Dear All;

I have 6 ABB thermocouple transmitters HT02 fixed in the HRSG (Heat Recovery Steam Generation) to measure Main chimney exhaust temperature. the problem now is that the transmitters sometimes give a correct reading and sometimes give bad reading. i checked all wiring it is ok, no lose. i simulate mA from local side, the cable ok and reading changing in DCS side. i need to ask what is the cause of this problem and How to solve it? and also if transmitter problem it is possible to read and sometimes not read?

Thanks all

 

1) What is an example of a 'good reading', for example, 215 Deg C?

2) What is an example of a 'bad reading', for example, 220 Deg C?

3) What does it mean "to read" and then "not to read"? What exactly happens when it 'does not read", using numbers?

4) How long have the installed transmitters worked properly? (5 days? 5 weeks? 5 months? 5 years?)

5) Are these 4-20mA loop powered transmitters or Foundation Fieldbus or Profibus?

6) Do all these transmitters share the same DC power supply?

7) How are the transmitters configured for failure mode? High output or low output?

8) Do all these transmitters share the same remote terminal panel?

9) Do all these transmitters share the same analog input card?

10) A web search for ABB HT02 returns nothing. What is an HT02?

 

check the resistance of the rtd's. you should be able to get a direct reading of the temperature that you can use to help evaluate if it is a transmitter problem or not.

 

msalem73,

David asks MANY good questions. But perhaps the best question is: When did this problem start?

It's probably a safe bet that these sensors/transmitters were working fine for some time but have recently developed problems. So, what happened right before the problems started?

Was there some kind of maintenance outage, whan welding was performed in or around the exhaust stack?

Was the ABB thingie replaced recently, or was it power-cycled (perhaps unintentionally)?

Do these inputs all have IS (Intrinsically Safe) barriers--specifically powered IS barriers?

Generally, when something like this occurs to a group of devices there's a reason and/or there is something that is common to them all that is causing the issue.

To receive the best responses it's best to provide as much information (called "actionable data") to prevent a lot of back-and-forth asking for more information and real, meaningful data.

Please write back with details and answers.

 

Hi,

I assume the TC's are directly connected to certain thermocouple module.

1-this problem may appear if the common mode voltage is high.

2- if the thermocouple type is not grounded-junction.

3-or it is grounded junction but with long distance.

usually the grounded junction thermo couple widely used, because it offer faster respond time, more accurate reading at short distant. It is a preferred junction type for high temperature applications. Precaution for ground loop at long distances and at low temperature usage.

 

first off all i am sorry for writing down transmitter model is ABB TH02. the answer of your question as below.

1) good reading is that 177 C.

2)reading of not good give a bad reading is -3bad in DCS

3)this mean the sensor give un logical reading like -3bad.

4)The transmitters was installed before 4 years

5)Are these 4-20mA loop powered transmitters.

6)no it is not sharing in same power supply .

7)The transmitter was configure in DCS if its value increase or decrease about UV and LV it will give Bad indication in DCS.

8)these transmitters not share in the
same remote terminal panel and install in the the different HRSG ( Heat Recovery Steam Generation).

9)It is in a different analog input cards .

Thanks and sorry for late

 

Okay,

You answered some of the questions.

Have you checked the thermocouples in the stack to see if the thermowells (I presume they are installed in thermowells) are clean or dirty?

Temperature sensing devices installed in thermowells should have conductive gel ("grease"; fluid; etc.) installed to assist with preventing air pockets and aiding heat transfer. Have you checked the thermocouples/thermowells?

It's very odd this group of T/Cs is experiencing similar problems so you have to look for something common to them. You say the power supply for the transmitters is not common. They are not connected to the same analog input card. You did not say if they pass through a junction box or marshalling cabinet with IS barriers, and what style of barriers is used (powered or unpowered).

Have you meggared the wires between the transmitter and the analog input card? If so, what were the results? It's possible that heat has damaged the wire/cable insulation and meggaring them may help prove that theory.

But, it's something like the above--again, something common to these thermocouples or the way they are installed or the cable they share or the stack they share.

Please write back with more information and let us know how you fare with troubleshooting and resolving the problem.

 

you appear to be describing sensor failures,

do you have details of the primary measurement and the fluid velocities in your piping/duct

 

6 different T/C transmitters with a 4 year history of proper operation which now fail-low intermittently, but none share a power supply or an input card. Hmmm.

It isn't clear whether the transmitter is configured to 'fail' or 'fault' at a high output (~22mA) or fail at a low output (~ 3.8mA). It would help to know this because a reading of -3C could be the transmitter indicating a failure at its fail-low output. But if the fail/fault is set to fail-high at 22mA, then the -3C reading is not being generated by the transmitter, it's coming from somewhere else, like a ground loop.

The transmitters are HART by design. Any chance that the DCS reading is the HART primary variable and not the analog current signal value?

The TH02 spec sheet says it has input-to-output isolation so even using a grounded thermocouple is not likely to produce an intermittent ground loop on the sensor side.

Thermocouples can fail open but 177C is not a killer temperature for thermocouples and failed thermocouples rarely self-correct their condition. Not likely.

Are the transmitters' power supplies grounded on one side?

Are there I/S barriers/isolators? Single or dual channel for each circuit?

Is there some point in the field wiring where these signals are in close proximity? conduit? cable trays? junction box? marshalling panel?

Are you in a wet climate where junction boxes where moisture accumulation has finally developed a ground loop?