How To Best Utilize A Horizontal Decanter Centrifuge
In A Hydro Vac Slurry Processing System
A centrifuge separates solids from water based on material density. The rate at which solids can settle out of water is increased using g-force which can reduce the settling time from hours to matter of seconds.
A slurry that requires dewatering is pumped into the horizontal bowl of the centrifuge. The fast rotation generates centrifugal force up to 4,000 x g. Under these forces, the solid particles with higher density are collected and compacted on the wall of the bowl. A scroll (also screw or screw conveyor) rotates inside the bowl at a slightly different speed and transports the settled particles along the cylindrical part of the bowl and up to the narrow end where the dewatered solids are discharged. The clarified liquid leaves through a paring disc at the wide end of the bowl.
This is a very simplified description of how a horizontal decanter centrifuge works with no mention or reference to the impacts of acceleration, differential speed, pond depth, filling volume, etc. Regardless, the description above is still accurate and all we need for this discussion.
The 3 Key Variables For Centrifuge Dewatering
For a decanter centrifuge to separate a water and solids mixture, there are 3 interdependent variables that need to be managed for optimal performance. These include …
- Solids Content in the processing feed.
- Processing Rate or Rate of Feed being pumped into the centrifuge
- Amount of Polymer Injected into the feed
The way in which these three input variables are collectively managed will dictate your ability to achieve your desired outputs which, for a Hydro Vac Slurry Processing System typically include …
- Water cleaned to allow for re-use or low-cost disposal.
- Processed Solids that do not leach water.
- Consistent Utilization of a high percentage of Processing Capacity (through put potential).
Solids Content and Processing Capacity
A large bowl centrifuge can intake up to 2,000 liters, or 2,000 kgs of water. The water can contain roughly 20% solids, depending on the density of the combined solution.
Remember that a centrifuge is going to be spinning at over 2,000 rpm, so in order for it to maintain the necessary balance to operate, the combined density of the solids and water in slurry cannot exceed 2,000 kgs in total weight and the weight of the slurry being fed into the centrifuge needs to stay consistent if you want to maximize processing capacity.
When the solids content goes up, the centrifuge control system senses the density change, and slows down the rate of feed. Failure to do this would potentially plug and possibly even damage the centrifuge.
For a highly consistent slurry feed that contains 20% solids, the optimal solids processing capacity of a large bowl centrifuge (21 x 72) is approximately 100 kgs a minute 6000 kgs an hour. This is of course a theoretical value. In practical terms, it’s possible to achieve an average processing capacity between 75% and 90% of the theoretical capacity, which would fall in a range from 450 kgs to 540 kgs an hour… for a highly consistent (homogenous) slurry feed that contains about 20% solids.
So the amount of available centrifuge processing capacity that can be effectively utilized primarily depends on the consistency of the slurry feed going into the centrifuge, which is more challenging within a batch processing system.
This will always be the case regardless of the make, model, or specific features of any given centrifuge unit.
Feeding Hydro Vac Slurry Into a Centrifuge
Once Hydro Vac Slurry is received and screened (with both receiving and screening having their own unique challenges), the remaining slurry mixture needs to be fed into a centrifuge for processing.
The big question at this point is how to feed it.
Feed Method #1 - Dewatering Tank
Most Hydro Vac Slurry Processing System that have a centrifuge as the primary processing unit, feed the centrifuge from the first compartment of a standard dewatering tank used commonly in oil field dewatering applications.
As the primary processing unit, the centrifuge is required to both clean the water and dry the solids at the same time while providing as much processing capacity as possible.
A dewatering tank is a large holding tank, typically about 10 feet high, 12 feet wide, with storage capacity usually between 60 m3 to 80 m3. The dewatering tank is divided into square compartments by vertical weirs that allow the flow of liquid from one compartment to the next.
In a typical dewatering tank design, screened slurry is pumped into the first compartment, which already contains hardened water for coagulation. The first compartment has an agitation system that is designed to keep as much of the solids in suspension as possible so that a water/solids mixture can be pumped and then fed into a centrifuge.
Once the slurry feed is processed by the centrifuge, the solids are dewatered and the clarified water is circulated back to the dewatering tank to provide additional retention time for sedimentation of any remaining suspended solids.
Feed Method #2 - Clarification Tank
Hydro Vac Slurry Processing Systems that have a Clarification Tank are able to feed the centrifuge from the build up of agglomerated solids at the bottom of the tank.
As the initial piece of dewatering equipment in the process, the Clarification Tank removes the suspended solids from the water and discharges or feeds the processed solids into a Centrifuge for further water removal. Working as a polisher, the Centrifuge is only responsible for removing additional water from the processed solids.
Properly designed Clarification Tanks allow the suspended solids to fall out of suspension and then build up at the bottom of the tank so a Centrifuge can be fed a more consistent water/solids mixture that allows for improved soil dewatering or solids polishing.
The water removed by the Centrifuge during solids polishing is returned to the Clarification Tank for additional cleaning of the remaining suspended material.
Feed Method #3 - Clarification Tank + Round Centrifuge Feed Tank
To further increase the consistently of the slurry feed to a Centrifuge, the discharge from a Clarification Tank is pumped to a Round Centrifuge Feed Tank where the combination of properly designed mechanical agitation in a round tank, the ability to control the slurry discharge from the Clarification Tank to Round Centrifuge Feed Tank, and the ability to adjust the water percentage of the slurry to keep the solids/liquid ratio within a desired range, allows for a more optimal feed to be sent to a Centrifuge for processing.
Creating A Consistent Slurry Feed - Rate of Feed
When a centrifuge is fed directly from a standard dewatering tank with no additional slurry conditioning steps, agitation is used to try and keep as much solid material as possible in suspension.
One of the key challenges with agitation is that the more inconsistent the material you’re processing, the less effective it can be in producing a consistent feed to a Centrifuge.
Here are some examples to further illustrate why agitation can be problematic.
The screened slurry that is being agitated is made up of a water, sand, silt, and clay mixture that is constantly changing in composition. Agitation systems, especially in square dewatering tanks, can allow heavier material to fall out of solution to the bottom of the dewatering tank. When the transfer pump sucks up mostly solid material from the bottom of the dewatering tank, a surge of solids will be fed into the centrifuge. The control system will detect the surge and slow down the centrifuge and/or shut down the pump until an acceptable slurry density is detected.
Slurry received from one Hydrovac Truck Load to the next will have different levels of total solids and different amounts of each type of solid material (sand, silt, and clay). High levels of overall solids will require water to be added as the centrifuge can only be fed up to approximately 20% solids in solution. Lower levels of solids will provide a diluted feed, reducing the volume of solids processed. And when the composition of solid material changes, there is a direct impact on the overall density of the feed which can also cause the centrifuge to be automatically slowed down.
Even if all Hydrovac Slurry received was exactly the same in terms of water content and solids composition, dewatering tank agitation by itself will always struggle to create a highly consistent slurry feed due to material settling and the constant adjustments made by the centrifuge’s control system to allow for sporadic changes in feed density.
The batch processing nature of Hydrovac Slurry Processing further exacerbates the challenges of feeding a centrifuge with just a standard dewatering tank as received material cannot always be processed in real time due to the processing capacity of a centrifuge not being able to keep up with the total slurry volume being received at any given time. In many cases, a Hydrovac Processing Plant will receive over 70% of its loads in 3 to 4 hour window during the afternoon period with receiving volume outstripping processing centrifuge capacity during different stretches of time in that receiving window.
When received and screened material needs to be retained prior to centrifuge processing, regardless if you’re using a tank or a pit, large amounts of the remaining soil material in the screened slurry are going to settle out of the slurry. When this happens, the screened material tank or pit is going to fill up quickly and instantly reduce storage capacity. The mixture in the screening tank that gradually gets pumped over to the dewatering tank is not only going to be difficult to move because of the constantly settling solids, but will be highly inconsistent in terms of a water to solids ratio, creating even more problems for the agitation and feeding process that comes next.
Premature settling of solids after screening reduces processing capacity because of the highly inconsistent feed now going to the centrifuge.
The combined impact of these different solids settling, agitation, and feeding challenges is that the realistic processing capacity of a centrifuge often gets cut in half from a range of 80 kgs to 100 kgs a minute to a range of 40 kgs to 50 kgs a minute.
Polymer Injection
The third interdependent variable is polymer injection.
Mechanical dewatering systems require chemistry to achieve rapid dewatering results. Polymer is added to a hard water slurry mixture to promote the development of larger particles with higher densities that can more easily be separated from water by gravitational force.
With a centrifuge, polymer is injected into the slurry feed immediately before the slurry enters the centrifuge. Polymer injection is a balancing act between generating clean enough water and dry enough solids. Too little polymer will result in runny solids and dirty water. Too much polymer may produce very dry solids, but it can also create murky water from polymer residue that will not allow the processed water to get clean enough for either re-use or low-cost disposal.
Getting the amount of polymer right when the centrifuge is the only piece of dewatering equipment can be extremely difficult due to 1) the constant change in slurry composition, and 2) the lack of retention time in the process that is necessary to continuously make both fast and effective adjustments.
Centrifuge as The Only Piece of Dewatering Equipment
In a dewatering system where a centrifuge stands alone as the sole piece of mechanical dewatering equipment, the system is asking the centrifuge to 1) get the water clean enough, 2) get the solids dry enough, 3) and maintain a high processing volume…all at the same time.
When you add in the inconsistency in material composition from one load to the next in a Hydro Vac Slurry Processing scenario, the degree of processing difficulty goes from high to near impossible some of the time, and to impossible most of the time.
Let’s go back to the three interdependent variables and how they work together.
- Solids Content in the processing feed.
- Processing Rate or Rate of Feed being pumped into the centrifuge
- Amount of Polymer injected into the feed
As an operator, you’re looking at the solids discharge, and it’s too runny or wet, so you increase the polymer. The solids start to get drier, but you notice that the water is getting cloudy from the polymer. So you reduce the polymer and slow down the feed to see if more processing time with less polymer can get you both clean enough water and dry enough solids. But then the solids content in the feed suddenly changes due to 1) the feed pump sucking mostly solids out of the bottom of the dewatering tank, 2) the next batch of slurry having too high or too low water content, and/or 3) the solids composition changing the slurry density. Now the water isn’t clean enough, or the solids aren’t dry enough, or both.
And so goes the constant tug of war between inflow, control, and output.
Slowing the whole process down may get you clean enough water and dry enough solids, but you may have to operate at or below 25% of your operating capacity to do so and that still may not work. Because the three defined input variables are interdependent, any change to one will have an impact on the others with any one change potentially having a positive or negative impact on one or more of the desired process objectives or outcomes.
As a result, The three desired process objectives …
- Clean enough water,
- Dry enough solids,
- High processing capacity
… cannot all be achieved a high enough percentage of the time so you must decide what you want to sacrifice.
In most Hydrovac Slurry Processing Systems where the Centrifuge is asked to do all three, the tradeoff tends to be dirty water for dry enough solids and greater processing volume.
Ironically, this is exactly what a centrifuge does when its fed by a properly designed Clarification Tank + Round Centrifuge Feed Tank, but with the added benefit of the overall processing system also being able to generate clean enough water.
***Note*** Clean water can be the most valuable element in any given Hydrovac Slurry Processing System. In many cases, trucks must pay to get water, or at least incur travel costs and time off site to procure it. If not cleaned for reuse, it can have a cost for disposal. Water available for truck re-use and/or truck clean out can both attract business and become a revenue stream in and of itself.
Centrifuge For Soil Dewatering or Solids Polishing
As a singular piece of dewatering equipment, a centrifuge is 1) removing suspended solids from water, 2) removing water from processed solids, and 3) struggling to do both at the same time to the levels required for all the reasons already mentioned.
As a polisher, the centrifuge has only one job and that’s to remove additional water out of processed solids.
Water removed by the centrifuge will be cleaned by the primary piece of dewatering equipment, which in this discussion is a Clarification Tank.
Further, because a Clarification tank can aid in the process of providing a properly conditioned and consistent water/solids feed, Centrifuge processing capacity can be optimized to a greater extent.
The inconsistent material being dumped from truck load to truck load is conditioned by the Clarification Tank which in turn pumps processed slurry into a Round Centrifuge Feed Tank in order to create a more homogenous slurry that can be continuously fed into a Centrifuge at or near the desired solids content level, which is what is required in order to optimize your available centrifuge processing capacity.
The processed solids discharged by the centrifuge can be made dry enough for re-use or disposal without sacrificing processed water quality or processing volume.
Summary
A centrifuge can be a highly effective piece of dewatering equipment within a Hydro Vac Slurry Processing System when it is utilized for the single purpose of soil dewatering or solids polishing.
In order to be used in this fashion, inconsistent Hydrovac Slurry being received from one load to the next needs to be conditioned into a more homogenous Centrifuge feed that can accommodate the batch processing demands of Hydrovac Slurry Processing. This will allow the Centrifuge to continuously operate at a high level of consistent performance, generating dewatered soil or solid material for either re-use or disposal.
A Centrifuge can also fail miserably as a primary dewatering unit where inconsistent input is turned into inconsistent and undesirable outputs.
It all comes down to how you feed it and how much you’re asking it to do.
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