During a maintenance call out on a steam drum, a set of three level transmitters were deviating with one deviating over 3 inh2o (shown in the picture below). Has the calibrated span widened 3 in or is the measurement off 3 in?
This was the ultimate process of troubleshooting the transmitter; as you can see, this is a manifold setup with tubing at the transmitter and also tubing at the high and low side of the steam drum. Initially, I block and bled the transmitter at the manifold and this caused the measurement to go to zero (in this case full scale) because the URV value is 0 (20mA). This validated that the high side reading was good. I then proceeded to pressure up the low side of the transmitter to 33 inh20. The transmitter responded by going to nearly -33.00 exactly. This proved that the low side of the transmitter was good, eliminating the transmitter as the issue. I then started looking for leaks and found that the tubing was leaking on the low side at the stream drum after the manual block valve. There was the culprit causing the drift and the deviation of a critical level transmitter. The "wet leg" water filled impulse line could have frozen during a recent freezing condition which could have compromised the fittings or tubing.
Avoided a process upset or steam drum failure due to inadequate level protection. Long term solution is to install a different level technology such as radar or ensure that adequate heat tracing is provided for installations with water filled impulse lines.