1. Bowl speed or G-force,
2. Feed rate,
3. Differential RPM
4. The liquid level in the bowl (pond level)

There are also very rare variables that occasionally benefit the separation, 5) Rare adjustments. And, 6), One more variable that can affect the separation results is by changing the feed slurry characteristics.

1) BOWL SPEED OR G-FORCE: Most decanters operate at 2500 to 3200 x G. G levels less than 2500 x G for fine or organic solids slurries generally are inefficient in the separation. G levels over about 3,000 x G very often have no significant improvement in results, use more energy than needed, and result in more maintenance over the long term. There are exceptions but not many. There are a few high G-force decanters (5,000 – 10,000 x G) for very special applications. Some applications in mining, the coal industry, and in the chemical and plastics industries have large particulates that are often heavy. G levels of 600 to 1200 Gs are enough in these cases. Since many of these separations are very abrasive the lower G levels are preferred which greatly reduces abrasive wear on the contact parts of the conveyor and bowl.

Two things occur in the clarification section of the decanter bowl (from the center of the feed zone to the liquid discharge end). These are (a) sedimentation between the surface of the pool and down to the bowl wall, or heel cake, or already settled solids mass and, (b) compaction of the solids mass under the pool of liquid.

(a) Depending on the diameter of the decanter the pool depth can be anywhere from 1 inch to about 8 inches in depth. As the incoming feed travels towards the liquid discharge end of the bowl the question arises as to how deep into the pool is the moving liquid stream as it travels down the bowl. At Sharples we set up a test with a P-3400 (I was there for one, possibly the only, test.) where we could feed pure water to the P-3400 at known feed rates and we had a plunger needle tied into the feed line just before the feed tube. The plunger tube was filled with blue dye. By calculation of the feed tube diameter and the feed rate we knew the time it took, upon a very quick plunge of the dye into the line, to reach the pool and start traveling along the bowl. We could visually see the liquid discharge and used stop watches to measure the time from the plunger use to the dye emerging in the centrate. By calculation of the pool surface area at the known pool depth we could calculate how deep the moving feed liquid penetrated the pool, at the G force level we were at. The depth of the moving layer was surprisingly very shallow, something in the range of 0.25 inches. Even at higher feed rates the moving layer depth increases very little. Therefore, if the centrate contains more solids as the feed rate increases, why? Answer, three possible reasons. First, the feed slurry just does not have time enough for the solids particles to get through the moving layer and into the non-moving liquid level (Where they are considered to have separated, but not yet settled completely and not yet compacted.) before the liquid and remaining solids discharge. Second, the higher feed rate causes more agitation hindering the settling. And third, the higher feed rate total solids under the pool may increase and the top of the pile of solids being conveyed along the bowl may impinge on the moving layer and get picked up and discharged with the liquid (This third reason can be eliminated with a higher differential RPM in most cases.). Items 2 & 3 contributed to the thoughts and design changes leading to the PM series of decanters at Sharples with deeper pools and axial flow of the liquid. More on this in another article.

(b) Once the solids settle to the bowl wall, or heel cake layer, they begin to accumulate and press down on the solids already settled. The cake solids under the pool can compact and reach a surprisingly dry form. Normally we say that almost all of the “free water” is squeezed out of the cake solids mass under the pool. Particle shape and malleability influence the final dryness. As the solids are conveyed up the dry portion of the beach most of any remaining free moisture drains back into the pool. The cake dryness is affected by differential, G-force to a certain extent, and pond depth slightly. Differential probably having the greatest affect since this variable affects the time under G. The longer the time, the drier the cake discharge, up to the dryness limit of the exact solids present. G-force also affects the cake dryness again up to the dryness limit of the solids. Excess G-force once the solids reach their dryness limit is ineffective and wasteful.

2) FEED RATE: The feed rate affects results. The greater the feed rate the less time the slurry has under G-force before the liquid decants out of the bowl. The effect of feed rate changes can be very gradual or quite dramatic. Normally feed rate increases do not cause the separation results to change equal to the feed rate change. For example, doubling a given feed rate (where the separation results are good) might result in only a 10-20 percentage point reduction in separation efficiency. The exact feed rate to a given decanter, or, the exact decanter model chosen for a customer feed rate requirement, depends on the objectives of the customer. Some customers need the cleanest centrate possible (and can accept a wetter cake discharge), some the driest cake possible (and can accept a centrate with some solids loss), some a combination of both goals. With the foreknowledge of what the customer needs, and with enough test data, it is not difficult (for an experienced decanter Engineer or Technician) to provide the correct size and design decanter for that particular customer requirement.

3) DIFFERENTIAL / BACKDRIVE: If we attach a motor (backdrive motor) to the small pinion shaft coming out of the gearbox we can rotate that shaft and cause movement of the gears inside the gearbox, which are connected to the conveyor via the spline shaft. Thus we can change the conveyor speed relative to the bowl speed. We call this the “differential” speed or RPM. Lowering the differential allows better settling and better clarity of the discharged liquid. Consider, we are trying to settle solids in a liquid pool at the same time we are agitating the pool by a series of conveyor flights continually churning through it. The less churning the better. However, we still have to push out and remove all the solids that do settle. If we do not they have to go someplace and will go out with the liquid discharge which we call the “centrate.” Therefore the differential cannot be too low. The proper differential is found by testing. This is done on start up as part of the optimization assistance we provide, and later by plant personnel should the feed slurry characteristics change.
Higher than needed differentials result in less clarity of the centrate, wetter cake solids, and sometimes a higher torque (often a lower torque due to wetter solids). Torque is a measurement of the resistance of the solids being conveyed along the bowl and up the conical section to the discharge ports, and the scraping of the flight edges on the heel cake.

4) POND LEVEL: The liquid level in the bowl, called the pond level, can be adjusted by stopping the centrifuge, lifting the cover and adjusting the plate dams on the liquid end of the rotating bowl assembly. A few decanters can adjust a skimmer in or out to change the pond level while rotating. The deeper the pond level the better the clarification of the liquid; however, there is a negative effect. As the pond level increases it covers more of the conical section of the bowl or beach. This reduces the dry beach length where the solids are draining prior to discharge. The result is that the cake discharge may contain more liquid. Should the customer desire a drier cake discharge, as opposed to the cleanest centrate possible, the pond depth is lowered. Most applications begin with a deep pond level with very little dry beach. Testing with the bowl RPM, differential, and feed rate may or may not yield the results the owner wishes. In that case the pond level may be lowered or raised and the optimization process repeated.

5) Rare adjustments are related to sedimented solids that will not convey up the beach to the solids discharge holes. The solids are thick but not firm, and tend to be slippery. They slide on the beach and on the face of the conveyor flights especially in the “dry” portion of the beach. (A BD design conveyor would normally be the answer, but might not be readily available.)

a) If adjustments in the pond level and the differential do not work, with a standard conveyor, another possibility is to provide a bed of rough solids as heel cake on the beach. An example of this that involved the author was in the grape juice industry. A 14” diameter decanter was sold to remove grape pulp and skins from the grape juice. The seeds had already been removed prior to the juicing operation. A Sharples P-3400 had been tested successfully on the application, but the P-3400 had been on other tests previously and had a hard and rough heel cake already present in the beach area. On start up of the purchased decanter with a clean bowl, no solids would discharge. The pond depth was increased, the bowl RPM adjusted, and the differential RPM lowered with no success. Drums of dried ground grape seeds were nearby and scoops of that material were dumped into the feed line and fed into the decanter until the seeds discharged like beebees into the casing. The grape pulp slurry was fed and now solids emerged with no problem and the centrate juice was clean. They continue to operate and now have several years of operation (2018).

b) On rare occasions the author has observed that lowering the bowl speed allows the cake to discharge. This happens with a solids type that does not need a high G to separate and compact, and can overcome a lower G when being conveyed up the beach.

6) FEED CHANGES: If all of the above adjustments just cannot produce the separation and cake dryness desired by the customer, it is possible sometimes to change the feed characteristics. The easiest way is to add a coagulant of some sort. With waste sludges, such as sewage or manure slurries, this might be a cationic polyelectrolyte. With water treatment (potable water) sludges it may be an anionic or nonionic polyelectrolyte. pH changes sometimes cause the solids particles to agglomerate as might a temperature change. Early in the author’s Process Engineering career with Sharples I was given the job of separating soybean protein with a P-5000 Super-D-Canter installed in the customer plant. After a week of failure with all of the adjustments discussed above I experimented with pH in the plant lab with little difference in the particle size resulting. Hot water added to the slurry did seem to agglomerate or coagulate the protein solids. Speaking with the plant personnel I found out that the feed slurry could easily be heated before fed to the decanter. They agreed to send it hot to the decanter. The goopy protein solids were coagulated and came out of the cake discharge like buck shot, and the centrate was clean. Sharples went on and sold dozens of decanters to this company, and other companies in the same industry.

The author hopes that the information shared here from his over 45 years of decanter experience may be of benefit to anyone working with a standard design decanter. Similar articles on BD decanters and screen bowl decanters are coming.

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Standard reconditioning of a decanter centrifuge includes the cleaning of the centrifuge and accessories, the complete disassembly of the centrifuge with all bearings, seals, gaskets, o-rings, screws and bolts, vee belts and the like discarded and replaced. All mating fits between the various bowl segments and bearing / seal surfaces are checked and made true to the original manufacturer’s dimensions and tolerances. This involves adding metal where needed and machining to the proper specifications. All hardsurfacing on the conveyor edge and face, feed ports and cake ports are inspected and repaired / replaced as needed. The grooves or ribs on the conical portion of the bowl are checked and repaired as needed. The bowl liner, if present, is repaired or replaced as needed. If the bowl has no liner it is inspected for any grooving or pitting especially at the point of the feed slurry entry. If the bowl is heavily grooved or pitted it is discarded and replaced with a new bowl. Any ribs in the bowl are repaired. The gearbox is completely disassembled with all gears and fits inspected and corrected or replaced as needed, with all bearings, bushings and seals replaced after solvent flushing, and new oil is added. The gearbox is then balanced independently. The feed tube is inspected and repaired / replaced as needed.

The main pillow block bearing housings are inspected by micrometer and reconditioned to the “true” dimensions and tolerances required for the new heavy duty bearings. New grease or oil is added. The centrifuge frame is inspected and the bearing housing surfaces are checked and made true and parallel. The frame feet are trued to each other. New vibration mounts are included matching the frame feet. The upper and lower casing segments are inspected especially in the cake discharge impact area, with repairs made as needed. The pulleys and backdrive in-line universal shaft (if present) are inspected and repaired / replaced as needed.

The bowl assembly with gearbox is balanced and test run after reassembly of the complete centrifuge. The conveyor with new bearings and seals is balanced independently. The guards are repaired as needed. The backdrive system and both motor mounts are checked and repaired / replaced as needed. All carbon steel components (frame, gearbox, pulleys, guards, etc.) are painted. The motors are sent out to a professional motor reconditioning shop for an overhaul or replaced by new motors. All items are assembled and test run once again with the new motors. If the main pillow blocks are lubricated by a circulating oil system that system is provided new. This includes all hoses and connections, the pump(s), the heat exchanger and all instrumentation. The oil reservoir may be solvent flushed and reused or may be new.

Any base plate or table included with the order is either new or reconditioned with a thorough cleaning by steam jet and/or bead blasting, and painted.

The customer is invited and encouraged to inspect during the final test run in the reconditioning shop. If we supply the new controls those controls are used for the final test run. We will not ship until the customer approves everything.

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Centrifuges may be scrapped or taken out of service due to:

1) Corrosive attack. The feed slurry fed to a centrifuge must not be corrosive to the metal making up the rotating bowl or any other part. Most decanters are manufactured in 316 stainless steel or a close variation of 316 with more strength under rotation (317, Duplex). Care should be taken in matching the centrifuge material of construction to the corrosive properties of the feed slurry, solids, and vapors. The objective is to have zero corrosion on the centrifuge metal. Other centrifuge materials of construction can be carbon steel (for mining or high oil content applications such as meat industry rendering), 304 and other stainless steels, several grades of Hastelloy, titanium, and one or two other exotic metals. Any special order metal centrifuges are extremely expensive compared to the standard metals.

2) Erosive attack. The three (3) major wearing parts of a decanter type centrifuge are the feed ports, conveyor flight edges, and cake discharge ports. Severely abrasive feed slurries can also attack the stainless of the feed zone inside the conveyor hub, or the abrasives containing cake solids can cut through the stainless cover at the point of cake discharge. The feed ports are normally protected with bolt on liners with an inner core of carbide or ceramic. The cake ports also have bolt (or glued) on liners with carbide surfaces. The conveyor flights can be protected with carbide or ceramic tiles, and welded stellite (weldable carbide matrix) all machined to the proper shape. The feed zone can be protected with a welded stellite surface covering, ceramic paste, or liners of rubber or plastics. The impact line in the cover and, to some extent, the lower casing can be lined with stellite, rubber, or other materials. With proper protection any erosive attack will never reach the relatively soft stainless steel which might affect the integrity of the decanter. Periodically the wear protection is inspected and replaced as needed. Properly performed, this protection will keep the decanter operational for many decades.

3) Obsolescence: Improvements in the separation efficiency of decanters continues to develop as the manufacturers conduct experimental research with that goal. This can be in the area of improved separation efficiency or savings in energy consumption. At this point of time (2018) it is tempting to say that the major improvements (for separation efficiency) have already occurred and that any improvements in the future will be incremental only; but, one never knows for sure. The three major improvements in basic decanter design were all invented or developed by Sharples Centrifuge personnel in the 1970s. These are the deep pool design, the axial flow design, and the BD (Biological Design) disc design. Dr. Allen “Charlie” Lee first conceived of the BD design while brain storming with Bob Missimer (Sharples Process Lab Test Technician) sitting in a 4 desk office at Sharples in Warminster, Pa. I was sitting at one of the desks and overheard the conversation. Missimer was astounded at the idea and immediately understood the concept. This was in about 1971. Since then every major manufacturer makes use of the BD design. The understanding of the benefits of increasing the depth of the pond and having an axial flow of the feed slurry down the bowl in decanters were realized by Sharples at about the same time (early 70’s) and the “deep pool axial flow” PM series of decanters were introduced. These were especially beneficial in the municipal waste sludge industry. More on this subject in another article.

4) Plant expansion can result in the existing decanters being too small for the new larger capacity needed. Rather than line up many small decanters management may decide to purchase one or two or even several larger decanters and retire the smaller ones. The smaller ones are sometimes used at another part of the process, or at another plant owned by the same company. Sometimes they are sold on the surplus market and sometimes scrapped. There is usually a large income benefit by selling the decanters on the surplus market versus scrapping them.

5) Sometimes the plant process will change making the decanters unneeded or ineffective for the new separation.

6) Sometimes new technology in filter design, and a good filter salesman, will cause the decanters to be replaced with filters. The final result can be better, or not.

7) Occasionally large companies or governmental agencies will decide to replace the decanters that are 20 or 30 years old due to company policy on machine age or the availability of funds for new equipment (like tax dollars). We also have seen almost unused decanters of this age put up for sale. The municipality or other type company did not want them but was forced to accept them by higher authorities, then they never ran them beyond a short start up period, if they ran them at all.

To conclude, the manufacturing date of a decanter centrifuge is irrelevant to its’ value for future use. Very fine decanter centrifuges, of whatever age, reach the surplus market all the time. With a thorough reconditioning and new controls they can be purchased for about half price of new, with a life expectancy of… 100 years!

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While the slurry is moving towards the liquid discharge end it is under a “moment of inertia” force due to the rapid rotation of the bowl. This can be calculated as an acceleration with 1 G-Force (Gravitational Force) being 32 feet per second per second. In daily life we know that a certain mass of atoms has a weight of one pound, just by definition under 1 G. We weigh so many pounds each as we stand at sea level on planet Earth. In Colorado with its’ higher elevation you weigh less than in Atlantic City.

A particulate will settle in water under 1 G in X amount of time. Just pour salt in a glass of water. If we increase the G to 2 G’s it will settle twice as fast (almost, viscosity of the water plays a small role). A decanter centrifuge is usually operating at nearly 3,000 x G’s, or more. Three thousand G’s! It takes very little time for a particulate to settle 2 inches or so in the liquid pond of a decanter under 3,000 G’s.

Note the decanter schematic included here. The dry beach is the part above the top of the liquid pond. Under the pond the conical section is called the wet beach. The cylindrical length of the bowl is where the settling and clarification of the feed liquid takes place.

When the particulates reach the bowl wall they fill the small gap between the conveyor edge and the bowl and keep piling up. Now along comes a conveyor flight, rotating at a few RPMs different from the bowl RPM, and pushes the settled solids towards the cake discharge end of the decanter. The solids pile up in front of all the flights with more and more solids the closer they get to the cake discharge end. The solids are pushed up the beach and out of the pool onto the dry beach where any excess liquid drains back down into the pond. The firm dry looking solids are conveyed over holes at the top of the cone where the G force causes them to be flung out into the casing and they then drop down usually to a belt or screw conveyor that moves them away for disposal or further treatment or packaging. Sometimes a dumpster or dump truck can be parked directly under the centrifuge cake discharge thus eliminating the conveyor. Some cake discharges are of a thick pudding or paste like consistency and a positive displacement pump can be used to transport the solids away.

The gearbox on the end (usually but not always on the liquid end) is bolted to the bowl and rotates at the bowl speed. The conveyor is separated from the bowl by bearings and is connected to the shaft of the gearbox and thus to the gears inside the gearbox. By action of the gears the conveyor is kept at a different speed or RPM compared to the bowl. This difference in RPM is needed to move the solids down the bowl, up the beach and out of the decanter. This difference in RPM is usually from 5-30 RPM. This is called the “differential speed” or “differential RPM” or just “differential.” The differential can be adjusted which is beneficial in optimizing the performance of the decanter in making the separation. A secondary or “backdrive” motor is connected to the external gearbox “pinion” shaft. The pinion shaft is connected to the gearbox gears on the opposite side of the gearbox from the conveyor. A change in the pinion RPM results in a change in the conveyor RPM.

Decanter centrifuges are usually manufactured in 316 stainless steel or a close variation of 316 with more strength under rotation (317, Duplex). Care should be taken in matching the centrifuge material of construction to the corrosive properties of the feed slurry, solids, or vapors. The objective is to have zero corrosion on the centrifuge metal. Other centrifuge materials of construction can be carbon steel (mining or high oil content applications such as meat industry rendering), 304 and other stainless steels, several grades of Hastelloy, titanium, and one or two other exotic metals. Any special order metal centrifuges are extremely expensive compared to the standard metals.

Additional articles on decanters can be found on our web site.

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