Thursday, July 16, 2015

Downpipe Liquid Level Sensor Purge Control

Downpipe bubbler type tank level indicator
Arrangement for bubbler type tank level indicator
with purge control
Courtesy King Gage
Industrial process control often requires the measurement of liquid inventory volume or mass. If the material is contained within a tank of known shape and dimensions, the volume can be determined accurately by measuring the level of the liquid. Various means are used to determine the liquid level. One of the simplest is the downpipe sensor, sometimes referred to as a "bubbler", where liquid level is inferentially derived using differential pressure. The downpipe is a length of pipe that is open at the end extending down into the liquid contained within the tank. The top end is connected to a purging device that provides a controlled flow of pressurized air or gas into the pipe. The pressure maintained within the downpipe will reach an equilibrium with the pressure produced by the hydrostatic force related to liquid level in the tank or vessel. Accurate measurement of this pressure, along with a thorough knowledge of the liquid's properties, can be used to determine the depth of the liquid within the tank and the corresponding volume.

King-Gage manufactures several industrial process level measurement devices incorporating downpipe purge technology and differential pressure measurement. They have applications in inventory monitoring, process control, hazardous and explosion zones, ballast monitoring, and other areas that benefit from their simple operation, low maintenance, and ruggedness. The company, in its own words describes the unit as an...
...extremely rugged unit designed specifically for hazardous areas requiring flameproof or ATEX (Ex d) rating. A proprietary wet check assembly ensures positive seal of fluids to ensure containment integrity. Loop powered transmitter provides 4-20mAdc output while components are isolated from the process media by a continuous air purge. Total consumption rate is less than 0.083 scfm for energy saving operation with external air/gas supply.The only internal element is a simple length of pipe extending into the tank.
Rugged design requires no setup or air flow adjustment due to its differential pressure regulation that avoids dynamic pressure drops common to other bubblers. This ensures highly accurate level measurement and repeatability within ± 0.2% while preventing turbulence or foaming of tank liquid. Applications include water/wastewater, sulfur pits, most free flowing liquids (including slurries) and temperatures in excess of 160 °C (320 °F) as the transmitter is effectively isolated from the process by the air purge.
The slides below illustrate various installation configurations, including one for explosion hazard areas. Contact an application engineer for more detailed information.