Lab Report Pump

1. INTRODUCTION

Pumps can be classified according to their basic operating principle as dynamic or displacement pumps. Dynamic pumps can be sub-classified as centrifugal and special effect pumps. Centrifugal pumps are widely used because they are generally are the most economical. A centrifugal pump is of a very simple design. The two main parts of the pump are the impeller and the diffuser. Impeller, which is the only moving part, is attached to a shaft and driven by a motor. Impellers are generally made of bronze, polycarbonate, cast iron, stainless steel as well as other materials. The diffuser (also called as volute) houses the impeller and captures and directs the water off the impeller. Water enters the center (eye) of the impeller and exits the impeller with the help of centrifugal force. As waterleaves the eye of the impeller a low-pressure area is created, causing more water to flow into the eye. Atmospheric pressure and centrifugal force cause this to happen. Velocity is developed as the water flows through the impeller spinning at high speed. The water velocity is collected by the diffuser and converted to pressure by specially designed passage ways that direct the flow to the discharge of the pump, or to the next impeller should the pump have a multi-stage configuration. The pressure (head) that a pump will develop is indirect relationship to the impeller diameter, the number of impellers, the size of impeller eye, and shaft speed. Capacity is determined by the exit width of the impeller. The head and capacity are the main factors, which affect the horsepower size of the motor to be used. The more the quantity of water to be pumped, the more energy is required.

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1.                OBJECTIVE

The objective of this experiment is to study the performance characteristics of a typical centrifugal pump. The test set-up can be run with a single pump, two pumps in series, or two pumps in parallel. The performance curves obtained will include pump head, power input, and efficiency as a function of flow rate for different pump speeds.  

1. THEORY

The energy created by the pump is kinetic energy according Bernoulli Equation. The energy transferred to the liquid corresponds to the velocity at the edge or vine tip of the impeller. The faster the impeller revolves or the bigger impeller is, the higher will the velocity of the liquid energy transferred to the liquid be. It is described by Affinity Laws.

The head of the pump in metric units can be expressed in metric as units as:

h= (p2-p1)/(ρg) + v22/ (2g)

where

h = total head developed (m)

p2 = pressure at outlet (N/m2)

p1 = pressure at inlet (N/m2)

ρ = density (kg/m3)

g = acceleration of gravity (9.81) m/s2

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 to drive a pump (Pi), is defined by:

Pi = (ρ g H Q)/ ɳ

where

Pi = input power required (W)

       ρ = fluid density (kg/m3)

       H = pump head (m)

       Q = flow rate (m3/s)

Flow rate, Q = (0.196 √h) × 10−3m3/s

      h = difference mercury reading, h (mm)

Power inlet, W1 =   W[pic 2]

    Where

      f = force in Newton

      velocity (V) = in rounds per minute

      Efficiency,

ɳ =  × 100(%)[pic 3]

The "Performance Characteristics" of a pump at a fixed speed are represented by the following graphical relationships:

Total Head (HP) versus Discharge (Q)

Power Input (W) versus Discharge (Q)

Efficiency (ɳ %) versus Discharge (Q)

Series Pump

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In this type of arrangement all the flow successively passes from one pump to the next with each pump adding more energy to the water. This is a typical arrangement in multi-stage turbine or submersible pump where the same discharge passes through all stages and each builds additional head. Often, series configurations are used when head requirements of the system exceed that which can be supplied by individual pumps. They are also used in systems with variable head requirements. A typical example is a small centrifugal pump used as a booster pump for corner irrigation on a center pivot system or, for that matter, any booster pump, in any water sys- tem, which works in addition to the main water pump. Figure 14 shows head-discharge curves for two pumps operating in series

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Parallel Pump

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A typical example of this arrangement is a situation where two or more pumps draw water from a single source and all the flows are discharged into a single pipe. Another example is a situation where several small wells are providing the required discharge. Parallel arrangements are also common methods of meeting variable discharge requirements of the system. Figure 16 shows a head-discharge characteristic curve for two pumps operating in parallel (Refer Appendix)

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When pumps operate in series, the flowrate is the same as for a single similar pump but the total head is increased. The combined pump head-capacity curve is found by adding the heads of the single pump curves at the same flow rate. For similar pumps twice the head gain for a single pump. The total head of the parallel pump increase remains unchanged but the flow rate is increased. The head-capacity curve is found by adding the flow rates of the single pump curves at the same head. For similar pumps twice the flow rate of a single pump.