Motion Magnified Video is a good tool for seeing potential rotating machinery system issues, such as, soft foot, piping support problems, foundation cracks, etc. This magnified video shows excessive system motion when the pump is operated below it's specified minimum operating speed. Vibration was acceptable within the specified running speed range (70% to 100% of maximum running speed). 

1.    Problem Statement (Challenge)

  • What (type of machine): Single stage double suction vertically mounted split case driven by 300HP motors through a long line shaft in low lift service.
  • Where (State/Country): Upper Midwest, United States
  • Why (problem/reason): Provided post-installation vibration testing services after a new motor and VFD were installed. The concern was converting the pumps from constant speed to variable speed while also adding new motors with different mass can introduce a resonance (high vibration) issue at start up or over the life of the plant.
  • Acceptance criteria: ANSI/Hydraulic Institute (HI) 9.6.4 plus ISO 10816-3 for the motor vibration measurements along with confirming that specified natural frequency separation margins were met.

2.    Work Performed

Methods:

3.    Results/Solution

Findings:

  • The pumps and motors had acceptable vibration within the running speed range:
    • Overall vibration limits set by HI 9.6.4 for a 300 HP system should not exceed 0.19 in/s RMS while running in the POR. 
    • Motor vibration limits set by ISO 10816-3 should not exceed 0.177 in/s RMS in its allowable running range.
  • Separation margin criteria was less than 10% indicating that a potential resonance (high vibration) issue was possible or even likely during the plant operating life. Separation margin was achieved after mass was correctly added to the motor.

MSI’s VibVue® MMV test results show acceptable motion within the running speed range but high motion when operating just outside of the running speed range (acceptable with added motor mass).

 Impact:

  • MSI recommended accepting the pump system from a vibration standpoint for operation within the 70% to 100% speed (498 rpm to 712 rpm) with or without the added motor mass.
  • The specified natural frequency separation margins were only met with the added motor mass.
  • The probability of avoiding high vibration issues due to resonance over the plant operating life was significantly increased with the MSI recommended motor mass addition.  

    Picture1

Summary of running speed range: The pumps were converted from constant speed to variable speed which can lead to exciting previously benign natural frequencies. New motors with different dynamic characteristics including mass were also changing the system natural frequencies.

Picture2

One example FRF plot from among many resulting from a properly specified and performed Experimental Modal Analysis (EMA, impact) test. This example shows acceptable separation margin from running speed. This means, high vibration due to resonance is unlikely at commissioning and over the plant operating life. Other FRFs indicted a separation margin issue.

 

Picture3

Overall vibration data showing that the pumps fall within the ANSI/HI specified vibration criteria at this measurement location in the vertical (blue), perpendicular (red) and parallel (green) directions relative to the pump discharge flange.

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