A false trip is when an instrument error, rather than an actual problem, sets off (“trips”) a safety alarm. All plant operators want to avoid false trips, especially the ones that lead to a full process shutdown – a very expensive scenario. WIKA’s SIL III certified compatible thermocouples maintains safety while greatly reducing the number of false trips.
Every once in a while, instruments malfunction and cause false trips. It’s like the boy who cried wolf, causing people to either slow down or stop their work in order to rectify the situation – only to find that there was no need for alarm.
In a refinery or petrochemical setting, errors in temperature measurement systems are expensive because they can cause temporary shutdowns. For example, shutting down a hydrocracking unit could mean up to a million dollars in losses each day. Even if the false trip is caught in time and doesn’t lead to a full shutdown, the associated labor and regulatory-compliance costs could reach five figures.
To minimize the expense and hassle of false trips, modern refineries and petrochemical production facilities rely on accurate, dependable instrumentation, such as the pressure and temperature measurement devices that WIKA makes, along with expert installation of monitoring systems.
False Trips Related to Thermocouple Failure
Even though a high-quality instrument with built-in redundancies can produce a false trip, it is exceedingly rare. On the other hand, devices with low manufacturing tolerances and few redundancies will obviously have a higher rate of false trips.
Historical records at petrochemical plants over the last few decades show that thermocouple failure is a known cause of false trips. While older thermocouple systems were reasonably robust and reliable, the latest temperature measurement solutions are much more reliable than their predecessors.
Extensive testing shows that WIKA’s SIL III certified compatible thermocouples have extremely low rates of failures and false trips. This high reliability, combined with accuracy and easy installation, is why WIKA multipoint thermocouple assemblies are currently in such great demand in the process sector today.
Downscale Break Failure Setting Is Generally Preferred to Upscale Break
When a thermocouple fails, it automatically sets off an alarm signal, typically set to make an “upscale break”. This means the temperature monitor will suddenly switch to “9999”, as if the temperature were so high as to be off the scale. Automated safety systems treat upscale breaks as highly urgent and very rapidly initiate safety procedures as a result – up to and including a unit shutdown.
Given that the loss of a single thermocouple point is not an emergency situation, WIKA suggests the best practice of setting the thermocouple failure alarm to “downscale break”. In this case, the temperature monitor will suddenly switch to “0000” if a thermocouple fails. Downscale breaks are still urgent situations requiring operators to investigate, but such breaks do not trigger automatic safety procedures like upscale breaks often do.
WIKA’s Gayesco Temperature Verification Service for Thermocouple Assemblies
After WIKA installs a Gayesco Flex-R® multipoint thermocouple system, every thermocouple point in the reactor vessel can be verified for functionality by a test heating to 700°F (371°C). If the client desires, this high-temperature verification test can involve a complete diagnostic check of each thermocouple, wiring, and transmitter, and confirms signal output to all desired locations.
WIKA USA also offers this same top-to-bottom post-installation multipoint verification service as a standalone option, usually undertaken during catalyst change-outs. The Gayesco Flex-R® is the only multipoint temperature measurement system that allows post-installation verification of thermocouple accuracy. Regular multipoint measurement systems verify thermocouples only while the reactor is in operation, which leads to an expensive trial-and-error startup process.
In addition, with the Gayesco Flex-R®, non-working hot junctions (thermocouple tips) can be repaired or swapped out with another point. This kind of flexibility allows older multipoint installations to have a substantially longer lifetime.
WIKA’s expert technical support staff are available to provide further details or answer any questions about the Gayesco Flex-R® multipoint thermocouple or the multipoint temperature verification service.
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