Operational Problems in Pumping Non-settling Slurries Resolved Using an Improved Laminar Flow Pipe Fitting Loss Model

A case history is presented pertaining to five pumping systems that operated satisfactorily until a new production requirement was imposed on the pumping systems. A new slurry product initially developed at lab scale was introduced into the production plant for an initial trial run. Problems began to surface immediately on three out of five batch process pumping systems when the slurry could not be pumped through the plant at contract rate. Additionally, significant "heels" (unwanted fluid levels) were left in some of the suction vessels that were unable to be pumped out, resulting in considerable yield losses. This manufacturing problem had not been anticipated by the team, and without quick resolution, a loss of customer confidence and a significant delay in the new product would have resulted. Investigation and analysis of the system revealed two major problem areas in pumping non-settling slurries in laminar regimes: • Initial prediction of head losses through suction piping fittings was flawed using traditional hydraulic loss methods. The original calculations for NPSHA values for the pumps predicted adequate NPSH margin. The fluid was non-Newtonian and was operating in the laminar regime. Upon further investigation, a weakness was revealed in predictions of fitting losses for laminar flow through the pipe fittings. An improved model for predicting losses through pipe fittings was identified and implemented. The improved model matched operational data much better and provided the critical insight needed to resolve the operational problems and get the facility operating. • Piping arrangements that allow for features such as clean-out ports (e.g., branch flow tees) can be counterproductive to unrestricted flow of the process liquid in systems with non-settling slurries operating in laminar regimes. Tees, elbows, diameter changes, and other fittings can introduce significant head losses in the pumping system. The authors present an improved method for analyzing fitting losses in pumping systems when dealing with nonsettling slurries operating in the laminar regime. In addition, design considerations are presented to minimize the impact that piping has on the pumping system when handling non-settling slurries operating in the laminar regime.