UFC 3 -430-11
14 February 2001
5-2.9.1 Thermowells. Thermowells are used to protect the temperature element
from the environment and for personnel protection. Thermowell design varies
depending on the application. Items affecting design include temperature, pressure,
type of fluid and fluid velocity. In general, thermowells can be classified into two types.
These are pressure service and non-pressure service. Thermowells used in non-
pressure service are commonly referred to as protective tubes.
Provide thermowells for all temperature elements in pressure service. Use
304 SS material as a minimum. Use the material best suited for the application of
protective tubes.
5-2.9.2 Thermocouples. In general, use the following thermocouples for the
different temperature ranges:
Type T, Copper constantan -- Below 17.77 to 371 degrees C (0 to 700
degrees F)
Type J, Iron constantan -- -17.77 to 593 degrees C (0 to1,100 degrees F)
Type K, Chromel alumel -- 315 to 1093 degrees C (600 to 2,000 degrees F)
Thermocouple a ssemblies can be single (one thermocouple) or duplex (two
thermocouples). Provide duplex thermocouples for all temperature control loops. Use
one thermocouple for control and the other for indication. Thermocouple and
thermocouple extension wire specifi cations are defined in ANSI/ISA-MC96.1,
and Thermocouple Wire.
5-2.9.3 Resistance-Temperature Detectors. Resistance-temperature detectors
(RTDs) are used where accurate temperature or temperature difference measurements
are required.
5-2.9.4 Filled Bulb Systems. Filled bulb systems are used for local temperature
indicators, recorders, and controllers in services where the fluid temperature is below
426 degrees C (800 degrees F). Mercury filled bulb systems are not allowed. Provide
armored capillary tubing for all filled bulb systems. Limit capillary length to 15.24 meters
(50 feet) maximum. In all cases provide a thermowell where the filled bulb system is
used in pressure ser vice.
5-2.9.5 Bimetallic Thermometers. Bimetallic thermometers are mostly dial type
thermometers used for local temperature indication. In all cases provide a thermowell
where the bimetallic thermometer is used in pressure service.
5-2.10
Pressure Measurements. Pressure instruments include gauges, switches,
and transmitters. In general pressure gauges use bourdon tubes to measure pressure
greater than 1.03 bar (15 psig), and bellows to measure pressure less than 1.03 bar (15
psig) or differential pressure. Transmitters and switches use diaphragms to measure
pressure. Use snubbers where pulsation dampening is required.
5-7
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