Q&A with Clean Slurry Technology

TBM: Tunnel Business Magazine caught up with industry veteran Jarrod Rice of Clean Slurry Technology to talk about the latest in slurry separation technology for tunneling and microtunneling applications. In these situations, slurry serves as the critical factor in maintaining peak production. Identifying and maintaining the right equipment and slurry chemistry for the job are paramount to effective and efficient operations.

Please give us a brief overview of Clean Slurry Technology and its role within the industry.

Clean Slurry Technology (CST) specializes in solids control and has developed global supplier relationships to bring the best resources to every customer. CST has exclusive sales distribution rights for Derrick Equipment in the U.S. Civil Construction Market; the CST rental fleet serves all markets. CST designs and builds separation plants to fit project specific needs. Expert personnel optimize every system and piece of equipment for its application. CST provides every client with the equipment and expertise to get the job done.

What basic equipment is needed for a slurry microtunnel operation?

The slurry system for a microtunnel project consists of a scalping shaker(s) followed by hydrocyclones. The scalping shakers should be able to process 110% of the TBM’s flow rate while producing stackable and truckable solids. The scalping screen(s) remove as many particles as it can to protect the slurry pumps which feed the hydrocyclones. Hydrocyclones are the most efficient technology to process the higher flows of the TBM and remove solids at the silt and sand size ranges. The solids fraction which is discharged from under the hydrocyclone is typically further dried out on a dewatering screen frame which is placed below the hydrocyclones. These components are typically all on a single multichambered tank with the last and cleanest chamber feeding the TBM. Additionally, a centrifuge is often added to help control the rheology of the fluid. The centrifuge removes particles down to 4-5microns operating in a “high speed” function. A contractor can remove solids down to 1 micron by utilizing a chemical addition before the centrifuge. This is typically called “dewatering.” In this mode the centrifuge typically spins slower but removes more solids. If the clay particles in the 0-5 micron size range are not removed, they continually build up and can be detrimental to the fluid. A clay content causes increased plasticity and thus increased pressure on the pumps, pipe, hoses, and formation. Some plasticity is good but too much can cause decreased tunneling rates.

One additional piece of equipment that we have seen provide huge benefits is a belt scalper, which is placed before the scalping shakers. This piece of equipment is worth its weight in gold when tunneling through clay. The large clay balls do not convey off a vibrating screen well and blind off the scalping screens. This results in tunneling fluid flowing right off the shakers and onto the solids discarded. At best this increases the contractors haul off, and at worst creates environmental impacts. A belt primer removes these clay balls via a belt and protects the scalping screens. We have seen projects come to a screeching halt because a belt primer was not used. Once this one relatively small piece of equipment is utilized the rest of the project carries on without a problem with the separations plant.

How can operators properly test the slurry to ensure they are getting optimum performance?
The tests that should be done at least twice a day are the mud weight, sand content, and viscosity. These are all quick tests that track the trends in the mud and can be used to identify most mud and equipment issues. The mud weight helps show the percent of total solids in the mud and how thick or difficult it is to pump. The sand content test shows the quantity of sand sized particles that are in the mud. Sand sized particles are particles greater than 74 microns or 200 mesh. A marsh funnel is used to track the viscosity of the fluid. The viscosity is typically managed based on the TBM manufacturer’s capabilities and ground conditions. The viscosity should be anywhere between 28-100secs/quart depending on the ground conditions. The finer the material the less viscosity needed and the bigger the rock the greater the viscosity.

Can you give us some tips for setting up and maintaining the equipment properly?

Safety is always paramount. Using the proper tools for the job is extremely important. Some separations plants require cranes or large excavators to assist with the set up and tear down. It is important to have knowledgeable people rigging this equipment. As far as operations are concerned there is a small amount of daily maintenance that needs to be done. However, the mud does need to be watched regularly to ensure it maintains the desired properties. Centrifuges should be cleaned out each shift and washed out each week. Screens on the shakers should be checked and replaced if there are large holes that allow solids to bypass the screen. Depending on the manufacturer of the equipment these tasks can vary in time and frequency.

A common misconception is that you only need one size slurry plant for all tunneling. Just as you need a particular TBM size for a job, slurry plants come in various sizes too. It is true that a slurry plant should be slightly oversized for the TBM, but we are seeing customers using slurry plants that are too small for the TBM, and the slurry plant becomes the limiting factor to production rates.

Some of the trends we see are tighter environmental controls, working within tighter footprints, longer drive distances, tighter curves, and larger diameters. Clients are starting to look at their “all in costs” broken down carefully. For example, tracking pump, pipe, cutting tool wear, as well as cost of bentonite and the disposal associated with the use of bentonite.

Disposal costs of spent slurry will be a significant factor on most jobs. With a professionally and intelligently designed separation plant, the project team will cut down the costs of spent slurry haul off and down time in a meaningful way.

We think that the trend soon will be to put more emphasis on separation plants that are appropriately designed for the specific project. We will also see a major increase in requiring each project to have a bespoke slurry separation plant, along with a fully qualified slurry engineer.

The more the contractor accurately tracks all costs and production rates, the better they will be able to determine the best place to invest to reduce these costs and make better purchasing decisions. Using a separation plant that results in a high sand content can cause extremely expensive repairs to the centrifuge, pumps, plumbing and the tunnel boring machine. These costs may not be realized for 6-24 months as a cost.

It is a difficult but worthwhile task to track your solids control costs to include wear and tear on all ancillary equipment and additional disposal costs. With a tight labor market and a young generation less than enthusiastic to not just enter this trade but all trades, mentorship of your current workforce and reasonable compensation is mandatory in our ever-evolving industry.

In addition to the disposal of solids is the increased regulation around the discharge of the water. We also build systems for treating the water prior to discharge. These permitted discharge criteria are becoming more and more stringent. Additional constituents of concern are being added and the current constituent requirements are becoming tighter and tighter. These additional regulations create an even greater need for systems to be designed and managed appropriately for the disposal of the solids as well as the liquid.

Concluding thoughts?

We believe that the outlook is solid for microtunneling in the foreseeable future. The largest challenge as an industry will be training qualified people. If we don’t, and some jobs go poorly, it could give the industry a black eye and lead agencies to look at performing the work differently.

Regarding evolving customer requirements, we see that in many cases clients are requesting an onsite slurry engineer to maintain favorable slurry characteristics and high uptime.

Slurry design plans are often requested by the client during the bid phase. Detailed plans of the separation plant and its capacities are also often cited by the owner during the bid phase and considered in awarding the bid. In fact, the owners and construction management teams are becoming aware that the separation system can be a bottleneck in the whole operation.

It is also becoming a factor to use local vendors, as there is much less chance of having a supply chain interruption. In the event of a breakdown of equipment, lead-times for repair can extend into days or weeks, or months if sourced overseas, which can be detrimental to the progress of the project. This can cause contractor downtime, which by its nature will be extremely expensive.

Some of the drivers behind these evolving project requirements are engineering firms writing more detailed specs to include the separation plant, as well as slurry engineering. This often takes into consideration the total operational costs, including pump rebuilds, wear on the cutterhead tools and other ancillary equipment.

It is also especially important to minimize waste disposal as this can be an extremely expensive effort. The separation plant can effectively recycle the slurry with no need to haul off the excess slurry. We are starting to see more projects with extremely limited space to accommodate excess waste. It is tremendously helpful and cost efficient if this excess slurry can be disposed of onsite as clear water.
As a leading supplier of slurry separation technology, Clean Slurry Technology has responded to these evolving customer requirements by designing separation plants that can adapt easily to accommodate different diameter tunnels as well as differing ground conditions.

We take into consideration the size of the machine, cutterhead design, process rate in gallons per minute, cubic meters per hour and most importantly, the maximum tonnage that this machine can produce under optimal conditions.

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