Hydrostatic Tank Gauging
Hydrostatic Tank Gauging
Fluid pressure is the pressure at some point within a fluid, hydrostatic pressure being the pressure at any given point of a non-moving (static) fluid. Fluid pressure occurs in open conditions, such as the ocean, and closed conditions, such as a tank. Pressure in open conditions is usually static, as the motion of waves and currents create only negligible changes in pressure. Closed bodies of water are either static (when non-moving) or dynamic (when moving, or compressed by air).
After determining the pressure within the fluid, the pressure’s constant of integration is equal to some reference pressure within the system. The properties (such as pressure) of a fluid can be determined from a control volume analysis of a cube of water. Since the stress of all sides of the cube must be normal and equal in magnitude, the pressure gradient can be found to be linearly increasing in a potential gradient. The pressure within a fluid will increase linearly as the product of the fluid’s density and gravity.
Purge Control
Hydrostatic level transmitters measure hydrostatic pressure using an air purge connection to the tank. An integral specialised airflow regulator ensures a linear response throughout the entire operating range. LiquiSeal™ or Purge Control differential pressure air flow regulators are highly refined versions of the basic bubbler air control. ‘Bubblers’ are usually a length of open pipe that extends downwards into the tank.
Semrad’s Purge Control offers continuous level measurement with pneumatic or electronic output, and is suitable for corrosive or aggressive liquids. The electronic transmitter can be applied to a maximum depth of 49m in water, while the pneumatic transmitter has a maximum depth measurement of 91m. Both have a flow rate of one nominal cubic foot per hour, and a temperature range between -18° and +82° C (though the actual process temperature limits may be significantly higher depending on the material used for the downpipe and purge airline).