RTD failure

J

Thread Starter

JAVAID

Hi fellows;

I am working in oil and gas field department. Here is a Centaur 50 gas turbine install. Most time unit tripped at RTD failure. I check the log and generate a graph which shows a spike occur for some mili seconds and in the mean time unit tripped.

The data shows temperature read by RTD is 86 Fahrenheit and suddenly it read 1500 and very next second again 86. But in the mean time unit trip and RTD FAILURE OR OVER RANGE.

I checked the RTD its seems ok It is PT100 type and the resistance is almost 101.5 ohm. It is three wire RTD and connect at Flex IO RTD module.

I also check the module and found healthy.
What is the probable causes for spike and how i prevent or suppress the spike; How i fix it.

Regards
Javaid
 
The "quick and simple" way to stop the trips is to put a time delay on them to filter the short duration spikes. If the spike lasts only milliseconds, use a 1 second time delay. You may need approval from the turbine supplier, but RTD's have a time lag anyway, and temperatures don't usually change so rapidly that a 1 or 2 second time delay would hurt anything. But, you still should get approval.

Best solution is to find out what is causing the spikes. Maybe a momentary open circuit in the RTD or the wiring, or some electrical "noise" (induced EMF) event. You don't say what the temperature being measured is. Have you tried replacing the RTD? Checked all terminations on the interconnecting wiring? Any other event occurring when the spikes happen - like a vibration event or electrical fault or breaker opening?
 
B

bob peterson

1500 is probably what the module reports when it detects an open circuit. I would be looking for some kind of intermittent wiring issue.
 
G

German Lopez

> The "quick and simple" way to stop the trips is to put a time delay on them to filter the short duration spikes. <

---- snip ----

my suggestion is simpler, install a 100ohms resistor instead of the rtd to see if the temp is stable now, if it is... then your rtd or wiring is defective.

another option if the last answers have not solved the issue is that the issue is created by electronic noise.... you might install a.. let's say a .01 to .001 microfarads non electrolitic capacitor in parallel with the RTD, sometimes there is "electronic" noise around, and the capacitor will filter the noise without affecting the temperature measurement. If this solution solves the problem then the issue is electronic noise entering to the metering device.
 
I am also had such type of experience for Solar machines. S-20, T-60 and C-40 were being tripped many times upon RTD failure as described by Javed.

Upon checking, the RTDs found healthy. Solar was contacted but no root cause was established.
 
O

Osanyintade Gbenga

I have experienced this before on 2 occassions. 1 on a outboard bearing RTD on a compressor module and the other on the turbine engine drain temperature. The actual root cause in both cases was exposed cable screen on the RTD cable (conduit extensions). The screens were grounding which is against the convention of grounding your instruments devices at only one end (usually at the Control Console). With exposed screens you have grounded at both ends and therefore allowing signal interference when ever there is a ground loop current in the grounding system.

I was able to correct this problem by just insulating the cable screen and VOALA!!!!! the problem was solved till date.
 
Re RTD failure:

I had experience the same issue but the only difference is that we made changes to cooling fans set point. The RTD went went from 0 to 463.3 sporadically. However I took out the RTD from the thermowell compare it with another device and verify that the ambient temperatures was the same and read the same in the field and control room and recalibrate the transmitter.
 
Hi fellows;

I am working in oil and gas field department. Here is a Centaur 50 gas turbine install. Most time unit tripped at RTD failure. I check the log and generate a graph which shows a spike occur for some mili seconds and in the mean time unit tripped.

The data shows temperature read by RTD is 86 Fahrenheit and suddenly it read 1500 and very next second again 86. But in the mean time unit trip and RTD FAILURE OR OVER RANGE.

I checked the RTD its seems ok It is PT100 type and the resistance is almost 101.5 ohm. It is three wire RTD and connect at Flex IO RTD module.

I also check the module and found healthy.
What is the probable causes for spike and how i prevent or suppress the spike; How i fix it.

Regards
Javaid
Very interesting topic because this is exactly the same problem that we are facing right now in our boiler feed water pump. In our case, the pump tripped due to winding temperature failure (RTD). Our Engineer tried to modify the logic by installing a time delay but when tried to run the pump it trips again.
 
An RTD AI card generates a constant current and detection of the interruption/opening of the current circuit and reporting a fault is quite common. RTDs can intermittently fail internally due to expansion from heat where the fault can be intermittent and disappear when the RTD cools.
 
In my experience the most nuisance happens when RTD's are directly connected to RTD Card by triad cable and the cable has a bad installation practice.. Best is to keep the triad cable short and install a Tx in between and take 4-20mA instead to an AI card.
But with a lot of RTD connections in the field, associated with prime movers,generators and pumps etc. installing Tx for every RTD is not a feasible option. In that case proper cabling,shielding,connection,termination etc. are the only option. Using a heavier gauge wire with shield, low in impedance, proper grounding, proper spacing in cable trays etc. should be done.
Every RTD modules can accept only a range of resistance, if the resistance goes beyond that it gives a high reading and as David_2 said that can happen when the excitation current is interrupted. S/N ratio is another factor. If noise is the problem a proper valued notch filter should be used. Sometimes limitation is also present with the cable lead resistance in RTD module.
Apart from that to mitigate nuisance tripping Rate Of Change should be incorporated in tripping logic which limits the amount of change in value that can happen at every sampling cycle for the tripping circuit to work. Wire open detection is another possibility but ROC is better.
 
I find the easiest thing to do for an RTD failure is to use a potentiometer in place of the RTD, and adjust the pot to obtain an acceptable reading, then use some silicone caulking to prevent the pot from changing position. You can connect the pot directly to the AI of the PLC/monitor.

Inexpensive. And simple. You just need to connect one lead of the pot to the center of the adjustment slider with a jumper. Easy and effective. And easy to undo when the motor is refurbished. The hardest part is connecting the pot to the AI channel; just follow the instructions in the PLC AI description.

Hope this helps!
 
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