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Instrumentation Accuracy and Performance Monitoring

How Instrumentation Can Effect Results

Tina L. Toburen, P.E.T2E3, Inc.

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Presentation Goal: Increase awareness of the effect of

small instrumentation errors on performance monitoring results

Increase understanding of the uncertainty bands surrounding key performance parameters

Improve the quality of performance monitoring programs at your facilities

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Instrumentation Accuracy and Performance Monitoring Why to Performance Monitoring Programs

Fail? No on site Champion for the system Lack of management support Operators lose confidence in the results

GARBAGE IN = GARBAGE OUT

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Instrumentation Accuracy and Performance Monitoring Improve Operators’ Confidence by:

Verifying performance calculations Compare results with expected, test and/or

design values Provide quick and easy access to results in

current value and trend formats Maintain the accuracy of results Train operators on what is a “significant”

change in results

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Instrumentation Accuracy and Performance Monitoring Improve Operators’ Confidence by:

Verifying performance calculations Compare results with expected, test and/or

design values Provide quick and easy access to results in

current value and trend formats Maintain the accuracy of results Train operators on what is a “significant”

change in results

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Temperature Measurement Thermistor

Range: 0-120 °F Accuracy: ±0.1 °C (best case)

Thermocouple Range: -250 – 4200 °F Accuracy (depends on type): From ±0.5°F at lower

temperatures to 1% or more at higher temperatures (1% of 4200 = 42°F)

RTD Range: 0 – 1750 °F (depends on type of RTD) Accuracy (depends on class): ±0.18 °F at low temps,

±8.0+ °F at higher temps Thermometer

Range: 0 – 120 °F (depending on equipment) Accuracy: ±0.1 °F and up (depending on equipment)

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Pressure Measurement Range and Accuracy depend on make, model

and calibration of equipment used. Differential Pressure

Gage Pressure

Absolute Pressure

Today’s transmitters can be 0.15% to 0.50% accuracy for their calibrated range

Note calibration range! this can have a significant impact on accuracy at measured values

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Fuel Flow Measurement Gas or Oil

Mass: Coriolis High accuracy, direct mass flow measurement

Differential: Orifice, Venturi, Nozzle High accuracy for code-level installations

Linear: Vortex, Turbine, Ultrasonic Lower accuracy and (potentially) lower cost Consider combining multiple measurements for

increased accuracy

Solid Fuels Difficult to measure accurately

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Natural Gas Fuel Flow Actual flow values (ACFM)

Measured Temperature and Pressure

Samples needed for constituent analysis or use of on site gas chromatograph

Tools for converting ACFM to SCFM and PPH

All Fuels – Also require a heating value to convert volume or mass flow to energy (MMBtu/hr)

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Electrical Output Power Metering

Revenue Watt-Hour Meters Revenue Class Potential Transformers (PT) and

Current Transformers (CT) Gross Versus Net Test Objective Unit Auxiliaries usually measured with lower

class (lower accuracy) PTs and CTs

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Instrumentation Accuracy and Performance Monitoring Accuracy – Instrument Calibrations:

Initial calibration of control instrumentation during startup

Annual calibration of control instrumentation as regular maintenance

Instrumentation not used for control often neglected from annual maintenance in order to save time and money

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Instrumentation Accuracy and Performance MonitoringThe Cost of Instrumentation Errors Control (Firing) Temperature

Reading lower than actual – leads to over-firing gas turbine Higher maintenance costs due to increased

degradation of hot gas path parts Reading higher than actual – leads to under-

firing of gas turbine Reduced available output due to fictional limit

placed on unit Reduced efficiency (higher heat rate) due to part

loading of gas turbine

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Instrumentation Accuracy and Performance Monitoring Barometric Pressure

Alters exhaust temperature control curve. Control curve x-axis (Pr) is a function of: Local station barometric pressure Compressor discharge pressure Gas turbine inlet filter losses

Pr = Pout / Pin Pout [psia] = CDP [psig] + Barom [psia] Pin [psia] = Barom [psia] – InLoss [psia]

Note: 1 inch H20 = 0.0361 psia

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Instrumentation Accuracy and Performance Monitoring

13.1

14.4315.88

1,050

1,075

1,100

1,125

1,150

1,175

1,200

1,225

1,250

12.0 12.5 13.0 13.5 14.0 14.5 15.0 15.5 16.0 16.5 17.0

Pressure Ratio (Pout/Pin)

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1200 F Isotherm

GE MS 7241 FA enhanced DLN

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Instrumentation Accuracy and Performance Monitoring Barometric Pressure

Reading low results in an higher Pout/Pin ratio, which will limit the gas turbine to a lower exhaust temperature, part-loading the unit.

Reading high results in a lower Pout/Pin, allowing the gas turbine to fire to a higher exhaust temperature, potentially over-firing the gas turbine.

0.2 psia (1.4%) error can result in a 15°F error in target GT exhaust temperature. >2% loss in output & 0.75% loss in efficiency

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Instrumentation Accuracy and Performance Monitoring What can the Performance Engineer Do

to optimize instrument accuracy? Identify all instrumentation used within the

performance monitoring program. Analysis, Reporting, Trending, etc.

Identify manufacturer’s stated accuracy for each instrument

Review calibration records, add instrumentation to maintenance schedules as needed

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Instrumentation Accuracy and Performance Monitoring Optimizing Instrument Accuracy (cont.)

Verify calibrations are correct for expected operating ranges

Determine Sensitivity of results to each measurement (an uncertainty analysis may be needed, see handout and ASME PTC 19-1)

Add and/or upgrade instrumentation if necessary

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Instrumentation Accuracy and Performance Monitoring Improve Operators’ Confidence by:

Verifying performance calculations Compare results with expected, test and/or

design values Provide quick and easy access to results in

current value and trend formats Maintain the accuracy of results Train operators on what is a “significant”

change in results

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Instrumentation Accuracy and Performance Monitoring Determining Significance of Changes in

Results Uncertainty Analysis Results as Tolerance

Band for Periodic Testing Systematic Uncertainty – Repeatable * Random Uncertainty – Non-repeatable **

* Repeatable errors cancel each other out when looking at changes over time

** Random errors are an indication of the effect of scatter in the data on the results

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HPSectionalEfficiencyUncertaintyAnalysis

n (A) (B) (AB/sqrt(n)) (AB)^2/n

MEASUREMENT PARAMETER (Design Value)

Number of Independent Measurement 

Points

Sensitivity (% PER %)

Systematic Uncertainty

Systematic Uncertainty Contribution

1. Throttle Pressure (2500 psia) 2 0.800 0.250% 0.141% 2.00E‐06

2. Throttle Temperature (1050F) 2 2.600 0.190% 0.350% 1.23E‐05

3. Exhaust Pressure (550 psia) 2 0.700 0.500% 0.247% 6.13E‐06

4. Exhaust Temperature (645F) 2 2.000 0.465% 0.658% 4.33E‐05

Total HP Section Efficiency Uncertainty 0.798%

Uncertaintytot = SQRT [(ABtot)2] (note: random compenent set to zero)

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HPSectionalEfficiencyUncertaintyAnalysis(adjusted)

n (A) (B) (AB/sqrt(n)) (AB)^2/n

MEASUREMENT PARAMETER (Design Value)

Number of Independent Measurement 

Points

Sensitivity (% PER %)

Systematic Uncertainty

Systematic Uncertainty Contribution

1. Throttle Pressure (2500 psia) 4 0.800 0.250% 0.100% 1.00E‐06

2. Throttle Temperature (1050F) 4 2.600 0.190% 0.248% 6.13E‐06

3. Exhaust Pressure (550 psia) 4 0.700 0.500% 0.175% 3.06E‐06

4. Exhaust Temperature (645F) 4 2.000 0.465% 0.465% 2.16E‐05

Total HP Section Efficiency Uncertainty 0.564%

Uncertaintytot = SQRT [(ABtot)2] (note: random compenent set to zero)

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HPSectionalEfficiencyUncertaintyAnalysis(includingRandomUnc)

fa (A) (B) (AB/sqrt(n)) (AB)^2/n (S) (AS/sqrt(n)) (AS)^2/n sqrt[(AB)^2+(2AS)^2]

MEASUREMENT PARAMETER (Design Value)

Number of Independent Measurement 

Points

Sensitivity (% PER %)

Systematic Uncertainty

Systematic Uncertainty Contribution

Standard Deviation of the Mean

Random Uncertainty Contribution

Total Uncertainty of Parameter

1. Throttle Pressure (2500 psia) 4 0.800 0.250% 0.100% 1.00E‐06 0.200% 0.080% 6.40E‐07 0.189%

2. Throttle Temperature (1050F) 4 2.600 0.190% 0.248% 6.13E‐06 0.143% 0.186% 3.45E‐06 0.446%

3. Exhaust Pressure (550 psia) 4 0.700 0.500% 0.175% 3.06E‐06 0.200% 0.070% 4.90E‐07 0.224%

4. Exhaust Temperature (645F) 4 2.000 0.465% 0.465% 2.16E‐05 0.310% 0.310% 9.61E‐06 0.775%

Total HP Section Efficiency Uncertainty 0.564% 0.377% 0.941%

Uncertaintytot = SQRT [(ABtot)2 + (2AStot)2]

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Presentation Goal: Increase awareness of the effect of

small instrumentation errors on performance monitoring results

Increase understanding of the uncertainty bands surrounding key performance parameters

Improve the quality of performance monitoring programs at your facilities

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Questions & Comments Tina Toburen

tinat@t2e3.com 206-227-2070 (mobile)