PERFORMANCE CHARACTERISTICS OF CENTRIFUGAL PUMPS

PERFORMANCE CHARACTERISTICS OF CENTRIFUGAL PUMPS

Instructed By

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Group Members :-

Mr. P.D.R. Lalantha R.S.V. Piyasena P.P.G.C. Prasanna R. Prasanthan M.G.M.M. Premathilaka T.G.P. Priyadarshana G.P.N.S.G. Punchihewa O.C. Ranawaka U.I. Ranganath   Name Index No Course Module Practical No Date of conduct Date of submission

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:- G.P.N.S.G.Punchihewa 080391T B.Sc. ENG. Mechanical ME 3020 30/09/2010 30/09 /2010 28/10/2010

Objective: To determine performance c haracteristics of centrifugal pumps.

Apparatus: y

Centrifugal pump apparatus bench

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Stop watch

Procedure: Operation was done for a centrifugal pump. A pump was operated with control rate of flow. This was done in two ways. One is varying the inlet ball valve and other is varying the outlet  ball valve. To assure the exact flow rate, pipe reading with stop watch was taken for each case. Correspondingly, measurements of pressures were ta ken for suction and delivery side. Also corresponding current was observed.

Theory: A centrifugal pump converts the input power to kinetic energy in the liquid by accelerating the liquid by a revolving device an impeller. Fluid enters the pump through the eye of the impeller  which rotates at high speed. The fluid is accelerated radially outward from the pump chasing. A vacuum is created at the impellers eye that continuously draws more fluid into the pump. The energy created by the pump is kinetic energy according the Bernoulli Equation. The energy transferred to the liquid corresponds to the velocity at t he edge or vane tip of the impeller. The faster the i mpeller revolves or the bigger the impeller, the higher the velocity of  the liquid energy transferred to the liquid. This is described by the Affinity Laws. It is important to understand that the pump will pump all fluids to the same height if the shaft is turning at the same rpm. Centrifugal Pumps are "c onstant head machines". The head of a pump in metric units can be expressed in metric units as: h = (p2  p1)/( g) + v22/(2 g)

Where; h = total head developed (m)  p2 = pressure at outlet (N/ m2)  p1 = pressure at inlet (N/m2)  = density (kg/m3) g = acceleration of gravity (9.81) m/s2 v2 = velocity at the outlet (m/s)

Energy Usage: The energy usage in a pumping installation is determined by the flow required, the height lifted and the length and characteristics of the pipeline. The power required to drive a pump (P i), is defined simply using SI units by:

Where; Pi - The input power required (W)  - The fluid density (kg/m3 ) g - The gravitational constant (9.81 m/s2) H - The energy Head added to the flow (m) 3

Q - The flow rate (m /s)  - The efficiency of the pump plant as a decimal The head added by the pump (H) is a sum of the static lift, the head loss due to friction and any losses due to valves or pipe bends all expressed in meters of water. Power is more commonly expressed as kW (103 W) or horsepower. The value for the pump efficiency  may be stated for the pump itself or as a combined efficiency of the pump and motor system. The energy usage is determined by multiplying the power requirement by the length of time the  pump is operating.

Performance curve: The performance curve is the easiest and most satisfactory way to show graphically the relationship between head, capacity, horsepower, etc., of any pump. For a given rotational speed and impeller size, the performance of a pump can be represented on a head  capacity curve of total developed head in feet of water versus flow in gallons per minute. Total dynamic head (TDH) is the difference between suction and disc harge pressure and includes the difference between the velocity head at the suction and dischar ge connection. The lines sloping downward from left to right represent the varying quantities of water delivered by the pump with variations in head or pressure for a given impeller size. The intersection of this line with zero delivery line shows the ³shut off head´, which is the  pressure developed by the pump when the discharge valve is shut. Starting from the shut off  head, as the pump delivers more water, the mechanical efficiency of the pump increases until a ³best efficiency point´ (BEP) is reached. Increasing the flow further decreases the efficiency until a point known as ³end of curve´ where the manufacturer no longer publishes the  performance. As the impeller gets s maller, the pump efficiency also decreases.

The power requirements are also shown on the performance curve. The horsepower line that does not cross the pump curve is called ³non  overloading´ horsepower because operation at any point on the published pump curve will not overload the motor.

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Discussion:  About the experiment:From the experiments, characteristics of pumps ran at different flow rate ar e observed and drawn in graphs. These characteristics curves are essential in defining the properties of a pump, since only size and shape cannot be sufficient to select a pump for certain purpose. Standard test were done according to the procedures defined by the lab manual and characteristics curves are drawn. To increase the efficiency of pumps we can use multiple pumps instead of a single pump. We have studied the variation of head available with change in flow rate. With pumps in parallel we can increase the flow almost twice for the same head delivered, while head delivered can be increased twice by arranging pumps in series. Hence, where a single pump is inappropriate for  large flow rate or high head, pumps can be arranged in series and parallel or combination of   both to suite our requirement. The main aim of the practical was to draw up the characteristics curve. With the help of  characteristics curve and the actual field conditions (Head available, input power required, required flow rate). We should have to compromise the efficiency for t he fulfillment of the actual field condition situations. So the characteristics c urves help to optimize the field conditions and to select a particular type of pump or a combination of pump for a particular  site.  Features of centrifugal pumps:Some of the salient features of centrifugal pumps include; y

Consistent and reliable flow

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Improved productivity

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Reliable seal integrity

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Low life cycle cost

All pumps (both the centrifugal pumps and the positive-displacement pumps) have operational limits. Specifically, centrifugal pumps have certain limitations which, if not properly eva luated can drastically reduce their working life. The BEP (Best Efficiency Point) is not only the maximum operating point but it is also the point where the speed and pressure at the impeller  and the spiral stator are equal.

As the operating point diverges from the Best Efficiency Point, the speed changes which in turn modifies the pressure acting on one of the sides of the impeller. This irregular pressure on the impeller manifests itself as a radial thrust which deflects t he pump shaft causing, among other things:

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an excessive load on the bearings

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an excessive deflection of the mechanical seal

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irregular wear on the shaft bearing

The damages that might occur then consist of a shorter bearing life or a broken shaft. Radial loading is maximized when everything closed. If operating outside of the recommended operating range, damage to the pump could be caused by excessive speed and turbulence. Vortexes can create cavitations damage which very quickly can destroy the pump and impeller  casing.  Advantages and disadvantages of centrifugal pumps:The advantages of centrifugal pumps include simplicity, compactness, weight saving, and adaptability to high-speed prime movers. One disadvantage of centrifugal pumps is their  relatively poor suction power. When the pump end is dry, the rotation of the impeller, even at high speeds, is simply not sufficient to lift liquid into the pump; therefore, the pump must be  primed before pumping can begin. For this reason, the suction lines and inlets of most centrifugal pumps are placed below the source level of the liquid pumped. The pump can then  be primed by merely opening the suction stop valve and allowing the force of gravity to fill the  pump with liquid. The static pressure of the liquid above the pump also adds to the suction  pressure developed by the pump while it is in operation. Another disadvantage of centrifugal  pumps is that they develop cavitations. Cavitations occurs when the velocity of a liquid increases to the point where the consequent pressure drop reaches the pressure of vaporization of the liquid. When this happens, vapor pockets, or bubbles, form in the liquid and then later  collapse when subjected to higher pressure at some other point in the flow. The collapse of the vapor bubbles can take place with considerable force. This effect, coupled with the rather  corrosive action of the vapor bubbles moving at high speed, can severely pit and corrode impeller surfaces and sometimes even the pump casing. In extreme instances, cavitation has caused structural failure of the impeller blades.

Whenever cavitation occurs, it is frequently signaled by a clearly audible noise and vibration (caused by the violent collapse of vapor bubbles in the pump). Several conditions can cause cavitation, not the least of which is improper design of the pump or pumping system. For  example, if the suction pressure is abnormally low (caused perhaps by high suction lift or  friction losses in the suction piping), the subsequent pressure drop across the impellers may be sufficient to 6-19 reach the pressure of vaporization. A remedy might be to alter the pump design by installing larger piping to reduce friction loss or by installing a foot valve to reduce suction lift. Cavitation can also be caused by improper operation of the pump. For instance, cavitation can occur when sudden and large demands for liquid are made upon the pump. As the liquid discharged from the pump is rapidly distr ibuted and used downstream, a suction effect is created on the discharge side of the pump. Think of it as a pulling action on the discharge side that serves to increase the velocity of the liquid flowing through the pump. Thus, as the pressure head on the discharge decreases, t he velocity of the liquid flowing across the impellers increases to the point where cavitation takes place. Perhaps the easiest way to avoid this condition is to regulate the liquid demand. If this is not possible, then increas e the suction pressure by some means to maintain pressure in the pump under these conditions.