VARIABLE SPEED PUMPING

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VARIABLE SPEED PUMPING
Techsys Corporation
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BASICS

• VARIABLE SPEED PUMPING is only advantageous when
  there is a
• VARIATION in the FLOW
• demand of a system
• If the system is fixed flow with no variations
  the VFD pumping is not beneficial

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PUMP CURVE
The Pump curve defines the Pump Capacity At
various flow rates
The Power curve is defined Vs Flow and Head
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SYSTEM CURVE
The System curve defines the System
Resistance At various flow rates
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Constant Flow Pumping
Power Absorbed at 100 Design Flow Rate
Static Head
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Variable Speed Pumping
The variable Speed Curve Can track the System
Curve Or a Fixed pressure
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Variable Speed Efficiency Curves

• Best Efficiency follows a quadratic equation
• The Affinity Laws cover Power, Speed Head at
  variable speed

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POWER LOSS PUMP SPEED
Power is proportional to the CUBE of the pump
speed
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PUMP Vs SYSTEM CURVE
If the flow rate changes the fixed speed pump
must follow the curve And take the system curve
with it
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PUMP SPEED CHANGE FOLLOWS SYSTEM CURVE

• Changing Pump Speed tracks the system Curve
• Can follow the system requirements without
• Excess loading on the pump or system

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POWER VARIATION

• For every 10 speed change
• POWER changes by
• 27
• 0.9 x 0.9 x 0.9 0.729 72.9 of full load

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TYPICAL PUMP SELECTION

• Most engineers select a pump and add a
• SAFETY FACTOR
• This Safety Factor is the cause of inefficiency.
• Typically any variation from B.E.P. of more than
  15 will cause major energy losses

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RUNNING COST SAVINGS
RUNNING COSTS Savings are the main advantage of
VFD Pumping
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RUNNING COST SAVINGS

• Based on
• 50 average flow
• demand
• Standard ISO pump
• 20c/kW hr
• BEP eff 76

Payback periods 6 12 months Motor size
dependant
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TYPICAL NON-VFD CONTROL

• There are three main ways for operators
• to attempt to match the system curve
• with Fixed speed pumps
• Throttle the discharge
• Re-circulate unwanted flow
• Start Stop the pump within
• pressure bands

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Throttled system
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Power Consumption with Throttled Control
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PUMP THROTTLING
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THROTTLED DISCHARGE

• Excessive power consumption
• Causes pump damage from cavitation
• Valve damage due to excess velocity
• Creates water hammer
• Not desirable

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Throttling Example
Wasted Power
Based on a 37kW pump with dump valve control
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FIXED SPEED PRESSURE CONTROL
Typical pressure control system CUT IN
Pressure CUT Out Pressure Suitable for operation
when Cycling is limited
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PRESSURE SYSTEM

• Best range of operation of a pump
• is generally 60 80 of Full Flow
• Pressure system needs to operate out of this
  range

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VFD Vs PRESSURE SYSTEM

• VFD System can vary the flow efficiently over
  most of the pump curve

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VFD Vs PRESSURE SYSTEM
Starting costs become significant at high cycle
rates
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VFD Vs PRESSURE SYSTEM
Any system that Operates below 50 of DESIGN
FLOW of the pump for over 20 of the time
should consider VFD pumping
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What Control Parameter ?

• PRESSURE
• Provides the ability to sense the load
• Easy to sense
• Provides a full picture of the system load via
  the system curve
• Suits over 90 of applications

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Typical Control Parameters
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WHY MULTI PUMP SYSTEM ?

• CAN HOLD BEST
• EFFICIENCY POINT
• OVER A GREATER RANGE
• MORE ECONOMICAL FOR THE FULL RANGE OF FLOW RATES
• COST OF COMPONENTS (VFD AND SWITCHGEAR) LOWER FOR
  SMALLER POWERS
• SEAL COOLING CONSIDERATIONS FOR MINIMUM FLOW RATES

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MULTI PUMP
Efficient Range 60-100 of design flow
rate If the flow span is greater than this then
the system is operating outside best efficiency
50
40
60
70
Pump curve
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MULTI PUMP
Pump Head
50
40
50
40
60
60
70
70
Efficient Range 30-100 of design flow rate
with multiple pumps
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The different types of pumps
Advantages using VFD
Centrifugal pumps Best performance
Reciprocating positive displacement pumps Fine
adjustment and tuning
Rotary positive displacement pumps Surge
dampening and adjustment
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VFD PUMP SYSTEM MANUFACTURERS
MODELS
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VFD PUMP SYSTEMS
There are numerous BMS and Electricians that
program PLCs for Pump control Operation -Buyer
Beware There is significantly more than meets
the eye Use reference site for evaluation
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DIFFERENT TYPES OF VFD CONTROL SYSTEMS

• Three types of VFD pump control
• On pump control Motor VFD
• Lead VFD backup soft start
• True VFD VFD rotates to all pumps.

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ON MOTOR VFD

• Neat Engineering solution
• Purpose built Inverter for pumps
• Very compact

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ON MOTOR VFD

• Suffer from Vibration and
• overheating
• Replacement components can
• be difficult to source
• Multi mump co-ordination
• difficult

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ONE VFD PER PUMP

• Operational VFD on each pump
• When extra pumps are required the system calls in
  additional VFD/Pump combinations
• The system can have all of the VFDs functioning
  at a speed
• All pumps vary the speed together

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ONE VFD PER PUMP

• Most expensive option
• Pumps operate furthest from B.E.P. as the load is
  shared on all pumps
• Tight control difficult
• Does not justify extra expense for risk
  management

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LEAD VFD - LAG SOFT STARTER

• Multiple pump
• First pump started is the VFD pump
• Lag pumps operate on fixed speed
• Set point 20

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LEAD VFD - LAG SOFT STARTER

• System pressure can
• Spike at changeover
• Requires rapid reaction
• time at changeover
• Needs damped reaction
• at operating set point
• Basic instability at
• SET Point

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ROTATING VFD SYSTEM

• VFD starts on First pump
• As the load increases the First pump is switched
  to fixed speed
• The second pump starts on VFD etc.

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ROTATING VFD SYSTEM

• Smoothest operation
• Best set point holding characteristics
• Lowest starting current
• Softest on pumps and reticulation

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RISK ANALYSIS OF SYSTEMS
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CURRENT DRAW FOR VARIOUS SYSTEMS
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SUMMARY

• VFD Systems are ideal for pumping with
• systems that have variable flow rates
• Variable speed systems generally operate best
  with Pressure control
• Multiple pumps typically are most efficient and
  are the least expensive
• Use a Brand Name control system