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8/13/2019 Low Flow High Head
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8/13/2019 Low Flow High Head
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A LONG
TIME AGO IN
A REFINERY FAR
FAR AWAY. . . A GALACTIC
WAR RAGES BETWEEN PUMP
MANUFACTURERS AND THE EVIL
EMPIRE OF PROCESS ENGINEERS.
FROM THIS PRESENTATION YOU WILL LEARN
THE SECRETS OF THE “ DARK SIDE ” OF LOWFLOW / HIGH HEAD PUMPS AND LEARN HOW
TO USE THE FORCE TO . . . . “CHOOSE WISELY”
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25,000
RPM
8000
RPM
3600
RPM
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Presentation
Why do we need “low flow,high head”
LF / HH pumps?• Centrifugal Pump Fundamentals
• Types of low flow / high head pumps
• Considerations and factors that impactyour pump selection choices
• Summary
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Classification of PumpsHy d rau l ic In s t i tu te A PI-610
OH1; OH2
OH3
Ov erh u n g 1- an d 2- s tag e OH4
Im p el ler OH5
OH6
R o t o d y n a m i c
(Cent r i fuga l ) B etw een 1- an d 2- s tag e B B 1
B ear in g s B B 2
Mu l t is tag e B B 3
B B 4
Ver t ic al ly B B 5
Supended
Kine t icReg en erat iv e Ov erh u n g Per ip h eral
Tu rb in e B etw een S id e Ch an n el
B ear ings
Sp ec ial E f fec t Ro tat in g Cas in g (p ito t )
P u m p s
Di rec t Ac t i ng S imp lex, Dup lex, e tc
Rec ip roca t i ng Power F rame API-674
Po si t i v e Co ntro lled Vo lu m e D iaphragm API-675
Disp lacmen t
Gear
Rotary Screw API-676
Vane
L o b e
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Reciprocating Pumps
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2. Centrifugal Pump Fundamentals
• Head vs. Tip Speed, Torque vs. HP• Affinity Laws
• Specific Speed—ns
• Suction Specific Speed—S
• NPSH
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Head vs. Tip Speed … Torque vs. HpTheoretical head is defined by the formula:
Where: HT = Theoretical Head (Ft.)
U = Impeller Tip Speed (ft./sec.)
g = Acceleration = 32.3 ft./sec2.
HT = U2
g
Tip speed (U) is found by the formula:
Where: D = Impeller Diameter (in.)
N = Speed (RPM)U =229
DN
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Constant
Speed
Q2 /Q
1= D
2 /D
1D=Impeller
Diameter
H2 /H1 = (D2 /D1)2
BHP2 /BHP1 = (D2 /D1)3
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Constant
ImpellerDiameter Q
2 /Q
1= N
2 /N
1 Q = capacityN = speed
H2 /H1 = (N2 /N1)2 H = head
BHP=brake
horse power
BHP2 /BHP1 = (N2 /N1)3
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Specific Speed ns
Where: Q = Flow (gpm) @ BEPN = Speed (RPM)
H = Head (per stage)
3/4s
H
Q Nn
x=
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Impeller Design vs. Specific Speed
D2/D1 > 2 D2/D1 = 1.5-2. D2/D1 < 1.5 D2/D1 = 1.
(D2 = Vane O.D. D1 = Vane I.D.)
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Pump Specific Speed ns
• Dimensionless number based on the affinity
laws used to select the impeller geometry formaximum efficiency.
• ns between 200 and 1000 = Barske
• ns between 1000 and 5000 = Francis vane
• ns between 5000 and 15000 = axial flow
pump (inducer)
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ns vs. Impeller Design, Curve Shape
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Curve Characteristics vs. ns
10,000 nsTypical
Axial Flow Machine
4000 nsTypical Mixed Flow
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Efficiency vs. ns by Pump Types
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Suction Specific Speed S
Where: Q = Flow (gpm) @ BEP“per eye”
N = Speed (RPM)
NPSHR = Net Positive Suction Head“required” by the pump
( ) 4/3R NPSHQ NS ×
=
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NPSH
• Net Positive Suction Head is the
amount of energy available in the fluidat the pump suction flange.
• Successful pump operation resultswhen the suction fluid has sufficientenergy to push liquid into the pumpfaster than the pump can pump it away.
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What Does NPSH Have To Do WithPump Maintenance?
• Inducer, impeller, cover , diffuser and pumpcase can suffer metal loss via “cavitation”
• Prevent Cavitation
by:
Increase NPSHA
(of the system)
Decrease NPSHR
(of the pump)
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NPSHA
• NPSHA = ABSOLUTE PRESSURE HEAD(Barometric Pressure +/- Fluid Vapor
Pressure converted to head)- VAPOR PRESSURE HEAD (Temp. at
Suction Flange)
- LINE LOSSES (Frictional Loss)
+/- STATIC HEAD (Difference in Elevationfrom the Liquid Level to Pump Centerline)
+ VELOCITY HEAD (Small, Often Negligible)
• Consistent Units of Feet or Meters
• Pump suction gage improves accuracy
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How To Increase NPSHA
• Increase Suction Vessel Pressure
• Decrease Vapor Pressure
(Decrease Suction Temperature)• Decrease Line Losses
• Increase Static Head
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How To Decrease NPSHR
• Add Inducers
• Operate unit near BEP
• Select lower speed unit
Typical S =18,000 to
23,000
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3. Types of Low Flow/ High Head Pumps
• Hydraulic Envelope• Sectional Views
• Methods for Handling Axial Thrust
• Ways to Handle Radial Loads
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Hydraulic EnvelopeLow Flow/High Head
• Product options are available
• Over-lap of product performance envelopes does
occur—you do have choices
• Understand your system requirements
• Gather details and discuss with the pump supplier
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Single Stage
3600 RPM
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Two stage3600 RPM
Pitot
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Single Stage
Medium Speed
Gearbox
Single Stage
Medium SpeedVFD
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Pitot
3600-5000 RPM
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Multi-stage
Between Bearings3600 RPM
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Single Stage
Integral High Speed
Gearbox
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Multi-stage Barske
3600 RPM
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Two Stage Barske
Integral High SpeedGearbox
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Three Stage BarskeIntegral High Speed
Gearbox
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Barske – single stage @ 3600 rpm
Pitot @ 3600 – 5000 RPM
or
Barske – two stage @ 3600 RPM
Barske – Single stage or two stage
• Gearbox @ 6000 – 17000 RPM
• VFD @ 8000 RPM
Pitot @ 3600 – 5000 RPM
Multi-stage barrel @ 3600 RPM
Barske – single stage
Gearbox @ 9000 – 25,000 RPM
Barske– multi-stage (barrel) @ 3600 RPM
Barske – two stage
Gearbox @ 9000 – 25,000 RPM
Barske – Three-Stage
Gearbox @ 9000 –25,000 RPM
A
C
H
I
G
F
E
D
B
Low Flow, High Head Pumps
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Two-stage Barske
M l i H i l d
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Multi-stage Horizontal and
Vertical Options
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Gearbox Driven Multi-stagePump with Barske Impellers
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Pitot Pump Sectional
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Pitot Principles of Operation
Pick-Up Tube
Generates 50% of Head
Rotor Assembly
Mechanical Seal on Suction Side Suction
Discharge
Centrifugal Rotor Cover
Generates 50% of Head
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Methods for Handling Axial Thrust
Pump-out vanes• Balance holes
• Wear rings
• Balance Drum
• Back-to-back Impellers
• Special Bearing Arrangements
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Multi-radial-blade Impellers
Shroud
HydraulicBalance Holes
Blades
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Closed Impeller
LABYRINTH STEPS (wear ring)
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Methods for Handling Radial Loads
• Volute Designs (circular vs. constantvelocity)
• Diffusers
Radial Load Trends
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Radial Load Trends
Principles of Developing Head in Diffuser Pumps
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A B C
P R E S S U R E
A B C
V E L O C I T Y
PUMP CASING
IMPELLER
DIFFUSERTHROAT
CONICALDIFFUSER
A BC
Principles of Developing Head in Diffuser Pumps
KMC B i
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KMC Bearings
Flexure Pivot™ Radial and Thrust
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4. Considerations & Factorsfor Your Pump Selection Choices
• Footprint
• NPSH
• Pump / System Interaction
• Life Cycle CostEfficiency Operational Flexibility MTBPM
Maintenance Practices Equipment Desirability
Service Support Personal Preference
Price & Delivery ISO 13709 (API 610) Compliance
• Equipment Field Experience
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ISO 13709 (API 610) Compliance
• Classifications OH3, OH4, OH5, OH6
• Pressure Containment
• Temperature Limits
• Mechanical Seal Designs
• Hydrocarbon Applications
• Handling Solids
• Pump Bearing Housing
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5. Summary• Do your homework to understand the
process system requirements• Select a pump type to fit your system for
normal and any upset conditions
• Evaluate the true benefit of any pump’s“special features”
• Evaluate Life Cycle Cost
Bottom Line… Choose Wisely
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Questions and Answers