(270b) The Difference Between L-Valves and Loop Seals

People often do not understand the difference between operation of non-mechanical L-valves and loop seals, and this can lead to solids transport design failures. In their most common operating modes, an L-valve is used to control the flow rate of solids in a system, while the loop seal does not control the solids at all, but automatically moves solids automatically from a cyclone back into a fluidized bed.

Loop seals can operate with Geldart Group A, B or D solids if they are designed correctly. Only a little aeration is required on the upleg of the loop seal to maintain this part of the loop seal in a fluidized mode. The L-valve, however, is generally operated to control the solids flow rate around a system. Adding a little aeration to the L-valve at the correct location causes solids to flow through the Lâ??valve. Adding more aeration causes a higher solids flow rate through the L-valve. While operating in this solids-control mode, the L-valve is practically limited to operating with solids greater than about 100 microns. Thus, only Geldart Group B and D solids can be used with an L-valve operating in the control mode. Group A solids do not work well with L-valves operating in the solids-control mode.

Another difference between the two devices is that the loop seal operates with an overflow, fluidized bed standpipe above it. The L-valve operating in a control mode cannot operate with this type of standpipe, but must operate only with an underflow, non-fluidized (packed bed) standpipe above it.

Why the L-valve operates differently than the loop seal will be explained by describing the differences in operation between the types of standpipes feeding the two non-mechanical devices.