It is important to check the heat tracing of the wet leg of the DP level transmitter which is giving erratic readings or not working. Since wet leg is being frequently drained every 1-2 hours, it can only mean that the heat tracing is not working to keep the temperature of wet leg above the dew point temperature of hydrocarbon.
Usually, for such applications, the wet leg should be filled up to the level where condensing vapours cannot enter and condense on top of Wet leg liquid to influence the pressure exerted by wet leg column. By Sloping the impulse tubing little downwards to let the condensate drain back to the tank, such situations can be avoided.
Better option for such applications is to have extended diaphragm DP transmitter with chemical seal on high and low pressure sides/ extended diaphragm on the high pressure side and extended pressure repeater on the low pressure side.
Author - Saurabh Bhardwaj