FIELD INSTRUMENTS

A PART OF TRANSMISSION SYSTEM AND FOR MONITORING

Performance of control loop system comprise of ; Process Transmission Control Earlier, IT is part of monitoring and control Now, IT is upgraded Control loop performance and Leak detection modeling Future, May be part of Field Distributed Digital Control System (Transmitters + Field Bus + Digital Positioner + Optical communication)

FIELD INSTRUMENTSTypes Measurement and Transmission of Pressure, Level and DP ValuesTransmission of Flow valuesField CommunicatorI/P ConvertersSmart Positioner (part of valve manufacturer)Remote IndicatorStandardsAPI RP 551 (Process measurement Instrumentation)API RP 552 (Transmission system)API RP 550 (Manual on Installation of Refinery Instruments and control system)

DesignSENSOR TECHNOLOGYFew of sensor cellCapacitance (Emerson, Fuji)Inductance (ABB)Piezo resistive (Honeywell)Silicon Resonating (Yokogawa)Diaphragm Transducer(Preferable as per API 521)Feature ofOver range protectionDamping

SELECTION AND PROCUREMENT CRITERIAFlow Transmitters Used to measure Flow across Variable Head sensors like Orifice, Meter run, Venturi, Flow Nozzle, AnubarLevel Transmitters More Suitable for Hydraulic Resonance (used to dampen) and Agitation Due to boiling (with seal) More Faster than Displacer Used in Bubbler level measurementPressure Instrument Direct measurement

SELECTION AND PROCUREMENT CRITERIATemperature measurementHead Mounted and Remote mountedRemote mounted is preferable in view of Amb Temp effect, Vibration, Accessibility, GroundingCold Junction for T/CTemperature MultiplexerField CommunicatorFor Configuration, Display process variable, DiagnosticsUsing FSK principle HART (Highway Addressable Remote Transducer) I/P Converter

Sample Block diagramElectronic Module MemorySensor ModuleSensor Module MemoryTemperatureSensor SensorD/AA/DCommunications-Sensor Linearization-Rerange-Damping-Diagnostics-Engineering Units-Communications-Range Values-Transmitter configuration- Transmitters Performance Curve-Correction Coefficients-Module InformationDigital Communications-Bell 202 FSKSmart ElectronicsLocal Zero and SpanAdjustment

PerformanceAccuracyResponse Time (Dead time Td + Time constant Tc 63%)Static Pressure effectTurndownTPE

Performance MeasurementTotal probable error (TPE) analysis provides a more accurate picture of how a transmitter can be expected to perform under specific conditions or changes in conditions.TPE = [(Accuracy)2 + (Temp eff)2 + (Static Press eff)2}1/2Errors to consider - Ref Accuracy at installed conditions, St Pressure Span error, Temp Effect Worse Case Error is the sum of individual error components. This error is unlikely to occur, since all the effects may not take place at the same time.

AccessoriesDiaphragm sealingMechanical barrier from viscous, congealing, plugging, corrosive,Sealing liquid (DC704, Syltherm)

PurgingTo seal and sweep clean lines from solid bearing, cocking, solidification

ImpulseTo reduce temp, vibration, pulsation, Heat TracingFor High viscous

MountingClose coupled is preferable

Remote used for accessibility and avoid vibration

For High temperature, use purging, long impulse and remote seal

FutureFieldBus - Local controlling - Self Diagnostics - Line Plugging - CharacteristicsOutput signal Compliance to EN 61158-2

*Also may be Gas detectors, receiver instruments, Limit switches, SOV, *Capacitance : advantage Over pressure protection, simple, API preferrable. Disadv Impedance, OilResonance Direct to Frequency , No A/D conversion, Repeatability is better Dis adv - Over range protection, responsePiezo resistive Adv : Linearity, Rangeability, Dis adv - Over pressure, Temp effect, si supplierInductance adv Over pressure, Dis adv - Temp effect*Sensor module has its own electronics board to store the characterization constants of the sensor.Electronics store the xmtr configuration, tag, descriptor, damping, performs the D/a conversion, and HART communications.Analog-to-digital (A/D) converter - converts the current from the sensor into a digital word. Microprocessor - controls the operation of the transmitter. It also performs calculations for sensor linearazation, reranging, eng. units conversion, damping, square root, sensor trim, diagnostics, and digital communicationsMemory module - holds all configuration, and digital trim data that can be changed by the transmitter software.Digital-to-analog converter - changes the corrected digital signal from the microprocessor to a 4-20 mA analog signal that is applied to the output loop.Digital communications - HART - provides an interface between the transmitter and external devices, such as the Model 275.

*

Power supply effect may be includedExample:A transmitter is calibrated 0 to 100inH2O at 75 F with no static pressure. The transmitter will be used at a line pressure of 1500psi and in a location where the temperature may vary by 50 F.Accuracy -0.2% of spanTemperature effect - 1.0% span/100FStatic pressure effect - 0.25% of reading per 1000psi

Error allowed within accuracy limits is 0.2inH2O (0.2% of 100inH2O)Error contributed by a 50F temperature change yields an additional error of 0.5 inH2O (1% of 100inH2O x 50/100)Error contributed by a 1500psi line pressure yields another error of 0.37inH2O (0.25% of 100inH2O x 1500/1000)

WORSE CASE ERROR = (0.2+0.5+0.37)inH2O = 1.07inH2OThis error is much worse than the expected accuracy of 0.2inH2O. A worse case error (total error) assumes that the errors will all be at the maximum amount in the same direction. This unlikely to happen and would not represent typical performance of a transmitter.

TOTAL PROPABLE ERROR= (0.2+0.5+0.37) inH2O= 0.49inH2O*Output signal is in Physical Layer level of OSI compliance to EN 61158-2 at 31.25 kbps , For FF H1 protocol for communication and for Profibus as per DIN 50170 Part 2