DP Transmitter Dry Leg & Wet Leg Calculations

In any process industry, level measurement is very important for safety and process purpose.

The level can be measured by two methods.

Direct method

Indirect method

Here we discuss the indirect method and which is using DP transmitter for level measurement.

A Differential pressure transmitter is a common and well-understood technology for liquid level measurement. If the tank is closed or pressurized, a DP measurement must be made to compensate for the vessel pressure.

DP Transmitter Level Measurement for Closed Tank

In closed tank DP level measurement, LP leg is connected to the top of tank.

There are two methods in closed tank DP Level Measurement.

Dry leg method:

Dry leg method is used in normal close tank where vapor is not condensate and temperature of process is equal to atmospheric.

Wet leg method:

Wet leg method is used where vapor has a tendency to make the condensate and temperature of process is high or low from the atmospheric .

For DP Transmitter Configuration, we have to find out Zero Level & Span Level. Accordingly we have to configure Lower Range Value (LRV) and Upper Range Value (URV) using HART communicator.

DP Transmitter Level Measurement for Closed Tank – Dry Leg Method

Simply when LP side of the DP transmitter is filled with any gas/air then we call it as Dry Leg & we apply Dry Leg Method for calculations.

At zero level (LRV) = pressure acting on HP leg – Pressure acting on LP leg

=H2 x specific gravity – 0

= 200 x 0.9 – 0

= 180 mmwc

At 100 % level (URV) = pressure acting on HP leg – Pressure acting on LP leg

= (H2+H1) x specific gravity – 0

= (200 + 500) x 0.9 – 0

= 630 mmwc

Range = URV – LRV = 630 – 180 mmwc = 450 mmwc

So, we have to set Lower Range Value (LRV) = 180 mmwc and Upper Range Value (URV) = 630 mmwc in the DP Transmitter using HART communicator.

DP Transmitter Level Measurement for Closed Tank – Wet Leg Method

Simply when LP side of the DP transmitter is filled with liquid then we call it as Wet Leg & we apply Wet Leg Method for calculations.

Y = H1 +H2 = 500 + 200 = 700 mm

At zero level (LRV) = pressure acting on HP leg – Pressure acting on LP leg

= H2 x SG1 – Y x SG2

= 200 x 0.9 – 700 x 1.0

= 180 – 700

= – 520 mmwc

At 100 % level (URV) = pressure acting on HP leg – Pressure acting on LP leg

= (H2+H1) x SG1 – Y x SG2

= (200 + 500) x 0.9 – 700 x 1.0

= 630 – 700

= -70 mmwc

Range = URV – LRV = -70 – (-520) = 450 mmwc

So, we have to set Lower Range Value (LRV) = -520 mmwc and Upper Range Value (URV) = -70 mmwc in the DP Transmitter using HART communicator.

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