How Underflow Standpipes Adjust to Maintain Pressure Balance Around a Circulation Loop

The standpipe has become a standard element of most circulation loops. In the past, standpipes have been thought of as a device to only allow solids to transfer from a low to a higher-pressure region. However, it also has another function in a solids circulation loop. It is also the device that adjusts its operation to balance the pressure around the solids circulation loop. An overflow standpipe adjusts the pressure drop across it by raising or lowering the solids level in the standpipe. However, the operation of the underflow standpipe is more complex. This self-adjusting function can cause underflow standpipe pressure drops to be negative, which seems counter-intuitive. How an underflow standpipe operating with a negative pressure build can occur in a circulation loop is described below using two commercial unit examples as illustrations of this phenomenon.