Chemical Processing - January 2008 - (Page 42) >> PLANT INSITES Bubbles can buoy level measurement tough services don’t necessarily demand expensive devices Operating cOnditiOns sOmetimes impOse reliability and maintenance requirements that force you into selecting specific equipment. One prime example is level measurement in high temperature, high fouling services. While non-contact devices often are the best choices to cope with fouling, high temperatures may make typical radar or ultrasonic units impossible to use. Other non-contact measurement techniques, such as gamma ray and neutron backscatter, may cost too much for your budget. However, there’re a couple of less-expensive options: purged differential pressure (dp) cells and bubblers. a standard level-measurement system uses a dp cell attached to the vessel at two points: one in the vapor space above the liquid and the second in the liquid at >> Purged DP system Level signal Liquid on collector Total draw Connected to bottom of draw sump Standard level measurement configuration Pumped return Compare the bubbler gas requirement to that of a purged DP cell. its lower level position; Figure 1 shows this connection to a collector tray in a distillation tower. the usefulness of the level signal depends upon the accuracy, precision and reliability of the cell. if the liquid is prone to plugging, then the impulse lines from the dp cell to the vessel may get clogged, which tehn can cause inaccurate readings. adding purges to either or both taps (Figure 1), as required, can keep the lines clean. typical installations use restriction orifices (rO) in the purge lines to prevent excessive flows. even so, the pressure drop from the purge connection point to the vessel can cause incorrect level readings —falsely high if the pressure drop is on the liquid side and falsely low if the pressure drop is on the gas side. Finding purge fluids compatible with the process and equipment may pose challenges. nitrogen gas is frequently available and may be suitable if it can be removed later. the liquid stream tends to be more difficult. Often the only compatible one is the actual process liquid itself. as long as existing pumped streams can be used, this may be feasible. However, incompat42 • January 2008 RO Gas purge Level signal Liquid purge Liquid on collector RO Connected to bottom of draw sump Level measurement with purge Total draw Pumped return Figure 1. Addition of gas and liquid purges to a standard level-measurement configuration can prevent clogging. ible liquids, density differences caused by cooled purge streams, and flashing liquids all can lead to major problems. if you’re okay with adding gas to the process, you do have another choice — a bubbler installation (Figure 2). a controlled gas stream is injected into a tube that descends into the process liquid. the flow rate and differential pressure between the gas and the vessel provide an indication of liquid level. Bubbler systems require accurate pressure measurement and flow control and a constant composition gas stream. they have more sources of error than a straightforward dp cell but may work better than a purged dp cell. www.chemicalprocessing.com http://www.chemicalprocessing.com
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