4.1.4 Condition monitoring instrumentation
This defined list of instruments is used on a periodic basis to confirm the absence of long term trends which might lead to unplanned outages and to define activities for future outages. To this extent, some of the instrumentation in this category is of a diagnostic nature.
The defined subcategories are:
- Heat rate test intrumentation
- Machine level instrumentation
- Shaft vibration instrumentation
The purpose and scope of each of these subcategories is described below.
Heat rate test instrumentation
This comprises all the equipment necessary to conduct a thermal performance test on the plant as described in British Standard 752. Since many of the transducers have to be precalibrated, UK practice requires that the plant is built with provision for fitting the instruments when a test is required. Thus the manufacturer supplies pockets for insertion of thermocouples and pressure tapping points up to and including the primary isolating valve. Where possible, steam and condensate flows are measured by differential pressure devices and so the provision made on the plant as-built can again be confined to fixed orifices with suitable tapping points. The main condensate flow measurement is one of the most critical. Provision is generally made to insert the calibrated orifice plates in sections of condensate pipework parallel to the main path. When tests are to be performed, the main path is isolated and all the flow passes through the orifice plates. Provision is also made for the connection of a transmission-type dynamometer to measure the power supplied by the boiler feed pump turbine (if fitted).
In some plants, a more limited range of parameters is measured on a permanent basis to provide a routine on-line efficiency monitoring scheme, usually combined with boiler plant. Such schemes inevitably provide less-accurate information than would be obtained from plant acceptance tests, using recently calibrated transducers. The value of on-line monitoring is that it enables trends to be identified and appropriate maintenance work scheduled. Where alternative plant operating strategies are possible, for example, selection of a standby feed pump instead of a faulty duty pump, then the operator can take measures to improve the efficiency of the plant on-line. The scheme is implemented by a series of algorithms processed by a dedicated computing system and is capable of displaying trends in overall performance given by changes in heat rate, as well as the performance of individual components.
Machine level instrumentation
This comprises equipment for automatically measuring and recording changes in the relative levels of the turbine-generator bearing pedestals. Primarily used for fault diagnosis, this equipment is sometimes supplied built-in by the turbine manufacturer. An alternative approach is to provide facings and connection points on the pedestals so that portable equipment can be used to make the checks during maintenance outages.
Shaft vibration instrumentation
The scope of the measuring points required for this is more complex than the provision made for turbine supervisory purposes. At each bearing, two transducers measure vibration in the same plane but displaced at 90° to each other. This facilitates the recording of shaft vibration signatures on run-down and also orbit plots may be taken on-load so that a complete picture of changes in shaft dynamics can be formulated by an expert or an expert computing system. Such an analysis can lead to improvements in the prediction of maintenance routines.