Instrument Required Temperature Rating

[Chance17]

Given: A vessel mechanical design 500-psig at 650-F.
Question: Does the instrumentation temperature rating have to match 650-F ??

In real life, the maximum fluid temperature is 350-F.
I am inclined to add a 50-F margin and specify instruments at 400-F.
In this case the instrument is a level transmitter.

The Client questions the lower temperature specification but would rather not pay the extra cost for a higher rating.

What is correct ??

 

[JLSeagull]

You will hate this but the instrument temperature rating depends upon the installation techniques. If a pressure differential level transmitter is connected using impulse tubing then some temperature can be disipated between the process and the transmitter. However if this is a level transmitter with a flange and diaphragm directly connected to the process fluid than your real life maximum + 50 degree F seems rational. Even this depends upon the failure mode. For example, would 650 degree F kill the electronics or cause the body seal to fail?

 

[IRstuff]

And, are you responsible for the instrumentation surviving extreme excursions? While your process, under NORMAL conditions, may not exceed 350°F, what about failures and the like.

In miltary specifications, there is a normal operating temperature range, say, -35°C to +50°C, and there is a survival max temp of 71°C. So, you need to discuss with your customer what abnormal temperatures there might be, and what your instrumentation needs to do about that. If it fails after such an event, who pay for the replacement?

TTFN

FAQ731-376