Closed Vessels
In closed vessels the pressure above the liquid will affect the pressure measured at the bottom. The pressure at the bottom of the vessel is equal to the height of the liquid multiplied by the specific gravity of the liquid plus the vessel pressure. To measure the true level, the vessel pressure must be subtracted from the measurement. This is achieved by making a pressure tap at the vessel and connecting this to the LP side of a DP transmitter. Vessel pressure is now equally applied to both sides of the transmitter resulting in the differential pressure proportional to the liquid height multiplied by the specific gravity.
However, these instruments are affected by changes in the process density and should only be used for liquids with fixed specific gravity or where errors due to varying specific gravity are acceptable.
Differential pressure devices require a constant head to be maintained on the external or reference leg. Two methods commonly available are:
•Dry leg
•Wet leg
Dry Leg
If the gas above the liquid does not condense the impulse piping to the low side of the transmitter will remain empty If the DP transmitter is installed below the bottom of the tank then zero suppression must be made to offset the constant static head that present, otherwise there will be an an incorrect level reading
Wet leg
If the gas above the liquid condenses in the piping the low side of the transmitter it will slowly fill up with liquid resulting in an incorrect level reading. To eliminate this potential error, the pipe is purposely filled with a convenient reference fluid that possesses a higher specific gravity than, and is immiscible with, the liquid in the vessel A common liquid used for this function is common anti-freeze. In figure 17
Output = [P (Sg vapour) + h (Sg liquid)] – [P (Sg vapour) + Z (Sg leg)]