Basic of Pumps

Category : IPCL Module No. : Mechanical NC : Training Module IPCLDSMEC001

Prepared by : SUMANT KUMAR Reviewed by : PGD Approved by : AKS

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INDIAN PETROCHEMICALS CORPORATION LIMITED

NAGOTHANE

TRAINING MODULE FOR

BASICS OF PUMPS

LEARNING CENTRE

NC



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MODULE IMPEMENTATION PLAN

TOPIC: BASICS OF PUMPS

CODE NO : IPCLCB071

FOR: SELF STUDY DATE:

REV:01 SITE:NAGOTHANE

SR NO

CONTENT

AUTHOR

RESOURCES AVAILABLE

LEARNING

VALIDATION

TRAINER

1 Introduction and classification

Hydraulic institute journal

N Self study

SK

2 Definition Pump handbook by F.POLLAK,KARRASIK

N Self Study

SK

3 Selection of pump

Pump handbook by F.POLLAK,KARRASIK

N Self Study

SK

4 Centrifugal Pump

Pump handbook by KARRASIK

N Self Study

SK

5 Reciprocating pump

Pump handbook by F.POLLAK

N Self Study

SK

6 Diaphragm pump

Pump handbook by F.POLLAK

N Self Study

SK

7 Gear pump Pump handbook by F.POLLAK

N Self Study

SK

8 Lobe pump Pump handbook by F.POLLAK

N Self Study

SK

9 Screw pump Pump handbook by F.POLLAK

N Self Study

SK

10 Vane pump Pump handbook by F.POLLAK

N Self Study

SK

11 Maintenance of pumps

Pump handbook by F.POLLAK KARRASIK

N Self Study

Quiz Duration ½ hour



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TABLE OF CONTENTS

SECTION

NO.

DESCRIPTION PAGE

NO.

1 INTRODUCTION

1.1 INTRODUCTION & CLASSIFICATION

1.2 DEFINITION

1.3 SELECTION OF PUMP

1.4 STANDARDS AND CODES

2 BRIEF DESCRIPTION OF VARIOUS TYPES OF PUMPS

2.1 CENTRIFUGAL PUMP

2.2 RECIPROCATING PUMP

2.3 DIAPHRAM PUMP

2.4 GEAR PUMP

2.5 LOBE PUMP

2.6 SCREW PUMP

2.7 VANE PUMP

3 MAINTENANCE OF PUMPS



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OBJECTIVE

In any industry ,various types of pumps are used to meet different requirements

of liquid handling. To understand these equipments better, it is desirable to have

basic knowledge of various types of pumps regarding their operating principle,

construction and application.

This module is prepared with a view to provide its reader basic knowledge of

various types of pumps which are commonly used in a process plant.

Hope this module will serve the purpose.



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SECTION – 1

INTRODUCTION TO PUMP



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1.1 INTRODUCTION TO PUMPS :

Pumps convert mechanical energy input into hydraulic or fluid enegy. They

fall into two distinctive categories depending upon the way in which the

enegy is converted from high liquid velocity at the inlet into pressure head

in a diffusing passage. Dynamic pumps and positive displacement pumps.

Pump classification chart is shown:



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APPLICATION OF PUMPS

Application of suitable type of pump for a given service is dependent on various

factors. A guideline is given in the following figure. This represents upper limits

of pressure and capacity available currently. From the figure it is evident that the

reciprocating pumps run off the pressure scale whereas the centrifugal pumps

run off the capacity scale. Upper limits of pressure and capacity of pump class

are shown in the following figure.



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Most of the centrifugal pumps find their application in column bottom pumps,

vacuum tower separation, hydrocarbon processing, removal of sulfur and

nitrogen. They are used in steam power plants as boiler feed water pumps,

condensate circulating pumps, fuel oil systems. In fire services, centrifugal

pumps are used as firewater pumps and as fire water jockey pumps (Smaller

pump which maintains pressure in the distribution system during periods of low

demand).

Reciprocating pumps are adapted to function as metering pumps for transfer of

fluids. The flow rates can be varied by changing the displacement per stroke.

1.2 DEFINITIONS :

Pumping is the addition of energy to a liquid to move it from one point to another.

Reciprocating pumps use pistons, plungers, diaphragms or other devices to

displace a given volume of liquid during each stroke of unit.

Centrifugal pumps employ centrifugal force to develop a pressure rise for moving

a liquid.

Impeller is the rotating member in a centrifugal pump through which liquid passes

and by means of which energy is imparted to the liquid.

Casing of centrifugal pump is the housing surrounding the impeller.

Critical speed of a centrifugal pump is that speed of the rotating shaft

corresponds to its natural frequency. At this speed, any minor imbalance of the

shaft is magnified and excessive vibration will occur.



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Rotary pumps use gears, vanes, screws, cams etc. in a fixed casing to produce

positive displacement of a liquid.

Packing is any material used to control leakage between a moving and stationery

part in the pump.

Mechanical seals are devices mounted on the shaft of centrifugal pump to seal

the liquid in the casing.

Cavitation is the phenomenon caused by vapourisation of a liquid inside a pump.

Viscosity is property of the liquid tat resists any force tending to produce flow.

Specific gravity of a liquid is that number which denotes the ratio of the weight of

the liquid to the weight of the equal volume of water.

Net positive suction head required is the energy needed on the suction side of

the pump to fill the pump to the discharge valve during operation.

1.3 SELECTION OF PUMPS

Following are important criteria for selection of any pump :

1. Fluid handled : Depending upon the process involved, fluid properties

like viscosity, density, boiling point, corrosiveness influence the on pump

and system design. Influence of operating pressures, temperatures on the

fluid also needs to be studied.



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2. System head curves : A preliminary layout of the pumping system

should be drawn which will include equipment layout, piping and

instruction diagram, elevation, various components like valves, their sizes.

This will help in developing system head curve, which will include

hydraulic losses. All such losses in the piping may then be added in this

graph. User can then determine the total head requirement of the

pumping equipment in order to overcome the system resistance.

3. Mode of operation : Whether operation of pump is continuous or

intermittent ? Criticality of the pump in service, which will decide upon the

stand by equipment.

4. Margins : Margins should be available for pumping parameters like

capacity and head in case of any unforeseen problems like sudden

voltage dips or malfunction of the check valve. Future expansions would

alter the operating parameters, which need to be taken into account in the

design stage only.

5. Life of the equipment need to be considered while design considering

wear of parts like sleeves, liners in abrasive services.



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Following guidelines can be adapted for selection of any type of pump in general

:



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1.4 PUMP STANDARDS :

Pumps are generally procured and manufactured in conformance to various

standards, which stipulate the design considerations, materials, manufacturing

process, inspection and shipping etc. Standards generally used for hydrocarbon

liquid include API and ANSI. For non-hazardous services like water, IS

standards are generally used.

The nature and criticality of service decide application of a particular standard.

Generally a project standard is developed by the Engineering Consultant which

also considers the design standard of Process Licenser. However it is subjected

to approval of plant owners as these could have huge financial implications.

In recent times, many stringent clauses are included in the project standard

looking into the safety and environmental issues, which are assuming significant

importance worldwide.

A Quick check of existing pump standards will reveal that there are a number of

them. The list includes:

• ANSI (American National Standards Institute) Standards for Chemical

Pumps

• B73.1 for Horizontal type.

• B73.2 for Vertical Inline

• Hydraulic Institute Standards

• API (American Petroleum Institute) 682 for centrifugal Pumps

• API 674 for Reciprocating Pumps

• API 675 for Controlled Volume Pumps

• API 676 for Rotary Positive Displacement Pumps



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• ISO (International Standards Organization) aimed at the medium duty

single stage pumps ( Metric)

• DIN. West German industrial norm standard

• VDMA West German standard for pump seals



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SECTION – 2

BRIEF DESCRIPTION OF VARIOUS

TYPES OF PUMPS



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2.1 CENTRIFUGAL PUMPS :

Principle of operation

A centrifugal pump is a one of the simplest types of equipment in any

process plant. Its purpose is to convert energy of a prime mover first into

velocity or kinetic energy and then into pressure energy of a fluid that is

being pumped. The energy changes occur by virtue of two main parts of

the pump, the impeller and the volute of the diffuser. The impeller is the

rotating part that converts driver energy into the kinetic energy. The volute

or diffuser is the stationery part that converts the kinetic energy into

pressure energy.

Generation of centrifugal force: The process liquid enters the suction nozzle and then into suction eye of a

revolving device known as the impeller. When the impeller rotates ,it spins

the liquid sitting in the cavities between the vanes outwards and provides

centrifugal acceleration. As the liquid leaves the eye of the impeller, a low

pressure area is formed causing more liquid to flow towards the inlet.

Because the impeller blades are curved, the fluid is pushed in a tangential

and radial direction by the centrifugal force. Figure below depicts a side

cross section of a centrifugal pump indicating the movement of the liquid:



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PUMP OPERATING PARAMETERS

Now, let us discuss various other parameters that are specified for a pump

independent of pump types:

Head, Capacity, Power, Speed, Efficiency and NPSH

Head:

The pressure at any point in a liquid can be thought of as being caused by a

vertical column of the liquid due to its weight. The height of this column is called

as the static head and is expressed in terms of meters of liquid. Head of a pump

is used to measure the kinetic energy created by the pump. In other words ,

head is a measure meant of the height of the liquid column that the could create

from the kinetic energy imparted to the liquid. Head is not equivalent to the

pressure .Head has units of meters pressure has unit of force per unit area. The



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main reason for using head instead of pressure to measure a centrifugal pump

energy is that the pressure from a pump will change if the specific gravity of liquid

changes, but the head will not change. Since any centrifugal pump can move a

lot of different fluids, with different specific gravities, it is simpler to discuss pump

head than its pressure.

Conversion of kinetic energy into pressure energy:

The key idea is that the energy created by the centrifugal force is kinetic energy.

The amount of energy given to the liquid is proportional to the velocity at the

edge or vane tip of the impeller. The faster impeller revolves or the bigger the

impeller is, then the higher will be the velocity of the liquid at the vane tip and the

greater the energy imparted to the liquid.

The kinetic energy of a liquid coming out of an impeller is harnessed by creating

a resistance to flow. The first resistance is created by the pump volute that

catches the liquid and slows it down. In the discharge nozzle, the liquid further

decelerates and its velocity is converted to pressure according to Bernoulli’s

principle.

Therefore, the head developed is approximately equal to the velocity energy at

the periphery of the impeller.

Capacity

Capacity means the flow rate with which liquid is moved or pushed by the pump

to the desired point in the process. Quantity of flow is defined as the amount of

liquid passing through the pump in unit time. It is measured in m3/hr.

The capacity depends on number of factors like:



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Process liquid characteristics like density and viscosity

Size of the pump and its inlet and outlet sections

Impeller size

Impeller rotational speed RPM

Size and shape of the cavities between the vanes

Pump suction and discharge temperature and pressure conditions

Power

The work performed by a pump is a function of the total head and the weight of

the liquid pumped in a given time period. Pump input or brake power is the

actual power delivered to the pump shaft and pump output or the hydraulic power

is the liquid power delivered by the pump and is expressed in KW.

Efficiency:

Efficiency of the pump takes into account all the losses in the system. When

specifying a pump, the rated point should be at or to the left of the best efficiency

point. Efficiency is expressed as

Efficiency = Power used by the pump / power supplied to the pump

NPSH

The satisfactory operation of a pump requires that the vapourisation of the liquid

being pumped does not occur at any condition of operation. This is so desired

because when a liquid vapourises, its volume increases very much.



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When the pressure at the impeller eye goes down below the vapour pressure of

the fluid at which liquid begins to boil at the existing temperature, the liquid will no

longer remain in the state of liquid but in vapour state. These air bubbles are

carried along till they meet a region of higher pressure where they collapse

damaging pump internals. This phenomenon is called cavitation.

NPSH is a measure to prevent liquid vapourisation.

It is expressed as additional head required above the vapour pressure of the

liquid at the pump centre line.

Specific speed

Specific speed is a non dimensional design index that identifies the geometric

similarity of pumps. It is used to classify pump impellers as to their type and

proportions. Pumps of the same Ns but different size are considered to be

geometrically similar , one pump being a size factor of the other.

Specific speed identifies the approximate acceptable ratio of the impeller eye

diameter D1 to the maximum impeller diameter D2 in designing a good impeller.

In British system,

Ns:500 to 5000 D1/D2>1.5:Radial flow pump

Ns:5000 to 10000;



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COMPONENTS OF CENTRIFUGAL PUMP

GENERAL Acentrifugal pump has two main components:

1. A rotating component comprised mainly of impeller,shaft,bearings

2. A Stationary component comprised mainly of casing,casing cover and

bearing housing.

A typical single stage , overhung process type pump is the heart of the

process industry.A centrifugal pump essentially consists of an impeller with

vanes,surrounded by a volute casing.impeller is mounted firmly on shaft

,which is supported by two bearings,which in turn is supported in the bearing

housing. Impeller rotates with close clearance with the casing.Stuffing box



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prevents leakage of the process fluid from the impeller area to the

atmosphere.At the other end of the shaft ,coupling is fitted which takes drive

from the primemover such as electric motor or turbine.

STATIONARY COMPONENTS:

CASINGS

Pumps with volute casings are generally called as Volute pumps in which the

casing section collects the liquid discharged by the impeller and converts

velocity energy into pressure energy. Volute increases in area from its initial

point till it encompasses the full 360 deg.around the impeller and flares out to

the discharge opening.The wall dividing initial section and the discharge

nozzle of the volute is called the tongue of the volute.

In a single volute pump casing,uniform pressure act on the impeller when a

pump is operated at design capacity.At other capacities, the pressures

around the impeller are not uniform and there is a resultant radial load on the

impeller which,may deflect the pump shaft and cause wear at the impeller

wear ring and seal faces.Hence single volute designs are used in slurry and

sewage services to minimize plugging at the throat and on low head pumps

where radial loads are nominal.Application of double volute casing design



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eliminates radial loads . This design consists of two 180 deg. Volutes ,a

passage external to second joins the two volutes into a common discharge.

SEAL CHAMBER AND STUFFING BOX

Seal chamber and stuffing box both have primary function of protecting the

pump against leakage at the point where the shaft passes through the pump

pressure casing. When the pressure at the bottom is below atmospheric, it

prevents air leakage into the pump. When the pressure is above

atmospheric, the chamber prevents leakage out of the pump. Both refer to a

chamber, either integral with or separate from pump case housing that forms

the region between the shaft and casing where sealing media are installed.

When the sealing is achieved by means of packing, the chamber is reffered

as stuffing box. The seal chambers and stuffing boxes are also provided with

cooling or heating arrangement for proper temperature control.



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GLAND PACKING

To stop the pumped fluid from escaping along the shaft, one method of

sealing is a packed gland. Rings of packing material are fitted in a packing

box and they fit around the shaft sleeve. Harder shaft sleeve is fitted for the

purpose of good wearing properties and a means of replacing a normally

wearing surface without having to replace the shaft.

BEARING HOUSING

The bearing housing encloses the bearing mounted on the shaft. The

bearings keep the shaft or rotor in correct alignment while stationery parts

under the action of radial and transverse loads. The bearing housing also

includes an oil reservoir for lubrication, constant level oiler, jacket for cooling

by circulating cooling water.

ROTATING COMPONENTS

IMPELLER

The impeller is the main rotating part that provides the centrifugal acceleration

to the fluid. They are often classified in many ways:

1.Based on major direction of flow in reference to the axis of rotation;

*Radial flow

*Axial flow



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*Mixed flow

2.Based on suction type

*Single suction :liquid inlet on one side

*Double suction:liquid inlet to the impeller symmetrically from both the

sides

3.Based on mechanical construction

*Closed :shrouds or side wall enclosing the vanes

*Open : no shrouds or wall to enclose the vanes

*Semiopen or vortex type

WEAR RINGS

Closed impellers require wear rings. Wear rings provide an easy and economical

renewable leakage joint between impeller and casing. As the wear rings wear

,the leakage loss increases and pumping efficiency goes down causing heat and

vibration .But, if the clearances are too tight, then both the both the impeller and

casing wear rings might seize resulting in jamming the pump. This is specifically

true for hot service pumps. Commonly used material for wear rings is 11-13% Cr

steel series, with casing wear ring material hardness more than the impeller wear

ring material hardness by about 50BHN to prevent galling. API 610 provides

guidelines on these clearances.

SHAFT



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The basic purpose of a centrifugal pump shaft is to transmit the torques

encountered when starting and during operation while supporting the impeller

and other rotating parts. It must do this job with a deflection less than the

minimum clearance between the rotating and stationary part.

SHAFT SLEEVE

Pump shafts are usually protected from erosion , corrosion and wear at the seal

chambers, leakage joints, internal bearings and in the waterways by renewable

sleeves.

MECHANICAL SEAL

A mechanical seal has a rotating face and a stationary face. Means such as

bellows , wedges and O rings are used to seal the rotating face against the shaft

sleeve. O-rings are normally used to seal the stationary face to the casing. One

face with the springs is held stationary in most of the cases and other face

rotates. Both the faces are matched so they fit together perfectly.

Stationary face is many times made of carbon and rotating face of silicon

carbide.

UNDERSTANDING CENTRIFUGAL PUMP PERFORMANCE

CURVES

The capacity and pressure needs of any system can be defined with the help of a

graph called a system curve. Similarly the capacity vs. pressure variation graph

for a particular pump defines its characteristic pump performance curve.



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The pump suppliers try to match the system curve supplied by the user with a

pump curve that satisfies these needs as closely as possible. A pumping system

operates where the pump curve and the system resistance curve intersect. The

intersection of the two curves defines the operating point of both pump and

process. However, it is impossible for one operating point to meet all desired

operating conditions. For example, when the discharge valve is throttled, the

system resistance curve shift left and so does the operating point.

Figure D.01: Typical system and pump performance curves

DEVELOPING A SYSTEM CURVE

The system resistance or system head curve is the change in flow with respect to

head of the system. It must be developed by the user based upon the conditions of service. These include physical layout, process conditions, and

fluid characteristics. It represents the relationship between flow and hydraulic



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losses in a system in a graphic form and, since friction losses vary as a square of

the flow rate, the system curve is parabolic in shape. Hydraulic losses in piping

systems are composed of pipe friction losses, valves, elbows and other fittings,

entrance and exit losses, and losses from changes in pipe size by enlargement

or reduction in diameter.

DEVELOPING A PUMP PERFORMANCE CURVE

A pump's performance is shown in its characteristics performance curve where

its capacity i.e. flow rate is plotted against its developed head. The pump

performance curve also shows its efficiency (BEP), required input power (in

BHP), NPSHr, speed (in RPM), and other information such as pump size and

type, impeller size, etc. This curve is plotted for a constant speed (rpm) and a

given impeller diameter (or series of diameters). It is generated by tests

performed by the pump manufacturer. Pump curves are based on a specific

gravity of 1.0. Other specific gravities must be considered by the user.

NORMAL OPERATING RANGE

A typical performance curve (Figure D.01) is a plot of Total Head vs. Flow rate for

a specific impeller diameter. The plot starts at zero flow. The head at this point

corresponds to the shut-off head point of the pump. The curve then decreases to

a point where the flow is maximum and the head minimum. This point is

sometimes called the run-out point. The pump curve is relatively flat and the

head decreases gradually as the flow increases. This pattern is common for

radial flow pumps. Beyond the run-out point, the pump cannot operate. The

pump's range of operation is from the shut-off head point to the run-out point.

Trying to run a pump off the right end of the curve will result in pump cavitation

and eventually destroy the pump.



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In a nutshell, by plotting the system head curve and pump curve together, you

can determine:

1. Where the pump will operate on its curve?

2. What changes will occur if the system head curve or the pump

performance curve changes?

2.2 GENERAL POSITIVE DISPLACEMENT PUMP

A positive displacement pump is one in which a definite volume of liquid is

delivered for each cycle of pump operation. This volume is constant regardless of

the resistance to flow offered by the system the pump is in, provided the capacity

of the power unit driving the pump or pump component strength limits are not

exceeded. The positive displacement pump delivers liquid in separate volumes

with no delivery in between, although a pump having several chambers may

have an overlapping delivery among individual chambers, which minimizes this

effect. The positive displacement pump differs from centrifugal pumps, which

deliver a continuous flow for any given pump speed and discharge resistance.

Positive displacement pumps can be grouped into three basic categories based

on their design and operation. The three groups are reciprocating pumps, rotary

pumps, and diaphragm pumps.

Reciprocating positive displacement pumps are generally categorized in four

ways: direct-acting or indirect-acting; simplex or duplex; single-acting or double-

acting; and power pumps.

Some reciprocating pumps are powered by prime movers that also have

reciprocating motion, such as a reciprocating pump powered by a reciprocating

steam piston. The piston rod of the steam piston may be directly connected to

the liquid piston of the pump or it may be indirectly connected with a beam or



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linkage. Direct-acting pumps have a plunger on the liquid (pump) end that is

directly driven by the pump rod (also the piston rod or extension thereof) and

carries the piston of the power end. Indirect-acting pumps are driven by means of

a beam or linkage connected to and actuated by the power piston rod of a

separate reciprocating engine.

A simplex pump, sometimes referred to as a single pump, is a pump having a

single liquid (pump) cylinder. A duplex pump is the equivalent of two simplex

pumps 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 the other piston is on its down stroke, and vice versa. This arrangement

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

comparable design.

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. One possible

arrangement for single-acting and double-acting pumps is shown in Figure 13.



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Power pumps convert rotary motion to low speed reciprocating motion by

reduction gearing, a crankshaft, connecting rods and crossheads. Plungers or



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pistons are driven by the crosshead drives. Rod and piston construction, similar

to duplex double-acting steam pumps, is used by the liquid ends of the low

pressure, higher capacity units. The higher pressure units are normally single-

acting plungers, 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 rarely be justified on the basis of

efficiency over centrifugal pumps. However, they are frequently justified over

steam reciprocating pumps where continuous duty service is needed due to the

high steam requirements of direct-acting steam pumps. In general, the effective

flow rate of reciprocating pumps decreases as the viscosity of the fluid being

pumped increases because the speed of the pump must be reduced. 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.

2.3 DIAPHRAGM PUMP

Diaphragm pumps are also classified as positive displacement pumps because

the diaphragm acts as a limited displacement piston. The pump will function

when a diaphragm is forced into reciprocating motion by mechanical linkage,

compressed air, or fluid from a pulsating, external source. The pump construction

eliminates any contact between the liquid being pumped and the source of

energy. This eliminates the possibility of leakage, which is important when

handling toxic or very expensive liquids. Disadvantages include limited head and



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capacity range, and the necessity of check valves in the suction and discharge

nozzles. An example of a diaphragm pump is shown in Figure 20.



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2.4 GEAR PUMP

There are several variations of gear pumps. The simple gear pump shown in

Figure 14 consists of two spur gears meshing together and revolving in opposite

directions within a casing. Only a few thousandths of an inch clearance exists

between the case and the gear faces and teeth extremities. Any liquid that fills

the space bounded by two successive gear teeth and the case must follow along

with the teeth as they revolve. When the gear teeth mesh with the teeth of the

other gear, the space between the teeth is reduced, and the entrapped liquid is

forced out the pump discharge pipe. As the gears revolve and the teeth

disengage, the space again opens on the suction side of the pump, trapping new

quantities of liquid and carrying it around the pump case to the discharge. As

liquid is carried away from the suction side, a lower pressure is created, which

draws liquid in through the suction line.

With the large number of teeth usually employed on the gears, the discharge is

relatively smooth and continuous, with small quantities of liquid being delivered to

the discharge line in rapid succession. If designed with fewer teeth, the space

between the teeth is greater and the capacity increases for a given speed;

however, the tendency toward a pulsating discharge increases. In all simple gear

pumps, power is applied to the shaft of one of the gears, which transmits power

to the driven gear through their meshing teeth.

There are no valves in the gear pump to cause friction losses as in the

reciprocating pump. The high impeller velocities, with resultant friction losses, are

not required as in the centrifugal pump. Therefore, the gear pump is well suited

for handling viscous fluids such as fuel and lubricating oils.



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There are two types of gears used in gear pumps in addition to the simple spur

gear. One type is the helical gear. A helix is the curve produced when a straight

line moves up or down the surface of a cylinder. The other type is the

herringbone gear. A herringbone gear is composed of two helixes spiraling in

different directions from the center of the gear. Spur, helical, and herringbone

gears are shown in Figure 15.

The helical gear pump has advantages over the simple spur gear. In a spur gear,

the entire length of the gear tooth engages at the same time. In a helical gear,

the point of engagement moves along the length of the gear tooth as the gear

rotates. This makes the helical gear operate with a steadier discharge pressure

and fewer pulsations than a spur gear pump.

The herringbone gear pump is also a modification of the simple gear pump. Its

principal difference in operation from the simple spur gear pump is that the

pointed center section of the space between two teeth begins discharging before

the divergent outer ends of the preceding space complete discharging. This



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overlapping tends to provide a steadier discharge pressure. The power

transmission from the driving to the driven gear is also smoother and quieter.



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2.5 LOBE PUMP :

The lobe type pump shown below is another variation of the simple gear pump. It

is considered as a simple gear pump having only two or three teeth per rotor;

otherwise, its operation or the explanation of the function of its parts is no

different. Some designs of lobe pumps are fitted with replaceable gibs, that is,

thin plates carried in grooves at the extremity of each lobe where they make

contact with the casing. The gib promotes tightness and absorbs radial wear.

There are many variations in the design of the screw type positive displacement,

rotary pump. The primary differences consist of the number of intermeshing

screws involved, the pitch of the screws, and the general direction of fluid flow.

Two common designs are the two-screw, low-pitch, double-flow pump and the

three-screw, high-pitch, double-flow pump.



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2.6 SCREW PUMP :

Two-Screw, Low-Pitch, Screw Pump

The two-screw, low-pitch, screw pump consists of two screws that mesh with

close clearances, mounted on two parallel shafts. One screw has a right-handed

thread, and the other screw has a left-handed thread. One shaft is the driving

shaft and drives the other shaft through a set of herringbone timing gears. The

gears serve to maintain clearances between the screws as they turn and to

promote quiet operation. The screws rotate in closely fitting duplex cylinders that

have overlapping bores. All clearances are small, but there is no actual contact

between the two screws or between the screws and the cylinder walls.

The complete assembly and the usual flow path are shown in Figure 17. Liquid is

trapped at the outer end of each pair of screws. As the first space between the

screw threads rotates away from the opposite screw, a one-turn, spiral-shaped

quantity of liquid is enclosed when the end of the screw again meshes with the

opposite screw. As the screw continues to rotate, the entrapped spiral turns of

liquid slide along the cylinder toward the center discharge space while the next

slug is being entrapped. Each screw functions similarly, and each pair of screws

discharges an equal quantity of liquid in opposed streams toward the center, thus

eliminating hydraulic thrust. The removal of liquid from the suction end by the

screws produces a reduction in pressure, which draws liquid through the suction

line.



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Three-Screw, High-Pitch, Screw Pump

The three-screw, high-pitch, screw pump, shown in Figure 18, has many of the

same elements as the two-screw, low-pitch, screw pump, and their operations

are similar. Three screws, oppositely threaded on each end, are employed. They

rotate in a triple cylinder, the two outer bores of which overlap the center bore.

The pitch of the screws is much higher than in the low pitch screw pump;

therefore, the center screw, or power rotor, is used to drive the two outer idler

rotors directly without external timing gears. Pedestal bearings at the base

support the weight of the rotors and maintain their axial position. The liquid being

pumped enters the suction opening, flows through passages around the rotor

housing, and through the screws from each end, in opposed streams, toward the

center discharge. This eliminates unbalanced hydraulic thrust. The screw pump

is used for pumping viscous fluids, usually lubricating, hydraulic, or fuel oil.



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2.7 VANE PUMP :

The rotary moving vane pump shown in Figure 19 is another type of positive

displacement pump used. The pump consists of a cylindrically bored housing

with a suction inlet on one side and a discharge outlet on the other. A cylindrically

shaped rotor with a diameter smaller than the cylinder is driven about an axis

placed above the centerline of the cylinder. The clearance between rotor and

cylinder is small at the top but increases at the bottom. The rotor carries vanes

that move in and out as it rotates to maintain sealed spaces between the rotor

and the cylinder wall. The vanes trap liquid or gas on the suction side and carry it

to the discharge side, where contraction of the space expels it through the

discharge line. The vanes may swing on pivots, or they may slide in slots in the

rotor.



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Positive displacement pumps deliver a definite volume of liquid for each cycle of

pump operation. Therefore, the only factor that effects flow rate in an ideal

positive displacement pump is the speed at which it operates. The flow

resistance of the system in which the pump is operating will not effect the flow

rate through the pump. Figure 21 shows the characteristic curve for a positive

displacement pump.

The dashed line in Figure 21 shows actual positive displacement pump

performance. This line reflects the fact that as the discharge pressure of the

pump increases, some amount of liquid will leak from the discharge of the pump

back to the pump suction, reducing the effective flow rate of the pump. The rate

at which liquid leaks from the pump discharge to its suction is called slippage.



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Positive displacement pumps are normally fitted with relief valves on the

upstream side of their discharge valves to protect the pump and its discharge

piping from over pressurization. Positive displacement pumps will discharge at

the pressure required by the system they are supplying. The relief valve prevents

system and pump damage if the pump discharge valve is shut during pump

operation or if any other occurrence such as a clogged strainer blocks system

flow.



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SECTION – 3

MAINTENANCE OF PUMPS



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IS THERE A RELIABLE METHOD OF INTRODUCING A CENTRIFUGAL PUMP PREDICTIVE MAINTENANCE PROGRAM?

Probably not! But if you want to try you are first going to have to define what you

mean by predictive maintenance. If you mean that you are going to inspect the

pump and based on your observation, you are going to accurately predict future

life, you are going to have a problem.

The relationship between life to date and future life is generally accepted as

valid. As an example:

• Measure the depth of the tread on your automobile tires, record the

distance driven on the tires, and if you do not change your driving habits,

you can accurately predict the life remaining.

• Do the same thing with the shoes you are wearing and you will come up

with a similar result.

These are items that tend to "wear out" so life to date is a valid measurement.

The problem with centrifugal pumps is that seals and bearings account for over

90% of premature pump failures and neither of these items ever "wears out".

Seals should run until the sacrificial carbon face has worn away, but a close look

at used seals will demonstrate that wear is actually a minor problem. In excess of

85% of mechanical seals leak with plenty of wearable face still visible.

Bearings do not "wear out" like mechanical seals. They have a predictive fatigue

life that is based on load and cycles. Properly loaded they could last a hundred

years, but like seals, they experience a very high premature failure rate. All of

this means is that the measurements you are taking today are no indication of

what is going to happen tomorrow. It is like trying to predict an automobile

accident. There are precautions you can take, but accidents still happen.



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Most companies base their predictive maintenance programs on vibration

analysis or interval timed, visual inspection. and that is why we find "reactive

maintenance" the norm in most plants. How many times have we heard the

expression "I did not have time to do the job correctly (realignment, dynamic

balancing, etc.) because I had to get the pump back on stream".

A more sensible approach to predictive maintenance is to monitor the equipment

for changes that could be destructive in the future, but allow you to correct them

before the destruction starts. I spent my formative years in nuclear power. If, as

an operator, you did something wrong that would be harmful to the atomic

reactor it would "scram" and shut down immediately. But if you took an action

that could be potentially dangerous, the reactor would start an "insertion" that

would start to slowly shut down the reactor and give you time to correct what

ever it was you did.

Medical people use a predictive maintenance program when they:

• Monitor your cholesterol level. If it exceeds some preset number (two

hundred in the U.S.) it means that your arteries are in danger of clogging,

so you should change your diet before it becomes serious. (insertion)

• If your blood pressure is too high you could get a stroke. (insertion)

• A high fever indicates a need to get medical attention before destruction

starts. (insertion)

• Some types of pains initiate an immediate operation. (scram)

• You do the same thing with your automobile:

• A high engine water temperature is a sign of engine failure in the future.

You better check the fan belt and look for water leaks. Nothing is serious

yet, but you should react to the warning signs. (insertion)

• High fuel consumption indicates a need for an engine tune-up. (insertion)

• A loss of oil pressure means shut off the engine and react immediately.

(scram)



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Pumps also "scram" and give "insertion" signals", unfortunately vibration analysis

indicates that destruction has already started (scram). Let's look at some of the

"insertion" signals:

The stuffing box temperature is increasing. If it gets too hot you are going to have

a problem. You had better correct the condition if you do not want to experience

a premature seal failure. What can happen if the stuffing box temperature gets

too hot?

• The product can change state. It can stop being a lubricant and quickly

become a destructive solid or vapor:

o It can vaporize, expand and blow the seal faces open&emdash;

leaving destructive solids between the faces.

o It can become viscous, interfering with the free movement of the

springs and bellows.

o It can solidify, gluing the faces together or making the moveable

components inoperable.

o It can crystallize and interfere with the moving parts of the seal.

o It can cause the product to build a film on the faces (hot oil as an

example) and sliding components, making them inoperable.

• Corrosion increases with increasing temperatures.

• Temperature causes materials to expand. Seal faces can go out of flat,

and pressed in carbon faces can loosen in their holder. Bellows vibration

dampers can stick to the shaft sleeve, opening the faces.

• Some seal faces can be damaged by high heat. Plated materials and filled

carbons are two such examples. Voids in some carbon faces can expand

causing pits in the lapped faces

• Elastomers can experience "compression set" problems, causing them to

leak or in some cases fail completely at higher heat levels.



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What could be causing this high heat? If you take no corrective action one of the

above will occur.

• A loss of flushing fluid. There are multiple reasons why this could happen

and I am confident you can think of many of them.

• Loss of barrier or buffer fluid between two mechanical seals, or the

convection of the barrier fluid has stopped for some reason. Keep in mind

that petroleum products need forced lubrication or a pumping ring

because of the petroleum low specific heat and poor conductivity.

• Loss of the quench in an A.P.I. gland.

• Loss of the discharge recirculation line because of a clogged filter, cyclone

separator or heat exchanger.

• Loss of suction recirculation because of solids in the fluid.

• Loss of cooling in the stuffing box cooling jacket because the circulating

water was "hard" and has deposited an insulating layer of calcium on the

inside of the cooling jacket.

• The seal is running dry because the stuffing box was not vented in a

vertical application.

• The seal was installed incorrectly. There is too much spring load on the

faces.

• You need a hydraulic balanced seal. The unbalanced design cannot

compensate for the high stuffing box pressure.

• Thermal shaft expansion is over compressing an outside seal design, or

one of the seals in a dual seal application.

• The open impeller adjusting technique can over compress some seal

designs.

• The stuffing box is running in a vacuum because the supply tank is not

vented properly or cold weather is freezing the tank vent.

• Water hammer, pressure surges and cavitation will all alter seal face

loading.



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A change in the stuffing box pressure can cause:

• The product to vaporize, opening the lapped faces.

• O-rings and other elastomer designs to extrude and jam the sliding

components.

• Lapped seal faces to distort and go out of flat.

• A stuffing box vacuum can blow open unbalanced seals.

• A differential pressure across the elastomer can cause ethylene oxide to

penetrate into the elastomer and destroy it as it expands in the lower

pressure side.

If you are monitoring temperature and pressure in the stuffing box area you will

note the changes mentioned and depending upon your knowledge of the above,

you will have time to react before seal failure occurs.

An increase in the bearing case oil temperature is significant because the life of

bearing oil is directly related to the oil temperature. Lubricating oil has a useful

life of thirty years at thirty degrees centigrade (86°F) and its life is cut in half for

every ten degree centigrade (18°F) increase in temperature. You can figure the

temperature in the bearing is at least ten degrees centigrade (18°F) higher than

the oil sump temperature. At elevated temperatures the oil will carbonize by first

forming a "varnish like" film that will turn into a hard black coke at these higher

temperatures. It is these formed solids that will destroy the bearing.

What is causing these elevated temperatures? There are a number of

possibilities:

• Loss of circulation in the stuffing box cooling jacket.

• Loss of cooling in the bearing case cooling sump.

• Some one is cooling the outside of the bearing casing causing the outside

diameter of the bearing to shrink, increasing the load.

• The bearing was installed incorrectly.



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• The bearing is over lubricated. The oil level is too high or there is too

much grease in the bearing.

• The lubricating oil is contaminated with water.

• The shaft is overloaded because the pump is operating off of the B.E.P.,

misalignment, unbalance, etc.

• There is too much axial thrust.

Oil sampling is always a good idea. It can tell you:

• If water is getting into the oil.

• If the oil additives are still present and functioning.

• If the oil is carbonizing due to high temperature.

• If there are solids due to corrosion, bearing cage destruction, or some

other reason.

If you monitor pump suction and discharge pressure and coordinate this

information with flow and motor amperage readings you can come up with a lot of

useful information such as:

• You can tell if you have the right size pump.

• You can estimate where you are in respect to the B.E.P. and know if the

shaft i deflecting, or is about to deflect.

• You can tell if the motor is close to an overload condition.

• You will know when the impeller needs adjusting or the wear rings need

replacement.

• You can spot poor operating practices if you have a chart recorder

installed, instead of pressure and temperature gages.

• You can tell if the tank you are pumping from is losing the proper level or if

the suction lines are clogging.

• You can tell if you are getting close to cavitation.



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It goes without saying that constant monitoring is the most sensible answer to

predictive maintenance. It is the same logic you use with your automobile. You

believe that the extra expense of installed gauges is a cheap investment for

longer engine life.

There is nothing wrong with vibration analysis (an E.K.G. is still part of taking a

physical) but do not substitute it for sensible monitoring. The "scram " is too

expensive in this very competitive world of ours.

To properly maintain these pumps, there are three areas that should be

addressed. This includes the lubrication, seal replacement and alignment.

Lubrication is probably the most important part of your pump maintenance

program. Begin with checking the manufacturers recommended schedule for

lubrication and use the lubricant that they suggest. Keep detailed records on

when you lubricated the pump. Make sure you record the suction pressure, the

discharge pressure and general conditions that the pump operates under. Is the

pump hot? Do you hear noises? Record it all so that you have a base line to

compare against in the future.

Proper Alignment Proper alignment is another area that needs to be addressed. Alignment is more

critical in higher rpm applications. Use a dial indicator if your pump is running at

3,500 rpm. For those of you running at 1,750 rpm a straight edge and taper

gauges will do the trick.

Replacement of Seals

The last area of maintenance is the replacement of seals. Whenever you replace

the seals make sure that you replace all the items and not just one part. Inspect



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the seals and try to determine why they wore the way they did. If there are

grooves in the seals, this could indicate that there is a high level of suspended or

dissolved solids that need to be addressed.

Be very careful when you replace the seals. Do not touch them with dirty hands

and be especially careful not to damage them when placing them on the shaft.

Also be careful not to scratch any of the machined surfaces that will result in a

leak path.

WHY PERFORM MAINTENANCE? Maintenance of mechanical and electrical plant is essential if equipment is to

remain in a safe and reliable condition and perform the duty it was designed to

do and in a cost effective manner.

We maintain equipment, to ensure:

• Reliability

• Efficiency

• Extend the asset’s service life

Maintenance levels vary depending on the complexity of equipment and the

consequence of failure. We must ensure our pumps and motors are maintained

in a safe and reliable condition. The level of maintenance and expenditure can be

evaluated by considering the cost of failure.

• Evaluate the risk to personal safety and environment damage

• Crop loss

• Cost of emergency arrangements

• Cost of emergency repairs

• Total loss of asset



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What type of maintenance is applicable for a block pump?

Maintenance can be performed by having procedures based on:

REACTIVE MAINTENANCE

• Maintenance is performed only when it has failed and you are required to

act immediately.

• This form of maintenance can be supported by spare parts or in some

cases a spare pump so that down time is kept to a minimum.

• Reactive maintenance is not a good method.

PREVENTATIVE MAINTENANCE

• This form of maintenance requires you to act just prior to failure.

• Deciding on what has to be performed and when is it a major problem.

PREDICTIVE MAINTENANCE

• By recording selected readings, noting operational conditions you can try

to predict when the unit may fail and of course act prior to this point being

reached.

• Predictive maintenance is the best method.

There are other preventative maintenance programs such as Continuous

Diagnostic Maintenance which takes constant readings and notes any significant

change in the readings. Machine history is also a good tool to predict the life of a

pump. It is based upon "like" operation in that it relies upon the history of the

previous unit relating to the present unit. Since the taking of readings and

observations form a vital part of most maintenance programs what should be

looked for?



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• Heat – in bearings and glands

• Pressure – pump discharge pressure

• Noise – cavitation, bearings

• Flow – a drop off in flow

• Leakage – glands – piping oil or grease

• Power consumption

• Vibration – an increase could indicate problems

Regardless of type of maintenance program practiced problems will still be

experienced. The aim of a maintenance program should be to reduce failure and

operational expenditure while still maintaining an efficient unit.

MONTHLY PREVENTATIVE MAINTENANCE Procedure

1. Ensure safety of plant and equipment before performing any work

2. Record all meter readings. Calculate efficiency

3. Check all valves, glands and supports

4. Test run unit, check for correct running, noise, heat, vibration

5. Adjust gland if required

6. Check oil levels if applicable

7. Check sump drain pump if applicable

8. Clean inside and outside of station

9. Check condition of electrical components for hot spots

IMPORTANT – if electrical work is to be performed ensure it is carried out

by licensed persons

10.Act on any findings



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REFERENCES: 1. PUMP HANDBOOK BY F.POLLOCK

2. PUMP HANDBOOK BY KARRASIK

3. PUMP BROCHURE OF KIRLOSKAR BROTHERS LIMITED

Basic of Pumps

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