Hydrostatic level gauge for closed tanks
In this article, it consists of two parts. In the first part, we introduced you to measuring the open tank level using a pressure difference transmitter (or DP transmitter). The second part of our collection introduces the output process of pressure difference level transmitters and open tank control loops and introduces you to measuring the level of closed DP tanks.
DP level measurement control loop
We looked at a control loop in which a pressure difference transmitter was connected to measure the level of the open tank.
The high pressure port is connected at 0 inches and the low pressure port is transferred to the atmosphere.
We said that: pressure in inches of water is equal to the relative density of the liquid multiplied by the height of the liquid surface in inches.
We talked about converting inches of water pressure to the pressure scale you need, such as psi, kPa or load.
DP level measurement considerations
Relative density
Well,… it works perfectly well if the liquid is in a container of water. What if the liquid is not stored in a container of water and has a relative density of 1?
The relative density of most liquids changes with temperature. For simplicity, we will for the time being ignore the effect of temperature.
Interestingly, when the container is filled to 200 inches, the pressure created is much lower because the liquid has a relatively low density. This is especially important when calibrating the pressure transmitter.
Suppose the transmitter has a pressure difference calibrated to produce 4-20 mA for the liquid level range from 0 inches to 200 inches.
Measure the level of closed DP tanks
So far, we have considered only open tanks. Let’s measure the level of closed tanks.
The main difference between measuring the surface of open and closed tanks is the fact that we have to consider the pressure in the high vapor of the liquid in the closed tank. This vapor pressure exerts a force on the liquid surface.
We can compensate for the steam pressure by connecting the low pressure side of the transmitter to the pressure difference at the top of the tank through a tube called the reference base.
Measuring the level of the closed tank DP – reference log
The reference leg may be dry or full of fluid. If the reference leg is dry, it is usually referred to as a dry leg. And if it is filled with liquid inside it, it is usually referred to as Wet Leg .
Measurement of closed tank level DP – dry log
In the Dry Leg system, steam pressure is applied to both high pressure and low pressure transistor pressure differences. The same pressure exerted on each side essentially cancels out each other.
Measuring the level of closed tanks DP – wet lag
Sometimes it is necessary to fill the reference foot with liquids. This base is now referred to as Leg Wet.
There are many reasons for Wet Leg, such as avoiding the measurement error caused by steam condensate in the reference leg.
Let’s look at an example of a Wet Leg system. Differential pressure, as always, high pressure minus low pressure. But it is not easy to predict the differential pressure because the liquid in the tank is usually not the same as the wet lag liquid!
Summary
Let’s examine what we discussed.
– The density and relative temperature of the liquid have a significant effect on measuring the level of the pressure transmitter.
– Reference log is used to compensate for steam pressure in closed tanks.
– Dry reference leg is called dry foot.
– Wet Leg is a reference foot that is filled with a liquid different from the liquid inside the tank.
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