An Overview of Process Instrumentation

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

2. Outline • The Evolution of Process Instrumentation • Choosing the Right Instrument • Temperature • Pressure • Flow • Level

3. 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

4. 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

5. 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

6. 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

7. 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

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 • 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

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 600ºF, 0.1ºF accuracy • Bi-metallic Thermometer • low cost • -80 to 800ºF, 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 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

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 • 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

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 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

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, 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.