Hydropower machines are now being operated at peak load, at increasingly higher loads, and under increasingly severe operating conditions. This has become possible, to a large extent, by the more efficient use of materials, although this has sometimes resulted in there being less of a margin for design and application errors.
At present, it is common for continuous operation to be expected and required for the 3 to 5 years between maintenance operations. Consequently, more restrictive requirements are being specified for operating vibration values for rotating machinery in order to ensure continued safe and reliable operation. There is, however, a wide range of different factors which need to be taken into account when specifying evaluation criteria for shaft or mechanical vibration measurements. Amongst these are the following: the bearing type, clearance, and diameter; the relative flexibility of the bearings, pedestals, and foundations; and the rotor mass and flexibility, stiff or heavy casing. This range of factors makes it difficult for standards to define unique evaluation criteria which can be applied universally to all machines.
ISO 10816 sets out general guidelines for the evaluation of mechanical vibration of machines by measuring the vibration response on non-rotating, structural members only. There are many types of machines, however, for which measurements on structural members, such as bearing housings, may not adequately characterize the running condition of the machine, although such measurements are useful. ISO 7919 sets out general guidelines for measuring and evaluating machinery vibration by means of measurements made directly on rotating shafts. This is for the purpose of determining shaft vibration with regard to changes in vibrational behavior, excessive kinetic load, and monitoring of radial clearances. The standards complement each other, and if the procedures of both standards are applied, the one which is more restrictive generally applies.
The disadvantage of only setting alarms based on a baseline when commissioning the unit, or from a statistical database, is that no consideration is taken of the loading of the bearings, brackets, or other structures. The load at a given displacement or static state will vary depending on the stiffness of the construction and the characteristics of the bearings. The results in this report propose that a dynamic vibration monitoring system could be installed in order to protect and monitor machines against unwanted high loads. The actual load would then be measured by the described methodology. By using a dynamic vibration monitoring system, the alarm and trip levels can be adjusted according to the design and production conditions, such as the operational mode, power output, rotational speed, temperature, etc. The upper limit for the alarm and trip levels is set according to the maximum permitted load without risking structural fatigue failure by a good margin. The alarm and trip levels for normal operation are set according to specified balancing demands and acceptable load levels on the bearings.
Alarm settings, bearing load, un-balance, displacement, rotor dynamics, theoretical assessment