Vibration – What is It? How does it Affect My Operation? What can I Do About It?

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- By: , , , ,

Courtesy of Water Environment Federation (WEF)

ABSTRACT
This paper describes how pump vibration can create problems in conveyance systems. Potential sources of vibration are reviewed as well as factors that affect pump vibration. Examples are presented that demonstrate pump vibration problems: (1) when a wet well level was lowered to obtain system storage capacity, (2) in a new installation, and (3) when one pump in an installation functioned properly and the adjacent pump had a high vibration amplitude. The successful use of vibration testing as a condition of routine startup and acceptance is described.

INTRODUCTION
End suction centrifugal, axial/propeller, and turbine pumps have been used for decades in wastewater systems. While all rotating equipment has some level of vibration, at what point does this vibration become excessive and damaging to the performance and operation of the equipment? More importantly, what can be done to mitigate vibration and allow the equipment to operate satisfactorily?

What exactly is vibration? Vibration is the oscillating motion of a machine or components of the machine about a reference point of equilibrium.

There are several potential sources of vibration in pumps:

  1. When variable speed drives are used to meet the variation of flows within a pumping system, owners have reported problems ranging from excessive vibration and accelerated seal and packing deterioration to accelerated bearing wear. The critical speed of the pump assembly, including the pump shaft and motor shaft, can have a dramatic effect on the vibration levels in a pump. When a pumping station operates in a variable speed mode, the critical speed of each pump and all its components must be at least 20 percent above the maximum operating speed of the pump. For example, if the full speed of a pump is 1,750 RPM, then the critical speed of all the pump components must be at least 2,100 cycles per minute or greater. If the critical speed of the pump is below its operating speed, the chances of having excessive vibration when operating in the variable speed mode are very high.
  2. Operating levels in the wet well have a dramatic effect on the level of vibration in a pump. Typically, low submergence of the pump suction will cause pre-rotation, which leads to irregular velocity variations across the pump’s impeller. This leads to excessive and damaging vibration levels.
  3. Another source of vibration in a pump can be the quality of the manufacturing of the pump and motor. If the pump and/or motor shaft do not have sufficient mass or are not balanced properly, this can be the source of excessive vibration.

A few of the factors that can affect pump vibration include:

  • Stiffness of bearings
  • Type of bearing housing
  • Wear rings
  • Packing
  • Lomakin effect (a hydrodynamic bearing effect caused by water flowing through small annular areas)
  • Wear
  • Hydraulic forces produced between the impeller vanes and the volute cutwater at vanepassing frequency
  • Recirculation and radial forces at low flows
  • Cavitation due to net positive suction head (NPSH) problems
  • Flow disturbances in the pump suction due to improper intake design
  • Air entrainment or aeration of the pumped liquid
  • Hydraulic resonance in the piping

ANSI/HI 9.6.4 Standards for Centrifugal and Vertical Pumps for Vibration Measurements and Allowable Values define the acceptable vibration levels for pumps. These levels of vibration are typically used to determine satisfactory performance in new and existing pump installations.

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