BS ISO 7919-2:2001 pdf – Mechanical vibration — Evaluation of machine vibration by measurements on rotating shafts — Part 2: Land-based steam turbines and generators in excess of 50 MW with normal operating speeds of 1500 r/min, 1800 r/min, 3000 r/min and 3600 r/min..
4.2 Criterion I: Vibration magnitude 4.2.1 General This criterion is concerned with defining limits for shaft vibration magnitude consistent with acceptable dynamic loads on the bearings, adequate margins on the radial clearance envelope of the machine, and acceptable vibration transmission into the support structure and foundation. 4.2.2 Vibration magnitude at rated speed under normal steady-state operating conditions 18.104.22.168 General The maximum shaft vibration magnitude observed at each bearing is assessed against four evaluation zones established from international experience. 22.214.171.124 Evaluation zones The following evaluation zones are defined to permit a qualitative assessment of the shaft vibration of a given machine and to provide guidelines on possible actions. Zone A: The vibration of newly commissioned machines would normally fall within this zone. Zone B: Machines with vibration within this zone are normally considered acceptable for unrestricted long-term operation. Zone C: Machines with vibration within this zone are normally considered unsatisfactory for long-term continuous operation. Generally, the machine may be operated for a limited period in this condition until a suitable opportunity arises for remedial action. Zone D: Vibration values within this zone are normally considered to be of sufficient severity to cause damage to the machine. NOTE The evaluation zones defined above are relevant to normal steady-state operation at rated speed. Subclause 4.2.4 provides guidelines for transient operation.
126.96.36.199 Evaluation zone boundaries Recommended values for the zone boundaries are given in annex A for both relative shaft vibration and absolute shaft vibration. These values, which are based on present accumulated experience of shaft vibration measurements in this field, are not intended to serve as acceptance specifications, which shall be subject to agreement between the machine manufacturer and customer. However, they provide guidelines for ensuring that gross deficiencies or unrealistic requirements are avoided. In most cases the values given in Tables A.1 and A.2 are consistent with ensuring satisfactory operation. However, in certain cases, specific features or available experience associated with a particular machine type may require different zone boundaries to be used (lower or higher). Examples are as follows. a) Where bearings with small clearance are used, or for bearings with built-in preloads (e.g. generator to exciter) which reduce the minimum bearing clearance, the values given in Table A.1 might be greater than the available bearing clearance. In such cases the zone boundary values will need to be reduced. NOTE This applies only if the shaft relative vibration is measured on the pedestal close to the bearing. The degree to which the zone boundary values are to be reduced will vary, depending on the type of bearing used and the relationship between the measurement direction and the minimum clearance. It is therefore not possible to give precise recommendations but annex B provides a representative example for a plain cylindrical bearing. b) For comparatively lightly loaded bearings (e.g. exciter rotor steady bearings) or other more flexible bearings, other criteria based on the detailed machine design may be necessary.
4.2.3 Operational limits for steady-state operation 188.8.131.52 General For long-term steady-state operation, it is common practice to establish operational vibration limits. These limits take the form of ALARMS and TRIPS. ALARMS: To provide a warning that a defined value of vibration has been reached or a significant change has occurred, at which remedial action may be necessary. In general, if an ALARM situation occurs, operation can continue for a period whilst investigations are carried out to identify the reason for the change in vibration and define any remedial action. TRIPS: To specify the magnitude of vibration beyond which further operation of the machine may cause damage. If the TRIP value is exceeded, immediate action should be taken to reduce the vibration or the machine should be shut down. Different operational limits, reflecting differences in dynamic loading and support stiffness, may be specified for different measurement positions and directions. 184.108.40.206 Setting of ALARMS The ALARM values may vary for individual machines. The values chosen will normally be set relative to a baseline value determined from experience for the measurement position or direction for that particular machine. It is recommended that the ALARM value be set higher than the baseline by an amount equal to of the zone boundary B/C. If the baseline is low, the ALARM may be below zone C.