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LEVEL MEASUREMENT BASICS THEORY
1.To understand level various terminology used under level measurement technique.
2. To know the principle and application of direct method of level measurement such as dip stick, tubular gauge glass, transparent gauge glass and float and type.
3. To know the principle and application of indirect method of level measurement such as displacer type, hydrostatic head method using differential pressure transmitter.
4. To know the open tank level measurement and closed tank level measurement and the calculations involved.
5. To Understand what is dry leg system and what is wet leg system.
6. To understand what is zero suppression and what is zero elevation.
Purpose of level measurement A typical process industry will contain numerous open or closed tanks or vessels. Level measurement devices determine the elevation of liquids and/or granular solids in tanks, containers, silos, or any other suitable container. If the vessels are holding caustic, hot, flammable, or hazardous materials, spills could lead to catastrophic results. This means that an accurate method of level measurement and indication is vital to safe and efficient plant operation. Other important reason for measuring level are the following.
1. To keep track of inventory in terms of volume or weight. Consumers want to know the amount of material available for a process.
2. To provide a measured variable representing the height or material presence within a vessel. The measured variable is then used in one or more of the following ways, where the measured variable becomes:
● a local level indication.
● a detection of material presence.
● an input signal to a level control scheme.
3. Many processes require a steady supply of inputs and outputs.
Please read also: DISPLACER TYPE LEVEL TRANSMITTER CALIBRATION INSTRUMENTATION
Precise control or monitoring of product level in a tank, reactor, or other vessel is important in many process applications. To provide good control, accurate measurement is essential. Let’s see the terminology you will need to understand level technology devices and how they work, as well as how various other material properties (e.g., volume, density) can be determined from a level measurement.
Measured Span
This is the distance between the lowest and the highest level that a level transmitter (LT) can measure in a particular application. For example in the figure shown , the measured span is 2-6 feet. The level in the tank could be expressed as 5 feet or 83% full or 75% of the measured span.
The properties which could be determined from a level measurement are
1. Volume
2. Density
3. Mass
4. Interface
5. Volume :
Volume is the space occupied by a quantity of material.
Density
Density is the mass of a material per unit of volume. Density is often expressed in terms of grams per cubic centimeter (g/cm3) or pounds per cubic foot (lb/ft3). Specific gravity is often used to describe the density of a material compared to the density of water at a common reference temperature.
Specific Gravity
Specific gravity is the ratio of the density of a material to the density of water at a common reference temperature. Water has a density of 1 g/cm3 (62.43 lb/ft3) at 4 °C. Glycerin, a compound often found in soaps, has a density of 78.66 lb/ft3. Therefore, glycerin’s specific gravity is 1.26 (78.66 ÷ 62.43).
Mass
Mass is typically expressed in terms of kilograms, grams, tons, or pounds. Mass is not affected by temperature. Thus, 10 kg of oil at 20 °C is still 10 kg at 50 °C—however, the overall volume of the oil may change due to expansion.
If we know the density, mass can be calculated from a level measurement by first calculating volume
Mass = Density x Volume
Interface
The boundary between the immiscible liquids is called an ―interface.‖ An interface measurement finds the boundary between two liquids stored in the same tank, each with a different density. For example, when oil and water occupy the same vessel, the oil floats on top of the water. The interface between the two fluids is the upper level of the water and the lower level of the oil.
Bottom-up and Top-down level measurement
Top-down measurements may or may not contact the process fluid. A top-down measurement poses less potential for leakage and enables level measurement devices to be installed or removed without emptying the tank (e.g., radar).
A bottom-up measurement typically contacts the process fluid . Level devices that use pressure transmitters are bottom-up measurement systems.
Direct and Indirect Measurement
Direct measurement indicates that level is measured directly. For example, when you use a dipstick to check the oil level in your car, you are making a direct measurement. A direct measurement is
independent of any other process parameters. Indirect measurement, also known as inferred measurement, indicates that a variable other than level is first measured and then used to determine a level measurement. For example, pressure transmitters use mass and the fluid’s specific gravity to calculate level.
Continuous, Single point or Multipoint Measurement Level measurement sensors are divided into two classes: point level switches and continuous level gauges. A continuous level-measurement system monitors the height of product within a range of all possible levels at all times. Continuous measurement is used for precise control, to maintain the level of a material at a particular point, and to ensure a consistent supply. Point measurement indicates whether a product is at least as high or low as a certain point, usually the high- or low-level limit. They are typically used to prevent over flow. Also in multipoint measurements, level indication is observed at two or more discrete points in the tank. Two single-point measurement devices may sound alarms or operate equipment at high and low limits. Several single-point devices located throughout the vessel could approximate a continuous level measurement system.