Centrifuge Decanters Enhance Microtunneling Construction Operation

By Daniel Lakovic, Flottweg Separation Technology

It’s generally uncommon to consider a Flottweg centrifuge when working on a microtunneling civil construction project, according to Bradshaw Construction Corporation Equipment Manager Ben Truschel.

“When people think of Flottweg centrifuges, they think of mining, chemicals, wastewater, and other industries,” Truschel explains. “They don’t really think of construction tunneling projects. In microtunneling, it’s all about the slurry—all slurry, all the time.”

However, while traveling in Europe and the Middle East, Truschel learned of a microtunneling project in Doha, Qatar, that was using Flottweg centrifuge decanters successfully. The job was canceled, and the equipment was available at a discounted price.

That’s when Truschel decided to explore an alternative equipment supplier for his field of expertise.

“Aside from the price, what was attractive about Flottweg is that they provided an entire kit on this particular job. It included a control panel, the feed pump, the piping—it was a complete system. You don’t normally see that. The guy I worked with was German, and he called Flottweg ‘The Mercedes of centrifuges.’ ”

Truschel liked that the Flottweg portfolio was all about centrifuges, and that they claimed separation technologies as their core competency. He decided to use the equipment combined with polymer machines from ClearWater Industries—a Wisconsin-based manufacturer of water clarification equipment and chemical preparation systems.

“We married the two companies’ equipment into a modular package,” he says. “I believe that was probably the first time it’s been done. Marrying technologies from two highly-specialized companies like Flottweg and ClearWater made perfect sense for our microtunneling civil construction projects, which is our core competency.”

The Microtunneling Process

Considered “Digital Age Tunneling,” microtunneling utilizes state-of-the-art, remote-controlled pipe jacking by a combination of PLC, HMI and a physical operator. The operator above ground operates the microtunneling system using a computer to advance and steer the microtunnel boring machine, jack the pipe, and excavate the soils.

Around 90 percent of Bradshaw Construction Corporation’s business is in microtunneling, mostly for water supply or water sewage projects. The tunnels, mostly created for future utilities, range from 20 to 120 in. in diameter.

The means of conveyance of the microtunneling machine is via a water-based slurry—similar to how a petroleum company drills down vertically for oil or gas extraction. Bradshaw uses a similar drilling fluid in their tunneling process, using a reverse circulation approach as opposed to direct circulation commonly found in the HDD industry.

“We must separate the ultimate liquid and the excavated material from the drilling fluid. That’s were Flottweg comes in,” Truschel explains. “In the mining or aggregate industry, their product is the aggregate—the solid. The material is mined and then a lot of times it must be cleaned, which uses water. The water gets dirty, and then you need to clean the water—usually for discharge or environmental reasons.

“In our world, it’s the opposite. The slurry is the product. We’re just trying to get the ground out of the way so we can get our tunnel in. The challenge for us is that the ground is typically not uniform. It could be sand, silt, clay, hard rock, weak rock, and everything in between.”

Bradshaw Construction is responsible for excavating that ground, safely supporting the ground, and conveying the spoils. Because the microtunneling machines convey via slurry, the solid must be separated from the liquid.

“Our product is really the slurry,” he says. “It’s the drilling fluid. We want to condition the drilling fluid best for the tunneling excavation process.”

Most all of Bradshaw’s projects are new construction and they have more than 30 years of experience. In those years, they have tried different types of separation equipment.

The first stage of separation is separating the actual cuttings that are coming out of the machine itself. This is typically being delivered to the plant in either a 4, 5 or 6-in. slurry line, depending on the size of the machine. The bigger the machine, the bigger the slurry line.

That supply of slurry is usually at least 30% solids content. The size of the material can be anywhere from 5 micron to 3-in. size aggregate. Clays and silts typically coming in at 74 micron.

“When we mine, the spoil-laden slurry enters a separation plant first, which has a primary scalping deck that removes all the large aggregate,” Truschel says. “What’s left is sand, silts and clays. After the first scalping deck, the material is usually picked up using a centrifugal slurry pump to pump through a series of hydrocycles onto a desanding deck. That is a second layer—the second deck is designed to take out all the sand.”

What’s left is silts and clay-sized material—fine particles generally 74 micron and less after the separation equipment does a significant amount of cleaning.

“The only way to remove the 74 micron and down—fine or small particles in the slurry—is to process it with small hydrocyclones with fine screening and then send it through a centrifuge,” Truschel says.

Combining Flottweg and ClearWater technologies, Bradshaw built a series of modular units that use a decanting centrifuge—two models for Flottweg (Z4E and Z5E). They have used this system since 2019 and have four systems in operation.

The Separation Process

The Flottweg Z Series centrifuges have a modular design, making them customizable to specific applications. When solid/liquid mixtures with a high portion of solids need to be separated, decanter centrifuges operate continuously using high centrifugal forces to separate the finely distributed solid particles from the suspension.

The centrifugal acceleration separates the solid phase and one or two liquids from each another. Because the sediment (solid phase) has a higher density, it collects on the wall of the bowl. The transport scroll moves the solid continuously to the outlet openings. The liquid phase(s) flows along the scroll.

A decanter can be regarded as a sedimentation tank that is wound around an axis. In the sedimentation tank, the solid particles, which are heavier than the liquid, move to the bottom by gravity and form a sediment (solid phase) at the bottom of the tank. The solid and liquid phases are separated by means of centrifugal acceleration.

In the rotating bowl of the centrifuge, the solid particles, which have a higher density and are therefore heavier than the liquid, move outward because of centrifugal force. The sediment is formed on the inner wall of the centrifuge bowl. Since centrifugal forces of approximately 3000 g are exerted in a centrifuge as opposed to 1 g in a gravitational field, the separation of the solid particles from the liquid is much faster and more efficient.

Advantages of Using Centrifuge Technology for Microtunneling

Measuring the results is the name of the game on a civil construction jobsite, Truschel says.

“If you walk onto a job and look at the effluent coming out of the discharge of the centrifuge and the water is crystal clear, then you can’t do any better than that,” he explains. “So even though it’s not traditional to use a Flottweg centrifuge for a microtunneling project, we have seen clear results.”

Truschel said there are many advantages to the system he has created using Flottweg equipment.

In the microtunneling process, the clean water is recycled back into the drilling fluid tank.

“At the end of the project, when the tunnel’s completed, we have all this drilling fluid. We can’t just dump it on the ground,” Truschel says. “With a good quality centrifuge from Flottweg and a good quality floculation unit from ClearWater, we can remove 99% of the solids out of the drilling fluid. Maybe you can discharge it into a filter system and then, literally, just discharge it right back to a storm drain.”

This produces a big reduction in disposal costs, as well.

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