Special Design Consideration for Agitators With Example of Repeated Breakdowns

The repeated failure of Agitator in the Phosphoric acid Reactor in MPC was a major incident causing high downtime and production loss. While doing the root cause analysis of this “Class A” incident many causes not considered in normal agitator design were considered and analyzed. This paper lists such special cases and the proposed solutions and calculations to be incorporated in design of critical systems with varying process fluid parameters for different applications. Agitators constitute almost 20% of rotating equipment in a fertilizer and similar plants but are given less priority compared to others until a severe failure happens. There are no standard analytical design criteria for agitator design due to large variations and uncertainties in the process conditions in the reactor or tank. Most designs are left to the EPC companies using preformatted design models based on experimental results, on the combination of few variables. (Like Power Number, Pumping Number, Geometrical ratios etc.) However, none these models considers the fluid flow conditions which changes inside the reactor which affects the agitator design in many ways like Localized flow, non-homogenous fluid, solid contents, Liquid additives or recycling etc. which can cause much higher alternating forces and bending stresses at the joints holding the blades causing fatigue and other failures. This paper gives more quantitative design calculations to determine the forces acting on the agitator and blades as listed above which can be used in similar application even with different fluids.