An innovative and effective straining method

Nicola Brittain

How automated strainers can be paired with macerators to eliminate solids from slurries.

In the power industry, industrial strainers are used to separate unwanted suspended solids from liquids, and slurries from boiler water slag; or for filtering Bunker C fuel supplied to burners for power generation. However, conventional strainer methods can be unreliable and lead to extensive maintenance, particularly in cases where the debris or solids are of substantial size and there is a significant amount of suspended solids present.

Fortunately, a novel blend of industrial wastewater technologies now allows for the efficient removal of solids without the need for extensive manual labour, worker exposure to messy or potentially hazardous substances, or constant maintenance. Specifically, the design involves a one-two punch combination of a macerator, which breaks down large solids into smaller fragments, and an automated scraper strainer flexible enough to efficiently filter out larger debris along with tiny particles. This innovative solution can even accommodate high solids loading without clogging.

The combination of these two established technologies is already being applied to some of the toughest, dirtiest straining applications in the industry including wastewater debris, power plant boiler water slag, asphalt transloading, and meat processing waste streams.

Overcoming traditional limitations

Duplex strainers are often used in continuous flow processes that cannot be shut down for cleaning purposes. Duplex basket strainers employ two distinct chambers that function independently. When one chamber needs cleaning, the flow is seamlessly diverted to the alternate chamber, enabling the removal and cleaning of the first basket.

Cleaning is a messy, laborious process that involves equalising pressure between the baskets, diverting flow to the off-line chamber, opening the cover, manually removing the clogged basket, and cleaning it before refitting the basket, ensuring the seal, and tightening the fasteners.

If an operator fails to adequately clean the basket strainers for any reason, both strainers can become clogged at the same time. This compromises the filtration process, resulting in quality issues or unexpected downtime until the problem is resolved. For many processors, this can occur simply due to having insufficient personnel to keep basket strainers clean along with their other duties.

“As an alternative, a combination of established complimentary technologies such as a macerator and an automated scraper strainer can essentially ‘knock out’ even the toughest problems related to large solids and high solids loading in an automated way,” says Robert Presser, vice president of Acme Engineering, a North American manufacturer of industrial self-cleaning strainers. The company is manufacturer of environmental controls and systems with integrated mechanical, electrical, and electronic capabilities.

In this configuration, a macerator would be installed upstream to reduce large solids down to a manageable size. The capabilities of the automated strainer are crucial to the process as well, according to Presser.

“Although the macerator cuts up the biggest solids, the strainer must still be able to separate both relatively large pieces and tiny particles while handling high solids loading without becoming obstructed,” he adds.

In the case of Acme, the OEM’s automated scraper strainer is designed to continually remove both very large and very small suspended solids from liquids and slurries. Cleaning is accomplished by a spring-loaded blade and brush system, managed by a fully-automatic control system.

A trash pump for additional pressure

If additional pressure is required to clean the screen, Acme Engineering can add an inexpensive trash pump to the blowdown line to assist in removing the solids from the strainer sump.

“Since the solids are small, a little trash pump can pressurise the blowdown line to evacuate solids from the strainer. The combination provides a decent ROI because operators no longer have to monitor and clean out heavily loaded basket strainers, resulting in substantially less labour and downtime,” says Presser.

Alternatively, the sump can be replaced by a cylinder bracketed by two gate valves that open and close as needed to remove the solids waste.

“When you are ready to empty the cylinder, you close the top gate valve momentarily and open the bottom one by depressing a button to dump the accumulated solids into a receptacle like a dump truck or a conveyor bucket so there is no manual handling required,” says Presser.

According to Presser, Acme has already worked with power plant operators and managers to implement a wide range of specialised straining systems for difficult applications with exceptionally large solids or very high solids loading.

Boiler water slag

One example of this is boiler water slag. Over time, boilers accumulate ash that forms slag deposits, which act as insulation on the boiler tubes. Consequently, more fuel is needed to achieve the desired temperature and output compared with a clean boiler. By removing slag deposits, boiler efficiency can be improved by 1-4%, leading to reduced emissions from power plants owing to decreased fuel consumption.

For one power plant, Acme Engineering addressed the slag issue by employing large cylinders and gate valves to strain out slag from a waste slurry produced during boiler washouts. The size of the cylinders used exceeded typical dimensions, with the largest cylinder measuring six feet tall and 18 inches wide.

In another application, the OEM employed a strainer, cylinder, and gate valve configuration to eliminate solid impurities and debris from Bunker C fuel oil.

According to Presser, adapting strainers for the specialised filtration of uncommon liquids and slurries requires not only expertise but also collaboration with the processor.

“For unusual applications, it may take a few attempts to get it right. You may have to adjust the timing and frequency of cleaning as well as adjust the screen slot size. There are quite a few variables involved,” he concludes.