pumps means that cycling times will be small. The inclusion of a modified check valve (Figure 15) will help overcome the problem. Figure 14 Aquacell Pressure Unit Figure 15 Operation of Modified Check Valve 16

17 The modified check valves allow full flow when the aquacell is draining but restricts the flow during filling, therefore increasing filling time and cycling time. P r o b e C a b l e k m P r o b e s i g n a l a c t i v a t e s s t a r t /s t o p s y s t e m 6 v o l t A / C p o w e r s u p p l y Motor M o t o r 3 phase power 3 p o w e r H i g h L o w P u m p N e u t r a l Figure 16 Probe Control System 4.3 Probe Control (Figure 16) In situations where water is being pumped to a storage tank the water level in the tank can be monitored using probes. The probes are set to detect maximum level (pump cut out) and minimum level (cut in). The third probe is a neutral probe. Electrical contact between the probes relies on the conductivity of the water. 4.4 Flow Sensing Control The low flow control has a valve which isolates the pressure switch from the main line at flows greater than 3-5 litres/minute, i.e. at flows above these, regardless of mainline pressure, the pump will not switch off. The potential exists for the pump to operate at or near maximum head and therefore pipelines must be able to withstand these pressures. 17

18 Recent developments have produced a flow sensing control unit which can eliminate the need for storage vessels. The use is made of two electronic sensors one of which detects the initial drop in pressure within the line when a demand for water is made. This first sensor starts the pump which satisfies the flow requirements but raises the pressure. But as soon as flow starts from the pump a lightly spring loaded valve is moved to a position where the second sensor can electronically override the pressure sensor and so flow will continue regardless of the demand, down to a minimum flow of about 20 litres/hr these units are only suitable for centrifugal pumps. A strong advantage of these units over a pressure switch is that if no water is able to flow for some reason, the pump will only run for a few seconds before switching off so preventing pump damage. This protective switching requires manual resetting after the flow problem is solved. Figure 17 Flow Sensing Pump Control 4.5 Float Switch Control Modern float control switches enable tank levels to be controlled by a simple plastic float with a mercury switch within and attached to a waterproof electrical cable. This cable is secured to the roof of the tank with an adjustable clamp which allows level control. When the level of water is low the float hangs from the cable and the switch causes the pump to start. As the water level rises the unit floats on the water and when it eventually comes to a horizontal position the pump is switched off. 18

19 Figure 18 Float Switch Pump Control 19