Pumps Introduction A pump converts the mechanical energy to hydraulic energy. The mechanical energy to the pump is

delivered by the prime mover, like an electric motor. Due to mechanical action pump creates a partial

pressure at its inlet, and it permits the atmospheric pressure to force the liquid into its inlet, the pump

then pushes the liquid into the discharge line. [1]

Pumps are broadly classified into two basic types

Displacement pumps

Centrifugal pumps

Displacement Pumps A displacement pump (also called positive-displacement, or just p-d) is a pump which imparts energy

to the pump-age (the material pumped) by trapping a fixed volume at suction (inlet) conditions,

compressing it to discharge pressure, then pushing it into the discharge (outlet) line.

Displacement pumps fall into two major classes: reciprocating and rotary as explained by the figure.

Figure 1 Classification of Displacement Pumps [2]

Uses and application Displacement pumps serve primarily in applications of low capacity and high pressure, those mostly

beyond the capabilities of centrifugal pumps. Some of these services could be performed by

centrifugal, but not without an increase in power requirements and /or maintenance. Because

displacement pumps achieve high pressures with low pumpage velocities, they are well-suited for

abrasive-slurry and high-viscosity services. A reciprocating pump must have special fittings to be

suitable for most slurries. [2]

Reciprocating Pump A reciprocating pump is a displacement pump which reciprocates the pumping element (piston,

plunger, or diaphragm). The capacity of a reciprocating pump is proportional to its speed, and is

relatively independent of discharge pressure.

Principle of Operation

Check the check valves in the discharge and the suction line, which allow flow in only one direction.

Displacement Pumps Reciprocating Pump

Rotary Pump

During the discharge stroke, piston moves to the right seating the check valve in the suction line and opening the check valve in the discharge line.

During the suction stroke, the check valve in the suction line opens and admits the water from the reservoir into the pump and closes the check valve in the discharge line.

Classification

Reciprocating Positive Displacement pumps are categorized in four following ways:

Direct acting or indirect acting pump

Simplex or Duplex

Single acting or double acting

Power Pumps

Direct and Indirect Acting Pump

Direct acting Pumps have a Plunger on the liquid end that is directly driven by the pump rod (piston

rod) and carries the piston of the power end.

Indirect acting pumps are driven by beams or linkage connected to and actuated by the power piston

rod of a separate reciprocating engine.

Direct acting Pump

In-direct acting Pump

Simplex and Duplex Pump

A Simplex pump sometimes referred to as a single pump, is a pump having a single liquid (pump)

cylinder.

A Duplex Pump is equivalent of two simplex pump placed side by side on the same foundation.

The driving of the pistons of a duplex pump is arranged in such a manner that when one piston is on

its upstroke (discharge) the other piston is on its down stroke (suction), and vice versa. This

arrangement doubles the capacity of a duplex pump compared to a simplex pump of comparable

design.

Single acting or double acting Pumps

A single – acting pump is one that takes a suction, filling the pump cylinder on the stroke in only one

direction, called the suction stroke, and then forces the liquid out of the cylinder on the return

stroke, called the discharge stroke.

A double – acting Pump is one that, as it fills one end of the liquid cylinder, is discharging liquid from

the other end of the cylinder. On the return stroke, the end of the cylinder just emptied is filled, and

the end just filled is emptied.

Power Pumps

Power pumps convert rotary motion (from prime mover) to low speed reciprocating motion by

reduction gearing, a crankshaft, and connecting rods and cross heads.

Plunger or pistons are driven by the crosshead drivers. Rod and Piston construction, similar to Duplex

double acting steam pumps, is used by the liquid ends of the low pressure, high capacity units.

The higher pressure units are normally single acting plunger and usually employ three (Triplex)

plungers. Three or more plungers substantially reduce flow pulsations relative to simplex and even

duplex pumps.

Power pumps typically have high efficiency and are capable of developing very high pressures. They

can be driven by either electric motors or turbines.

They are relatively expensive pumps and can really be justified on the basis of efficiency over

centrifugal pumps. In general, the effective flow rate of reciprocating pumps decrease as the viscosity

of the fluid being pumped increases because the speed of the pump must be reduced. [3]

In contrast to centrifugal pumps, the differential pressure generated by reciprocating pumps is

independent of fluid density. It is dependent entirely on the amount of force exerted on the Piston.

Problems and their solutions Reciprocating pumps have some disadvantages, the most common being pulsating flow. Because of

the pulsation, special consideration must be given to system design.

In most applications, the initial and maintenance costs for a reciprocating pump will be greater than

for a centrifugal or rotary pump. The packing in a typical power pump lasts about 2,500 hours, less

than a mechanical seal on a rotating shaft.

Most problems with reciprocating pumps can be minimized by selecting pumps to operate at

conservative speeds, by carefully designing the pumping system, by careful operating procedures, and

by maintenance practices which preserve the alignment of the plunger (or rod) with the stuffing box.

[2]

Discharge of a reciprocating Pump Discharge of a single acting reciprocating pump is given by the formula,

𝑄 =𝐿𝐴𝑁

60 𝑚3 𝑠⁄

What will be the discharge of a double acting pump?

Discharge of a double acting reciprocating pump is given by the formula,

𝑄 =2𝐿𝐴𝑁

60 𝑚3 𝑠⁄

What is the slip of a pump and negative slip?

In practice, the actual discharge is less than the theoretical discharge. The difference between the

theoretical and actual discharge is called slip of the Pump.

Sometimes, the actual discharge of the reciprocating pump, is more than the theoretical discharge. In

such cases, the coefficient of discharge is more than unity and the corresponding slip is known as

negative slip of the pump.

Centrifugal Pumps The Pump which displaces the pumpage by the action of centrifugal force is known as centrifugal

Pumps. [5]

Main parts of Centrifugal Pumps Following are the main parts of the pump,

Impeller

Casing

Suction line

Discharge line

Impeller The rotating part of a centrifugal pump is called impeller. It consists of a series of backward curved

vanes. The impeller is mounted on a shaft which is connected to the shaft of an electric motor (or a

prime mover).

Casing The casing of a centrifugal pump is an air tight passage surrounding the impeller and is designed in

such a way that the kinetic energy of the water discharged at the outlet of the impeller is converted

into the pressure energy before the water leaves the casing and enters into the delivery pipe. The

following are the three types of the casings are commonly applied:

Volute casing

Vortex casing

Casing with the guide blades.

Volute casing

It is of spiral type in which area of flow increases gradually. The increase in the area of flow, decreases

the velocity of the flow. The decrease in the velocity of the flow increases its pressure. It has been

observed that in the case of volute casing, the efficiency of the pump is lower as compared to other

casings because of the formation of the eddies.

Vortex casing

If a circular chamber is introduced between the casing and the impeller as shown in the figure. The

casing is known as the Vortex Casing. By introducing the circular chamber, the loss of energy due to

the formation of eddies is reduced to a considerable extent, thus the efficiency of the pump is more

than the efficiency when only volute casing is provided.

Volute Casing Vortex Casing Casing with Guide Blades

Casing with Guide Blades

This casing is shown in Figure in which the impeller is surrounded by a series of guide blades mounted

on a ring which is known as diffuser. The guide vanes are designed in such a way that the water from

the impeller enters the guide vanes without stock.

Suction Pipe with a foot Valve

A pipe whose one end is connected to the inlet of the pump and other end dips into water in a sump

is known as suction pipe. A foot valve which is a non-return valve is fitted at the lower end of the

suction pipe. The Foot valve opens only in the upward direction. A Strainer is also fitted at the lower

end of the suction pipe.

Discharge Pipe

A pipe whose one end is connected to the outlet of the pump and the other end delivers the water at

a required height is known as discharge pipe.

How a centrifugal pump works Pump creates partial pressure at the inlet of the pump, which forces the liquid to be sucked into the

inlet. The liquid then enters the impeller, through which they are given the kinetic energy as they

rotate with the vanes and have the increased kinetic energy at the end of the impeller edge, this kinetic

energy is then converted to the pressure energy in the casing surrounding the impeller, providing the

air tight passage.

Figure 2 Working of A Centrifugal Pump

Classification of Centrifugal Pump The manner in which fluid flows through the pump is determined by the design of the pump casing

and the impeller. The three types of flow through a centrifugal pump are

Radial flow

Axial flow

Mixed flow

Radial Flow In a radial flow pump, the liquid enters at the center of the impeller and is directed out along the

impeller blades in a direction at right angles to the pump shaft. The impeller of a typical radial flow

pump and the flow through a radial flow pump are shown in figure.

Figure 3 Radial flow centrifugal pump

Axial flow pumps In an axial flow pump, the impeller pushes the liquid in a direction parallel to the pump shaft. Axial

flow pumps are sometimes called propeller pumps because they operate essentially the same as the

propeller of a boat. The impeller of a typical axial flow pump and the flow through a radial flow pump

are shown in figure.

Figure 4 Axial Flow Pump

Mixed flow pump Mixed flow pumps borrow characteristics from radial and axial flow pumps. As liquid flows though the

impeller of a mixed flow pump, the impeller blades push the liquid out away from the pump shaft and

to the pump suction at an angle greater than 900.

The impeller of atypical flow pump and the flow through a mixed flow pump are shown in the figure.

Figure 5 mixed flow centrifugal pump

Multi-stage Centrifugal pump A pump stage is defined as that portion of a centrifugal pump consisting of one impeller and its

associated components. Most centrifugal pumps are single-stage pumps, containing only one

impeller.

A pump containing seven impellers within a single casing would be referred to as seven-stage pump

or generally considered as a multi-stage pump. A centrifugal pump with a single impeller that can

develop a differential pressure of more than 150 psig between the suction and discharge is difficult

and costly to design and construct.

A more economical approach to develop high pressure with a single centrifugal pump is to include

multiple impellers on a common shaft within the same pump casing at moderate speed. Internal

channels in the pump casing route the discharge of one impeller to the suction of another impeller.

The water enters the pump from the top left and passes through each of the four impeller in series,

going from left to right. The water goes from the volute surrounding the discharge of one impeller to

the suction of the next impeller.

Figure 6 multistage centrifugal pump

Efficiencies of a Centrifugal Pump A centrifugal pump has following three types of efficiencies

Manometric efficiency

Mechanical efficiency

Overall efficiency

Manometric efficiency It is the ratio of Manometric head to the energy supplied by the impeller/KN of the water.

Mechanical Efficiency It is the ratio of the energy available at the impeller, to the energy given to the impeller by the prime

mover.

Overall efficiency It is the ratio of actual work done by the pump, to the energy supplied to the pump by the prime

mover.

Frequently asked questions Q. 1. Difference between plunger and pistons pumps?

Plunger pumps share the same operating principles of the piston pumps but use a plunger instead of

a piston in the cylinder cavity. However, the plunger pumps can provide higher pressure conditions

than the piston pumps ranging up to 200MPa.

• Plungers have solid plunger instead of a piston inside the cylinder cavity.

• Plunger pumps produce pressures up to 200MPa, while piston pumps produce pressure at a

maximum of 150Mpa. [4]

Q.2. what is priming?

When first put into service, the waterways are filled with air. If the suction supply is above

atmospheric pressure that is the suction head is positive, then this air will be trapped in the pump

and form an air lock.

References

[1] A. Esposito, Fluid power with applications, PearsonEducation.

[2] I. J. K. J. L. B. T. L. HENSHAW, “Fans, Pumps, and Compressors”.

[3] “slide share,” [Online]. Available: http://www.slideshare.net/orgasmic/positive-displacement-

pumps. [Accessed 11 05 2015].

[4] R. S. Khurmi, A Textbook of Hydraulics, Fluid Mechanics and Hydraulic Machines.

[5] “differencebetween,” [Online]. Available: http://www.differencebetween.com/difference-

between-piston-and-vs-plunger/. [Accessed 13 05 2015].