[FREE COST] Decanter Centrifuge Handbook by Pdf books by Scope of Publication A reference work for process designers and users of. Decanter Centrifuge Handbook 1-st Edition. Year: Author: Alan Records Ken Sutherland | Elsevier Science.. Genre: Handbook. Format: PDF. Quality: OCR. Decanter Centrifuge Handbook. You've reached the end of this preview. Sign up to read more! Start your free 30 days. Page 1 of 1.
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ayofoto.info Tam Ninh. Decanter Centrifuge Handbook 1st Edition This Page Intentionally Left Blank Decanter Centrifuge Handbook. MORE THAN A MACHINE: DECANTER. A centrifuge decanter; consists of a solid cylindirical bowl rotating at high speed, a scroll rotating at the same axis with. Purchase Decanter Centrifuge Handbook - 1st Edition. Open - Buy once, receive and download all available eBook formats, including PDF, EPUB, and Mobi.
Thus the density of the process materials in use can have a major effect on the safe working speed of the bowl. W h e n used as a decanter gearbox, the input shaft, called the pinion shaft in decanter terminology, is held stationary or is braked to a speed below bowl speed. In some applications it is found necessary to surround the cake discharge area with a stationary collector, fixed to the casing, to prevent cake sticking to the main casing. By comparison, therefore, with: The quality of the sinter depends upon the processing conditions, the binding alloy, and the composition and size distribution of the carbide. It aims also to be a useful guide for the centrifuge engineer, both in equipment manufacturing companies and in the end-user companies, and their associated contractors and consultancies. The latest development of this component, by Alfa Laval, is to use a saddle shape, w h i c h improves the wear life of the component.
Elsevier Science B. Scope of Publication A reference work for process designers and users of decanters, this book aims to bridge the information gap in this field - that between academic theory promoted in student textbooks and case study data in manufacturers sales literature. Design It includes information on design and specification, preparing the reader to select and correctly size equipment.
Purchase As a design or project engineer working with vendors to make final equipment selection, this work provides the readers with the full facts before they start talking to product vendors. Operation Once an equipment purchase is made, the user needs to be made aware of how to optimally operate decanters. Read more Click link to access ScienceDirect Click for electronic access to e-book. Show all links. Allow this favorite library to be seen by others Keep this favorite library private.
Find a copy in the library Finding libraries that hold this item Print version: Records, Alan. Decanter centrifuge handbook.
Document, Internet resource Document Type: Alan Records ; Ken Sutherland Find more information about: Alan Records Ken Sutherland. A reference work for process designers and users of decanters that includes information on design and specification, preparing the reader to select and correctly size equipment. It covers relevant process operating issues such as instrumentation and control and the use of flocculents.
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Primary Entity http: Book , schema: CreativeWork , schema: Clarification, classification, dewatering, extra dry solids, washing, reslurry wash, thickening -- Theory -- Clarification -- Conveying -- Conveying -- Power Consumption -- Polymer Addition -- Pressure developed -- Scaling -- Particle size distribution -- Moving layer theory -- Calculating the effect of vanes and discs -- Fundamental studies -- Flocculents -- Types of flocculents -- Dosing -- Make-up -- Test work and data Calculations and scale-up -- Options -- Identifying the pertinent method -- Data needed -- Preparing the data -- Mass balance -- Recovery -- Polymer Dose -- g-level -- Power Calculations -- Worked examples -- Plant Instrumentation and control -- Decanter Features for special application -- Tables of decanter data -- Nomenclature, acknowledgements, references, index.
The Decanter Centrifuge Handbook covers relevant process operating issues such as instrumentation and control and the use of flocculents. Intangible ;. Click link to access " ;. InformationResource , genont: Home About Help Search. All rights reserved.
Suspended solids sediment to the bowl wall, where they are picked up by the conveyor and scrolled as a saturated cake to the conical end of the bowl, over the heel of cake which builds up in the small clearance between the bowl and conveyor. The resulting clarified liquor flows to the opposite end of the bowl and decants over weirs into the casing for discharge.
The heel, the thin layer of process solids which builds up between bowl and conveyor, can progressively consolidate with coarser particles bedding themselves into it.
This, while providing an aid to scrolling efficiency, can be an u n w a n t e d source of abrasion for the conveyor. However, generally, there is a tendency for the heel to move, albeit at a m u c h lower rate t h a n the cake itself.
Thus there is a tendency for the heel slowly, but continuously, to regenerate itself. Materials of construction are i m p o r t a n t considerations in the basic design. Most decanters are constructed with the parts in contact with the process in some form of stainless steel. Although some m a n u f a c t u r e r s successfully use carbon steel, others have not been as fortunate, due to severe corrosion and associated problems. These need to be described in more detail.
The four major c o m p o n e n t assemblies are the rotating assembly, the flame and casing together, and the drive and back-drive assemblies.
The rotating assembly includes the bowl, beach, conveyor and gearbox. It is the most important and expensive part of the decanter, where all the work is done, and which contains the most sophisticated technology, both process and mechanical.
For such a heavy component, weighing up to several tons and producing a force field of several t h o u s a n d g, a high level of precision engineering is required, followed by precise balancing.
Bearings and seals used in the rotating assembly and gearbox are an important part of the decanter.
Bearings in general have to be lubricated to work properly. To do this, seals separate the lubricated bearings from the process environment, both to protect the bearings and to avoid c o n t a m i n a t i o n of the product or environment, by the lubricant.
Seals are also needed to contain process liquids and vapours within the centrifuge casing. Seals are especially important where the process requires a positive pressure or v a c u u m , and where vapours are flammable or toxic. The vertical designs are most frequently used for special applications and are described in Section 2.
Thus the horizontal design will be taken from here on as the basic design. In the co-current design, both solids and liquid travel in the same direction, axially, in the separating zone, with the clarified liquid diverting to the opposite end to the solids discharge t h r o u g h off-take channels.
Both designs have strong proponents and arguments. For the moment, countercurrent flow is assumed, and co-current flow is discussed further in Section 2. Conventionally the front end of the decanter is the liquid discharge end and the solids end is referred to as the rear. The solids discharge is more usually referred to as the cake.
While defining flow and positional conventions, it is worth mentioning that later in the book when discussing the interior of the bowl, terms such as "up", "over", and "bottom" for instance will be used. These terms relate to the centrifugal field, and thus "bottom" refers to the bowl wall, "up" and "over" mean towards the bowl axis. Centrate Cake Figure 2. Countercurrent flow. Co-current flow. DecanterDesign 21 2.
It is more usual to make the contact parts, particularly in the rotating assembly, of some form of stainless steel. This is to avoid assembly problems and misalignment due to corrosion on mating surfaces. This has to be avoided with high speed rotating equipment. Nevertheless, it must be said that there are m a n y decanters in operation with bowls of carbon steel, where their m a n u f a c t u r e r claims to be able to o v e r c o m e , or avoid, corrosion.
For stationary contact components there is no need for a high grade of stainless steel. When the process used is non-arduous, simple neoprene seals and gaskets will suffice.
Supporting framework will be in ordinary or even cast steel. Materials of construction for the decanter are discussed in more detail in Section 2. The first cylindrical bowls used a filler piece in the end of the bowl to form the beach. On modern bowls, particularly the larger ones, the beach is bolted to a flange at one end of the cylindrical section, although with some overlap to provide mechanical location. The thickness of the bowl wall is dictated by the material of construction used, the m a x i m u m speed at which the bowl will be rotated, and the m a x i m u m weight of process material, feed, centrate or cake, likely to be held in the bowl.
Thus the density of the process materials in use can have a major effect on the safe working speed of the bowl. Basic bowl assembly. However, some effort is often m a d e to encourage cake to stick to the bowl, to aid scrolling instead of slipping r o u n d with the conveyor.
The means of doing this could be by knurling the inside of the bowl for instance. This can w e a r smooth relatively quickly. More often longitudinal ribs are welded, or a liner with similar ribs is fitted see Section 2. At each end of the bowl the outside bowl diameter can be increased to provide, if necessary, excess metal for removal during balancing. In particular, it can provide a position for m a c h i n i n g grooves, which will mate with corresponding baffles in the casing.
Together, the grooves and baffles form labyrinths to c o u n t e r a c t cross-contamination of the products discharging at either end of the casing. It has an inner spindle to locate the conveyor, its bearing and seals, and an outer spindle for the fitting of the front main bearing and pillow block.
Seals will also be fitted to the outer spindle as required. The discharging liquid is commonly known as the centrate. A decanter fl'ont hub. These weirs, sometimes called dam plates, cause a pond to form in the bowl. The simplest form of weir is a r e c t a n g u l a r plate with slotted holes bolted to the outside face of the front hub. The pond height is adjusted by loosening the bolts, repositioning the plate and then re-securing the bolts see Figure 2.
Accurate location of the weir plates is necessary to enable a d j u s t m e n t of the pond level to within, say, 1 m m or better. This has necessitated the development of better designs see Section 2. For best process control, the weir width needs to be maximised to reduce the level of cresting over the weir.
The crest is the extra level of liquid above the weir inner edge, necessary to effect flow, as seen over weirs in rivers. This cresting varies with feed rate, but will be an inverse function of the weir length. Thus the larger this is, the smaller is the variation due to feed rate, or more properly, centrate flow changes.
A simple centrate weir. On the inside surface of the sheet will be welded longitudinal strips. The liner is to combat erosion, but more particularly to form a key for the settled cake to improve scrolling efficiency. The liner will be held in position in the bowl by tack, or spot, welds. On the smaller sizes of centrifuge the liner will be full length. On larger machines it can be full length, but sometimes it will cover only a partial length of the bowl from the beach junction forward to a little way past the feed zone.
The diameter of the conveyor and the profile of the larger end of the beach need to be adjusted to accommodate the liner. Thus the use of a liner should be decided before the centrifuge is built. Fitting a bowl liner is not an easy thing to do as an afterthought. It is supported in a housing and sealed with a non-contacting flinger, wind back and labyrinth parts on each side.
The housing is accurately mounted to the main flame and aligned with the bearing housing at the opposite end of the rotor. The bearing shown in Figure 2. Lubrication is usually by oil, static, circulating or mist. Circulating oil, while usually the most expensive, is the best and most reliable.
Most actual bearing failures are due to lubrication failure or foreign contamination, not load. A circulating system flushes out contaminants and introduces only cooled, filtered oil to the bearing.
The oil drains from each housing must be large enough to discharge the oil quickly, after it passes through the bearing. Components of a main bearing assembl!
Decanter Design 25 Housing Fliger Figure 2. An alternative main bearing assembly. Smaller decanters are often grease lubricated to reduce cost. The bearings of smaller decanters are often cylindrical roller, or ball type. Bearing housing seals must have sufficient axial clearance to permit t h e r m a l expansion of the rotor, and at least one bearing must float axially.
The seal between the casing and front hub is usually a close clearance bushing. The space between the bearing housing and the casing is best vented to ensure separation of the oil and the process liquid. Most leakage is from outside air entering the casing, due to the slight v a c u u m produced by the rotating front hub.
If a positive seal is required, both axial mechanical and radial mechanical seals are used. A radial seal, which uses two split, floating, carbon rings with a gas buffer riding on a tungsten carbide coated runner, is an example of an advanced design, permitting axial movement and both high- and low- t e m p e r a t u r e operation.
The front hub and the beach together enable a pool of liquor to be held in the bowl. Being a component in contact with the process liquor, the beach will be fabricated in the same material as the bowl.
The beach will be flanged and bolted to the end of the bowl or inserted into the end of the bowl as a filler piece. To the rearmost end of the beach is fitted the bowl's rear hub. The hiilf'inrluded iinglr ol' t. A different beach iingle, o r ;i combination o"Igles in a compound beach, cvuld be selected lo t'acilitate better dryness, better washing. A brach angle of 8 to 10 dcgrccs is a common valuc choscn for many prnccsscs. Alternative designs are dr: The rear h u h m;iin hearings arid seals ;1rr similar to t h a t ofthc front h u b SCC Section 2.
I ,The rear h u h si1pport.
A hall or roller hearing also resists the axial thrust ofthc convcyor. This is possible sincc theseal rubbing velocily ir; low. A rear bowl hub.
Larger decanters usually use a tension bar, to transfer the axial load to a bear- ing located in the driven pulley. This prevents the axial load produced by scrolling torque from being imposed on the bowl shell bolts. All well-designed decanters permit the re-greasing of the conveyor bearings without the requirement to disassemble the casing.
In its simplest form, the cake discharge will be a series of radial holes around the beach end. These holes will usually be lined with some form of erosion protection, quite often in the form of a sintered tungsten carbide cylinder in a steel holder.
For process reasons it is important to have a defined cake discharge diameter. This is the diameter of the inner edge of the beach radially, outer axially , over which the solids decant into the casing. Thus, prior to the discharge ports will be a ring or ledge providing a definite discharge level.
It is therefore c o m m o n to provide a scrolling aid in the form of grooves or ribs. The grooves would be machined into the beach surface, whereas the ribs would be welded on or form a part of an inserted liner.
Figure 2. Beach ribs and cake discharge apertures. It has a iiurnbcr of functions. Not only does il. In its simplest form, t. Somewhere in between the bearings will be a charnbcr c,allcd the feed zone. The flocculant can be added upslrearn of the decanter, but there are inany circumstances where, for best efficiency. On these occasions there will be an extra c h a m h w , t.
It could. A conveyor hub. This bushing could be splined, keyed or specially shaped, e. The feed zone will be built into the hub to discharge at the start of the cylindrical section of the bowl adjacent to the beach. Next to the feed zone, a second chamber for flocculant or rinse may be fabricated within the hub. A buffer chamber between the feed and additive chamber will sometimes be built, with simple exit ports into the pond.
By putting distance between the feed and the additive chamber bv use of the buffer chamber, there is less chance of the additive chamber being contaminated by feed material. The natural vibrational frequency of the conveyor can be a limiting feature controlling the maximum speed of the centrifuge.
If the hub diameter gets smaller, the conveyor flexibility increases, thus lowering the natural frequency. Increasing the hub diameter will solve this problem, but with modern decanters using deeper ponds in m a n y applications, the hub becomes immersed in the pond. Immersed hubs can result in more hydraulic turbulence, and thus lower separation due to friction on the liquor surface, and possible build-up on the hub due to a sticky floating phase.
Surface non-concentricity results in mechanical vibration due to non- symmetrical buoyancy effects, so high precision is needed in geometry. Air flow and degassing of the feed stream become more complicated with submerged hubs.
Some new designs avoid these problems by permitting small hubs designed with high stiffness and high natural frequency . However, within the last decade, immersed hubs have been designed to float on the pond, considerably reducing potential vibration and enabling higher speeds [ 2 ].
Decanter Design 31 2. Naturally the helix profile has to be tapered to suit the beach section. Each section is welded in turn to the conveyor hub and then welded to the adjacent section.
Double welding both sides with grinding afterwards is essential where hygiene is of importance. However double welding is c o m m o n practice, even w h e n hygiene is not required. The flights will be normally perpendicular to the decanter axis or bowl wall. It is not always appreciated w h a t a complex shape the surface of a flight is. The pitch angle is the angle the tip of the flight subtends to a right circle of the bowl. If the flights are not to be protected from wear, then their tips will be ground smooth and perhaps chamfered, to provide a m i n i m u m of area in contact with the heel, to minimise torque.
The feed enters the feed zone c h a m b e r from the feed tube. Once in the II Figure 2. A conveyor flight section before welding to the conveyor hub. A typical feed zone. To assist the feed up to speed, a c c e l e r a t o r v a n e s will sometimes be found on the " t a r g e t " , the plate opposite the feed tube end.
These vanes could be radial, at an angle to the radii, or curved. In extreme cases of wear, parts inside the feed zone are hard surfaced or specific erosion resistant c o m p o n e n t s are used.
Hard s u r f a c i n g is often used on the accelerator plate, p a r t i c u l a r l y on leading edges and the tips of the vanes. Some shaped accelerators have been m a d e completely of u r e t h a n e rubber. W h e n the feed leaves the feed tube, in most cases it is at a high axial velocity. W h e n it hits the r o t a t i n g target, some splashing inevitably occurs. In fact a dense aerosol mist is often produced. At the back of the feed zone a tube is sometimes built in, to s u r r o u n d the end of the feed tube.
On the outside of this tube, small accelerator v a n e s are welded to accelerate and condense the mist and also to accelerate liquor up to speed, should the feed zone become flooded. Ideally, air is allowed to enter the feed zone from a r o u n d the feed tube.
It will be d r a w n in by the fan effect of the feed zone and t h r o w n out of the feed zone exit ports. The air would t h e n pass along the bowl to exit over the centrate. This d r a u g h t helps to prevent splash back of feed from the feed zone. The exit ports from the feed zone are themselves subject to a considerable range of designs and i n n o v a t i o n s.
It is not usual to h a v e just one exit port. For s y m m e t r y and balance an even n u m b e r of ports is usual, two, four, six or eight. The basic design has these ports fitted with t u b u l a r nozzles, often lined with a ceramic w e a r protection. New feed zones have been introduced recently to reduce feed particle attrition, by slowing and e x t e n d i n g the acceleration time to bring the feed up to speed, and to reduce the inlet t u r b u l e n c e in the s e p a r a t i o n zone.
Decanter Design 33 2. Flocculant c a n be relatively expensive, and on an effluent application c o n t r i b u t e s a large p e r c e n t a g e to the total t r e a t m e n t costs. An additive chamber. Floc Addition. A seal will be used to retain the grease and a second outer seal will be used to prevent ingress of process fluids and solids into the bearing. Thus two seals are fitted back to back at each end.
Areas of wear protection. A typical conveyor bearing assembly with seals. The differential rotation between the two races of each bearing is low and thus the life of these bearings should be good when adequately greased and sealed. There are two main types of gearboxes used on decanters. These are the epicyclic gearbox and the Cyclo gearbox, made by Sumitomo of Japan.
However there are a number of decanters which have eliminated the gearbox by using a hydraulic system called a Rotodiff manufactured by the Swiss company Viscotherm.
The Rotodiff and the Cyclo gearbox will be covered in more detail in Sections 2. The epicyclic system consists of a pinion shaft and gear, which engages three planetary gears mounted on carrier plates which in turn engage a ring gear fixed to the gearbox casing.
For the decanter the epicyclic gearbox involves two stages, although recently three stages have been in use. The carrier plate of the first stage holds a second pinion shaft carrying the sun gear for the second stage. The ratio of the gearbox is the product of the ratios for each stage.
A two stage epic! Decanter Design 37 m a x i m u m ratio for a two-stage epicyclic gearbox of to Three stage epicyclic gearboxes with ratios over have been used on decanters. If the central pinion shaft is held stationary, the differential speed between conveyor and bowl will be the bowl speed divided by the gearbox ratio.
If the pinion shaft is allowed to rotate at some speed below the bowl speed, then the differential between bowl and conveyor will be the difference between bowl speed and pinion speed, divided by gearbox ratio.
If the pinion speed is controlled by using a brake, or a variable speed motor, differential speed m a y be varied from close to m a x i m u m , w h e n the brake is at its slowest speed, to nearly zero, w h e n the brake is almost at bowl speed. Reducing the pinion speed below zero, i. Using an epicyclic gearbox causes the conveyor to rotate slower than the bowl, whereas it is normally faster w h e n using a Cyclo gearbox. Generally the conveyor flight helix is "left h a n d e d " with an epicyclic gearbox and right handed with a Cyclo gearbox.
More usually it is fabricated from steel channel or box sections.
The flame needs to be a rigid support for the rotating assembly. The surfaces for the main bearing pillow blocks are accurately machined in the same plane, and in line, to ensure no end-to-end misalignment of the rotating assembly, which would cause p r e m a t u r e bearing failure. Some m a n u f a c t u r e r s fill part of the main flames of their larger machines with concrete effectively to form an inertia J Figure 2.
Some flames have been used as a reservoir for the lubricating oil for the main bearings. The flame and casing see Section 2. When oil lubricated, the pillow blocks will be piped to an oil system, which will include a circulating pump, an oil reservoir and cooler with associated pressure, flow, and t e m p e r a t u r e i n s t r u m e n t a t i o n see Section 2.
A clamp or flange holds it on a support extension from the main flame. It extends to the feed zone and within a few centimetres of the accelerator in the feed zone. A main hearing and pillow block assemblH. Decanter Design 39 Figure 2.
Feed tube. The geometry of m a n y decanters is such t h a t there is a risk of r e s o n a n t vibration of the feed tube at frequencies a r o u n d the bowl speed.
To c o u n t e r this, feed tubes have been made slightly tapered, and made of lighter materials such as glass fibre and even carbon fibre, and sometimes composite material. Entering the conveyor with the feed stream is a flow of leakage air, w h i c h passes through the clearance between the conveyor and the feed tube.
This air flow must eventually be vented, and in those applications where odour or toxicity is an issue, minimising this air in-flow is important. A simple, lightly contacting lip seal is often used.
On critical sealing processes, m e c h a n i c a l seals are required. When a sub- flame is used the mounts are placed under the sub-flame. Even with a well balanced m a c h i n e , out-of-balance can occur when solids build up unevenly, when there is uneven wear, or when some u n p l a n n e d mechanical movement occurs within the rotating assembly. Considerable oscillations of the rotating assembly can occur during run up and shut down, w h e n the speed of the bowl passes t h r o u g h critical speeds.
Owing to the presence of these isolators, all process connections to the decanter must be flexible. Likewise, oil lubrication connections m u s t also be flexible. A feed tube seal. A decanter vibration isolator. T h e c a s i n g must, obviously, keep the separated products apart. Upper Casing a. Decanter casing. Decanter Design 41 Simply stated, this casing is a stationary collector for cake at one end, and centrate at the other. There are m a n y design variants and each m a n u f a c t u r e r has its own recognisable design w h e t h e r it be just its finish or its shape and functionality.
In its more usual format, a lid is hinged onto a bottom half and bolted with a flat rubber gasket between the two halves.
This close fit may be to the plain surface of the bow], or to a shoulder on the bowl, or into a labyrinth groove machined into the bowl. It is necessary to ensure that this cake does not stick to the casing, and is directed down into the receiver.
Casing baffles. The pipe size needs to be sufficient to allow free flow of the copious quantities of air at this end.
Alternatively a separate vent pipe will be introduced. The centrate tends to continue its circular motion on leaving the bowl and swirls around in the casing. It is therefore sometimes seen t h a t the centrate discharge is offset from the centre line to take a d v a n t a g e of this tangential flow.
This will be of, say, neoprene, Viton or silicone rubbers, depending upon the application. Sometimes these gaskets are moulded with various cross-sectional shapes to provide good location and secure positioning. The clearance between the casing and the outer spindles of the bowl hubs can be open, or have fitted some form of seal depending upon the degree of sealing required.
Horizontal decanters, operating at a slight positive or negative pressure. Some higher pressure designs avoid this problem by h a v i n g the casing cylindrical, with disassembly in the axial direction, thus using simple O-ring seals. However, m a c h i n e disassembly is then complicated and added floor space is required for m a i n t e n a n c e.
Sealing process discharge vents, as well as feed lines, requires the use of commercial and special flexible connectors. These connectors m u s t be carefully designed to limit the forces imposed on the centrifuge and m o d u l a t e the forces transmitted to the plant piping and structure. The material of these flexible connectors must be chosen to resist the process temperature, pressure and corrosive characteristics.
This turbulent motion of air, called windage, tends to move outwards due to centrifugal action, and in doing so drags air towards the centre line to replace it. Windage needs to be channelled and vented rather t h a n suppressed.
Suppressing windage can cause cross c o n t a m i n a t i o n rather t h a n prevent it. It can be advantageous to allow air into the centre c o m p a r t m e n t to satisfy the windage created at either end of the bowl. In so doing the air travels from the centre outwards and thus helps to prevent escape of products from their designated discharge c o m p a r t m e n t s.
Decanter Design 43 If air is allowed in, then it has to be allowed out, and thus it is n e c e s s a r y to e n s u r e that the product lines or receivers are vented a n d the lines are a d e q u a t e in size to carry both p r o d u c t and the air flow. This e n t r a i n e d liquor has a propensity to migrate a r o u n d baffles.
T h u s the bottom of the casing is generally slightly sloped to drain any spilled liquor back to the c e n t r a t e. In thickening applications, the slope will be in the opposite direction, as a little extra dirty liquor in a fluid cake is preferable to dirty liquor in a clean c e n t r a t e.
The sub-flame forms a base for the m a i n f l a m e c a r r y i n g the r o t a t i n g assembly plus the main drive motor and back-drive system w h e n used. Vibration isolators are strategically placed u n d e r the sub- f l a m e to share the load properly The use of a s u b - f l a m e subjects the drive m o t o r to more vibration, but at the same time it m a k e s installation easier.
A purpose built belt g u a r d will cover the two pulleys and belts. Such large motors are more often directly m o u n t e d on the floor, in w h i c h case a special belt-tensioning device is incorporated to allow for the differential m o v e m e n t of the rotating assembly.
Howcver, a largc niiijority o f dt. Once a t speed. I t i s seldnm stopped and restarted. There is, however, o n e d u t y that Ilie maiii motor has t o perform t h a t difkrs from most other drive niotrir applications. It has to Iiavc the thermal capacity to accc1er;itc a high irierlial load to speed. Thc uscr's requirernrnts. It is deprndent upon the inertia ul' the niotor and dccanter combined.
Decanter Design 45 Motors can be two-, four- or six-pole giving s y n c h r o n o u s speeds of 3 0 0 0 , or rpm at 50 Hz, respectively. The most commonly used motor is the four-pole, which is a more usual standard in motors and is capable of being better balanced t h a n the two-pole.
Because of the low speed the six-pole m o t o r would be an u n u s u a l choice. The torque available from the main motor varies according to the method of start up, w h e t h e r it is star-delta or direct-on-line. A m o t o r connected in star produces a starting torque one third of that provided w h e n starting direct-on-line.
Direct-on-line starting torque can be two and a half times the motor's full load torque, with a starting current of six times full load current. Main motors need starter overload and short circuit protection. High rupture fuses HRC will protect the motor against short circuit conditions, and will interrupt the electrical supply in milliseconds of the fault occurring.
It is essential that fuses of this type are always fitted. Conventional overload protection, thermal or magnetic, can offer no protection to a motor with an extended acceleration time.
Thermistor overload protection is the only true protection for a motor under these conditions. A thermistor is embedded in each of the motor's three windings and connected in series. The resistance of these thermistors is designed to increase rapidly at a set temperature, depending upon the insulation class of the motor. The thermistors are connected to an electronic amplifier control unit in the starter enclosure, and will trip the starter contacts w h e n required.
The device will not reset until the motor has sufficiently cooled. In Europe the main motor is usually an AC motor, using a star-delta starter.
An inverter for the main motor is becoming more common, particularly with the smaller decanters. The inverter enables a soft start, and allows speed adjustment for different process requirements.
Inverter motors can cause u n w a n t e d electrical interference, and harmonic wave forms, on the main electrical supply lines. These problems can be minimised by using electrical filters and the latest advanced electrical technology. This could be offset from the gearbox shaft, in the same m a n n e r as the main drive, and connected by a belt. This belt would be a timing belt because of the accurate control required.
Normally the back-drive is connected directly and in line with the gearbox pinion. A decanter back-drive system.
The main component of the back-drive assembly can be an eddy c u r r e n t brake, inverter motor or a DC motor. The Viscotherm Rotodiff hydraulic conveyor drive is a variable speed device, powered by a fixed speed hydraulic motor. It is now possible to discuss the alternative designs of these components. The g force is at right angles to the axis in both cases, so rotors that are equivalent in diameter, speed, geometry and design will give comparable separation.
There is one main alternative to the basic horizontal orientation and that is vertical. However, the Flottweg Company does have an inclined decanter, inclined at the angle of the beach, which facilitates full emptying of the bowl for clean-in-place processes. The availability of inexpensive vertical decanters is very limited, such that in some circumstances one has to select a horizontal model even though a vertical one might have been preferred.
The vertical orientation lends itself to better and more reliable pressurisation, with only one end to be provided with rotational seals. The amount of engineering required for pressurisation and sealing makes the vertical decanter more expensive than the non-pressurised and open horizontal decanter. While each type requires three slow speed seals for the conveyor and one slow speed seal for the gearbox, these seals, due to their very low surface rubbing speeds, do not cause problems if designed properly.
The horizontal type requires two large high-speed seals to be mounted on the rotor, and one smaller high speed feed tube seal. The vertical type, in which the rotor and gearbox are pendulum suspended from a flexibly mounted spindle, requires only one high speed seal between the bearings in the spindle and the casing. A vertical decanter By courtesy of Alfa Laval.
Comparison of vertical and horizontal decanters. In addition, with the casing and flame of the horizontal decanter flexibly m o u n t e d and vibrating, all process and electric connections must be flexible, thus needing periodic attention.
The vertical decanter flame and casing are rigidly mounted. Therefore there is no need for a vibration isolator between it and the external connections. With no second bearing and seal on the vertical decanter, thermal expansion and alignment issues are almost eliminated. Almost all of the vertical decanters installed have been designed to meet the Unfired Pressure Vessel, Explosion Proof Code requirements and chemical industry piping, vessel lubrication and i n s t r u m e n t a t i o n codes.
Due to this, the cost of this design is higher t h a n horizontal m a c h i n e s of equal process capacity. For those horizontal m a c h i n e s that meet the same e n v i r o n m e n t and code requirements, the cost is comparable.
In general, the emitted noise level of vertical m a c h i n e s is m u c h lower t h a n horizontal machines due to less vibrating surface exposed to the work area. This is m o u n t e d on the spindle assembly cartridge, which is separable from the gearbox by m e a n s of a tapered joint, thus allowing seal and bearing m a i n t e n a n c e w i t h o u t removal and disassembly of the main rotor. Maintenance can then be done in a clean e n v i r o n m e n t.
Most seals installed are titanium bellows with carbon nose, rubbing on a hard coated stainless steel or solid carbide or ceramic m a t i n g ring. The materials used are chosen to meet the most difficult of corrosive and t e m p e r a t u r e and pressure environments. Seal assemblies having two seals and one m a t i n g ring, and a buffer liquid between, are available.
The axial load upward on a vertical design reduces the axial bearing load due to the rotor weight. This reduction in bearing load is equal to the process pressure times the high-speed spindle seal area. In fact, with a high enough process pressure, the axial load can be near zero on the m a i n bearing and can even push up on it. This load reversal must be considered in the design. The device most used is a single reinforced elastomeric bellow, sufficient to resist the internal vapour pressure in the casing, and lined with PTFE to resist the corrosion of the process.
In this design, the feed enters a feed c h a m b e r situated close to the bowl front hub, from a feed tube t h r o u g h the front main bearing.
Both the cake and the clarifying liquor travel together towards the beach end. Prior to the beach, the clarified liquor decants into channels built onto the conveyor hub, dipping into the pond, which direct the centrate back to the front hub for discharge over adjustable weirs in the normal way, already described. In this design the gearbox is fitted to the rear of the bowl.
The drive is usually at this end also. The co-current design allows the shortest feed tube. It also means that the finer solids, settled the furthest distance from the feed zone, do not have to return u n d e r the turbulent area of the feed zone, and so risk being re- suspended. The co-current design requires horizontal channels built onto the conveyor hub, dipping into the pond, to direct clarified liquor back to the front hub. These can suffer from fines settlement.
However, in the c o u n t e r c u r r e n t design the majority of the solids are removed from the clarification zone early, allowing more room for clarification. Separation problems, which result in more solids in the centrate than usual, can necessitate the decanter to be shut down for rodding out of plugged return tubes. Turbulence at the entrance to the r e t u r n tubes, near the solids discharge, can cause solids re-suspension.
Despite these differences in behaviour, there are m a n y t h o u s a n d s of decanters of each design successfully operating in the field, and both are currently sold. It thus can be concluded that performance differences are marginal. There are only a few applications where the supplier might claim the physical difference of his preferred design offers an advantage.
A vertical decanter casing seal By courtesy of Alfa Laval. A co-current decanter BI! The most critical parts, the c o n t a c t parts of the r o t a t i n g assembly, are m o s t frequently m a d e in one of the m a n y stainless steels.
A c o m m o n r e a s o n for corrosion on d e c a n t e r s is the presence of chlorides in the process material. The chlorides c a n c a u s e pitting corrosion, crevice corrosion a n d stress corrosion of the d e c a n t e r parts in c o n t a c t with t h e process. To avoid pitting and crevice c o r r o s i o n in severe e n v i r o n m e n t s , m o r e corrosion resistant materials, n o r m a l l y special stainless steels with a h i g h e r c o n t e n t of alloying elements, are used.
Some bowls are m a c h i n e d from a simple cast ingot a n d others from a special centrifugal casting. Quality control of casting, and especially rolled and welded parts, is critical. In the case of contact parts that are not rotated, such as the casing, the specification of the steel need not be so stringent.
Bowls in stainless steel and casings in a non-stainless steel are not u n k n o w n. Materials for erosion protection are described in Section 2. Elastomers used for seals and gaskets again depend upon the e n v i r o n m e n t. Neoprene is a standard material, but Viton and m a n y other materials, even PTFE, are used when necessary. In some parts of the decanter, such as bowl and beach liners and feed zone liners, they can be made of a similar material to the bowl, w h e n they just act as a preferential sacrificial wear component, to protect a more expensive component.
In the cake discharge c o m p a r t m e n t of the casing, linings of rubber, PTFE, PVC, steel and even Stellite have all been used variously to combat wear or to overcome cake sticking. Flexing of some materials here is encouraged to aid the desticking process. Some of the longer bowls are made in sections, which are flanged and bolted together to give a set of standard bowl lengths. This allows c o m m o n components with the shorter designs. The m a c h i n i n g of the inside diameter will depend upon w h e t h e r a liner is to be fitted or not.
The outside is machined to mate with baffles in the casing and with grooves when required to form a labyrinth with the casing baffles. However these variations are generally for mechanical considerations, having little effect, by themselves, on process performance. The thickness of the hub is dictated by the mechanical strength and stiffness requirements. When centrate discharge ports are drilled radially into it, as for some three-phase designs, extra thickness is needed.
With m a n y different designs for centrate discharge, which are incorporated on the front hub, the mechanical design is made to suit.
The precise bearings specified for the inner and outer spindles of the hub will need their own m a t i n g diameters. Devices added to the front hub, such as centripetal pumps, noise a b a t e m e n t rings if used, and the gearbox, all require their own special designs and fixings.
Decntiter Design 55 When simple weirs are used, then the hub will be machined with recesses to locate the weirs and their support plates. On the smaller diameter bowls the number of weir plates will probably be four, while Iarger models may take six or even eight. A thicker cover plate would secure each weir plate. The outer edge of the weir and cover plate would be circular to locate in the recess. The longer this edge is the better, as this minimises cresting, which facilitates easier process control.
The cover plate would have its inner radius larger than the shallowest weir plate usable. The deeper the weir plate the more important it is to secure it with a support plate.
This is because the dam, being relatively thin because of the range of sizes necessary, can easily distort with the pressure of the pond at speed. The better cover plates will have a protruding lip fabricated on their inner edge. This is to ensure that the discharging centrate separates from the bowl at.
This minimises power consumption. One alternative to the multi-plate weir is to have a complete circle split in two, t n facilitate fitting, and bolted over the ports. These hairrings have t o be very thick to avoid distortion and loss of seal. Another alternative is to have a complete circular plate n7itb slots cut at a specitic radius to cover the hub holes. Other sets of slots. To change the pond setting the plate is unboltcd and rotated. These last two variants tend to bc uneconomic unless it is known that only a small number or pond settings will be requircd.
A further type of pond adjustment is provided by a set of close-fitting circular inserts in the bowl hub apertures. An eccentric hole is turned in each casting insert. By rotating the circular casting in its seat, an almost infinite range of pond settings can be obtained. The shortcoming of this design is that the overflow edge is locally circular, not matching the pond surface, such that a large crest forms over the weir.
This does not allow good process control when needed. When the weir height is set above the cake discharge height a considerable amount of feed can flow from the cake discharge at start up.
There are various ways of combating this. This method of operating the decanter liquid discharge radius smaller than the cake discharge radius is sometimes k n o w n as "negative pond operation". It is used to improve scrolling efficiency, and to enhance the control of scrolling, by making use of the hydraulic head difference between the discharges.
Alternatively a recess can be machined into the bowl to take the thickness of the liner with its ribs. However, in this case and operating without a liner, a m u c h bigger heel results, which could give out of balance problems, should part of the heel dry out and break away after the bowl remains stationary for any length of time. An alternative to the ribbed liner, recently introduced, is an expanded metal sheet "Expamet" fitted in a recessed bowl.
This is now used frequently with success. Instead of a liner, m a n y bowls have longitudinal ribs spot welded to the inside of the bowl. A n o t h e r alternative to a liner is knurling or r o u g h e n i n g of the bowl, which is a simple procedure, but would normally be done only as a temporary measure as such surfaces will be quickly worn smooth.
Some applications do not require the assistance of a bowl liner as the process material has sufficient friction, even with a smooth m a c h i n e d bowl surface, to provide e n o u g h keying to allow an adequate scrolling efficiency. Using a liner on these applications could raise the scrolling torque to an unacceptable level.
Centrate Discharge Figure 2. A centrate "moon"dam with cover plate.
A split dam. A single piece dam. Sornc manufacturers. When selecting milin bearings lor [Iec;jnters sevltral p;ir;jmet. The rn;iin par;imeters ;ire l h e rotat. F o r a giver1 decanter. Some bearings usedfor supporting the rotating assembl! The design of main bearings, their sealing, and their lubrication is a complicated task, which requires a high degree of expertise in bearings and decanter design.
See also Section 2. A major beach parameter is the beach angle. Here the bulk of the solids are discharged t h r o u g h special nozzles in the bowl wall at the foot of the beach and only the coarsest solids discharge conventionally. Some alternative beach designs. Decanter Design 61 conveyor on the beach is reversed to convey any solids which pass the nozzles back to them.
Another variant of the beach angle is to have a compound angle. In this design, some distance from the foot of the beach the angle is increased or decreased resulting in a concave or convex design. The concave design is an attempt to make a more gradual transition from the horizontal bowl section to the final beach angle. The convex design is used to maximise the dry beach length where this is a limiting factor, and to facilitate conveying.
Rarely a completely, or near completely, curved concave contour is used. This is used for very difficult scrolling problems. The large increase in cost of fabrication of this beach and corresponding conveyor section inhibits the wide use of this design. In some old designs a beach is not used at all. A simple flat rear hub is used with cake discharge holes. The conveyor is tapered as if there were a beach, and the cake then forms its own beach. This design can work reasonably well with stiff cohesive cakes, but very high conveyor torques can result.
Scrolling improvement and abrasion protection add to the beach variations. These include ribbing, grooving, tiling and liners. The cake discharge is generally a part of the beach fabrication but this will be covered in Section 2. There will be variations according to the method of lubrication of the rear conveyor bearing.
The bowl design speed and the type of bearings used will affect the detailed design. These details will not affect process performance. The main parameter, which will give rise to the major hub variations, is the design of the cake discharge.
If the cake discharge is wholly in the beach wall then there is a simple a t t a c h m e n t of the hub to the beach. Sometimes, however, the cake discharge is extended into the hub or is completely in the hub. The cake discharge variants are more fully described in the next section.
An i n a d e q u a t e design will reduce process performance. A cornrriori solution i s to end. Sornc nianufacturcrs. If t h e decanter has deep groove ball bearings. When selecting milin bearings lor [lec;jnlers several p;ir;jmelers must bP considered. The m;iin p;ir;imeters ;ire I tie rolai. For a giver1 decanter size a large convcyor h u b shaft diamctcr is oftcn dcsirablt.
A n nil-lubricated bcaring will have a higher iIllowilhle speed a r i d I. An advance on this is to provide matching castellations on the rear hub to improve discharge area. As designs have developed there has been a tendency to enlarge and shape holes and i n t r o d u c e castellations. On very large decanters the thickness of the beach wall in the area of the cake discharge is such as to allow the m a c h i n i n g of exit ports of a helix snail shell shape to minimise contact of the cake with the surface of the discharge ports.
This reduces wear in the area. These pillars enable the a t t a c h m e n t of the rear hub. The pillars are restricted in length by mechanical strength considerations, but provide sufficient axial gap for free solids discharge. In some applications it is found necessary to surround the cake discharge area with a stationary collector, fixed to the casing, to prevent cake sticking to the main casing. The impellers sweep clean the inside of the collector, although they also add to the noise level of the decanter.
The type and design of hard surfacing add to the variants of cake discharge. For irregular shapes, flame applied hard surfacing is used on the discharge area or on a steel casting made to fit the discharge.
Alternatively a plain steel sacrificial c o m p o n e n t is used. The latest development of this component, by Alfa Laval, is to use a saddle shape, w h i c h improves the wear life of the component. However in extreme cases of erosion, a beach liner of say Stellite with its own Stellite ribs is used. Alternatively the beach can be covered with small carbide or alumina tiles like a R o m a n mosaic. Tiles of two different thicknesses allow the formation of in-built ribs or grooves as desired.
It thus has a large number of variations in design. In general the shape of the conveyor has to match the inside profile of the bowl and beach assembly, with from 0. A beach liner. The large n u m b e r of conveyor variants arise from the p e r m u t a t i o n of the different hubs, flight design, feed and floc zone types, and the different types and degrees of hard surfacing.
Devices added to the conveyor, and the type of flow, two-phase, three-phase see Section 2. The co-current flow conveyor will have its feed zone at the front large diameter end and some return channels will be built into the conveyor hub to lead the clarified centrate from the foot of the beach back to the front hub, between the exit ports of the feed zone. The c o u n t e r c u r r e n t conveyor has its feed zone in the conveyor but abreast of the foot of the beach.
Clarified liquor is free to flow a r o u n d or t h r o u g h the conveyor flights to the front hub while the cake is scrolled to the rear, and up the beach.
The orientation of the bowl vertical or horizontal does not usually give rise to a variation in conveyor design. The conveying action can induce a large axial thrust towards the end of the bowl opposite to the beach. On the larger horizontal decanters, Alfa Laval employs a tension bar on the conveyor to c o u n t e r b a l a n c e this thrust and take off the load t h a t could be placed on the front bowl hub. The tension bar consists of a substantial hollow bar bolted to the rear face of the conveyor.
It projects t h r o u g h the spindle of the rear hub and is locked in position with a large nut o u t b o a r d of the spindle. The hollow centre affords access for the feed tube. Also the size and type of c o n v e y o r bearings and seals will make some differences to the e n g i n e e r i n g design of the ends of the hub.
However, these last differences will not affect process performance. The diameter of the h u b is usually minimised w i t h i n limits of, for example, mechanical s t r e n g t h and the size of the feed c h a m b e r to maximise pond volume. However, in some applications it is necessary to increase the h u b size to shorten the flight height and so e n h a n c e the scrolling torque capacity of the conveyor.
To the hub will be a t t a c h e d a n y baffle discs or similar devices w h i c h the process needs. These are discussed in Section 2. The simplest design has holes cut out of the flights before they are welded onto the hub. A more sophisticated design see also Section 2. The rake or cant of the flight is varied for some applications, as is the c o n v e y o r pitch or pitch angle see Sections 2.