Rotor / Casing Position Monitoring
In order to obtain maximum operating efficiency, large turbines are typically designed to operate with very tight internal clearances; maintaining these clearances during turbine startups and run downs is absolutely vital and requires careful monitoring to avoid rubbing or rotor-to-stator contact, which will invariably wreck the turbine.
Perhaps surprisingly, large turbines are typically designed to operate with internal clearances of the order of 5-30 mils of radial clearance and perhaps 100 to 500 mils clearance axially, so when considering the size of a typical turbine/generator machine, this may seem to be incongruous. However, maintaining these clearances during turbine startups and run downs is vital and requires careful monitoring to avoid rubbing or rotor-to-stator contact which will invariably wreck the turbine.
During start-up when the turbines are thermally cycled from ambient temperature – in the region of 60°F – up to working temperature, which might be as high as 1000°F or more, they undergo significant dimensional changes, often expanding by as much as 1 to 1.5 inches axially. It is for this reason that axial clearances should be carefully monitored, and the machine should be protected against unexpected casing or differential expansions or thrust collar movements.
Typically, the axial clearances are monitored using non-contact sensors (eddy current probes) which are usually located close to and directly monitor the position of the shaft thrust collar relative to the thrust bearing. These values are typically small (less than 100 mils) and therefore, a standard eddy current probe is usually sufficient for the purpose. But moving firther away from the thrust bearing, the thermal expansion values increase significantly due to the large masses and long turbine shafts, and therefore either a different type of sensor is usually required or a mechanical ‘amplifier’ is used to increase the range of a short range eddy current probe. For larger expansions of stationary parts, a Linear Variable Differential Transformer (LVDT) can be used, or for rotating parts, ramps machined onto the rotating shaft or in some cases, cantilever ‘amplifiers’ are used to increase the range of a standard eddy probe.