Vertical pump natural frequency 1

[DUISENM69]

We have three vertical pumps installed on cooling tower. Pumps are supplied by Flowserve. During commissioning we faced a problem with tripping on high vibration during start-up. We have measured on natural frequency on the casing of the pump and motor (bump test). It is 5.41 Hz whereas the running speed frequency is 7 Hz. Shouldn't the natural frequency be above the running speed frequency?

 

[electricpete]

Above or below is ok in my mind. Some people look for 20% separation from running speed to nearest resonance which I believe you have.

We have some vertical motors which we bump test while secured. In this configuration all of the water drains out of the casing. I have often wondered if the actual natural frequency during running will be lower than measured due to additional water mass present while running (missing during bump). In your case of measured bump test below natural frequency this is not a concern.

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[MikeHalloran]

I'd take a hard look at what, exactly, is vibrating, e.g. the cooling tower itself.

Structures comprising beams designed to "deflection as a percent of span" rules will have a natural frequency around 6 Hz.

A small stepping motor, attached rigidly at the pump mount, swept slowly through the frequency band from 4..8 Hz might provide some surprises. It doesn't even need an unbalanced load.





Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[MachineryWatch]

Above or below, it shouldn't matter, as long as the running speed frequency is outside the range of influence of the natural frequency. In fact, sometimes the appropriate fix for a vertical pump natural frequency is to install a softer mounting system such as rubber pads or Bellville Spring washers in order to lower the natural frequency away from the running speed.

Skip Hartman

http://www.machinerywatch.com

 

[Hatch]

If the natural frequency is far enough from the running speed it does not matter if it is above or below. However, if it is below, then you need to reset your vibration trip limits for the startup (and shutdown), since the unit is going to be passing through the natural frequency on startup. This is exactly what you are experiencing. Since you did not mention high vibration levels at running speed, I'll assume that it is ok.

In summary, set a higher trip limit for the startup, and then use the normal trip units once the unit is up to speed. If your system is not programmable, then press the "reset" button after startup, and use the normal trip limits.

 

[DUISENM69]

Yes, at running speed we do not have any vibration problem. We have implemented trip delay (10 sec) in order to avoid tripping of the pump during startup.
After conducting change-over test on the pumps when we started stand-by pump and stoped running one we experianced tripping of already running pump on high vibration when stand-by one starts. So I think not only natural frequency is contributing to this problem but may be the valve arrangement on the discharge of the pumps as the pumps work to common header through butterfly valves at each pump discharge (no check valve).