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Vibration Limits – Steam 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 steam 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:

  • For example, a 10 MW steam turbine with flexible foundation and 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”.

  • Alternatively, a 50 MW steam turbine rotating at 3000 rpm with shaft vibration of 200 µmp-p would be classified in zone C (large steam turbine 40+ MW, zone C is all values between 165 µmp-p and 240 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-to-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). The limits shown are for small steam turbines of any speed, and large turbines with speeds 1,500 rpm or 3,000 rpm. Turbines with speeds of 1,800 rpm or 3,600 rpm are also covered by the relevant standards but have different vibration limits and are not displayed here due to simplicity.

 

(2) Small steam turbines: - These vibration limits are based on the standards ISO-10816-3 group 1 and ISO-7919-3. The bearing vibration limits depend on the flexibility of the foundation, whereby either a “flexible” or a “rigid” support for the foundation is present. If the turbine is mounted directly to a solid foundation without any spring supports then the “rigid” limits are to be used. If spring supports are used then the “flexible” limits are typically used. Technically, the decisive factor for the foundation type 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) Large 1,500 rpm steam turbines: - These vibration limits are based on the standard ISO-20816-2. Only applicable to steam turbines with fluid-film bearings, power outputs above 40 MW, and operating speed under load of 1,500 rpm.

(4) Large 3,000 rpm steam turbines: - These vibration limits are based on the standard ISO-20816-2. Only applicable to steam turbines with fluid-film bearings, power outputs above 40 MW, and operating speed under load of 3,000 rpm.