We were requested to witness field balancing on a primary air fan being performed by third party. They were using handheld vibration data collector. Our engineer installed temporary vibration sensors at both drive end (DE) and non-drive end (NDE) bearings of the fan plus a phase marker sensor and used multiple channels portable vibration analyzer for data collection. Once the unit was started, rubbing was clearly identified in just a few minutes and informed operator to stop the unit immediately. As a consequence, it was recommended to inspect both bearings for rubbing evidence. As suspected, severe rubbing was identified during inspection as shown above. The bearings were then replaced and then we were requested to take over the mission. The work progress gone smoothly with final vibration less than 0.7 mm/s, rms (original vibration at full speed was 3.5 mm/s, rms) after correction run. This resulted about 200,000 USD lost mainly bearing cost and associated manpower.
Background:
The unit was overhauled during plant outage with general inspection and fan cleaning. Low-speed shop balancing was performed before the rotor was installed back to the unit. Initial vibration amplitude was 3.5 mm/s, rms while acceptable vibration limit is 1.0 mm/s, rms (agreement prior to overhaul commencement). Therefore, field balancing was required and third party was hired by machine inspection team. However, outage schedule is very tight and acceptable field balancing result did not seem to arrive since final vibration amplitude was increasing to more than 10.0 mm/s, rms after fourth run. End user started to worry about plant startup schedule and engaged our services to oversee the whole work process as a consultant.
Lesson learned:
Field balancing is generally simple and straightforward, but it should be carried out properly with correct work steps/procedure. There are some key notes about this case below.
-selecting contractor both shop and field balancing is important as their capability and qualification should be taken into consideration and evaluation. Shop balancing work should be witnessed by competent person and balancing machine calibration certificate should be checked and reviewed.
-proper vibration measurement is required with the right tools, transient and steady state vibration data is required in most cases so handheld portable vibration data collector might not be sufficient as transient data could not be evaluated properly unless some systems can do so. For example, rotor bow/slow roll data, heavy spot, and critical speed are not available.
-correct diagnostic is a key to ensure if the root cause is unbalance, not other malfunctions which can produce similar dynamic vibration characteristics, for example, 1X vibration can be generated from other problems as well such as misalignment, rubbing, looseness, rotor crack, etc.
-rotor heavy spot (or residual unbalance angle) should be identified correctly, hence, calibration weight can be installed properly to get safe vibration response during calibration/trial run. This can prevent possible risk from excessive vibration amplitude and consequence damages. If there is no previous balancing record on the same unit, ten percent rule might be used as a starting point for amount of calibration weight. Amount of calibration weight is installed to produce centrifugal force equal to approximately 10% of rotor weight. be careful about the unit!
We hope this helps so mistake in field balancing can be prevented.
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