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Explore the evolution and advancements in process instrumentation from the early technologies to modern developments. Learn about temperature, pressure, flow, and level measurements, along with selecting the right instruments for different variables.
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An Overview of Process Instrumentation CM4110 Unit Operations Lab October 2008
Outline • The Evolution of Process Instrumentation • Choosing the Right Instrument • Temperature • Pressure • Flow • Level
Background:Important Discoveries • 1592 – 1st thermometer • 1701 – first practical thermometer • late 1700’s – temperature is not a fluid! • 1821 – thermocouple effect • 1880 – first controller • 1885 – effect of temperature on conductivity • late 1800’s – metals have different thermal expansion effect Fisher Type 1 pump controller, 1880
Background:Several Early Technologies Optical Pyrometer – Color used to measure high Temp Bi-metallic Temperature measurement – connection to dial is similar to pressure gage Bourdon tube for Pressure or Temp measurement
Background:Beginning of Industrial Revolution to 1920’s • Temperature readings by a Thermometer or colorimetric method or Bimetallic Device • Pressure by Bourdon Tube gages • Level by Sight Glass • dP by Manometer • Pen Chart Recorders
Background:Need for Signal Transmission Arises 1930’s • Transmitters used to convert sensing device signal to pneumatic signal • Feedback controllers invented • Improvements in valve design • Valves fitted with pneumatic actuators Foxboro Flow Controller w/ 24-hr. Chart Recorder
Background:1960’s - Need Greater X-mission Distance • Control rooms w/ centralized control panels are common • Most process signals can be converted to low-level electric by transmitter • 4-20 mA current loop becomes standard for analog instruments
Background:More Recent Developments Industry recognized weaknesses of 4-20 mA devices • need continuous re-zero and re-range • transmits PV as a linearly scaled value only • Digital Instrumentation-1988 • Self-Calibration, Transmits PV in EU, Self-Diagnostics • Networked Instrumentation-1998 • Bus systems for process instrumentation • Wireless Transmitters-2004 • Self-Organizing Networks
Selecting the Right Instrument • What variable do I want to measure? • What accuracy and precision are required? • What are the process conditions? • How should the measured variable be displayed? • Does the measured variable have to be used by another device?
Local Temperature Measurement • Glass stem Thermometer • low cost, long life • local readout, difficult to read, no transmitter • -200 to 600ºF, 0.1ºF accuracy • Bi-metallic Thermometer • low cost • -80 to 800ºF, 1ºF accuracy
Local Temperature Measurement/ Control • Fluid-filled Thermal Elements • low cost, long life • -300 to 1000ºF, ±½% of full scale accuracy • low accuracy, great for some applications where tight control is not req’d • self-contained, self-powered control (can use fluid expansion to proportionally open control valve) • dial read-out for indication, can be remotely located
Local or Remote Temperature Measurement • Thermocouples • low cost sensor • needs transmitter/readout • -440 to 5000ºF, typically 1 to 2ºF accuracy • wide temperature range for various types • rugged, but degrades over time • many modern transmitters can handle T/C or RTD
Local or Remote Temperature Measurement • RTD’s • -300 to 1150ºF, 0.1ºF accuracy or better • more fragile, expensive than T/C • very stable over time • wide temperature range • also needs readout/transmitter
Pressure Measurement • Pressure Transmitters • available in gage pressure, absolute pressure and differential pressure • typically ±0.075% range accuracy • 50:1 turndown • same transmitter and sensor body as in dP flow measurement and dP level
Flow Measurement • Differential Pressure – Orifice Meter • well-characterized and predictable • causes permanent pressure (energy) loss in piping system, typically 8 ft. head loss (3 to 4 psi loss) • 5:1 rangeability • requires straight run of 20 pipe diameters upstream, 5 downstream • suitable for liquid, gas, and steam • accuracy is 1 to 2% of upper range
Flow Measurement • Turbine Flow Meter • accuracy is ±0.25% of rate • good for clean liquids, gases • 5 to 10 pipe diameters upstream/downstream • 10:1 turndown • 3 to 5 psig pressure drop
Flow Measurement • Magnetic Flow Meter (Mag Meter) • 0.4 to 40 ft/s, bidirectional • accurate to ±0.5% of rate • fluid must meet minimum electrical conductivity • head losses are insignificant • good for liquids and slurries • upstream/downstream piping does not effect reading • linear over a 10:1 turndown
Flow Measurement • Vortex Flow Meter • suitable for liquids, steam, and gases • must meet min. velocity spec • 0.5 to 20 ft/sec range for liquid • 5 to 250 ft/sec for gases • non-clogging design • not suitable if cavitation is a problem • accuracy is ±½% of rate • up to 5 psig head loss • linear over flow ranges of 20:1
Flow Measurement Coriolis Effect Mass Flow Meter • used for steam, liquids, gases • measure mass flow, density, temperature, volumetric flow • expensive, but 0.2% of rate accuracy • very stable over time • 100:1 turndown • negligible to up to 15 psig head loss
Level Measurement • Non-Contacting – Radar Level • suitable for liquids and solids • foaming, turbulence, vessel walls and internals can effect signal if not installed correctly • can use “stilling leg” if turbulence is extreme • typically ±0.1 inch accuracy
Level Measurement • Contacting – dP Level • suitable for liquids only • foaming and turbulence will effect signal • can use “stilling leg” if turbulence is extreme • typically ±0.05% range accuracy • 100:1 turndown • uses same dP transmitter as in dP flow measurement
References Miller, Richard W., Flow Measurement Engineering Handbook, 3rd Ed., McGraw-Hill, New York, 1996. Taylor Instrument Division, The Measurement of Process Variables, no date. www.emersonprocess.com/rosemount/, Rosemount, Inc., Oct. 2006. www.emersonprocess.com/micromotion/, Micro Motion, Inc., Oct. 2006. www.ametekusg.com/, Ametek, Inc. Oct. 2006.