written by: Bob Agnew, editor
A leading Midwest manufacturer of organic and inorganic color pigments, dyes and intermediates reports a dramatic increase in process flow by incorporating the DCF-800 Series self-cleaning filter technology supplied by Ronningen-Petter Engineered Filter Systems, Portage, Michigan.
Previous filtration methods incorporated an air-operated double diaphragm pump located beneath a blend tank which fed the pigment through tubular or bag filters and onto its final destination, a filling machine, rail tank car, tank truck etc... Depending on the size of a given batch the current filter would slowly accumulate unwanted solids which resulted in a decrease in process flow since the rising differential across the filter would require additional work to be performed by the diaphragm pump. The inherent performance characteristics of air-operated double diaphragm pumps dictate a fall in capacity when sensing a rise in system pressure, the amount of energy imparted into the fluid being pumped. The resultant decrease in flow would continue as solids loading increased. Not only did the decrease flow result in longer fill times, on occasion solids loading would reduce flow to virtually a no-flow condition, a point where the compressed air supplied to the diaphragm pump would be close to the differential pressure across the filter and added process pressure.
 courtesy: Wilden Pump & Engineering Co., Colton, CA The diaphragm pump performance curve (above), illustrates the reduction in flow resulting from an increase in the bag filter differential pressure during the filling process. Should the filter bag not be cleaned or changed additional solids loading will swing the pump left toward shut-off, a no flow condition. Generally speaking, once a bag filter's differential pressure approaches 12psid it will rapidly blind. In this application, when the pump sees a process pressure of 60psig the pump will stall as the compressed air supply pressure to the pump will equal the process pressure. |
When the filter would blind,
a no flow condition, the operator would interrupt the
filling process and maintenance would be called to change
the bag and the filling process would resume. On large
batches, and those where end user specifications required
tight retention's this procedure might be repeated
several times. Fluid laden bags once removed from the
filter would require cleaning, drying and storage, among
other maintenance tasks. The inherent product loss due to fluid laden bags, filter drainage, and spillage during disassembly and removal, the effects of reduced flows, and the numerous man-hours involved in this portion of the process lead process engineering's effort to source a filtration technology which would address and eliminate these issues. After a thorough investigation into optional filtration methods which included vibrating screen filtration and backwashing filter systems the DCF-800 Series self-cleaning filter manufactured by Ronningen-Petter was selected and put into service. The DCF's ability to clean (regenerate) itself during the process and purge unwanted solids virtually eliminated any reduction in flow and dramatically reduced operator and maintenance involvement. |
The DCF-800 filter was located at the point in the process previously taken by the bag filter. As pumped fluid from the air-operated double diaphragm pump would enter the top of the DCF it was directed down through the inside diameter of a slotted wedge wire element where the undispersed pigment and other unwanted debris would lay against the screen while the fluid would pass the element and exit the lower side outlet of the filter. As debris collect on the screen a pneumatically actuated circular cleaning device is initiated which travels downwardly then upwardly removing the debris from the screen. The natural downward flow of the fluid combined with the action of the cleaning disc would direct the removed solids toward the bottom of the filter. As debris collect in this area they are periodically purged from the filter using process pressure. Opening and closing of the purge valve is occasional and rapid limiting the volume of fluid exiting with the solids.
Cleaning of the DCF element is independent of differential pressure, it is based on time, and during an upset condition cleaning can be initiated automatically or by the operator. The DCF filter element is cleaned prior to developing any significant pressure differential. Keeping the filter at its clean differential allows the air-operated double diaphragm pump to maintain its maximum flow at process pressure throughout the entire batch. This consistently higher flow dramatically reduces filling time. With its ability to purge removed solids there is no interruption in the process due to solids removal. Operator and maintenance involvement has decreased, there is no longer a need to stop the process, open the filter, remove and clean the filter media, and resume the process.
The DCF-800 self-cleaning filter has decreased filter bag purchases and affiliated disposal costs, increased finished product volume per batch which is a result of the DCF's top inlet bottom outlet design permitting stripping of viable product at completion, and is a primary contributor to a cleaner, safer, environmentally friendly workplace.
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