Centrifugal and Submersible Pumps

Report
Don Davis, CIC
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
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Submersible Pumps
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Deep well applications
Open water applications with suction lifts above
25’
Open water applications with excessive elevation
requiring higher output pressure
Applications where a visible pump is undesirable
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How does a pump….pump?
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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)
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The spinning impeller creates inertia, increasing
pressure and discharging the water.
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Electrical motor
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Voltage, phase varies by application
Impeller
Plastic, cast iron, brass material options
 Rotating 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 ¾ horsepower
pump in a 1000 foot deep well can supply water)
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Horsepower
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Power required to lift 33,000 pounds or 3750 gallons
of water 1 foot in one minute
Feet of Head
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A 1’ high column of water contains the potential
energy of 1’ of head
The 1’ high column of water will have a pressure of
0.433 PSI at the base
Feet of head divided by 0.433 = PSI
PSI x 2.31 = feet of head
What is the pressure required to pump water
to the top of a 25’ column?
25’ / 2.31 = 10.8 PSI
25’
0r
25’ x .433 = 10.8 PSI
How much pressure do we have at the top
of the column?
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How does this affect pump applications?
Convert 14.7 PSI at sea level to feet of head:
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1 PSI loss for each 2000’ increase in elevation
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14.7 PSI x 2.31 = 33.9 feet of head
Insufficient atmospheric pressure to push water into the
impeller above 33.9’
4000’ elevation would equal 12.7 PSI atmospheric pressure
12.7 PSI x 2.31 = 29.3 feet of head
Rule of thumb: DO NOT exceed 25’ suction lift
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Curve provides performance data for a specific
pump
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Curve notes GPM the pump provides at a specific
feet of head
Selection of pump should be ABOVE your specific
design point on the curve
Selection of a pump in the center of the curve is ideal
System design criteria is critical for pump selection
Vertical elevations: measure feet
Horizontal distances: measure friction loss
PSI, convert to feet
Friction Loss Tables
Friction 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.
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Assume system requirements are 12 GPM at 50 PSI:
Suction lift (assume submersible for this example)
Elevation change from pump to highest point on site
Mainline friction loss (500’ of 1” SCH 40, 12 GPM)
 3.36 PSI loss/100 feet * 500 feet mainline = 16.8 PSI
 16.8 PSI * 2.31 = 38.8’
Desired operating pressure of 50 PSI converted to feet:
 50 PSI * 2.31 = 115.5’
0’
17’
38.8’
115.5’
Total Feed of Head
171.3’
Will need to use the 1 hp submersible from the pump curve
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Reducing feet of head requirements may allow selection of smaller
pump. Take the previous example:
Suction lift (assume submersible for this example)
Elevation change from pump to highest point on site
Mainline friction loss (500’ of 1 ¼ ” SCH 40, 12 GPM)
 0.89 PSI loss/100 feet * 500 feet mainline = 4.45 PSI
 4.45 PSI * 2.31 = 10.2’
Desired operating pressure of 45 PSI converted to feet:
 45 PSI * 2.31 = 115.5’
0’
17’
10.2’
New total feet of head:
Can use the ¾ hp submersible from the pump curve
131.1’
103.9’

Most pumps fail due to improper plumbing on
the suction side.
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Minimize fittings and bends
Size 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.
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Discharge plumbing tips:
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Use galvanized pipe/fittings
Install an isolation valve to aid in priming
Pressure relief valve/priming port should directly above
discharge
Install a union for maintenance purposes
Add filtration to all non-potable water sources
Install a pressure gauge
Install a high temperature sensor, low pressure sensor
Proper installation using
galvanized fittings.
Avoid using PVC for direct connections to
centrifugal pumps. Heat generated
during operation or no-flow situations will
cause problems!
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Pump sled to include
an inlet strainer and
outlet well seal
Install union in
discharge line near
shore for maintenance
purposes
Include safety line for
retrieval
Install check valve in
suction line
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Pump 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 spin
Secure wiring to discharge pipe. Leave excess wire at
shore line for maintenance purposes.
Install isolation valve upstream of pump for
troubleshooting purposes.
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A 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.
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Use in situations where a continuously
pressurized mainline is not desired
Does not require pressure tank installation
Pump activates only when irrigation controller
signals operation
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Pump Start Relay
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2 wire pumps can use a standard PSR
3 wire pumps require a control box with start
capacitor
Refer to manufacturer’s cable sizing charts to
determine wire gauge requirements
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Use in situations where a PSR is not feasible
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Multiple controllers using same pump
Quick couplers or hydrants desired on site
Controller and pump are not in close proximity
Special requirements:
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Shelter large enough to accommodate pressure
tank(s)
Drain to exterior for PRV
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.
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Cycle Stop Valve
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Restricts 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.
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Variable Frequency Drive Motor (VFD)
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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 required
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Formation 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.
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Increase net positive suction head (NPSH)
available by:
Increase the diameter of suction line
 Minimize fittings in suction line
 Reduce flow rate through pump
 Reduce suction lift elevation
 Reduce suction line distance
 Create artificial pressure on the discharge by
installing smaller diameter discharge pipe or
throttling valve
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Brass Impeller
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MOTOR STARTS TOO OFTEN
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Check 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.
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MOTOR RUNS CONTINUOUSLY
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Check pressure switch for welded contacts; adjust
settings as necessary
Pump intake blocked
Check valve stuck closed
Low water level or loss of prime
Leak in discharge
Worn pump: symptoms similar to low water level
or drop pipe leak; reduce pressure switch setting and
pump will shut off indicates warn parts
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PUMP DELIVERS LITTLE OR NO WATER
Low line voltage to motor
 Incomplete priming of pump
 Air lock in suction line
 Drop pipe has disconnected from pump
 Low water level
 Clogged or defective foot valve / strainer
 Worn pump parts or plugged impeller
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Check for correct line voltage.
Overheated control or starter may require
ventilation.
Defective control box.
Defective motor or cable.
Worn pump or motor.
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Ohm reading < 500,000 indicates insulation
damage.
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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 + ohms
 Existing motor (without drop cable): 10,000,000 + ohms
 New motor in well: 2,000,000 + ohms
 Existing motor in well: 500,000 – 2,000,000 ohms
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Refer to manufacturer’s charts for ohm values.
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2 wire motors: measure resistance from line to line
3 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 damaged
 If any one value is < normal, the motor is shorted
 If any one value is > normal, the winding or cable is
open / bad splice
Control Box
Schematic
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Power OFF for ohm tests:
Overloads should ohm less than 0.5
 Capacitor should ohm near 15,000
 Relay coil should ohm 4500-7000
 Relay contact should zero ohm
 Start and run capacitors should ohm near zero and
then move toward infinity
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Motor under load for amperage tests:
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Red lead current should start high and then fall to
manufacturer’s chart reading. Black and yellow lead
current should not exceed chart reading.
 Relay or switch failure: Constant high red lead current
and overload tripping
 Open run capacitor: lower than normal red lead amps,
and higher than normal yellow and black lead amps
 Failed start capacitor or open switch / relay: red lead
current is not momentarily high at starting
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Water source type
Minimum and maximum flow, GPM
Desired pressure at sprinklers
Vertical elevation—water line to pump
Vertical elevation—pump to highest point
Mainline (size, type, length)
Suction line (size, type, length)
Well depth, yield, water level, pump set depth
Well pump HP, GPM

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