THE SEW

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THE SEWAGE PUMPING HANDBOOK

Foreword

3

The use of submersible pumps in sewage anddrainage pumping applications has increasedgreatly in the last decades since they entered themarket. The introduction of heavy-duty submers-ible pumps with motor power ratings exceeding500 kW has also made them available for centralmunicipal pumping duties. The good servicerecord and high quality standard attained bythese pumps has all but excluded the use of con-ventional pumps in municipal service.

By the same token, the special characteristics ofsubmersible pumps have also required the devel-opment of new knowledge on their implementa-tion, such as the design of pumping stations. Thiswork has been advanced by both pump manu-facturers and municipal engineers. The intention of this book is to bring the newestinformation on both submersible pumps andpumping stations to the use of all concernedprofessional people in a concise form. The book isdivided into Sections according to the relatedtopics.

Basic pump theory is described in Section 1, pro-viding a reference background for the assessmentof pump performance. Submersible pump design

and construction is described in Section 2. Pumpperformance is dealt with in Section 3, offeringmethods for the calculation of pump performancein various installations. Factors affecting pumpselection are also discussed. Section 4 offers infor-mation on pump testing. Basic design of pumpingstations is discussed in Section 5, offering designinformation for both large and small applications.Continuous regulation of submersible pump oper-ation by frequency control is described in Section6. The concept of whole-life cost for pumps andpumping installations is presented in Section 7.Matters relating to pump commissioning are pre-sented in Section 8, whereas pump operation andservicing is described in Section 9. Section 10 dealswith pumping station control and monitoring.Appendix A offers information on the hydrauliccharacteristics of common pipe components forpipeline loss calculations. Appendix B presents amethod for the determination of sewage pump-ing station capacity and pump starting frequency.

One objective of the book has been to make thecontents easy to read and comprehend. The pre-sentation is therefore enhanced with a large num-ber of illustrations, providing examples of andcomplementary information on the matter athand.

Foreword

Table of Contents

5

Table of Contents

1 Pump Theory ............................................. 71.1 The Head Equation .........................................71.1.1 Flow with Losses or Addition of Energy ....71.1.2 Fluid Flowing from a Container .................. 81.2 The Basic Pump Equation ............................. 81.3 Pump Curve and Losses ...............................101.3.1 The Effect of Finite Number of Vanes ......101.3.2 Friction Losses Hf ..........................................101.3.3 Discontinuity Losses Hs ...............................101.3.4 Leakage Losses Hv .........................................101.3.5 Other Losses ....................................................111.4. Cavitation and NPSH .....................................111.4.1 Definition of NPSH ....................................... 121.4.2 Reference Plane ............................................. 121.4.3 Required NPSH ............................................... 121.4.4 Available NPSH ..............................................141.4.5 NPSH Safety Margin ..................................... 151.4.6 Damming up of Suction Wells................... 15

2 Pump Construction ................................. 162.1 General ............................................................162.2 Pump ................................................................182.2.1 Impellers ..........................................................182.3 Motors ............................................................. 272.3.1 General ........................................................... 272.3.2 Explosion-proof Motors .............................. 272.3.3 Motor Cooling ............................................... 272.3.4 Motor Tightness ...........................................292.3.5 Motor Bearings ..............................................312.3.6 Motor Protection Devices .......................... 322.4. Pump Connection ........................................ 342.5 Construction Materials,

Corrosion and Wear .....................................362.5.1 Corrosion Resistance ...................................362.5.2 Wear Resistance ........................................... 372.5.3 Abrasive Liquids ............................................ 37

3 Pump Performance .................................. 383.1 Pump Head .................................................... 383.1.1 Submersible Pumps ..................................... 383.1.2 Dry-installed Pumps ....................................393.2 Pump Performance Curves ........................393.2.1 H Curve ...........................................................393.2.2 Efficiency Curves ..........................................403.2.3 Power Curves .................................................403.2.4 NPSH Curve ....................................................403.3 Pipe Losses and Rising Main

Characteristic Curves ...................................413.3.1 Friction Losses ................................................41

3.3.2 Local Losses ....................................................433.3.3 Rising Main Characteristic Curve .............433.4 Rising Main Size ............................................443.4.1 Economy .........................................................443.4.2 Free Passage for Solids ................................453.4.3 Avoiding Settling of Solids and Sludge ...453.4.4. Water Hammer ..............................................453.4.5 Avoiding Water Hammer ........................... 473.5 Pump Duty Point ..........................................483.5.1 Single Pump Operation ...............................483.5.2 Parallel Operation, Identical Pumps ........483.5.3 Parallel Operation, Different Pumps ........483.5.4 Serial Operation ........................................... 493.5.5 True Duty Point ............................................ 493.6 Sludge Pumping ........................................... 493.7 Complex Rising Mains ................................ 493.7.1 What Goes on in a Complex

Rising Main? ................................................. 493.7.2 Determination of Head ................................513.7.3 Pipe Size and Flow Velocity .........................513.7.4 Choice of Pump ..............................................513.7.5 Confirming Measurements ........................513.8 Duty Point Evaluation for

Parallel Pumping Stations .......................... 52

4 Testing of Pumps .....................................544.1 Testing Arrangements ................................. 544.1.1 Production Testing ....................................... 544.1.2 Field Testing, Duty Point .............................564.2 Acceptance Tests .......................................... 574.2.1 Testing Standards ......................................... 57

5 Pumping Stations ....................................595.1 Pumping Station Basic Design ..................595.1.1 Wet Well Volume and Surface Area ........595.1.2 Pumping Station Inlet Pipe ....................... 605.1.3 Wet Well Floor Shape ................................. 605.1.4 Stop Levels ...................................................... 615.1.5 Start Levels .....................................................625.1.6 Suction Pipe Dimension and Design .......625.1.7 Pumping Station Internal Pipework ........635.1.8 Flushing Devices ...........................................635.1.9 Odour Problems in Pumping Stations .... 645.1.10 Pumping Station Design Examples ......... 645.1.11 Dry-installed Pump Positions ....................675.2 Package Pumping Stations ........................ 685.2.1 Out-of-doors Pumping Stations .............. 685.2.2 Indoor Pumping Stations ...........................705.3 Pumping Stations with

Column-installed Pumps ............................705.4 Pumping Station Dimension Selection ... 725.4.1 Regular Sewage Pumping Stations .......... 72

Table of Contents

6

5.4.2 Stormwater Pumping Stations ..................725.4.3 Combined Sewage Pumping Stations

and Retention Basins ...................................735.5 Pump Selection ..............................................745.5.1 Pump Selection Based on Pump Curves ..745.5.2 Observing Pump Efficiency .........................745.5.3 Number of Pumps .........................................755.6 Special Considerations ................................765.6.1 Pump Vibrations ............................................765.6.2 Pump Noise ....................................................77

6 Frequency-controlled Sewage Pumps ........... 786.1 General ........................................................... 786.1.1 Pump Motor Selection ................................ 786.1.2 Maximum Frequency .................................. 786.1.3 Minimum Frequency and Minimum

Performance.................................................. 796.1.4 Pump Frequency Curves ............................. 796.1.5 Pump Clogging .............................................806.1.6 EMC Cable Requirement ............................806.1.7 Bearing Currents ..........................................806.1.8 High Tension ..................................................816.1.9 Explosion-proof Motors ..............................816.1.10 Guaranteed Values .......................................816.1.11 Tests with Frequency Controller

(String Tests) ...................................................816.1.12 Collaboration with the Pump

Manufacturer .................................................81

7 Pump Whole-life Cost Evaluation .......... 827.1 General ........................................................... 827.2 Calculation Period ........................................ 827.3 Investment Costs ......................................... 827.4 Energy Costs .................................................. 837.4.1 Efficiency Over Time .................................... 837.4.2 Energy Usage Calculations ........................ 847.5 Maintenance Costs ...................................... 847.6 Cooperation With Pump Suppliers .......... 857.7 Life Cycle Cost Publication ......................... 85

8 Commissioning ....................................... 86

9 Operation and Service ............................ 879.1 Safety .............................................................. 87

10 Pumping Station Control and Condition Monitoring ............................. 88

10.1 Local Control Methods ............................... 8810.1.1 Manual Control Units ................................. 8810.1.2 Relay-based Control Units ......................... 8810.1.3 Programmable Logic Controllers .............. 8810.2 Sensors for Pump Control and

Condition Monitoring .................................89

10.2.1 Wet Well Water Level Sensors ..................8910.2.2 Current Sensor ............................................. 9010.2.3 kWh Meter .................................................... 9010.2.4 Phase Failure Relay ...................................... 9010.2.5 SARI 2 Monitoring Device .......................... 9010.2.6 ASM 3 Alarm Status Module ......................9110.3 Pump Control Units ......................................9110.3.1. Control Features ............................................9110.3.2 Condition Monitoring Features ................9210.3.3 Parameters and Signals ..............................9210.3.4 Data Logging and Analysis .........................9310.3.5 User Interface ................................................9310.4 Remote Control and Monitoring System 9310.4.1 Different Levels for Remote Control ........9310.4.2 Software and Hardware .............................9410.4.3 Data Transmission ....................................... 9510.4.4 Alarm Transfer ............................................... 9510.4.5 System Integration ......................................9610.5 Internet & WAP Based Remote Control

and Monitoring .............................................96

Symbols .............................................................98

APPENDIX A ...................................................... 100

APPENDIX B....................................................... 108

Pump Theory 1

7

1 Pump Theory

This section is a primer of fluid pumping theoryand provides the reader with the theoretical back-ground knowledge essential for deeper under-standing of the pumping process.

1.1 The Head Equation

Figure 1 shows part of continuous fluid flow in aduct. Between the two observation sections 1 and2 no energy is transferred to or from the fluid andthe flow is assumed to be frictionless. Thus thetotal energy of the fluid relative to a horizontalreference plane T at the two sections must beequal. The total energy comprises components forpotential energy, pressure energy and kineticenergy, and for a fluid particle with a mass m theenergy at the observation sections is as follows:

where is the fluid density and g the accelerationof gravity.

For a flow without losses the total energy in sec-tion 1 and 2 will be equal, thus

.

Dividing both sides of the equation with the termmg it is obtained

(1)

This equation is called Bernoulli's equation afterthe engineer who first derived it. The terms of theequation are expressed as heads, and the terms

are consequently called static head, pressure headand kinetic head, respectively. The equation is essential for fluid mechanics andcan be used to account for many hydrodynamicphenomena, such as the decrease in pressure thataccompanies a reduction in a flow cross sectionarea. In this case the fluid velocity increases, andfor the total head to remain constant and assum-ing the potential head remains unchanged, thepressure term or static head, must decrease.

1.1.1 Flow with Losses or Addition ofEnergyIf there are losses in the flow between section 1and section 2 in Figure 1, the head equation 1 canbe written

(2)

where Hr is the head loss.

If energy is added to the flow by placing a pumpbetween section 1 and section 2 in Figure 1, theequation 2 can be written

(3)

where H is the pump total head.

Section 1 2

PotentialEnergy

PressureEnergy

KineticEnergy

mgh1 mgh2

mgp1g------ mg

p2g------

12---mv1

2 12---mv2

2

mgh1 mgp1g------

12---mv1

2+ + mgh2 mg

p2g------

12---mv2

2+ +=

h1p1g------

v12

2g------+ + h2

p2g------

v22

2g------+ +=

v2

p2

p1

v1

h2

h1

2

Q

T

1

Fig. 1

Section showing flow of liquid through two obser-vation cross sections. T is a reference plane for the po-tential heads h1 and h2, p1 and p2 are the prevailingpressures and v1 and v2 the fluid velocities at sections1 and 2.

h1p1g------

v12

2g------+ + h2

p2g------

v22

2g------ Hr+ + +=

h1p1g------

v12

2g------ H+ + + h2

p2g------

v22

2g------ Hr+ + +=

1 Pump Theory

8

1.1.2 Fluid Flowing from a ContainerAn example of the application of the Bernoulliequation is the calculation of the flow rate of afluid flowing freely from an open container.

Figure 2 shows an open container with an outletorifice near the bottom. For practical purposes thearea A1 is assumed much larger than the orificearea A2, and the atmospheric pressure p1 in thecontainer is equal to that outside the orifice, p2.

Choosing the centre line of the orifice as the refe-rence plane T, the term h2 is equal to zero and h1equal to h. Because A1 is much larger than A2, the

kinetic head can be assumed as zero. Thus

the head equation 1 can be written

(4)

whence

(5)

For volume flow without losses is obtained

(6)

To accommodate for losses present, a flow coeffi-cient is added to equation 6, whence

(7)

The flow coefficient is dependent on the shapeof the orifice, and can be obtained from textbooks on the subject. If the fluid level in the con-tainer is allowed to recede, the level height h willchange, which will have to be accommodated forin calculations.

1.2 The Basic Pump Equation

The basic pump equation is used to calculate anddesign geometrical shapes and dimensions ofcentrifugal pumps. The basic pump equation isalso used to deduce the pump Q/H curve.

A pump impeller vane and its associated velocityvectors are shown in Figure 3.

v = absolute fluid velocityw = velocity relative to the vaneu = perimeter velocityvu = tangential component of absolute velocityvm = radial component of absolute velocity

The relative velocity is parallel to the vane at anygiven point.

Also and

Assuming the flow to be without losses and thenumber of vanes infinite (), the familiar basicequation of pump theory can be derived using thelaws of mechanics. This relationship is known asthe Euler equation and is expressed as:

. (8)

where the index t refers to a flow without lossesand refers to the assumption of infinite numberof vanes ensuring complete fluid direction.

In an actual pump neither of these assumptionscan be satisfied, as friction losses are alwayspresent, and the finite number of vanes will notdirect the flow entirely in the direction of thevane.

p2 = p2

p1

v1h

T

v2

A1

A2

Fig. 2

Section of a fluid container with an outlet orifice nearthe bottom. A1 and A2 are the cross section areas of thesurface and the outlet orifice, h the height differencebetween surface and orifice centre line, v1 surface re-cession velocity and v2 liquid outlet velocity throughthe orifice. Ambient pressure is constant.

v12

2g------

hv2

2

2g------=

v2 2gh=

q1 A2 2gh=

q1 A2 2gh=

vu1 v1 1cos= vu2 v2 2cos=

Ht1g--- u2vu2 u1vu1( )=

Pump Theory 1

9

The reduction in head caused by losses in the flowis taken into account by the hydraulic efficiency h,and the reduction due to the deviation of the flowfrom ideal angle 2 is accounted for by a vanecoefficient k. With these modifications, the Eulerequation for an actual pump reads as follows:

(9)

It can be shown that both h and k are less thanunity. They will not be discussed in further detailhere.

Centrifugal pumps are normally designed with 1 = 90 , whence vu1 = 0.

Thus the basic pump equation is simplified to

(10)

Hhg

------ ku2vu2 u1vu1( )=

H khu2vu2

g---------------=

u2

v2

vm2

2

2

vu2

1

w1

12d

dvm 1

11

v1

u1

vu

Fig. 3

Pump impeller vane with the velocity triangles at leading and trailing edges. Fluid absolute velocity v, relative velocityw, vane perimeter velocity u, liquid absolute velocity tangential component vu and radial component vm.

1 Pump Theory

10

1.3 Pump Curve and Losses

The ideal head obtained from the Euler equationis independent of the volume rate of flow Q. If theQ/Ht curve is plotted, Ht is indicated by astraight line. The real Q/H curve is derived fromthis by subtracting the effects of the finite num-ber of vanes and various other losses that occurwithin the pump. Please refer to Figure 4.

1.3.1 The Effect of Finite Number of VanesAs noted earlier, a finite number of vanesdecreases the head by the vane factor k. Takingthis into account, the theoretical head Ht isobtained. It can be written:

(11)

Ht is not perfectly linear because the vanecoefficient is slightly dependent on the volumerate of flow Q. The head reduction from Ht

to Htis not caused by flow losses, but by deviation ofthe fluid from the ideal flow angles due to thefinite number of vanes.

1.3.2 Friction Losses HfFriction losses occur as the fluid flows through thepassages of the impeller and the pump casing.They increase approximately with the square ofthe flow rate Q.

1.3.3 Discontinuity Losses Hs

Discontinuity losses are generated in the follow-ing areas: At the vane leading edge where the fluid hits

the vane tip. The loss is smallest at the pumpdesign point, where the fluid contacts the vaneat the vane angle 1. The losses increase withincreasing deviation of the contact angle fromthe vane angle 1, see Figure 5.

At the vane trailing edge losses occur due toeddies shed by the vane. These increase ap-proximately with the square of the flow rate.

In the pump casing at flow rates other thanthe design value, when the flow velocity at thecasing differs from that at the impeller perime-ter. The effect is shown in Figure 6. The velocitydifferences create turbulence leading to losses,growing with increasing difference of actualflow from design flow.

The effects of the discontinuity losses are shownin Figure 4.

1.3.4 Leakage Losses HvLeakage losses occur at the clearance betweenimpeller and pump casing. Even if the clearance iskept as small as possible, a small backflow passesfrom the high pressure area at the impeller rim tothe low pressure area of the impeller eye. Thusthe flow through the impeller is slightly largerthan the flow out of the pump casing, and thepump head is met with a reduced flow, the differ-ence being the leakage loss Hv. The effect of theleakage loss is shown in Figure 4. As the pumpwears out, this loss will increase.

Ht kHt=

Reduction of flow, Q

Effect of finite number of vanes Ht

Friction losses Hr

Discontinuity losses HsHN

H

QN Q

caused by leakage losses, Hv

Fig. 4

True pump Q/H curve (H) reduction from theoreticalpump head Ht .

losses

Q > QN

Q = QN

Q < QN

w1'

v1'

v1

w1

v1"

w1"

u1

1

Fig. 5

Vane leading edge relative velocities (w) and lossesat various flow rates. Minimum losses occur at thepump nominal flow when the fluid angle of attackis equal to the vane leading edge angle 1.

Pump Theory 1

11

1.3.5 Other LossesThere are further losses in a centrifugal pump, notaffecting the Q/H curve, but that will increase themotor shaft power requirement. These include: friction losses at the impeller outside surfaces shaft seal friction losses bearing friction losses

For submersible pumps, the last two items areincluded in the motor losses.

1.4. Cavitation and NPSH

Cavitation is caused by the formation and collapseof vapour bubbles in a liquid. Vapour bubblesform when the local static pressure in a flowingliquid decreases to or below the liquid vapourpressure at ambient temperature. When the bub-ble, or void, moves with the flow to an area with ahigher pressure, it will rapidly collapse. The implo-sion causes a transitory, extremely high localshock wave in the fluid. If the implosion takesplace near a surface, the pressure shock will, ifoccurring repeatedly, eventually erode the surfacematerial.

The cavitation phenomenon will typically occur incentrifugal pumps at a location close to the impel-ler vane leading edge, see Figure 7. Cavitation mayalso lower the pump Q/H curve and efficiency. Acavitating pump emits a typical rattling noise, likesand being pumped through the pump. No pumpmaterial will completely withstand cavitation, socare should be exercised if the pump operatingconditions present a risk of cavitation.

Wear marks from cavitation typically occur locallyand consist of deep pittings with sharp edges. Thepittings can be several millimetres deep, pleaserefer to Figure 8.

Normally pump curves published for submersiblepumps are drawn so that a pump in normal sub-merged installation will not cavitate as long asthe duty point is on the allowed section of the Q/H curve.

Velocity in casing

Absolute velocity after impeller (vu)

Resulting losses

QQN

Fig. 6

Effect of the difference of velocities in the pump cas-ing and at the impeller perimeter. Pump casing di-mensions are designed to accommodate nominalflow at the perimeter speed, leading to losses at otherflow rates.

Vapour bubbles

Implodingvapour bubbles

( Q > QN )

Fig. 7

Pumped fluid hitting the vane leading edge at anangle different than the vane angle. Eddies and lowpressure zones will form on the other side of thevane. If the pressure falls below the vapour pressure,vapour bubbles will form. Moving entrained in theflow to an area with higher pressure, they will even-tually implode. The consequent high pressure im-pact may lead to pitting and erosion of the adjacentstructure.

Fig. 8

Typical cavitation pitting in impeller.

1 Pump Theory

12

If the submersible pump is installed dry with asuction pipe, the installation must be checked forcavitation. In these cases the concept of NPSH isused.

1.4.1 Definition of NPSHNPSH is the acronym for Net Positive SuctionHead. The following pressure heads are used forthe calculation of NPSH:

ht = inlet geodetic headhA = height difference between reference

plane and tip of vane leading edge.Hrt = flow losses in inlet pipe

= pressure drop caused by inlet velocity

h = local pressure drop at vane leading edgepb = ambient pressure at liquid levelpmin = minimum static pressure in pumppv = liquid vapour pressure at prevailing

temperature

The pressure heads are shown in Figure 9.

In order to avoid cavitation, the minimum staticpressure in the pump (pmin) must be larger thanthe liquid vapour pressure, or

Figure 10 shows the principle of static liquid pres-sure distribution in inlet pipe, pump and pressurepipe of a dry pump installation.

1.4.2 Reference PlaneThe reference plane is the plane on which theNPSH calculations are performed. It is the hori-zontal plane through the centre point of the circledescribed by the tip of the vane leading edge. Forhorizontal pumps the reference plane coincideswith the shaft centre line. For vertical pumps thelocation of the reference plane is stated by thepump manufacturer.

1.4.3 Required NPSHThe required NPSH is obtained from the followingequation:

(12)

This is also called the pump NPSH value. It can bepresented as a function of flow as shown in Figure11. It is independent of temperature and type ofliquid being pumped. The pump manufacturer isrequired to state NPSH as a numeric value orcurve.

Any pump will actually have different NPSH-val-ues, depending of definition of occurrence, as can

hA

HORIZONTAL PUMP VERTICAL PUMP

Reference Plane

Minimum PressureNPSH

pming

Hrt

h

v02

2g

ht pb

pbg

required

Fig. 9

Dimensions and reference pressures for NPSH calculations.

v02

2g------

pmin pv>

NPSHrequired hAv0

2

2g------ h+ +=

Pump Theory 1

13

be seen in Figure 12. According to the testing stan-dards used by pump manufacturers, the NPSHr isdefined as the situation where pump head isdecreased by 3% due to cavitation. This value isdefined as NPSH3.

Light cavitation can be harmless to the pump ifthe vapour bubbles do not implode near thepump structural parts, such as the impeller vane.

The difference between the various NPSH valuesis greater in pumps with impellers with fewvanes. Thus single-vane impellers have the great-est differences in NPSH values with the differencebeing caused by the NPSH3 curve dropping, andthe tests thus giving too favourable readings.Therefore an NPSHr curve based on the 3% rule ofthe standard is a poor base for a cavitation riskassessment in pumps with few vanes. The NPSHr

Liquid static pressure

Vapour pressure

Absolute 0 pressure

Lowest pressure in pump

pbg

pming pv

g

pb ht

ht

Fig. 10

Pressure variation in a dry pump installation. Distribution of static liquid pressure in inlet pipe, pump and pressurepipe.

NPSH required

(m)

Q N Q

Fig. 11

Typical variation of required NPSH with pumpflow rate.

1 Pump Theory

14

curve published by a pump manufacturer shouldin principle guarantee that no damages will occurin the pump if the pump is operated above it. Thisis especially the case for wastewater pumps,which have a low number of impeller vanes. Theproblem is that there is no accurate way of testingand establishing such an NPSH value.

1.4.4 Available NPSHThe available NPSH indicates the pressure availa-ble for the pump suction under the prevailingconditions. This may be called the pumping sta-tion NPSH.

(13)

The term ht is positive when the reference plane isabove the liquid surface and negative if below it.Available NPSH is determined by the pumpingstation designer.

Figure 13 shows vapour pressure for water as afunction of water temperature.

Figure 14 shows atmospheric pressure as functionof elevation above sea level.

NPSHavailablepbg------ Hrt ht

pvg------=

Fig. 12

Different NPSH curves.

NPSH NPSHF (Cavitation free)

NPSHonset of noise

NPSHonset of material loss

NPSH0 (0% Head drop)

NPSH3 (3% Head drop)

Q

Temp ( C) Head (m)

pvg

10010

10

20

30

40

50

60

70

80

90

9

8

7

6

5

4

3

2

1

0,5

Fig. 13

Vapour pressure for water as a function of temper-ature.

Barometric

Altitude km

m H2Opressure

Fig. 14

Atmospheric pressure as function of elevation abovesea level.

Pump Theory 1

15

1.4.5 NPSH Safety Margin

NPSHavailable NPSHrequired + Safety margin

The NPSH margin must be sufficient to allow forvariations in a situation where the real conditionsmay differ from those theoretically calculated.The suction pipe flow losses may be inaccuratelyestimated and actual pump operation point maydiffer from the theoretical because of variations inthe Q/H curve and inexact pressure pipe resis-tance calculations. Harmful cavitation may occurearlier than expected, or at greater NPSH valuesthan NPSH3 (Figure 12). Manufacturing technicalvariations of the shape of the vane leading edgemay affect cavitation behaviour. The requiredNPSH may also be affected by the inlet pipeshape.

For horizontally installed pumps with straightsuction pipes, a safety margin of 1 to 1,5 m is suit-able.

For vertically installed pumps the safety marginshould be set at 2 to 2,5 m, provided that a reduc-ing bend is used before the pump inlet. Bend cen-treline curvature radius should be at least D1 + 100mm, where D1 is the diameter of the larger open-ing.

The matter of NPSH, safety margins and measur-ing methods for NPSH are discussed in details inthe EUROPUMP publication NPSH FOR ROTODY-NAMIC PUMPS,REFERENCE GUIDE (1997).

1.4.6 Damming up of Suction WellsIn practical installations situations may arise,where the liquid level on the suction side risesand the pump head decreases so that the pumpduty point moves to a sector where NPSHr > 10 m.No cavitation will occur, however, since NPSHavail-able will also rise and still be larger than NPSHre-quied. Typical installations where this situation willarise are dry dock drainage pumping, sewer block-age situations and drainage pumping with vary-ing suction liquid levels.

2 Pump Construction

16

2 Pump Construction

This section describes the construction of modernelectric submersible pumps. Various designs andthe main parts of the pumps are discussed as wellas topics concerning pump operation and mainte-nance. The study is limited to pumps for munici-pal sewage, drainage and raw water.

2.1 General

The submersible pump is a unit combining apump and an electrical motor to an enclosed unit,suitable for submersible installation in a wet wellholding the liquid to be pumped. The submersiblepump may be connected to the pressure pipingwith a special baseplate connection at the bottomof the wet well for ease of installation andremoval, or it can be installed connected with aflexible hose or other arrangements with riserpipes. Power to the unit is fed through one ormore flexible cables, supplied with the pump inlengths suitable for the installation.

Many submersible pumps can also be installeddry like conventional pumps. This type of installa-tion ensures uninterrupted operation of theinstallation in case of flooding of the dry well.

Submersible pumps are available for a number ofapplications with different requirements, and dif-ferent designs for various special applicationshave been devised.

A submersible pump comprises a waterproofmotor and matching pump components. Thepump components include the impeller, the pumpcasing and the required connection parts for dif-ferent installation alternatives. These include aguide shoe for submersible installation onto amatching connecting baseplate, a stand for porta-

Fig. 15

Section of a GRUNDFOS 2,4 kW submersible pumpshowing details of motor and pump. The pump is fit-ted with a guide shoe for use with a submerged base-plate in the wet well, facilitating easy pumpinstallation and removal.

Section of a GRUNDFOS 17 kW submersible pumpshowing details of motor and pump. The pump is fit-ted with a guide shoe for use with a submerged base-plate in the wet well, facilitating easy pumpinstallation and removal. The pump casing is adjust-able with set screws for maintenance of impeller suc-tion clearance.

Fig. 16

Pump Construction 2

17

ble pumps and the necessary connection flanges,stand for dry-installed pumps and seat ring forcolumn-installed pumps.

The motor is a dry squirrel-cage electric motormatching a range of pump parts for variousduties. Motor and pump have a common shaftwith the bearings and shaft seals housed in themotor. The motor also includes watertight cableinlets and a handle for lifting the unit.

Figure 15 shows a modern small submersible sew-age pump and Figure 16 a medium-sized submers-ible sewage pump. Submersible sewage pumpsare available with motors rated from under 1 up to500 kW for duties ranging from light portable useto large city sewerage system main pumpinginstallations. A submersible pump for dry installa-tion is shown in Figure 17.

a b c

S

SS

closed semi-open open

Fig. 18

Different impeller designs. The closed impeller has integral shrouds on both sides of the vanes, whereas the semi-open impeller incorporates only one shroud on the back side. An open impeller consists of only a hub and vanes, re-lying on close clearances (s) to the pump casing for its function.

Fig. 17

Section and outline of a 160 kW submersible pump.The pump is intended for horizontal dry installationand connects with integral flange joints to both suc-tion and pressure pipework. The submersible designpermits flooding of the installation without risk ofdamage to the pump.

2 Pump Construction

18

2.2 Pump

The pump comprises the impeller and the pumpcasing as well as ancillary devices and fittings.

2.2.1 ImpellersSubmersible pumps are fitted with differentimpeller designs depending on intended use. Thevarious impellers can be classified as impellers for sewage pumps impellers for macerating pumps propellers for axial pumps

Impellers can also be classified according to con-struction as closed, semi-open or open impellers.These are shown in Figure 18. Semi-open impellersand open impellers rely on the close clearancebetween impeller and pump casing (about 0,5mm) for their function. The efficiency of theseimpellers is very sensitive to wear and willdecrease rapidly as the clearance increases. Figure19 shows the effect on pump efficiency from wearon closed and open impellers. The open and semi-open impellers are also susceptible to impuritiesbecoming jammed between impeller and suctionplate, slowing down or completely stopping thepump.

Impellers for Sewage PumpsIn order to avoid pump blockage, or clogging, spe-cial impellers have been developed for sewagepumping. These include single-channel impellers,double-channel impellers and vortex impellers.The design principles of these are shown in Figure20. For very large sewage pumps, impellers with amultitude of vanes may also be used.

Free Passage The concept of free passage is of special relevanceto sewage pumps. It refers to the ability of thepump to let solids in the pumped liquid pass

Closed Impeller

Open Impeller

Operating time

Fig. 19

Test results from a comparison of the effect of wearon pump efficiency for different impeller types.

Winglets

Counterweights

Vortex impeller 1-channel impeller 2-channel impeller

Fig. 20

Impeller types for sewage pumps.

Pump Construction 2

19

through, and thus to the capacity of being non-clogging. The dimension of the free passage usu-ally refers to the largest spherical object that maypass through the impeller and the casing open-ings. If the free passage is described with twonumbers, it refers to the largest oblong objectthat can pass crosswise through the pump.

The ability of a pump to operate without cloggingrelates strongly to the free passage, as can beseen in the diagram in Figure 21. Normally a freepassage of 80 mm will be sufficient forunscreened sewage in small and medium-sizedpumps. In larger pumps (flow > 100 l/s) the mini-mum free passage should be at least 100 mm.

Free passage alone does not ensure good proper-ties against clogging in a pump. Impeller and vanegeometry must also have features that preventblockages. Pumps from different manufacturershave varying qualities in this respect. There arecases where a pump clogging problem has beensolved by changing pump make, even with thepumps having equal free passages, same numberof vanes and pump speed. The tendency of sew-age to choke a pump may vary from one locationto another, with easy and difficult pumpingstations. The design of the sewer line leading upto the pumping station is important for the func-tion of the pumps, because they must be able tohandle any agglomeration of solids originatingthere. Real conditions in sewage systems cannotbe simulated in laboratories, and the good proper-ties of the Grundfos sewage pumps against clog-ging are based on long-term practical experience.

Single-channel ImpellersA single-channel impeller is shown in Figure 22.The single vane is designed as long as possible forbest efficiency within the limits set by the require-ment of free passage. The impeller having onlyone route of passage for the pumped liquidensures good inherent characteristics againstclogging. The asymmetric shape requires theimpeller to include integral counterweights forbalance. Highest attainable efficiency is 7075%.

Clogging probability

Free passage (sphere) [mm]

40 60 80 100 120

Fig. 21

A diagram showing the relation between probabilityof clogging and pump free passage. Good safetyagainst clogging is achieved with 80 mm free pas-sage.

Fig. 22

A GRUNDFOS S-1 single-channel impeller for sew-age use. The impeller is semi-axial in design withone long continuous vane, ensuring good proper-ties against clogging. The asymmetric design re-quires the casting to include counterweightmasses to facilitate static and dynamic balancingof the impeller

2 Pump Construction

20

Double-channel ImpellersA double-channel impeller is shown in Figure 23.The inherent difficulty with double-channelimpellers is that long, fibrous impurities mayenter both channels and get caught by the vaneleading edges, causing the pump to clog. This sit-uation can be alleviated by good vane leadingedge design, and this can be found only by devel-opment work under real conditions in difficultpumping stations. With the right design and afree passage of at least 100 mm, double-channelimpellers can be designed to handle unscreenedsewage without clogging. Highest attainable effi-ciency is 8085% for double-channel impellers.

Three- and Four-channel ImpellersIn very large pumps impellers with three or fourvanes can be used and still have a free passage ofat least 100 mm and an impeller with good prop-erties against clogging. Also for these impellersthe design of the vane leading edge is decisive.Highest attainable efficiency is 8286% for theseimpellers.

Vortex ImpellersThe principle of the vortex impeller is to induce astrong vortex in the open pump casing. Thepumping action of the vortex pump is thereforeindirect, with the impeller being situated outsidethe main liquid flow. Vortex impeller pumps haveinherently excellent properties against clogging,and the pumps run very smoothly. The use ofsmall vortex impeller pumps for sewage isincreasing largely due to improved design andefficiency in later years. They are also used as sandseparation pumps in sewage treatment plants. Avortex impeller is shown in Figure 24. Highestattainable efficiency is around 50% for vorteximpellers. It is essential to note that the efficiencyin the flow range 315 l/s of vortex pumps isroughly equal to that of single-channel pumps.

Flow and Head (Q/H) Ranges for Differ-ent Impeller and Submersible PumpTypesFigure 25 shows the typical application areas fordifferent sewage pump and impeller types of theGRUNDFOS range. It can be seen that withincreasing flow and pump size the number ofvanes of the impeller also increases. The diagramalso shows the Q/H area for which submersiblepumps are available for sewerage use. The largestpump in the Grundfos range has a motor of 520kW power.

Fig. 23

A GRUNDFOS S-2 double-channel impeller. Goodproperties against clogging are achieved with re-cessed vane leading edges and a semi-axial design.The symmetric design is inherently in balance.

Fig. 24

A GRUNDFOS SuperVortex impeller. The design in-cludes patented vane winglets. The winglets pre-vent the formation of secondary eddies over thevane edges, greatly improving pump efficiency.

Pump Construction 2

21

Fig. 25

Vortex 1-channel 3-channel 4-channel2-channel

Flow and head (Q/H) ranges for different impeller types.

Fig. 26

GRUNDFOS Grinder pump. The macerating unit is made of hardened stainless steel.

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22

Impellers for Macerating PumpsFor installations with very small amounts of sew-age, macerating pumps have been developed.Typical applications are pumping stations for sin-gle homes, small developments or camping areas.The required flow is very small, sometimes lessthan 1 l/s, but the total head may be high becauseof long and narrow rising mains. The flow for amacerating pumps are typically 15 l/s, withheads as high as 50 m.

In macerating pumps the solids are shredded intosmall fragments of around 10 mm, which makes itpossible to use rising mains of small dimensions,usually DN 40DN 80. For the very small flowsfrom single pumping stations even smaller pipingwill be used, in order to attain a flow velocity of atleast 0,5 m/s.

Macerating pumps may not be used for sewagewith sand content, since the shredding unit is verysusceptible to wear. Where macerating pumps areconsidered for larger installations comprising sev-eral buildings, it is always advisable make a tech-nical and economic comparison with a solutionbased on conventional pumps.

Figure 26 shows a GRUNDFOS macerating pump.Outside of the impeller is a macerating unit withsharp cutting elements installed. The maceratingunit is made of hardened stainless steel.

Propellers for Axial PumpsAxial pumps, using the submersible motors fromsewage pumps, have been developed by manymanufacturers. Figure 27 shows a GRUNDFOSaxial-flow pump with adjustable-pitch propeller.The design incorporates trailing fixed vanes thattransform the rotating movement of the waterinto pressure energy, increasing pump efficiency.Propeller pumps are normally column-installed.

Propeller pumps are used for stormwater andflood water pumping, drainage, irrigation and rawwater pumping as well as for effluent pumping insewage treatment plants. Propeller pumps are notsuitable for unscreened sewage because of risk ofclogging. Small and medium-sized propellerpumps are not suitable for sewage treatmentplant internal circulation pumping of e.g. returnsludge, since they may clog and get jammed bythe fibres present in these fluids. Highest attain-able efficiency for propeller pumps is 7585%.

The operating range (Q/H area) of the GRUNDFOSpropeller pumps is shown in Figure 28. Part of therange is also covered by channel-impeller pumpsfor column installation, which may be a more suit-able choice for many applications. The final choicebetween these should be based on desired dutypoint and application. The pump manufacturershould be consulted in the selection process indifficult projects.

Fig. 27

GRUNDFOS propeller pump. Blade angle isadjustable for best efficiency.

Pump Construction 2

23

For recirculation pumping in sewage treatmentplants special axial pumps have been developedas shown in Figure 29. They are intended to oper-ate at very low heads, only 0,3 1,0 m, and highflow rates, up to 2000 l/s. These pumps aredesigned non-clogging with back-swept blades,large (10 mm) clearance between blade tips andcasing, and by omitting the lead vanes. Highestattainable efficiency for circulation axial pumps is3550%. The delivery loss at the exit of the thim-ble is significant for the head. Additional head canbe attained by using a conical thimble, loweringlosses.

10

0 100 200 500 1000 2000 4000 5000

8

6

4

2

0

Fig. 28

GRUNDFOS propeller pump flow and head (Q/H) range.

Fig. 29

Submersible recirculation pump for sewage treat-ment plant. The pump is lowered in place alongguide rails.

2 Pump Construction

24

Impeller Auxiliary VanesAuxiliary vanes on the outside of the shrouds arean important feature of the impellers of smallsewage pumps. The auxiliary vanes increase thevelocity of the flow of fluid in the space betweenthe impeller and the pump casing. Figure 31shows the location of auxiliary vanes on a single-channel pump impeller.

Auxiliary vanes assist the pump operation byperforming the following functions: Decrease axial loads on bearings, particularly if

semi-open impellers are being used Reduce impeller and casing wear at the suction

clearance Prevent the wedging of fibres in the suction

clearance Prevent fibres and rags from wrapping around

the pump shaft behind the impeller.

The use of auxiliary vanes extending to theshroud perimeter is not possible on large impel-lers, since at high flow rates they could cause apressure drop below the vapour pressure of theliquid, leading to cavitation. Large pumps are,however, less prone to jamming because of highmotor torque. Auxiliary vanes are therefore notincluded on the inlet side of large impellers.

Suction ClearanceThe clearance between the impeller and thepump casing should be as small as possible inorder to reduce leakage losses. The suction clear-ance is in the order of 0,5...1,0 mm for most cen-trifugal pumps. The clearance can be designedcylindrical or axial as shown in Figures 32 and 33.

Fig. 30

GRUNDFOS recirculation pump

Auxiliary

Pressure distribution

Pressure distribution

ps

p's

S

with auxiliary vanes

without auxiliary vanes

vanes

Fig. 31

The effect of the auxiliary vanes on the suctionshroud is a lowered pressure difference p's over thesuction clearance. With less back-flow the suctionclearance will last longer and the risk of jamming isreduced.

Pump Construction 2

25

Pump performance and efficiency over time aredependent on the suction clearance being keptwithin specified limits. The lowering effect of thesuction clearance on pump efficiency and headcan be calculated with the following empiricalequation:

(14)

where

Q= flow [l/s]H= head [m]s= clearance [mm] and H are proportional.

For semi-open impellers the effect is increased bythe factor 1,5.

Figure 34 shows the results of a test where apump was operated with varying suction clear-ance.

If the suction clearance widens to 2...3 mm forimpellers without auxiliary vanes and to 4...5 mmfor impellers with auxiliary vanes, it is necessaryto restore the clearance in order to retain pumpperformance. If the suction clearance is madeadjustable, this procedure is easily done by servicetechnicians in the field, whereas a pump with

Fig. 32

Cylindrical suction clearance. The design is suscepti-ble to jamming, since fibres that get wedged in thespace between impeller and casing may accumulateand drag down the pump. In case of wear, a wearring on the suction cover and the impeller need to bereplaced or re-machined.

Fig. 33

Axial suction clearance. This design is less prone tojamming, since drag forces will remove wedged ma-terial towards the pump suction. The clearance canbe made adjustable for ease of maintenance andwear compensation.

H K2

K+ K

K 0 008 s2 H

Q---- ,=

Fig. 34

Effect of different suction clearance dimensions onpump curve and efficiency.

2 Pump Construction

26

fixed impeller suction clearance will have to bebrought to the shop for overhaul, or worse,scrapped for high costs of spare parts and work.

In pumps with adjustable axial suction clearanceperformance can always be guaranteed by check-ing and adjusting the suction clearance duringroutine maintenance. Figure 35 shows a submers-ible pump design, where the suction clearance isadjusted with the help of three set screws.

For dry-installed pumps GRUNDFOS has devel-oped a patented design (SmartTrim) allowing thesuction clearance to be adjusted and restoredwithout the need of removing the pump or open-ing pipe connections. Adjustment does not affectpipe connections or require re-alignment of these.Figure 36 show the principle. The adjustment isdone by first closing the clearance and then back-ing up the adjustment screws 1 mm, after whichthe suction cover is tightened against the setscrews with the fastening bolts.

The adjustment margin on the GRUNDFOS pumpsis 10...15 mm, depending on pump size. It is dimen-sioned to last the lifetime of the impeller.

Impeller AttachmentThe attachment of the impeller onto the shaftmust be both reliable and easy to dismantle.Removal is necessary for shaft seal maintenance,and for impeller replacement if the pump is usedfor pumping abrasive materials. The impeller mayhave either a cylindrical or tapered fit onto theshaft end.

A shaft joint tapered to the right angle is easy todismantle. The tapered joint is additionally tight-ened with a screw, which makes the joint free ofplay and rigid.

The joint is keyed for transmission of torque. Solidimpeller mounting is a key component in pumpoperational reliability, and great care shouldalways be exercised when the impeller is disman-tled. It is good practice to always use a torquewrench when setting the impeller screw. Thepump manufacturer issues correct tighteningtorque information and possible recommenda-tion of screw lubricant in each case.

Fig. 35

Suction clearance adjustment system with threeset screws.

Fig. 36

Suction clearance external adjustment system ondry-installed pumps.

Pump Construction 2

27

2.3 Motors

2.3.1 GeneralSubmersible pump motors are squirrel-cage elec-tric motors wound for regular three-phase or sin-gle-phase alternating current supply. Single-phase pump motors are available only for smallpumps (2 kW or less). Motors are available for 50or 60 Hz supply and a number of voltages. Themotors are built for submersible operation, ClassIP 68 according to IEC. The electrical features ofsubmersible motors are described in detail later inthis book.

The submersible pump is a fixed combination of amotor and a pump with a common shaft andbearings. The motor is short-coupled to the pump,and some of the pump parts, such as the volutecover may be integral with the motor attachmentflange. For best results the pump and motor aredesigned together, with one motor frame fitting arange of pump parts for different duties and dif-ferent operational ranges by the same manufac-turer. Motor and pump sections are selected anddesigned so as to exclude overloading at any dutypoint on the pump curve.

Submersible motors are normally air-filled. Smallmotors (1,5 kW and less) are also made oil-filled.The oil used in these is low-viscosity oil used alsoin transformers, in order to keep the rotor frictionlosses a small as possible. The growing losses andlower efficiency prevent manufacturers frommaking larger oil-filled motors. Oil-filled motorsare cheaper than air-filled motors because ofsmaller number of parts.

2.3.2 Explosion-proof MotorsSubmersible pumps are available in explosion-proof versions for use in environments where thepumped liquid or ambient atmosphere may con-tain explosive gases. This condition may exist, forexample, at or near petrochemical works but aspace can also be defined as explosive elsewhere,if deemed necessary for safety reasons.

The principle of explosion-proof motors is theirsafety against causing a potentially explosiveatmosphere from igniting. The following twoalternative technical solutions are available to fillthe requirement: The motor is designed so that the enclosure

can withstand any internal explosive blaze andprevent it from spreading into the explosivesurroundings. This is referred to as Class D.

The motor is designed so that no sparking orhigh temperatures may occur inside the motor.This is referred to as Class E.

An explosion-proof motor is designed and builtaccording to the rules set forth by internationalgoverning bodies (for example, Euronorm 50014and 50018). The requirements for class D motorsare detailed, involving among others the selectionand gauge of construction materials, casing jointdesign and manufacturing tolerances, motor inte-rior volume utilization as well as strength of thestructure and fasteners. The essential require-ment for the joints is that the mating surfaceshave to be longer, as they are supposed to serve asextinguishing gaps. Certification and approvalof a design is always subject to extensive tests,where the actual ability to withstand internalexplosions, is determined.

Class E explosion-proof motors do not requireextensive structural modifications, but are testedfor internal temperature rise at certain loads. Alsointernal sparking must be prevented by adequategaps between rotating and stationary parts.

Usually explosion-proof motors are based on theregular designs of a manufacturer, and form acomplement to these. The power characteristicsare normally not altered, and the pump parts arecommon for both. The structural requirements onexplosion-proof motors make these more expen-sive than regular motors.

2.3.3 Motor CoolingMechanical and electric losses in the motor areconverted to heat, which must be dissipated. In aregular submersible pump motor (see Figure 14)the heat is transferred from the stator casing tothe liquid through submersion. For cooling pur-poses it is normally sufficient if the motor is sub-merged to about half of the motor depth. Theliquid level may be pumped down all the way forshort periods without risk of overheating themotor.

A motor operating in water this way is in fact veryefficiently cooled, since cooling continues afterthe motor has stopped. Thus it is possible to allowfrequent starts and stops of submersible motors,

2 Pump Construction

28

which is beneficial for the design of pumpinginstallations.

Allowed Water TemperatureCooling of submersible motors relies on thepumped liquid, either by submersion or other-wise. Water temperature is therefore essential.Usually the motors are rated for +40 C liquids.Higher liquid temperatures may be allowed, butthen the pump selection should be referred to thepump manufacturer. Also the cavitation risk mustbe assessed for higher temperatures with anNPSH analysis, because of higher vapour pressureof the liquid.

Submersible Motor Cooling in Dry Instal-lationsMany submersible motors are installed dry forvarious reasons. Adequate cooling of thesemotors must be ensured, and it can be arranged ina number of ways:

With a cooling water jacket that encases themotor or parts thereof. Part of the pumped liquidis diverted through channels from the pump cas-ing into the cooling jacket where it recirculatesafter the casing has filled up. The water enters thespace behind the impeller through a filteringclearance (about 0,5 mm) and is circulated by theauxiliary vanes on the back of the shroud aroundthe motor stator housing inside the jacket. Excessheat is conveyed to the water through forced con-vection, ensuring efficient cooling. The principle isshown in Figure 37. The usage of a filtering clear-ance and wide enough cooling channels hasensured that the system is non-clogging also inpractice. A cooling jacket is often optional onsmall and medium-sized pumps for dry installa-tions, whereas very large pumps are oftenequipped with a cooling jacket as standardregardless of installation method.

In some cases, where the liquid being pumped isunsuitable for circulation in the water jacket,external cooling water may be used. In thesecases the pump is modified with external waterconnections in the jacket and by plugging theentrance channels from the pump casing. A safetycircuit is necessary to protect the pump fromoverheating due to disruption of the externalcooling water supply.

With thick stator housing walls. This design, suit-able for small submersible pumps, employs athickened stator housing that conveys the heatfrom the stator to the pumped liquid. In this con-struction the stator housing flange may contactthe pumped liquid directly or through the oilhousing cover flange. The flange can be shapedwith a recess or channel for good contact with theliquid. The stator casing may also be made of alu-minium in dry-installed pumps to furtherenhance heat dissipation. Figure 38 shows theconstruction.

For dry-installed pumps only a cooling waterjacket offers equal or even superior motor coolingto submergence. Other motors may have to bedown rated for dry installations, limiting theselection of pump components from the match-ing range.

Fig. 37

A dry-installed GRUNDFOS submersible pump withmotor cooling jacket. Part of the pumped liquid isfiltered through a gap of about 0,5 mm and remainsrecirculating in the cooling jacket, circulated by thepumping action of the impeller back shroud auxilia-ry vanes. Efficient cooling is provided by heat con-vection from the stator through dissipation into thepumped liquid.

Pump Construction 2

29

With an internal cooling circuit, where a coolingfluid, e.g. glycol, is circulated by a separate smallimpeller on the pump motor shaft. The pumpincorporates a heat exchanger between pumphousing and the motor, where the cooling fluidyields heat into the pumped liquid. System com-plexity may pose problems.

2.3.4 Motor TightnessWater intrusion in the motor leads invariably todamage, or, if detected by safety devices, at leastto pump outage. The chief requirement anddesign consideration of submersible pumpmotors is therefore complete integrity againstleakage. Motor tightness is ensured with gooddesign and continuous quality control includingtests during manufacturing.

All submersible motor joints are machined to fit,and O-rings are used throughout. The O-rings are

renewed each time a joint is opened for service toensure tightness.

The electrical cable inlet to the motor must bereliably watertight. A good design uses compress-ible rubber grommets that match both cable andthe inlet recess. The grommet is compressed toprescribed tightness by the shape of the matchingparts when assembled. A cable clamp external tothe sealing carries all outside tensile loads on thecable, preventing pulling at the seal.

The possibility of water intrusion through thecable is a reality. If the cable free end is allowed tobe submerged, water may travel by capillaryaction between the copper strands of the leads tothe motor. This action is enhanced by the temper-ature changes of the motor, and water may thisway enter an otherwise undamaged motor. Thecondition can arise in new pumps that have beenstored outdoors prior to installation with thecable free end unprotected.

Most pump manufacturers deliver their pumpswith protecting sleeves on the cable free ends.Warning labels are attached to warn the storageand installation personnel of the risk of submerg-ing the cable free end.

Securing submersible motor tightness requiresspecial knowledge and special tooling, and it istherefore advisable to return the pump to anauthorized shop for repairs. Pump manufacturesoffer training and special tools to their customers.For owners of large numbers of submersiblepumps an in-house authorized shop may be war-ranted.

Shaft SealsThe shaft seal, providing safety against leakage ofthe pumped liquid into the motor, is one of themost important elements in a submersible pump.

Modern submersible pumps almost exclusivelyuse a shaft sealing arrangement with doublemechanical seals separated by an oil-filled cham-ber. The arrangement, developed and refined overthe years, provides adequate protection againstleakage and motor damage in most cases.

Figure 39 shows a mechanical seal arrangementused in submersible pumps. There is a lower orprimary seal and an upper or secondary seal. The

Fig. 38

A GRUNDFOS submersible pump suitable for dry orsubmerged installation. The thick-walled lower sec-tion of the motor serves as a heat conduit to thepumped liquid. The stator casing may be made ofaluminium to further enhance the effect.

2 Pump Construction

30

seals, being separated by an oil bath, operateunder different conditions. This is reflected intheir construction with different materials. Bothseals comprise two contacting slip rings, one sta-tionary and one rotating with the shaft. The ringsare pressed against each other by spring forceand, for the primary seal, in addition by the pumppressure.

Sealing between the slip rings is based on theextremely smooth and flat contact surfaces of theslip rings. The surfaces are in such close contactthat no or only a very minute leakage can passbetween them. The flatness and smoothness ofthe rings are in the magnitude of 0,0005 mm andthe faces are finished by lapping. The slip ringsseal against the stationary seat or shaft with O-rings. The material of the O-rings is selected towithstand high temperature and the corrosiveand dissolving action of the seal oil and the impu-rities in the pumped liquid.

Notches in the stationary slip rings of the primaryseal secure them in the seat against turning. Therotating rings are locked similarly with drive pins.Spring clips or washers keep the stationary ringsin their seats during abnormal pressure situa-tions.

The material of the primary seal faces is normallyhard, because of the abrasive action of the

pumped liquid. The material used today is siliconcarbide (SiC), which has a hardness around 2000on the Vickers scale and ranks next to the dia-mond. The silicon carbide rings can be either solidor converted. Converted carbide rings are sinteredto SiC to a depth of approximately 1 mm, leavingthe ring interior unchanged. SiC also has verygood resistance against corrosion, and can beused in all wastewater and dewatering applica-tions.

If the secondary seal is oil lubricated, a combina-tion of materials may be used. A stationary ring ofa softer material with good friction properties incombination with a hard rotating ring provides forlow seal rotation resistance. The oil lubricationprotects the seal against wear. Modern secondaryseals normally have faces of silicon carbide andcarbon .

Modern submersible pumps utilize mechanicalseals custom-designed for the purpose. Gooddesigns have been developed by most major man-ufacturers. A proprietary design combining pri-mary and secondary seal is shown in Figure 40.

Fig. 39

A GRUNDFOS double mechanical seal with primaryand secondary seals.

Fig. 40

A GRUNDFOS integrated double mechanical seal.

Pump Construction 2

31

All mechanical seals used in submersible pumpsmust allow rotation in either direction, sincepumps frequently get started in the wrong direc-tion or may be turned backwards by back-flowingwater in installations without check valves.

All submersible pumps with double mechanicalseals have an oil space between the seals. The oilserves the following functions vital to the func-tion of the seals and the pump: Seal lubrication, especially of the secondary

seal Seal cooling Emulsification of possible leakage water, thus

rendering it less harmful Seal condition monitoring. By checking the

seal oil during maintenance, the seal conditionand rate of leakage can be estimated.

Overfilling of the seal oil chamber should beavoided in order for the oil to be able to absorbleakage water by emulsification and to preventpossible overpressure due to thermal expansionof the oil. The pump manufacturer provides infor-mation on oil quantity and filling and monitoringmethods.

In special applications, where the pumped liquidcontains very fine materials, the primary seal mayopen due to material build-up on the slip ringfaces. In these cases it may be warranted toarrange for continuous external flushing of theseal. These installations are always consideredseparately for each case by manufacturer and cus-tomer.

Mechanical seal life expectance cannot be deter-mined theoretically or even by lab tests. Perfor-mance over time is also difficult to predict. Seallife varies greatly from case to case, with servicerecords from a few years to over 15 years reported.

2.3.5 Motor Bearings

Bearing loadsThe submersible pump bearings carry the com-bined load of the pump and motor as exerted onthe common shaft. The following forces act on thebearings, either radially or axially: hydrodynamic radial force hydrodynamic axial force magnetic radial force the weight of the rotating parts

The significant forces acting on the bearings arethe hydrodynamic forces.

The hydrodynamic radial force is the resultant ofthe pressure distribution at the impeller perime-ter in various relative positions to the pump cas-ing. The radial force is dependent on a number ofdesign factors as well as on the pump operatingpoint.

The hydrodynamic axial force is the resultant ofthe forces induced by the impeller diverting theflow from axial suction to radial discharge, andfrom the pressure difference between suction andpressure side of the impeller. The axial force isalso strongly related to the pump flow and oper-ating point.

BearingsRolling bearings are used throughout in submers-ible pump motors. Ball bearings are used for theirability to carry both axial and radial loads. In verylarge motors a combination of ball bearings androller bearings are used because of the largeforces on the components.

To allow for heat expansion of the shaft and formanufacturing tolerances, the shaft upper bear-ing is allowed axial movement, whereas the lowerbearing is locked axially.

Bearing selection is governed by internationalstandards with regard to bearing life. According tothe pump standard ISO 5199, Bearing rating life(B10) should be at least 17500 hours.

Submersible pump bearings are normally lubri-cated for life at the pump factory, using specialgrease suitable for the high operating tempera-tures allowed in submersible motors.

2 Pump Construction

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2.3.6 Motor Protection DevicesSubmersible motors are equipped with variousprotection devices for prevention of damages forthe following reasons: overheating water intrusion seal failure bearing failure winding insulation deterioration

Some protection devices are standard issuewhereas others may be available as extra equip-ment on request only. Large pumps need betterprotection devices because of the greater eco-nomic values of these pumps.

The protection devices can be divided into inter-nal devices with sensors inside the motor andexternal devices in the pump motor control panel.

Internal Protection DevicesThe following protective devices are mountedinside the motor: Thermal switches in stator windings. These are

normally bimetal miniature switches thatopen at a fixed, preset temperature, pleaserefer to Figure 41. Three switches, one in eachphase, are used in three-phase motors. Theswitches are connected in series in the controlcircuit, which is wired to stop the motor whenopening. The switches reset upon cooling, clos-

ing the circuit, making re-start of the pumppossible. Thermal switches in the windingsprotect the motor against overheating frominsufficient cooling, and are especially impor-tant in pumps depending on submergence forcooling.

Water intrusion into the sealed motor can bemonitored with a moisture switch that reactsto excess moisture. Normally the moistureswitch is connected in series with the thermalswitches in a circuit that disconnects the cir-cuit breaker coil and stops the motor uponopening. Figure 42 shows a moisture switchthat operates when the humidity reaches100%. The moisture switch is non-reversingand does not reset after tripping. In a commoncircuit with moisture and thermal switches, itcan be determined which device has opened,since only the thermal switches close againafter cooling. The motor must be opened anddried out before any attempts to restart it afterthe moisture switch has tripped.

Water intrusion into the sealed motor past theshaft seals can be monitored with a leakagedetector sensor in the seal oil chamber. Regu-lar motor oils used as seal oil in submersiblepumps can form an emulsion with up to 30%water content. The leakage detector eitherreacts on a water content exceeding 30% (con-

Fig. 41

Thermal switch. The unit consists of a miniature bi-metal switch that opens according to the switchtemperature rating. The switch is connected in thecontrol panel to break the current in case the motoris overheating.

Fig. 42

A GRUNDFOS moisture switch. The unit consists of anumber of moisture-sensitive disks stacked onto anactuating rod, and a micro switch. The hygroscopicdisks expand upon contact with excess moisture,pulling the actuating rod. A cam at the rod end tripsthe micro switch and breaks the circuit. The unit isnon-reversing and must be replaced after use.

Pump Construction 2

33

ductive detectors) or monitors the water con-tent continuously (capacitive detectors). Thelatter may be calibrated to trip at any watercontent and used to indirectly observe primaryseal condition by monitoring water intrusionover time (leakage rate). Leakage detectors areusually not standard, but available as extraequipment.

Water intrusion into the sealed motor throughcapillary flow through the supply cable beforepump installation can be prevented by fitting atight protective sleeve over the cable free endat the factory. The sleeve is not removed untilthe cable is connected at the control panel.

The condition of the bearings and/or bearinggrease can be monitored with thermal sensorsin the bearing bracket. These are installedclose to the bearing outer race, and calibratedto register bearing temperature. Thermal sen-sors are extra equipment.

External Protection DevicesThe following protective devices are mounted inthe motor control panel: Short-circuit protection is accomplished by

means of fuses, circuit breakers or electronicmotor protectors. Fuses and circuit breakersshould be dimensioned to withstand themotor starting current, but the rating must notexceed that of the supply cable or switchgear.Where fuses are used, these should be of theslow type.

Overload protection is required in a suddenoverload situation, such as when the impeller

develops operational difficulties or getsjammed, when the pump becomes clogged orduring loss of phase in the mains supply. Over-load protection is frequently provided by over-load relays coupled to the motor contactors.These consist of ambient temperature-com-pensated bimetal elements, that trip the cur-rent to the contactor coils in case the currentexceeds the set specified value. Overloadrelays provide good protection against loss ofphase in the supply. The overload relay shouldbe set according to the motor nominal current.When star delta start is used, the currentthrough the overload relay is reduced by thefactor 0,58 (1/3), which must be taken intoaccount when setting the relay. Figure 43shows an overload relay.

The stator winding insulation is monitored byan automatic resistance measuring device thatmeasures the resistance between the phasesand between phases and earth each time thepump stops. Alarm levels for resistance can beset, preventing damages short circuits anddamages to windings.

Thermal overload relay. The relay connects to themotor contactor and breaks the current in case of theelectric load exceeding the set value.

Fig. 43

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34

2.4. Pump Connection

A submersible pump, when installed submerged,is connected only to the discharge pipe. For fixedinstallations a self-connecting baseplate arrange-ment is normally used.

Submersible BaseplateThe concept of the submersible baseplate hasbeen developed over the years for use with sub-mersible pumps. The arrangement allows for thepump to be lowered into the pump well andfirmly connect to the discharge pipework withoutthe need of the operating personnel to enter thewell. Likewise the pump can safely be hoistedfrom the well for service. The system includes railsor pipes that guide the pump down onto thebaseplate. A special flange, or guide shoe, on thepump discharge mates with the joining surfacesof the baseplate for a firm connection. Well-designed systems are made to precision and havemachined surfaces and rubber seal rings for a

sturdy and tight connection. The pump is kept inplace by its own weight. Figure 44 shows a sub-mersible pump baseplate and guide rails.

Figure 45 shows a flexible seal designed in a waythat the seal action is further enhanced by pumppressure, ensuring a tight connection at all times.

Some pump manufacturers offer conversion kitsfor the connection of pumps to older baseplatesor as replacement pumps to some other manufac-turer's baseplate. Thus the upgrading or conver-sion of existing pumping stations may be donewith a minimum of work and costs.

Fig. 44

A GRUNDFOS submersible baseplate. When seated,the weight of the pump keeps it firmly in place. Pre-cision-machined connecting surfaces and a rubberdisk seal ensures tightness. Clearance between theguide shoe and the rails ensures unobstructed hoist-ing of the pump even in fouled conditions

Fig. 45

Flexible seal between pump pressure flange andconnector. The seal is designed in a way that theseal action is further enhanced by pump pressure,ensuring a tight connection at all times.

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35

Hose ConnectionFigure 46 shows a submersible pump installationwith hose connection. It may be used for tempo-rary installations or in applications where thepump is shifted around the wet well for sludgepumping.

Column InstallationThe concept of column installation of submersiblepumps has been developed during the past fewyears. The pump is lowered into a vertical pipe orcolumn, where the circular pump casing fits ontoa seat ring installed at the lower end of the col-umn, please refer to Figure 47. The pump stays inplace by its own weight and from the pressureforce from the pumping action. The pump casingis special-designed for the installation, and is fit-ted with trailing vanes. The seat ring is conical,ensuring a tight connection between pump andcolumn. The tight connection and dowels preventthe pump from spinning loose at start-up

Column installation is ideal for submersible pro-peller pumps, but also for sewage pumpsintended for large flows and low to moderateheads. Figure 48 shows the Q/H area on whichcolumn-installed Grundfos pumps are available.For this range column installation is likely to leadto lower investment costs, but each projectshould be assessed individually. Column-installedpumps have the same efficiency as pumpsintended for other installation modes, but thepump curves will differ slightly because of theopen pump casing. Column installation is verysuitable for return sludge pumping in sewagetreatment plants. The column pipe may be madeof stainless steel or hot dip galvanized steel.

For seawater installations a column made ofstainless steel may create a strong galvanic ele-ment between pump and column, leading topump corrosion. Especially galvanized pump partswill rapidly corrode from the galvanic action ofthe large cathode area of the column surrounding

Fig. 46

Submersible pump on stand with hose connection.This installation version is used for temporary orshifting installations.

Seat ring

Fig. 47

Pump column installation. Pump rests on conicalseat ring installed at the bottom of the column

2 Pump Construction

36

the pump. A lifting chain left in place, for instance,will have to be made of stainless steel. The castiron pump should be protected by sacrificinganodes that are replaced at regular intervals.Painting the column with a paint layer of at least200 m thickness prevents the cathode surfacefrom forming and thus pump corrosion.

2.5 Construction Materials, Corrosion and Wear

2.5.1 Corrosion ResistanceCast iron is the main construction material in sub-mersible sewage pumps, with fasteners and hard-ware made of stainless steel. The pump shaft iseither made entirely of stainless steel or pro-tected against contact with the pumped media.Where the pump or baseplate includes fabricatedsteel parts, these are hot-dip galvanized. Thesematerials will last for decades in regular sewerageduty.

In cases, where the pumped liquid contains indus-trial effluents, the corrosion resistance of cast ironmay not be sufficient, especially for parts subjectto fast flow velocities, such as impellers and pumpcasings, which will be subjected to erosion corro-

sion. In these applications the natural corrosionlayer, providing the underlying material with nat-ural protection becomes scrubbed away, leadingto rapid corrosion. The use of stainless materialsfor these vulnerable parts may be warranted.

Corrosion in seawater is dependent on a numberof factors, such as salinity, oxygen content, pollu-tion and temperature, and the right materialselection must be considered for each case. Sacri-ficial zinc anodes may offer protection againstcorrosion in certain cases.

The supply cable sheath material must be able towithstand oils and other pollutants present insewage. Other rubber parts, such as O-rings areusually made of Nitrile or Neoprene for resistanceagainst sewage, oil and chemicals.

Submersible pumps are also available madeentirely of stainless steel for use in highly corro-sive liquids, such as process industry effluents.Stainless steel submersible pumps are 34 timesas expensive as pumps made of regular materials.In difficult applications the pump manufacturermay not be able to guarantee the corrosion prop-erties for a specific case, but will cooperate withthe client to find the right solution for the case.

Fig. 48

GRUNDFOS column-installed pump flow and head (Q/H) range.

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37

2.5.2 Wear Resistance The sand content in sewage is on averagebetween 0,002 and 0,003 % (in volume). The con-tent may periodically, e.g. during heavy rainfalland snow melting be much higher in areas withcombined sewage and stormwater drain systems.Regular cast iron will last in most applications foryears, but special material may have to be consid-ered for highly abrasive effluents, such as sewagetreatment plant sand trap pumping.

2.5.3 Abrasive LiquidsPump performance in an abrasive liquid isstrongly dependent on the content of abrasives inthe liquid. The standard abrasive is commonquartz or silicon sand, to which the following canbe applied directly.

The sand content is either expressed as volume orweight content, which are related as follows:

pm 3pv (15)

where pm is weight content and pv is volume con-

tent in %. Thus pv = 5% equals pm =15%.

With increased sand content the density of theliquid/sand mixture increases. Since requiredpump power is directly related to the density ofthe pumped liquid, required power will have tobe checked separately in each case to ensurepump performance, whenever liquids with highsand content are being pumped. For sand trappumps in sewage treatment plants, a powerreserve of 30% has proven adequate.

The density of a mixture water and sand can bewritten

= 1 + 0,007pm (16)

where pm is expressed in %.

Thus, if pm = 15%, = 1,1 kg/l.

The following factors affect the wear of a pump: sand content sand quality pump material pump head type of impeller

Figure 49 is a diagram showing the relationsbetween the pump wear rate and the sand con-tent and pump head. High sand contents in theliquid will have a dramatic effect on pump servicelife. The effect of the sand content is exacerbatedby high pump head.

Pump wear can be minimized using suitablewear-resistant materials and through appropriatedesign. For best results, materials with a hardnessover 500 HB should be used. The difficultmachineability of hard materials, such as Nihardand some alloyed irons, may require specialimpeller and pump casing designs where machin-ing is minimized.

The use of submersible pumps in abrasive envi-ronments must be considered separately on acase by case basis and using sound engineeringjudgment.

Sand contentPm [%]

Pump head H0 [m]

Pump service life [h]1

0,110000100010010

0,2

0,5

1

5

10

20

50 2010 5

Fig. 49

Pump wear rate as a function of sand content andpump head. H0 is pump head at Q=0. Wear rate isexpressed as expected service life of a cast iron im-peller and is strongly dependent of sand content andpump head. The graph is based on experiments, andcan be used generally.

3 Pump Performance

38

3 Pump Performance

Pump performance is the result of the interactionbetween pump and rising main or pressure pipe-line. An introduction to pump selection and thecalculation of rising main resistance characteris-tics are presented.

3.1 Pump Head

3.1.1 Submersible PumpsIn the following the concept of head is applied tosubmersible pumps. For practical reasons thepressure in the pump well, or lower well, isassumed to be equal to the pressure prevailing inthe receiving, or upper container. Should thesecontainers be under different pressure, the pres-sure difference would have to be taken intoaccount. The difference in atmospheric pressurecan also be disregarded in all practical installa-tions, since the difference in atmospheric pres-sure between a receiving container situated, forinstance, 100 m above the pump well is only 0,001bar or 0,01 m of water.

Figure 50 shows how the head is defined in asubmersible pump installation. The followingunits are used:H = pump total head (m)Hst= pump static head (m)

Hd = pump dynamic head (m)

Hgeod= geodetic head (m)

HJ = pipeline losses (m)

pL = atmospheric pressure in pump well

pU = atmospheric pressures in upper container

v2 = flow velocity at outlet (m/s)

g = acceleration of gravity (9,81 m/s2)

If an observation pipe is installed at the pumpoutlet flange, the pumped liquid will rise in it to aheight Hst from the well level. This height repre-

sents the pump static head. In addition, the liquidhas a velocity v2 at the pump discharge, which

can be converted to pressure or dynamic head Hdwith the following equation:

(17)

The sum of the static head and the dynamic headis the pump total head, thus

H = Hst + Hd (18)

According to international agreement (StandardISO 2548), the total head H according to equation18 is used when plotting characteristic curves forsubmersible pumps.

The total head H is thus available to pump the liq-uid through the rising main. The pressure or headrequired to pump a given flow through a pipelineis made up by the geodetic head and the flowlosses. Thus can be written:

H = Hgeod + HJ (19)

The geodetic head Hgeod is the actual physical

difference in height between the liquid levels inthe pump well and the receiving container. Pipe-line flow losses consist of pipe friction losses, locallosses from various fittings in the pipeline

Hdv2

2

2g------=

Fig. 50

Head components in submersible pump installations.

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39

(elbows, valves, etc.) and the outlet loss at thepoint of discharge.

Losses due to liquid flow in the well to the pumpare considered as pump losses in submersiblepump installations. If a suction pipe is installedbefore the pump, it will have to be taken intoaccount when calculating pipeline losses.

3.1.2 Dry-installed PumpsWhen calculating heads of dry-installed pumps,the situation before the pump will also have to beconsidered. Figure 51 illustrates the situation.

In this case it is assumed that the suction well andthe receiving container are open to the atmo-sphere and that the pressure at the liquid surfacesis constant. Thus the pump head is the sum of thegeodetic head and the flow losses in the suctionand pressure pipelines. Thus

H = Hgeod + HJt + HJp (20)

where HJt represents flow losses in the suctionpipeline and HJp flow losses in the pressure pipe-line.

3.2 Pump Performance Curves

Centrifugal pump characteristics are normallypresented as a set of curves, where the data hasbeen established through the testing of thepumps or assessed by the manufacturer for e.g. aspecial impeller diameter. For submersible pumpsthe following important information is normallyplotted as curves against the flow rate Q: H head curve efficiency curve(s) P power curves

Figure 52 shows a typical pump performancecurve sheet with information important for theuser.

3.2.1 H CurveThe head or H curve gives the pump total head asa function of the flow Q. The curve may containadditional information on pump usage, such aslimits due to cavitation, vibration or motor over-load.

Fig. 51

Pipeline loss components for dry-installed pumps.

Fig. 52

Typical pump performance curve sheet for submers-ible pump. The dashed sections of the curves indicateareas, where pump prolonged use is prohibited. Thereasons for the limitations may be cavitation, vibra-tions or motor overload.

3 Pump Performance

40

3.2.2 Efficiency CurvesPump efficiency is also a function of the flowrate Q. The efficiency can be indicated as a ratio orpercentage. For submersible pumps both thepump efficiency and the overall efficiency gr aredefined, where gr includes motor losses. It isimportant to distinguish between these defini-tions for efficiency, especially when comparingpump performance. The losses leading to thepump efficiency are discussed in Section 1 of thisbook. Thus it can be writt