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An Overview of Process Instrumentation

CM4110

Unit Operations Lab

October 2008

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Outline

The Evolution of Process Instrumentation Choosing the Right Instrument

– Temperature– Pressure– Flow– Level

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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

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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

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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

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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

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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

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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

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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?

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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

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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.