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6:00 am
May 1, 2007
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Field Balancing

0507_field_balancing1For sure. It’s not good for every situation. When it is and when it’s done right, downtime can be reduced and dollars can be saved.

Today’s business environment, with its emphasis on maximizing plant output and efficiency, demands that machinery downtime be minimized. Excessive vibration adversely impacts the health of the affected machine and can shorten repair intervals, thus increasing downtime. The increased vibration also can lead to catastrophic failure, posing a threat to people and surrounding equipment. Unbalance is one cause of this vibration.

Most people observe the effects of unbalance in common rotating machinery like washing machines and ceiling fans. In these situations, restoring the proper balance may be as simple as rearranging a load of clothes or cleaning fan blades. Reducing or eliminating the unbalance in rotating plant equipment is usually more complex, however. Still, it often can be accomplished on site with the proper tools and techniques.

Is balancing the answer?
Attempting to balance a machine that has other problems may lead to unpredictable, possibly catastrophic results. Before attempting to balance a machine in the field, you need to determine the following:

  • Is unbalance the cause of the high vibration?
  • Is an in-place balance going to be sufficient, or does the rotor need to be removed for shop balancing, rebuilding or replacement?

A thorough inspection of the machine should be conducted and other causes of increased vibration should be corrected before balancing is undertaken. If the other defects are not corrected, satisfactory balancing may not be possible, or the correction may be short-lived. Some of the most common causes of increased vibration include:

  • Machinery support problems such as soft foot
  • Defects of mechanical transmission, such as loose or broken couplings, gear or pulley wobbles or cracked gear teeth
  • Bearing defects, including increased bearing clearances, non-uniform lubrication or non-uniform wear of friction surfaces
  • Shaft-line issues, such as misalignment or rubs (As we’ll discuss later, it is important to ensure the machine has reached thermal stability before measuring the effects of balance corrections.)
  • Rotor problems (Increased vibration can be the result of a shaft crack, machine fouling, or a broken or loose rotating component [i.e. blades, vanes, buckets, etc.].)

The best solution for conditions like these is to correct the root cause, rather than attempting to compensate for their effects by balancing.

Balancing the machine
Correcting unbalance is conceptually easy—place a corrective weight on the rotor to counter the effect of an excessive radial mass. Accomplishing this in the field, however, is usually less straightforward.

There are several software packages available to assist in developing balance solutions. They are particularly useful in solving complex, multi-plane balance problems or when the machine needs to be balanced for multiple operating speeds. Solving complex balance problems, though, is best left to properly trained and experienced technicians and is beyond the scope of this article.

Preparation…
Before the actual balancing begins, you must understand the machine’s normal mode of operation. The relationship of the machine’s normal operating speed to its “critical speed” is crucial to understanding the machine’s response to the unbalanced condition. Since we cannot directly identify the rotor’s “heavy spot, ” we must infer its location by observing the point of maximum radial displacement, the “high spot.” If a machine operates below its critical speed—that is, if it doesn’t go through a resonance point while accelerating to normal operating speed—the heavy spot will coincide with the high spot. As the rotor accelerates through the critical speed, the heavy spot and the high spot will separate until they are about 180o out of phase.

Measurements during balancing runs should be taken after the machine has reached thermal stability and when other conditions, such as load or bearing conditions, are as close to normal operation as possible. This minimizes the influence of other sources of vibration, such as a temporarily bowed rotor.

If the machine is to be balanced based on measurements at a single operating speed, the speed must be held steady during the balancing runs as the centrifugal force caused by the unbalance is proportional to the square of the shaft speed. If the machine is equipped with sensors to measure the shaft speed and determine the phase and amplitude of vibration, it is better to collect measurements over a transient event, especially if the operating speed is above the machine’s critical speed. The collection of transient measurements allows for better analysis of the influence of weight changes on the machine, particularly near resonance. (This article focuses on the constant speed method since many 0507_maintenancelog2common smaller machines are not equipped with permanently installed vibration monitoring equipment.)

For machines that lack installed vibration measurement equipment, you will need to measure vibration and shaft speed. Use the measurement points specified by the equipment manufacturer. If not specified, use a point, such as a bearing end cap where the vibrations are transferred from the shaft to the machine casing. A phase reference also will be needed to determine the angular position of maximum displacement.

Before taking measurements, the trial weights and attachments should be prepared. Again, the manufacturer will usually specify the weight placements and may even prepare locations, such as balance holes or dovetailed slots, on the rotor to place balance weights. If the weight placements are not specified, then the weights should be attached on an accessible rotating surface.

0507_maintenancelog3

All weights should be held rigidly in place to prevent them from shifting or flying off during operation. Either of these events will invalidate a balance run. Moreover, if a weight flies off, it may present a hazard to people or nearby equipment.

If you have no reliable prior balancing data, the first trial weight should produce a centrifugal force not greater than 10% of the weight of the rotor. This limit is imposed because the addition of weight to an unfamiliar rotor may cause undesirable vibration during startup, particularly near resonance.

If there is more than one balance plane, a separate trial run must be used for each plane. The required correction weights are calculated independently for each plane. The verification run is performed after all the individual correction weights have been calculated and applied.

Determination of baseline condition…
With the machine at normal operating conditions, measure the vibration. Determine the amplitude and phase of maximum vibration and plot on a polar plot. Furthermore, record the condition of the machine—temperatures, rotor speed, process state, etc. All information recorded during the balancing procedure should be retained to facilitate future balancing procedures.

Calibration or trial run…
Install a calibration weight (Wcal). Record the weight and position. Bring the machine up to normal operating conditions. The more closely you can duplicate the conditions of the baseline run, the better. Measure and record the machine’s vibration response to the weight. Plot the resulting response vector on the polar graph.

Calculation of balance weight…
Calculation of the balance weight is done with vectors. Subtract the baseline response vector O from the trial run response vector TR. The resultant vector C is the response due solely to the calibration weight. The correction weight should be placed to produce a desired response vector – O that is equal in magnitude and opposite in direction from the baseline response.

The size of the correction weight (Wcor) is calculated by multiplying the size of the trial weight by the ratio of the amplitude of the baseline run vector to the trial weight vector. The correction weight is placed at an angle equal to the angular difference of the desired response vector and trial weight vector. If the trial weight was placed at a position other than 0Y, add this angle to the angular difference to determine the correction weight position. Apply the correction weight to the machine (see Fig. 1).

Verification and documentation…
Bring the machine up to normal operating conditions, then measure and record its response to the correction weight. Verify that the vibration has changed as expected and is within tolerance.

One correction may not produce the desired results. There often are small errors in vibration measurements, differences in the parameters (size and placement) of the actual correction weights compared to the calculated ones or other influencing forces. These small variances may require the addition of a trim weight.

The size and position of the trim weight is calculated in the same manner as the correction weight. Another validation run is required after the trim weight has been applied. (Keep in mind that zero or near-zero vibration is not the end goal as achieving this state would usually involve several iterations. In most cases, there are some manufacturer- recommended or ISO-recommended acceptable vibration levels.)

Once the final weights are placed, document the steps of the balancing process. This documentation can serve to reduce the number of runs in future balancing events. In typical balancing events, the majority of time is spent either securing the machine to make the required adjustments or bringing the machine back up to speed and reaching thermal stability and normal operating conditions. Eliminating just one run can shave hours or even days off the balancing job—which means the machine can be returned to profitable operation more quickly.

A quick case study
The importance of having good machine balance data cannot be overstated—even if it’s not for the machine in need of balancing. Consider the following situation involving a gas turbine at an integrated cogeneration plant that powers a combined refinery and chemical complex. Downtime costs in such facilities are enormous, often several million dollars a day.

One of the two gas turbines in this facility began to exhibit high vibration levels. Unbalance was determined to be the most likely source of the problem. Since this machine was fairly new, there was no previous balance information associated with it.

This machine needs a two-plane balancing. With no balancing history, however, a two-plane balance would normally require at least three runs—one trial run for each plane and the correction verification run.

Fortunately, machinery diagnostic engineers and rotor dynamic experts were able to use a database of similar machines and calculate the required correction weights. This allowed the machine to be balanced in just one run. The owner/operator noted that the savings in this case were “…well over a million dollars.” [Ref. 1]

References

  1. Y. Lee and W.C. Foiles, “One-Shot Balancing for a Gas Turbine,” ORBIT Magazine, pgs. 5-13, Vol. 25, No. 1, 2005

Patrick Hamilton is a licensed Professional Engineer with 20 years of experience in plant operational and maintenance management. Prior to joining GE Energy, he spent 12 years in the U.S. Navy, which included service as the Engineer Officer of a nuclear submarine. GE Energy’s optimization and control group includes the Bently Nevada™ Asset Condition Monitoring and Optimization and Control Services product lines. In addition to providing hardware and software solutions, the company has an MDS (Machinery Diagnostic Services) group to assist in solving complex machinery problems.


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