Instrumentation Calculators

DP calculator for Dry leg Level measurement – closed tank

Dry leg method used to determine level in closed tank. Any types of liquids which do not evaporate can be used with this technique. There is no fluid in the transmitter impulse piping’s low pressure side or the leg. The low side of the transmitter’s pipe will stay empty if the gas above the fluid does not condense. 

  If the gas phase is capable of condensing, such as steam, condensate will develop in the low-pressure impulse line, creating a column of liquid that increases the pressure on the transmitter’s low-pressure side. Adding a knockout pot below the transmitter on the low-pressure side is one way to address this issue. Condensate in the knockout pot should be periodically drained to keep the impulse line dry.

DP calculator for Dry leg Level measurement

The influence of gas pressure is eliminated, and only the pressure caused by the liquid’s hydrostatic head is sensed. A dry leg is produced whenever the low-pressure side impulse line of the level transmitter is linked directly to the gas phase above the liquid level in the closed tank.

h = The liquid level range is need to be measured.

SG = specific gravity of the process liquid

Pressure on H.P. side of transmitter P.HP = Pgas + (SG*h)

Pressure on LP side of the transmitter P.LP= Pgas(process gas pressure inside the tank)

The differential pressure of the transmitter is computed as follows:

DP = P.HP – P.LP

DP = Pgas + (SG*h) – Pgas

DP   = SG*h 

LRV Calculation

To achieve LRV, the pressure on the HP and LP sides of the tank must be empty or level below the transmitter’s HP side tapping. When the tank is empty, there won’t be any pressure on either the HP or LP side. This is because of the absence of liquid height.

So h = 0

Then DP = Pgas+(SG*h) – Pgas

PH.P = Pgas + (SG*0)

PH.P = Pgas

PL.P = Pgas

LRV = PH.P. – PL.P

LRV = Pgas – Pgas

LRV = 0

Estimating the URV

Determine the differential pressure (DP) of the transmitter when the tank level is reached to the maximum range  with liquid (h) .

PH.P = Pgas +(SG*h)

PL.P = Pgas


URV= Pgas+(S.G*h) – Pgas

URV = S.G*h

Range of the level transmitter = 0 to (S.G*h)

DP calculator for Dry leg Level measurement 

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Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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