Choose the right instrumentation
Instrumentation used to record test measurements should be selected based on a pre-test uncertainty analysis (see "Understanding test uncertainty"). This analysis is important to fine-tune the instrumentation to ensure that the quality of the test meets expectations. The test instruments themselves are usually a combination of temporary units installed specifically for testing, permanently installed plant instrumentation, and utility instrumentation (billing or revenue metering). Temporary instruments are typically installed to make key measurements that have a significant impact on results and where higher accuracy is needed to reduce the uncertainty of test results. Among the advantages of using a piece of temporary instrumentation is that it has been calibrated specifically for the performance test in question following National Institute of Standards and Technology (NIST) procedures.
Another benefit of installing temporary instrumentation is to verify the readings of permanent plant instruments. Plant instrumentation typically lacks NIST-traceable calibration or has been calibrated by technicians who are more concerned with operability than with accurate performance testing. There's a good reason for the former: Performing a code-level calibration on plant instrumentation can be more expensive than installing temporary test instrumentation. An additional benefit of a complete temporary test instrumentation setup is that the instrumentation, signal conditioning equipment, and data acquisition system are often calibrated as a complete loop, as is recommended in PTC-46 (Overall Plant Performance).
All performance instruments should be installed correctly, and any digital readings should be routed to a central location. Choosing a good performance data center is very important. A performance command center should be out of the way of site operations yet close enough to observe plant instrumentation input and operation.
Obviously, performance instrument readings should be checked against those of plant instruments, where available. This is one of the most important checks that can be made prior to a performance test. When a performance tester can get the same result from two different instruments that were installed to independent test ports and calibrated separately, there's a good chance the measurement is accurate. If there's a difference between the readings that is close to or exceeds instrument error, something is likely to be amiss.
Typically, when plant guarantees are tied to corrected output and heat rate, the two most important instrument readings are measured power and fuel flow. If either is wrong, the test results will be wrong. For example, say you're testing a unit whose expected output is 460 MW. The plant instrument is accurate to within 1%, and the test instrument is even more accurate: +/–0.3%. In this case, the tester prefers to see the two readings well within 1% of each other (4.6 MW) but they still may be as far apart as 5.98 MW (1.3%) and technically be within the instruments' uncertainty.
When setting up for a performance test, it is not uncommon to find errors in permanent plant instrumentation, control logic, or equipment installation. These errors can influence the operation of a generating unit, for example by causing over- or underfiring of a boiler or gas turbine and significantly impacting the unit's output and heat rate. In cases where the impact on actual operation continues undetected, the corrected test report values may still be in error due to corrections made based on faulty instrument readings. If these reported values are used as the basis of facility dispatch, a small error could have an enormous impact on the plant's bottom line, ranging from erroneous fuel nominations to the inability to meet a capacity commitment.
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