Unexplained peak frequency of the pump

Case definition:

Concerns "unexplained" peak frequency measured at the level of the pump and pressure pipe. 
Near a centrifugal pump, pressure, and suction pipe, a peak in the vibrations was measured resulting in 3750RPM. This was not present before adjustment. The closer we get to the housing of the pump, the higher the elevation of the vibration. The vibration is monitored on the pump in horizontal direction. Instinctively, the vibration is caused by the fluid pulsation in the pipes (we observed the needle of the pressure nanometer vibrate intensely). In the pipes, we measure levels up to 20mm/s and more. Towards the collector, the amplitude decreases again.

Overview of the vibration levels: 

PUMP NOT CLUTCH HORIZONTAL (?)

  • Measurement 02-Jun-06 13:46 : 1.468 mm/Sec  
  • Measurement 29-Jun-06 09:26 : 1.803 mm/Sec  
  • Measurement 30-Jun-06 06:25 : 1.661 mm/Sec 

PUMP NOT CLUTCH VERTICAL 

  • Measurement 02-Jun-06 13:47 : 1.410 mm/Sec  
  • Measurement 29-Jun-06 09:28 : 5.953 mm/Sec 
  • Measurement 30-Jun-06 06:25 : 6.123 mm/Sec 
Unexplained peak frequency of the pump

Matching spectrum in trend: 

Acknowledged changes during adjustments: 

  • During adjustments, the pump was replaced. 
  • After adjustments, the pumps combined process 45-50 tons/hour more. 

To determine how the vibrations develop in reaction to a decrease of the load (settings before the discharge), we have placed three sensors ñ one on the pump, the pressure pipe, and the suction pipe. Subsequently, the tests stated below were conducted, while the device continuously registered the vibration levels.

Unexplained peak frequency of the pump

Action taken: 

  • 09h49: discharge c-pump closed. 
  • 09h55: Combined discharge valve of the three pumps closed.
  • 10h00: Discharge c-pump opened. Combined discharge valve remains closed. 
  • 10h03: Normal function. Combined discharge valve back opened. 

Monitoring position

Unexplained peak frequency of the pump

Overall values of the vibrations:

Unexplained peak frequency of the pump

After closing discharge valve, we even observed that the vibrations of the pump increased slightly. None of the actions taken eliminate the resonance phenomena. 

Decision:

As the phenomenon does not depend on the flow rate, we made further inquiries with the department that was responsible for the overhaul of the pump. Apparently, they had increased the washer slack to reduce the risk of blockage in the stainless steel pumps (only increased by 20%). After adjusting this slack, the vibration measured returned back to normal.

Unexplained peak frequency of the pump