Rotary Airlock Case Study - Specialty Minerals

Specialty Minerals operates a small facility in Wisconsin Rapids, Wisconsin. The plant processes 100 Tons/ day of ¾” minus pebble lime for use as a filler & brightener by an adjacent paper mill. As part of the process, pebble lime is off-loaded from railcars, transferred into the plant, and stored in one of two silos. As needed, pebble lime is pneumatically transported from the silos to a slaking operation, which prepares the lime into slurry by mixing with water.

For years, the company used Smoot FT-12, Type 6 ni-hard valves w/ u-cup packing type shaft seals to feed pebble lime from the silos into the pneumatic convey line. After 3 months service, the 6 PSI internal convey line pressure would blow out the shaft seals, causing pebble lime to leak onto the ground. The spilled lime compromised the safety of plant personnel and replacing seals required significant costs in labor and maintenance.

Specialty Minerals also experienced shortened life from the valves. Being abrasive in nature, the pebble lime would wear the sealing edges of the valve rotor and corresponding bore. As the clearances on the valve increased, excessive amounts of pneumatic air would leak upward thru the valve and into the storage silo. The air disrupted mass flow of product into the valve, reducing its displacement and ultimately lowering production line capacity. In just 15 months, Specialty Minerals was forced to shut down the line and replace the valve.

Due to the limited time and resources available to combat these problems, Rick Kniprath, Plant Manager for the Wisconsin Rapids facility contacted Prater-Sterling through their local representation, Matt Schaldach of MKS Processing & Packaging. Having successfully used Inpro Brand bearing isolators in another area of the plant, Rick asked Matt if Prater could supply a rotary airlock with the same type of seal. Matt relayed this information to Prater-Sterling, who agreed to build the valve as requested.

As a result, Prater supplied their Model PAV-12 HD Rotary airlock valve with 12” throat. The valve was a direct fit replacement for the Smoot FT-12. All Prater valves come with standard outboard mounted, sealed for life bearings and a fabricated, open-end rotor. Working closely with Rick to provide a specific engineered solution, the valve was custom designed with Inpro Brand Air Mizer shaft seals designed to eliminate shaft seal leakage and a ceramic lined housing with tungsten coated rotor blades to provide longer wear life against the abrasive pebble lime.

To properly apply the custom seals, Prater worked closely with Inpro to select and install the Air Mizer onto their valve. Like all Inpro products, the Air Mizer seal is comprised of a stator fixed in place on the valve’s end plate and a rotor pressed onto the valve’s shaft. When fitted within one another, the two parts form a non-contacting, non-wearing labyrinth. For further isolation, compressed air is purged into the stator, which, when combined with the labyrinth, forms a positive barrier to keep product inside the valve, even when opposed by a positive internal pressure within the valve. Fitting the stator to the end plates required custom modifications to remove the existing stuffing box where packing would normally be fitted. This was easily accomplished at Prater’s factory, where final valve assembly and checkout took place.

To combat wear, Prater lined the valve with their exclusive 1/8” thick, alumina ceramic tiles. Unlike competitors’ spray on ceramic coatings, Prater’s glued and vacuum bonded ceramic tiles will not separate from the valve body under normal use. The alumina ceramic composition provides 20-40 times more hardness than conventional Ni-Hard castings. Furthermore, to ensure consistent tolerances and a positive air seal, all Prater ceramic lined valves are diamond honed to original dimensions. This added step ensures that all Prater ceramic lined valves come with the same 0.008 inch or less operating clearance offered on their normal products. This minimal clearance limits air loss across the valve, resulting in improved mass flow from bins and higher displacements over the life of the valve.

Specialty Minerals purchased and installed the first Prater PAV-12 valve in September 2006. Over the course of 15 months operation, no seal leaks were reported. In January 2008, it was removed for inspection, and found to be in good working order with no wear. A second valve has been operating in its place since then. Currently both silos operate the Prater PAV-12 valves, with a third available as a spare.

Rick Kniprath was asked about the impact of the improvements. He stated that each new Prater valve has eliminated almost $2,000 in annual costs attributed to lost production time, clean up of wasted product and parts/ labor to change the seals. He stated that Specialty Minerals’ production line now runs more consistently and at higher capacity with no downtime. The valves are also conservatively estimated to last a minimum of 3 to 5 times longer, significantly reducing costs to replace or repair worn valves on a more frequent basis. Overall, each new Prater valve is expected to save Specialty Minerals $9,500 per year, providing payback in less than two years.