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Thermocouplesor TemperatureTransmittersin

Safety Systems?

I discussed the use of RTDs and thermocouples for temperature measurement

last week in a post, Temperature Measurement and Installation Practices.

Emersons Len Laskowski, a certified functional safety expert (CFSE), was

recently asked about the use of thermocouples in safety instrumented systems.

Len has shared his thoughts in a number of process safety-related posts.

The question was raised because of the number of temperature sensors required

and the difference in price between thermocouples and temperature transmitters

from a capital purchase point of view. The temperature measurements were in

a SIL 2reactor train application.

In some applications, a risk reduction factor (RRF) greater than 100 (SIL 2range) can be achieved assuming low demand for one-out-of-two (1oo2) voting

architecture of duplex thermocouples wired directly to a safety logic solver. This

could be done, if and only if, external comparison is done (preferably in the SIS)

to validate the temperature signal. However, in an Intech article,Handle with

care: Temperature sensors need know-how, author Bud Alder noted:

For a thermocouple, a short becomes a second measurement junction in parallel

with the original thermocouple. This type of fault is termed a nondiagnosed

dangerous failure. In safety-instrumented functions, a dangerous failure is a

measurement error in excess of 2% and is undetectable by diagnostics.

Thermocouple extension wire degrades over time, with an unpredictable

contribution to measurement drift. Yearly drift of 10F (5.5C) or more is

common.

Although use of thermocouples in safety instrumented systems are possible, Len

raised several points why this approach should not be taken. For his initial point,

he cited William Gobles and Harry Cheddies book, Safety Instrumented Systems

Verification, and their well-designed system concept:

A simplistic definition of such a system would be one where all techniques andmeasurements presented in our functional safety standards to prevent systematic

failures are followed.

SIL-rated temperature transmitterssuitable for SIL 2 applications are available

where a raw thermocouple is not SIL rated. Hence, there is a systematic

capability of SIL 0 assigned to this design. Len notes that good engineering

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practices for a basic process control system (BPCS) temperature control loop

would require a temperature transmitter on the loop if it were to be used for DCS

control. A process safety system should also follow this same logic. Reasons for

this include accuracy, signal robustness, diagnostics and reduction of induced

noise on the signal.

The duplex thermocouple in a 1oo2 voting arrangement also has the issue of a

spurious trip rate on the order of once every three years. Typical spurious trip

rate targets are on the order of 50 years or greater. The high number of

thermocouples required in the train of reactors makes the aggregate spurious trip

rate worse. The cost of even one spurious trip can dwarf the difference in capital

costs between thermocouple and temperature transmitter. The longer the

extension wiring and the more terminals involved the greater the potential for

inhomogeneous wires, terminals, terminal corrosion and stray pickup ofelectromagnetic field (EMF) interference that are difficult to detect (Instrument

Engineers Handbook Vol 1, Liptak).

Also from a lifecycle standpoint, the useful life of thermocouples needs to be

carefully examined and temperature elements need to be replaced routinely to

insure calibration, based on the established useful life span. For validation or

proof testing, testing these devices, pipe to pipe is recommended by the

standards. While a portable temperature bath can be used for short sensors, it is

not practical for thermocouple bundles. The verification process includes the

remote duplex sensors, connected cabling, and the I/O subsystem channels.

Len noted the diagnostics available in temperature transmitters. These

diagnostics can be used to degrade the voting schemes to increase overall reactor

availability in logic solvers that can access the HART temperature transmitter

values.

With these points, Len presents a strong case for the use of temperature

transmitters in safety instrumented systems.

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