RTD Reading low

hello everyone. I have one question regarding the RTD Platinum PT100 output.

we have 3 RTD install on heat exchanger after replacing shell and tube reading reduces from 117 to 111

As per our process indication and energy consumption the Temp Transmitters it actually should read higher than current reading.

What will be the possible cause for temperature reduce while process temperature should be around 118 C.

Several possibilties that I have seen over the years for low reading RTD’s:

  1. Presumably the RTDs are in thermowells. If the shell and tubes were replaced and the thermowells also, then the thermowell placement might be different than the previous installation, resulting in temperature sensing at different points.
  2. 4-20mA transmitter LRV/URV scaling mismatch with control system (the most common problem during commissioning)
  3. 4-20mA output/analog input calibration mismatch.
  4. RTDs read correctly and something in the process is affecting the temperature readings
  5. RTDs inaccurate. Very low probability if sourced from a reliable vendor. I am aware of this only one time in 33 years.
  6. RTD’s not inserted to full depth in the thermowell so each ‘reads’ a temperature in the thermowell that is not the expected temperature at bottom of the thermowell bore. The air gap at the bottom also slows and dampens the RTD response.
  7. ‘Stem conduction’: Removal of previously installed ‘lagging’ insulation now allows heat loss through the thermowell/transmitter head lag fittings and eventually the RTD sheath/MgO/element that reduces the temperature ‘seen’ at the RTD tip.
  8. 2-wire RTD used instead of 3-wire RTD with a decent run of extension wire to a direct-reading analog input. A short jumper was used at the analog input to mimic the 3rd wire connection that was not representative of the resistance of the extension wire’s copper wire run. The jumper substitution caused an offset error, but I cannot remember whether the error was positive or negative. Paralleling the extra wire on a 4-wire RTD for a 3-wire connection will produce a measurement error.
I fully agree with David_2's answer.
If using a 3-wire RTD temperature reading may be either over-estimated or under-estimated in case of contact resistances on screwed connections (corroded terminals or loose connections). The reading will be correct only if the 3 wires have the same resistance. A 1 ohm difference will result in a 2.5°C error approximately
If using a 2-wire RTD temperature reading will be always over-estimated in case of contact resistances on screwed connections.
The 4-wire connection is the only one which may not be affected by contact resistances (there are two types of 4-wire measuring principles : one is not sensitive to contact resistances , the other is).
Most often temperature readings are erroneous due to poor thermal exchange between the temperature probe and the process medium and adverse thermal exchange with the external environment. Whenever possible a thermowell should be filled with high thermal conductivity oil or grease (air is a thermal insulator) to improve the thermal exchange with the process medium, and, ideally, the part of the probe which protrudes out of the piping should be covered with insulating material (including the probe head). I hope this may help (sorry for my poor english).
I forgot one reason for RTD's to read low - the pollution of the MgO insulation inside the RTD sheath with moisture, i.e. water. MgO is the white powder that electrically insulates the RTD element from the sheath. Electrical insulation is necessary because the excitation current should not be shunted elsewhere, which could happen if the element were grounded.

Dry MgO has a very high resistance, on the order of 100Mohm at 60°C. If water moisture gets through the 'transition' and into the MgO, it reduces the insulation dramatically.

A lowering of the MgO resistance, generally due to moisture intrusion, causes a parallel path for the excitation current where the MgO now acts as a resistor in parallel with the actual RTD element. As you'll remember from basic electricity, parallel resistors always have a combined resistance lower than either of the individual resistances. A combined lower resistance results in a low reading RTD element.

If the sensors got wet during the change over of the heat exchanger, then wet MgO could be a likely cause for low reading.