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Nuisance Process Trips Due to Nucleonic Detector
Nuisance process trip due to nucleonic level transmitter

We have a nucleonic density gauge level measurement system on a vessel which is measuring the bulk oil level in the vessel. The source (emitter) is Cesium CS-137 and the detector is scintillation type detector both (source and detector) mounted external to the vessel i.e. non-intrusive.

The way this technology works is if there is high level in vessel, then the detector (scintillation crystal) picks up low counts thus initiating HiHi Trip. If the vessel is empty then the detector (scintillation crystal) picks up high counts thus initiating a LoLo Trip. This is commonly used in Oil and Gas industry and is considered to be extremely reliable once setup/calibrated correctly during commissioning. In our case, we only have a LoLo trip on this vessel. There is no HiHi Trip. Our system has been in service for over 2 years now and generally has been reliable except following issue:

The problem we are having is we are having unexpected nuisance process trips from this device. Looking at the trends in ESD system, the level drops below the trip setpoint for around 10 seconds and automatically recovers to healthy state again. This 10 seconds drop in level is sufficient to cause ESD system to initiate process trip. We can confidently say that it is impossible for level to drop for 10 seconds in that vessel below LoLo trip setpoint. We have also cross checked it with the trends from Guided wave radar level transmitter which is used for control purposes. This on average has happened once every two months. There was no radiography happening at that time so it can't be radiography.

We have replaced all the components in the loop but the problem is not going away. Both nucleonic detector manufacturer and ESD System vendor have done extensive investigation at site but we still can't figure out what is causing this issue. Does anyone has experienced any issue similar to this in the past?

Hello at you,

Which output signal it's use with nucleonic density gauge? If suppose it's a on/off relay contact (non analogic), on the board in the nucleonic density gauge transmitter, because only the peak of counts it's change the state in the process application. I'm encourage you to check the output (relay, board, circuit, signal) and simulate a local fault. The ESD certainly just respond to the fail from transmitter, and it's correct!

It is a analogue input to ABB latest 800xA. The system has built-in diagnostic. Since it's SIL rated any failure is pretty much self revealing. System vendor was mobilised and they have done extensive checks to ensure there is no fault in the system but problem doesn't go away. I am pretty much having this issue on all nucleonic loops (we have around 8 loops) from different nucleonic detects on different vessels. And at one time only one detector behaves in this way. Then after couple of months it would be different detector behaving in this way.