Wednesday, August 20, 2014

Level Measurement

level measurement
Level Indicator
(King-Gage)
Level measurement is an important process measurement. In many processes, it is important to measure the level of various liquid and solid materials. Effective level measurement provides reliable and continuous operations by maintaining optimum material inventory, by maximizing plant availability, and the prevention of spillages and other process disturbances.

Level is measured at the interface between two material "phases", that is, between liquid and air or solid and air, or other different density materials. The different phases must be clearly separated. When liquids are thoroughly mixed together, level cannot be measured because there is no separation of phases. The interface must be definite in nature.

In the case of an open tank filled with water, the point at which water and air contact is the definite interface, or phase change. Another interface example would be between oil and water. In this example, the specific gravity of oil is less than water, so oil floats on top, but in separate form. Liquids that mix well together do not have a clear interface, so it is difficult to determine level of each.

There are various types of level measurements devices. They generally fall in two major groups:
  • Point level measurement devices: Monitoring or controlling level at a preset point, which could be either a low level or a high level. Point level detection is useful in applications where action needs to be taken at a specific set point. For example, shutting off a pump (for protection) if level has dropped below a required point, or to shut a valve to avoid over-filling a tank.
  • Continuous measurement devices: Monitoring or controlling level continuously, noticing any fluctuations. They give continuous readings and can provide an output which can be seen or recorded in remote places. This type of measurement allows for continuous monitoring and control of parameters of any process.
There are two classifications of level measurement defined by application parameters:
  • Direct level measurement: The direct method employs physical properties like fluid motion, buoyancy, etc. To measure level directly, the position of interface may be monitored visually through sight glass or by any device which is floating on surface. Displacers are also used for direct level measurement.
  • Indirect level measurement: Indirect level measurement involves measurement of some other quantity, such as pressure, and converting it into a level measurement. An example is installing a pressure gauge at the lowest point on a tank. The change in pressure as the tank level increases or decreases can be used as an indicator of the change in the level of liquid inside the tank.