1 An Overview of Process Instrumentation CM4110 Unit Operations Lab October 2008

2 Outline n The Evolution of Process Instrumentation n Choosing the Right Instrument – Temperature – Pressure – Flow – Level

3 Background: Important Discoveries n 1592 – 1 st thermometer n 1701 – first practical thermometer n late 1700s – temperature is not a fluid! n 1821 – thermocouple effect n 1880 – first controller n 1885 – effect of temperature on conductivity n late 1800s – metals have different thermal expansion effect Fisher Type 1 pump controller, 1880

4 Background: Several Early Technologies Bi-metallic Temperature measurement – connection to dial is similar to pressure gage Optical Pyrometer – Color used to measure high Temp Bourdon tube for Pressure or Temp measurement

5 Background: Beginning of Industrial Revolution to 1920s n Temperature readings by a Thermometer or colorimetric method or Bimetallic Device n Pressure by Bourdon Tube gages n Level by Sight Glass n dP by Manometer n Pen Chart Recorders

6 Background: Need for Signal Transmission Arises 1930s n Transmitters used to convert sensing device signal to pneumatic signal n Feedback controllers invented n Improvements in valve design n Valves fitted with pneumatic actuators Foxboro Flow Controller w/ 24-hr. Chart Recorder

7 Background: 1960s - Need Greater X-mission Distance n Control rooms w/ centralized control panels are common n Most process signals can be converted to low- level electric by transmitter n 4-20 mA current loop becomes standard for analog instruments

8 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 n Digital Instrumentation-1988 – Self-Calibration, Transmits PV in EU, Self-Diagnostics n Networked Instrumentation-1998 – Bus systems for process instrumentation n Wireless Transmitters-2004 – Self-Organizing Networks

9 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?

10 Local Temperature Measurement Glass stem Thermometer low cost, long life local readout, difficult to read, no transmitter -200 to 600 º F, 0.1ºF accuracy Bi-metallic Thermometer low cost -80 to 800 º F, 1ºF accuracy

11 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 reqd self-contained, self-powered control (can use fluid expansion to proportionally open control valve) dial read-out for indication, can be remotely located

12 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

13 Local or Remote Temperature Measurement RTDs -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

14 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

15 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

16 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

17 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

18 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

19 Flow Measurement 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 Coriolis Effect Mass Flow Meter

20 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

21 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

22 References Miller, Richard W., Flow Measurement Engineering Handbook, 3 rd Ed., McGraw-Hill, New York, Taylor Instrument Division, The Measurement of Process Variables, no date. Rosemount, Inc., Oct Micro Motion, Inc., Oct Ametek, Inc. Oct