LOW FLOW / HIGH HEAD Centrifugal Pump Options Calgary Pump Symposium November 18,2005 Presented by Frank Korkowski & Larry Glassburn
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 LOW FLOW / HIGH HEAD PUMPS AND LEARN HOW TO USE THE FORCE TO . . . . “CHOOSE WISELY”
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 LOW FLOW / HIGH HEAD PUMPS AND LEARN HOW TO USE THE FORCE TO . . . . “CHOOSE WISELY”
8000 RPM 3600 RPM
25,000 RPM
Presentation Why Why do do we we nee need d “lo “low w flo flow, w,hi high gh he head ad”” LF / HH pumps? • Ce Cent ntri rifu fuga gall Pum Pump p Fu Fund ndam amen enta tals ls • Ty Type pes s of of low low fl flow ow / hig high h hea head d pum pumps ps • Co Cons nsid ider erat ation ions s and and fa fact ctor ors s tha thatt imp impac actt your pump selection choices • Summary
. Why & How we got to LF/HH Pumps • In the the beg begin inni ning ng,, proc proces esse ses s were were pre prett tty y basi basic, c, low low pressure…no LF/HH demand, 1 and 2-stage OH pumps were sufficient • 19 1930 30’s ’s Mo More re ef effi fici cien entt and and im impr prov oved ed ch chem emic ical al reactions required higher process pressures and temperatures…LF/HH temperatures…L F/HH demand is born, multistage pumps and PD pumps were employed • 1960’s Partia iall Emissio ion n (Barske Barske)) pumps with a Gearbox for higher impeller speeds to 25,000 rpm and Pitot Tube pumps were introduced. • 1980’s Various types of Barske wheels and increased number of stages were introduced • 2000’s (so far) VFD’s to increase impeller rotational speed….. speed…..
Classification of Pumps H y d r au l i c In s t i t u t e
A P I-610
O v er h u n g Im p el l er
1- an d 2- s t ag e
B et w een B ear i n g s
1- an d 2- s t ag e
OH1; OH2 OH3 OH4 OH5 OH6
Rotodynamic (Centrifugal)
M u l t i s t ag e V er t i c al l y Supended
BB1 BB2 BB3 BB4 BB5
Kinetic R eg en er at i v e Tu r b i n e
O v er h u n g B et w een B e ar i n g s
S p ec i al E f f ec t
P er i p h er al S id e C h an n el
R o t at i n g C as i n g (p it o t )
Pumps Reciprocating Po sitiv e Displacment
Direct Acting
Simp lex, Dup lex, etc
Power Frame
C o n t r o ll ed V o lu m e D i a p h r a g m
API-674 API-675
Gear
Rotary
Screw Vane Lobe
API-676
Reciprocating Pumps
2. Centrifugal Pump Fundamentals • Head vs. Tip Speed, Torque vs. HP • Affinity Laws • Specific Speed—ns • Suction Specific Speed—S • NPSH
Head vs. Tip Speed … Torque vs. Hp Theoretical head is defined by the formula: 2
HT = U g
Where: HT = Theoretical Head (Ft.) U = Impeller Tip Speed (ft./sec.) g = Acceleration = 32.3 ft./sec2.
Tip speed (U) is found by the formula: U =
DN 229
Where:
D = Impeller Diameter (in.) N = Speed (RPM)
Constant Speed
Q2 /Q1 = D2 /D1 D=Impeller Diameter
H2 /H1 = (D2 /D1)2 BHP2 /BHP1 = (D2 /D1)3
Constant Impeller Diameter
Q2 /Q1 = N2 /N1
Q = capacity N = speed
H2 /H1 = (N2 /N1)2
H = head BHP=brake horse power
BHP2 /BHP1 = (N2 /N1)3
Specific Speed n s
ns
=
N x
Where: Q = N = H =
H
Q 3/4
Flow (gpm) @ BEP Speed (RPM) Head (per stage)
Impeller Design vs. Specific Speed
D2/D1 > 2 (D2 = Vane O.D.
D2/D1 = 1.5-2.
D1 = Vane I.D.)
D2/D1 < 1.5
D2/D1 = 1.
Pump Specific Speed
ns
• Dimensionless number based on the affinity laws used to select the impeller geometry for maximum efficiency. • ns between 200 and 1000 = Barske • ns between 1000 and 5000 = Francis vane • ns between 5000 and 15000 = axial flow pump (inducer)
ns vs. Impeller Design, Curve Shape
Curve Characteristics vs. ns
600 ns Typical Partial Emission Open/Semi Open Impellers
1000 to 1500 n s Typical Multistage Closed Impellers
Curve Characteristics vs. ns
4000 ns Typical Mixed Flow
10,000 ns Typical Axial Flow Machine
Efficiency vs. ns by Pump Types
Suction Specific Speed S
S
=
N
×
Q 3/4
( NPSH R ) Where:
Q = Flow (gpm) @ BEP “per eye” N = Speed (RPM)
NPSHR = Net Positive Suction Head “required” by the pump
NPSH • Net Positive Suction Head is the amount of energy available in the fluid at the pump suction flange. • Successful pump operation results when the suction fluid has sufficient energy to push liquid into the pump faster than the pump can pump it away.
What Does NPSH Have To Do With Pump Maintenance? • Inducer, impeller, cover , diffuser and pump case can suffer metal loss via “cavitation” • Prevent Cavitation by: Increase NPSHA
(of the system) Decrease NPSHR (of the pump)
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 Elevation from the Liquid Level to Pump Centerline) + VELOCITY HEAD (Small, Often Negligible) • Consistent Units of Feet or Meters • Pump suction gage improves accuracy
How To Increase NPSHA • Increase Suction Vessel Pressure • Decrease Vapor Pressure (Decrease Suction Temperature) • Decrease Line Losses • Increase Static Head
How To Decrease NPSHR • Add Inducers Typical S = 18,000 to 23,000
• Operate unit near BEP • Select lower speed unit
What Does NPSH Have To Do With Pump Maintenance? • Inducer, impeller, cover , diffuser and pump case can suffer metal loss via “cavitation”
• Resulting high speed rotor unbalance can lead to higher vibration and possible bearing failure. • Seal problems
3. Types of Low Flow/ High Head Pumps • Hydraulic Envelope • Sectional Views • Methods for Handling Axial Thrust • Ways to Handle Radial Loads
Hydraulic Envelope Low 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
Single Stage 3600 RPM
Two stage 3600 RPM Pitot
Single Stage Medium Speed Gearbox
Single Stage Medium Speed VFD
Pitot 3600-5000 RPM
Multi-stage Between Bearings 3600 RPM
Single Stage Integral High Speed Gearbox
Multi-stage Barske 3600 RPM
Two Stage Barske Integral High Speed Gearbox
Three Stage Barske Integral High Speed Gearbox
Low Flow, High Head Pumps A
Barske – single stage @ 3600 rpm
B
Pitot
@ 3600 – 5000 RPM or Barske – two stage @ 3600 RPM
C
Barske – Single stage or two stage • Gearbox @ 6000 – 17000 RPM
• VFD
@ 8000 RPM
D
Pitot
E
Multi-stage barrel @ 3600 RPM
F
Barske – single stage Gearbox @ 9000 – 25,000 RPM
G
@ 3600 – 5000 RPM
Barske– multi-stage (barrel) @ 3600 RPM
H
Barske – two stage Gearbox @ 9000 – 25,000 RPM
I
Barske – Three-Stage Gearbox @ 9000 –25,000 RPM
Single-stage Barske
Two-stage Barske
Multi-stage Horizontal and Vertical Options
Gearbox Driven Multi-stage Pump with Barske Impellers
Pitot Pump Sectional
Pitot Principles of Operation Centrifugal Rotor Cover Generates 50% of Head
Pick-Up Tube Generates 50% of Head Discharge
Rotor Assembly
Mechanical Seal on Suction Side
Suction
Methods for Handling Axial Thrust Pump-out vanes • Balance holes • Wear rings • Balance Drum • Back-to-back Impellers • Special Bearing Arrangements
Multi-radial-blade Impellers
Blades
Shroud
Hydraulic Balance Holes
Closed Impeller
LABYRINTH STEPS (wear ring)
Methods for Handling Radial Loads • Volute Designs (circular vs. constant velocity) • Diffusers
Radial Load Trends
Principles of Developing Head in Diffuser Pumps CONICAL DIFFUSER
IMPELLER PUMP CASING
A
B
C DIFFUSER THROAT
Y T I C O L E V
E R U S S E R P
A
B
C
A
B
C
KMC Bearings Flexure Pivot™ Radial and Thrust
4. Considerations & Factors for Your Pump Selection Choices • • • •
Footprint NPSH Pump / System Interaction Life Cycle Cost Efficiency Operational Flexibility MTBPM Maintenance Practices Equipment Desirability Service Support Personal Preference Price & Delivery ISO 13709 (API 610) Compliance
• Equipment Field Experience
ISO 13709 (API 610) Compliance • • • • • • •
Classifications OH3, OH4, OH5, OH6 Pressure Containment Temperature Limits Mechanical Seal Designs Hydrocarbon Applications Handling Solids Pump Bearing Housing
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
