PSI-Polymer Systems

PSI-Polymer Systems Inc. Extended-Area Continuous (EAC) Screen Changer for High Volume Material Filtration to be Featured at NPE 2015 Conover, NC (March 23, 2015)

PSI-Polymer Systems recently sold a very large Extended Area Continuous (EAC) screen changer to one of the world’s largest materials producers. The producer utilizes a massive extruder with a 4 ft diameter die that can produce enough material to fill an entire railcar in under an hour when running at full capacity. Because of this, downtime is extremely costly for the company and they targeted their old screen changers as one of the main targets causing their downtime.

Changing market conditions have resulted in demand for virgin materials filtered to higher purity levels.  Where once, 20 mesh screens were the norm, now 100 mesh filters are becoming more common.  The chief benefit for the secondary processor market, such as extruders of packaging products, is minimizing the frequency of screen changes and not compromising production output.   However, running higher filtration levels at the primary processing level has its own production costs.  Output rates are reduced 5%-10% to compensate for the increase in back pressure caused by the finer screen mesh.  Additionally, finer screens trap more impurities, but do not last as long.  On a reactor line, screen life can drop to a matter of days and perhaps up to a week before the extrusion line must be stopped.  The increased frequency of stoppages costs production output on these big lines and also increases the real risk that the line will not start in time to prevent a reactor shut down.

The screen changer design on the supplier’s old machine had significant issues. The process involved pelletizing virgin polymer on a reactor-fed line. Periodically, the line needed to be shut down to change filters. While the reactor is idled for the changeout, minimized output is collected in receiving vessels. These vessels have a finite capacity, which necessitates getting the screen change done and restarting the machine before they fill. If it is not done in time, the reactor has to be shut down which can cost days of downtime for cleanout, potentially costing the operator anywhere from $500,000 to $1 million dollars.

Seeing the design of the existing screen changer as the problem, this materials producer decided to seek out new screen changer suppliers to help improve their process. The producer approached PSI-Polymer Systems and two major European screen change suppliers asking for them to first, calculate how much filter area would be required to meet a specified production rate and delta pressure while filtering through 100 mesh screens, and second, to supply the equipment.

One of the European suppliers offered equipment with less filter area than was currently in operation; the other offered a solution whereby the processor would be required to undertake a major building expansion to fit the recommended screen changer. PSI alone approached the problem with a clean sheet of paper and determined that whereas the existing screen changer offered 670 in2 of online filter area, in fact 2,800 in2 was needed to achieve the goal.

PSI determined that four (4) online filter positions utilizing extended area tube filters would be the most process efficient approach and would not require physical plant modifications other than repositioning the large underwater pelletizer. The screen changer would provide the plant with the means to run in a continuous mode, changing screens without stopping production, and would run the higher mesh screens within the required pressure window. Additionally, the PSI design would eliminate all air inclusion at screen change, providing the added benefit of reducing off-spec pellet and scrap generated by air passing through the die plate during screen change. The project moved forward and PSI and the buyer worked closely together, performing CFD (computational fluid dynamics) analysis until arriving at the ideal design. 

Increased Filtration = Increased Material Production

The new PSI EAC screen changer contains four massive oval pockets, each one of which is 10” tall x 14” wide. Employing the desired “candle filter” design, the new machine has 700 square inches of filter area per position, a huge increase that allows for higher volume and higher quality filtration, as well as less downtime. The screen changers are 100% PLC controlled and run automatically, requiring very little operator intervention.

This particular customer had the bad experience of following the old screen changer manufacturer’s recommendation that they shift on the fly. When the slide plate failed to complete the shift, the resulting polymer spill was catastrophic. As such, the operators were reluctant to follow PSI’s instructions to change the filters without a line shut down, instead opting to maximize run time and change all filters at once. After gaining confidence with the operation of PSI’s EAC Screen Changer, the plant with PSI’s engineering manager on-hand, made the attempt. Success followed success and today, line shutdowns for screen changes on this line are not required. Production rates have not only increased to the desired filtration level, but overall yield has measurably improved by eliminating periodic line stoppages. It has been running for 8 months now with no issues yet reported.

“We saw an opportunity to use our diverse skill set to make a difference at this company, and that’s exactly what we did,” says Don Macnamara, General Manager at PSI. “At PSI, we pride ourselves in being able to meet any customer’s needs, no matter what the process or the machinery requires. This is just one more example of us being innovative to find a solution for a customer.”