Evaluating Starvation of Pumps

Notes:

Starvation occurs when not enough liquid is present to fill each vane on the impeller every revolution of the shaft.

Pump starvation can often be confused with unbalance since they both show up as 1x vibration frequency in a frequency spectrum, which is generally considered to be a frequency representing unbalance.

However, starvation can be easily distinguished by the varying amplitude of 1x frequency component in a vibration spectrum (either seen during capture of the spectrum or on a waterfall diagram). This happens at constant speed with reduced load on driving motor.

When the vanes on the impeller are striking something, the vane pass frequency (the number of vanes times the rotational speed) is excited. Because the striking causes a force on the shaft, an unbalance component (the 1x frequency component) would be present.

The frequency spectrum response would show a response at 1x and vane pass frequency. The time waveform will also show a high frequency response (vane pass frequency) riding on a frequency response at 1x. The vane pass frequency would be in phase with the shaft speed.

If the impeller or the bearing (outer or inner race) is loose on the shaft, the vane pass frequency would be modulated by the shaft speed.

An Illustrative Case

A Eight Stage Pump (Fig 1)

Abnormally high vibration was measured in a multi stage pump (eight stages).

After replacing the pump bearings some improvement was observed in pump vibration levels. Vibration came down from 44 mm/sec to 11 mm/sec. Still vibration level was in the unacceptable range.

Spectrum analysis indicates 1X Rpm (fundamental) as dominant frequency. Harmonics of fundamental (of minor amplitudes) were also found on both pump bearings. However, vane pass (modulated) also dominated the spectrum.

Trends of Overall Vibration Readings

Overall Vibration readings on the Pump in three directions (Fig 2)

Before Change of Bearings

Spectrum as on 03/10/2016 (Fig 3)

Spectrum as on 03/10/2016 (FIg 4)

Spectrum as on 03/10/2016 (Fig 5)

Spectrum as on 03/10/2016 (Fig 6)

After Change of Bearings

Spectrum as on 26/10/2016 (Fig 7)

Spectrum as on 26/10/2016 (Fig 8)

Spectrum as on 26/10/2016 (Fig 9)

Spectrum as on 26/10/2016 (Fig 10)

Analysis:

The Present Situation:

The present level of vibration of the pump (ref Fig 2). especially at Pump Drive End (PDE) is not acceptable for a pump. It is already in the warning zone, which over time is expected to reach the alarm zone rather quickly.

Causes:

At present, this system fundamentally suffers from three faults, which are as follows:

1. Starvation problem — Starvation occurs when not enough liquid is present to fill each vane of the impeller with every revolution of the shaft. In this case there are eight such impellers. This can be seen by the high vane pass frequency with modulation on one side of the vane pass frequency. Moreover, as is evident the vane pass frequency dominates the vibration spectrum. Also note that 1x rises gradually with time.

2. Resonance Problem. — Resonance Problem is due to the unsupported piping structure. In this case the pipes exerts spring force which amplifies the vibration in one direction.

3. Piping layout not as per standard engineering convention. The layout of flanges, straight runs of pipes before flanges are fitted, pipes running parallel to the shafts are fundamental mistakes made during erection and commissioning. Check vibration at the midpoint of the suction and discharge pipes. If the vibration at the mid point of the pipes is equal to or more than 30 mm/sec then we have a fundamental problem to live with, if the situation is not corrected.

Possible Solutions:

1. For Starvation Problem — the client can discuss with the pump manufacturer to ensure how flow rate may be increased in the pump so that enough liquid is present to fill each vane of an impeller for all eight stages with every revolution of the shaft.

2. For Resonance Problem — the suction and discharge pipelines may be supported asymmetrically so that the vibration of the pipes doesn’t pass on to the pump to amplify vibration.

3. For Piping layout — This is an inherent problem the system was born with. It would be extremely effective if corrections may be done, keeping the basic principles of piping design in mind. However, the client may discuss with the OEM (Original Equipment Manufacturer)to make necessary corrections, which may possibly be carried out along with the rectification of the ‘Starvation Problem.”