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 fluidsWashing & cleaningMachining centers  Machine coolingFilter systemsOthers? 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 Municipality Arizona

Domestic Water Supply within a High Rise Building Colorado

Supply Side Tank Level Control in a Copper Mine New 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 Q 2 = Q 1 X ( RPM 2 / RPM 1 ) H 2 = H 1 X ( RPM 2 / RPM 1 ) 2 BHP 2 = BHP 1 X ( RPM 2 / RPM 1 ) 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

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 GPM115 GPM172 GPM 220’ Pump is sized for Peak Demand of ’ ( 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 130 GPM  227 FT of Head  12.2 HP  74.5% Hydraulic efficiency 90 GPM  312 FT of Head  10.5 HP  67.6% Hydraulic efficiency 41 GPM  330 FT of Head  7.89 HP  43.3% Hydraulic efficiency

PRV Domestic Water Supply to a Hospital Arizona 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 GPM172 GPM 220’ Pump is sized for Peak Demand of ’ ( 95 PSIG ) 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 GPM115 GPM172 GPM 220’ Pump is sized for Peak Demand of ’ ( 95 PSIG ) 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 130 GPM  91% of Full speed  9.7 HP  74.6% Hydraulic efficiency 90 GPM  86% of Full speed  7.08 HP  71.1% Hydraulic efficiency 41 GPM  82% of Full speed  4.63 HP  49.3% 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 to VFD System Comparison – Flow Reduced to 90 GPM PRV - Full speed  BHP required = 10.5 HP  Hydraulic efficiency = 67.6% VFD - 86% of Full speed  BHP required = 7.08 HP ( 32% less )  Hydraulic efficiency = 71.1%

PRV to VFD System Comparison – Flow Reduced to 41 GPM PRV - Full speed  BHP required = 7.89 HP  Hydraulic efficiency = 43.3% VFD - 82% of Full speed  BHP required = 4.63 HP ( 41% less )  Hydraulic efficiency = 49.3%

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