Variable Speed Pumping

Transcription

1 Variable Speed Pumping Jim Vukich Application Engineer ITT Corp. Malvern, PA

2 Why Do It?

3 Why Do It? Flow-Matching Minimize Starting & Stopping Flexibility Different Discharge Points, Flows Changing Conditions Present & Future

4 How To Do It? Adjustable-Speed Devices

5 How To Do It? 2-Speed Motor Two Windings in a Single Stator

6 How To Do It? Transmission Belts & Pulleys, Gears, etc.

7 How To Do It? VFD The Most Common Method Today

8 VFD Variable Frequency Drive

9 VFD Variable Frequency Drive We can change the speed of the motor by varying the frequency of the signal Frequency = Cycles per second = Hertz In the U.S. systems operate on 60Hz nominal

10 VFD Variable Frequency Drive

11 Types of VFDs Current Source Inverter Simple, but Noisy Flux Vector Control Precise, but Expensive Pulse Width Modulated Most Common for Pumps

12 Pulse Width Modulation Uses a Constant Voltage Pulsed with Integrated Bi-Polar Power Transistors Vary the Width of the Pulses to Generate a Sine Wave Switching Frequency Higher is Better (to a point)

13 Pulse Width Modulation

14 Effects on the Motor Harmonics Noise Torque Ripple Overheating Modern VFDs have eliminated most of these problems!

15 Reduced Torque

16 Effects on the Motor Steep Voltage Transients Short Rise Times Stress on the Insulation Material Reduced Motor Efficiency

17 Sizing the VFD What You Need to Know: Motor Full-Load kva Ratio of Maximum to Normal Torque

18 Sizing the VFD Do NOT size the VFD based on the horsepower!!! Size it on current and voltage Pumps are variable torque machines Pump VFDs often need to be oversized

19 Sizing the VFD Full-Load kva (Volts x Amps x 3) 1000 Size the VFD at least 10% higher than the motor

20 Sizing the VFD Older motors or cheap motors need to be de-rated 10% to 15% de-rating is typical Insist on a VFD-ready motor! Class H insulation High-fill impregnation

21 Sizing the VFD Make sure the VFD is rated for the same number of starts per hour as the motor

22 Why VFDs are Used in Pumping

23 Why VFDs are Used in Pumping A VFD allows you to change the flow and head performance of your pump

24 Why VFDs are Used in Pumping Flow Matching Match the inflow rate Maintain the outflow rate Maintain a constant liquid level in the sump Minimize the sump volume

25 Why VFDs are Used in Pumping Changing Conditions Over Time Increase flow without new pumps Maintain flow as new pump stations come online

26 Why VFDs are Used in Pumping VFDs can be used as soft start and soft stop VFDs can be used as phase converter

27 Why VFDs are Used in Pumping A VFD allows you to change the flow and head performance of your pump

28 Single Speed Curve

29 The Golden Rule for Pumps The pump can only operate at ONE point Where it intersects the system curve

30 The Golden Rule

31 Variable Speed Field

32 Many Possible Operating Points!

33 Efficiency vs. Speed

34 The System Curve

35 The Allowable Operating Range

36 Specific Energy kw-hrs per Million gallons

37 Different Kinds of System Curves

38 Risks & Limitations of VFD Pumping

39 Operation Outside of Allowable Range

40 The Allowable Operating Range

41 When you operate outside of the allowable range: Recirculation Clogging, Wear, Heat Rise Vibration Shaft Bending Stress on Seals and Bearings Cavitation

42 Recirculation Wear

43 Wear Heat Rise

44 Clogging Dirty little secret: There is no such thing as a non-clog impeller Any kind of impeller can clog if operated under the wrong conditions

45 Clogging Impeller energy drops off rapidly at reduced speeds If speed = 50%, impeller energy = 25% Turbulent zone moves further from leading edge Rags build up on the impeller Less available torque to pass the occasional mophead

46 The Turbulent Zone

47 Unstable Region Stall Point

48 Motor Cooling Motors with cooling jackets may require a minimum speed To provide heat transfer To prevent settling of solids Check with the equipment manufacturer

49 Natural Frequencies

50

51 Natural Frequencies All systems have a natural, or resonant, frequency. If that frequency is fed into the system from an external source, the system will begin to vibrate.

52 Natural Frequencies As the VFD ramps up and down, it may hit certain frequencies that excite the system The controls must be set to lock out those dangerous frequencies

53 Natural Frequencies Always install flex connectors to isolate the pump from the piping Always support the piping independent of the pump Pipe hangers Thrust blocks Base elbows

54

55 Running Above 60Hz Be very careful!!! 10% overspeed requires 33% more power NPSH goes up risk of cavitation recommends 63Hz maximum

56 Reliability A VFD is an in-line device Any in-line device will reduce the reliability and efficiency of the system

57 The Downside of Constant Liquid Level

58 Grease Ring Floating Debris

59 Operational Strategies or The Smart Way to VFD Pumping

60 Operational Strategies The #1 problem with VFD pumping is clogging Clogging usually results from running the pump too slow for too long

61 Prevent Clogging Know the limitations of your pump Set the minimum speed to stay within the A.O.R. Consider running in on/off mode at the minimum speed

62 Prevent Clogging Start at full speed and then ramp down This will clear clogs, and will open check valves fully Ramp up to full speed once in a while, to clear clogs and flush out cooling jackets

63 Grinder Pumps Don t run a grinder pump on a VFD Grinder pumps depend on torque to do their work Available torque is reduced when run on VFD

64 Keep the Sump Clean Vary the on/off level to prevent grease rings Run the pumps to snore to suck up debris

65 Keep the Sump Clean The controls may automate this for you!

66 Running Multiple Pumps on VFD Always ramp the 1st pump up to full speed before bringing the 2nd pump online Slow down the 1st pump to match the 2nd This is called synchronous operation

67 Use the Right Settings If using VFD as soft-start, ramp up to full speed as quickly as possible If using VFD as soft-stop, choose the correct setting! Otherwise, water hammer may occur

68 Use the Right Settings Make sure the VFD is set for a variable torque load This will maintain: Magnetization level Motor efficiency

69 Use the Right Settings Some VFDs have built-in presets for equipment: Pumps Blowers Compressors Fans Make sure you are using the pump settings!

70 Use the Right Settings Switching frequency Should be in the 3 to 4 khz range Any higher can damage the motor Any lower can cause audible noise

71 Installation Guidelines

72 Pump Sensors Some types of sensors may be sensitive to noise and harmonics generated by the VFD Filters may be required Check with the pump mfgr. Check with the VFD mfgr.

73 VFD Output Filter Reduces Noise Can eliminate need for motor de-rating Can reduce nuisance tripping (false alarms)

74 Pump Sensors Use shielded wire to carry the sensor signals, especially on long runs Run the power and sensor cables in separate conduits, spaced at least 12 inches apart

75 Power Factor Correction Capacitors PWM drives will keep the power factor at 95% or better Capacitors are not required Capacitors can actually be destroyed by the VFD!

76 VFD in same cabinet with other components VFD must be properly constructed to prevent interference

77 Bypass Contactor Insist on getting a bypass contactor installed Eliminates the VFD in emergency situations Helps with troubleshooting

78 Use Caution when Taking Readings You need a true RMS meter to take accurate readings of voltage, current, and power

79 Conclusions

80 Conclusions VFDs are NOT a cure-all!! They have their problems and you must use caution Be aware of the limitations of your equipment Pumps Pipe system VFDs

81 Conclusions VFDs are NOT appropriate for all systems High static head (flat system curve) may not work well with VFDs Don t use a VFD with grinder pumps

82 Conclusions VFDs can result in off-peak operation of the pumps This will lead to problems! Clogging Vibration Wear Lower efficiency Shorter life

83 Stay within the Allowable Operating Range

84 Restrict operation outside A.O.R. to short or intermittent duty ONLY

85 Conclusions Avoid VFDs if possible Consider the alternatives: Use different-sized pumps (jockey pumps) Make the wet well larger Choose pumps that can start more times per hour

86 Conclusions Avoid VFDs if possible If not correctly applied, VFDs may: Increase cost Increase complexity Decrease efficiency Decrease reliability

87 ?? Questions??