HSP ? Herrenknecht Separation Plants: separation plants for microtunnelling, pipe jacking and associated applications

Oct 12, 2007

In the past, it has been proven time and time again that separation is a vital and sensitive integrative module in the use of tunnel boring machines with hydraulic slurry systems. The separation plant separates the slurry suspension components – solids and liquid – in order to re-provide the slurry circuit with the supporting medium and to make the separated solids disposable. Hydraulic transportation is, however, limited by the concentration of the solids and by grain size. Only an efficient separation can guarantee high performance rates. Insufficient separation obligatorily leads to difficulties regarding the removal of the material from the tunnel face to the separation plant above ground, ensuing downtimes and reduced output.

The absorption capacity of any liquid, including the drilling fluid or so-called slurry, is limited. If the specific gravity rises to approx. 1.3 – 1.35t/m³ the carrying capacity of the mud decreases and a complete saturation is reached. It is then no longer possible to absorb any more material and a complete change of the slurry becomes necessary. An insufficient slurry system is not able to cope with the continuous accumulation of ultra fine solids in the slurry.
Conventional screening and cyclone methods fail regarding the separation of ultrafine particles already after a short period of time, since these particles cannot be sufficiently separated. This leads to a successive densification of the slurry. Similar to sugar in a cup of coffee, which can no longer dissolve additional sugar after reaching a certain saturation level, it is not possible to further absorb and transport additional material, once the slurry is saturated. Experience showed that wear on all system components increases dramatically due to the content of fine particles in the slurry and that the performance of the pumps, and thus operating costs, also rise considerably due to higher pipe resistance and pressure losses.
Herrenknecht has designed its new, innovative Herrenknecht Separation Plant (HSP) technology with a sequential modular design to overcome these problems. During each separation stage, a maximum of solid particles are removed from the slurry. The system technology, which has been specially adapted to the requirements of microtunnelling, guarantees continuous and fast advance rates, reducing downtimes to a minimum. State-of-the-art screening technology combined with Hydro Vacuum Cyclones (Hydro-VACsTM) – the Vertical Clarifier HVC 27 and the Centrifuge HKD 46150F constitute a complete recycling concept, providing optimal removal of critical fine particles and guaranteeing constant slurry properties.
BHX steel screens for coarse screening and dewatering
Subject to the required capacity, flow, tunnel diameter, predicted geological conditions and advance rate, HSP 170, HSP 250, HSP 400 and HSP 500 separation plants are ideally designed for any kind of tunnelling up to an outside diameter of 4.5m. The use of different shakers, which are driven linearly or circularly and operate with an accelerating force of up to 6.3 G in combination with wire woven steel screens, guarantees optimum screen performance.
In comparison to often used polyurethane screens, the fine pre-tensioned BHX steel screens with hexagonal structure also allow the separation of fine particles in the critical range of 100 to 45 micron. This prevents sand from settling in the tank, slurry densification, and consequently a considerable decrease of performance rate and high disposal costs. The complete plane surface of the hexagonal design, which can be easily repaired using screen rubber plugs, provide maximum dewatering efficiency and a long product life of the screens. In addition, the hammer-wedge attachment system provides a fast and uncomplicated exchange of the panel screens. Subject to the encountered geology, the shakers can be equipped quickly with different screens from 4 to 325 mesh size (4.76 to 0.045 mm) in order to achieve an optimum separation performance.
Second separation stage: Separation with Hydro-VACTM Cyclones
Unaffected by density changes of the feed, the 12“ (301 mm) and 15“ (381 mm) large Hydro- VACTM Cyclones with under-pressure control, achieve - in comparison to conventional hydro cyclones - a more uniform and dryer underflow, and reduce the danger of clogging caused by oversize particles by means of the 76 mm inlet and outlet openings. The main feature of the Hydro-VACTM Cyclones is the low operating pressure of 0.8 to 1.0 bars that reduces the energy demand of the centrifugal pump as well as the wear within the cyclone body. Moreover, the lower operating pressure avoids a mechanical grain fracturing of the encountered grain fractions and refinement, i.e. degradation of the solids’ weight. The 4“ (100 mm) hydro cyclones, which are often used in tunnelling, have a considerably higher operating pressure of more than 2.5 bar. They continuously shear the excavated material to smaller particles during each process cycle. The capacity of the Hydro-VACTM Cyclones, which is higher than the process volume of the entire system, allows not only to separate the solids, generated by tunnel excavation, but also to clean the slurry in the storage tank of the circuit at the same time in order to increase the efficiency of the entire separation plant.
The generated centrifugal force in the Hydro-VACTM Cyclones moves the particles towards the outside at a simultaneously downward motion. Due to the different densities, lighter particles, which are located closer to the rotation center, are put in a counterwise upward motion and are removed towards the overflow. The form of the cyclone and the entry velocity determine the cut point that is about 45 micron (D50). (Note: D50 means that 50 % of all removed particles are not larger than 45 microns). Particles larger than 45 micron are discharged via the underflow. Fine particles smaller than 45 micron are directed back to the storage tank via the overflow. Agitators, which are vertically mounted in the tanks, keep the slurry in suspension in order to avoid fine particles to settle.
Removal of particles smaller than 45 micron
In many cases, the use of a HSP unit without additional separation stages is sufficient. In silty and/or clayey ground conditions with a high content of fine particles it becomes, however, necessary to introduce a Centrifuge in combination with a Vertical Clarifier into the system. This design allows the complete removal of all particles from the slurry nearly down to a size of 0 micron. The Vertical Clarifier HVC 27 is a special thickener tank that can process higher slurry feed volumes and will provide concentrated slurry to feed the centrifuge HKD 46150F for dewatering. The advantage will be a higher capacity of clean water returns from the Vertical Clarifier while the centrifuge dewaters the high density sludge from the Clarifier outlet.
The slurry that has been cleaned and desanded by HSP screens and Hydro-VAC TM Cyclones is transferred to the Vertical Clarifier HVC 27. Before the slurry enters the Vertical Clarifier, polymers or rather flocculants are introduced to the slurry by means of the Herrenknecht Flocculation Unit (HFU). Due to the different electrical loads of the polymers and the particles, the polymers attract fine particles and form larger flocs, which sink into the lower conical section of the Clarifier by gravity. The clean water rises into the upper part of the Clarifier, where it overflows a weir and is fed back to the slurry circuit. The sludge, that consists of flocculated particles, is pumped to the Centrifuge.
The HKD 46150F Centrifuge with a bowl diameter of 46 cm, a bowl length of 150 cm and a flow rate of 60 to 70 m³/h operates at a constant rotational speed of 2000 rpm and is designed to remove 7-8 t/h of solids. A heavy-duty, purpose-built impeller accelerates the slurry to bowl speed, thus normalizing the solids to the high-G environment. The feed goes through four longitudinal passageways, which spread the slurry over a long area of the bowl. This reduces the potential to disturb the solids as the scroll conveys them up and out of the beach. The quasi-axial flow hub design of the scroll induces a near laminar flow condition within the liquid pool. Liquid freely travels to the overflow weir with little or no turbulence. The removed solids have a moisture of about 25-35 % solids by weight that can be easily handled and disposed.
Conclusion
The innovative modular Herrenknecht HSP separation plants provide a solution, which is ideally adjusted to microtunnelling requirements. The process technology of the separation systems are an integral part of all AVN tunnelling systems and, as a proven system element, it assures optimum technical and efficient operation.

Contact

Timo Seidenfuß, M.Eng. (Herrenknecht AG)

77963 Schwanau (Germany)

Phone:

(+49) 7824 302-7897

Fax:

(+49) 7824 302-364

E-Mail:

Seidenfuss.timo@herrenknecht.de

Internet:

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