API - Centrifugal Pump [Section 1,2,3].pdf

Training For Professional Performance

This manual is one of a seri es for your use in learning more about equipment that you wo rk with in the oilfield. Its purpose is to assist in developing your knowledge and skills to the point that you professional manner.

In order for you to learn the contents of the manual, you must dig out the pertinent facts and relate them to the subject. Simply reading the material and answering the questions is not enough. The more effort you make to learn the material the more you will learn from the manual.

The manual was prepared so that you can learn its contents on your own time, without the assistance of an instructor or classroom discussion. Educators refer to learning by self -study as Programmed Learning. It is a method widely used in all industries as a means of trai ning employees to do their job properly and teach them how to perform higher rated jobs.

Teaching yourself requires seifdiscipline and hard work. In order to prepare yourself for the sacrifice you will have to make, you should set goals for yourself. Your ultimate goal is to perform your work in a more professional manner. Training is one step in reaching tha t goal. Application of what you learn- is another. Seeking answers to questions is a third.

You can demonstrate your desire to be a professional by taking a positive attitude toward learning the contents of this manual and others that are applicable to your job.

Once you have established your final goal, you must determine the means for reaching that goal. You may decide, for example, that you must complete a series of 10 or 15 manuals. to get the basic knowledge and skills you need. After you decide which training mate rial is required, you should set a time table for completing each section of the material.

can

perform

your

work

in

a

more

The au thor of tliis manual has years of experience in operating petroleum He also has t he tech nical equipment. knowledge of how and why petroleum equipment functions. The text was written _ . for use by personnel with little or no ? previous experience with petroleum equipment. Consequently, some of the mate rial may be familiar to yo u if you have experience with oilfield equipment. From such experience, you have observed the effect of making operating changes. The manual will help explain why the changes occurred that you observed. It will also teach you how and why equipment functions.

Achieving your final goal may take more than a year, and will require hours of hard work on your part. You will know you have achieved your goal when you understand how and why to operate oilfield equipment in order to obtain the maximum product at the lowest cost. Your sacrifice will have been worth-while from the satisfaction of knowi ng that you can perform your job in a methodical professional manner, instead of a trial-and-e rror-approach.

Instructions For Using This Manual This is your manual. You should write your name on the cover. Upon completion you will find it helpful to keep it in an accesslble place for fu ture reference. Problems may be included throughout the text. The solutions to the problems are given at the end of the book. The manual is used in traini ng programs all over the world. In some countries, English of measurement such as feet, gallons, etc., are used. In other countries Systems Internationale (SI) or Metric units, such as meters, liters, kilograms, etc., are used. In order for the manual to be of maximum use, both SI and English units are shown.

The following general procedure is recommended for using this manual :

1.

Turn to Page 1. Read the material until you come to the first problem or question.

2.

Work the first problem or answer the question and enter the answer in the proper space in ink. If the problem or question is shown in both SI and English units of measurement, answer only t he part in units of measurement tha t you use.

3.

Compare your answer with that shown at the end of the book; be sure to use solut ions to the problems in the units you are working in.

The SI unit always appea rs first, and the English unit follows in brackets []. Example: the temperature is 25'C [77'F], The English equivalent of the SI Unit will be rounded off to the nearest whole number to . plify the text and examples. A distance of m may be shown as 33 ft when the exact equivalent is 32.81 f1. If you are working in English umts, you may find it helpful to mark out the parts that are in SI units, and vice versa. Some of

the Figures have units of ~ 'ffieasurement. In such cases, two Figures are included. The first one has Sl units, and the Figure number is follow ed by the .letter A (Example: Figure lA). The second Flgure wllI be on the next page and will have English units. It will be the same number as the first one but it will be followed by the leiter 8 (Figure 18). If you use SI units, be sure to refer to Figures followed by the letter A; lf you use English units, refer to Figures followed by the letter 8.

If your answer is correct, continue reading unti 1 you come to the next problem and work it. If not, restudy the manual until you understand th e reason for your error. Rework the problem if necessary. Leave your wrong answer and note the correct one. This will keep you from making the same mistake later on. 4.

Proceed stepwise as shown above until you have completed the text.

The above approach will require thought, mak ing mistakes, and re t hinking the situa ti on. Concentrate on two things - the how and the why. Do not cheat yoursel f by taking shor t-cuts or looking up the answers m advance. It saves time and errors but produces no real understanding. Your future depends on how efficlentl y you perform your job and not on how rapidly you proceed t hrough this manual. Since this is your manual, any errors you make are private.

Abbrevjations Used In This Manual

Units Of Measurement

SI UNIT ABBREVIATIONS

SI UNITS OF MEASUREMENT

s, min h, d mm cm m km • 2 m m'

m'ld L g kg kPa MPa kPa(a) bar kJ MJ W,k W M

second, minute hour, day millimeter cen timeter meter kilometer square meter cubic meter cubic meters per day liter gram kilogram kilopascal megapascal kilopascal absolute bar (1 bar = 100 kPa) k ilojol~ e

megajoule watt, kilowatt meta

time time length leng th length length area volume volume rat e

MM

METRIC UNIT Pressure bar

SI UNIT

weight weight pressure pressure pressure pressure hea t, work heat, work power million

time second, minute t ime hour, dgy length inch, foot area square inch square foot area volum e cubic foot volum e gallon volume bal'rel (42 US gal) volume rate barr els per day weight pound pressure Ibs per square inch Ibs per sq in absolute pressure British thermal unit hea t thousands of Btu heat hea t millions of Btu power wat t , kilowatt power horsepower gas flow ra te cubic feet per day gas flow ra te thousands of cfl d gas flow rate millions of cfl d thousand million

Hea t

CONVERSION

ki lopascal, kPa bar =

volum e

ENGLISH UNIT ABBREVIATIONS s, min h, d in, ft sq in sq ft cu ft gal bbl BPD Ib psi psia Btu MBtu MMBtu IV , kIV hp d id Md/d MMcf/ d M

Most of th e SI units of measurement used in the oilfield are traditional metric units. The exceptions we are concerned wi th are pressure and hea t units, which differ as follows:

kil oca l kilojoul e, kJ

kPa

IOU

kJ kcal =[2

STANDARD CONDITIONS FOR GAS VOLUME Measuremen t units for gas volume are cubic met ers (m ' ) or cubic feet (cf). Th e lett ers st or s are some times used with t he units to designate volume at standard temperature and pressure: m ' (51) or scf. In this manual, st andard volumes are corrected to a temperature of 15 °C and a pl'essure of 101.325 kPa(a), or GO °F and 14.7 psia. To si mplify the tex t, the letters st and s are omitted However, aU gas volumes shown are at st andard conditions unless specifica lly stated otherwise.

HEAT CAPACITY AND REL ATIVE DENSITY Specific heat and specific gravity are traditi onal t erms that have been used in both Metri c and English uni ts for many years. Th ese names are being I'eplaced with th e words: hea t capacity and relative density. The new names are used in thi s manual. Wh en you see the term hea t capeci ty (H t Cap), it will have the same meaning as specific heat; and rela ti ve density (ReI Dens ) means specific gravity.

CENTRIF UGAL PUMPS

TABLE OF CO NTENTS

INTRODUCTION ...... . ......... . ... ... . . .... .. .. . . . ... . ..... . ...... I I.

DESCRIPTION OF CENTRIFUGAL PUMPS... . . .. . . ... . . . . •.. . .. . . .. 2 A.

Basic Pump Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . • . . . . . . . . . 2 1.

B. C. D. II .

III.

. ... ... . .... . ... ... .... . ... . . . .. . . .. . ... .... . . . . . . 2 Impeller. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Shaft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Bearings . ..... . . _. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2. 3. 4. 5. Seal or Packing . .... . .... . ............... . ... .. . .. . . ... . 3 Couplings ... . ....... . . . ...• .... .. .... . .. ... .. .... . . . .. . .... 4 Types of Cent rifgual Pumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6 Alternate Sealing Systems . .............. . . . .. . . . .. .. ... .. .. 10

PRINCIPLES OF CENTRIFUGAL PUMPS . . .. .......•....•......... 14 A. B. C. D E F.

Flow Through Pumps ............ . . .. . . .... ... .. ... . .. . ... ... Centrifuga l Force ... ... . .... ....... . ................ . .... . Head Pressure . ....... . ... . ..... .. ....... . .........•... .. ... Cavitation a nd Vapor Lock .. . . . . ... ............. . . . ........ . Performance Curves . . . . . .. .... . ..... . ... .... . ... .... .. .. ... Pump Efficiency ... . . .. . ... . ... . ..... ... ... . ......•. . .. . ...

G.

Driver Power .... .. ...... . ....... ... . . . ..... .. .. . . . ...... . . 24

H.

Liquid Suction Head

I. J.

Thrust . . ... .. . . . ... .. .. . . " ... ... .. . . . ..... .. ... . .... .. . . . 27

14 14 16 17 19 21

............ .. . ... . ... . .. ......... . . .... 25

Pump Curve Application

.. ... . .... .. .... . .•... . .. .. .. ... . . . . 29

OPERATION .... . ... . .. .. . .... . .... .. ... ... . ... . . . ... . . .. .. .. .. 35 A. B. C. D.

IV.

Case

Start-up Procedure . .... . . .. •.. . . . ..... . . . . .. . .. .... . . . . .. .. Control of Pump Flow Rate . . . . . . ... ... ... .. .. .. . .. .. . ... .. .. Shutdown Procedure ............ . ... ... .. .. ... ... .. . ........ Routine Operating Checks .. . .... . .... . . ... .. ... . .. .• . .. . .. ..

35 36 40 41

TROUBLESHOOTING .. . . .... . .. . ......... . ...... . . . ...... . ..... 42 A. B.

Troubles hooting Procedure for Vapor Lock . .. . . . . . . . . . • .. . ..•.. 42 Troubleshooting Procedure for Low Flow Rate .. . ......•........ 43

VALIDATION, SI UNITS .... . ... . .......... ......... . .. . . . ... . .. .. ... 45 SOLUTIONS TO PROBLEMS, SI UNITS . . .... . .... ... .•.. . . . . . .. . .. . ... 46 VALIDATION, ENGLISH UNITS

........ .. ... .. . . . . ... . . . . . . .. . . ... . .. 47

SOLUTIONS TO PROBLEMS, ENGLISH UNITS

............ . ........ ... . 48

LIST OF DRAWINGS, GRAPHS AND ILLUSTRATIONS

..... ... .............. . .. ......... ... .. ..... . .. ........

Impellers

2

Cut-away Picture of Pump

3

Packing and Seals

.. ....... .. .... .. .. ....... . .. .. ... . ............. .

4

.... .. . ... ... .... ... .. ... . ........ ..... . .. . ............

6

Couplings

Pump with 2 Seals . . .................. ... ... . .. ... .... .. . ...... .. . 10

Seal Oil Pots

. . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .. . . . . . . . . . .. 11

Circulating Seal Oil System

13

Flow Through P-um p ... .... . . ......... .. .... .. .... .. . . . .... ...... ... 14 Head Pressure

. .. . .......... . . ..... ..... .. . .. . ........ . . . ..... ... 17

Procedure to Clear Vapor Lock 18 Pump Performance Curves .• . ... .•. .. . ..• .. . . ..... . ... . .... . 20, 22, 23 Liquid Suction Head Thrust

.•....... • .. . .... •. . .... ... .... ....... . . . .. ... 26

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..

27, 28

Balance Piston . . ... . .... .. ................ . .. .... .. .......... . ... 28

Design Conditions for Stabilizer Feed Pump .. . .. . .. .... . .•........... 29 Performance Curves for Stabilizer Feed Pump

...... •. . .. • ....... . 30, 31

Start-up Procedure .............. • .... .. ........ . .•............. . Flow Control with Control Valve in Discharge Line Regulated with Level Controller

35

. .. ... .. .. .. .. .. . .. 36

Flow Control with Control Valve in Discharge Line .. ... ........ . .. .. , 37 RJ',!/ulated with Pressure Controller Low Flow Recycle .. ........ .. .•.. ...•. .. . ........ . •...... • ... . .. 38 Flow Contl'ol By Changing Driver Speed

... ........• .. . . •.. . ....... • 39

Effect of Pump Speed on Capacity, Pressure Head and Power . .. ... ...•. 40

CENTRJFUGAL PUMPS

INTRODUCTION

Pumps are used to force a liqu id to flow from a point of low preso,;;ure to one of highcr p,·c,-,u,·e. Ther Are two general cia

ifieutions o f pumps:

l. Positiv e Displuc mcnt Pumps

2. Centrifugal Pumps In thi., manual we will di ... cuss the Ccnlr'ifugal Pump. Posi tiv e Displacement Pumps

are discussed in Manual £-17 .

NOTE :

Thi s manual includes bo t h SI and Engli sh Units of measurem ent. I f you use English Unit ... , disl'egard the i\ietl'ic Units, and vice ve rsa. Refer to the instruc t ion page at the front o f the manual.

. . .

••• CENTRIFUGAL PUMPS

I. DESCRIPTION OF CENTRIFUGAL PUMPS

2

A. Basic Pump Parts A t ypical Centnfugal Pump is show n on the opposi t e page,

It has fi ve basic parts

which arc deseribed below : l.

Case - The pump ease

01'

othel' par'ls arc enc losed within it. other special mAteJ'ials.

casing is the visible part of the pump,

Most of the

It is usuAlly made of case iron or steel, plHStiC, or

1n the oilfie ld, casings on pumps operating at 8 pl'essure below

1000 kPa'[150 psiJ usually arc made of cast Iron,

Pumps opel'ating at highel' pressu ]'e

generally will have casing made of steel.

2,

Impeller - The Impelle]' is the part of t he pump that causes the liquid pl'essul'e to

rise, It is fu'mly attached to the shaft with a key and/or pI'essed on the shaft, It rotat es inside the case at the speed of the shaft. l'he Impeller on most oilfield pumps is made of cast iron . However', stainl ess steel, pla~llcl

or oUler' special ma ter ialS ca n be used for corrosive or chemical serv ice. 'J'h€!'c

are two gener.1 types of Impeller's; the open vane and the closed vane, The closed vane develops a higher pressure, but has a lower capaci ty.

CLOSED VANE IMPELLER 3,

OPEN VANE IMPELLER

Shaft - The shaft I'otates inside the case at the speed of the dl'iver, It usually is

made of .teel.

The portion or shaft exposed to the seal

01'

pack ing may have a sleeve

made of some hard metal, such as tungston carbide, to resis t corrosion or

WeBI'

at tl1a t

point.

-t.

Bearings - Bearings serve two functions on a pump:

a, To hold the shaft so that it does not wobble inside the pump easing, b, To prevent lateral movement or the shaft so that the rotating par ts do not touch the pump casing,

Th,'ust forces, developed as the impeller I'otates, are the main

PUMP PARTS

3

- --.

Bearings

Case

Shaft Seal

PARTS OF CENTRIFUGAL PUMP

cause of lateral shaft movement.

One or more of the bearings must be designed to

withst and the thrust forces . On small process pumps, the bearings may be contained in the pump casing. On larger pumps, the bearings are contained in housings located on one or both ends of the shaft. The bearings reqUIre lubrication.

The bearing housing shown above

is partially

filled with oil for lubricatrion. A sight glass indicates the level of oil in the housing. The bearings shown on the end of eacll shaft on Page 5 are a grease lubricated type. 5. Seal or Packing - The seal or packing is used to preven t liquid under pressure inside the pump from leaking out the pump. The mechanical seal is used in most oilfield cen trifugal pum ps. components:

It has two basic

PUMP PARTS

4

Sea l Gland Stationary Sea l Ri ng Rotating

fni~i;;z:;~seal Ring

,"

PACKING RINGS

Sha ft

MECHANICAL SEAL

a. 1\ statiooary ring thll t is secured in the sea l gland. b. A rotating "ing that is part of tile sea l element attached to the shaft. One of the seal ri ngs is made of cal'bonj the other is made of hardened steel, ceramic

Ot'

othe r speci al non-cor r'os ive material. Some seal manufacturers use a carbon

stati onary ring and oth er's a cHrbon "otating ring. Pack ing often is used in low pressUl'e service, or in pumps handling abrasive liquids such as mud or s]urr'y. P ac ~ ing

is composed of a series of pliable ri ngs contained in a packing gland. The

.. ings arc comp ressed by tighte ning the gland nuts. Th is squeezes the dngs aga inst the shaft and p,'events liq uid fro m leak ing ou t, "'Iecilanical seals generally req uire much less mainte nance than pack ing, so they are used whenever poss ible. When they are used, liquid must be free of sand, dirt, or other so lid partic les tha t

CUll

scratch the seal faces and cause leakage.

B. Couplings

The pump sha ft connects to the driver with a coupling. Coup lings trans mit ,'otation from the dri ver sha ft to the pump shaft. If a gearbox is between the drive,' and the pump, a coupling attaches the dr ive,' shaft to the inlet gearbox silaft. alld another coupling attaches the outlet bearbox shaft to the pump shaf t.

COU PLIN GS

5

Bearing s Coupling

The couplings ill ilst be able to withstand the shock of a sudden change in pump load, 0 1'

stoppage of t he dr ivel'.

They must be flexible enough to t,' ansmit power from the

dri ver to t he pump at high speed when the two shafts al'e not pcl'fectly al igncd, In fact, it is almost impossible to perfectly align the two shafts, because the operati ng tempel'a ture diffel'ence between the d,' iver and pump resul ts in one expanding slightly more than the otehr. The coupling must be able 10 1wobble' enough to overcome the misalignment.

Some of the more com man types of coupli ngs are shown on Page 6. In each type, the drivel' shaft attaches t o one half or hub of the coupling, and the pump shaft att aches to the other. The shafts al'e usually keyed to the coupling hubs.

Pl'oblem 1 Match each itcm in the column on the rig ht with one on t he left. Im pe ll er

a,

Prevents liquid inside pump f rom leaking out.

Case

b,

Pre ven t shaft movement.

Seal

c,

Connects pump and dr iver,

Shaft

d.

Open or closed vane,

Bear ings

e,

Rotates inside pump.

f,

Enclosure for rotating pump par t s.

__ Couplings

Ii

PUMP TYPES

FLEXlflU DISC COUI'UNG \ lubs attach to compo sitIOn di:.;cs that arc

casily replacl:d.

Hub

Sleeve CR It) COU,'UNG Hubs art" se rpent I

fils

'"

GEA R COUPLING

·;'".hed

with ,, ;"lng that

il' :";IU:~ In

Hubs with gear t eeth me sh with sleeves having rnatching t eeth.

each hub.

c . Types of Cent r ifugal Pu mps (\,'Iltl'igu:d pumps can eithC'1' be hOl'jzontaJ or vCl'ti('ul. Tht' hOl'i7Clnla Lpump I'l'quircs l:

'~rll'

plpirl~.

'olillda li on ror its mounti ng.

wh e r' ea~

the vertical pumr Can he nttaeilcd to the

l-.ilil 0 minimum of support bcnenth the pump.

/,i! ~~gvd P1lillp

The horizontal pump is 11 more

wl1i('11 will I'c~js t any vibration present.

V('I'll(,'al pump" are commonly used in process ph-Jots i n IOt'ations where vibrution is :!I)t "pf'Obk nt. I\nolilel' advantage or the vel' tical pump is thul the ulignlTlen t bf'twcen the !110({'I[' lind Dump

j-;

much easier to maintain than that of the horizontal pump.

7

PU MP TYPES

HORIZONTAL PUMP

VERTICAL PUMP 1. Mu l tistage Pumps As we win learn later, there will be occasions when 2 or more impellers are needed

for the pump to delivel' the required presslIre. called multistage pumps. impe llers is

Pumps with more than one impeller are

Each impellel' is referred 10 as a stage.

A pump with 5

a 5-slage pump.

There are three common types of mu lli-slage pumps;

1. Submersible

2. Can

3. HOl'izon tal The submersible pump is an integral pump-motOJ' un i t in a sealed enclosure. well, Ihe pump is insert ed inside the casing. tile motor.

In a

An electric cable runs from Ihe surface to

These pumps are used for lifting watel'

OJ'

oil from any depth.

The pump

capacity is limited by the size of the casing. For example, a submel'sible pump which will fit inside a 20 cm [8 inch J casing will deliver a maximum now rate of about 68 m 3/d

[300 gplll

J.

An electr ic power source is required to operate the pumps.

8

PUMP TYPES

C AN TYPE: VER TIC AL PUMP

SUBMERSIBLE PUMP

9

PUMP TYPES

HORIZONTAL MUL TlST AGE PUMP

Can-type Pumps are used to 1ift liquids from storage tan ks or sub-sur race sources.

The pump driver is located at

01'

above the liquid surface, and the shaft extends f rom the

driver to the pump, which may be located some dist ance bel ow the liquid sUt'face,

Th is

type is used f requently on offshore pl stfol'ms to provide an emergency f irewatel' supply, Both the can and submersible pumps can have up to 60 stages or impellel's, depending upon the depth at which thc pump is se t. [f one impeller developes a pressure rise of 1300 kPa [43,5 psi], and a total pressure rise of 9000 kPa [1305 psi J is I'equ ired to lift the liquid to the surface, then the number of st ages wi ll equal: SI UNITS

ENGLISH UNITS

T otal P['essure Requ ired

9000 kPa

1305 psi

Pl'es;ure rise pel' stage

360 kPa

43,S psi

Number of stages

9000 = 30 300

1305 = 30

43.5

Horizontal multistage pumps are used in process plants and oil pi pelines where th e pump must raise the liq uid pressure seve!'al thousand kPa [sevel'81 hundred psi]. Thel'e is no lheoreticallimit to the numbel' of impeJlers in 8 hori zonta l pump, but more than 8 are

seldom used,

SE AL SYSTEMS

10 D. Alternate Seal Systems

A pump handling liquid hydroca rbon can cause a hazardous situat ion if hydrocarbon

leaks out the pump seal to t he surrounding atmosphere. One way to avoid this is to install tw o seals on the pump wi th a pressure guage between the two. When the inner seal starts leaking, pressure will rise between the two seals and it can be observed on the pressure gauge. In so me cases, a press ure swi tch is provided between the two seals so that a rise in pressure trips the switch a nd s ignals a n alal'm or may even shut dow n the pump.

Rise in pressure between

seals indicates leaking inner seal.

Impeller

Shaft

Inner Seal

Outer Seal

PUMP WITH TWO SEALS Anot her way to prevent liquid inside the pump from leaking to the atmosphere is that of using a seal oil system, which also has two seals. A simple seal oil system is shown opposite. There are two sea l oil pots with water in lhe bottom of lhem. Pump discharge liquid fills one pot above waler level; the other pot is filled above lhe waler level with seal oiL The seal oil is piped to the space between the two seals on lhe pump. Water in the bottom of the pots preven ts pump discharge liquid fro m mixing with the seal oil. Since pump discharge pressure is imposed on the seal oil pots, the press ure in the pump seal oil chamber (bel ween the two seals) is pump discharge pressure. The purpose of the bypass line is to allow liquid on l he pump side of the inner seal to flow inlo the suction side. This will hold the presure on the pump side of the inner seal at suction pressure. Pressure on the other side of lhe inner seal is pump discharge pressure. Wilh this arrange ment, a leaking seal will resull in seal oil leaking into the pump, becau e

SEAL SYSTEMS

11

Pump LIquid At Dllctl.rge Pre"ure

I J BypllSil

SEAL OIL POTS

Used for IiqllLd CW"I In! PlIrT"9 side 01 the inoel Jeal 10 flow

to the aoction tide of the pump, This Jowerl press"re on pump .ide of IlYler IIIal to lion prfllllUtt.

~~

_ _ _ ..bd-.. __ ..-.

Inne r Seal

---.

Seal Oil Chamber

ll;!akirw:l see! will result in Ioeel oil lellking into pump.

l eak ing aeal wlU Te...,lt in luI oil leelcing into lilt atmosphere.

SIMPLE SEAL OIL SYSTEM PUMP WITH SEAL OIL POTS seal oi l press ure is higher than pressure on the pump side of the seal. The outer seal is prov ided to prevent seal oil from leaki ng to the atmosphere. The seal oil pots are used on small process pumps - usually less than 35 kw [5 0 hp J. The effectiveness of the system is limi ted by the volume of sea l oil conta ined in t he system . If a large leak occurs in the inner seal, pump discharge liquid will even tu ally displace sea l oil in the pots, and the liquid between the two seals will be pump liquid. In th is sit uat ion, failure of the out er seal will result in pump liquid leaking to the surrounding atmosphere and crea te a hazard. Lal'ge pu mps hand ling vo latile or hazardous liquids are often eq uipped with a circulatirg seal oil system as shown on page 13. This system has two pump seals just as the sea l oil pot system did. The primary difference is tha t seal oi l is cont inuously pumped through the seal chamber at a pressure higher than the pressure inside the pum p, pressure controU er in the seal oil outlet line is set to hold this desired press ure.

A

12

SEAL SYSTEMS

The drawing on the opposite page shows a mult i-stage pump with a balance piston used to offset thrust for ces in t he pump.

Pressure on the ou tboard side of the balance

piston is held at suction pressure by allowing liquid that leaks across the balance piston t o flow back to the suction side of the pump t hrough t he balance line . This particular pump has scals at each end of the shaft. The bal ance line holds suc t ion pressure on the pump side of both sea ls. Consequently, as long as the sea l oil pressure is above pump suction pressure, leaking seals will result in seal oil leaking into the pump rathel' than pump liquid leaking to the seal oil system. Seal oil is a non-volatile liquid that docs not contaminate the liquid inside the pump when it leaks into it. Some fOI'm of lubricating oil is often used for seal oil in hydrocarbon pumps.

PI'oblem 2 List th type of pump and seal to use in the follow ing serv ices:

Service a.

Process pump used in gasoline plant

b.

Pump water from a well

c.

High pre ssure cI'ude oil pipeline pump

Pump Type

Seal

located in an enclosed building d.

Fire water pump on offshore platform

CENTRFUGAL PUMPS USED IN ffiACTIDNA TlNG SECTION 0; REFINERY

13

SEAL SYSTEMS

PRESSURE CONTROLLER

l

1 DISCHARGE

SEAl.. OIL COOLER

WATER

t

flLTERS SEAl OL TAN<

F'l.MP

SEAL OIL SYSTEM

14

D. PRINCIPLES OF CENTRIFUGAL PUMPS

A. Flow Through Pump Liquid enters the pump at the eenter or eye of the impeller. In most process pumps, the impeller rotates at a speed of 1200 to 36.00 revolutions per minute. At this speed, the liquid enters the center of the impeller and is thrown into an enlarged chamber called the volute. Liquid flows around the volute and exits in the outlet nozzle. Liquid Outlet .

Liquid • Inlet

b=l,,'; ~F

1?~--;r.I---lmpeller-----t7t--"'*'

-n'f----Volute------+_

UQUJD FLOW IN CENTRIFUGAL PUMP

B. Centrifugal Force Suppose you take a bucket that is almost completely filled with water and swing it in a circular motion around your body. If you swing it very slowly, some of the water will spill out of the bucket. However, if you swing it fast enough, none of the water will spill out of the bucket. The centrifugal force generated by swinging the bucket pushes the water against the bottom of the bucket so that it does not spill out. Now suppose we have a small hole in the bottom of the bucket. As you swing the bucket, water will come out of the hole. The faster you swing the bucket, the farther the water will travel that leaves the bucket. This is the principle of centrifugal action. When you move the bucket fast, you use more energy. The distance that the water travels from the hole in the bucket will depend upon the amount of energy that you use in spinning the bucket. Before we attempt to understand the principle of centrifugal pumping, let us look at the pump unit first. It includes a driver and a pump. The energy used by the driver -

CENTRIFUGAL FORCE

15

motor, turbin e, or engine - is transferred to liquid in the pump in the form of pl'essure by

the pump. In other words, a pump is a device for transfer ring energy from the driver to t he liquid. It is important that we recognize that energy is enter'ing the liquid in order to under'stand pumping. El ectri c energy used by a motor-driven pump is transferred to liquid by the pump in t he form of pressure. Another thing we need to realize is that ener'gy can exit in sever al form s. A ri fle shell contains ener gy in the form of powder .

When t he shell is fired, energy

burning powder transfer s to the bullet in the form of velocity.

of th e

That energy converts to

pressure when t he bullet strikes an objec t and l oses its velocity.

Vel ocity energy is

converted to pressur'e energy, A cen trifugal pump uses the same velocit y-pressure concept to mise liquid pressure. Liquid enters an impeller at the eye.

The speed of the impeller' Cl'eates a cen t rifuga l .

16

HEAD PRESSURE

force that throws the liquid to the outer edge at a high velocity. It leaves t he

i,~p e ll er

at

high velocity and enters the volute, which is an en larged chamber where the velocity is quick ly reduced. This veloci ty reduction results in a pressure increase. The liquid flow ca n be compared to that of the moving bullet .

The now in the

impeller at a high veloc ity cor responds to the movement of a bullet through the air. The liquid slow ing dow n in the volute with a resultant pressure rise is comparable to the force of a bu llet striking an object. The amoun t of pressure an impeller will develop depends upon its diameter and the speed at which i t rotates.

A large di ameter impeller operat ing at a high a speed will

develop t he highest pressure.

The p"essure developed by the impeller is l imited by the

materials of which the impeller is made. It is subjec t to the sa me cen trifugal force as the liquid and will fly apart if the cen tr igual force is excess ive.

If a si ngle impell er will not develop thc p" essure requi"ed, two or more impellers can be inst alled in ser ies to increase the press ure rise across the pump. A pump with three impellers can be compared with t hree pumps which operate in series.

Discharge liquid

from the first pump enter s the second one, and liquid from t he second pump flows to the third one. There is no theoretical limit to the number of impell ers which can be instaUed in a pump. However, horizontal pumps seldom have more than eight impeUers in one casing. If this is not enough to produce the desired pressure, a second pump will be used. Submersible or can pumps can have 50 or more impellers. Vertical pumps are usually built in segments, so t hat there is no theoretical mechanical lim it to the number of impellers which can be installed. C. Head Pressure The purpose of a pump is to raise the pressure of liquid. The amoun t of pressure rise is called the head pressure, or si mply head

It equals the discharge pressure minus the

suct ion pressure. The pressure developed by the pump - head pressure - will be constant fOl' any suction pressure. In other words, a pump that develops a head pressure of 300 kPa [45 psi], wi ll ha ve a discharge pressure that is 300 kPa [45 psi] more than the suction pressure, regardless of wha t the suct ion pressure is. Obviously, the pump casing must be designed to withstand t he highest discharge pressure expec ted in the servi ce for which i t is used.

17

CAVITATION AND , VAPOR LOCK

It is important that you remember the t er m head pressure, as it will be used frequently in the following discussions. Suction Pressure ·

Sue t i on .1IIIII.1IIIII.1IIiII1IIIII.1II!~1 Liquid

DIScharge Pressure

Discharge 11IIIII1II!1IIII1III1IIIII1IIIl1IIIII1IIIII-. Liquid

CENTR IFUGAL PUMP

HEAl) PRESSURE

= DISCHARGE PRESSURE - SUCTION PRESSURE

Problem 3 The discharge pressure gauge on a pump reads 1000 kPa [145 .psi J. . Suclio.n pressul·e . is 400 k.Pa [58 psi _ _ _ kPa

J. The head pressure developed by the pump is [psi J.

D. Cavitation and Vapor Lock Cavitation and vapor lock are ter[lls often used interchangeably to describe pump failure due to the presence of vapor in it.

Although caviation and vapor lock, both occur

when gas is present in a pump, they each have different effec ts on the operation of the pump.

Cavitation occurs when the liquid entering a pump contains a few bubbl es of gas. The gas flows through the impeller with th e liquid and as its pressure is increased in the pump, some or all of tti", gas ·liquifies (the vapor ' bubble.s collapse.)

A high centripetal

force results from this collapse and. m·ay ;cause severe vibration and poss ible pump damage. The pump will continue to ·pump liquid, but it will be noisy and may vibrate.

Vapor lock occurs when gas. enters the pump· with liquid and separates from the liquid inside the pump and fills all ar a part of the pump. The pump will compress the gas a slight amou nt, but not nearly enough for tlie gas to flow out the. discharge line. tr~pped gas prevents liquid

through the pu mp.

The

tram entering .t he pump. The effect is that no liquid flows

-

VAPOR LOCK

18

When a pump vapor locks, the discharge pressure gauge reads about the same as suction pressure while the pump is running.

In order to clear the conditi on, the vapor

must be removed from the pump. In some cases, this can be done by opening a vent valve while the pump is r unning. Quite often, the pump must be shutdown and the casing vented unti l liquid flows out the vent line. At this point, the pump is restarted. Some pumps are more prone to vapor lock tha n othe rs.

A procedure for starting

these pu mps is: 1.

Close a val ve in the discha l·ge line. Suction valve is open .

2.

Open the casing ven t valve until a steady strea m of liquid comes ou t. Partiall y close the vent valve, but keep a steady steam of liquid flowing.

3.

Start the pump and observe the discharge pl·essure.

It should rapidl y increase

and then level off. 4.

Slowly open the valve in the discharge line.

5.

Close the valve in the vent line .

Observe the disoharge pressure during Step 4.

If i I drops to suction pr essure, the

pump has vapor locked again, and you will have to shut it down and start over.

cP r.M.l

SUCTION

Sto" d,I .. ,

la~ :

)

Open "eot ",lve uotU 1[811dy

.tream of liquid come. out.

8

Open .... 1.... in IlUCtion line ~--'-----...

1

1-------'

5 Quer ... e dllch8rqe preMUU!. /~""" ": /I t ahoi,Jld rill! rapid ly and then level off. Slowly open .... I... e In dischllrge line.

PROCEDlRE TO START - UP AFTER CA VIT AnON

Cavitation or vapor lock occur when gas is present in the pump. A few gas bubbles will cause cavitation. More will cause vapor l ock. Both are prevented by preventing gas from entering a pump. This can be done by raising the suction pressure to the pump, or raising the l evel of liquid in the vessel that is being pumped.

PERFORMAN CE CU RVES

19

E. Performance Curves

It will help us in operating our centr ifugal pumps i f we understand how pumps are selected in the fi r st place, and what their operati ng limitations are. Suppose have need for a centrigual pump that will operate at the follo wing conditions: Flow Rate :

40 m J I hr [ 17 5 gpm]

Head Press ure:

600 kPa [ 87 psi]

Relat ive Densit y of Liqu id:

0. 80

Maximum Discha rge Pre!)'Sure:

3450 kPa [5 00 psi]

We give thi s inform at ion to a pump manufacturer and t ell him to supply us with a pump driven by an elec tric mot or. The manufacturer has a number of st andard size pump casi ngs and impell cl·s. He must selec t the standard unit that will fit our des ign conditions and operate at a high efficiency so that we don't wast e a lot of elec tricity in dri ving the motor. We wi ll discu ss efficiency later. The pump manufact ure r has pel'formance curves fOi' cach standard si ze pump that he makes. These curv es show the relation bet ween flow rat e and head pressure f or differe nt size impellel's operating at differen t speeds tha t can be used in the same pump ca sing. Typica l cUI'ves for a pump operati ng at 35 00 rpm are shown on the following page. The top curve is for the largest diameter impeller that can be used in that pum p casing. It has the highest head pl'essure of any of the impellers. It also requires the largest driv er. The bottom cu r ve shows t he smallest dia met er impell er wh ich can be used in that pump casmg.

In our application, a 200 mm [8 in] diamet er impeller will deliver th e head

pressure at the flow rate we have spec i fi ed.

This will be the size impeller that the

manufacturer wiU use in our pump.

The pump curves show the head pressure t ha t different sizes of impellers will develop at various flow I'ates at a 3peed of 35 00 I·pm . A di ffer ent se t of curves for the same casing and impellers wi ll apply at a speed other than 3500 rpm .

As the speed is

reduced, the head pressure at a given flow ra t e will be less. We will discuss the effects of speed later.

For the time being, we wi ll confine our discussion to pumps operat ing at a

const ant speed. The pump manu fac turer uses the pump curves to select the pump casing, impeller size, and speed that will sa t isfy our process requ irements at the lowest pow er consumption by the driver.

PERFOR MAN CE CU RV ES

20

51 UNITS

, 230 . _ Iotl.O _ _ ,JA",·'r,·, '"

800

' IMPELLE R

,

,.

,"

215 lotH

"t

-,, t .,

OES TG

POINT'

500

4 00

.,

, ..

., I

0

-

,

20

10

, 1 - ..! -

,.

:

30

40 PU-1P C APACITY, M I /Kt

ENGLISH UNITS

,. i

11 0

.. 100 T

., . . 71.

"1

'\

INC"

..

S I GNPO I NT

,. , ~

7

'

_.

60 0

T

' ,. I",

3500 RPH

t 50

100

150

200

250

'00

CAPACITY CLRVES FOR VARIOUS DIAMETER IMPELLERS IN Sf ANOARD PUMP CASING AT ) 500 RPM

350

21

PERFORMANCE CU RVES

The pump cUi'ves also t ell us something else:

that the pump will deliver the flow

rate and head press ure shown on the curve. In the example we have cited, we selected a 200 mm [8 in] impeller whi ch will deliver head presure of 600 kPa at a flow rate of 40 m '/hr [head pl'ess ure of 87 psi at a flow rate of 175 gpm] . Suppose when we sta rt to o"perate the pump that we r equire a head pressure of only 550 kPa [80 psi] at the design flow ra teo I f we look at tile 200 mm [8 in] diameter pump curve, at 500 kPa [80 psi ] the flow rate through the pump wi 11 be 56 m '/hr [ 25 0 gpm ]. In othel' words, the pump is goi ng to de li ver a flow rate and head pressure along its operating curve. Even though we do not need as much head pressure as it will deve lop, we cannot reduce the flow rate without increasing the head pressure. What th is means from an opera ti ng standpoint is thi s:

if a constant speed pump

develops more head pl'essure than we need, we must have a pressure reducing device on the pump discharge that wi ll take up t he excess head pressure that t he pump develops. Using a pressure reducing dev ice is wasting the energy t hat was used by the pump dr iver to put up the pressure drop we are taking across tile pressure reducing device.

We can save t hat wasted energy by installi ng a smaller impeller in the pump. Look at t he pump curves again.

At our operating flow rate of 40 m '/hr [175 gpm]

and

opera t ing head pressure 550 kPa [80 psi ], we need an impell er having a diameter of 195 mm [7-3/4 in]. We can purchase t his size impeller from the pump manufacturer.

When

we put the smaller diameter impeller in the pump, we will have a new curve shown in the

dotted lines of the pump curves. This is our new per formance curve. ru n the pump at our actual operating conditions wihtout wasting

It will allow us to

POW€ I'

in the driver.

We purchase the pump with a 200 mm [8 in] diameter impeller to give some excess capacity. The manufacturer suppli es us wi t h performanc.e curves for that pump as shown on the next pages. The top curve is the sa me as th'e 200 mm [ 8 in ] diameter impeller on Page 20 . It shows the pre ssure head at di fferent flo w rates. The maximum head pressure the pump will develop 650 kPa [9 3.5 psi]. the pump.

This head is developed with no flow through

In other words, if we turn the pump on and close the discharge valve, the

pressure gauge on the disch ar ge will read 65 0 kPa [ 93.5 psi] more than the suction pressure gauge.

If the occasi on should arise t hat we want to increase the flow rate

through this pump f rom 40 m '/hr to 60 m '/hr [175 gpm to 265 gpm developed by the pump will drop to 540 kPa [77.5 psi].

J,

the head pressure

22

PUMP EFFICIENCY It is important to rec-

ogni ze that a pump will oper-

.

-

ate at some ' press ure and

.;

SILNTS ...:..

'

' -I ~

>

•

flow rate on or near its operaling curve. stage

Large multi-

pumps

may

deviate

slightly from the operaIi ng curve. As a pump wears and

clearances increase, some in-

<

~ 600

~ ~

~

ternal leakage from the dis-

:

,.

-

.~

and

the

.....

.. _ . ·i

- .. --

-.. ~

.. ..;.

-,

,- _ . ' I

500

problems.

pump

We observe the

.....

head pressure and flow rate through the pump and compare

it

to

the

operating

"

'

.

60

. - . .;

- .'

!.,.

I

.

.. - - -... .., ,

,

-1

~

,

20 ,

is

,

10

- I..

-' --

,

••

,

:

-J.

,

o

F.

_.

"

'" .-

t ;" .

,- .-

..

- ",.-

..:

I

,

:

-"

,

.."'

1 T

:

.-

; , i

-

-

.0

~-

.,.

-..,

".

-

o

Pump Efficiency Centrifugal pumps are

'

".

~ 500~

f

'i

.. .;:. = .1 : ~ 400~ B .. . , .,

·

. ,

10

~

"

" '- a

.; .:. I .;

, ,

• r -•.

~-.

.-

.....

"

,-

, i

.!. .

20

40

o

~

' " •

OO:J

.... "f

60

Pl..M' CAPACITY. M J I'rfl

PERFORMANCE CURVES FOR PUMP WITH ioo MM IMPEl.LER AT }500 RPM

not high effi ciency energy transfer devices.

In other

wordS, only part of the energy used by the driver is actually transferred by the pump into pressure. The pump efficiency is the percentage of energy that transfers from the driver to the liquid in the form of presure.

~

-,

1"- -

I -.

energy

driving the pump.

,

I.

Otherwise,

wast ing

".

..

:- ,- ..

- .....

t _ .

are

•,:

-

the curve, it may be time to we

oj

i -!

.. ;. , ..

. .,..,

curve. If it is too far below repair the pump.

- : .- - -

- "- '... ' ....

~

~ K

..

'-

.

2

' r- .

I

,

.,.- ..,

.

T--

•"

·

_

POINT

line parallel wit h the original troubleshoot ing

"

~

_.

opera ting

This is a way of

....

.! .

curve moves downward to a

curve.

·

; - DESIGN '

550

charge back to the suction occurs,

"_1

- .. . .

~ .

"

,

.

-

,

"

The efficiency cu rve for the pump we selec ted

indicates the maximum efficiency for this particular pu mp is about 63%.

PUMP EFFICIENCY

23 ecuSHl.NTS

This is the highest efficiency we can get for th is pump.

At our design flow

rate of 40 m 3/ hr [175 gpm 1, the pump efficiency is about 61%. The efficiency drops off

-

,-

,

~

i ~

t

.~

•

-r "DESICN - POINT

80

-

I.

_._

-. ,

,

rapidly as the flow rate reduces.

70

. -. -.

60

.-t-r-

The ene rgy supplied by the driver which does not

- -

..

,- -,-

transfer into pressure energy

r

, I

,-

inside the pump has to go somewhere.

Part of it goes

,-

.:.. 40

to friction; part of it makes up for internal leakage; the

-,

remainder enters the liquid in the pump in the form of heat. As long as the pu mp is opera-

L

o

• 20 ~

"

ting at an effic iency of 30% or more, the heat energy that

-

•

~

_.

~--d-

•

(AD '

. SU ' ClIONt-i · _ . _ .. , \QllID

- .,

transfers from the driver to

10

Q

the liquid in the pump will cause only a degree or two rise in the liquid tempera-

o

'"

100

150

200

250

o 300

PUMP CAPACITY I GPM

PERHRMANCE CURVES FOR PUMP WITH B N::H IMPELLER AT J500 RPM

ture.

S ~

~

However, at low pumping rates, the efficiency may drop as low as 10-20%, which means that a larger percentage of the driver energy is entering liquid in the pump in the form of heat. In this situation, the temperature rise may be several degrees, which may vaporize part of the liquid or expand the internal parts of the pump to the point that damage may occur.

Problem 4 What is the head pressure and efficiency at a flow ra te of 50 m 3/ hr (220 gpm J?

§

DRIVER POWER

24

A pump having several impellers will have a temperature rise ac,'oss each impeller. In some instances, this limits the number of impellers tha-t can be installed in a pump

case.

65.

G. Driver Power The

power required

to drive the pump is indica ted on the curve. power

increases

as

flow rate increases.

The the The

51 LNT5

, <

•~

.

~'

- ,

~

,

"

of

,~

>0. ..

;

1

. ,

,

.. ,

I.",,,

'. • . -. !-.

r . . , . . . . r

20

-

." '

;-

1

, ,

1

.~

·, · • r

operating at a higher flow rate, we size the driver for

,

• -!

.

,

I

~(.p.

'"

~\\ltfI. 90 ...

,

'! I

•

.

,

.

_

pump perform-

,

,

.,...

1·;; · ·. _ ' -' •

•

10

.

The same pump with

'"

•

t

.

-,

•

t

, ."

,~.

... _..

-

,

,

,,

t

I "T" .j .

-.

I . 1 ..

"-

,

20 PUMPCAPAOrv,

a com mon motor speed.

--

.

,

I

..... __ ..... ..,. • 40 '"

,. -,

I

I -

. ~, .r-110l'1 H(.AO

UQUID~ "

1'

-

•

r-

.•

' •

.

"

.. . . . . , . . .; • • I ~,---"

ance curves are fot' a pump

This is

~

- ,

hp J motor.

per minutes (rpm).

-

•

.-

L ~ ,"

'

.

•

f. -,

• __ •

." l

.

•

flow rate,

speed of 3500 revolutions

•

.... .,..".

"-,"

the pump is capable of

The

I

"

-I"

which requires a 14 kW [20

"

,

m' / hr [175 gpm J is 12 kW

the maximum

,

,

40

However, since

• .

,.

,

[16 hp J.

-,

I.

'1,

'

,

rate

-

...

,

In this particular

flow

. I'

,

case, the power required at design

. j.

DESIGN POINT .. . -,

T ' -'

pump.

--

, .. -

power curve is used for se-

l ect ing the driver for the

!

I

~,.,

"

55. "

,

'

1-ti:4D"~' . ~E"Ssr....

-

600

,J

~~

,

"

40

"'-/Hit

"

PERFORMANCE CURVES FOR PUMP WITH 200 MM IMPELLER AT }500 RPM

a different speed motor (or speed control) would have a different set of performance curves. If we have a centrifugal pump driven with a variable speed .engine or turbine, it will have pel'formance curves at

each different speed. The effect of speed on the head pressure developed by the pump is a square l'OOt function. Cut the speed in half and the head pressure developed will be one fourth the original.

LIQ UID SUCTION HEAD

25

H. Liquid Suction Head

The impeller on a centrifugal pump pulls liquid into it from the suct ion line t o the pump. Liquid moves at a high velocity from the point that it enters the pump to the eye of the impeller.

This dis-

tance may be on ly a fe w em

signi ficant pressul'e drop inside the pu mp. This pressure drop that occurs within th e pump can cause some of the liquid t o vapor ize in the sucti on chamber of t he pump.

•

When this occurs, t he pump

i

will ""vitate or vapor leek. We

norma lly

prevent

locating the pump far enough below

the

vessel

we

pumping out of, so that the pressure

head

due

to

the

~

l ~

height of the liquid in the ~ vessel is mor e t han the pressure dl'op inside the pu mp and connecting piping. The pressure drop inside the pump i s expressed as height of liquid required at the suction line to t he pump.

P\..Np CAPACITY, GPM

It will var y

with flow to the pump as

PERFORMANCE CURVES FOR PUMP WITH 8 INCH IMPELLER AT J500 RPM

shown by the cur ves. The liquid sucti on head represents a pressure drop as liquid Dows from the pump inlet flange to the impeller.

We norm ally add about 10%to the liquid head to allow for

pressure drop in piping between the vessel we are pumping out of and the pump. In other words, the height of the liquid in the vessel above the pump will be 110%of the height

26

UQUID SUCTION HEAD

LIQUID SUCTION HEAD

r-F~~~~~~D I SC H ARGE ~~~~ LIQUID t.:::::.::> PUMP

shown on the suction head curve, If the pump is loca ted some dista nce from the tank or vessel it is pumpi ng out of, we will calculate the press ure drop in the piping and add it to the liquid suct ion head to ge t the to ta l pressure drop, and then adjus t the level in our separa tor so that we have enough liquid head p,'ess ure to ove,'come press ure drop in the piping and in the pump, If we allow the leve l to drop below this point, vapor will form in the pump and it will cavi tate a" vapor lock, Since piping pressure drop depends on size, it is importan t tha t the suction line be la"ge enough, [n cases whe re adj ustm ents in level cannot prevent cavitat ion, a larger suction line may be needed, Example

At a {low ra te of 40 m ' /hr [ 175 gpm], the suction head required at the pump is 340 c m [ 11 feet],

We determin e t he pressure d,'op in the piping

between the seporator and the pump is 100 cm [ 3,3 fee t], This must be added

to the head taken f rom the curve in orde,' to get the total heigh t of liquid above the suction to the pump, When the two are added, we get

Q

liquid head

requirement of 440 cm [ 14,3 ftl. If the level in the separat or fall s below this

paint, the pump will vapor lock and stop pumping , suction line is not completely open, it can cause

Q

If a valve in the pump pressure drop which will

reduce the suction head to the pump to the point that vapor lock will occur,

The suct ion head is referred to as NPSH by engineers, an abbreviation fo r Net Positive Suct ion Head, It is particularly important when pumping volat ile liquids, such as ethane, propane, or unstabi lized crude Oil; or if the pump is loca ted so me distance from t he vessel con ta ining liquid, On offsho,'e producti on pla tforms, crude o il pipeline pu mps often are located some distance fro m the separa tors or tanks, A booster pump is often

THRUST

27

used to pump liquid from the separa t or into the pipeline pumps.

The purpose of the

booster pump is simply to maintain suction head to the pipeline pumps so they will not vapot· l ock. Pump cavitation and vapor lock are major operating problems of centri f ugal pumps. As we mentioned earlier, when a pump vapor locks it simply stops pumping liquid. pump will continue to run.

The

If the problem is not co,·,·ected, the pump will overhea t

because no liquid is circulati ng through it to cool it. In this case, the pump is transfe,,·ing some of the energy from the driver in the form of heat, because no liquid is flow ing through the pump to remo ve energy in t he form of pressure. The im por t ant thing to remember about suction head is that it increases as the flow rate inc,·cases through the pu mp. Suppose we are operating the pump with the . curves shown on pages 24 and 25. It was sized f or a flow r at e of 40 m J Ihr [175 gpm]. The sucti on head required is 340 cm [ 11 ft J. If the fiow rate to the pump increases to 60 m J Ihr [ 265 gpm

J,

the liquid suct ion

head must be 585 cm [19.2 ft J or the pump will cavitate. If we design the elevation of our separator for a 340 cm [ Jl ft J suction head, we will not abe able to opera t e the pump above 40 m J Ihr [17 5 gpm

J unless we raise th e level of liquid in our separator. Remember

that the suction head is the pressure drop inside the pump, and we must add about 10%t o it to allow for presure drop in piping bet ween th e separa tor and the pum p.

Problem 5 Wha t liquid suction head is required at 50 m J Ihr [ 22 0 gpm J?

t

I. Thrust As a pump impeller rotates, a thrust force

SUCT ION ....-.

I

PRESStJRt.

develops which is transm itted through the pump fLOW

The

for ce

developed

in si ngl e impeller

pumps i s relatively low, and can be overcome with

. , . PRESSURE

I

shaft. The force is similar to that of an airplane prope ller which pulls the airplane through the air.

.... OISCHARCE

...

/

SHAFT

+ DIRECTION

Of THRUST

th,·ust bearings locat ed on the pump shaft as shown in the pho t ograph on Page 3. SINCLE It.flEl...LER EXER 15

n . fUJST TOWARD !iJCTD-I EN)

28

THRUST

j

t

Thrust forces in multistage pumps are compounded at each impeller.

Special design considera-

tions are required to contain these

forces.

One way of neutralizing

two forces is to install some of the impellers in opposite direction to others, so the thrust forces equal-

FLOW

SHAFT

+ DIRECTION Of THRUST

DIRECTION Of'THHUST

ize one another. This design does not totally balance thrust forces,

Tl-RUST NElJlRAUZED WITH OPPOSING IMPELLERS

but it red uces them enough so that small thrust bear ings can be used.

Some multi- stage pumps have all impellers facing the same direction.

This

arrangement results in the maximum thrust force. It can be neutralized by installing a balance piston on the high pressure end of the shaft. Pump discharge pressure is imposed on one face of the piston. A small amount of discharge liquid leaks around the piston to the outer face, and flows to the suction of the pump. This results in a press ure on the outer face of the piston of suction pressure. The force exerted on the inner side of the piston will equal discharge pressure times the area. The piston is sized so that the net force resulting from the piston is approximately equal to, and in the opposite direction of, thrust force from the impellers. This arrangement minimizes the size of thrust bearings required. Selection of a multi-stage untt having opposed impellers, or having in-line impellers SUCTION

DISCHARCE , - -BALA>CE

PISTON

-THllUST BEAR..,

7 STADE PUMP WITH BAlAf'CE PISTON

29

PUMP CURVE APPLICATION

with a balance piston, depends upon the pump service and the cost of the two un its. The balance piston is att ached to the pump shaft and rotat es in the casing. The clearance between the piston and the casing must be very low to prevent excessive discharge liquid from leaking around the piston. This requires a clean liquid inside the pump so that dirt does not get between the blance piston and the casing and wear one or the other parts. Multi-stage pumps having opposed impell ers require spec ial passageways through the casing for liquid to flow from the fi nal stage of the first se t of impellers to the first stage of t he opposing se t of impellers. This adds considerable cost to t he casing.

J. Pump Curve Applicati on Now let us app ly what we have learned to an operating situation. Liquid from a sevarator must be pumped into a st abilizer. Operating conditions of pressure and flow are as shown below. The pump selected for this service has performance curves as shown on the following pages. The basic design point is for a flow rate of 68 m 3/hr [300 gpm J and a head pressure of 345 kPa [50 psi J. At these conditions the pump efficiency is 73%, and the dr iver requires a horsepower of 12 kW [ 16 hpj. A 15 kW [20 hpj motor was provided wi th the pump.

1 ]O}~ kPa [1$0 p1i]

-

690 kPa [100 kP~

STABIUZER

",""p

DESIGN COI'VmONS FOR ST ABIllZER FEED PUMP

First of all, look at the power curves to deter mine what maximum continuous flow rate can be ma intained in the pu mp without overloading the motor. The maximum power required by the pump, is 15 .8 kw [21 hp J, which is 5%above the power of the motor. We can safely operate at 105% motor load for extended periods, so we can say that the motor does not Ii mit the flow t hrough the pu mp. \

Let's get back to the design point on our pump.

Checking the efficiency at the

PUMP CURVE APPLICATION

30 des ign

flow

rate

SI LNTS

of 68

m ' l hr [300 gpm I we find that it is 73% efficient at that point. This means that 73% of the electrici ty used

~ LoT

., "

300

~ ~

in driving the motor is con- ~ verted into pressure energy

'. ,, '

.. , 200

inside the pu mp. The other

,

leakage

in

,

,. ,,

,

the

"

80

•

,

27% is lost to frict ion, to internal

I '

,

, I

70

~~

,

t ,

pump, and to temperature

t

, .0

"

rise in the liquid. •

Refer to the suct ion the design flow ra te of 68 m ' l hr [3 00 gpm 1, a suction

f

head of 270 cm [9 ftl is

this

distance

above

the

I"

,.

means that the level in the separator must be at least

..

...

between the separator and

,

I,

'

..

,

,,

... , ,

t

I'

,

~

I

~

,

. •

I' I

lJ(JJ\O

I'

, i 20

.. .-....Il-IE,o.O •

SUC1\U"

'0

.

.,

60

..l-

<

.. I

I , ...

i ..

r

"

,

, 80

'00 3

,

, .... !

I

o

,,

t - t • -

pump. We normally add 10% for pressure drop in piping

I

t

. '

I

,

,

f

.~~

ck:\...j~""

,

, ,, ,

- . , , ' .. iJ' • 4

required at the pump. This

.,

.,

head curve. It shows that at

I ..

100

,

oo~

300~

.,

,. 200 120

the pump, so the level in the separator must be 297 cm

PERFCRMANCE CURVES FCR STABlllZER FEED PUMP 51 UNTS

[ 9.9 ft I above the pump. Look at the suction head curve at a flow rate of 114 m 'lhr [500 gpm I. It shows that the liquid head to the pump 'must be at least 455 cm [15 ft). If we add 10% for pressure drop in piping to the pump, we get a height of about 50 0 cm [16.5 ft I. Suppose the maximum level we can maintain in the separator is 455 cm [15 ft I above the pump. We deduct 10%to allow for pressure drop in piping, which leaves 410 cm [13.5 ft I of suction head. At this height the maximum flow rate the pump will deliver

PUMP CURVE APPLICATION

31

OGJSH \..NITS

without vapor locking is about 108 m3/ hr [470 gpm). Assume we learn that the flow

to

the

separator and

,,

through the pump will increase to 102 m 3/hr [450 gpm J. Also assume that the pump discharge pressure must be 1035 kPa [ 150 psi)

in order to pump liquid

•

into the stabilizer.

!

Refer to the head curve at 102 m 3/hr [450 gpm) : the pump will deliver a head presSure of 300 kPa [43 psi). If we

,

deduct this from the discharge pressure, we get a suction presSure of 737 kPa [107 psi ). This is the pressure we will have to

t

hold on the separator at a flow rate of 102 m 3/hr [450 gpm). Now look at the liquid suction head curve at 102 m 3/hr [450 gpm). It shows that the level of liquid must be at least 380 cm [12.5 ft) above the pu mp. Adding 10%for safety gives us a

400 PUMP CAPACITY. GPM

PERIU~MAt«.:E

CURVES FOR STABIUZER FEED PUNP ENGUSH LNlTS

total liquid height of 418 cm [13.75 ft J. We will have to raise the level in the separator to this height above the pump,

and hold the separator pressure at 735 kPa [107 psi) in order for the pump to deliver 102 m 3/ hr [45 gpm) at a discharge pressure of 1034 kPa [150 psi). Suppose the pump has been in service for a few years, and we are checking its capacity against the design curve. We have a flow meter in the line which shows 79 .5 m 3/ hr [350 gpm J. Pressure gauge at the pump discharge reads 1035 kPa [ 150 psi),

"

,.

-,'>',

3Q.' ~pd . ~

a. hc, it, at'}"i~; t\~e. ,\Ic>wmr, we!stlQu1(1. .p1te.\!~ " . ',,

As

long

1

'.

-.,.'

.. ,

.....

-

-

.,

' ';' ';

>

'

••

"

,

,.

', '

,

'fhe p,eriorll)a;r:tee ;per)pdjf1!t1,ly.,jn: 6I;lI"t to see 'if .J he ,condition' g,grs woi'st;, ' Qyr,le·~, " T.

,~"

J"

.•..

'

" " . ,

~

,freque~tly" onc~ .wea.r!kgtns: ,!t a~~~Le.~al~f r!!pidly:,,1'~~ e;((eet 'if, !~j,S iS I~, reduG~, i~,~:\:>;~ ;'~ea~l p.t~~.~re :t~e,;~4m-p ~ii}~:-~~~'i~~~~; :~"~f :, Each centrifugal :discussed. ~

'b~mp

"

has

'p",,'{9rm~nce

~

, •

,,",r,ves 'sjm ilar to the .. 'o~s we:'Jilive'

OJ1r,ves;, Pr\)v,td,i d," ~y. ;,the; have fi.~9.w.n ,is \~ ~ i~e h~?,a pr.~~ui~ "'~~" I11~A~f;;,:q~Uj:~,i~ , ,

The only differeli'ee b'etw,fen 'ihe pe'rforman'c e

'!l'ariufit~turei' . arid itlo~e

,we

•

t,

,r

". .

, '

PUMP CURVE APPLICATION

33

performance curve is given in meters [ feet J of liquid rather than kPa [psi J. The he ight of liquid is conver ted to pressure by the following equations: SI UNITS, kPa

ENGLISH UNITS, PSI

LIQUID HEAD PRESSURE = (Height,m) (Rei Dens) x 9.8 = (Height,ft) (Rei Dens) x 0.43 The term relative density used in the head pressure equation is the new term in Sl nomenclature

that replaces the

traditional term specific gravity.

the same thing and are found by

LNJTS

dividing the density of liquid by the

:

density

of

water

at

the

,I

Both mean I

I

same

~

conditions. The performance of each pu mp should be checked at 3 to 6 month

~ ~

intervals to see if the pump is opera- ~ ting near its curve.

When the head

pressure drops below the curve, performances should be checked more

•

frequently so that the point at which

I

the pump will fail to deliver the required flow rate can be ant icipated and repairs made before this occurs. In checking the suction and discharge pressures of the pump, it is

~

best to use the same pressure gauge,

ffi

or use two gauges that have been

•~

~ gauge is preferred as it will give 5 recently calibrated. Use of th e same more accurate readings. Our primary concern is that of pressure difference and not the actua l pressure readings. Two different gauges may each be slightly

in

error

so

that

the

difference in readings of the gauges will not be accura teo

300 P\..Jt.ofI CAPACITY, GPM

400

PERfeRMANCE CLRVES FeR Sf ABIUZER FEED PUMP

EU;USHLNTS

PUMP CURVE APPLI CATION

34

Another use of the performance curves is that of estimating the fl ow ra t e through a pump.

This can be done vel'y easily by measuring the curr ent and voltage to a motor

dr iven pump . Power equations for 3-phase alternating current motors are: MOTOR POWER:

KW = Volts x Amps x 0.00l5

HP = Volts x Amps x 0.002

Example The st abiliz er (eed pump with curves shown on pages 32 and 33 is driven

with an AC motor that has 440 volts and 20 amps. Calculate the Power and (low rate through the pump.

SI UNITS POWel' Equa tion

Volts x Amps

ENG LISH UNITS

x 0.0015

x A mps x 0.002

Volts

Motor s volts

440

440

Motor amps

20

20

Substitute in equation

= 13.2 kW

440

x 20 x 0.0015

440

x 20 x 0.0002

= 17.6 hP

From pump curve, jlow rate at above power

76 m '/hr

335 gpm

Problem 6 Refer to the stabilizer feed pu mp curves on Pages 32 and 33 and answer the following:

A.

Flow rate is 80 m' Ihr [ 350 gpm )

C.

We are checking the per formance of

Head pressure is

t he pump after 2 years of operation.

Effi ciency is

Flow Rat e: 75 m' Ihr [330 gpm )

Dr iver power is

Discharge Pressure: 1070 kPa [155 psi)

Liquid Suction Head is

Suction Pressure : 740 kPa [107 psi J Head Pressure is Flow ra te should be _ __

B.

The current to the driver is 10.6

D.

We are

having difficulty with the

amps and voltage is 660 v.

pump

Driver power is

gOm '/hr. [400 gpm J

Fl ow rat e is

Height of liquid in separat or must be

Head pressur e is

vapor

locking

at

flow

of

ID. OPERATION

35

A. Start-Up Procedure 1.

Check for bearing lubrication - observe oil level In bearing housing or other

form of lubrication. 2.

Open valves in the suction piping between the pump and the vessel containing

liquid to be pumped. 3.

If the pump is to be started with no pressure at the discharge side, close the

discharge valve.

If there is normal pressure on the discharge side of the pump, the

dischal'ge valv e can be left open during start -up if a check valve is included in the discharge piping. 4.

Vent vapol's from the pump casing until a continuous liquid str eam nows from

the vent valve. 5.

Start the motor or d,·iver.

s,~"

?

SUCTION

drlwer

Is";J o

TA

'

,

Open

jI,

"'Ilve III p.plng.

IUCt U)rl

~

Open ... ent .... 1". U'llil t tuOy strum

of liquid come. ouL Then clo-e

11-_

,

:

Cibtetyt dllcn,rge pre.ure. II it i. 7

... eper locked.. Shutoown end repel! Slep II.

,. r

NfTlC .. '\oICUOIl j)l1!aute, PUI'l'll ".,

1 r-~lV=(~N='~~==~~~==~ DISCHARCE J

"there ill pl1! .. ure In db· chMge pipifl9, open '1.1" ..

DRAIN

(0 Cheek lor bearing

V

oV

Check lor nol.e or IIlb,.lIoo. Shutdown If either i, noted.

l<

%

'"....

-

EXHIBIT 2

10,000 100,000

.2

liorTIll l llll l ll l l l H 1111111 11111 11111 11111 11111 11111 11111 11t t Ll1 111 11111 11111 16 ... w

1001111 1111

::lfIrm l1i l fll ffff.mJif!iTImlIIII I III J1II I Mm; rnn

901I I I I I I t 451

__ "z I II I ITIIIITI 11 1111 I I I I I I1 11 H-! I 1111Il'kl 11111111111111111111111111111113

;as! Ill flit4PI III! 1IIII IIIIti tlll llll ll·.. ..... 1-

7Oi-H+ 7H:;; 3

1

V'

(}«-~

-

...w

«- RADIAL MOVEMENT

In most pumps, more of the area of the impeller is exposed to discharge pressure than to suction pressure. This unbalanced pressure causes a to be exerted in an axial direction.

force

125. Movement can also occur if the pump has a long, un-

supported shaft, or if the impeller is out of balance. This is (axial/radial) movement. 126. Both radial and axial movement must be _ _ _ _ __

radial controlled, or minimized

if the impeller is to remain in position. 127. Bearings support the shaft and allow it to rotate with

very little friction. Bearings also control ______ and ____ __ movement of the shaft.

axial; radial

128. The bearing lubricant provides a fluid film between the

rotating shaft and the bearing. This fluid film prevents the shaft and its stationary supports from against each other. 129. A radial (journal) bearing on which the shaft rests controls movements.

rubbing radial

130. A thrust bearing limits end-to-end movement of the shaft.

A thrust bearing limits the amount of (axial/radial) movement. 50

axial

131. Some pumps use ball bearings to control both radial and thrust movement.

RING SHAFT

TWOIIST

BALL BEARING

The shaft of this pump is supported by both _ _~''"'