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.
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 = P.HP – P.LP
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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