InstrumentationLevel Measurement



In industries level measurement is a crucial process.there are different types of storage tanks in this session we are gonna discuss about open & closed tank measurement.

Open Tank Measurement

The simplest application is the fluid level in an open tank. Figure shows a typical open tank level measurement installation using a pressure capsule level transmitter

If the tank is open to atmosphere, the high-pressure side of the level transmitter will be connected to the base of the tank while the low-pressure side will be vented to atmosphere. In this manner, the level transmitter acts as a simple pressure transmitter.

We have:

Phigh = Patm + S⋅H

Plow = Patm

Differential pressure ΔP = Phigh – Plow = S⋅H

The level transmitter can be calibrated to output 4 mA when the tank is at 0% level and 20 mA when the tank is at 100% level


Closed Tank Measurement


Should the tank be closed and a gas or vapour exists on top of the liquid, the gas pressure must be compensated for. A change in the gas pressure will cause a change in transmitter output. Moreover, the pressure exerted by the gas phase may be so high that the hydrostatic pressure of the liquid column becomes insignificant.

For example, the measured hydrostatic head in a CANDU boiler may be only three meters (30 kPa) or so, whereas the steam pressure is typically 5 MPa. Compensation can be achieved by applying the gas pressure to both the high and low-pressure sides of the level transmitter.

This cover gas pressure is thus used as a back pressure or reference pressure on the LP side of the DP cell. One can also immediately see the need for the three-valve manifold to protect the DP cell against these pressures.

The different arrangement of the sensing lines to the DP cell is indicated a typical closed tank application

We have:

Phigh = Pgas + S⋅H

Plow = Pgas

ΔP = Phigh – Plow = S⋅H

The effect of the gas pressure is cancelled and only the pressure due to the hydrostatic head of the liquid is sensed. When the low-pressure impulse line is connected directly to the gas phase above the liquid level, it is called a dry leg.



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