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

2. 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)

4. 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

7. MDE ACCELERATION
PDE ACCELERATION

8. PNDE ACCELERATION

9. 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.

10. Check proper lubrication at all monitored bearings.
Flow parameter needs to be maintained as per design.
Trend monitoring is required for further analysis.

12. 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

13. 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.

14. 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.

15. 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.

16. 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)

17. 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

18. 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.

19. 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