Back to Basics: Understanding the Flow of Powders and Bulk Solids

Funnel flow is where many problems with storing and discharging solids become evident, Maynard explained. It’s often at the mercy of the environment—humidity and heat in particular—which causes solids and powders to stick together, cling to the walls of the hopper, and stop discharging.

"I was at a pet food plant in Brazil, and [the workers] got frustrated with the dog food not coming out of the hopper. Can you imagine dog food in Brazil, where it’s extremely hot and humid?” Maynard says. “The dog food is coated with flavored fat, so when that fat heats up on the surface of the kibble it tends to stick together and bridge. So, the operators there beat the sides of this hopper with a sledgehammer to try and get it to flow again.”

The Persuader

Speaking of sledgehammers, Maynard says it’s far too common in plants all over the world to have what he calls “the persuader” nearby to dislodge blocked hoppers, which not only damages the hopper over time, but it also puts worker safety at risk.

“I’ve been involved in projects over my nearly three decades of doing this, where an operator was injured. They can get tennis elbow or a hurt shoulder from using a sledgehammer,” Maynard explains, “or, as was the case with a project I worked on years ago in West Virginia, one operator accidentally hit another operator in the head with a sledgehammer. All because they were trying to get sticky material out of the hopper.”

Arching/Bridging

Not all powder and bulk solid jams are created equal, so Maynard detailed the most common obstacles in a hopper that everyone in the industry should be able to identify, and why that material ends up blocking the discharge of product.

“Some materials don’t like to flow, they don’t like pressure, and they tend to interlock,” Maynard says. “We call this arching or bridging. Big rocks or material of different sizes literally form an obstruction over an outlet, and they lock together. The Roman aqueducts, for example, use bridging to hold those arches in place.”

Maynard says applying a sledgehammer or having a vibrating hopper to shake the material loose doesn’t work when bridging occurs. “If you shake it, guess what happens? Even less chance of flow because you’re just packing the material in tighter with nowhere to go,” he says, adding that an easy way to see bridging in action is at a hotel breakfast buffet, where the cereal dispensers stop working because flakes or puffs, for example, are interlocking and bridging together over the discharge area.

Poor powder and bulk solids handling—which includes environmental factors in a processing facility—can affect production costs, time to market, and worker safety, says Eric Maynard, vice president at Jenike & Johanson.Michael CostaRatholing

Ratholing—named because the vertical tunnels that form within stagnant material resemble rat burrows—happens when powder sticks together due to environmental humidity or moisture being present, and on an industrial scale can be a costly problem. Ratholing doesn’t happen with mass flow—only funnel flow, according to Maynard.

“Ratholing is fundamentally due to the particles not flowing along the walls of the hopper and being stagnant. It can only flow down through itself in narrow tunnels, overcoming its own cohesion to open up this vertical channel. I don’t care how much you hit the side of a hopper with a sledgehammer, you’re not going to break a rathole—that material is stuck and stagnant,” says Maynard. “An operator can only realize about 10% to 20% of a hopper’s capacity if there is ratholing.”

In industrial-sized hoppers, there can be two or more ratholes surrounded by stagnant material. In some plants Maynard has visited, workers were directed to stand on top of the stuck powders and attack the rathole with a pole or air hose to loosen the jam. “Do you think that’s safe?” Maynard asks. “If the worker falls in, it’s game over, right? I was at a plant in South America where I saw this happening, and the plant manager said it was OK, because the worker had a hose wrapped around his waist so they can pull themselves out or be retrieved if they fall through. That is a horrible job for anyone, and you never want your personnel in that situation.”

Caking

Caking is similar to what happens with ratholing, except there are no tunnels or holes—all the material clings together because of high humidity and clogs the entire discharge. Flour, Maynard notes, is a prime candidate for caking, and if left stagnant for a certain amount of time, will have to be disposed.

“I worked on a project recently with a flour recall because flour beetles grew in that caked, stagnant flour inside a silo,” Maynard remembers. “That’s why [processors] heat-treat or cold-freeze flour before they process it into something like a cookie dough, as a precaution against something like this.”

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Powder separation

One of the fastest-growing segments of powder processing is packeted drink mixes, where different types of powder are mixed—sweetener, flavoring, nutritional/functional granules, etc.—and dispensed into packets through a hopper. But those drink mixes, along with other products that start as mixed powders, like vitamins, will only discharge fully mixed if the hopper is working in mass flow. Separation will occur if there is funnel flow.

“The powders can separate by size, shape, or density. So, when [an operator] goes to fill this mix into packets, some of the packets get a lot of sugar, some get a lot of citric acid, some get a lot of vitamins because of particle separation,” Maynard explains. “The finer powders move to the center in a vessel, and the course powders roll off to the periphery. So how you discharge from a vessel is important. If you have a silo that predominantly discharges from the center, you’ll get the fine powder first, then the course material, and the blend is lost. But when you come down in mass flow, the fine and the course particles meet at the outlet and they re-blend.”