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sirisha
ICE Student
Joined: June 11, 2007
Posts: 28
Location: Karnataka
Status: Offline
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 Posted:
Aug 29, 2007 - 08:57 AM |
 
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| Post subject: Compensating cable for K type thermocouple |
What are selection factors for a K type thermocouple compensating cable? How do we select AWG of compensating cable? What is the permissible length of compensating cable from field to a transmitter or control room, and what is the permissible resistance of compensating cable?
I have got 6 thermocouples in built into our furnace. Recently there is a sudden drop of temperature in two points where as the rest of the points are alright. All the thermocouple heads are not covered with dust where as the compensating cables connected from t.cs to the control room are immersed in dust( collection of iron ore and coke). Is this effecting the signal from t.c? |
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fundu_instru
ICE Expert
Joined: June 20, 2006
Posts: 180
Location: Singapore
Status: Offline
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| Posted:
Aug 29, 2007 - 02:43 PM |
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Giving you reply point wise:
What are selection factors for a K type thermocouple compensating cable?
Wherever Thermocouple is used, K type thermocouple compesating cable to be used between T/C to transmitter or thermocouple to control room.
How do we select AWG of compensating cable?
AWG is nothing but american standard for wire thickness (diameter), wherever we want to lay the cable for longer lenght, we require to minimise resistance to transfer mV signal generated at thermocouple sensor end. so as we require higher AWG.
In addition to that, some project specification always asks for specific minimum AWG to be followed.
What is the permissible length of compensating cable from field to a transmitter or control room, and what is the permissible resistance of compensating cable?
The resistivity of extension and compensating cables varies according to the different conductor metals; the limit to cable lengths which can be accommodated by measuring instruments therefore depends on both the thermocouple type and instrument specifications. A general rule for electronic instruments is that up to 100 Ohms loop cable resistance (i.e. total of both legs) will not result in measurement errors.
One example is that of Type K extension cable which has a combined loop resistance of 4.5 Ohms with 7/0.2mm conductors; in this case, 20 to 25 (100/4.5) meters is the maximum permissible cable run. The use of larger diameter wires will permit greater lengths of course.
Hope this satisfies your query regarding thermocouple cable. |
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fundu_instru
ICE Expert
Joined: June 20, 2006
Posts: 180
Location: Singapore
Status: Offline
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 Posted:
Aug 29, 2007 - 02:51 PM |
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I have got 6 thermocouples in built into our furnace. Recently there is a sudden drop of temperature in two points where as the rest of the points are alright. All the thermocouple heads are not covered with dust where as the compensating cables connected from t.cs to the control room are immersed in dust( collection of iron ore and coke). Is this effecting the signal from t.c?
For furnace, there may be many possibilities.
1. It may be possible that, thermocouple itself is damaged inside the furnace, which makes the hot junction un equal.
For this you can interchange thermocouple and measure value again.
2. There may be possibilities that the iron ore and coke which are deposited on cable have burnt or damaged the cable, which gives erratic value.
For this you can wash cables with water. The best way.
3. Third possibility is at the end of refractory inside furnace is covering sensing element such a way which is not giving direct contact of flue gas.
You can check this only at shutdown or by poking if manhole or poking hole is available along with cleaning air. (dangerous !!)
4. You may check connection at system side, JB end or instrument end. Which is least possible thing.
Dont check this, if thermocouple is connected to trip or control loop. |
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sirisha
ICE Student
Joined: June 11, 2007
Posts: 28
Location: Karnataka
Status: Offline
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Sep 01, 2007 - 11:23 AM |
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Hey..thanx for the reply..As there was so much dust and gas leakage i cudnt check my thermocouple at head. But tday i cud do it finally. I found that at head also i m getting only 60deg where we are expected to get above 400 deg. So there is no problem with cable as i m getting same 60deg in control rm also.I cant replace with another t/c and see bcz it is embedded into the f/c. I feel that t/c is alright and there might be a blockage of the charging material over the sensing element and bcz of that its giving a lower temperature. But i m not able to prove it to the production departments as our GM and ED are after them..they are also taking this very seriously. So..how can i convince them? |
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fundu_instru
ICE Expert
Joined: June 20, 2006
Posts: 180
Location: Singapore
Status: Offline
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| Posted:
Sep 02, 2007 - 01:55 AM |
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i feel that still we can not conclude that problem is due to blockage only.
You should propose that,
1. There may be overlay of ore which is blocking temp. measurement. Which may be removed via soot blower if available. Otherwise this can be done only in shutdown.
2. Second thing is faulty thermocouple . equally possible. These also can be done in shutdown only. If thermocouple is in built and no thermowell is mounted separately then there is not remedy to replace thermocouple.
For convincing your higher authorities , you need to do replacement of thermcouple and prove that, it is not faulty. |
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sirisha
ICE Student
Joined: June 11, 2007
Posts: 28
Location: Karnataka
Status: Offline
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| Posted:
Sep 08, 2007 - 11:00 AM |
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My thermocouple itself is not ok. I found no output at head of thermocouple. I tried to replace it but coudnt do it as it is embedded into the furnace. |
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fundu_instru
ICE Expert
Joined: June 20, 2006
Posts: 180
Location: Singapore
Status: Offline
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Sep 08, 2007 - 03:54 PM |
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So problem identified 
Thums up ! |
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ankur
ICE Newbee
Joined: July 18, 2008
Posts: 2
Location: New Delhi , INDIA
Status: Offline
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| Posted:
July 18, 2008 - 05:17 AM |
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| Post subject: Difference between Compensating Cables , K , Vx & S Type |
Wanted to know the Difference between various Compensating Cables of Type Vx , K, S, R etc & what are their applications along with the testing methods & kind of intruments required for testing , also can the conductor resistance of the above can be used for differenciating the above if so where can we find the specific values of the same. |
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