Flanged Tail Shaft Redesign for OCI Chemicals in Green River

General Description

At the OCI Chemicals facility in Green River, WY, Trona ore is mined, conveyed to the surface and refined into soda ash. Trona is a mix of sodium carbonate, sodium bicarbonate and water. The world’s purest, largest and most accessible deposit is in Wyoming’s Green River Basin. Soda ash is used in a variety of finished products including glass, powdered laundry detergent and various food and chemical products.

Screw conveyors are used throughout the soda ash refining process. Downtime is not an option so screw conveyors must be designed for mine-duty operations and constructed from heavy-duty stainless steel. High torque drive units are required to handle the dense material and any type of upset condition. Special designs are required for screw conveyor components including the drive, coupling shafts, tail shafts and screw sections to reduce downtime and facilitate replacement.

One of the screw conveyors at the OCI Green River facility kept experiencing failures of a tail shaft. The tail shaft would fail every few weeks due to high radial loads causing extensive down time. Production of soda ash was interrupted every time the tail shaft broke. It would take approximately twelve hours to replace or repair the tail shaft. The screw conveyor was not originally manufactured by KWS but OCI needed help in solving their problem. The reliability engineer at OCI contacted KWS for help. OCI had past experience with KWS and knew that KWS was the company to solve difficult conveying problems. KWS provided a long term solution and eliminated the tail shaft failures from the screw conveyor.

Design Parameters

KWS Advantages

Engineers from KWS worked directly with the reliability engineer at OCI Chemicals to understand the application and determine the loads on the tail shaft and screw section. Using Finite Element Analysis (FEA), KWS was able to model the loading conditions on the shaft and screw to determine the location of stress concentrations. By redesigning the tail shaft, KWS was able to reduce the stress concentrations and design the shaft for infinite life. Since the shaft had to fit back into an existing screw conveyor, critical dimensions of the shaft could not change.

Special Features

The new tail shaft was designed to bolt to the screw with a flanged disc eliminating the use of CEMA coupling bolts. The flanged design allowed OCI maintenance personnel easy access to install and remove the tail shaft without having to disassemble one end of the screw conveyor.

KWS reduced the down time to change out the tail shaft by 75-percent, if ever needed. KWS also modified the existing screw section to add the mating flange. All of the new components were machined to very close tolerances and shaft run-out was less than 0.005-inches at the bearing. The new KWS design was so successful that OCI had KWS retrofit another existing screw conveyor with the same issue.