Write Up on Pumps

PUMPS…… CRITICAL ENTITIES OF A UNIT

BY : VINAY KUMAR SHARMA

A Brief Introduction of Pumps:-

PREFACE

This report is on “Pumps” and deals with various hardware specifications

and the process involved in the selection of a pump for a specific work

purpose in hydrocarbon industries. Pumps are critical entities of any

industry and are crucially linked to the layout development. Fluids, as per

their property, have a tendency to flow from higher potential to lower

potential. But when it is required to bring fluids to a higher altitude from

lower or transfer of fluid, pumps have their very indispensible role. Pumps

are devices that impart kinetic energy to fluids to bring them to higher

altitudes.

Hopeful of appreciation by pioneers of the industry, every possible

detail to the best of knowledge has been incorporated. Anything and

everything including constructive criticism and suggestions that adds value to

it is highly welcome.


CONTENTS

1. INTRODUCTION

1.1 Pumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Pump classification. . . . . . . . . . . . . . . . . . . . . . . . 5

1.3 Selection of Pumps. . . . . . . . . . . . . . . . . . . . . . . . 6

2. CENTRIFUGAL PUMPS 9

2.1 Working Principal. . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.2 Important Parameter. . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3 System Curve. . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.4 Constructional features. . . . . . . . . . . . . . . . . . . . . . . 21

2.5 Criterion in Pump Design. . . . . . . . . . . . . . . . . . . . . . 45

2.6 Criterion for selection of Motor. . . . . . . . . . . . . . . . . . . 47

2.7 Sundyne Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . 49

2.8 Double Suction Pump. . . . . . . . . . . . . . . . . . . . . . . 50

2.9 Axial Flow Pump. . . . . . . . . . . . . . . . . . . . . . . . . . 51

2.10 Barrel Pump or Can Pump. . . . . . . . . . . . . . . . . . . . . . 52

2.11 Pump classification. . . . . . . . . . . . . . . . . . . . . . . . 53


3. POSITIVE DISPLACEMENT PUMP 55

3.1 Types of Positive Displacement Pump. . . . . . . . . . . . . . . 55

3.2 Difference between Centrifugal & Positive Displacement Pump 55

3.3 Advantage of Positive Displacement Pump. . . . . . . . . . . . 56

3.4 Pump Characteristic. . . . . . . . . . . . . . . . . . . . . . . . 56

3.5 When to Use PD Pumps. . . . . . . . . . . . . . . . . . . . . . 57

3.6 Plunger Pump. . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

3.7 Reciprocating Diaphragm type of Pump. . . . . . . . . . . . . . 59

3.8 Rotary types Pumps. . . . . . . . . . . . . . . . . . . . . . . . 60

4. PROCUREMENT/ ORDERING SYSTEM IN GAIL 71

4.1 Categorization of MR. . . . . . . . . . . . . . . . . . . . . . 71

4.2 Summary of ordering systems of Pumps unit in GAIL PATA. . . 71

4.3 Evaluation Criterion. . . . . . . . . . . . . . . . . . . . . . . . 72

4.4 Evaluation of Prices in case of Pumps. . . . . . . . . . . . . . . 72

4.5 Problem faced & Lesson learnt. . . . . . . . . . . . . . . . . . 73

4.6 Areas of Improvement. . . . . . . . . . . . . . . . . . . . . . 74

4.7 Important Documents for Review. . . . . . . . . . . . . . . . 74


CHAPTER-1

INTRODUCTION: CLASSIFICATION AND SELECTION OF PUMPS

1.1 PUMPS

A pump is a hydraulic machine which receives a flow of liquid at a certain inlet pressure,

raises this pressure to a higher value and discharges the liquid through the outlet.

The purpose of a pump is to add energy to a fluid, resulting in an increase in a fluid

pressure, not necessarily an increase of fluid speed across the Pump.

1.2 Pumps Classification

Pumps may be classified on the basis of applications they serve, the materials from

which they are constructed, the liquids they handle and their orientation in space. A

more basic system of classification could be the principle by which energy added to

the fluid.

Progressive Cavity

1.2.1 Dynamic-in which energy is continuously added to increase the fluid velocities

within the machines to values in excess of that occurring at the discharge such that

Single / Two Stage

Multistage

Horizontal

Sump Pump

Turbine Type

Barrel Pump

Vertical

Medium to large flows

Low to medium pressures

Viscosity < 200 Cst

Dynamic(Centrifugal)

Screw

Gear

Liquid Ring

Rotary

Metering

Piston / Plunger

Reciprocating

Low to medium flows

Pressure no limitation

Positive Displacement

PUMPS

Sundyne pump


subsequent velocity reduction within or beyond the pump produces a pressure

increase.

1.2.2 Positive displacement- in which energy is periodically added by application of

force to one or more movable boundaries of any desired number of enclosed, fluid

containing volumes, resulting in a direct increase in pressure up to the value

required to move the fluid through valves and ports in to the discharge line.

Displacement pumps are essentially divided in to reciprocating and rotary types,

depending upon the nature of the pressure producing members.

1.3 Selection of Pumps

Selection of right type of pump for different fluid and operating conditions

can be daunting because of the large number of options to fit various

operating conditions. Before proceeding to the actual pump selection, it is

necessary to have the complete knowledge of the location and the basic job to

be performed. The proper pump selection requires a careful study of the

hydraulic system also.


1.3.1 Major Parameter for Selection of Pumps

The following points are to be considered at the time of pump selection

i) Fluid Characteristics: A detailed study of the fluid characteristics

is usually the most important factor for the proper selection of

pumps. i.e. Chemical identity of the fluid pumped such as pH,

dissolved oxygen, corrosive or abrasive nature, Concentration,

suspended solids and temperature, etc.

ii) Absolute Viscosity: It plays an important role while selecting the

pump. It causes the liquid to resist flow, the higher the viscosity, the

greater the head loss due to friction in the pipeline and in the pump

casing as well as in the whole system. The suction head and the

available Net Positive Suction Head (NPSH) both decreases with an

increase in liquid viscosity for the same pumping rate. At the same

time, discharge and total head both increases with an increase in

liquid viscosity for the same pumping rate. In other words, the

power requirement also increases with liquid viscosity.

iii) Specific Gravity: It affects the pump life along with performance

of the pump.

iv) Temperature: The operating temperature at the pump is an

important factor affecting overall performance of the pump. While

considering temperature, the combined ambient and liquid

temperature along with the temperature rise due to evaluation of

heat from the resistance in the system shall be taken into

consideration. As per general experience, pumps can perform

efficiently with trouble-free operation over an approximate

temperature of up to 80°C.

v) Space available for pump: It helps in selecting the pump, i.e.,

horizontal or vertical. It also influences the model and size of the

pump.


vi) Self Priming Requirement: A pump where suction nozzle

elevation is above the source, there self priming capacity may be

necessary. Positive Displacement pump such as a piston pump or

a rotary screw or gear pump are used which are able to self-prime.

vii) Variable Head/Flow requirement: Centrifugal pumps and

Axial Flow pumps are most suitable pumps for variable head /

flow requirement. For high flow and high head combinations, a

multi-stage centrifugal pump can be used. Various designs of this

type of pump are available for wide range of condition. (High

temperature, cryogenic, water, hydrocarbon, and so on).

viii) Low Flow with Precise Flow Adjustment Ability: For low-flow

applications where accurate flow metering is necessary, a

proportioning pump is appropriate. This type of pump can also be

provided with variable flow capability. Certain types of gear,

plunger, and diaphragm pumps can also be used in combination

with a variable speed drive for flow rate regulation.

ix) Low Available Net Positive Suction Head: If the available net

position suction head (NPSHA) is low, specially designed

centrifugal pumps can be considered, like Sundyne Pump.

Depending upon how low the NPSHA is, either horizontal end

suction with a suction inducer or a horizontal double suction

arrangement may be applied. A vertical turbine pump may also be

used, either immersed in the process fluid (possibly in a tank or

vessel) or in a specially designed vessel (known as a suction can)

that can be installed below grade to increase the NPSHA.


CHAPTER-2

CENTRIFUGAL PUMP

A centrifugal pump is one of the simplest pieces of devices whose purpose is to convert

energy of an electric motor or engine into velocity or kinetic energy and then into

pressure of a fluid that is being pumped. The energy changes occur into two main parts

of the pump, the impeller and the volute. The impeller is the rotating part that converts

driver energy into the kinetic energy. The volute is the stationary part that converts the

kinetic energy into pressure.

Centrifugal pumps are entirely dynamic in action, that is to say they

depend upon rotational speed to generate a head which is manifest as a difference of

pressure between inlet and outlet branches. The quantity of liquid pumped and the power

involved depends upon the system of liquid contained, static pressures and pipeline etc.

to which pump is attached.

Centrifugal pumps are used for large discharge through smaller heads.

2.1 Working Principle

2.1.1 Centrifugal Force

Liquid enters the pump suction and then the eye of the impeller. When the impeller

rotates, it spins the liquid sitting in the cavities between the vanes outward and imparts

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

is created at the eye allowing more liquid to enter the pump inlet.

2.1.2. Conversion of Kinetic Energy to 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 the 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.

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

a resistance to the flow. The first resistance is created by the pump volute (casing) 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.


2.2 Important Parameter of Centrifugal Pump

2.2.1 Capacity (min/nor/rated): Capacity means the flow rate with which liquid is moved or pushed by the pump to the

desired point in the process. It is commonly measured in either gallons per minute (gpm)

or cubic meters per hour (m3/hr). The capacity usually changes with the changes in

operation of the process. For example, a boiler feed pump is an application that needs a

constant pressure with varying capacities to meet a changing steam demand. The

capacity depends on a number of factors like:

Process liquid characteristics i.e. density, viscosity

Size of the pump and its inlet and outlet sections

Impeller size

Impeller rotational speed RPM

Size and shape of cavities between the vanes

Pump suction and discharge temperature and pressure conditions

For a pump with a particular impeller running at a certain speed in a liquid, the only

items on the list above that can change the amount flowing through the pump are the

pressures at the pump inlet and outlet. The effect on the flow through a pump by

changing the outlet pressures is graphed on a pump curve.

Minimum Capacity: Although a constant speed Centrifugal pump will operate over a

wide range of capacity but at low flow it will encounter following troubles:

i) Abrasive wear: Liquids containing a large amount of abrasive particles,

such as sand or ash must flow continuously through the pump. If flow

decreases, the particles can circulate inside the pump passages and quickly

erode the impeller, casing and even wear ring and shaft. Each pump has a

value of minimum continuous flow which is a characteristic of that

ii) Thermal: Inescapable energy conversion loss in the pump warms the

liquid.

iii) Hydraulic: When the flow decreases far enough, the impeller encounters

the suction or discharge recirculation or both.

iv) Mechanical: Both constant and fluctuating load in the radial and axial

directions increases as pump capacity fall. Bearing damage, shaft and

impeller breakage can occur. Its value can be obtained from the

characteristic curves provided in the vendor’s catalogue. The pump should


be selected so that the minimum flow during the service does not fall below

minimum continuous flow.

2.2.2 Pressure Pressure is usually measured in gauge that registers the difference between the pressure

in vessel and current atmospheric pressure. Therefore the gauge does not indicate the

true total gas pressure.

To obtain the true pressure or pressure above zero, it is necessary to add the current

atmospheric or barometric pressure, expressed in proper units. This sum is the absolute

pressure.

2.2.3 Pump Head

Head/ static head is the distance between two horizontal levels in a liquid. It is also the

measure of the pressure exerted by a column or body of liquid because of the weight of

the liquid. The term pump head represents the net work performed on the liquid by the

pump. It is composed of four parts. The static head (Hs), or elevation; the pressure head

(Hp) or the pressures to be overcome; the friction head (Hf) and velocity head (Hf),

which are frictions and other resistances in the piping system.

Significance of using the “head” term instead of the “pressure” term


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 the static head

and is expressed in terms of feet of liquid.

The same head term is used to measure the kinetic energy created by

the pump. In other words, head is a measurement of the height of a liquid column that

the pump could create from the kinetic energy imparted to the liquid. Imagine a pipe

shooting a jet of water straight up into the air, the height the water goes up would be the

head.

The head is not equivalent to pressure. Head is a term that has units of a length or feet

and pressure has units of force per unit area or pound per square inch. The main reason

for using head instead of pressure to measure a centrifugal pump’s energy is that the

pressure from a pump will change if the specific gravity (weight) of the liquid changes,

but the head will not change. Since any given centrifugal pump can move a lot of

different fluids, with different specific gravities, it is simpler to discuss the pump's head

and forget about the pressure. So a centrifugal pump’s performance on any Newtonian

fluid, whether it's heavy (sulfuric acid) or light (gasoline) is described by using the term

‘head’. The pump performance curves are mostly described in terms of head.

A given pump with a given impeller diameter and speed will raise a liquid to a

certain height regardless of the weight of the liquid.

i) Pressure head:- Pressure can be converted to head with following formula:-

H = Pressure/Density = P/ρg

ii) Velocity Head: - It is the head required to impart velocity to liquid. It is

eequivalent to the vertical distance through which the liquid would have to fall

to acquire the same velocity.


Velocity head = V2 / 2g

iii) Friction Head: - It is the force or pressure required to overcome friction and

is obtained at the expense of the static pressure head. Unlike velocity head,

friction head cannot be “recovered” or reconverted to static pressure head.

Thermal energy is usually wasted, therefore resulting in a head loss from the

system.

Hf= f(L/D)(V2/2Zg)

Where f is friction factor, L is length of pipe, and D is

dia. of pipe

Total dynamic head = Hs + Hp + Hv + Hf

Suction lift: Suction lift exists when the source of supply is below the center line of the

pump.

Suction head: Suction head exists when the source of supply is above the centerline of

the pump.

2.2.4 Net Positive Suction Head (NPSH)

NPSH is what the pump needs, the minimum requirement to perform its duties. NPSH

takes into consideration the suction piping and connections, the elevation and absolute

pressure of the fluid in the suction piping, the velocity of the fluid and the temperature.

Some of these factors add energy to the fluid as it moves into the pump, and others

subtract energy from the fluid. There must be sufficient energy in the fluid for the

impeller to convert this energy into pressure and flow. If the energy is inadequate we say

that the pump suffers inadequate NPSH.

The Hydraulic Institute Standards defines NPSH as the total

suction head in meters absolute, determined at the suction nozzle and corrected to datum,

less the vapor pressure of the liquid in meters absolute. Simply stated, it is an analysis of

energy conditions on the suction of a pump to determine if the liquid will vaporize at the

lowest pressure point in the pump.

The pressure, which a liquid exerts on its surroundings, is dependent upon its

temperature. This pressure, called vapor pressure, is a unique characteristic of every

fluid and increases. When the vapor pressure within the fluid reaches the pressure of the

surrounding medium, the fluid begins to vaporize or boil. The temperature at which this

vaporization occurs will decrease as the pressure of the surrounding medium decreases.

A liquid increases greatly in volume when it vaporizes. One cubic foot of

water at room temperature becomes 1700 cu. Ft. of vapor at the same temperature.


It is obvious from the above that if we are to pump a fluid effectively, we

must keep it in the liquid form. NPSH is simply a measure of the amount of suction head

to prevent this vaporization at the lowest pressure point in the pump.

Net Positive Suction Head (NPSH) is a statement of the minimum

suction conditions required to prevent cavitations in a pump.

i) NPSHA (Available):- NPSHA is a total available suction pressure - over

the vapor pressure - (expressed in feet/ meter of head). In other words, it is Net

Suction Head - vapor pressure (expressed as head). This is the energy in the

fluid at the suction connection of the pump over and above the liquid’s vapor

pressure. It is a characteristic of the system and we say that the NPSHA should

be greater than the NPSHR (NPSHA > NPSHR).

ii) NPSHR (Required):-It is the energy in the liquid required to overcome the

friction losses from the suction nozzle to the eye of the impeller without

causing vaporization. It is a characteristic of the pump and is indicated on the

pump's curve. It varies by design, size, and the operating conditions. It is

determined by a lift test, producing a negative pressure in inches of mercury

and converted into feet of required NPSH.

How to increase NPSHA?

a) INCREASE P SOURCE:

-Increase Vessel/Reactor Pressure

-Install Booster Pump Upstream Of Main Pump

Ps

H friction

NPSH (A-R)

NPSH A

NPSH R

P vap

H vap


b) INCREASE delta H:

-Increase Vessel Level

-Increase Minimum Liquid Level

-Lower Pump Installation Level

c) DECREASE SUCTION LOSSES:

-Increase Suction Pipe Size

-Reduce Suction Pipe Length

-Minimize No. Of Bends

-Use Low delta P Valves/Strainers in Suction

d). REDUCE VAPOUR PRESSURE:

-Lower Liquid Temp.

-Minimize Heat Pick-up in Suction Line

How to decrease NPSHR?

a) Select pump at lower speeds

b) Use double suction impellers

c) Modify pump geometry

d) Use inducer

2.2.5 Cavitations

It is the formation and subsequent collapse of vapor filled cavities (such as bubbles,

vapor filled pockets etc) in a liquid due to dynamic action. Inadequate NPSHA

establishes favorable conditions for cavitation in the pump. If the pressure in the eye of

the impeller falls below the vapor pressure of the fluid, then bubble formation begins. As

bubbles flow from low pressure to higher, they implode against metal surfaces with high

energy. These micro-hammer-like impacts erode the material, creating cavities – thus

“cavitation”.

2.2.6 Vapor pressure

The vapor pressure of a liquid is the absolute pressure at which the liquid vaporizes or

converts into a gas at a specific temperature. The vapor pressure of a liquid increases

with its temperature. For this reason the temperature should be specified for a declared

vapor pressure.

Thoma’s cavitation factor:- The thoma cavitation factor is used to indicate the onset of

cavitation. It is defined as:

σ = (Ha – Hs – Hv)/ Hmano = NPSH / Hmano

Ha = atmospheric pressure expressed in meters

Hv = vapor pressure in meters.


Hs = Total suction head.

Hmano = Manometric Head = Head imparted by the impeller to

Liquid – loss of head in the pump.

How does vapor pressure effect pump performance? When cavitation occurs in a pump, its efficiency is reduced. It can also cause sudden

surges in flow and pressure at the discharge nozzle. The calculation of the NPSHR (the

pump’s minimum required energy) and the NPSHA (the system’s available energy), is

based on an understanding of the liquid’s absolute vapor pressure. The effects of

cavitation are noise and vibration. If the pump operates under cavitating conditions for

enough time, the following can occur:

Pitting marks on the impeller blades and on the internal volute casing wall of the

pump.

Premature bearing failure.

Shaft breakage and other fatigue failures in the pump.

Premature mechanical seal failure.

These problems can be caused by:

A reduction of pressure at the suction nozzle.

An increase of the temperature of the pumped liquid.

An increase in the velocity or flow of the fluid.

Separation and reduction of the flow due to a change in the viscosity of the liquid.

Undesirable flow conditions caused by obstructions or sharp elbows in the suction

piping.

The pump is inadequate for the system.

2.2.7 Brake Horse Power (BHP)

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 horsepower (BHP) is the actual horsepower delivered to the pump

shaft.

Pump output or hydraulic or water horsepower (WHP) is the liquid horsepower

delivered by the pump.

2.2.8 Efficiency

The ratio of power output of the pump to the power input to the pump is called

efficiency of the pump.

Ƞ= WKW/ 366.9× BKW


2.2.9 Best Efficiency Point (BEP)

Best Efficiency Point (BEP) is the capacity at maximum impeller diameter at which the

efficiency is highest. All points to the right or left of BEP have a lower efficiency.

Significance of BEP

BEP as a measure of optimum energy conversion

When sizing and selecting centrifugal pumps for a given application the pump efficiency

at design should be taken into consideration. The efficiency of centrifugal pumps is

stated as a percentage and represents a unit of measure describing the change of

centrifugal force (expressed as the velocity of the fluid) into pressure energy. The B.E.P.

(best efficiency point) is the area on the curve where the change of velocity energy into

pressure energy at a given gallon per minute is optimum; in essence, the point where the

pump is most efficient.

BEP as a measure of mechanically stable operation

The impeller is subject to non-symmetrical forces when operating to the right or left of

the BEP. These forces manifest themselves in many mechanically unstable conditions

like vibration, excessive hydraulic thrust, temperature rise, and erosion and separation

cavitation. Thus the operation of a centrifugal pump should not be outside the furthest

left or right efficiency curves published by the manufacturer. Performance in these areas

induces premature bearing and mechanical seal failures due to shaft deflection, and an

increase in temperature of the process fluid in the pump casing causing seizure of close

tolerance parts and cavitation.

BEP as an important parameter in calculations

BEP is an important parameter in that many parametric calculations such as specific

speed, suction specific speed, hydrodynamic size, viscosity correction, head rise to

shutoff, etc. are based on capacity at BEP. Many users prefer that pumps operate within

80% to 110% of BEP for optimum performance.

2.2.10 Pump specific speed

Pump specific speed is a dimensionless quantity which is similar for geometrically

similar pumps and is defined as


Ns = NQ1/2

/ H3/4

Specific speed is calculated at best efficiency point (BEP) with the maximum impeller

diameter.

Specific speed as a measure of the shape or class of the impellers

The specific speed determines the general shape or class of the impellers. As the specific

speed increases, the ratio of the impeller outlet diameter, D2, to the inlet or eye diameter,

D1, decreases. This ratio becomes 1.0 for a true axial flow impeller. Radial flow

impellers develop head principally through centrifugal force. Radial impellers are

generally low flow high head designs. Pumps of higher specific speeds develop head

partly by centrifugal force and partly by axial force. A higher specific speed indicates a

pump design with head generation more by axial forces and less by centrifugal forces.

An axial flow or propeller pump with a specific speed of 10,000 or greater generates its

head exclusively through axial forces. Axial flow impellers are high flow low head

designs.

Specific speed identifies the approximate acceptable ratio of the impeller eye diameter

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

Ns: 500 to 5000; D1/D2 > 1.5 - radial flow pump

Ns: 5000 to 10000; D1/D2 < 1.5 - mixed flow pump

Ns: 10000 to 15000; D1/D2 = 1 - axial flow pump

Specific speed is also used in designing a new pump by size- factoring a smaller pump

of the same specific speed. The performance and construction of the smaller pump are

used to predict the performance and model the construction of the new pump.

Pump Suction Specific Speed:-

Pump suction specific speed provides an assessment of pumps susceptibility to internal

recirculation. It is mathematically defined as:-

S = NQ1/2

/ NPSHR3/4

Suction specific speed is also calculated at BEP with maximum impeller diameter.

Also

(Thoma’s cavitation factor) = (Ns/ S)3/4

2.2.11 Priming of a centrifugal pump

The operation of filling the suction pipe, casing of the pump and a portion of the delivery

pipe completely from outside source with the liquid to be raised, before starting the

pump, to remove any air gas or vapor from the parts of the pump is called priming of

centrifugal pump. If the pump is not primed before starting, air pockets inside the


impeller may give rise to vortices and cause discontinuity of flow. Further dry running of

the pump may result in rubbing and seizing of the wear rings and cause severe damage.

2.3 System Curves

Ordinarily a centrifugal pump is worked under its maximum efficiency conditions.

However the pump is run at conditions different from the design condition, it performs

differently. Therefore to predict the behavior of the pump under varying conditions of

speeds, heads, discharges or powers tests are usually conducted. The results obtained

from these tests are plotted in form of characteristic curves. These curves generate useful

information about the performance of a pump in its installation.

The following four types of characteristic curves are usually prepared for

centrifugal pumps:

1. Main characteristic curves

2. Operating characteristic curves

3. Constant efficiency or Muschel curves

4. Constant head or constant discharge curves

2.3.1 Main characteristic curves

The main characteristic curve is obtained as follows:

The pump is run at a constant speed and the discharge is varied over the

desired range.

Measurements are taken for manometric head and shaft power for each

discharge.

Calculations are made for pump efficiency.

The curves are plotted between Q and Hmano; Q and P; and Q and ƞ for that

speed.

The same procedure is repeated by running the pump at another speed.

The family of curves is obtained as shown in the fig.1

Fig. 1

2.3.2 Operating characteristic curves


When a centrifugal pump operates at the design speed the maximum efficiency occurs.

Evidently from optimum performance, the pump needs to be operated at the design

speed and the discharge is varied, as in the case of main characteristic curves. The

operating characteristics are shown in the fig.2 below. The design discharge and head are

obtained from the corresponding curve where the efficiency is maximum.

Fig.2

2.3.3. Constant efficiency or Muschel curve:

The constant efficiency curve also called as iso- efficiency curve, depicts the

performance of a pump over its entire range of operation. The curves are obtained from

main characteristic curves as follows:

For a given efficiency, the values of discharge are obtained from fig.1. these

points are projected on the head v/s discharge curve of fig1.

Similarly for other values of efficiency and speed, the points are obtained and

projected.

The points corresponding to one efficiency are joined.

The constant efficiency curve helps to locate the regions where the pump would operate

with maximum efficiency.

2.3.4. Constant head and constant discharge curves

The performance a variable speed pump for which the speed constantly varies can be

determined by these curves. When the pump has a variable speed, the plots between Q

and N, and Hmano and N may be obtained.


2.4 CONSTRUCTIONAL FEATURES

General Components of Centrifugal Pumps

A centrifugal pump has two main components:

I. A rotating component comprised of an impeller and a shaft

II. A stationary component comprised of a casing, casing cover, and bearings.

Various parts of a centrifugal pump are briefly described below:

2.4.1 Impellers:

Impellers are the components of pump which impart dynamic energy to the fluid, which

gets converted to pressure energy. This may be classified on the basis of

(a)Mechanical design of impellers

According to this impellers may be classified as:

(i) Completely open

It consists of vanes attached to a central hub for mounting on the shaft without any form

of sidewall or shroud. The disadvantage of the impeller is its structural weakness and it’s


more sensitiveness to wear. Due to more wear the efficiency deteriorates rapidly. One

advantage of open impeller is that they are better suited for handling stringy materials.

Also they are better suited for handling liquids containing suspended matter as the

possibility of clogging is not there. They are used in small inexpensive pumps.

Fig: Open impeller Fig: Semi- open impeller

(ii) Semi open

It incorporates a single shroud usually at the back of the impeller. This shroud may or

may not have pump out vanes which are vanes located at the back of impeller shroud.

This function is to reduce the pressure at the back hub of the impeller and prevent

foreign matter from lodging in the back of the impeller and interfering with

proper operation of the pump and stuffing box

(iii) Closed

It is almost universally used in pumps handling clear liquids, incorporates shrouds or

enclosing side walls that totally enclose impeller water ways from suction eye to the

periphery. Although this design prevents the liquid slippage that occurs between an open

or semi-open impeller and its side plates a running joint must be provided between the

impeller and casing to separate the discharge and suction chambers of the pump. This

running joint is usually formed by a relatively short cylindrical surface on the impeller

shroud that rotates within slightly larger cylindrical surface.

Fig: Closed impeller (single suction) Fig: Closed impeller (double suction)

(b)Based on suction type

Single and double suction impellers:


In single suction impellers the liquid enters the suction eye on one side only. A double

suction impeller is in effect two single suction impellers arranged back to back in a

single casing, the liquid enters simultaneously from both sides, while the two casing

suction passage ways are connected to a common suction passage and a single suction

nozzle. For the general service axially split casing design, a double suction impeller is

favoured because it is theoretically in axial hydraulic balance and because the greater

suction area of a double suction impeller permits the pump to operate with less absolute

head or its better NPSH characteristics.

2.4.1 i) Axial thrust

Axial thrust in single stage pump

Axial hydraulic thrust on an impeller is the sum of the unbalanced forces acting in an

axial direction. The ordinary single suction impeller with the shaft passing through the

impeller eye is subjected to axial thrust because a portion of the front wall is exposed to

suction pressure and thus relatively more backwall surface is exposed to discharge

pressure. If the discharge chamber pressure was uniform over the entire impeller surface,

the axial force acting toward the suction would be equal to the product of the net

pressure generated by the impeller and the unbalanced annular area. Generally speaking,

axial thrust toward the impeller suction is about 20 to 30% less than the product of the

pressure and the unbalanced area.

Theoretically, a double-suction impeller is in hydraulic axial balance, with the

pressure on one side equal to and counterbalancing the pressure on the other. In practice,

this balance may not be achieved for the following reasons:

1. The suction passages to the two suction eyes may not provide equal or uniform

flows to the two sides.

2. Unequal leakage through the two leakage joints can upset the balance.

3. External conditions, such as an elbow located too close to the pump suction

nozzle, may cause unequal flow to the two suction eyes.

Combined, these factors can create axial unbalance. To compensate for this, all

centrifugal pumps, even those with double suction impellers, incorporate thrust bearings.

2.4.1 ii) Axial thrust in multistage pumps

Most multistage pumps are built with single suction impellers in order to simplify the

design of inter stage connections. Two arrangements are possible for the single suction

impellers:

1. Several single suction impellers mounted on one shaft, each having its suction inlet

facing in the same direction and its stage following one another in ascending order of

pressure. The axial thrust is then balanced by following hydraulic balancing devices;

a. Balancing drums


b. Balancing disks

c. Combination balancing disk and drum

2. An even number of single suction impeller may be used, one half facing in one

direction and the other half facing in opposite direction. With this arrangement, axial

thrust on the first half is compensated by the thrust in the opposite direction on the

other half. This mounting of single suction impellers back to back is called opposed

impellers.

Impeller Axial Thrust Diagram

Rotor Assembly Axial Thrust With Balancing Disc & Drum


2.4.2 Shaft and Shaft Sleeves

i) Shaft

The basic function of a centrifugal pump shaft is to transmit the torque

encountered in 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 rotating and stationary parts.

Loads involved- (1) Torque

(2) Weight of the parts

(3) Both radial and axial hydraulic forces

In designing a shaft, the maximum allowable deflection, the span or overhang and the

location of the loads all have to be considered, as does the critical speed of the resulting

design.

Critical speed: Any object made of elastic material has a natural period of vibration.

When a pump rotor or shaft rotates at any speed corresponding to its natural frequency,

minor unbalances will be magnified. These speeds are called the critical speeds.

Rigid and Flexible shaft:

A rigid shaft means one with an operating speed lower than its first critical speed.

A flexible shaft means one with an operating speed higher than its first critical speed.

It is possible to operate centrifugal pump shaft above their critical speed for the

following two reasons-

1) Very little time is required to attain full speed from rest.

2) The pumped liquid in the stuffing box packing and the internal leakage joints

act as a restraining force on the vibration.

ii) Shaft sleeves:

Pump shaft are usually protected from erosion, corrosion and wear at stuffing boxes,

leakage joints, internal bearings and in the waterways by renewable sleeves.

The most common shaft sleeve function is that of protecting the shaft from

wear at a stuffing box.


Fig: Shaft and Shaft sleeve

Material for stuffing box sleeve: Stuffing box shaft sleeves are surrounded in the

stuffing box packing , the sleeve must be smooth so that it can turn without generation

too much friction and heat. Thus the sleeve materials must be capable of taking very fine

finish, preferably a polish; therefore cast iron is not suitable for shaft sleeve. Hard

bronze is suitable for pumps handling clear water. Hardened chrome or other stainless

steels for pumps subjected to grit. Etc;

2.4.4 Bearings

The function of bearings in centrifugal pump is to keep the shaft in correct alignment

with the stationary parts under the action of radial and transverse loads. Bearing that give

radial positioning to the rotor are known as line bearings and those locate the rotor

axially are called thrust bearings. In most application the thrust bearings serve actually

both as thrust and radial bearings.

In horizontal pump with bearings on each end, the bearings are usually

designated by their location as inboard and outboard. Inboard bearings are located

between the casing and the coupling. Pumps with overhang impeller have both bearings

on the same side of casing so that the bearing nearest the impeller is called inboard and

the one farthest away outboard. In a pump provided with bearings at both ends, the thrust

bearing is usually placed at outboard end and the line bearing at the outboard end.

2.4.5 Wear rings

Wear rings are used on close clearance areas of casings and impellers where they form

leakage joints between suction and discharge pressure. They act as replaceable wear

surfaces and are respectively called impeller and casing wear rings. In case of open

impellers, they are provided only on casing and are called wear plates. Since wear rings

are close clearance parts, they have galling/seizing tendencies. Therefore minimum wear

ring clearances are insisted upon even though this may mean lower efficiencies.


Fig: Wear rings

2.4.6 Casing

The casing is an airtight chamber surrounding the pump impeller. It is the part that

contains the pump components and is used for converting the velocity energy to pressure

energy. It contains suction and discharge arrangements, supporting for bearings and

facilitates to house the rotor assembly.

The essential purposes of the casings are:

I. To guide the water to and from the impeller, and

II. To partially convert the kinetic energy into pressure energy.

a) Solid and split casings

Solid casing implies a design in which the discharge waterways leading to the discharge

nozzle are all contained in one casing, or fabricated piece. The casing must have one side

open so that the impeller may be introduced into it.

A split casing is made of two or more parts fastened together. Axially split

casing is a casing divided by a plane through the shaft centerline or axis. Since both the

suction and discharge nozzles are usually in the same half of the casing, the other half

may be removed for inspection of the interior without disturbing the bearing or the

piping. Radial split casing is a casing split in a plane perpendicular to the axis of

rotation or shaft centerline.

b) Volute Casing

In this type of casing the area of flow gradually increases from the impeller outlet to the

delivery pipe so as to reduce the velocity of flow. Thus the increase of pressure occurs in

volute casing.

c) Single and Double volute casing

In a single volute casing design, uniform or near uniform pressure act on the impeller

when the pump is operated at design capacity (which coincides with the best efficiency).

At other capacities, the pressure around the impeller is not uniform and there is a

resultant radial reaction (F).


F= KD2W2H(S.G) / 10.21(104) in metric units

Where K – Radial thrust factor (depends on % of design capacity

and pump specific speed.)

D2 – Impeller diameter

W2 – Impeller width

H – Pump total head

This unbalanced radial thrust increase as capacity decrease from that design flow. Thus a

high head pump with a large impeller diameter will have a much greater radial reaction

force at partial capacity then a low head pump with a small impeller diameter. Radial

thrust is least in the region of Best Efficiency point (discussed later.)

Because of the increasing application of pumps which must operate at reduced

capacities, it has become desirable to design standard units to accommodate such

conditions. One solution is to use heavier shafts and bearings. Expect for low head

pumps in which only a small additional load is involved, the solution is not economical.

The only practical answer is a casing design that develops a much smaller radial

reaction force at reduced capacities. One of these is the double volute casing design, also

called twin volute or dual volute design.

This design consists of two 180 degree volutes. A pressure unbalance exists at

partial capacity through each 180 degree arc, the two forces are approximately equal and

opposite. Thus little if any radial forces act on the shaft and bearings.

Fig: A double volute casing pump

d) Vortex Casing

If a circular chamber is provided between the impellers and the volute chambers, the

casing is known as vortex casing. The circular chamber is known as vortex or whirlpool

chamber. The efficiency of a volute pump fitted with a vortex chamber is more than that

of a simple volute pump.

2.4.8 Seal Chamber and Stuffing Box

Seal chamber and Stuffing box both refer to a chamber, either integral with or separate

from the 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 a mechanical seal,

the chamber is commonly referred to as a Seal Chamber.


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

referred to as a Stuffing Box. Both the seal chamber and the stuffing box have the

primary function of protecting the pump against leakage at the point where the shaft

passes out through the pump pressure casing. When the pressure at the bottom of the

chamber is below atmospheric, it prevents air leakage into the pump. When the pressure

is above atmospheric, the chambers prevent liquid leakage out of the pump. The seal

chambers and stuffing boxes are also provided with cooling or heating arrangement for

proper temperature control.

2.4.9 Mechanical Seal or Packing

Packing or seal are required to prevent the process fluid from leaking to atmosphere and

to prevent loss of pressure energy. Packing are now used for only water services. For all

hydrocarbon services mechanical seal may be used in single, tandem or double

configurations. Single pusher type seal are most commonly used. Tandem seal are used

for hazardous fluids where leakage to atmosphere cannot be allowed.

Advantages of mechanical seals:- 1. Greater sealing capability

2. Lower leakage

3. Tolerance of liquids

4. No adjustment- self compensation for wear, therefore do not require adjustment.

Fig: Gland packing Fig: Mechanical seal


Fig: Tandem seal

i) Principle (Mechanical seals)

The sliding seal interface is affected between the flat, polished mating faces of

two rings, one connected and sealed to pump’s rotor, the other to its casing. One

of the ring is flexibly mounted to accommodate manufacturing tolerances, axial

movement of pumps rotor and wear of the seal faces.

Lubricants are used to lower the coefficient of friction and remove the heat

generated.

For mechanical seal to function, the forces tending to close its faces must exceed

those tending to open the faces. This net closing force gives rise to what is

termed as face loading has a upper limit beyond which the lubricating film

between the faces break down.

ii) Seal material

Depends upon the type of fluid to be handled and on operating conditions.

Most commonly used material is carbon and silicon carbide.

iii) Mechanical seal classification


By design:- Unbalance, Balance, rotating seal ring face seal, stationary face ring

seal, single spring seal.

By installation:- Single, Double, Internally mounted, Back to back mounted,

Face to face mounted, Tandem seal.


2.4.12 Couplings

A coupling is used wherever there is a need to connect a prime mover to a piece of

driven machinery. The principal purpose of a coupling is to transmit rotary motion and

torque from one piece of equipment to another. Couplings may perform other secondary

functions, such as accommodating misalignment between shafts, compensating for axial

shaft movement, and helping to isolate vibration, heat, and electrical eddy currents from

one shaft to another.

Centrifugal pumps are connected to their drivers through couplings of one

sort or another, except for close-coupled units, in which the impeller is mounted on an

extension of the shaft of the driver.

Classification:- i) Rigid Couplings:- Rigid couplings are used to connect machines where it is desired

to maintain shafts in precise alignment. They are also used where the rotor of one

machine is used to support and position the other rotor in a drive train. Because a rigid

coupling cannot accommodate misalignment between shafts, precise alignment of

machinery is necessary when one is used.

ii) Flexible Couplings:- Flexible couplings accomplish the primary purpose of any

coupling; that is, to transmit a driving torque between prime mover and driven machine.

In addition, they perform a second important function: they accommodate unavoidable

misalignment between shafts. A proliferation of designs exists for flexible couplings,

which may be classified into two types: mechanically flexible and materially flexible.

A. Mechanically flexible coupling:- Mechanically flexible couplings compensate for

misalignment between two connected shafts by means of clearances incorporated in the

design of the coupling.

B. Material flexible coupling:- These couplings rely on flexing of the coupling

element to compensate for shaft misalignment. The flexing element may be of any

suitable material (metal, elastomer, or plastic) that has sufficient resistance to fatigue

failure to provide acceptable life. Material flexible shaft couplings can be divided into

two basic groups: elastomeric and non-elastomeric.

Elastomeric couplings use either rubber or polymer elements to achieve flexibility.

These elements can either be in shear or in compression. Tire and rubber sleeve designs

are elastomer in shear couplings; jaw and pin and bushing designs are elastomer in

compression couplings.

Non-elastomeric couplings use metallic elements to obtain flexibility. These can be one

of two types: lubricated or non- lubricated.

Lubricated designs accommodate misalignment by the sliding action of their

components, hence the need for lubrication. The non-lubricated designs accommodate


misalignment through flexing. Gear, grid and chain couplings are examples of non-

elastomeric, lubricated couplings. Disc and diaphragm couplings are non-elastomeric

and non- lubricated

Fig: Elastomer coupling (flexible type coupling)

2.4.13 Suction and Discharge Nozzle

The suction and discharge nozzles are part of the casings itself. They commonly have the

following configurations.

1. End suction/Top discharge - The suction nozzle is located at the end of, and

concentric to, the shaft while the discharge nozzle is located at the top of the case

perpendicular to the shaft. This pump is always of an overhung type and typically

has lower NPSHR because the liquid feeds directly into the impeller eye.

Top Discharge

End Suction


2. Top suction /Top discharge nozzle -The suction and discharge nozzles are

located at the top of the case perpendicular to the shaft. This pump can either be

an overhung type or between-bearing type but is always a radially split case pump.

Top Suction Top Discharge

3. Side suction / Side discharge nozzles - The suction and discharge nozzles are

located at the sides of the case perpendicular to the shaft. This pump can have

either an axially or radially split case type.

Side Suction

Side Discharge


2.5 CRITERIA IN PUMP DESIGN

Following points are taken into consideration, according to American Petroleum

Institute (API) standards, while selecting a pump:

2.5.1 BASIC DESIGN CONDITIONS

1) Pumps shall be designed & constructed for a minimum service life of 20 yrs.

& at least 3 yrs. of uninterrupted operation.

2) Pumps shall be capable of operation at the normal & rated operating points &

any other anticipated operating conditions.

3) Pumps shall be capable of at least 5% head increase at rated conditions by

replacement of impeller.

4) Pumps shall be capable of operating at least upto the maximum continuous

speed (equals 105% of rated speed).

5) Pumps that have stable head/ flow rate curves are preferred for all applications.

6) Pumps shall have a preferred operating region of 70% to 120% of best

efficiency flow rate of the pump.

7) The best efficiency point for the pump should be between the rated point & the

normal point.

8) For pumps with heads greater than 200m & more than 225kw the radial

clearance between the diffuser vane & the impeller shall be at least 30% of the

maximum impeller blade tip radius for diffuser design & at least 6% for volute

design.

% clearance, P = 100(R2 – R1)/R1

R2 – Radius of volute

R1 – Maximum impeller blade tip radius

9) Cooling system if specified by the purchaser is used. To avoid condensation,

the minimum temperature at the cooling water inlet to bearing housing should

be above the ambient air temperature.

10) Spares & all replacement parts of the pump & all furnished auxiliaries

shall as a minimum meet the criteria of the API codes.

2.5.2 WEAR RINGS AND RUNNING CLEARANCES

1. Mating wear surfaces of hardenable materials shall have difference in Brinell

hardness no. of at least 50 unless both the stationary & the rotating wear

surfaces have Brinell hardness no. of at least 400.

2. Renewable wear rings if used shall be held in place by a press fit with locking

pins, screws or by tack welding.


3. The diameter of hole in a wear for radial pin shall not be more than 1/3 the

width of the wear ring.

4. Running clearances shall be sufficient to assure freedom from seizure under all

specified operating conditions.

5. For materials with low galling tendencies the clearance shall be used within

the range 010 – 037 inches & for materials with high galling tendencies

operating at above 200 C, 125 um shall be added to the above diametric

clearances.

6. For non-metallic wear rings materials with very low galling tendencies

clearances less than above should be used.

2.5.3 MECHANICAL SHAFT SEALS

1. Seal cartridge shall be removable without disturbing the driver.

2. Seal chamber face runout should not exceed 5 um/mm of seal chamber box.

3. Seal chamber & seal gland shall have provisions for for only those connections

required by the seal flush plan.

4. Provision shall be made to ensure complete venting of the seal chamber.

5. If specified jackets shall be provided on seal chambers for heating.

2.5.4 BEARINGS & BEARING HOUSINGS

1. Each shaft shall be supported by 2 radial bearing & 1 double acting axial bearing.

2. Thrust bearings shall be sized for continuous operation under all specified

conditions including maximum differential pressure.

3. Rolling element bearings shall be mounted directly on the shaft & shall be

retained on the shaft with an interference fit.

4. Bearing housings shall be arranged so that bearings can be replaced without

disturbing pump drives or mountings.

5. Sufficient cooling, including an allowance for fouling shall be provided to

maintain oil & bearing temperature.

6. Bearing housing for rolling element bearing shall be designed to prevent

contamination by moisture, dust & other foreign matter.

7. Shielded or sealed bearings shall not be used.

2.5.5 LUBRICATION

For lubrication of bearing & bearing housings following points shall be taken into

consideration:

1. Unless otherwise specified, bearings & bearing housing shall be designed for oil

lubrication.


2. The operation & maintenance manual shall describe how the lubrication system

circulates oil.

3. If specified, provision shall be made for either pure oil or purge oil lubrication.

4. If specified, rolling element bearing shall be grease lubricated.

2.5.6 MATERIAL

Material for pump elements is selected on the basis of following points:

1. The purchaser shall specify the material class for pump parts.

2. The material specification of all components shall be clearly stated in the

vendor’s proposal.

3. The vendor shall specify the optional test & inspection procedures that are

necessary to ensure that materials are satisfactory for the service.

4. Pump casing parts of double casing pumps that are to handle flammable or

hazardous liquids shall be of carbon steel or alloy steel.

5. The purchaser shall specify any erosive or corrosive agents present in the process

fluids & in site environment.

6. The purchaser shall specify the amount of wet H2S that may be present.

2.6 Criterion for selection of motor

The power of the motor should be taken as the greatest of the following three values:

1. At the rated point

2. At the end of curve

3. At the minimum continuous flow

After selecting the largest value it should be multiplied by suitable safety factor

depending on its value. The safety factor is to be chosen as follows:

1. 1.25 for P less than 22KW

2. 1,15 for 22KW < P < 55KW

3. 1.1for P greater than 55KW


FORMAT FOR PUMP DATASHEET

1

GENERAL

2

Project: Job No.:

3

Owner: Site:

4

Purchaser: Max./Min.

Ambient C:

Unit: Unit No:

5

Item No.: Service:

6

No. Reqd.: Working Standby Parallel Operation Required: Yes No 7

Applicable to Proposal Purchase As Built 8

Scope option & Information specified by purchaser Information Reqd. from & option left to vendor. Vendor to cross the selected option. 9

Driver: Working Standby Driver Supplied & Mounted By: Pump Mfr. Other 10

OPERATING CONDITIONS

11

Liquid Handled Capacity (m3/hr): Min/Nor/Rated:

12

Pumping Temp. ( C): Normal Max. Discharge Pressure (kg/cm²,A):

13

Specific Gravity at P.T./15 C: Suction Pressure: Nor./ Max. (kg/cm²,A):

14

Vapour Pressure at P.T. (kg/cm²,A): Diff. Pressure (kg/cm²) @ Rated Capacity:

15

Viscosity at P.T. (cP/cst): Corr./Eros. By: Diff. Head (m) @ Rated Capacity:

16

Solids in suspension Yes No Size: % NPSH Available (m):

17

MANUFACTURERS SPECIFICATIONS

18

Pump Manufacturer: Model No.:

19

CONSTRUCTION PERFORMANCE

20

Casing Mounting: Centerline Foot Inline Proposal Curve No.

21

Casing Split: Axial Radial Visc. Corr. Factor: C CQ CH

22

Type: Single Volute Double Volute Diffuser NPSH Reqd. (Water) (m): F/L Speed (rpm):

23

Casing Connection: Vent Drain Gauge No. of stages: Efficiency (%):

24

Nozzles Size ANSI Rating Facing Position Rated BKW(0% Tol.): kW Max.BKW rtd. Imp.: kW

25

Suction BKW @ MCF( =1.0): kW Rec. Driver Rating: (kW) kW

26

Discharge Max.head rtd imp.(m): Cap@ BEP(m3/hr):

27

Imp. (mm) Max: Rated: Min: Type: MCF (m3/hr):Stable Thermal

28

Brg.: Type/No. Radial: Thrust: Lub: M.A.W.P @ 15 C/P.T./Design Temp.(kg/cm²,G):

29

Cplg.:Make/Type: Fleximetl w spacer Nonspark Guard Yes No Hydrostatic Test pressure (kg/cm²,G):

30

Driver Half cplg. mounted by: Pump Mfr. Others Rotation facing coupling end: CW CCW 31

Packing Type: Size: No. of rings: Seal flush/ Quench plan: Material :

32

Mech. Seal: Make Model::

API Code : Ext. seal flush fluid: LPM: @ kg/cm²G/ C

33

Base Plate Drain Rim Type : Yes No Fdn. Bolts: Yes No Seal Barrier fluid: LPM: @ kg/cm²G/ C

34

Throat Bush: Yes No No Matl.: Bal. Device: Yes No Ext. quench fluid: LPM: @ kg/cm²G/ C

35

Materials (API-610 Matl. Class): MOC ASTM Grades C.W. Plan : LPM: @ kg/cm²G/ C

36

I - Cast Iron Casing Weight(kg): Pump+Base+Coupling: Driver:

37

B - Bronze Impeller AUXILIARY PIPING INTERFACE CONNECTIONS 38 B - Bronze Impeller

(All interface conn.shall be termntd.with a f/l. block valve) 39

S - Carbon Steel Inner Case parts (All interface conn.shall be termntd.with a flng. block valves)

40

C - 11-13% Chr. Stl. Sleeve Packed Size Rating(ANSI) Facing 41

h - Hardened Sleeve Seal Lantern Ring Inlet/Outlet

42

f - Faced Casing ring H-BHN Ext. Seal flush fluid Inlet/Outlet

43

K -SS 304 Impeller ring 50(min) Seal Quench fluid Inlet

44

L -SS 316 Shaft Seal pot vent/ drain

45

X Throttle Bush Casing vent/ drain

46

Y Throat Bush C.W Inlet/ Outlet

47

Z Balance Drum Base plate drain (only flanged)

48

Driver suitable for Pump starting with open Disc. Valve condition. Casing steam jacket

49

INSPECTION & TESTS (EACH PUMP) SHOP INSPECTION & TESTS (EACH PUMP) 50

Witness Observe Witness Observe

51

Shop Test / Inspection NPSH As Reqd. Per Spec. Mandatory 52

Material Certificates Dismantle Insp. & Re-assembly after Test 53

Hydrostatic Unitisation/Dimensional Check 54

Performance/Sound Level Check for direction of rotation of pump & driver.

55

Applicable Specification: API Std. 610, Edition, alongwith EIL Std. Spec.No. 6-41-

56

REMARKS:- 1) Max. allowable casing working pressure shall not be less than kg/cm²G @ C.

57

2) Down Stream Design Pressure is kg/cm²g.

58

3) Accessories and Instrumentation shall be as per EIL approved vendors only.

59

4) Unitization of Pump and Driver shall be done in pump manufacturer's shop.


2.7 SUNDYNE PUMPS:

It is a centrifugal vertical type of Pump(API OH 6) ,they are able to achieve high heads

with very low NPSH(a) using their conical diffuser and straight vaned impellers by

running at relatively high speeds. Unlike most centrifugal pumps, it operates at impeller

speedup to 23400 RPM.


2.8 Double Suction Pumps

DOUBLE SUCTION, SPLIT CASE, HORIZONTAL CENTRIFUGAL PUMP

Higher flows moderate head

Used when low NPSHA


2.9 Axial Flow Pump

Applications

- Pumping from a pit


2.10 Barrel or Can Pump

Applications

Barrel Pump is used when no NPSHA

Submersible pump is used in Tank


2.11 Multistage Pump


Applications

High head applications (Around 400 m & above)

SINGLE STAGE:

In this type of pump head is developed by a single impeller.

MULTISTAGE PUMP:

In this type of pump head is developed by the use of two or more impeller operating in

series each taking suction from the discharge of the preceding impeller

Approx 150 m head can be achieved per impeller


CHAPTER-3

POSITIVE DISPLACEMENT PUMP

Positive displacement- in which energy is periodically added by application of

force to one or more movable boundaries of any desired number of enclosed,

fluid containing volumes, resulting in a direct increase in pressure up to the value

required to move the fluid through valves and ports in to the discharge line.

Displacement pumps are essentially divided in to reciprocating and rotary types,

depending upon the nature of the pressure producing members.

3.1 Types of the Positive Displacement Pumps.

A. RECIPROCATING:

PISTON

PLUNGER

DIAPHRAGM

B. ROTARY:

TWIN SCREW

SINGLE SCREW (PROGRESSIVE CAVITY)

GEAR

VANE

LOBE (TWO, THREE LOBES)

3.2 Difference between Centrifugal Pump & Positive Displacement

Pump


3.3 Advantages of PD pumps over centrifugal Pumps

1. Flow is independent of pressure. You can change the flow without upsetting the

pump's efficiency.

2. The pump can handle high viscosity fluids efficiently.

3. The pump is self priming

4. You can get the desirable high head low flow combination that is need in many

high pressure applications.

5. They give you a non-shearing act that will not degrade sensitive

petrochemicals and polymers.

3.4 Pump Characteristic for PD Pumps

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.

The dashed line 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.

Some commonly used terms

Slip: Leakage flow within a rotary positive displacement pump from the discharge

back to the suction caused by the clearances needed between the rotating and

stationary parts.

Pulsation: The variation of pressure in line due to flow variations caused by piston,

plunger or diaphragm which are creating a pumping action.


Self priming: The ability for a pump to draw liquid into itself and start pumping

liquid by evacuating the air or vapour. PD pumps are inherently self priming.

3.5 When to use Positive Displacement Pumps

RECIPROCATING PUMP:


3.6 PLUNGER PUMPS

Power end is similar to that of Piston Pumps. The difference lies in the fluid end,

where the plunger runs through the packing like a piston rod.


3.7 Reciprocating Diaphragm type Pump:

Diaphragm Pumps are displacement pumps with flexible membranes clamped at their

peripheries in sealing arrangement with a stationary housing. The central portion moves

in a reciprocating manner through mechanical means such as crank or eccentric cam or

by fluid means such as compressed air or liquid under alternating pressure. .


3.8 ROTARY TYPES PUMPS

3.8.1 Main Features of Rotary Positive Displacement Pump

Positive Displacement

Slow and medium speed

Self Priming

Fairly constant discharge

Less vibration

Weight per unit flow is lower when compared to Reciprocating Type Pumps

Because of less number of parts in contact with each other, lower friction

(hydraulic and mechanically), higher efficiency.

Rotary pumps are often employed in systems where small flows at relatively high

pressures are required. Rotary Pumps are used in the lubricating and control

systems of turbine sets, large pumps and compressors, hydraulic systems.

An advantage of all Rotary Pump types is that they can be directly coupled to the

drives, which make the unit compact.


3.8.2 Screw Pumps

Screw Pumps are operating on the principle of progressively moving a fluid between sets

of counter-rotating screws. Screw Pumps have been traditionally chosen to pump viscous

fluids and impart minimum shear forces on the fluid with relatively low discharge

pressures.

Screws Pumps do not have drawbacks like speed limitations and discharge pulsations

and are characterized by uniform discharge high pressures, high speed, quiet operations

and high efficiency.

Screw Pumps are more expensive than gear and rigid vanes pumps of the same

performance because of manufacturing of specially profiled screws involve complicated

techniques.

Working of Twin Screw Pump:

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.


3.8.3 Working of Triple screw pump

The three-screw, high-pitch, screw pump, 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.

Twin v/s Triple screw pumps

Twin screw Pumps

Very effective with Viscous

fluids

Dry running is also permitted

as rotating elements operate

without contact.

Entrant gas or air can also be

pumped without interrupting

the flow

Four mechanical seals are

required

Large size, More expensive

Triple Screw pump.

Handles only clean

lubricating fluids

Must not run Dry as the

screws are in close contact

If the liquid being handled

congeals at low temp, then

heat the pump casing

sufficiently otherwise the

pump element would adhere

to each other.

One or maximum two seals

are required.

Compact in size and less

expensive.

3.8.4 GEAR PUMPS

Gear Pump traps the liquid between the gear teeth on the suction side and carry it around to the

discharge side from where it forced out into the discharge pipe.


Advantage Disadvantage

Two moving parts

One stuffing box

Positive suction, non-pulsating

discharge

Ideal for high viscosity liquids

Constant and even discharge

regardless of varying pressure

conditions

Low NPSH required

Easy to maintain

Low speeds usually required

Medium pressure

One bearing runs in pumped

product

Overhung load on shaft

bearing

Working of Gear Pump

3.8.4 .i) External Gear Pump



High speed

Medium pressure

No overhung bearing loads

Relatively quiet

Design lens itself to use of a wide

variety of materials

Four bushings in liquid area

Four stuffing boxes

No solids allowed

Applications

Industrial and mobile applications

Fuel and lubrication

Metering

Mixing and blending (double pump)

Hydraulic applications

OEM configurations

Precise metering applications

Low-volume transfers

Light or medium duty

3.8.4.ii) Internal gear pump



Two moving parts

One stuffing box

Positive suction, non-

pulsating discharge

Ideal for high viscosity

liquids

Constant and even

discharge regardless of

varying pressure conditions

Low NPSH required

Easy to maintain

Low speeds usually required

Medium pressure

One bearing runs in pumped

product

Overhung load on shaft

bearing

working of Internal Gear Pump:

Internal Gear V/s External Gear

Ideal for High Viscosity Low to medium viscosity

Use less of space Use of more space

One stuffing box Four stuffing Box

Overhung load of the shaft Load is divided with between bearing design


3.8.5 VANES PUMP

Liquid is drawn into and discharged from an axial hole in the rotor, which is

divided into suction and discharge chambers by tight fitting end covers.

As the rotor rotates in the direction indicated, space between the vanes grows in

volume, the result being that the liquid is drawn in from suction chamber through

radial holes.

As the vanes run along the volume of space is decreased and the liquid is

discharged into discharge chamber.

Working of Vane Pumps:

SLIDIDNG VANE PUMP . SWINGING VANE PUMP


Advantages Disadvantages

Medium capacity

Medium speed

Thin liquids

Sometimes preferred for

solvents, LPG

Can run dry for short

periods

Can have one seal or

stuffing box

Develops good vacuum

Can have two stuffing boxes

Medium pressure

Complex housing

Not suitable for high

viscosity

Not good with abrasives

Applications

Aerosol/Propellants

Aviation Service - Fuel Transfer, Deicing

Auto Industry - Fuels, Lubes, Refrigeration Coolants

Barge Unloading

Bulk Transfer of LPG and NH3

Chemical Process Industry

LPG Cylinder Filling

Ethanol/Alcohol Refining

Fertilizer Production - CO Transfer

Lubrication Blending - Solvents, Oils

Mobile Transport - Chemicals, Fuels, LPG, NH3

Petroleum Industry - Crude Oils and Hydrocarbons

Power Generation - Fuels, Lubrication

Pulp and Paper

Railroad Transfer - Fuels, Lube Oils, Coolant

Refrigeration - Freons, Ammonia

Rubber and Plastic

Solvent Distribution


3.8.6 Lobe pump


Pass medium solids

High acceptance

Little galling possibility

Timing gears

More space required

May require factory service to repair

Two seals

Working of lobe Pumps:

Applications: Food processing.

Beverages.

Dairy Produce.

Personal Hygiene Products.

Pharmaceutical.

Biotechnology.

Chemical.

Industrial.

Medium and heavy duty cycles.


3.8.7 PROGRESSIVE CAVITY PUMP

It is used for pumping difficult materials such as sewage sludge contaminated with

large particles or highly viscous liquid containing solid particle, this pump

consists of a helical shaped rotor, about ten times as long as its width.

This can be visualized as a central core of diameter , with typically a curved spiral

wound around . This shaft fits inside a heavy duty rubber sleeve. As the shaft

rotates, fluid is gradually forced up the rubber sleeve. Such pumps can develop

very high pressure at quite low volumes.

1>Rotor,2>Stator,3>Drive train with joint,4>Shaft Seal,

5> Suction / Discharge housing, 6>lantern (housing) with flanged

drive

3.9 Selection between Reciprocating Pumps


3.9.1 Depending on solid particles in the fluid for rotary Pumps

3.9.2 Depending on the flow requirement for rotary pumps


CHAPTER-4

PROCUREMENT/ORDERING SYSTEM IN GAIL PATA

4.1 Categorizations of Material Requisition

The following considerations were made for categorizations of MR (Material

Requisition):

1. SINGLE STAGE CENTRIFUGAL PUMP

2. MULTISTAGE CENTRIFUGAL PUMP

3. SUNDYNE PUMP

4. RECIPROCATING PUMP

5. VERTICAL PUMP

6. MEETING THE BIDDERS CRITERIA FOR NPSHA

7. COSTING OF MR SHOULD BE LESS THAN 10 CR (POA CASE)

4.2 Summary of Ordering system of Pump for Cracker Unit of GAIL PATA

MR No.

MR Description

MR QTY.

PO QTY.

Bids Recd. Date

TBA Relsd.

Price Bids

Opening

Sch. Date

of Order

Act. Date

of Order

5580 CENTRIFUGAL HORIZONTAL PUMP

65 63 21 JUL 12 AUG 25 AUG 22 JUL 02 SEP

5570 PUMP CENTRIFUGAL HORIZINTAL

5 5 01 SEP 19 SEP 26 SEP 20 JUL 19 OCT

5560 PUMP CETRIFUGAL PUMP

10 10 05 SEP 07 OCT 25 OCT 17 JUL 09 NOV

5620 PUMP CENTRIFUGAL PUMP

18 18 8 NOV 07 DEC 14 DEC 30 NOV 20 DEC

5550 VERTICAL PUMP 16 14 9 NOV 21 NOV 19 DEC 25 OCT 22 DEC

5590 RECIPROCATING PUMP

8 8 27 DEC 06 FEB 17 FEB 13 AUG 04 MAR

5552 PROGRESSIVE CAVITY PUMP

2 2 31 OCT 5 JAN 18 JAN 15 NOV 28 JAN


4.3 Evaluation Criterion:

1. The offered Model of Pump by bidder should meet Pump datasheet parameter.

2. The NPSHA should normally be at least 0.6m above the NPSHR of offered

model.

3. By Analyzing the Pump Characteristic Curves, i.e.

Pump Curve shall meet the rated condition of Pump Datasheet

BEP should be between the rated point and the normal point

The head capacity characteristic curve should continuously rise as flow is

reduced to shutoff (or zero flow).

Operating Point should be between MCF and 120% of BEP

Do not select the pump at max. Impeller dia. (5% head rise should be

possible)

4. By reviewing the Performance Track Record of offered model.

5. Power loading

4.4 EVALUATION OF PRICES IN CASE OF PUMPS

TOTAL EVALUATED COST = A+ B + C

A Total Capital cost of Package/ item comprising:

Basic quoted price of equipment including

Commissioning, special tools & tackles and

Mandatory Spare

Freight

Taxes & duties i.e Excise Duty + Educational

Cess (10.3%), Sales Tax (CST with

concessional Form i.e. 2% / VAT), Service Tax

(applicable on freight & services @10.3%)


B Differential Operating Cost, i.e.

B= N(OP) * (BKWE – BKWR) * CF * 8000*DF

WHERE,

N(OP) = NO. OF OPERATING UNITS

BKWE = GUARANTEE SHAFT POWER(KW) FOR

PUMP QUOTED BY BIDDER UNDER

EVALUATION.

BKWR = LOWEST QUOTED (GUARANTEE) PUMP

BKW (AMONGST THE TECHNICALLY

ACCEPTABLE BIDDERS)

CF = COST OF ENERGY i.e. 3.84 RUPEES PER KWH

8000 = NUMBER OF OPERATING HOURS PER YEAR

DF = DISCOUNTING FACTOR TO ARRIVE AT NET

PRESENT VALUE(NPV) BASED ON NO. OF

OPERATING YEARS (i.e. ∑ N=2 TO N = K+1 [ 1 / {1

+(X/100)}]N = 2.915)

X = PERCENTAGE RATE OF INTEREST = SBI BASE

RATE ON THE DATE OF PRICE OPENING + 5% =

10% + 5% = 15%

NOTE:- Power loading applicable for Centrifugal Pump

only.

C Cost of Supervision of Erection , Testing &

Commissioning (i.e. no. of pumps X no. of Mandays X

per diem rate)

4.5 Problems faced & Lessons learnt

1. Changes in data sheets by Process after floating of enquiry.

2. Revised MR issued rather than amendments showing key changes.

3. Though limited enquiry was issued as per EIL MSL, PTR requirement

was there and many vendors didn’t submit along with offer.

4. Though zero deviation tendering TQ’s were issued.

5. Cost estimate data bank not accurate always.

6. Type of pump changed by Licensor based on our feedback regarding

Charcoal particle presence in fluid (type of pump changed from screw

to progressive cavity).


7. Seal related issues (packing / dual 53A & 53B)

8. Limited source for supply of Vertical pumps - Landed in single vendor

case.

4.6 Areas for improvement

1. Pre tendering / Pre Bid must

2. Data bank for various types of models offered to be available with RED

rather than seeking PTR for each MR

3. Costing data bank to be strengthened

4. MR to be concise rather than bulky

5. Only addendum covering changes in MR clauses to be issued rather

than revision of entire MR

6. TBA format needs revision.

7. MR should preferably specify end – top arrangement for ease of piping

4.7 Important Documents for review

1. Pump General arrangement drawing

2. Pump Cross sectional drawing

3. Pump Datasheets and Characteristic curves.

4. Utility Data

5. Pump Motor data or Turbine datasheet with P&IDs.

Write Up on Pumps

Documents

Transcript of Write Up on Pumps