centrifugal pumps may 4, 2006
TRANSCRIPT
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CENTRIFUGAL PUMPS
Justin Bjork Senior Sales Engineer Flowserve Corp.
Barry Erickson Key Account ManagerFlowserve Corp.
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Session Overview
Session 1 Centrifugal Pumps
IntroductionConstructionPump FundamentalsPump and System curvesControlReliabilityVibration Characteristics
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Learning Objectives
Understand various pump constructionsIntroduce pump and system curvesUnderstand relationship between flow rate and reliabilityBe able to relate typical vibration spectra to operational parameters
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Construction
Pumps are divided into Roto-dynamic or centrifugal pumps andPositive displacement pumps
Within these main groups there are many different types of pumps
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Construction
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Construction
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Construction
Mechanical ConstructionBetween Bearing
Impeller/s supported between two sets of bearings
Overhung Impeller Impeller overhangs a bearing support bracket
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Construction
Overhung Impeller
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Construction
Between Bearing
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Construction
Radial Flow Pump
High head low flow
Mixed Flow Pump Axial Flow Pump
Low head, high flow
Hydraulic Types Specific Speed
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Construction
Semi Open Impeller
Closed ImpellerFully Open Impeller
Impeller Types
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Pump Fundamentals
PressureHeadKinetic EnergyPotential Energy
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Pump Fundamentals
A pump adds energy (pressure) to a fluidPumps can deliver: high pressure / low flow or high flow / low pressure (and everything in between)Reliability and energy use are highly dependent on operating point
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Pump Fundamentals
Pressure = Force per unit area
0 105
15
5 lbs. steel
A = 1 in2PSI
Gage Pressure (psig) Pressure above surrounding atmospheric pressure.
Atmospheric pressure at sea level is 14.7 psig
Absolute Pressure (psia) Pressure above an absolute vacuum.
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Pump Fundamentals
Head vs. Pressure
0 105
15
5 lbs. steel
A = 1 in2PSI
11.5 ft. of water
Head (ft) = 2.31 x psi / Specific Gravity
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Pump Fundamentals
Effect of Specific Gravity on Head
100 ft
43.3 psi
Water 68ºF
S.G. = 1.0
55 ft 43.3 psi
Sulfuric Acid
S.G. = 1.8122 ft
43.3 psi
Oil
S.G. = 0.82
133 ft
43.3 psi
Gasoline
S.G. = 0.75
Head (ft) = 2.31 x psi / Specific Gravity
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Pump Fundamentals
Centrifugal pumps add energy byincreasing the kinetic energy of the fluid V2/2g
Higher impeller tip speeds increasekinetic energy
Impeller diameterImpeller speed
Higher flows through impellerdecrease kinetic energy
Rotation
Flow thru impellerImpeller speed
Kinetic energy of fluid
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Pump Fundamentals
Volutes catch and convert liquid kinetic energy to pressure energy
Flow Pattern at less than BEP
Flow Pattern at greater than BEP
Flow Pattern at BEP
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Pump Fundamentals
Effect of Specific Gravity on Pump Performance
Water 68ºF
S.G. = 1.0
100 ft
43.3 psi
10 HP
100 ft
32.4 psi
Gasoline
S.G. = 0.75
7.5 HP
77.9 psi
Sulfuric Acid
S.G. = 1.8
100 ft
18 HP
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Pump Fundamentals
Effect of Fluid Velocity
Vs
Vd
Velocity head is the kinetic energy of the fluid
Often suction and discharge velocity is different
The pump delivers energy to effect the velocity change
Velocity head
hv =V2
2g=
0.00259 GPM2
D4
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Pump Fundamentals
Gage Height Correction
Pressure readings must be corrected to a common datum
Normal datum is the center of the suction
hs
hd
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Pump Fundamentals
Total Differential Head TDH
TDH = Total Discharge Head Total Suction Head
Total Head = Discharge Pressure + Velocity Head + Static head
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Pump Performance Parameters
HeadFlow RatePowerEfficiencyNet Positive Suction Head (NPSH)Characteristic Curves
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Pump Characteristics
Head
Flow
Duty Point or Operating Point
Pump Curve
System Curve
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Pump Characteristics
Power
Flow
Radial Flow Pump
Mixed Flow Pump
Axial Flow Pump
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Pump Characteristics
Every pump exhibits internal losses The size of the losses depend on where the pump is operated on its curve
The losses can be minimal or substantial The pump is designed for a specific flow and pressure at a specific RPM When the flow deviates from the design flow, the liquid does not hit the vanes at the correct angle and extra turbulence and losses occur.
Losses lowest / efficiency highest, at the Best Efficiency Point (BEP)The ratio between output power and input power is the efficiency of the pumpLosses can be measured by comparing delivered hydraulic power to input power
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Pump Characteristics
Pump Efficiency
= What is sought / What it costs
p = Water Power / Pump input power
p = GPM x TDH / (HP x 3960)
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Pump Characteristics
Efficiency
Flow
Best Efficiency Point
(BEP)
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Pump Characteristics
Net Positive Suction Head (NPSH)NPSH Required (NPSHR)NPSH Available (NPSHA)
NPSH is a measure of the energy (pressure) in a liquid above the vapor pressureIf the pressure drops below the vapor pressure the liquid boils
That condition is called cavitation
All pumps require the NPSHA to be > 0How much, is called the NPSHR
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Pump Characteristics
Pre
ssur
e
Friction
Entrance Loss Turbulence,
Friction, and Entrance Loss at Vane Tips
Increasing Pressure in
Impeller
Suction Piping
Suction Flange
Impeller Inlet
NP
SH
R
Vapor Pressure
Net Positive Suction Head
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Pump Characteristics
NPSHR
Flow
BEP
NPSHR increases
quickly beyond BEP???
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Pump Characteristics
Cavitation Process
Vapor Bubble FormsBubble Expands into
colder liquid and begins to condense
Bubble Collapses creating intense
pressure (10,000 psi) and shock waves
Head
FlowLarge vapor volumes can cause
reduction in head and loss of prime.
Surging and unstable flow often results
Intense pressures on metal surfaces exceed material strength resulting in surface fatigue failure
Creates a pitted surface similar to coral or course sandpaper
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Pump Characteristics
CavitationDamage
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Cavitation Damage
CavitationDamage
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Pump Characteristics
Preferred Operating Range (POR)That range of operation where normal life can be expectedTypically 40% - 110% of BEPOften not shown on pump curvesPrimarily used in the petroleum and refining industries
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Pump Characteristics
Allowable Operating Range (AOR)That range of flow rates over which the pump will operate with some reduction in reliability and increase in noise and vibrationTypically 10% - 120% BEPOften labeled on characteristic curves as Minimum Flow
Maximum flow often limited by NPSH margin
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Pump Characteristics
Pump Characteristic Curve
50
100
150
200
250
300
350
0 50 100 150 200 250 300 350 400
GPM
TD
H -
ft
10"
7"
8"
9"
10 HP
30 HP
20 HP
50% 55%
63%
Min Flow
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Pump Characteristics
Effect of RPM
0
50
100
150
200
250
300
0 50 100 150 200 250 300 350
Flow Rate - GPM
TD
H -
ft
0.0
5.0
10.0
15.0
20.0
25.0
30.0
HP
1750 RPM
1180 RPM
GPM2 = GPM1 x (RPM2/RPM1)
TDH2 = TDH1 x (RPM2/RPM1)2
HP2 = HP1 x (RPM2/RPM1)3
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System Curves
Static HeadDynamic HeadPipe FrictionFitting Losses
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System Curves
It takes Energy to move fluid though a system of pipes and other equipment.
The pressure (head) used to overcome friction is called the dynamic head.The head required is proportional to the square of the fluid velocity
It takes Energy to lift fluid from one level to anotherThe pressure used to lift fluid is called static head , The head required to lift a certain volume of fluid is independent of velocity
System Head = Static Head + Dynamic Head
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System Curves
Energy required = 10000 ft-lb, or
10 gal.(83.3 lb)
m
(less than one M&M)3.24 calories
120 ft static head
THIS IS INDEPENDENT OF SPEED
Energy Requirements to Lift a Fluid are
Proportional to Mass and Height
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System Curves
Power required = 65 calories per hour or 0.1 horsepower
10 gal.(83.3 lb)
120 ft in 180 seconds
Ideal Power Dependson How Fast it is Moved
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System Curves
Static Head
hs
Static Head = hs Static Head = hs + 2.31 x Pt / SG
hs
Pt
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System Curves
Dynamic Head
The friction head loss:
Function of water velocityLower flow gives lower head lossProportional to the square of velocityReduced to 25% when velocity is cut
in half !Increased by a factor of 4 when the
velocity is doubled !
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System Curves
Sources of FrictionPipe wallsValvesElbowsTeesReducers/expandersExpansion jointsTank inlets/outlets
(In other words, almost everything the pumped fluid passes through, as well as the fluid itself)
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System Curves
What parameters influence frictional losses in piping?
Hf = pressure drop due to friction (ft)f = Darcy friction factorL = pipe length (ft)V = velocity(ft/sec), g = gravitational acceleration(ft/sec2)d = pipe diameter (ft)
= velocity head (ft)
Hf = f Ld
V2
2g
V2
2g
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System Curves
Standard Pipe Head Loss TablesTabulated values for head loss per 100 ft of pipeAvailable for most common pipe
Flow Rate gpm
Velocity fps
Vel HeadHead
Loss per 100 ft
Velocity fps
Vel HeadHead
Loss per 100 ft
500 3.21 0.16 0.42 3.51 0.19 0.52
1000 6.41 0.64 1.55 7.03 0.77 1.95
2000 12.8 2.56 5.91 14.1 3.1 7.46
Sch 40 Sch 80
8" New Steel Pipe
Cameron Hydraulic Data
Flowserve Corp
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System Curves
For pipe components, frictional losses have generally been estimated based on the velocity head.
K is determined by pipe size, valve type, % valve open, type of component and other physical factors.
Hf = K V2
2gK = Loss coefficient
= velocity headV2
2g
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System Curves
Component Loss Coefficient(K)90° elbow, standard 0.2 - 0.390° elbow, long radius < 0.1 - 0.3Square-edged inlet (from tank) 0.5Discharge into tank 1Check valve 2Gate valve (full open) 0.03 - 0.2Globe valve (full open) 3 - 8Butterfly valve (full open) 0.5 - 2Ball valve (full open) 0.04 - 0.1
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System Curves
Head
Flow
Design Flow
TotalHead
Friction Loss orDynamic Head
Static Head
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System Curves
Long pipes: Mostly
frictional head
Short fat pipes: Mostly static head
Two System Types
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System Curves
Head
Flow
Static only
Dynamic only
Combined, low frictionCombined, high friction
TYPES OF SYSTEM CURVES
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Pump and System Curves
The operating point will be found when the pump The operating point will be found when the pump and system curves are drawn on the same and system curves are drawn on the same diagramdiagram
The operating point is The operating point is alwaysalways where these where these curves intersectcurves intersect
The pump will operate where there is balance The pump will operate where there is balance between the head the pump can deliver and between the head the pump can deliver and what is demanded by the systemwhat is demanded by the system
Where will the pump operate?
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Pump and System Curves
Control ValvesPump ChangesParallel PumpingSeries Pumping
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Pump and System Curves
0
50
100
150
200
250
300
350
0 50 100 150 200 250 300 350
Flow Rate - gpm
TD
H -
ft
System CurvePump Curve
Operating Point
System Curve No Control Valve
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Pump and System Curves
0
50
100
150
200
250
300
350
0 50 100 150 200 250 300 350
Flow Rate - gpm
TD
H -
ft
System Curve - CV 100%
System Curve - CV 25% System Curve - CV 50%
System Curve With Control Valve
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Pump and System Curves
Effect of Impeller Diameter
0
50
100
150
200
250
300
350
0 50 100 150 200 250 300
Flow Rate - gpm
TDH
- ft
10" Impeller
9.2" Impeller
21 HP27.4 HP
30% Power increase to get 10% more flow
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Pump and System Curves
Effect of RPM
0
50
100
150
200
250
300
350
0 50 100 150 200 250 300 350
Flow Rate - GPM
TD
H -
ft
1750 RPM
1180 RPM
6 HP
21.4 HP
250% Power Increase to get 47% more flow
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Pump and System Curves
At the same head flow rates addPumps must be matched for effective operationProvision must be made to observe minimum flow criteriaCan be a good way to handle wide flow rate variations
Parallel Pumps
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Pump and System Curves
Parallel Pumping System
0
50
100
150
200
250
300
350
0 100 200 300 400 500 600 700
Flow Rate - gpm
TD
H -
ft
Tw o PumpsSingle Pump
0
50
100
150
200
250
300
350
0 100 200 300 400 500 600 700
Flow Rate - gpm
TD
H -
ft
Tw o PumpsSingle Pump
5 gpm increase in flow rate!
Each Pump Operates Here
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Pump and System Curves
Parallel Pumping System Low Friction
050
100150
200250
300
0 100 200 300 400 500 600 700
Flow Rate - gpm
TD
H -
ft
Tw o PumpsSingle Pump
Not a Good Operating Point
Each Pump Operation Parallel Pump Operation
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Pump and System Curves
Parallel Pumping Mismatched Pumps
0
50
100
150
200
250
300
0 100 200 300 400 500
Flow Rate - gpm
TDH
- ft
Pump BPump A + B
Pump A
Combined Flow
Pump B Flow
Pump A Flow
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Pump and System Curves
Series PumpingHeads add at the same flow rateSecond stage pump must be rated for discharge pressureStart up and shutdown procedures are critical
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Pump and System Curves
Series Pumping
0
100
200
300
400
500
600
0 50 100 150 200 250 300 350
Flow Rate -gpm
TDH
- ft
Single Pump
Tw o Pumps in Series
39% Increase in Flow Rate
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Pump Vibration
What are Acceptable Vibration LevelsHydraulic Institute Standards: www.pumps.org
ANSI/HI 9.6.4 Covers Horizontal and Vertical Centrifugal PumpsRecommends use of RMS velocityDistinguishes between types of pumpsLimits flow rates to the Allowable Operating RangeLower limits within the Preferred Operating Range
~ 40% - 110% of BEP
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Pump Vibration Characteristics
NormalAbnormal
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Pump Vibration
Flow Rate
Vib
ratio
n O
A r
ms
BEP
Typical Vibration Characteristic
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Pump Vibration
Normal CharacteristicsWithin the Preferred Operating Range
Dominated by rotation frequency and it s multiples
Outside POR , within AORBlade pass will began to dominate
Number of vanes x rotational frequency (single volute pumps)More prominent in pumps with few impeller vanes (wastewater)More prominent when impeller is near maximum diameter
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Pump Vibration
Abnormal OperationCavitation
Broad Spectrum toward higher frequenciesVibration levels may, or may not, be highMore likely to be high in higher HP pumps (> 50 HP)More likely to be high in higher speed pumps (2 pole)
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Pump Vibration
Abnormal OperationLow flow (< 20% BEP)
Broad spectrum, toward lower frequenciesHigh vane pass frequency content (80% of total)More severe in high HP pumps (> 100 HP)More severe with higher speeds (2 pole)
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Pump Vibration
Natural FrequencyShafts
The lateral natural frequency of most shafts is above operating speed (2 pole)Shaft torsional natural frequencies can be a problem, particularly on long vertical drives
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Vibration
Natural FrequencyPump Structure
Horizontal pumps rarely have natural frequencies in the operating rangeVertical pumps often have structural natural frequencies in the operating rangeParticularly a problem when equipped with variable speed drives
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Questions?Questions?