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Vibration Limits – Gas Turbines

If you operate turbomachinery you should be aware of the important vibration limits applicable to your machinery. There are a lot of standards for gas turbine vibration, and their specifications for measurement and evaluation are extensive and not easy to understand. However, a few essential guidelines can help you evaluate the vibration severity and, ultimately, enable you to decide on whether to investigate any faults.

 

Generally, a machine’s level of vibration is classified into one of four evaluation categories. These categories give you an indication of the severity of the vibration and the likelihood of damage occurring. These categories are as follows:

A

The vibration of machines newly comissioned normally falls in this zone

B

Acceptable for unrestricted long-term operation

C

Unsatisfactory for long-term operation

D

Sufficient severity to cause damage to the machine 

The exact vibration limits for each of these vibration evaluation zones are given below. The limits depend on the machine size, the speed of rotation and the type of vibration being measured.

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Examples:

  • 2 MW gas turbine with bearing vibration of 4 mm/srms would be classified in zone B (small turbine,  zone B is all values between 3.5 – 7.1 mm/srms)  and would therefore be considered “Acceptable for unrestricted long-term operation”

  • 30 MW gas turbine rotating at 1500 rpm with shaft vibration of 250 µmp-p would be classified in zone C (medium gas turbine 3 – 40 MW, zone C is all values between 9000 /√1500 and 13200 /√1500, which is equal to 232 µmp-p and 341 µmp-p) and would therefore be considered ”Unsatisfactory for long-term continuous operation”

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(1) General Notes: -

Bearing vibration is evaluated as r.m.s vibration in mm/srms only (also known as “overall” or “effective” vibration). The frequency range of measurement is 10 Hz to 1,000 Hz. The direction of measurement is radial, and the location of measurement is on bearing pedestals/housings. The transducers may be placed at any angular location, although usually one sensor each in the vertical and horizontal directions are preferred.

It is not common practice to measure axial vibration on main radial load-carrying bearings during continuous operational monitoring. However, in some cases the limits can be applied to axial vibration when measured on a thrust bearing with axial vibration correlating to the axial pulsations which could cause damage to the axial load-carrying surfaces.

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Shaft vibration is evaluated as peak-peak vibration in µmp-p only. If peak vibration in µmpk is measured by the operator then the shaft vibration limits displayed here must be divided by 2. The transducers must be located at or close to the bearings. The shaft vibration must be measured relative to the bearing housing. The machine must be operating under steady-state conditions at the rated speed (or within the specified speed range). The limits do not apply when the machine is undergoing a transient condition (i.e. changing speed or load).

 

(2) Small gas turbines: -   

These vibration limits are based on the standards ISO-10816-3 group 1 flexible foundation and ISO-7919-3.

The bearing vibration limits depend on the flexibility of the foundation. The limits shown are for a turbine supported by a foundation with spring supports i.e.  “flexible” support. If the turbine is mounted directly to a solid foundation without any spring supports then the “rigid” limits are to be used (Zone A up to 2.3 mm/s, Zone B up to 4.5 mm/s, Zone C up to 7.1 mm/s). (Technically, the decisive factor for the foundation is whether the lowest natural frequency of the combined machine and support system in the direction of measurement is higher than the main excitation frequency (this is in most cases the rotational frequency) by at least 25 %. If this is the case then the support system may be considered rigid in that direction. All other support systems may be considered flexible). The bearing vibration limits are also valid for axial vibration on thrust bearings.

The shaft vibration limits are valid only for turbines with fluid-film bearings (the limits are not valid for rolling-element bearings) and only for turbines with speeds between 1,000 rpm and 30,000 rpm.

(3) Medium gas turbines: -   

These vibration limits are based on the standard ISO-20816-4. Only applicable to gas turbines with fluid-film bearings, power outputs above 3 MW, and operating speed under load of between 3,000 rpm and 30,000 rpm. Aero-derivative gas turbines are not covered by these limits.

 

(4) Large gas turbines: -   

These vibration limits are based on the standard ISO-20816-2. Only applicable to gas turbines with fluid-film bearings, power outputs above 40 MW, and operating speeds of either 1,500 rpm or 3,000 rpm. Turbines with speeds of 1,800 rpm or 3,600 rpm are also covered by this standard but have different vibration limits (not displayed here due to simplicity). Turbines with any other operating speed are covered by ISO-20816-4 (see Medium Gas Turbines limits).