Fundamentals of Centrifugal Pump notess
- 1. FUNDAMENTALS of CENTRIFUGAL PUMPS JIM VUKICH APPLICATION ENGINEER XYLEM – FLYGT PRODUCTS MALVERN, PA
- 2. TOPICS •DEFINITION •COMPONENTS •PUMP CURVES •THE PIPE SYSTEM •NPSH •VFD OPERATION
- 3. DEFINITION A CENTRIFUGAL PUMP IS A ROTODYNAMIC MACHINE THAT CONVERTS ROTARY MOTION INTO PRESSURE ROTO = SPIN DYNAMIC = CHANGE
- 4. Why Use a Pump? Water does not flow uphill Pumps are used to lift water from a lower elevation to a higher elevation Pumps are also used to generate flow and/or pressure •in a jet aeration header •to pump downhill
- 5. Why Use a Centrifugal Pump? Available in a wide range of sizes – discharges from 1” to 12ft and bigger Handle a wide range of head and flow conditions Limitations: •cannot handle entrained air – 3% max •cannot handle viscous liquids
- 6. PUMP TYPES Centrifugal pumps come in a wide variety of styles end suction split case in-line double suction vertical multistage horizontal multistage submersible self-priming axial-flow regenerative
- 7. Split case Suction and discharge on opposite sides Double-hung impeller Shaft is perpendicular to flow Casing is split into top and bottom halves Top half is lifted off for service
- 8. End suction Suction is inline with the shaft Overhung impeller mounted on end of shaft Can be close-coupled, or Frame-mounted – pump and motor are separate units on common base plate
- 9. End suction
- 10. A submersible pump is a close-coupled end suction pump with an integral electric motor Submersible
- 11. Vertical turbine Long-shaft pump The bowl is down in the water The discharge head is mounted above Line shaft runs up the discharge column Add multiple bowls in series for high head
- 13. The driver provides the power to do the work Can be coupled to, or integral with, the pump shaft Must be carefully aligned and supported DRIVER ELECTRIC MOTOR DIESEL ENGINE HYDRAULIC STEAM
- 14. DRIVER The amount of power needed to move the liquid depends only on the wet end In theory, you can mate any size wet end with any size driver BUT USING AN UNDERSIZED DRIVER WILL OVERLOAD THE MOTOR!
- 15. Horsepower Power is a measure of work per time Work = Force x Distance Power = Work ÷ Time One horsepower is the amount required to raise 33,000 lbs up 1 foot in 1 minute
- 18. WET END Also called the “liquid end” The part of the pump that contains the pumped liquid
- 19. IMPELLER Draws liquid into the wet end Imparts velocity to the liquid Throws liquid against the inside of the volute
- 20. VOLUTE Captures the liquid exiting the impeller Converts kinetic energy (velocity) into potential energy (pressure) Directs liquid into the discharge pipe
- 21. VOLUTE Also called the casing or pump housing Volute = spiral Converts the motion of spinning liquid into pressure Forces liquid into the piping at high pressure
- 22. WEAR RINGS Form a running seal between the suction (low pressure) and discharge (high pressure) Increase efficiency by preventing recirculation Can be easily replaced to maintain close running clearance
- 23. Seals Packing Lip seal Mechanical seal Dynamic seal Purpose: seals off the opening where shaft enters volute
- 24. Seals Lubricated with water Use external flush water for pumping grit and rags Seals are designed to leak, at a slow rate
- 25. IMPELLER TYPES OPEN & CLOSED CHANNEL NON-CLOG GRINDER VORTEX CHOPPER PROPELLER
- 27. IMPELLER TYPES NON-CLOG •High efficiency •Smooth passageways •No abrupt turns •Clog resistant
- 28. IMPELLER TYPES GRINDER •Like a garbage disposal •Cutting ring, wheel, etc. •Low efficiency •Used with small piping
- 29. IMPELLER TYPES VORTEX •Also called recessed, swirl, or torque flow •Creates a “tornado” effect to suck up liquid •Liquid does not pass through impeller – good for abrasive liquids
- 30. IMPELLER TYPES CHOPPER •Screw shape •Cutting plate •Low efficiency
- 31. IMPELLER TYPES PROPELLER •Pure axial flow •High flow, low head •Sensitive to inlet flow
- 32. PUMP CURVES A centrifugal pump can deliver a wide range of flows. As flow increases, the head decreases. Pump efficiency varies across the flow range. We plot head vs. flow to get the pump curve, or performance curve.
- 33. PUMP CURVES
- 34. PUMP CURVES WE TALK ABOUT FLOW AND HEAD FLOW = GPM The rate of liquid (volume per time) passing through the pump. Mgd, cfs, m3/hr, etc. HEAD = FEET The amount of energy added to the liquid by the pump.
- 35. PUMP CURVES WE TALK ABOUT HEAD IN FEET OF LIQUID IF A PUMP RUNS AT 20 FEET OF HEAD, IT WILL SUPPORT A COLUMN OF LIQUID 20 FEET HIGH
- 36. PUMP HEAD The term “head” likely derives from the elevation difference available to power a waterwheel
- 37. Parallel and Series Pumping Parallel: Pumps operate side-by-side Provides more flow Series: Pumps operate in-line Provides more head
- 38. PUMP CURVES WHERE WILL THE PUMP OPERATE? WHERE IT REACHES A BALANCE WITH THE PIPING SYSTEM
- 39. THE PIPE SYSTEM THE PIPE SYSTEM CREATES RESISTANCE TO FLOW THE AMOUNT OF RESISTANCE IS BASED ON STATIC AND DYNAMIC COMPONENTS
- 40. THE PIPE SYSTEM HOW HIGH? The elevation difference is the static head – it is independent of the flow rate HOW FAR? Longer pipe runs create more resistance THROUGH WHAT? There is friction as the liquid runs through the pipe. Friction depends on the pipe size and roughness.
- 41. Pipe Diameter & Roughness
- 42. EFFECT OF PIPE LENGTH
- 43. EFFECT OF FLOW RATE OR ROUGHNESS
- 44. EFFECT OF PIPE DIAMETER
- 45. THE SYSTEM CURVE WE CAN SHOW THE RESISTANCE OF THE PIPE SYSTEM AS A SYSTEM CURVE System Head - 500 ft. of Ductile Iron Pipe - 20 Feet of Static Head 0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 Flow - Gpm System Head - Ft.
- 46. THE PUMP RUNS WHERE IT MEETS THE SYSTEM CURVE
- 47. System Head Curves A system head curve is valid for only one condition in the pipe system Changing the system (partially closed valve) will change the shape of the system head curve Changing water surface elevation will change the static head and shift the system curve up or down
- 49. VERIFYING THE OPERATING POINT Using Pressure Gauges
- 51. BEST EFFICIENCY POINT •The “sweet spot” •Operation at BEP results in lowest operating cost and longest service life •BUT PUMP SELECTION ALWAYS INVOLVES TRADE OFFS •SOLIDS HANDLING SIZE •SPEED •COST
- 52. OFF-PEAK OPERATION •Reduced efficiency •High bending forces - wear on seals, bearings •Vibration •Clogging •Temperature rise •Cavitation •Motor overload
- 53. POWER CURVE
- 54. NPSH CURVE NPSH = NOT PUMPING SO HOT? NPSH = NET POSITIVE SUCTION HEAD IT’S THE POSITIVE PRESSURE REQUIRED AT THE PUMP INLET NEED TO COMPARE AVAILABLE TO REQUIRED NPSH
- 55. NPSH AVAILABLE
- 56. WHAT HURTS THE NPSH? •INSUFFICIENT SUBMERGENCE •AIR ENTRAINMENT •SUCTION PIPING (DRY-PIT PUMPS) •HOT LIQUID NPSH AVAILABLE
- 57. NPSH CURVE
- 58. Cavitation Cavitation is the formation of vapor bubbles in any flow that is subjected to an ambient pressure equal to or less than the vapor pressure of the liquid being pumped. Cavitation damage is the loss of material produced by the collapse of the vapor bubbles against the surfaces of the impeller or casing. Cavitation may be present in combination with erosion and corrosion – especially in wastewater
- 59. Cavitation - Causes 1 – Insufficient NPSH available Occurs on the low-pressure, or visible, surface of the impeller vane 2 – Recirculation – partial reversal of flow through the impeller Occurs on the high-pressure, or invisible, surface of the impeller vane
- 62. Cavitation, Corrosion, and/or Erosion?
- 63. CHANGING THE FLOW AND HEAD •BIGGER OR SMALLER IMPELLER •CHANGE SPEED WITH A VFD
- 64. BIGGER OR SMALLER IMPELLER Caution! A bigger impeller might overload the motor
- 65. CHANGE SPEED WITH A VFD VFD = Variable Frequency Drive
- 66. Preferred Operating Region (POR): 70% to 120% of BEP flow (per HI) Allowable Operating Region (AOR): 50% to 125% of BEP flow (per mfr.) CHANGE SPEED WITH A VFD
- 67. CHANGE SPEED WITH A VFD CAUTION! When you turn down the VFD, the pump may run at a different spot on its curve. You might be doing more than simply changing the flow rate!
- 68. CHANGE SPEED WITH A VFD CAUTION! RUNNING THE PUMP TOO SLOW FOR TOO LONG CAN CAUSE PROBLEMS •CLOGGING •VIBRATION •HIGH BENDING FORCES
- 69. ? QUESTIONS ?
- 70. THE END!