New Methodologies for Instrument Set point and Loop Uncertainty Calculations

Transcription

1 New Methodologies for Instrument Set point and Loop Uncertainty Calculations NuPIC 01 mposium for Nuclear Power Plant Instrumentation and Control) Manik Singh (Instrumentation and control Engg. Dept) KEPCO E&C

2 Table of Contents 1. Introduction. Basic Terms and definition 3. Types of Uncertainties 4. Module and Total Channel Uncertainty Equations 5. Revision Points in New ISA-67.04, I&II standards 1

3 1.Introduction Why is Instrumentation set point and Uncertainty calculations needed anyways? - Uncertainty of measurement is the doubt that exists about the result of any measurement. We might think that even the best calibrated instruments would show the True reading of the process parameter that it measures. But in reality there are many small errors that are introduced in to the system due to environment conditions like temp, pressure, humidity, vibrations, ageing effect of the instrument, some design inaccuracies (PM errors) etc. - So in order to account for these errors we need to do uncertainty calculations, to determine near approximate set point (TS) which would trip the system before it reaches the Analytical limit. - To Present a method of calculating and documenting the alarm set point such that the analytical limit, as derived from plant design basis and safety limit, would not be exceeded.

4 .Basic Terms and Definitions Uncertainty: The amount to which an instrument channel's output is in doubt (or the allowance made for such doubt) due to possible errors, either random or systematic. The uncertainty is generally identified within a probability and confidence level. Instrument channel: An arrangement of components and modules as required to generate a single protective action signal when required by a plant condition. Module: Any assembly of interconnected components that constitutes an identifiable device, instrument, or piece of equipment. Safety limit: A limit on an important process variable that is necessary to reasonably protect the integrity of physical barriers that guard against uncontrolled release of radioactivity. 3

5 Basic Instrument Channel 4

6 .Basic Terms and Definitions Analytical limit (AL): Limit of a measured or calculated variable established by the safety analysis to ensure that a safety limit is not exceeded. Limiting safety system setting (LSSS): Limiting safety system settings for nuclear reactors are settings for automatic protective devices related to those variables having significant safety functions. The setting must be so chosen that automatic protective action will correct the abnormal situation before a safety limit is exceeded. It can be said that it s equal to LTSP (Because it s the least conservative value of Trip set point) As-left value: The condition in which a channel, or portion of a channel, is left after calibration or final set point device set point verification. As-found value: The condition in which a channel, or portion of a channel, is found after a period of operation and before recalibration (if necessary). 5

7 .Basic Terms and Definitions Limiting trip set point (LTSP): The limiting value for the nominal trip set point so that the trip or actuation will occur before the analytical limit (AL) is reached, regardless of the process or environmental conditions affecting the instrumentation. Nominal trip set point (NTSP): A predetermined value for actuation of a final set point device to initiate a protective action. Drift: A variation in sensor or instrument channel output that may occur between calibrations that cannot be related to changes in the process variable or environmental conditions. Random Errors: Random errors - arise from random fluctuations in the measurements. Random errors are reduced when the experiment is repeated many time and they get a mean value. Generally they are specified in the form of ± % of span or URL. 6

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9 .Basic Terms and Definitions Bias: An uncertainty component that are always present in the system and even with repeated experiments they do not change in magnitude and sign. Reference Accuracy: A number or quantity that defines a limit that errors will not exceed when a device is used under specified operating conditions. It mainly takes into account mainly four factors i.e, Linearity,Dead band,hysteresis, and Repeatability. 8

10 .Basic Terms and Definitions Linearity: Linearity is the deviation from the calibration curve of an instrument from a straight line relationship between 0% and 100% input. Generally this is the deviation in the output due to the changes in temperature and pressure of the environment. Dead band: Its simply the ability of the instrument to react to small process variable input changes. It is defined as the range of different input values over which there is no change in the output. Hysteresis: The non-coincidence between loading and unloading curves of instrument when it is measures process parameter in increasing and decreasing fashion is known as Hysteresis. Hysteresis can occur instruments which have electrical windings around magnetic core, due to magnetic hysteresis. Repeatability: It is the variability of the measurements obtained by one person while measuring the same item repeatedly. This is also known as inherent precision of measuring equipment. 9

11 Deadband &Hysteresis 10

12 3.Types of Uncertainties 11

13 Module Uncertainties Reference Accuracy (RA): Reference accuracy is the percentage of error associated with the instrument operating within designed constraints under reference conditions. Power supply variations (PS): Most electronic instruments exhibit a change in output because of variations in power supply voltage. A typical manufacturer's specifications may read: ± 0.01% span per volt variation Environmental Effects (EE): Most instruments exhibit a change in output when they are exposed to environment conditions different than what there were during calibration. Normal Environmental Conditions: It signifies the those conditions which are present during normal operating conditions of the plant. Accidental/DBE Environmental conditions: Conditions like high temperature, pressure, humidity, and radiation environment, along with the seismic response, which occur during an accident,must be included in the uncertainty calculation to determine their effect on Set point calculation. 1

14 Module Uncertainties Drift (DR): An assumption for the time intervals between recalibration for each instrument should be included in the set point uncertainty calculation. It can be calculated by ways: For example,for a PT with URL of 1000psig is calibrated to psig. And Vendor specified Drift is±0.5% URL for a six month period. Therefore Drift = [±(0.5%)(URL/SPAN)]x (18months/6 months) Drift=[±(0.5)(1000/500)]x (18months/6months) Drift=± 1.5% span Digital Signal Processing Err: 13

15 Calibration Uncertainties M&TE (Measuring and Test Equipment) Uncertainty: M&TE uncertainty includes the reference accuracy of the M&TE, the uncertainty associated with the calibration of the M&TE, and the readability of the M&TE by the technician. MTE = (RA MTE + RA STD + RD ) 1/ Where MTE = uncertainty of the M&TE RA MTE = reference accuracy of the M&TE RA STD = reference accuracy of the controlled standard RD = reading Uncertainty 14

16 Calibration Uncertainties For example, using the diagram of Figure 4, the sensor M&TE uncertainty (SMTE) may be calculated as follows: SMTE = [(MTE1) + (MTE) ] 1/ As Found/As left calibration values: The difference between the as-found data of the current calibration and the as-left data from the previous calibration represents the net effects of several uncertainties. Calibration corrections: Frequently, calibration procedures include corrections to account for the difference in instrument performance or readings between calibration conditions and normal operating conditions, such as static pressure and head pressure corrections. These effects are generally taken care of while calibration of the instrument. 15

17 Calibration Uncertainties Calibration Tolerance: Calibration tolerance is the acceptable parameter variation limits above or below the desired output for a given input standard associated with the calibration of the instrument channel. To give a better picture of the tolerances, as left (MALT) and as found (MAFT) values are calculated for the modules present in a particular channel. For (MALT) only those effects will be considered,which might be present during surveillance test, since the module is calibrated under controlled environment. Module as Left tolerance: (MALT)=± (RA + M&TE ) 1/ Module as found tolerance: (MAFT)=± (RA + M&TE + Drift ) 1/ Once the module tolerances are calculated then Channel as left and as found tolerances are calculated: (CALT)=± (MALT 1 + MALT + MALT 3 + MALT n ) 1/ (CAFT)=± (MAFT 1 + MAFT + MAFT 3 + MAFT n ) 1/ 16

18 4 Module Uncertainty Equation Module Uncertainty Equations(Normal & DBE Condition): e + = + [RA + DR + TE + RE + SE + HE + SP + MTE + PS ] 1/ + B + e - = [RA + DR + TE + RE + SE + HE + SP + MTE + PS ] 1/ B - Where e + RA DR TE RE SE HE SP MTE PS =Uncertainty of the Module =Module Reference accuracy =Drift of module over a specified period =Temp. Effects (Normal + DBE) =Radiation effects (Normal + DBE) =Seismic or vibration effect (Normal + DBE) =Humidity effect (Normal + DBE) =Static pressure effect (Normal + DBE) =Measurement and Test equipment effect =Power supply variations for the module 17

19 4 Total Channel Uncertainty Equation Total Channel Uncertainty (Normal & DBE condition): CU + N =+[PM +PE +e 1 + e +... e n ] 1/ + Bt + CU - N =- [PM +PE +e 1 + e +... e n ] 1/ - Bt - CU DBE + =+[PM +PE +e 1DBE + e DBE +... e ndbe ] 1/ + Bt + CU DBE - =- [PM +PE +e 1DBE + e DBE +... e ndbe ] 1/ - Bt - Where CU PM PE e n Bt + Bt - =Total Channel Uncertainty (Normal & DBE) =Process Measurement uncertainty =Primary element accuracy =Module uncertainty of each module in an Instrument channel (Normal +DBE) =Total of positive biases associated with an Instrument channel =Total of negative biases associated with an Instrument channel 18

20 5 Revision Points in New Edition S.NO ISA RP 67.04,Part-1,000 (Set points for Nuclear Safety related Instrumentation) 1 (TS)Trip set point-a predetermined value for actuation of a final set point device to initiate a protective action. TS=AL± (CU+Margin) (AV)Allowable value- A limiting value that the trip set point may have when tested periodically beyond which appropriate action shall be taken. AV= TS± CU(Normal conditions) ISA RP 67.04,Part-1,006 (Set points for Nuclear Safety related Instrumentation) (NTSP)Nominal trip set point-a predetermined value for actuation of a final set point device to initiate a protective action. NTSP=AL± (CU+Margin) (LTSP)Limiting Trip set point-the limiting value for Nominal trip set point so that the trip or actuation will occur before the AL is reached. LTSP=AL±CU Explanation Allowable value presents a limiting surveillance as found test value. The use of single value to verify that the channel met all requirements failed to identify any conditions where the channel was not performing within identified expectations. 19

21 5 Revision Points in New Edition, Part II S.NO ISA RP 67.04,Part-,1994 (Methodologies for the determination of Set points for Nuclear safety related Instrumentation) ISA RP 67.04,Part-,010 (Methodologies for the determination of Set points for Nuclear safety related Instrumentation) 1 There is no discussion about MALT MALT (Module as left tolerance)-to compute the acceptable as left values, consider only normal conditions or those module & channel errors which may be present during surveillance test. (MALT)=± (RA + M&TE)1/ There is no discussion about CALT CALT(Channel as left tolerance)- Its simply the SRSS of all modules as left tolerances: (CALT)=± (MALT 1 + MALT + MALT 3 + MALT n ) 1/ 3 There is no discussion about MAFT MAFT(Module as found tolerance)-since it includes the operation time of the module, therefore the formula includes the Drift value: (MAFT)=± (RA + M&TE + Drift)1/ 4 There is no discussion about CAFT CAFT (Channel as found tolerance): It s simply the SRSS of all modules as found tolerances. (CAFT)=± (MAFT 1 + MAFT + MAFT 3 + MAFT n ) 1/ 5 There is no discussion about the use of newly found drift value for new set of uncertainty calculations. 6 The instrument was deemed inoperable if the As found data was outside the Allowable value. Initially the vendor given drift value maybe used for channel uncertainty calculation. But after 18months of operation, we must use the new as found Drift value in uncertainty calculations. New set of uncertainty calculations must be done for a particular channel with new found Drift value, if the As found criteria are within the limits (CAFT). However if the Instrument shows the As found value outside the defined limit (CAFT), then the instrument is simply discarded. Therefore the original uncertainty calculations could be used. 0

22 Q&A 1

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