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How to identify cavitation in pumps and rectify it....

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  1. 1. Monitored Item: Motor Details: 74 kw,1475 RPM Pump Details: Hot well Pump, Acceptable: Still Acceptable 2.8 mm/sec (Rms) 7.1 mm/sec ISO-10816 (Class II) ISO-10816 (Class II)
  2. 2. Monitoring Point Direction Rms Velocity (mm/sec) Acceleration G MNDE H 1.61 0.55 V 3.29 A 2.50 MDE H 2.09 1.19 V 1.78 A 4.22 PDE H 2.44 1.02 V 2.13 A 2.57 PNDE H 1.83 0.41 V 1.72 A 2.63
  5. 5. Over all vibration velocity and acceleration values were recorded at all motor and pump side bearings in load condition. Vibration velocity values are within the limit for this class of machine as per ISO 10816 standards. Acceleration values are high at motor and pump bearings. Spectrum captured by the impeller indicating cavitation at pump.
  6. 6. Check proper lubrication at all monitored bearings. Flow parameter needs to be maintained as per design. Trend monitoring is required for further analysis.
  7. 7. Monitoring Point Direction Rms Velocity (mm/sec) Acceleration G MNDE H 2.72 0.36 V 1.26 A 3.16 MDE H 2.26 0.49 V 1.73 A 1.46 PDE H 2.66 0.96 V 1.81 A 1.18 PNDE H 1.88 0.89 V 1.58 A 1.09
  8. 8. NET POSITIVE SUCTION HEAD (NPSH) Total pressure head (m) at the suction of the pump over and above the vapor pressure at the pumping temperature. 1) Velocity of water entering the impeller eye depends on the static head. 2) Velocity is a function of pump speed. Greater the speed, greater the velocity. 3) NPSHR N4/3 * Q2/3 4) NPSHR increases with Q.
  9. 9. NET POSITIVE SUCTION HEAD (NPSH)  Net positive suction head is another term that needs to be determined for the cavitation free design & performance of the pump.  It is the absolute static pressure above the vapor pressure of the liquid in the pump.  Available NPSH (NPSHA) should be more than required NPSH(NPSHR).  If NPSHA is less than NPSHR, then vapor can accumulate and form bubbles to cause erosion of impeller vanes.
  10. 10. NET POSITIVE SUCTION HEAD (NPSH) NPSH = Pb – hS – hL – PV Pb = Barometric pressure (abs) in ft. or Mtr. hS = Vapor pressure of liquid in ft. or Mtr. hL = Suction head + ve in Negative suction. - ve in Positive suction. PV = Head loss in suction line.
  11. 11. Capacity change effect on NPSH R Q1 Q2 Q3 NPSHA NPSHR Recommended operating range Designed flow Limiting flow Increased flow Critical range Non-recommended operating range Flow (Kg/Hr)
  12. 12. Effect of pump temperature on NPSH QD NPSHA NPSHR Pump temperature was designed for 160OC. At 180OC, NPSHA was inadequate and cavitation will occur. Flow (Kg/Hr) 160OC 170OC 180OC
  13. 13. Effect of pump Location on NPSH QD NPSHA (old location) NPSHR Flow (Kg/Hr) NPSHA (New location) As the location of the pump has been changed from the original position, NPSH has decreased.
  14. 14. Effect of Speed change on NPSH Q-high NPSHA NPSHR-low speed Flow (Kg/Hr) With speed increase, NPSH R also goes up and will reduce the margin available in NPSH A. Q-low NPSHR-high speed