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Back Mechanical Mechanical Engineering Dictionary Hydraulics Introduction to Pumps - Mechanism of Work and Equations

Introduction to Pumps - Mechanism of Work and Equations


Pump is used to transform mechanical work into fluid energy. It is a mechanical device. Pumps increase static fluid pressure (increase flow energy) so that steady flow can be maintained. The steady flow rate through a given system depends on physical characteristics of the system (the fluid and its boundaries) and upon the characteristics of the pump(s) in that system. Pump has two major point pumps suction area and pump discharge area. Through suction it get mechanical work as input and delivered energy with the discharge area as output. Centrifugal pumps are the most popular types because of their simplicity, low cost, and ability to operate over a wide variety of conditions.

Work done on the fluid is result to increase in fluid energy is known as pump head. It is expressed in unit of feet. Fluid flow through the system is to decrease in fluid flow energy is known as Head Loss. It is also expressed in unit of feet.

An expression for pump work, expressed in terms of fluid properties, can be derived from the General Energy Equation (GEE). If a system consists of constant mass flow rate of an incompressible fluid as it move from the pump section to the pump discharge. The general energy equation (GEE) will be used to derive the formula for solving pump work.

KE1 + PE1 + FE1 + U1 + Won + Qin = KE2 + PE2 + FE2 + U2 + Wby + Qout

In this equation the fluid kinetic energy, fluid potential energy, fluid flow energy, internal energy and work done to the fluid and heat added to the fluid (enter to the pump) is equal to the fluid kinetic energy, fluid potential energy, fluid flow energy, internal energy and work done by the fluid and heat added to the fluid (leave to the pump).

Rate of work done by the pump is the power of pump or it is the work done by the pump to maintain steady flow in a fluid flow system. If the fluid mass flow rate through the pump is multiplied by the specific work (work per pound mass of fluid) done by the pump, the pump power is obtained. Pump fluid work was defined as the increase in fluid flow energy as it passes through the pump. Therefore, pump fluid power is the product of the fluid’s mass flow rate and its flow energy increase through the pump. Relationship of head to pressure can be determined by the ratio of head by the fluid specific volume. Pump efficiency is defined, As the ratio of the power delivered to the fluid (fluid horsepower) to the power supplied to the pump shaft (brake horsepower), and is denoted by the Greek letter ? (eta).

In order to increase the volumetric flow rate in a system, or to compensate for larger flow resistances, pumps are often used in parallel or series. When two pumps are operating in parallel, each pump is generating the same pump head. The system flow rate (total flow past point 2) will be the sum of the flow rates through each pump. Centrifugal pumps are used in series to overcome a larger system head loss than one pump can overcome individually. Centrifugal pumps operating in series have the same volumetric flow rate through each pump, and the head added to the flow is the sum of the heads provided by each pump.

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