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DP Transmitter Dry Leg & Wet Leg Calculations

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

Level can be measured by two methods.

  1. Direct method
  2. Indirect method 

Here we discuss about 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.

  1. 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.
  2. 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.

DP Transmitter Level Measurement for Closed Tank Dry leg Method

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.

DP Transmitter Level Measurement for Closed Tank Wet leg Method

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.

Article by :
Ashish Agrawal

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  1. Dear Sir
    I have question, why you neglect the LP side When you do a calculation on a dry leg mode? could you explaine that to me please?

    Best regards
    Ahmed

    • when we consider a dry leg, it means that LP side is subject to only Vapor / gas pressure, in the other hand the vapor/gas pressure is applied to both sides (LP and HP). so we don’t consider it (ie. the vapor/gas pressure) altogether.

  2. Dear sir,
    how to calculate single liquid LP & HP both side same spesifc graviti (0.838)

    H1=90inch
    h2=10inch
    sg.f=0.838
    sg.p=0.838

    i have calculate and my answer had came LRV=-67.04 and URV=0 but once i hace feed same value but transmiter showing 167% like.
    my transmiter is Husser+Endrees .

    Pls let me know ASAP

  3. HELLO,

    ACCORDING TO MY KNOWLEDGE:
    RANGE IS NOTHING BUT URV-LRV
    SO,HERE IN THE ABOVE CASE i.e for DP Transmitter Level Measurement for Closed Tank – Wet Leg Method
    To Cross Verify the Range: H1*SG1 = 500mm*0.9= 450mm;
    Here in the above case at during verification Range:Range = URV – LRV = -210 – (-840) = 630 mmwc the LRV value instead of -660mmwc it is mentioned -840mmwc.I hope that is wrong and we cannot get the range that is to be attained.

  4. there are two mistakes in this problem.
    1. In diagram you specified a SG2 as 1.0 but in calculation you used as 1.2
    2.range valve is URV-LRV= -210-(-660)= -210+660 =450mmwc.

    so please verify before posting.. thnak you

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