By Barry L. Sorteberg
Separation plants have historically been the bottleneck in many tunneling projects, resulting in lost time and money to the contractor. It is vitally important to plan ahead to have the properly sized and correct equipment in place before the start of the project. Remember, poor planning results in poor performance, and poor performance equates to lost profits to the contractor.
Many contractors focus on the tunnel boring machine (TBM) and the performance that can be expected from this machine. Generally the contractor will bid the project on the upper end of the parameters that have been specified by the machine manufacturer. In most cases, these machines can meet or exceed the expected advance rate. However, contractors easily can overestimate the capabilities of their separation plant, resulting in the slowdown of the advance rate due to the inability of the plant to keep up with the amount of solids that are being generated by the TBM.
Choosing the correct separation plants is every bit as important as choosing the correct TBM for the project. Using the geotechnical data, the separation plant should be designed to accommodate the majority of the material expected to be processed. Due to the unknown factors that can affect the tunneling advance rate, the contractor must have adequate separation equipment to allow the contractor to move forward at an unimpeded rate when the tunneling conditions are favorable.
Too often, the contractor can find favorable tunneling conditions that can produce fast advance rates, only to find out that the separation plant cannot handle the amount of material, and the advance rate has to slow down to accommodate the separation plant’s inability to keep up with the TBM. Generally, when this happens, there are also several occurrences that can be very expensive to deal with, such as lost fluid over the shakers, filling tanks with solids and creating a major problem with the slurry rheology that can cause excessive wear on the TBM and ancillary components. If this cost was to be put on a spreadsheet, it would be phenomenal and be an eye-opening experience for the contractor.
However, there is excellent equipment available in the marketplace today that can alleviate the majority of the problems being encountered by contractors. Unfortunately, given the current economic conditions, it is difficult for a contractor to be able to bid a job and allow for the proper separation equipment to be purchased and still stay competitive.
Many contractors need to rely on existing equipment in their inventory and much of this is severely worn, bordering on obsolete or not favorable to the existing geotechnical stratum that is being encountered in the tunneling formations. It is imperative that the contractor anticipates the ground conditions and then selects from his existing fleet of equipment the best components available and use them in the best possible way for anticipated conditions. Each piece of equipment should be evaluated thoroughly as to handling capacity and suitability for the ground conditions that will be encountered on the specific project. If it is determined that this specific piece of equipment will be adequate for the conditions expected, then it needs to be refurbished to the best of the contractor’s ability. If this recommendation is followed rigorously, the end result should be acceptable to complete the project on time and within budget.
Components and Performance of a Properly Designed Separation Plant
Separation plants are designed around the anticipated geotechnical information that is available for the project and should be evaluated to incorporate the best equipment available for the specific size solids that are expected. To do this, general knowledge of separation equipment should be studied and researched. The basic separation plant consists of the following components:
Clay removal equipment, such as gumbo chains and belt scalpers. Typically, the clay removal machines will remove solids at ½-in.-plus size material.
Primary screening machines, which consist of high frequency, high “G” units to remove as much of the course sand and gravel as possible before the slurry is introduced into the slurry tank. Primary screening should be sized to handle 120 percent of the highest anticipated return rate and should be able to remove material larger than 500 microns to allow the secondary equipment to work efficiently and minimize wear on pumps and cone parts.
Secondary screening machines, which consist of various size hydrocyclones mounted over high frequency, high “G” dewatering screens. These units should be sized to handle 130 percent of maximum anticipated flow and be able to utilize screen media of 200 mesh or finer to remove all sand size particles (74-500 microns).
Centrifuges, which are used for removing solids that are silt sized (5-74 microns). Generally centrifuge capacity should be sized to handle a minimum of 33 percent of the highest anticipated slurry pump rate. This will help to keep the slurry rheology favorable for maximum advance rate with the TBM.
Polymer make-down and delivery systems, which are utilized along with a centrifuge, or other dewatering devices such as belt presses or filter presses, to completely remove solids in the 0-5 micron range. This stage is necessary to control adverse rheology encountered when the slurry starts to become saturated with ultra-fine particles in the clay range. Ultra-fine particles can cause problems with density and viscosity, which can cause adverse conditions throughout the entire tunnel circuit. This stage is also used to produce clean water for disposal when the need arises.
Rules of Thumb
When designing and selecting slurry plants, the following general rules can be used to ensure that the plants are properly sized.
Size separation plant for fastest anticipated advance rate.
Size separation plant for maximum tonnage that can be produced by the TBM and then add 20 percent redundancy to allow for the wear and inefficiency of mechanical equipment.
Size all components to maximum flow rate that can be anticipated by pumps being used to in the slurry circuit. Then build in 30 percent redundancy to allow for slurry inconsistency and wear and tear on cyclones and pump parts. Keep in mind that pumps are rated on water and “brand new” efficiency curves. Rates begin to drop quickly when wear is occurring.
Adjust components of the separation plant to meet the needs of anticipated geotechnical data. If you are anticipating mostly sand and gravel, you should concentrate your efforts on the primary screens and the hydro cyclones, as well as the dewatering screens underneath the cones. It is imperative that most sand-size particles are removed at the front of the separation plant. Any sand-size particles that pass the first defense of shakers and cones will quickly become an abrasive component to the slurry that can cause severe damage to the pump parts and ancillary components. Remember, sand is easily removed from the slurry the first time around. After it has been fractured by mechanical means, such as pumps, it can quickly be degraded to silt and clay-size particles, which are much more expensive and more difficult to remove.
If the geotechnical data indicates a large amount of clay with high plasticity, then your efforts should be geared to centrifuges and polymer delivery systems. The more times that a solids particle is circulated through the system, the smaller the particle will become and the more expensive it is to remove.
If the contractor does not have an in-house expert on separation equipment, consult experts with the equipment suppliers or an independent consultant. These people are trained and have the experience to alleviate many of the problems before they have a chance to occur. This is a very inexpensive insurance policy, and in many cases should be utilized prior to the bidding process. In many instances, contractors rely on their field foremen, who have vast experience in construction of the tunnel, but may not have specialized knowledge in separation technology. In some cases, the knowledge that is gleaned on this subject is erroneous or has been learned through trial and error. This can result in losses of 10 percent of the total cost of the project.
In conclusion, the functions of a properly designed slurry separation plant are an absolute necessity to complete tunneling projects successfully, on time and within budget. A properly designed separation plant will save time and money, and allow the contractor to concentrate on putting pipe in the ground, instead of dealing with adverse slurry parameters and the mess that this can create.
Barry L. Sorteberg is the principal of Clean Slurry Technology Inc. and a recognized expert in the field of slurry separation design and equipment.