Self-cleaning strainers eliminate pipeline crud in powder coating operation

July 18, 2017

An aging water supply system presents a filtration challenge for a powder and paint manufacturer.

Powder coating, a widely used alternative to liquid paints, is preferred in many applications because it does not require the use of a volatile solvent. The process uses fine, dry thermoplastic or thermoset powders that are electrostatically attracted to the object being coated and then thermally set and fused to form a tough, hard skin that typically is thicker and more durable than conventional paint.

A successful powder coating operation depends on consistent powder characteristics from batch to batch including purity, particle size, color, chemical composition and thermal properties. Naturally, powder production technologies are closely guarded trade secrets.

Powder problems

Major powder suppliers tend to be experienced paint manufacturers because the two technologies are so closely related. When one of the largest powder producers moved an East Coast operation into a new-to-them location in a 20-year-old industrial building, the producer immediately began to experience production and product quality issues.

The production issues centered around clogged spray nozzles used in the production process, chillers fouled with sediment and excessive wear on other production equipment. More importantly, customers were noticing a decrease in the quality of the powder they were supplied. They saw a higher-than-acceptable number of inclusions that produced defects in the finished surface of the customer’s products.

Both issues were quickly traced to the plant’s aging water supply system. The process lines in the plant use 600 gallons per minute (gpm) of water originally sourced from the municipal supply. The water is chilled to various temperatures used in the process and then recirculated through a cooling tower in a semi-closed loop that requires only makeup water from the municipal system.

Upon inspection it was discovered that the plant piping was heavily corroded and clogged with sediment. One section of 6-inch supply piping, for example, was more than 50 percent blocked with scale and sediment, robbing the downstream processes of adequate flow.

Plant managers initiated a program to replace the worst of the piping and performed an acid wash to free up accumulated scale in the system. Given the extent of the contamination, however, it simply was not economically feasible to replace all piping in the plant, and an alternative solution was required.

Upon inspection it was discovered that the plant piping was heavily corroded and clogged with sediment. One section of 6-inch supply piping, for example, was more than 50 percent blocked with scale and sediment, robbing the downstream processes of adequate flow.

Considering the options

At this point the facilities’ management team called in Engineering Sales Associates (ESA) of Charlotte, North Carolina, to explore the possibility of creating a filtration solution to eliminate the contamination at point of use. The task was assigned to ESA Solutions Manager Brandon Pue.

“The customer’s initial request was for point-of-use strainers at each critical device in the production system,” Pue said, “so that’s where we started.” He said they installed eight 2-inch strainers with 400 mesh elements to protect key equipment, but before long they found they needed a more comprehensive solution.

“The strainer installations included flowmeters and valving that allowed us to monitor each station independently,” Pue said. “What we found is that the amount of contamination still in the system quickly filled the strainers, which created a maintenance requirement, negatively impacting production. We needed an additional solution to clean up the whole system and not just the points of use.”

ESA’s analysis determined that the optimum solution was to continuously filter the entire 600 gpm flow to 25 microns. Achieving that result was complicated by the customer’s strong preference for a nonbackwashing filter system and the physical space limitations of the plant.

The first solutions they looked at — a bag filter and a self-cleaning filter — required more floor space than they had available. Working with engineers, they found the best solution was a pair of magnetically coupled self-cleaning strainers.

These self-cleaning strainers are mechanically cleaned by moving a disc over the surface of a cylindrical wire filter screen. In this model the cleaning disc is magnetically coupled to the actuating piston, which eliminates the need for shaft and external drive seals and greatly reduces system maintenance requirements. Process liquid is introduced in the center of the media and flows through it to be discharged on the outside.

Contaminants are collected on the inside surface of the media, and when the flow restriction reaches a predetermined value, the disc moves up and down to remove the contaminants and deposit them at the bottom of the filter housing where they can be purged. Very little process fluid is lost during the contaminant purge, and the entire process is accomplished while the filter is operating at full capacity with no interruption of liquid flow through the system.

A pair of mechanically cleaned filters installed in a large powder coating operation handle the 600 gpm flow in the main circuit. Two self-cleaning strainers were installed to meet both the flow and retention requirements of the system.

Installation details

The pair of self-cleaning strainers was installed to handle the full 600 gpm flow in the main circuit and make sure they could meet the system’s flow and retention requirements.

“They are installed on a common header with a butterfly valve to divert all of the flow through one strainer during maintenance,” Pue said. “We designed it that way to eliminate costly downtime whenever one of the strainers was out of service for maintenance or any other reason.”

The installation also includes a bag filter in a side loop of water from the cooling tower, and point-of-use strainers were left in place as an extra precaution.

“The (bag filter) is really just there to catch anything that may have been picked up in the cooling tower before the water is returned to the process loop,” Pue said.

Once installed, the filtration solution immediately produced an increase in powder production primarily due to the elimination of contamination-related maintenance downtime. The new strainers are serviced annually, and even that does not interrupt production since the flow is simply diverted from one strainer to the other.

The efficient, self-cleaning strainers have allowed the plant to increase production while eliminating the downtime previously required to service clogged spray nozzles and fouled chillers, as well as the cost of replacing hundreds of feet of contaminated pipe. Even more important, however, is the fact the customer complaints about powder quality have diminished from a roar to a whisper.

Douglas Drummond is a writer who specializes in writing about technology stories for the trade press. He has written for Eaton for the past 12 years, covering industrial and off-highway equipment applications.

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