General Requirements of Temperature Instruments in Process Industry

This article is about General Requirements of Temperature Instruments, Thermocouples Design Requirements, RTD Design Requirements, Thermowells Design Requirements, Application Considerations, Temperature Transmitters, Temperature Recorders, Temperature Measurement – Low Temperature and Cryogenic Service.

General Requirements of Temperature Instruments in Process Industry

Specifications for Temperature Instruments

  • Thermocouples Design Requirements

a. All thermocouples used in plants shall be ungrounded unless they are supplied as part of package units.

b. Type T thermocouples shall be used when working temperature is between -195ºC and 95ºC.
c. Type K thermocouples shall be used when working temperature is between 95ºC and 1090ºC.
d. Thermocouples shall be used for indication loop. Thermocouple with temperature transmitter shall also be used for control and shutdown loops if high temperature does not permit use of RTD.
e. Comments: Above requirement supercedes paragraph 5.4 of SABIC Engineering Standard R02-E01.
f. A single thermocouple signal shall not be connected to more than one device. To connect to more than one device, the signal shall first be converted to standard voltage or current signal.
g. Two separate temperature assemblies shall be provided when indication is also required for process control and shutdown.
h. The thermocouple signal transmission shall be shielded for minimum noise.
i. The thermocouple wiring shield shall be grounded at only one point.
j. Specifications for thermocouples shall be based on the following criteria:

i. Copper – Constantan (CC)
CC couples shall be of the standard accuracy in accordance with ANSI MC 96.1
Type-T, grade special.
ii. Chromel -Alumel (CA)
CA couples shall be of the standard accuracy in accordance with ANSI MC 96.1
Type-K, grade special.
iii. Metal-Sheathed Mineral-Insulated Type
The material for the protective sheath shall be stainless steel 316, unless otherwise
specified, and the diameter of the sheath shall be a minimum of 6 mm.
iv. Temperature-emf (electromotive force) relationships shall conform to ANSI MC 96.l
v. Thermocouples for the measurement of furnace tube skin temperature shall be knife- edge type.

k. All thermocouples shall be of dual elements terminated in head.

  • Resistance Temperature Detectors RTD Design Requirements

a. RTDs with temperature transmitters shall be used for all control and shutdown loops unless high temperature application.
b. RTDs shall be of 100-ohm resistance (at 0ºC), and utilize the three-wire system. Material of RTD shall be platinum.
c. RTDs shall be metal-sheathed type.
d. The material for the protective sheath shall be stainless steel 316, unless otherwise  specified, and the diameter of the sheath shall be a minimum of 6-mm (1/4-inch).
e. RTD shall have 3-wire configuration.
f. RTD head shall be weather proof and made of aluminum.
g. All RTD shall be of dual elements terminated in head.

  •  Compensating Lead Wires and Extension Wires
    a. The compensating lead wires and extension wires shall conform to ANSI MC96.1. Thermocouple extension wire shall be installed in one-continuous length. If intermediate terminating points are required, as in case of multiconductor cables, then the connecting blocks shall be of the same material as the extension wire.
    b. Wire insulation shall be compatible with ambient temperatures. For ambient temperature up to 100ºC, polyvinyl is acceptable. Above 100ºC, non-asbestos insulation is required.
  • Other Requirements

a. The terminal head body and cover shall be of die-cast aluminum. The cover shall have female threads, be furnished with gasket, and be fastened to the body by a chain.
b. Multi-thermos elements shall not be used, except for multipoint temperature measurement of reactors or similar equipment, unless otherwise specified.
 Thermocouples and RTDs shall be furnished with terminal blocks enclosed in a corrosion resistant metallic weatherproof head having a female threaded, gasketed cover.
 Mineral insulated metal sheathed (normally 316 SS) thermocouples shall be used and shall be 1/4 inch OD For general service, the nominal thermocouple wire size shall be 1.3mm but within the temperature limits as recommended by the manufacturer.

  • Type K thermocouples in hydrogen service shall be furnished with titanium gettrs at the junction or along the length of the metal sheath for the purpose of absorbing trace amounts of oxygen, a potential cause of preferential oxidation in the presence of hydrogen.
  • Thermocouple calibrations shall conform to ISA MC96.1, Temperature Measurement Thermocouples, with cold junction reference at 0ºC (32ºF).

Thermowells Design Requirements

The following criteria shall be taken into account when selecting and specifying thermowells:

  • In general, all thermowells shall be flanged, unless fluid conditions (i.e. pressure, temperature, corrosion, etc.), piping specifications, equipment requirements, and/or natural frequency analysis results dictate otherwise. In general, threaded type thermowells will not be used, unless piping specifications does not permit flanged thermowells.
  • Thermowells shall be provided for all temperature sensing elements except surface temperature measurements and temperature measurements in low-pressure ducts where the element does not need protection from harmful atmospheres, corrosive fluids, or mechanical damage. 
  • Thermowel design as defined herein shall not be applicable to those included in the scope of furnace design. The design of such wells, and requisitioning and purchase of it, shall be by Furnace supplier. 
  • In general thermowell material shall be 316 SS. where the nature of the fluid is such as to require another material, it shall be suitable for the service.
  • All thermowells shall be drilled barstock (not built-up type) except pipe type wells for use in fired heaters. 
  • Flanged thermowells shall have flanges and weld filler of the same material as specified for the thermowell, and shall have full penetration welds. 
  • Test thermowells shall be furnished with threaded SS 316 plugs and chains. Where plant ambient conditions require another material, it shall be suitable for the service. 
  • The thermowells shall be hydrostatically tested to an internal pressure of 207 barg (3000 psig). 
  • Thermowells shall be suitable for stresses due to stream velocity conditions. The wake frequency (commonly referred to as Strouhal or the Von Karman Trail) shall not exceed 66 percent of the natural frequency of the thermowell in accordance with ASME PTC 19.3. In general Natural and wake frequency calculations shall be performed for all thermowells. 
  • Thermowells for dial thermometers and filled system instruments shall be purchased with the instruments to assure proper fit. 
  • Thermowells threaded connection shall be 1 inch or less and flanged connection shall be minimum 1 ½ inch. Pressure rating of flanges shall conform to the piping service or equipment specification. For water, air and small size pipelines of package units such as lube oil lines, 1-inch thread connection may be used. In general, minimum 2-in. connection shall be used for equipment (columns, vessels/drums, and tanks). 
  • Thermowell insertion length shall be selected such that the tip falls in the middle third of the pipe diameter, considering velocity rating of thermowell. 
  • For thermowell dimensions see attachment 1.

Thermistor Design Requirements

  • Thermistors with negative temperature coefficients shall be used only.
  • Thermistors shall be used where extreme accuracy is required.
  • The use of thermistors shall be limited to alarm applications.
  • Thermistors have low mass and limited heat dissipation capabilities. Therefore, manufacturer data shall be used in calculations of heat dissipation to ensure self heating does not occur.
  • Thermistors shall not be used for wide span measurement because the resistance versus temperature relationship is nonlinear.
  • The application of thermistors shall be at the discretion of Owner.

Filled System Instruments

1 Materials shall be corrosion resistant suitable for the environment.
2 Bulbs shall have bendable extension union connected with a pressure tight gland.
3 Thermometers shall be over range protected.

4 Vapor filled and mercury filled system is not acceptable.
5 Furnish thermometer with thermowell as specified in paragraph above.
6 The systems are self-contained; no auxiliary power is necessary.
7 The maximum distance from the sensing bulb to the readout device is quite limited when compared to thermocouple and resistance thermometer sensors. The maximum distance ranges between 15 to 60 meters, depending upon the thermal system class.

Bimetallic Thermometers

Requirements

a. Thermometer accuracy shall be within 1 percent of range.
b. Thermometer case diameter shall range from 1 to 6 inches with stem length from 2 ½  to 24 inches.
c. Thermometers shall be of industry standard lengths.
d. Outside diameter of protecting tube shall be ¼ inch.
e. Thermometer case shall be made of stainless steel with waterproof construction.
f. Thermometer head shall be hexagonal or wrench flat.
g. The installation of Bimetallic Thermometers shall conform to API RP 551.
h. Thermometers shall be purchased with thermowells as one assembly.

Application Considerations

a. Bimetallic Thermometer shall be used for local indications only.
b. The use shall be restricted to temperature ranges of –70 deg C (-100 deg F) to 500 deg  C (1000 deg F).
c. Hermetically sealed elements shall be used for temperatures below –30 deg C (-20 deg F).
d. The use of thermometers above 425 deg C (800 deg F) shall be limited to intermittent  duty.
e. Back connected or bottom connected models shall be specified depending on applications.
f. When subjected to vibration, silicon oil filled thermometers shall be used.

Temperature Transmitters

1 The transmitter shall be smart of the two-wire types.
2 The transmitter shall provide linearized output with respect to the input temperature.
3 The transmitter shall have electrical isolation between the input and output signals. This feature allows the use of grounded thermocouples.
4 Installation of temperature transmitters shall conform to API RP 551.

Temperature Recorders

  • Potentiometric servo-type recorders with a chart width of 150 mm or greater should be used.
  • One or two continuous and six, twelve or twenty-four multipoint recorders shall be used.
  • For multipoint recorders, the interval between prints shall be ten seconds or faster per point and pen traveling time for the full scale shall be five seconds or less.
  •  Cold junction compensation and upscale or downscale drive features on thermocouple burnout shall be provided to drive the instrument to safe condition as follows:

a. Control loop to the specified failure mode
b. Alarm loop to the specified alarm direction
c. Shutdown loop away from specified shutdown direction

  • A single thermowell with dual thermocouple shall be provided for checking of thermocouple actuated board mounted control, alarm, or interlock circuits.

Temperature Measurement – Low Temperature and Cryogenic Service

  • Generally for temperatures below 0 deg C, thermocouples shall be of the copper constantan type Resistance thermometers shall be used, for narrow spans, differential temperature measurement, and when high accuracy is required.
  • The use of local temperature indicators shall be minimized. Where used they shall be gas filled types.
  • Material of themowells shall normally be AISI 316 stainless steel Thermowells shall be welded in the piping for all cryogenic applications.
  • The fluid velocity limitations shall be calculated for all thermowells in cryogenic services.
  • The thermocouple heads shall extend at least 200 mm outside the insulation.
  • For skin temperature, measurements along the inner wall of cryogenic storage tanks, the elements shall be installed in a welded pad with suitable expansion loop between inner wall and connection head on the outer wall.

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