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Where to get a threshold value for permissible rotor unbalance (imbalance)?

Option 1:
Ask your OEM (Original Equipment Manufacturer a.k.a. Turbine manufacturer)!

  • The Value provided by the OEM should be the governing number and typically would overrule all thresholds determined or established by any other means. For many turbines it can be found in the “Turbine Type Certification” Document.
     

  • Admittedly it might not be easy to find or locate the number in such documents.

Option 2:
    If for whatever reasons OEM data are not available, you can calculate a sophisticated estimate of the unbalance threshold by using the operational data of the turbine and formulas provided in ISO 1940-1, with G16 for turbines with Gear box and G6.3 for direct drive turbines.

  • It could be shown in the past that a value provided by the Manufacturer, is close to or nearly matches the  ISO 1940-1 value limit and is therefore a suitable substitute.
     

  • For instance :
    1MW+ GB Turbine      OEM 250kg*m   vs    ISO1940-1 257.41 kg*m
    1MW+ GB Turbine      OEM 330kg*m   vs    ISO1940-1 353.60 kg*m
    2MW+ GB Turbine      OEM 650kg*m   vs    ISO1940-1 569.69 kg*m
    2MW+ GB Turbine      OEM 500kg*m   vs    ISO1940-1 592.04 kg*m
    1MW+ DD Trubine      OEM 60kg*m     vs    ISO1940-1 86.68 kg*m

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In general a value provided by the manufacturer would overrule any limit established by ISO 1940-1!!!
ISO 1940-1 is not a law or binding standard ! Turbine design features may allow for higher values.
Option 3:
This is the fall back worst case scenario. There are two reasons you might have to use Option (3):

  1. In advance to testing done by third parties, the reference value provided by the OEM, a particular division of the OEM, the OEM’s  successor or any entity in its place,  is obviously significantly higher than the value calculated with the help of ISO 1940-1. It should be concerning if the number exceeds more than twice the ISO 1940-1 number.
     

  2. Mass turbine testing for unbalance was performed without provision of a predetermined unbalance Value by the OEM, a particular division of the OEM, the OEM’s successor or any entity in its place. In this case it might happen that the OEM seemingly adjusts the unbalance threshold to declare the vast majority of turbines below a “now” presented threshold which exceeds a ISO 1940-1 value by more than the factor 2.
     

    • In this case it is necessary to look for the technical data for the next turbine part or component which will be directly affected by rotor unbalance. This part is the main bearing. ISO 1940-1 provides a formula on how to calculate the maximal permissible unbalance based on the maximal permissible radial force the bearing is specified for.
       

    • This value is typically higher than permissible imbalance since it also needs to compensate for load scenarios not directly related to mass unbalance.  It typically is in the range of 2 to 3 times the G16 Value. If the value provided for permissible unbalance is near or even above the number resulting from a calculation based on the  maximal permissible radial force the bearing is specified for than bearing failure should be expected and the number should be adamantly questioned.

Example of all three values for one turbine:
OEM 650kg*m       v.s.       ISO 1940-1 G16 569.69kg*m       v.s.       max. perm. radial force Calculation 1017.52 kg*m

In some cases the main bearings were specified to withstand 8 or 10 times the OEM provided permissible unbalance.

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