Damping

DAMPINGĀ D004

Any time you touch the rear view mirror in your automobile to stop it from vibrating you are applying vibration damping.

Damping is the physical touching of a component to arrest or interrupt vibration. This becomes necessary in mechanical seals if you are pumping a product with poor lubricating qualities such as a gas or hot water.

The seal faces tend to stick together and slip as the drive lugs engage the face or the metal bellows winds up and releases. This alternating “sticking and slipping” of the faces causes the seal components to vibrate.

Along with “slip stick” the seals and bearings are subject to other types of vibration that include:

Mechanical causes of vibration

  • Unbalanced rotating components. Damaged impellers and non-concentric shaft sleeves are common.
  • A bent or warped shaft.
  • Pump and driver misalignment.
  • Pipe strain. Either by design or as a result of thermal growth.
  • The mass of the pump base is too small.
  • Thermal growth of various components, especially shafts.
  • Rubbing parts.
  • Worn or loose bearings.
  • Loose hold down bolts.
  • Loose parts.
  • Product attaching to a rotating component.
  • Damaged parts.
  • There is not enough mass in the pump pedestal. If you weigh the base plate, pump and driver there should be a least five times that mass in the pump pedestal.
  • The pedestal is not wide enough. If you drop a vertical line from the center of the motor two lines radiating out thirty degrees from this centerline should pass through the base, not the sides of the pedestal.

Hydraulic causes of vibration

  • Operating off of the best efficiency point (B.E.P.) of the pump.
  • Vaporization cavitation.
  • Impeller vane running too close to the pump cutwater.
  • Internal recirculation
  • Air getting into the system through vortexing.
  • Turbulence in the system (non-laminar flow).
  • Water hammer.

Other causes of vibration.

  • Harmonic vibration from nearby equipment.
  • Operating the pump at a critical speed. Watch out for this problem in variable speed and pulley driven pumps.
  • Seal “slip stick” at the seal faces.
  • The product is vaporizing at the seal faces
  • A pump discharge recirculation line aimed at the seal faces.

In an O-ring mechanical seal the dynamic elastomer provides a damping effect. Metal bellows seals do not have a dynamic elastomer so they have to be provided with some method of providing damping

The most common solution is to let the rotating seal face holder bounce off the shaft or sleeve.

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Posted

  • On February 15, 2018