We have two steam boilers and turbine. what is happening turbine inlet steam temperature is always higher than the outlet temperature of both boilers. we are using temperature transmitters of rosemount. Problem couldn't be sorted out as down stream temperature should be lesser than upstream. why can anyone sort this problem. How could i analyze this.

 

Without any information on the type of sensor or the magnitude of the reputed error, it's hard to know which category of error, so here's a bunch:

Some reasons why temperatures vary:

1) The configured temperature range of the transmitter does not match the configured temperature range of the controller input

2) Transmitter configured for wrong type thermocouple.

3) Upstream boiler outlet is in Deg C
Downstream turbine inlet in in Deg F

4) Someone replaced the copper signal wire from the transmitter to the controller/indicator with thermocouple extension wire and there's a gradient across it.

5) Sensors are not bottomed in the thermowells

6) Sensor sheaths are not coated with thermal conductive paste in the thermowells.

7) Exposed thermowell is not insulated and loses measurement heat in the lag portion of the sensor

8) False junction in thermocouple sensor or lead wire (crimped wire).
False junction at transmitter junction block due to stray wire strand

9) Bad thermocouple, large drift component. Drifted TC's have much higher resistance than new thermocouple elements, which are low, < 1 ohm resistance.

10) Transmitter configured for wrong type of cold junction compensation

11) HMI/control system misconfigured, readings attributed to device A are for device B.

 

I think that maybe your measuring the boiler outlet temperature to close to the Desuperheater outlet where the spray is not sufficiently "mixed" with the steam.

 

All very good troubleshooting steps, to which I would add the question: When did this problem start? Has it been ongoing since day one of the commissioning? Or did it start after a maintenance outage?

A temperature transmitter uses some kind of sensor, a T/C or an RTD (not usually an RTD for this type of application, but stranger things have happened). The transmitter "reads" the sensor and converts the reading, usually, to a linear output of something link 4-20 mA for a specific range.

The transmitter has to be correctly configured for the type of sensor being used, and there are many different types of T/Cs (and RTDs). It's entirely possible that one transmitter is using a Type J T/C and the other transmitter is using a Type K T/C, and both transmitters are configured for Type J (or Type K). It's entirely possible that someone replaced the T/C in one location with the wrong T/C type and didn't change the configuration of the transmitter.

If the transmitters are SMART and you're using a SMART calibrator, then you're not really testing the transmitter using the sensor.

Have you tried swapping the transmitters to the opposite locations and observing the readings?

But, the likely problem is that someone has either replaced the sensor with the wrong type, or the interconnecting wire/cable between the sensor and the transmitter is the wrong type (T/Cs require the proper T/C extension cable, properly terminated, to work properly), or the transmitter is not configured for the type of sensor that's being used.

Or, it could be a combination of more than one thing. I recall going to a site where a large AC motor's protective relay would actuate very shortly after the motor was started, and sometimes even when the motor wasn't running. It was found that the relay was configured for the wrong type of RTD that was in the motor's windings and the wrong type of wire was used (non-shielded, not-twisted cabling run in the same steel conduit as the high-voltage motor leads), and the wires were not terminated correctly at the relay. All the wiring issues had occurred after the relay had been relocated; the relay had been improperly configured for the RTD type since the original installation.