Know and Understand Centrifugal Pumps Episode 1 pps

Contents Prologue About the Authors 1 Basic Pump Principles Introduction How do pumps work?. Pressure measurement Atmospheric pressure ATM Absolute pressure psia Gauge pressure psig Vac

Know and Understand

CUSTODIO

c

Know and Understand Cen t r if ug a I Pumps

Know and Understand Cent r if u g a I

Pumps

by Larry Bachus and Angel Custodio

ELSEVIER

U K

USA

JAPAN

Elsevier Ltd, The Boulevard, Langford Lane, Cdlington, Oxford OX5 l G B , UK

Elsevier Inc, 360 Park Avenue South, New York, NY 1001 0- 171 0, USA

Elsevier Japan, Tsunashima Building Annex, 3-20- 1 2 Yushima, Bunkyo-ku, Tokyo 11 3, Japan

Copyright 0 2003 Bachus Company, Inc

Published by Elsevier Ltd

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British Library Cataloguing in Publication Data

Bachus, Larry

Know and understand centrifugal pumps

1 Centrifugal pumps

I.Title II.Custodio, Angel

6 2 1.6’7

ISBN 1856174093

N o responsibility is assumed by the Publisher for any injury and/or damage to

persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein

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Elsevier Advanced Technology,

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Contents

Prologue

About the Authors

1 Basic Pump Principles

Introduction

How do pumps work?

Pressure measurement

Atmospheric pressure (ATM)

Absolute pressure (psia)

Gauge pressure (psig)

Vacuum

Pump head

Specific gravity

Pressure measurement

Pressures inside the pump

2 NPSH, Net Positive Suction Head

Introduction

Definition of NPSHr (required)

Definition NPSHa (available)

3 Cavitation

Introduction

Vapor pressure

Cavitation

The effects of vapor pressure o n pump performance

Cavitation: A practical discussion

Review for preventing cavitation

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Contents

Cavitation review

4 The Affinity Laws

Introduction

The Laws

The Affinity Laws and the impeller diameter

What’s the practical application of these laws?

5 Useful Work and Pump Efficiency

Useful work from a pump

Flow determination

Pump efficiency

Factors that affect the efficiency

Calculating pump efficiency

6 Pump Classification

Introduction

Positive displacement pumps

Centrifugal pumps

Conceptual difference

Centrifugal volute pumps

Types of centrifugal pumps

Overhung impeller

Impeller between the bearings

Turbine pumps

Specific duty pumps

API (American Petroleum Institute) pumps

Vertical turbine pumps

Non-metallic pumps

Magnetic drive pumps

Canned motor pumps

Pump impellers

Turbine impellers

Conventional impellers

Suction specific speed, Nss

Open impellers

Semi open impeller

Totally enclosed impeller

Wear bands

Specific speed, Ns

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7 Understanding Pump Curves

Pump performance curves

History

Head versus pressure

H-Q

Pump efficiency

The energy (BHp) curve

The pump’s minimum requirements (NPSH)

Special design pumps

Family curves

The system controls the pump

The elements of the Total Dynamic Head ( T D H )

Determining the Hs

Determining the H p

Calculating the H f and Hv

The dynamic system

Variable elevations

The happy zone

Dynamic pressures

Variable resistances

Short term resistance changes

Long term resistance changes

Pumps in parallel and pumps in series

Pumps in parallel

Three tips

Pumps running in series

Combined parallel and series pump operation

9 Shaft Deflection

Introduction

Operation, design and maintenance

Signs of shaft deflection

Interpreting the evidence

The sweet zone

The dual volute pump

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Contents

10 Pump and Motor Alignment

Introduction

Types of misalignment

Alignment techniques

Equipment alignment sequence

Coupling alignment

11 Bearings

Introduction

Bearing lubrication

Bearing failure

Bearing maintenance

Bearing seals

12 Pump Shaft Packing

History

Vegetable fibers

Reciprocating action

Packing

Rotary action

Synthetic fibers

Compression packing

The lantern ring

The packing lubricant

Stages in the life of packing

13 Mechanical Seals

Pump packing

History

The mechanical seal

The single, unbalanced, inside mounted mechanical seal

The single, outside-mounted, unbalanced seal

The single, balanced, internal mechanical seal

The single, balanced, external mechanical seal

Advantages of O-rings

The balance effect

Advantages of balance

Balance explained by math

Cartridge mechanical seals

Double seals

The tandem dual seal

The back-to-back double seal

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Contents

The face-to-face dual seal

Support systems for dual seals

The thermal convection tank

The pumping unit

14 Failure Analysis of Mechanical Seals

Causes of premature seal failure

O-ring (the elastomer) failure

The elastomer sticks to the shaft

The shaft frets under the shaft seal

Incorrect installation dimension

Environmental controls for difficult sealing applications

Difficult pumping applications for mechanical seals

Environmental controls

Proper pump repair alignment methods

15 Common Sense Failure Analysis

Pump maintenance files

Failure analysis on centrifugal pumps

Why is this pump in the shop?

16 Avoiding Wear in Centrifugal Pumps

Introduction

Erosion

Corrosion

Wear rings

Fluid velocity accelerates wear

Turbulence

Throttling

17 Pump Piping

Introduction

The Submergence Laws

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Index

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Prologue

About the Authors

Angel specializes in the installation and imple- mentation of Preventive Maintenance Systems through his consulting engineering company formed

in 1987 in Puerto Rico His installations have given

approaches to hands-on maintenance and operator

Engineers

Know and Understand Centrifugal Pumps

/ \ r

suction piping The fluid must be available to the pump with sufficient energy so that the pump can work with the fluid’s energy The pump cannot suck on or draw the liquid into the pump The concept of the fluid being available to the pump is discussed in detail in Chapter 2 of this book

Positive displacement (I’D) pumps take the fluid at the suction nozzle and physically capture and contain the fluid in some kind of moveable enclosure The enclosure may be a housing with a pulsing diaphragm,

or between the teeth of rotating gears There are many designs The

the fluid into the pump The captured fluid is physically transported

Inside the pump, the expanded moveable enclosure then contracts or the available space compresses This generates a zone of high pressure inside the pump, and the fluid is expelled into the discharge piping,

that a PD pump can generate is mostly a function of the size of the pump housing, the speed of the motor or driver, and the tolerances between the parts in relative motion The pressure or head that a PD pump can develop is mostly a function of the thickness of the casing

diaphragm, or meshed gear teeth As these parts wear, the pump will

changed with a degree of frequency based on time and the abrasive and lubricating nature of the fluid Changing these parts should not be

Basic Pump Principles

- ROTATION

Fiaure 1-3

viewed as breakdown maintenance Nothing is broken This periodic servicing is actually a production function to return the pump to its best or original efficiency

Centrifugal pumps also require that the fluid be available to the pump’s suction nozzle with sufficient energy Centrifugal pumps cannot suck or draw the liquid into the pump housing The principal pumping unit of

The impeller is attached to a shaft The shaft spins and is powered by the motor or driver We use the term driver because some pumps are

impeller and is trapped between the impeller blades The impeller blades contain the liquid and impart speed to the liquid as it passes from the impeller eye toward the outside diameter of the impeller As the fluid accelerates in velocity, a zone of low pressure is created in the

the pump with sufficient energy

The liquid leaves the outside diameter of the impeller at a high rate of speed (the speed of the motor) and immediately slams into the internal casing wall of the volute At this point the liquid’s centrifugal velocity

Bernoulli Principle in reverse) Because the motor is spinning, there is also rotary velocity The fluid is conducted from the cutwater around

pathway increases, the rotary velocity decreases and even more energy