LEVEL MEASUREMENT

Authorized by ; OGUNNUSI O.O

SELECTING MEASUREMENT AND CONTROL DEVICES

The instrumentation and control (I&C) designer must first understand the process if he or sheis to be able to implement the required control system with the proper instruments. The properselection of instruments and controls typically involves considering the following:

1. Compliance with all code, statutory, safety, and environmental requirements in effect atthe site.2. Process and plant requirements, including required accuracy and speed of response.3. Good engineering practice, including acceptable cost, durability, and maintainability.Selecting instrumentation and control items entails several important aspects other than thespecific technology. These include:

• safety• performance• equipment location• air supply• electrical supply• grounding• installation and maintenance

PROCESS MEASUREMENT

Process Measurement and Control (also known as Process Automation, Process Instrumentation and Control, or just Instrumentation) is needed in modern industrial processes for a business to remain profitable. It improves product quality, reduces plant emissions, minimizes human error, and reduces operating costs among many other benefits.

The production quantities and requirements define the type of process required to make a certain product. In the process industries, two types are commonly used: continuous process andbatch process. Often, a combination of the two processes exists in a typical plant.

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LEVEL MEASUREMENT

Level measurement is defined as the measurement of the position of an interface between two media. These media are typically gas and liquid, but they also could be two liquids. Over time the methods of measurement has been upgraded.

Eventually, level measurement was used on pressurized tanks by connecting the upper end of the tube to the vessel. With equal pressure in the tube and the vessel, the liquid level in the tube was at the same point as the level in the tank.

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Classification

Level devices operate under different principles. They can be classified into three main categories that measure.

• the position (height) of the surface.• the pressure head.• the weight of the material through load cells.

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Measurement of Solids

When plants are measuring the level of solids, sensors located near the bottom of a bin may need to be protected from falling material when the bin is being filled. Solid material often arches.

The top level of solids material in a bin is rarely horizontal since most solids have an angle of repose. Therefore, the location of the measurement point should provide a representative average of the overall level, and in some cases several probes may have to be used for this purpose.

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Differential Pressure (or Pressure/Static Head)

Differential-pressure level measurement (see figure 1-1), also known as “hydrostatic,” is based on the height of the liquid head (the U-tube principle). Level measurement in open tanks is based on the formula that the pressure head is equal to the liquid height above the tap multiplied by the specific gravity of the fluid being measured. In closed tanks, the true level is equal to the pressure measured at the tank bottom minus the static pressure above the liquid surface. To compensate for that static pressure, a leg is connected from the tank top to the low side of the differential pressure transmitter (see figure 1-1). Two options are available: dry leg and wet leg. In dry leg applications, it is expected that the low side will remain empty (i.e., no condensation). If condensation takes place, an error will occur because a pressure head will be created on the low side. This error is avoided by intentionally filling the low side with a liquid—hence the term wet leg. Where filled systems (with diaphragm seals) are used between the transmitter and the tank, calibration of the transmitter should allow for the specific gravity of the fill fluid. The user should refer to the vendor’s instructions when setting the zero and span values.

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1-1

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Gas station

The air or nitrogen is slowly fed into the bubbler system until the pressure is equal to the hydrostatic pressure of the liquid in the tank. At this point, the flow of bubbles goes out of the end of the tube and rises to the top of the tank. For every inch of water height the bubbles raise, it equals the amount of hydrostatic pressure it takes for the bubbles to reach the top of the solution to equal the height or level of that solution.