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High-precision measurements prevent long-term losses

In a new power station, safety, environmental compatibility, availability and efficiency all hinge on how supplied components perform. As even minor shortfalls can cause considerable losses over a plant's service life, independent high-precision measurements to verify performance guarantees are indispensable, claims TÜV SÜD, which recently carried out extensive tests on the state-of-the-art Severn Power plant.

Ralf Szamer,TÜV SÜD Industrie Service, Germany

In practice, the significance of measuring the performance of components used in thermal power stations is frequently underestimated. Measurement accuracy considerably influences data quality, which is of paramount importance in performance tracking such as verification of plant profitability analyses. Measurement campaigns are generally aimed at: verifying contract performance guarantees; ensuring periodic inspection of the state of repair to identify time- and operation-related degradation of equipment and the resulting changes in performance guarantees; and obtaining a set of data which can be used in optimising the plant's mode of operation.

For new plants in particular, verification of contract performance guarantees is a major economic factor. Throughout the service life of a plant, performance losses even of less than 1 per cent can have enormous consequences. The contractually agreed efficiency, electricity output and other performance indicators are critical for the type and extent of performance tests. In addition to defining the national and international standards, directives and regulations in accordance to which performance testing will be conducted, the reference conditions of the respective plant or component must be established. Factors such as the state of repair of a plant, the characteristics of the fuel, the load cases to be examined and the environmental and ageing-related influences need to be documented in a contract.

Codes and standards

Essentially, performance tests for individual components are set forth in codes and standards. At an international level two different sets of codes and standards are used: the German DIN standards, which are successively transposed into European standards, and the US ASME (American Society of Mechanical Engineers) codes.

The standard or code applicable to acceptance testing should either be named in the delivery contracts of the plant or components or agreed in advance by all contracting parties. This is all the more important as the two sets of standards or codes indeed differ significantly in some key areas.

Table 1 summarises the key codes and standards for acceptance testing of power stations, listed by plant component. These sets of standards and codes are complemented and expanded by additional standards and codes governing measurement processes, such as temperature and performance measurements.

Guaranteed performance testing

All contracting parties must agree in advance on the precise terms and details of the performance tests. Specifically, the parties should draw up a test programme in advance regulating issues including the plant's state of repair observed during test runs, the consideration of ageing-related degradation, all necessary load cases, the evaluation of measurement results and their correction in line with reference conditions.

This test programme also includes a detailed presentation of the measurement instruments, systems and sensors and their calibration standards. Before the start of performance tests, the test programme must be approved by all contracting parties. This ensures a high level of acceptance for the performance tests, which in turn is of significant advantage whenever shortfalls are identified.

Plan for measuring locations and performance indicators

For acceptance testing of a gas turbine, a plan showing the locations where measurement instrumentation will be placed is necessary. Measurements cover the conditions of air intake (ambient pressure, relative humidity, temperature of the air), the conditions downstream of the compressor (pressure, temperature) and the turbine (pressure, temperature), the pressure loss at the air intake filters and the combustion chambers.

Further parameters determined include fuel quantity, fuel temperature and fuel pressure and the quantity, temperature and pressure of the injection water where appropriate. The key electrical performance guarantees that must be measured are the generator's voltage and current levels, its output wattage and power factor. In addition, intrinsic consumption and excitation losses are also measured.

This type of instrumentation enables the key performance indicators to be determined; in this context, exhaust mass flow and turbine inlet temperature can be calculated from mass and energy balances. In accordance with the relevant equations, the terminal output and the efficiency of the turbine can be calculated directly from the measured value. To calculate turbine power, the excitation loss must be deducted from the output measured. The efficiency of the turbine is defined as the ratio between turbine output and input fuel energy. Turbine inlet temperature, the mass flow at the compressor inlet and the exhaust mass flow are determined via the turbine's mass and energy balances.

Correction and accuracy

As the performance data of the turbine depend highly on the prevailing test conditions – particularly ambient conditions including temperature, relative humidity and barometric pressure – the measured values must be corrected for reference conditions to enable performance to be compared with the contract performance guarantees. Correction to reference conditions is generally effected on the basis of correction curves supplied by the turbine manufacturers, which therefore need to be an integrated element of the contract.

Severn Power combined-cycle power station in Wales underwent a comprehensive measurement campaign to validate the performance guarantees of its components Source: Siemens

As some of these correction curves include steep gradients, the measurement of these influencing factors must be highly accurate. The accuracy class of the measuring equipment used is critical for the quality of results such as that for terminal output. Examples of result tolerances for a 100 MW gas turbine are provided for two typical equipment accuracy classes (about 0.5 per cent for operational measurement instrument and 0.1 per cent for the high-precision equipment).

Measuring ambient conditions, in particular pressure and temperature and measurements of the pressure loss upstream of the compressor and downstream of the turbine, is equally important. Accuracy is ensured by decentralised high-precision measurement units with 22-bit A/D-converters which can deliver resolution of up to 120 nV. To avoid long analogue transmission paths that are vulnerable to electrical and electromagnetic interference, measurement units use a digital optical-fibre cable network with central recording unit, ensuring 100 per cent data security along the transmission path and avoiding electrical interference. This quality standard permits highly precise measurement and optimum determination of guaranteed turbine performance data.

Case study: Severn Power station

The Severn Power combined-cycle power station in Uskmouth, Wales, UK, is one example of a successful measurement campaign. On behalf of its client DONG Energy, Siemens Energy Sector erected a turnkey plant there. TÜV SÜD Industrie Service was commissioned to carry out comprehensive performance measurements.

The core of the plant consists of two single-shaft units, with the main components (gas turbines of the type SGT5-4000F, Heat Recovery Steam Generators by Cockerill Maintenance and Ingenierie – CMI, SST5-5000 steam turbines) arranged in a single line of shafting. With a total capacity of 834 MW and excellent efficiency of 58 per cent, the Severn Power power station supplies over 1.5 million households with clean energy.

The terms of reference defined in the run-up to the performance tests were to carry out precise measurement and correction of the performance guarantees of the main components, namely the gas turbine, waste heat boiler and steam turbine. The central challenges of the Severn Power power station included the high number of measurement locations where calibrated high-precision measurement instruments had to be placed within certain deadlines. This required painstaking preparation, logistics and practical implementation.

Implementation of The measurement campaign

The performance test of the overall plant was based on a test programme agreed between DONG Energy, Siemens and its sub-contractors including CMI. The waste-heat boiler installed by a sub-contractor was subject to separate provisions. Essential elements of the test programme were the detailed requirements expected of the high-precision measuring instruments, such as accuracy classes and calibration standards, a detailed plan of measurement locations which outlined the limit values of the individual components and the expected ranges of the measured variables of pressure, temperature, flow quantity, electricity output including power factor and grid frequency, and a description of the evaluation procedure.

To keep measurement uncertainty to a minimum, the types of sensors were selected for all measurement locations and calibrated in accordance with the defined standard within the scope of preparation. When setting up the instrumentation on site at the power station, the test engineers had to comply with relatively high safety, health and environmental protection standards. Numerous country-specific features also had to be taken into account. In a first test run, the settings of the gas turbine were optimised and the insulation of the turbine cycle checked to ensure exact determination of the inlet and outlet mass flows. In this context, the temporary measuring instrumentation installed underwent repeated quality assurance. The measurement data were also used to verify the plant's standard operational I & C.

Following alignment with the grid operator, the actual measurements were performed. The performance test experts also took fuel samples for analysis in an accredited testing laboratory at a later stage. After completion of the measurements, the experts carried out temporary and final data evaluations and summarised their results in a final report. As the measured performance, for example that of the gas turbine, is extensively influenced by ambient conditions including temperature, barometric pressure and relative humidity, it cannot always be compared 'one-to-one' with the contract performance guarantees. In this case, defined correction factors may have to be applied and the measured values corrected for reference conditions.

Contract performance guarantees validated

The components delivered at the Severn Power power station outperformed the required performance guarantees in all points. This was confirmed by comprehensive performance measurements carried out by TÜV SÜD. This approach brings security of supply, profitability and environmental compatibility into line. The measurement process, which included several instances of quality assurance, ensured maximum accuracy of the power station's performance-related data.

About the author

Ralf Szamer is head of the Measurement Department at TÜV SÜD Industrie Service.
Email: ralf.szamer@tuev-sued.de

In the forefront of the trend: combined-cycle power stations

With the expansion of the renewables sector, certain features including operational flexibility and the possibility of partial load operation are moving into the centre of the requirement profiles of power stations. As wind and solar power stations supply fluctuating amounts of energy, the startup and shutdown frequency of fossil power stations rises to ensure the stability of the grid. "Parking" of the power station in low-load operation is also part of these features. Possible consequences are increased wear and maintenance costs.

Combined-cycle power stations are highly efficient and boast ideal load cycle properties and fast startup and shutdown, making them a much-favoured alternative. In combined-cycle technology the waste gas from the gas turbine generates steam in an unfired boiler (waste heat boiler). This steam is then converted into electricity in the steam turbine. To assure the profitable operation and maximum service life of combined-cycle plants, the exploitation of all potential areas of optimisation and the optimum fine-tuning of plant components to each other is also of paramount importance in this case.

Expertise from TÜV SÜD

Third-party performance tests are aimed at verifying the contract performance guarantees of a power station and supplying starting-points for improving the plant's settings and thus mode of operation. In addition to acceptance testing of new plants, TÜV SÜD's test engineers also periodically check existing plants for degradation. Further services include noise and vibration measurements. The interdisciplinary approach enables highly accurate results when determining the state of repair and checking the success of turnaround, retrofit and modification measures.

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