Until systems utilizing permanent instrumentation become installed or proven, specially-instrumented tests are likely to be needed. These should be run prior to a scheduled major overhaul, mid-cycle to the next scheduled overhaul, and as results and experience show. Cotton (1983) recommends enthalpy-drop efficiency (discussed later in this chapter) tests be run no less frequently than every six months, as well as prior to an overhaul outage.
A test after a major overhaul is required to reset the baseline, particularly if modifications have been made.
Test readings during a test run of an hour or so each are carefully made using test instruments, calibrated before and after use, with preferably two separate measurements at each critical point. Readings can be taken manually on older type instruments with good results, but modern data acquisition and processing systems are more productive once set up. IEC 60953-2 recommends readings over an hour at one minute intervals for flow and power, and three to five minutes for other variables, provided fluctuation is acceptable. A table is given of acceptable instrumentation and average uncertainties.
For special tests, wireless sensors can save on wiring from transducers to the data collection system.
The ASME Performance Test Code Committee has estimated the repeatability as one-half the overall instrument uncertainty (ASME PTC 19.1-2005).
Tests also give a field check of the condition and calibration of key service instruments.
Tip from Steam Turbine Performance and Condition Monitoring by Ray Beebe