In-field 1 & 2 Plane Balancing

It has been estimated that unbalance and misalignment account for more than 75% of vibration problems.   Finding a lot of customers were unable to access balancing services nor perform accurate alignment corrections, ISS provides an in-field balancing service. 

Does the Machine Really Need Balancing?

While unbalance is claimed to be one of the most common causes of problem vibration, we are often called in to balance machines that we find do not need balancing.  So there are a few questions to be answered and checks to be made before wasting time and money on specialist balancing services.  ISS will do these checks before we start any balancing runs anyway.  However, if we have travelled to the site, only to find the machine does not need balancing, we will have to charge you anyway to cover our time and travelling.  Perhaps save some money by doing some simple checks before calling us.

Why do we need to balance the machine, presumably it was correctly balanced when installed? So what has changed.  If it has just been repaired or painted then OK, balancing may be justified.  However, there are some details you can check before you call us.   For example if a cooling tower fan is vibrating because of what appears to be unbalance some quick checks should be made before starting a balance run.:-

  • Make sure the structural integrity of the fan is OK.
  • Are the blades clean?
  • Has their been any ingress of water into the structure (is the balance {shaft order vibration} varying with time or constant)?
  • Be aware that aerodynamics can cause shaft order vibration (blade lead, lag and lift all cause 1 per rev vibration) so check for aerodynamic change. You can actually balance out an aerodynamic force with an equal and opposite unbalance force, but this will work only at one speed and this does leave a stress in the rotating system.
  • Have any previous balance adjustment weights been lost.
  • Does the vibration increase exponentially with rpm.
  • For Boiler Induced draft fans with wet scrubbers the most common cause of unbalance is a build up of `carry-over' on the fan blades.  In some industries (eg. sugar) this carry over can be very hard to remove and requires very high pressure jet washing or even needle guns.

 

Ready to do a balance run.  What next?

The accuracy of dynamic in-field  balancing can be greatly reduced or even made impossible by influences beyond our control.  ISS has a set of rules we apply in each case to minimise these influences and avoid wasting our customers time:

Rule one.  Safety.  Obey all site safety regulations.  Attaching test weights to large rotating machinery can introduce the additional hazard of a weight detaching during the test runs, minimise these risk by using great caution in choosing mounting methods, we prefer to weld the test weights on.  Carry out a risk assessment for the operation. Use warning tape to rope off any potential hazard area. Be aware most ID fans will be classed as enclosed spaces. Ensure everybody involved applies their own safety tags and locks.  

Rule two. You may have been called in to rectify the balance of a machine.  However take the time to prove that balance is the real problem; a thorough vibration analysis  at the bearings of the rotor to be balanced takes very little time and it is important to remember, experience has shown that there is very rarely just one fault in a machine.  

Rule three. If the vibration data from the above analysis indicates any significant alignment errors then these should be corrected prior to balancing, preferably using a laser aligning system. 

Rule four Establish any resonance frequencies, it is all but impossible to balance a machine operating at or near to a resonance condition. Produce Bode plots for run up/down through normal running range to aid selection of balancing speed.  We like to balance with at least 2/3rd maximum operating speed.

Rule five. If  the fan is determined to be out of balance, find an acceptable reason for the unbalance before correcting erosion  rebuild, repair, painting etc.  One fan that came up for repeat balancing was found to be cracked and distorting (our records showed that over a two year period  our 2-plane balancing was adding further weight to previously corrected light spots).  

Rule six. By balancing a fan we are adding a stress into an already stressed rotor in an effort to unload the support bearings. If the fan is being balanced because of erosion/corrosion, we will also be adding the correction weights to the light (thin?) side. Always consider structural integrity.  The structural integrity of the rotor should be verified by the customer or an independent NDT inspection.

Rule seven. If you balance a rough and corroded machine it will run smooth, often giving the impression of good condition and perhaps a false sense of security. Make sure in your report you mention the poor condition and call for repair and/or crack detection if the slightest doubt exists.

Rule eight. Be aware. If possible before commencing a balance action listen to the trades people who `know' the fan history.  

Rule nine.  If you have time educate the customer staff around you.  You are playing with their machine.  Recognise and encourage their proprietary attachment to the machinery by explaining what you are doing and how.  Balancing is not rocket science, the basic principals are relatively simple.  Inevitably there is "waiting time" in the balancing procedure so use this time productively.  Most plants have some basic vibration analysis equipment and skills, if so explain how to do a three circle single plane balance without phase data.  This is unlikely to loose you any future business, but one day may get your customer out of trouble with an acceptable solution  fix adequate until a more accurate balance.

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