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Centrifugal and Submersible Pumps

Date post: 10-Feb-2016
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Centrifugal and Submersible Pumps. Don Davis, CIC. Pump Applications. Centrifugal Pumps Booster applications Open water applications (25’ maximum suction lift) Shallow well applications Applications where electrical lines can’t be installed in open water. Submersible Pumps - PowerPoint PPT Presentation
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Centrifugal and Submersible PumpsDon Davis, CICPump ApplicationsCentrifugal PumpsBooster applicationsOpen water applications (25 maximum suction lift)Shallow well applicationsApplications where electrical lines cant be installed in open water

Submersible PumpsDeep well applicationsOpen water applications with suction lifts above 25Open water applications with excessive elevation requiring higher output pressureApplications where a visible pump is undesirable

Centrifugal Pump OperationHow does a pump.pump?

An airtight intake creates a vacuum during impeller rotation.14.7 psi of atmospheric pressure exists at sea level. This is the pressure pushing water into the impeller.(Atmospheric pressure decreases 1 psi for each 2000 feet increase in elevation)The spinning impeller creates inertia, increasing pressure and discharging the water.

Whats Inside?

Internal ComponentsElectrical motorVoltage, phase varies by applicationImpellerPlastic, cast iron, brass material optionsRotating impeller pushes water against pump casing or volute and increases pressure. Add more impellers to increase pressure and create a multi-stage pump (that is how a horsepower pump in a 1000 foot deep well can supply water)Pump TerminologyHorsepowerPower required to lift 33,000 pounds or 3750 gallons of water 1 foot in one minuteFeet of HeadA 1 high column of water contains the potential energy of 1 of headThe 1 high column of water will have a pressure of 0.433 PSI at the base

Feet of head divided by 0.433 = PSIPSI x 2.31 = feet of head ExampleWhat is the pressure required to pump water to the top of a 25 column?2525 / 2.31 = 10.8 PSI0r25 x .433 = 10.8 PSI

How much pressure do we have at the top of the column?Maximum Suction LiftHow does this affect pump applications?Convert 14.7 PSI at sea level to feet of head:14.7 PSI x 2.31 = 33.9 feet of headInsufficient atmospheric pressure to push water into the impeller above 33.9

1 PSI loss for each 2000 increase in elevation4000 elevation would equal 12.7 PSI atmospheric pressure12.7 PSI x 2.31 = 29.3 feet of head

Rule of thumb: DO NOT exceed 25 suction lift

Pump Curve DataCurve provides performance data for a specific pumpCurve notes GPM the pump provides at a specific feet of headSelection of pump should be ABOVE your specific design point on the curveSelection of a pump in the center of the curve is idealSystem design criteria is critical for pump selection

Pump Curve - Centrifugal

Pump Curve - Submersible

Calculating Feet of Head

Vertical elevations: measure feetHorizontal distances: measure friction loss PSI, convert to feetFriction Loss TablesFriction loss tables provide the PSI loss per 100 feet of pipe at a given flow.

Larger diameter pipe results in lower PSI loss at the same flow.

Calculating Feet of HeadAssume system requirements are 12 GPM at 50 PSI:

Suction lift (assume submersible for this example)0Elevation change from pump to highest point on site17Mainline friction loss (500 of 1 SCH 40, 12 GPM)38.83.36 PSI loss/100 feet * 500 feet mainline = 16.8 PSI16.8 PSI * 2.31 = 38.8Desired operating pressure of 50 PSI converted to feet:115.550 PSI * 2.31 = 115.5

Total Feed of Head171.3Will need to use the 1 hp submersible from the pump curvePump SelectionReducing feet of head requirements may allow selection of smaller pump. Take the previous example:

Suction lift (assume submersible for this example) 0Elevation change from pump to highest point on site17Mainline friction loss (500 of 1 SCH 40, 12 GPM)10.20.89 PSI loss/100 feet * 500 feet mainline = 4.45 PSI4.45 PSI * 2.31 = 10.2Desired operating pressure of 45 PSI converted to feet:103.945 PSI * 2.31 = 115.5

New total feet of head:131.1Can use the hp submersible from the pump curve

Proper Plumbing - SuctionMost pumps fail due to improper plumbing on the suction side.Minimize fittings and bendsSize the suction line 1-2 pipe sizes larger than the inlet thread size.Make it as short as possible.Use a straight, level length of pipe into the suction. (length = 5-10 times the pipe diameter)Foot valve/strainer must be in clean water.

Proper Plumbing - DischargeDischarge plumbing tips:Use galvanized pipe/fittingsInstall an isolation valve to aid in primingPressure relief valve/priming port should directly above dischargeInstall a union for maintenance purposesAdd filtration to all non-potable water sourcesInstall a pressure gaugeInstall a high temperature sensor, low pressure sensor

Avoid using PVC for direct connections to centrifugal pumps. Heat generatedduring operation or no-flow situations will cause problems!

Proper installation using galvanized fittings.Submersible Pump InstallationPump sled to include an inlet strainer and outlet well sealInstall union in discharge line near shore for maintenance purposesInclude safety line for retrievalInstall check valve in suction line

Submersible Pump InstallationPump sled can be constructed from PVC pipe and fittings.Use galvanized (or stainless steel) fittings between the pump and discharge pipe. Pump start-up torque WILL unscrew the pump from PVC fittings!Well seal prevents torque spinSecure wiring to discharge pipe. Leave excess wire at shore line for maintenance purposes.Install isolation valve upstream of pump for troubleshooting purposes.Pump Sled or SleeveA casing is mandatory for a submersible pump! The water intake is located above the motor. Placing the pump in a casing forces all of the intake water to pass over the motor for cooling purposes.A pump left in open water WILL overheat. Cistern or dock applications: install pump inside a sleeve. On-Demand Pump SystemsUse in situations where a continuously pressurized mainline is not desiredDoes not require pressure tank installationPump activates only when irrigation controller signals operation

On-Demand Pump ControlsPump Start Relay2 wire pumps can use a standard PSR3 wire pumps require a control box with start capacitorRefer to manufacturers cable sizing charts to determine wire gauge requirements

Pressurized Pump SystemsUse in situations where a PSR is not feasibleMultiple controllers using same pumpQuick couplers or hydrants desired on siteController and pump are not in close proximity

Special requirements:Shelter large enough to accommodate pressure tank(s)Drain to exterior for PRV

Pressurized Pump Controls

Size bladder tank at minimum one gallon drawdown for each GPM of pump capacity. Multiple tanks can be installed in series for higher GPM requirements. Set tank pressure at 2 PSI below pump cut-in pressure.Tank tee allows for pressure switch, pressure gauge, pressure relief valve, drain valve, and check valve installation in a compact location.

Varied GPM RequirementsCycle Stop ValveRestricts pump output to match GPM demand. As demand decreases, the Cycle Stop Valve increases back pressure on the motor. Increased back pressure decreases the gallon requirement. This decrease in gallon requirement reduces the load on the motor, resulting in reduced amperage draw and therefore power consumption.Pressure downstream remains constant within the allowable flow rates for the particular unit.Byproduct of Cycle Stop Valve operation is the elimination of water hammer.

Varied GPM RequirementsVariable Frequency Drive Motor (VFD)Varies the frequency and voltage supplied to an electric motor. As frequency (or hertz) increases, motor RPM increases.While a standard motor will operate at full RPM regardless of GPM demand, a VFD has potential for energy savings when operating at a lower frequency during lower GPM demand.3 phase motor requiredCavitationFormation of air bubbles in a liquid that occurs when the pressure falls below the vapor pressure.The vapor will turn back to a liquid and explode, causing damage to the components.Preventing CavitationIncrease net positive suction head (NPSH) available by:Increase the diameter of suction lineMinimize fittings in suction lineReduce flow rate through pumpReduce suction lift elevationReduce suction line distanceCreate artificial pressure on the discharge by installing smaller diameter discharge pipe or throttling valveCavitation DamageBrass Impeller

Troubleshooting All Motors MOTOR STARTS TOO OFTENCheck setting on pressure switch. Reset limit or replace switch.Damaged or defective check valve will not hold pressure.Check for waterlogged pressure tank. Change air charge or replace tank.Examine system for leaks and repair as necessary.MOTOR RUNS CONTINUOUSLYCheck pressure switch for welded contacts; adjust settings as necessaryPump intake blockedCheck valve stuck closedLow water level or loss of primeLeak in dischargeWorn pump: symptoms similar to low water level or drop pipe leak; reduce pressure switch setting and pump will shut off indicates warn partsPUMP DELIVERS LITTLE OR NO WATERLow line voltage to motorIncomplete priming of pumpAir lock in suction lineDrop pipe has disconnected from pumpLow water levelClogged or defective foot valve / strainerWorn pump parts or plugged impellerCircuit Breaker, Fuse, Overload TripsCheck for correct line voltage.Overheated control or starter may require ventilation.Defective control box.Defective motor or cable.Worn pump or motor.Insulation Resistance TestOhm reading < 500,000 indicates insulation damage.With power off and motor leads disconnected, test resistance between any one of the motor leads and equipment ground. A normal ohm value for all leads indicates the motor is not grounded and the cable insulation is not damaged.New motor (without drop cable): 20,000,000 + ohmsExisting motor (without drop cable): 10,000,000 + ohmsNew motor in well: 2,000,000 + ohmsExisting motor in well: 500,000 2,000,000 ohms

Winding Resistance TestRefer to manufacturers charts for ohm values.2 wire motors: measure resistance from line to line3 wire motors: measure resistance Y to B (main winding) and Y to R (start winding)If all ohm values are normal, motor is not grounded and cable insulation is not damagedIf any one value is < normal, the motor is shortedIf any one value is > normal, the winding or cable is open / bad spliceControl Box Schematic

Control Box-Ohm TestsPower OFF for ohm tests:Overloads should ohm less than 0.5Capacitor should ohm near 15,000Relay coil should ohm 4500-7000Relay contact should zero ohmStart and run capacitors should ohm near zero and then move toward infinity

Control Box-Amperage TestsMotor under load for amperage tests:Red lead current should start high and then fall to manufacturers chart reading. Black and yellow lead current should not exceed chart reading.Relay or switch failure: Constant high red lead current and overload trippingOpen run capacitor: lower than normal red lead amps, and higher than normal yellow and black lead ampsFailed start capacitor or open switch / relay: red lead current is not momentarily high at starting

ReviewWater source typeMinimum and maximum flow, GPMDesired pressure at sprinklersVertical elevationwater line to pumpVertical elevationpump to highest pointMainline (size, type, length)Suction line (size, type, length)Well depth, yield, water level, pump set depthWell pump HP, GPM