Temperature Measurement

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-OE-021

TEMPERATURE MEASUREMENT:
RESISTANCE ELEMENTS AND THERMOCOUPLES

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CONTENT

TEMPERATURE MEASUREMENT: CONSIDERATIONS
SPECIFICATION OF FUNCTION
DESCRIPTION OF FLUID
NORMAL OPERATING TEMPERATURE
REQUIRED TEMPERATURE RANGE
ALARM SETTINGS
TRIP SETTINGS
FLUID VELOCITY
REYNOLDS NUMBER
LINE SIZE
LINE REFERENCE
EQUIPMENT REFERENCE
NOZZLE SIZE
MINIMUM DESIGN PRESSURE
CORRESPONDING TEMPERATURE
MAXIMUM DESIGN PRESSURE

OTHER DESIGN INFORMATION
REQUIRED DESIGN INFORMATION

APPENDICES
A

VIBRATION INDUCED BY VORTEX SHEDDING

B

STANDARD TEMPERATURE RANGE AND ACCURACY

FIGURES
1 VORTEX SHEDDING
2 CURVE SHOWING RELATIONSHIP BETWEEN STROUHAL No. AND
REYNOLDS No


TABLES

1 PREFERRED TEMPERATURE RANGES
2 PREFERRED TEMPERATURE RANGES

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE


OPERATIONAL BEST PRACTICES:

TEMPERATURE MEASUREMENT

SPECIFICATION OF FUNCTION
The function is a brief description of the purpose of the temperature
measurement and is used as a descriptive title for it. The information may be
supplemented by an ELD grid location or Line Reference or Equipment No. to
identify the measurement location.
DESCRIPTION OF FLUID
The name of the fluid should be given together with its concentration or
composition where the latter are important from the point of view of corrosion or
safety. It should also be used to describe mixtures of fluids with unusual or vastly
differing chemical or physical properties.
NORMAL OPERATING TEMPERATURE
The normal operating temperature is the temperature at normal flowsheet
conditions, expressed in degrees Centigrade.
Give the minimum value of the temperature range, expressed in degrees
Centigrade, over which meaningful temperature measurement is required.
Give the maximum value of the temperature range, expressed in degrees
Centigrade, over which meaningful temperature measurement is required. Note
that the wider the range, the lower is the accuracy.
ALARM SETTINGS
The need for alarms is identified on the ELD and the required HIGH and/or LOW
alarm settings should be given. The default entry is ''not yet defined''. Alarm
settings should be reviewed during Hazard Study III and once approved at that
stage should not be altered without a formal review.


TRIP SETTINGS
The need for trips is identified on the ELD and the required HIGH and/or LOW
trip settings should be given.
Trip settings should be reviewed during Hazard Study III and once approved at
that stage should not be altered without a formal review.
Note that for safety trip systems the instrument may have an associated alarm.
FLUID VELOCITY
The fluid velocity past the thermowell is used to check for potential fatigue failure
of the thermowell through vibration induced by vortex shedding (see Appendix
A).
The gas velocity (or the gas velocity component of a two-phase mixture) should
always be provided.
On liquid service, where gas-liquid two-phase flow is unlikely to occur, only
velocities in excess of 2 m/s need be stated.

GBHE can provide simplified correlations of pocket length against velocity
for various types of pocket.

REYNOLDS NUMBER
The Reynolds number based on the pipe diameter should be provided where the
velocity is given.
When two-phase flow occurs, the Reynolds number should be calculated on the
basis of the bulk properties.
LINE SIZE
For pipe lines, the nominal pipe size should normally be taken from the ELD. The
units should be either NOMINAL mm or NOMINAL inches and not exact
conversions.


LINE REFERENCE
In the case of new plants, the Line Reference provides the key to the information
on line specification and material of construction requirements. In the case of
existing plants, the flange rating and material of construction should be specified.
The data is normally taken from the ELD.
EQUIPMENT REFERENCE
For plant items, the equipment number should be given.
NOZZLE SIZE
For plant items, the branch size specification and materials of construction
should be given. The units should be either NOMINAL mm or NOMINAL inches
and not exact conversions.
MINIMUM DESIGN PRESSURE
The design pressures determine the mechanical rating and should normally be
consistent with the conditions applied to the plant item or pipe.
The minimum design pressure refers to vacuum conditions, expressed in bar
GAUGE.
The temperature corresponding to the minimum design pressure is required for
considering mechanical integrity and materials of construction constraints.
MAXIMUM DESIGN PRESSURE
The design pressures determine the mechanical rating and should normally be
consistent with the conditions applied to the plant item or pipe.
The maximum design pressure is the highest pressure, in bar GAUGE, to which
the temperature measurement device is designed to withstand.
The temperature corresponding to the maximum design pressure is required for
considering mechanical integrity and materials of construction constraints.


OTHER DESIGN INFORMATION
If the Process engineer has any particular requirements for the temperature
measurement, they should be recorded.

REQUIRED DESIGN INFORMATION
If any special features affect the choice of the temperature measurement device,
they should be recorded.
The following is a list of topics which are likely to call for comment:
(a)

Brief description of any unusual or special factors affecting the selection of
data entered on the temperature measurement data sheet.

(b)

Any special reasons for the choice of type of temperature measurement
device.

(c)

Any special reasons for the choice of materials of construction.

(d)

Constraints on the location of the temperature measuring device to cater
for fluid mixing (e.g. 6 pipe diameters downstream of a mixing Tee), heat
losses and/or two-phase flow.

(e)

Any special requirements for the measurement of the temperature of
particulate solids in a conveyor or pipe.

(f)

Any other information considered relevant by the Process engineer.

There may be occasions when information other than that specifically requested
by the Process Engineer. The following list is intended to give examples and
indicate where it is especially important for the Process and Control & Electrical
engineers to liaise closely.
(1)

Accuracy of temperature measurement.

(2)

Fast response (less than 2 minutes) to a step change. Special
requirements of speed of response, e.g. safety shut-down systems,
require discussion.


(3)

Special requirements for sterile or hygienic operation.

(4)

Special fluid properties, e.g. suspended solids, scale formation/
solidification, high viscosity, etc.

(5)

Need for extended range.

(6)

Need to cater for start-up and/or pre-commissioning conditions, possibly
involving fluid of different composition and thermal properties.

(7)

Special requirements on measurement or transmitter failure mode.


APPENDIX A
A.1

VIBRATION INDUCED BY VORTEX SHEDDING

VORTEX SHEDDING

Vortex shedding occurs as a consequence of fluid flow past a bluff body or
cylindrical object such as a temperature pocket. Vortices detach from one side,
then the other, establishing a vortex ''street'' (see Figure 1). The frequency with
which this happens is called the ''shed frequency'' and is determined by the
'STROUHAL number (S)'.

The cylinder experiences periodic transverse forces due to the drag/pressure
forces generated by the vortices as they shed on alternate sides of the body.
When the ''shed frequency'' coincides with the natural frequency of the body, it
will begin to oscillate. The amplitude is limited by mechanical damping.


A.2

THEORY

The separation of the flow from the back of bluff bodies is a function of Reynolds
number:

where:

v
V
D

is the kinematic viscosity of the fluid
is the bulk velocity in the pipe
is the outside diameter of the temperature pocket

Figure 2 shows a typical relationship between the Strouhal number (S) and the
Reynolds number (Re).


The Strouhal number (S) is the proportionality constant between the predominant
frequency of vortex shedding (fs) and the free stream velocity (V) divided by the
temperature pocket outside diameter (D). fs is therefore determined from:

The temperature pocket is assumed to act as a uniform cantilevered beam, and
as a result its fundamental natural frequency is given by:

If fs > fn, then damage due to vortex shedding is likely and the thermopocket will
have to be redesigned. Control engineers will double the data sheet velocity as a
safety factor In assessing potential damage.


APPENDIX B
B.1

STANDARD TEMPERATURE RANGE AND ACCURACY

RESISTANCE ELEMENTS
Resistance elements to BS 1904.
Resistance bulb input 100 ohms at 0°C.
3-wire system.

The accuracy of the temperature measurement using resistance elements
typically is ± 0.75% of the temperature range.


B.2

THERMOCOUPLES
Electrically insulated base metal thermocouples Type K, J and T to BS
4937.

The accuracy of the temperature measurement using thermocouples is typically
± 1.0% of the temperature range.

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:

BRITISH STANDARDS
BS 1904
Specification for Industrial Platinum Resistance Thermometer
Sensors (referred to in Appendix B)
BS 4937
International Thermocouple Reference Tables
Part 3:
Iron/Copper - Nickel Thermocouples. Type J
Part 4:
Nickel Chromium/Nickel Aluminium Thermocouples.
Type K
Part 5:
Copper /Copper - Nickel Thermocouples. Type T (all
referred to in Appendix B)


Temperature Measurement

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