"As a rule, whatever is fluid, soft, and yielding will overcome whatever is rigid and hard. This is another paradox: what is soft is strong.” - Lao Tzu

In this PTOA Segment #103 PTOA Readers and Students will finish their assessment of the  benefits and limitations of fluid-filled temperature measuring systems and determine where these instruments work best in process industries.

GIMME A REAL-WORLD ASSOCIATION, PLEASE!

Throughout this PTOA Segment #103, PTOA Readers and Students should keep in mind that the components of a fluid-filled  temperature measuring system could be found connected together in an instrument cabinet like the one shown on the left.

When the cabinet is opened, the
components that measure and make it possible to indicate the process temperature are visible and accessible.

On a P&ID, the capillary that links the temperature measuring element (TE ... the bulb in this case) to the temperature indicator (TI ... the scale and pointer) would be a line of evenly spaced Xs.

 

This PTOA Segment 103 also features an important focus on bourdon tubes.

Bourdon tubes are a type of mechanical transducer commonly found in pneumatic instruments.

Transducers are instrumentation hardware components that change one form of energy into a different kind of energy.

In fluid-filled temperature measuring systems, helically-wound bourdon tubes are used to change a force created by thermal expansion into a mechanical movement.

 

RESPONSE LAG
OF FLUID-FILLED SYSTEMS CONTINUED

PTOA Readers and Students who are reading the PTOA Segments in the intended sequential order already know that a most of the measurement error associated with  fluid-filled systems derives from measurement response lag.

PTOA Segment #102 described how measurement response lag can be reduced with different styles of bulbs.

The style of bourdon tube can also impact temperature measurement response and accuracy.

Bourdon Tube Styles

The bourdon tube is THE DEVICE that changes  molecular movement of a heated fluid into a  mechanical movement.

Did you know that getting something that is just sitting around to move requires force?

Did you know that what we call "pressure" is just force applied over an area?

That tidbit of information will be thoroughly explored during the upcoming PTOA Process Variable Pressure Focus Study Area.

 

 

The C-Shaped Bourdon Tube is used to measure and indicate gas pressure.

Thanks to pumpfundamentals.com, PTOA Readers and Students learned in PTOA Segment #98 how the C-Shaped Bourdon Tube transduces a force from a process gas stream into a mechanical movement.

The animated graphic shows how the mechanical movement of the tip of the bourdon tube (right side) is translated into a dial-reading with the help of gears and linkages (left side).

The below photo is a blow up of a pressure indicator (PI).

Hopefully, the graphic shows how the C-Shaped Bourdon Tube and a temperature-compensated rotary gear would convert the mechanical movement made by the tip of the C-shape Bourdon Tube into the rotary movement of a pivoting pointer ... after the Instrument Tech reassembles the PI.

When a gadget is disconnected as shown above, it is kind of hard to tell that the process pressure being measured enters the hollow C-Shaped Bourdon Tube on the bottom, below the socket seal.

The relatively small amount of tip movement made by the C-Shaped Bourdon Tube is not sufficiently sensitive for other industrial applications that require the molecular movement to be transduced into a mechanical movement.

The Spiral-Type Bourdon Tube.

Pretend the C-Shaped Bourden Tube is straightened, flattened out a bit, made longer, and then rolled around the same center in wider and wider circles ...

Voila! The Spiral-Shaped Bourdon Tube shown at the left is created!

Note that this design does not need gears or linkages to convert the mechanical movement into a form that human beings can understand.

The expanding fluid in the capillary applies pressure to the Spiral Bourdon Tube tube...

The spiral tries to uncoil ...

which creates considerably more tip movement than a C-Shaped Bourdon Tube can manage.

The increase in sensitivity means the Spiral Bourdon Tube transduces molecular movement into a wider range of mechanical movement ... and no motion is lost to gears or linkages!

The temperature measurement made by a Spiral Bourdon Tube is more accurate than the measurement made by a C-Shaped Bourdon Tube.

Helical-Shaped Bourdon Tube is used to measure Temperature.

The Helical-Shaped Bourdon Tube is found in  many standard fluid-filled temperature measurement systems.

The Helical-Shaped Bourdon Tube is made by winding the multiple turns of a Spiral Bourdon Tube into a stack instead of around the same center.

There could be as few as two or three turns in the stacked coil or as many as twenty.

The helix shape creates the most mechanical movement per applied force and the increase in sensitivity translates into improved measurement accuracy.

 

RELIABILITY OF FLUID-FILLED SYSTEMS

PTOA Readers and Students learned in PTOA Segment #100 that functionality of the instrument refers to its range of operation, accuracy, and power requirements.

Low and High Measurement Ranges

As stated above, the effective temperature measuring range of fluid-filled systems is -185 °C to 650 °C (-301 °F to 1202 °F).

Process stream temperature measurements that cannot tolerate the inherent instrument error range of ± 4 °C to ±17 °C should use a different technology to measure temperature.

However, in some process applications, the ± 4 °C to ±17 °C range of inherent measurement error will not harm product quality.

Resistance to Breakdown

Fluid-filled temperature-measuring systems do not depend upon fancy electronics or technology.

The working fluid's relationship of volume expansion/contraction when the temperature increases/decreases is very predictable.

The noteworthy caution is the non-linear relationship of the volatile working fluids used in vapor-pressure fluid-filled systems.

When vapor-pressure systems are used for measuring temperature, the temperature range of the process stream being measured must stay within the linear vapor pressure/temperature range of the working fluid.

Physical Limitations/Considerations

The Averaging Bulb and Capillary Bulb described in PTOA Segment #102 cannot be used to measure the temperature of high velocity process streams.

Only the Plain Bulb style is sufficiently rugged to withstand high velocity process streams. Ergo, ruggedness is gained by sacrificing response time.

The capillaries that link the bulb to the bourdon tube are expensive, thin and fragile. They must be protected in durable, flexible wrap.

The length of a capillary that connects the TE to the TI is also limited.

Back in the olden days, the length limit of the capillary was around 400 feet and the response lag issues were significant.

Modern applications that use fluid-filled systems limit capillary length to 100 feet.

Longer capillaries reduce temperature accuracy below the point of usefulness; for this reason fluid-filled temperature systems are only used for local temperature indication, recording, and control.

Power Requirements

No electrical power requirements are needed for the bulb-capillary-bourdon tube to generate a readable temperature indication on a round, dial-type TI.

However, the investment of fluid-filled system measurement technology is often used to  simultaneously convert the changing position of the bourdon tube into a transmitted pneumatic (air pressure) signal.

PTOA Readers and Students read all about signal types in PTOA Segment # 14 entitled "I Just Gotta Get a Message to U."

That all-important conversion of a mechanical movement into a 3-15 psi transmitted pneumatic air signal makes it possible for the process stream's temperature measurement to be locally recorded on a strip chart and even locally controlled by the Outside Process Operator.

Working Environment

Pneumatic signals cannot generate a spark.

A fluid-filled system can be used to indicate, record, and control temperature without fear of generating a spark.

 

 

 

SUMMARY OF FLUID- FILLED SYSTEM LIMITATIONS

No matter what enhancements are designed into a fluid-filled temperature measurement system, the measurement response lag will always be a severe limitation to its application in process industry.

 

SUMMARY OF FLUID-FILLED SYSTEM BENEFITS

Fluid-filled temperature measuring systems can be installed in areas that prohibit any possibility of sparks.

At the time this PTOA Segment was written, liquified natural gas (LNG) production was increasing in the USA.  LNG facilities have production areas that require intrinsic safety because natural gas and sparks are an explosive combination.

 

GOOD USES FOR FLUID-FILLED TEMPERATURE MEASUREMENT SYSTEMS

Due to measurement lag, this technology will only be used for local temperature control, indication, and recording of process temperatures that are not expected to dynamically change from a specified range.

Fluid-filled temperature measurement systems will also be found in industrial areas that prohibit sparks yet still need a means to indicate, record, and even control the local process temperature.

TAKE HOME MESSAGES: PTOA Readers and Students determined the best industrial use of fluid-filled temperature measuring systems would be described by a need for:

• No-spark local temperature indication, recording, and control for
• Process streams that are not expected to have a dynamically changing temperature range.

A list of Bourdon Tubes styles with increasing sensitivity and accuracy is below:   

• C-shaped Bourdon Tube
• Spiral Bourdon Tube
• Helical Bourdon Tube

Bourdon Tubes are important, common transducers found in process industry instrumentation.

A transducer changes one form of energy into another. 

©2016 PTOA Segment 00103
PTOA Process Variable Temperature Focus Study Area
PTOA Process Industry Automation Focus Study Area

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