Instrumentation - Temperature
August 22, 2002

Introduction Thermocouples Resistance Thermometry Thermowells         Thermowell Materials         Length         Installation

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Introduction

There are many types of temperature instruments that can be used to indicate or measure process temperatures; thermocouples, radiation and optical pyrometry, resistance thermometry, filled-system thermometer, liquid-is-glass thermometers, bimetal thermometers, pyrometric cones and temperature-sensitive crayons and paints.

Of these many methods, only three are typically used in acid plant design for temperature measurement; thermocouples, resistance thermometry and bimetal thermometers.

Thermocouples

When the two junctions of an electric circuit having two different materials as wires are at different temperatures, a potential will exist at the terminals in an open circuit.   If the circuit is closed, a current will flow.  The magnitude of the potential depends on the materials used and the temperature difference between the hot (or measuring) junction and the cold (or reference) junction.  This principle is used in thermocouples to measure temperature.

There are seven standard material combinations used in thermocouples.  Each combination is given a letter designation (i.e. type B, E, J, K, R, S, T).  Type S is the international standard for purposes of calibration and comparison.

Type J thermocouples will cover the majority of applications in sulphuric acid plants.  Higher temperature applications (i.e. sulphur furnace, regen. furnace) will use a type K thermocouple.

(a) Silicon, or aluminum and silicon may be present in combination with other elements.

Resistance Thermometry

All pure metals increase in electrical resistance with rise in temperature in a repeatable and predictable manner.  A resistance temperature detector (RTD) is simply a length of appropriate wire, suitable insulated, would on a form, adjusted to a definite resistance at a specified temperature and contained in a protecting tube.  A small electrical current passing through the detector produces and output signal proportional to the temperature.  Accuracy depends on a suitable power source and proper selection of precision bridge resistors which do not vary with temperature.

RTD's are generally used in measuring temperatures in rotating equipment (ie. bearings, electric motors, etc).

Bimetal Thermometers

When a composite metal strip consisting of two pieces of different metals or alloys welded together is heated, the differential expansion of the metals causes the strip to bend.  This principle is use in dial thermometers.  In sulphuric acid plants, bimetal thermometers will be used where only local display of temperature is required.

Thermowells

A thermowell is a pressure-tight, leak-free receptacle adapted to receive a temperature element and provided with external threads or other means for pressure-tight, leak-free attachment to a vessel.  The thermowell will also protect the temperature element from the process fluid which may be corrosive.

Thermowell Materials

A variety of materials are use for thermowells depending on the conditions of service.  The most common materials and their application to sulphuric acid plants are given in the following table.

Process Fluid	Material	Remarks
Sulphuric Acid (90-100% H2SO4)	Alloy 20
Oleum (0-30%)	304 or 316 SS
Weak Acid (0-15% H2SO4)	304 or 316 SS w/tantalum sheath Alloy C-276	Material selected depends on exact composition of weak acid including HF and HCl
Cooling Water	304 or 316 SS
Steam	304 or 316 SS
SO2 Gas, wet	304 or 316 SS w/tantalum sheath Alloy C-276	Material selected depends on exact composition of weak acid including HF and HCl
SO2 Gas, dry (< 650°C)	316 SS
SO2 Gas (> 650°C)	Ceramic

Length

In most cases, the thermocouple/thermowell should be reach to the mid-point of pipe, duct or vessel.  Shorter lengths may be used where the thermowell would be unsupported and there is a risk of it breaking or putting too much stress on the nozzle.

Installation

The installation of the thermowell will vary depending on the service and the material of the process line.