temperature measurement

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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 CORRESPONDING TEMPERATURE

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. REQUIRED TEMPERATURE RANGE Give the minimum value of the temperature range, expressed in degrees Centigrade, over which meaningful temperature measurement is required. REQUIRED TEMPERATURE RANGE 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. CORRESPONDING TEMPERATURE 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. CORRESPONDING TEMPERATURE 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 VIBRATION INDUCED BY VORTEX SHEDDING A.1 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 is the kinematic viscosity of the fluid

V is the bulk velocity in the pipe D 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 STANDARD TEMPERATURE RANGE AND ACCURACY B.1 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)