variable speed applied to pumps. life cycle costs - courtesy of hydraulic institute and europump...

Variable Speed Applied to Pumps

Life Cycle Costs - Courtesy of Hydraulic Institute and Europump

• Initial cost is not the only cost associated with a pump

Initial cost rarely accounts for more than 5% of a pump’s life cycle cost

Operation & maintenance costs usually account for approx. 10% of a pump’s life cycle costs

Power consumption usually accounts for up to 85% of a pump’s life cycle costs!

Ways to Reduce Energy Costs

• Select a pump with a high hydraulic efficiency

• Select a motor with a high motor efficiency

• Operate the pump close to the BEP

• Consider varying the pump speed when there’s a changing demand Flow Pressure Level Others

Potential Pump Applications with a Changing Demand

• Water supply to a home, neighborhood, business or municipality

• Water supply within a building

• Boiler & condensate

• Filtration / Water purification • Ambient temperature control – Heating or cooling

• Maintaining a constant sump or tank level

• Process applications Process fluids Washing & cleaning Machining centers Machine cooling Filter systems Others?

• Others?

Constant Pressure Control with Variable Speed

Constant Level Control with Variable Speed

Constant Temperature Control with Variable Speed

Constant Flow Control with Variable Speed

Domestic Water Supply to a MunicipalityArizona

Domestic Water Supply within a High Rise BuildingColorado

Supply Side Tank Level Control in a Copper MineNew Mexico

Boiler & Chiller Applications in a Retail / Residence Building Colorado

Benefits of Varying the Speed of a Pump

• Greatly reduce energy costs

• Reduce pump and / or pump system wear

• More precise control of the liquid

• Reduce water waste – In an irrigation application as an example

• Reduce noise levels produced by the pump or pump system

Varying the Speed of a Pump - The Affinity Laws

• The Affinity Laws describe how changes in RPM effect Flow ( Q ), Head ( H ), & Brake Horsepower ( BHP ) in centrifugal pumps The same laws can be applied for changes in impeller diameter ( at a constant speed )

• There are three formulas

Q2 = Q1 X ( RPM2 / RPM1 )

H2 = H1 X ( RPM2 / RPM1 )2

BHP2 = BHP1 X ( RPM2 / RPM1 )3

When impeller diameter is constant

The Affect of Applying the Affinity Laws

Reduce the pump speed to 75% of full speed and

Flow is reduced to 75% of the original flow

Head is reduced to 56% of the original head

BHP is reduced to 42% of the original BHP

Losses due to reduced motor speed and the VFD will reduce the energy savings, but not by much

Variable Speed Pump Performance Curve

• Curve shifts down and to the left when the speed is reduced

• Efficiency shifts with curve

• Pump can operate anywhere in the shaded area

Testing the Affinity Laws

• Full speed Flow = 210 GPM Head = 146 FT BHP = 13.7 HP

• 75% of Full speed Flow = 157 GPM Head = 81.2 FT BHP = 5.78 HP

24 Hour Consumption Profile (Ave. GPM per hour)

0

50

100

150

200

250

300

350

400

450

Hour of the Day

Av

e. G

PM

Typical varying flow demand for a community ( or apartment bldg., house, etc. )

Constant Pressure – Varying Flow Demand Application

Three Common System Types to Deliver Constant Pressure

• Pressure Reducing Valve ( PRV ) System Most often used for a building water supply system

• Hydro Pneumatic System Most often used for a municipal water supply system

• Variable Speed ( Variable Frequency Drive ) System Used for building & municipal water supply systems

Main Components Needed for Each Type of System

Hydro Pneumatic System:

• Constant speed pump(s)

• Control panel

• Pressure sensor

• Large hydro tank

• Air compressor ( large systems )

Variable Speed System:

• Variable speed pump(s)

• Control panel

• Pressure sensor

• Small hydro tank

PRV System:

• Constant speed pump(s)

• Control panel

• Pressure Reducing Valve

• Large hydro tank

Operation of a Pressure Reducing Valve System

• Pump(s) started and stopped at full speed High motor inrush current ( potential short cycling problem ) High starting and stopping torque on motor(s) & pump(s) ( increased wear ) Soft start can be added to reduce pump starting speed ( cost close to VFD cost )

• Pump(s) run constantly when there is a flow demand

• Pump(s) always run at full speed Pump(s) develop higher pressure than needed ( energy waste ) Pump(s) may tend to run at low efficiency points ( energy waste ) Pump shaft deflection may be a problem ( increased wear )

• Large hydro tank is needed to reduce pump cycling

• PRV is used to create a “false head” to deliver desired pressure ( energy waste )

• Pressure to network is relatively constant

Constant Pressure With a Pressure Reducing Valve System

60 GPM 115 GPM 172 GPM

220’

Pump is sized for Peak Demand of 172 GPM @ 220’ ( 95 PSIG )

Excess pressure = WASTED ENERGY

Excess pressure =WASTED ENERGY

Operation of a PRV System as Expressed on Hydraulic Curves

• 172 GPM 220 FT of Head 13.4 HP 71.5% Hydraulic efficiency




PRV Domestic Water Supply to a HospitalArizona

Pressure measured at the pump( before the PRV )

Pressure of the system( after the PRVs )

218 PSIG ( pump ) -112 PSIG ( system ) = 106 PSIG loss through the PRV

The energy required to develop 106 PSIG ( at this flow ) is WASTED!

Operation of a Hydro Pneumatic System

• Pump(s) started and stopped at full speed based on pressure sensor High motor inrush current ( potential short cycling problem ) High starting and stopping torque on motor(s) & pump(s) ( increased wear ) Soft start can be added to reduce pump starting speed ( cost close to VFD cost )

• Pump(s) start when pressure in tank reaches low pressure setting

• Pump(s) stop when pressure in tank reaches high pressure setting

• Pump(s) always run at full speed Pump(s) develop higher pressure than needed ( energy waste ) Pump(s) may tend to run at a low efficiency point ( energy waste ) Pump shaft deflection may be a problem ( increased wear )

• Large hydro tank is needed to reduce pump cycling

• Air compressor ( energy consumer ) is used to maintain air pressure in the tank

• Pressure to network varies by greatly (typically by a range of 20 PSI)

Constant Pressure With a Hydro Pneumatic System

140 GPM 172 GPM

220’


Excess pressure = WASTED ENERGY

Hydro Pneumatic System Operation Expressed on Hydraulic Curves

• The pump is sized to deliver the

maximum required flow at the minimum acceptable pressure

The pump is turned on when the tank pressure is 95 PSIG

• The pump develops a higher pressure than the end users require

This results in energy waste The pump is turned off when the

tank pressure is 115 PSIG

Operation of a Variable Speed System

• Pump(s) start and stop at slow speed and ramp up & down to meet flow demand

Low motor inrush current ( no short cycling problem ) Low starting and stopping torque on motor(s) & pump(s) Pump(s) only develop the pressure desired Pump(s) tend to run a higher efficiency points Pump shaft deflection is decreased

• Pump(s) started and stopped based on pressure sensor and controller

• Pump(s) start when pressure in discharge line drops below setpoint

• Pump(s) stop when pressure in discharge line is at slightly over setpoint

• Small hydro tank is needed to stop the pump(s) with no flow demand • Pressure to network varies by +/- 3 PSIG

Constant Pressure With a Variable Speed System

60 GPM 115 GPM 172 GPM

220’


Pump slows down & curve shifts

Pump slows down& curve shifts

Operation of a VFD System as Expressed on Hydraulic Curves

• 170 GPM 100% of Full speed 13.4 HP 71.5% Hydraulic efficiency




PRV to VFD System Comparison Maintaining 220 FT ( 95 PSIG )

PRV to VFD System Comparison – Flow Reduced to 130 GPM

• PRV - Full speed BHP required = 12.2 HP Hydraulic efficiency =

74.5%

• VFD - 91% of Full speed BHP required = 9.7 HP ( 20% less ) Hydraulic efficiency = 74.6%


• PRV - Full speed BHP required = 10.5 HP Hydraulic efficiency = 67.6%



• PRV - Full speed BHP required = 7.89 HP Hydraulic efficiency = 43.3%


Reasons to Consider Using Variable Speed Systems

• Lower Energy Costs Reduced speed reduces

energy consumption

• Less System Wear Reduced speed reduces

pressure in pump, piping and valves, resulting in decreased wear

Reduced speed helps the pump

operate closer to the BEP which

results in decreased bearing &

seal wear

• Lower Noise Levels Reduced speed reduces

motor and pump noise level

• Reliability Continuing electronic

technology advances

• Easy Start Up & Changes “Plug N Pump”

• VFD Cost Electronic equipment costs

continue to decrease, mechanical equipment costs continue to increase

variable speed applied to pumps. life cycle costs - courtesy of hydraulic institute and europump...

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