Every liquid in storage tank will have hydrostatic pressure equal to the height of the liquid. With an assumption that the liquid has constant density.
The DP level transmitter indication is a measure of the head pressure in the level of the tank whose level is to be detected.
Head Pressure, DP, is calculated as
DP = (Height).(Specific Gravity)
Level Transmitter Errors
For example, let’s say 50% level is displayed in the control room, the specified SG = 0.8.
The DP transmitter is actually measure a differential pressure (50%)*(0.8) = 40 units of the differential pressure.
Now, what if the SG change. maybe because the liquid aerated, or the feed to the vessel has become lighter.
For some reason, the SG has dropped from 0.8 to 0.6. The real level will increase to 66.67% (40/0.6) but the indicated level in the control room is still 45% because the DP output is constant at 36 units of DP.
This is happened because DP level transmitter cannot discriminate between height and density.
This is something to worry about. The defective level transmitter can cause liquid overflow and vapour cloud explosion can be ignited if ignition source available in adjacent area
Author : Iqbal Matondang