The difference from the atmospheric tank, the pressurised tank is an enclosed container in which has vapour pressure apply over the liquid and the value of the vapour pressure may vary along the time. Since the vapour pressure can not be neglected therefore, it should be measured by both pressure inlets of DP level transmitter. This could be accomplished by routing the vapour pressure into the top port using an impulse piping.
There are two methods for this installation: Dry Leg and Wet Leg. The selection between both of them depends on the vapour properties.
Dry Leg Level Measurement
When no vapour condensation, the Dry leg can be considered for the level measurements. Upper tapped leg always dry, and no liquid need to fill the upper leg. In this case, no additional liquid pressure will apply to high port other than the vapour pressure. Hence, Top Port only measures the vapour pressure (see picture 1).
Wet Leg Level Measurement
Suppose condensation of the vapour is possible, then the wet leg should be chosen. As the condensation would fill the leg and subsequently will be accumulating on the upper port side, which adds additional pressure resulted from condensate hydrostatic force.
In this situation, the upper leg shall fill with water or any liquid that is no reaction to the process liquid. Hence the pressure from the upper port needs to be taken into account as a constant pressure in the calculation of the calibration range.
In conclude Wet Leg is still recommended in more of the level application, this to prevent the unforeseen condensation happens in the top port.