Nuisance Process Trips due to Nucleonic Detector (Density Gauge Level Measurement System) 1

[robot65]

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. The way these nucleonic detectors works, it appears to me that the detector (scintillation crystal) is picking up high counts radiation from somewhere else for 10 seconds and goes back to normal. 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?

 

[georgeverghese]

What is the nature of this "bulk oil" in terms of frothing or foaming at the vapor-liquid interface - how would this (top mounted?) LT respond if this were to happen ?

 

[hacksaw]

You might take a look at the possibility of RF interference with the electronics,

 

[itsmoked]

It also 'feels' like electrical noise is the problem to me too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[robot65]

Hi georgeverghese,

The source is side mounted and the receiver is mounted on the other side of the vessel and both are non-intrusive. The source CS-137 is continuously emitting radiation which are being picked up by the receiver on the other side of vessel. The no. of counts received at receiver end determines the actual level in the vessel (higher the counts, lower the level). There is no foam etc. The actual nuisance trip which we are getting comes quite unexpectedly (may be once every two months) and then after few seconds problem goes away.

Hi Hacksaw,
Again the problem we are getting (nuisance trip) where signal becomes abnormal for approximately 10 seconds. If there was actual RF interference I believe that would remain there for longer than that and more frequent trips...

 

[hacksaw]

The walkie-talkie 2-ways have been known to cause intermittent interference,

rare to be sure, so the question ultimately involves the wiring and shielding of your field devices.

 

[itsmoked]

That's either interference or electrical failure of the counter electronics. I'd say it's probably interference since it seems to work fine for an extended period between triggers. I've seen cellphones do this.

Got thrusters or VFDs anywhere near the receiver?


Keith Cress
kcress -

http://www.flaminsystems.com

 

[robot65]

Thanks all for response.
We are having this issue on number of vessels. So the source (CS-137) is only one and there are two separate nucleonic detectors (receivers) mounted on other side of vessel next to each other. One is used for DCS (alarms) and the other is used for ESD (trip - safety instrumented loop). Now if there was interference with walkie talkie both detectors (DCS + ESD) should get affected as they are mounted right next to eachother. But this is not the case. We only get problem with the detector going to DCS and then after few months probably we get issue with the ESD one. However I have asked platform to take the walkie talkie radio (offshore uses UHF for communication) and stand next to detector for couple of minutes and talk on it. We will then look at the trends in the DCS/ESD System to check if there is any change. Platform going to try this over the weekend so hopefully I should get to know by Monday.

 

[itsmoked]

I can imagine all sorts of possibilities as to how one could be affected but not the other with nearby RF interference. It's very possible.

Good to run this test though. The test should be done with your person walking all about and definitely not standing in one place. If there is any chance cell phones are ever used in the vicinity they should be tried also as they run different power and frequencies and the detectors can have aspects that lend them to receiving one frequency in a negative way over another.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[robot65]

Right... Test conducted for RF interference. Nothing happened which I was expecting. So it must be something else..

 

[itsmoked]

OK we can draw a light line thru that.

Next would be:
Conducted interference from other electrical equipment
or
power disturbances.

At this point I'd put a scope on the power to the detector and look for high frequency noise from things like VFDs or dips and spikes from other equipment starting or load cycling.

Is the detector walk-up-to or is it buried in flammable goo in a no-mans land?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[robot65]

We do have water injection pumps which is injecting water in the reservoir to improve the production. The actual motor speed is variable and is getting controlled via some control panel which I am not familiar with. I will look into this in more detail exactly what we got and get back with some explanation. We also got "Variable Speed Drives". Now we got approximately 8 loops with following configurations:
1) Scintillation based detector which goes to gateway in safe area and then to Analogue input card in ICSS (Integrated Control & Safety System). Please see loop diagram on page 3 of this pdf (

http://www.scanargentina.com/pdf/smartdensitygauge.pdf)

2) Geiger Miller tube based detector which goes to MTL 5042 repeater power supply (safe area), and then to Analogue input card in ABB 800xA ICSS (Integrated Control & Safety System). Please see loop diagram on page 2 of this pdf (

http://www.scanargentina.com/pdf/smartlevelgauge.pdf)

I was speaking to system vendor and he told me that if there was any power fluctuation (let's say more than power supply tolerance whatever it is), the ICSS will generate fault alarm to alert. However looking at the event logs, I don't think there has been any power supply alarm, I will double check.

Its a good point though about power supply fluctuations but do you know any data logger (small enough to fit in the DIN Rail panel) which I can install on each loop to check the power supply fluctuations. Since we don't know when this problem will come back and on which loop (as only one loop get's affected at any one time), I wouldn't need to install some sort of power supply monitoring device on each loop possibly 2-3 months until we come across the problem again. Also wouldn't power supply fluctuations effect all these loops at same time as it's the same power supply source for all DCS loops and same for all ESD loops.

 

[itsmoked]

VFD's are notorious for causing these kinds of sensor issues.

You actually do not want a DIN mounted type logger because just logging is not what you need. To find this with a logger would be a herculean effort. What you need is a way to trigger logging only when the supplies get out of spec and then that logging needs to be fast and heavy. If you have long term logging you would need to store gigabytes of info piling up for months then you'd need to search it for anomalies.

This is why I suggested a scope. All scopes let you set triggering levels. I'd set it up and watch if for an hour or two to see what's typical. Then set trigger limits a little higher and leave it to trigger. When that happens see what was captured and then possibly set the limits higher and higher and over a day or two you will learn the 'dark secrets' of what's in the power.

I suspect that your system is being subjected to power disturbances frequently or even continuously but they aren't enough to trigger a problem for the system. That doesn't mean you can't see those issues and do things to weed them out and mitigate them.

For instance, you may see some nasty stuff when the VFDs are running. If you do you take actions to eliminate them. You essentially scour out all the issues you see. Next thing you know, you never get false alarms anymore because the issue was when the VFDs are running and a walkie-talkie and the heat exchanger's extra fans are all running to cause the perfect storm that disturbs the sensor.

Get an EE in with a scope to give your system a check-up.

Whereabouts are you located?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[robot65]

Thanks for your advise. I am only onshore support and will recommend offshore to do utilize scope, etc. I am based in UK north sea sector.

 

[itsmoked]

I guess I won't be driving by to help you out...

Keith Cress
kcress -

http://www.flaminsystems.com