by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge
A disc stack centrifuge is a specific industrial centrifuge that utilizes a stack of conical discs within its rotating bowl to enhance its separation efficiency. It is a flow-through centrifuge that continuously separates liquids from liquids and solids.
Gustaf de Laval invented the disc centrifuge in 1894 in the form of a cream separator from milk. He was a native of Sweden who went on to form AB Separator company which today is known as Alfa Laval.
The first centrifuge to utilize a disc-stack to affect high-efficiency separation was in the form of a cream separator invented by Gustaf de Laval.
The disc stack centrifuge works on the principle of differential settling of fluids of different densities. This centrifuge exerts a centrifugal force that is multitudes of the force of gravity to separate the liquids and solids having different densities instantaneously.
The name disc stack centrifuge originates from the stack of conical plates (known as discs) inside the bowl of this centrifuge. These discs are the distinguishing feature of the centrifuge from other industrial centrifuges.
A disc stack centrifuge separates two immiscible (not soluble) liquids with different densities from each other. This centrifuge performs this separation continuously as a flow-through liquid separator. The mixture of liquids enters the centrifuge through an inlet pipe, and the two separated liquids exit the centrifuge through discrete outlets.
The purpose of the discs in the centrifuge is to drastically reduce the settling distance for the solids in the fluid mix. The reduction of this distance allows the solid particles to settle quickly, imparting a high separation efficiency to the centrifuge. Additionally, the discs separate the liquid column in the bowl into thin layers in the inter-disc space. The rotating discs can accelerate the incoming, non-rotating liquid subjecting it to the high centrifugal force, which is the critical aspect of centrifugal separation.
The discs in a disc stack centrifuge bowl vary from 50 discs in a small centrifuge to over 200 discs in a high-capacity centrifuge. The number of discs also depends on the thickness of the discs, which in turn varies depending on the centrifuge application.
The typical thickness of the discs in a disc stack centrifuge is 1 mm with 1 mm separating caulks for inter-disc spacing. The disc caulks are thicker for light viscosity liquids such as water or milk. A thicker caulk reduces the total number of discs in the bowl.
The stainless steel discs are standard in a disc centrifuge. 316 stainless steel is the material for Alfa Laval bowl discs. Cheaper or generic centrifuge discs sometimes use lower-grade stainless steel or even carbon steel.
There are three types of disc stack centrifuges. The type of bowl each centrifuge utilizes categorizes it; therefore, we have a Manual Clean, Self-Cleaning, and a Nozzle type of disc stack centrifuges. The primary difference between these types of bowls is the mechanism to extract the separated solids. The manual clean centrifuge requires the operator to stop the centrifuge and extract the separated solids. The self-cleaning centrifuge bowl ejects the separated solids automatically. The nozzle bowl centrifuge has sludge discharge nozzles in the bowl wall that constantly eject the solids as they accumulate.
All disc stack centrifuges can separate 3 phases, namely liquid, liquid and solids, and therefore these centrifuges are also known as 3-phase centrifuges.
A disc stack centrifuge has one inlet for the fluid/solid mixture and two-fluid outlet pipes for the separated liquids. The bowl also incorporates space for the accumulation of the separated sludge, which is the third phase.
A disc stack centrifuge cannot separate three different liquids because the centrifuges have two liquid outlets for the separated liquid phases. If one were to process a mixture of three immiscible liquids through a 3-phase disc centrifuge, two of the liquid phases would discharge the centrifuge through one outlet, and the third liquid phase will exit through the other outlet.
A disc stack centrifuge cannot separate liquids that dissolve in each other. Oil dissolves in diesel, and the resulting liquid is one homogenous phase. A disc stack centrifuge uses the high centrifugal force to affect the differential settling of liquids with different densities. Oil dissolved in diesel becomes one liquid of set density.
A disc stack centrifuge cannot separate solids dissolved in a liquid such as salt in water or sugar in water. The centrifuge uses the difference in densities between solids and liquids to separate them. Once a solid dissolves in a liquid, i.e., salt in water, the resulting liquid has the characteristic of a single, homogeneous liquid that is not separable by centrifugation.
A manual clean disc stack centrifuge has a solid bowl that retains the separated solids during the process. The operator needs to extricate the separated solids periodically by stopping the centrifuge. This requirement limits the applicability of manual-clean centrifuges to low or zero solid applications.
A self-cleaning disc centrifuge has a unique bowl design that has a moving bowl bottom inside the bowl. This feature allows self-cleaning to eject out the separated solids while the bowl is rotating. The ability to eject the solids during the process makes the self-cleaning centrifuge applicable for liquid, liquid, and solids separation applications.
There are some advantages to a manual clean disc centrifuge due to its simplicity of construction. Unlike the self-cleaning disc centrifuge, the manual-clean centrifuge does not need an additional operating water system for sludge ejection. The absence of this requirement makes the installation and operation of manual-clean centrifuges simpler than the self-cleaning centrifuge.
Though a self-cleaning centrifuge requires an operating water system to operate the sludge ejection mechanism, it does have some significant advantages over the manual-clean centrifuge. The ability to eject the separated sludge vastly expands the applicability of self-cleaning centrifuges to fluids with an appreciable amount of solids.
It is impossible to convert a manual clean disc centrifuge to a self-cleaning centrifuge because the manual clean centrifuge bowl does not incorporate the moving parts and sludge ejection slots that are critical to the sludge ejection process.
A self-cleaning centrifuge is useable for liquid-liquid separation without the presence of any solids. In such applications, the deactivation of the sludge ejection cycle makes the self-cleaning centrifuge operate as a manual clean centrifuge.
A manual clean centrifuge is well suited for liquid-liquid separation applications. The separation of turbine lube oil from water, diesel fuel, water, hydraulic oil dehydration, etc., are examples of such applications.
A self-cleaning centrifuge incorporates a sequence of steps for the sludge ejection process. This process is a time-bound set of events that a PLC typically controls. However, the operator can also execute this sludge ejection process by following a set of manual operations. A simple timer-based controller can also operate the sludge ejection process of a self-cleaning centrifuge. Both these alternate processes involve the operator, making the process dependent on human intervention, which is not always possible or desirable.
An automated controller (PLC) triggers a sequence of actions by controlling various disc centrifuge components to facilitate the centrifuge operation. For example, the controller energizes the motor, waits for the centrifuge to reach operating speed, triggers the bowl close solenoid valve, starts the feed pump motor, monitors the pressures in the pipes, executes the sludge ejection cycle, etc. To summarize, though an operator can use the self-cleaning disc stack centrifuge manually, the PLC controller allows the centrifuge to operate automatically without the need for an operator with several other valuable features such as alarms, pump operation, etc.
A self-cleaning disc stack centrifuge uses a hydraulic mechanism to operate the sludge discharge mechanism. This system uses water to move the internal parts that enable the accumulated sludge to eject out of the centrifuge bowl. Due to the design differences between centrifuges, the water pressure varies between centrifuge models.
The operating water in a disc stack centrifuge hydraulically actuates the sludge discharge mechanism. This water passes through small orifices in the centrifuge bowl to exit the bowl. Small debris or sediment can impede the bowl operation, and therefore, the operating water needs to be filtered.
The operating water in the disc stack centrifuge is in a water chamber under the process liquid space. Ambient temperature water cools down the process fluid, which is not desirable when the fluid is viscous, and the reduction in temperature will affect separation efficiency. The operating water can be at ambient temperature when processing water or other liquids of low viscosity.
The operating water in a disc stack centrifuge does not contact the process fluid. The operating water enters and discharges from a separate water chamber below the centrifuge bowl, totally isolated from the fluid processing chamber in the bowl.
The operating water in a disc stack centrifuge has to be at a specific pressure because the design of the sludge discharge mechanism is pressure-sensitive. The bowl rotation amplifies the operating water pressure significantly. Therefore, a slight deviation from the specified pressure can cause the sludge discharge system not to function correctly.
It is possible to reuse the operating water of a disc stack centrifuge under certain conditions. The process fluid can contaminate the water in rare occurrences, and, therefore, monitoring the operating water before reuse is desirable.
One can discharge the used operating water from a disc stack centrifuge into the local drain. However, under rare circumstances, the process fluid can contaminate the water, which may not be suitable for direct drain discharge. Therefore, collecting the used operating water and checking it for contamination before drain discharge is always advisable.
A disc stack centrifuge cannot use any fluid besides water to operate the sludge discharge mechanism. The centrifuge bowl design is specific to the physical properties of water, especially the density and viscosity. The physical properties of liquids don’t allow the centrifuge bowl to function correctly.
Disc stack centrifuges have a limit on the amount of solids they can process. Typically the percentage of solids is limited to 8% by volume. However, a larger centrifuge can process higher amounts of solids at a lower flow rate given their higher sludge holding capacity.
Self-cleaning disc stack centrifuges eject the separated solids intermittently through an automatic sludge discharge cycle. However, if the percentage of solids exceeds the threshold for a given centrifuge, the sludge discharge frequency increases to a point where it affects the centrifuge process efficiency. Therefore, there is a limit on the amount of solids a disc centrifuge can handle.
A disc stack centrifuge ejects the separated solids through a hydraulic mechanism integral to the rotating bowl. The lower part of the bowl has a moving part known as the piston. Water is introduced into the chamber below the piston to move it up, which seals the bowl. The discharge cycle involves the water under the piston being allowed to escape letting the piston slide down. The lowering of the piston opens the bowl to the sludge discharge ports in the bowl body. The separated sludge ejects the bowl instantaneously due to the high centrifugal force. The reintroduction of the water below the piston moves the piston up, sealing the bowl cavity. This opening and closing of the centrifuge bowl comprise the sludge ejection cycle.
The sludge outlet of a disc stack centrifuge should never be connected to a sealed or enclosed tank. A sludge ejected from the disc centrifuge accompanies a pressure pulse. If the sludge outlet connects to an enclosed tank, the pressure causes the sludge to bounce back into the centrifuge, causing operational issues.
A disc stack centrifuge is limited by the amount of solids it can handle. In addition, the particle size in the process fluid cannot be larger than the disc spacing. Another disadvantage is that disc stack centrifuges require an additional operating water system to function, which is an added expense.
The main advantage of a disc stack centrifuge is its ability to separate liquids from other immiscible liquids. The other advantages include separating microscopic particles in the 1 to 50-micron size and long service life often exceeding 50 years.
Disc stack centrifuges are safe to operate, as proven by their field performance over 100 years and typical service life of over 30 years. Like any machinery, it is essential to operate and maintain disc centrifuges per the manufacturer’s recommendation to ensure safe and reliable operation.
The crucial indication of a disc stack centrifuge operating in an unsafe manner is excessive vibrations. Immediate stoppage and a thorough investigation of excessive vibrations in disc centrifuges are critical.
Theoretically, there is no time limit on how long a disc stack centrifuge can operate without fluids. However, the air friction on the bowl assembly heats the bowl and can potentially damage the bowl seals. When operating with the process fluid, the fluid maintains the bowl temperature and prevents excessive temperatures. Therefore, in general, we don’t recommend operating a disc stack centrifuge without fluids for more than 2 hours.
The manufacturer balances the rotating bowl in a disc stack centrifuge as part of the manufacturing process. The manufacturer balances the bowl as a complete assembly with all the components assembled. Therefore, it is critical not to exchange parts between disc centrifuge bowls.
Disc stack centrifuges operate at very high rotational speeds. Any imbalance of the rotating mass (bowl) causes vibrations that are detrimental to the centrifuge. Excessive vibrations are a safety risk due to the possibility of catastrophic damage to the centrifuge and operator.
The main advantage of a disc stack centrifuge is that it separates the solids without media use, as in filters. The use of media adds to the material and labor cost of using filters. Also, disc stack centrifuges can separate liquids from other immiscible liquids, which is impossible with filters.
A disc stack centrifuge is a flow-through type of centrifuge. As the fluid enters the centrifuge, the separation occurs within the centrifuge bowl, and the clear fluid continuously discharges from the centrifuge. Therefore, the flow-through design allows a disc-stack centrifuge to operate without the need for stoppage for long periods.
A disc stack centrifuge is a continuous process centrifuge because, unlike batch centrifuges, a disc stack centrifuge can operate continuously. It continuously discharges the separated phases, eliminating the need for batch cleaning cycles.
Alfa Laval, Westfalia (GEA) are the primary, reputed manufacturers of disc stack centrifuges who have established a reputation for high-quality, durable centrifuges over decades. Other brands such as Veronesi, Flottweg, etc., also manufacture disc stack centrifuges, but they are not as prevalent as Alfa Laval or Westfalia.
The rotating bowl assembly is the critical component of a disc stack centrifuge. The material of construction of the bowl is the most crucial factor in the long-term durability of the centrifuge. Generic or copy centrifuge bowls are often made of low-grade stainless steel, making them prone to premature failure and frequent service interruptions.
Alfa Laval disc stack centrifuges are the industry standard of centrifuge durability and longevity. It is not uncommon to see Alfa Laval centrifuges that are operating 50 years after their manufacture.
The efficiency of a disc stack centrifuge is a factor of bowl speed. Therefore, as long as the centrifuge is operating at the design speed, the separation efficiency of the centrifuge is maintained. A properly remanufactured disc stack centrifuge has the same separation efficiency as that of a new centrifuge.
Disc stack centrifuges are simple to maintain due to their robust but simple mechanical design. As with all machinery, an adequately trained technician can service and maintain the centrifuge.
The end-user of a disc stack centrifuge can efficiently operate and maintain the centrifuge. Initial operator training and service guidance from an established manufacturer or specialized centrifuge company will ensure the ability of the end-user to service and maintain the centrifuge.
A disc stack centrifuge has a set sequence of operations for optimum separation. Also, operator familiarity with automatic centrifuge controls is helpful for smooth centrifuge startup and operations. Therefore, we recommend some basic, hands-on training of the centrifuge operator.
Since mechanical parts of a disc stack centrifuge are intricate and have stainless steel or other alloys, they can be expensive. However, high-quality, less-expensive generic parts are widely available for Alfa Laval disc stack centrifuges. Consumable parts such as seals, gaskets, and bearings are not expensive and readily available.
Though disc stack centrifuges are not a typical industrial machine, replacement OEM and generic parts are readily available for Alfa Laval centrifuges. However, replacement parts for Westfalia centrifuges are primarily available from the OEM, and generic parts are hard to find. The same goes for Mitsubishi separators.
The discs in a disc stack centrifuge do not wear out under normal operating conditions. It is uncommon to see worn-out discs in disc stack centrifuges. In rare cases where abrasive solids pass through the centrifuge, disc wear is a possibility.
There are certain areas in the bowl of a disc stack centrifuge prone to wear in abrasive material applications. The sludge ejection ports are the areas that experience maximum wear in such applications. Protective liners and wear-resistance coatings are available for the centrifuge.
A disc stack centrifuge bowl body cannot be repaired by welding or other material addition methods. Welding of bowl components is to be avoided under any circumstances due to the potential safety risks. The welding heat can change the microstructure of the bowl steel, making it brittle and cause a loss the strength.
The cost of electricity primarily drives the operating costs of a disc stack centrifuge. Operating labor and material costs are negligible because most disc stack centrifuges are set up for automatic operation.
Disc stack centrifuges are inexpensive to operate because the only cost related to their operation is power, i.e., electricity. Since these centrifuges do not use any kind of media (filters, cloth, etc.), the operating cost is significantly reduced.
The only operating cost of a disc stack centrifuge is electricity. Therefore, the cost to process a gallon of fluid depends on the centrifuge's processing capacity and the motor's size. For example, a 20 GPM centrifuge has a 5 kW motor. Assuming $0.15 per kWh, the cost to process 1,200 gallons of fluid will be 5 x 0.15 = $0.75. So the cost to process one (1) gallon of fluid is 0.75/1,200 or .06 cents!
Positive displacement pumps are best for disc stack centrifuges. For example, progressive cavity pumps, screw pumps, and gear pumps are ideal for disc centrifuges. Positive displacement pumps do not churn the fluid, which helps with better separation through the centrifuge.
Centrifugal pumps and air diaphragm pumps do not work well with disc stack centrifuges. Centrifugal pumps tend to churn the process fluid, which is opposite to the purpose of the centrifuge. Air diaphragm pumps feed the process fluid with varying pressure that destabilizes the liquid interface in the bowl, degrading centrifuge efficiency.
A disc stack centrifuge can be gravity fed. The user should note that the flow rate reduces with gravity feed as the feed tank level goes down.
The clean, separated fluid discharges under pressure from a disc stack centrifuge. The disc centrifuge bowl has a built-in centripetal pump that converts the rotational energy of the fluid into pressure.
The discharge pressure of the centrate is up to 70 psi, depending on the size of the centrifuge. However, it is not advisable to use this centrifuge-generated pressure to its full extent because the resulting back pressure on the centrifuge affects the separation efficiency.
The clean fluid outlet of a disc stack centrifuge should not be connected to any suction device such as a pump. The suction of the pump will destabilize the liquid-liquid interface within the centrifuge bowl. Excessive suction will cause the light phase to break over to the heavy phase side.
Though the internal pressure generated by the centrifuge can theoretically transfer the clean fluid to remote or tall tanks, it is not advisable to do so. Using the centrifuge created pressure exerts a back pressure on the centrifuge, which affects the centrifuge performance. Excessive back pressure on the centrifuge can also result in the liquid breaking over.
The back pressure on a disc stack centrifuge is the pressure exerted on the centrifuge by partially restricting the outward flow of the centrate. A controlled amount of pressure helps centrifuge performance by stabilizing the liquid columns within the centrifuge bowl. The backpressure also helps the centrifuge controller detect liquid breakover conditions.
Backpressure around 20 psig is best for a typical centrifuge performance, while it also serves to detect centrifuge malfunctions. Backpressure higher than this level will result in reduced throughput and can potentially affect centrifuge performance.
Outdoor installation is allowable for a disc stack centrifuge provided the centrifuge system design accommodates the environmental factors. However, one should pay careful attention to the outdoor temperature range because the disc stack centrifuge should never freeze or operate under freezing temperatures.
The minimum operating temperature for a disc stack centrifuge is 0 C or 32 F. The lower temperature limit is to prevent the operating water within the centrifuge bowl from freezing. Operating a disc centrifuge below 32 F (0 C) is also a safety hazard due to the freezing water possibly damaging the rotating centrifuge bowl.
The maximum ambient temperature for a disc stack centrifuge is 50 C or 130 F. This temperature limit allows the centrifuge to dissipates the internal heat generated during operation.
Disc stack centrifuge installations are generally at ground level; this is primarily due to the inherent vibrations experienced by these centrifuges. However, disc stack centrifuges have been installed on elevated platforms, provided the platforms are structurally sound and designed for the dynamic load of the centrifuge vibrations.
The smallest disc stack centrifuge is 2’ x 3’ x 2’ (H), and the large ones are 5’ x 6’ x 8’ (H).
The smallest disc stack centrifuge weighs around 250 Lbs, while the largest ones can weigh over 3 tons.
Disc stack centrifuges do vibrate due to the minor imbalance on the rotating bowl. The balancing of the bowl mitigates the vibrations to an acceptable level.
Though the rotating bowl assembly of the disc stack centrifuge is factory balanced, it is not possible to remove all imbalance of the bowl. Therefore, a disc stack centrifuge vibrates during operation.
The typical vibration observed in a disc stack centrifuge is between 2 mm/s and 4 mm/s RMS value.
A disc stack centrifuge in good working condition has a noise level of approximately 85 dB. An operator can converse normally (with voice elevation) at 6 feet from the centrifuge.
Though hearing protection is not necessary while operating a disc stack centrifuge, it is always advisable to check local OSHA protection guidelines to determine the need to have hearing protection.
Electric motors drive disc stack centrifuges.
The size of the disc stack centrifuge motors depends on the capacity of the centrifuge. The smallest disc centrifuge uses a 1 HP motor, while the large capacity centrifuges use a 100+ HP motor.
It is possible to drive a disc stack centrifuge with hydraulic power using a hydraulic motor. Fitting a hydraulic motor requires special centrifuge modifications.
Disc stack centrifuges operate on 3 phase AC motors that operate at 230 volts or 460 volts.
A disc stack centrifuge bowl RPM ranges from 10,000 RPM for the smaller centrifuge to 4,000 RPM for the large capacity centrifuge.
The stress exerted on the centrifuge bowl limits the maximum rotating speed of a disc stack centrifuge. In addition, the bearing capacity is another factor.
Disc stack centrifuges have different RPMs based on the size of the centrifuge. For example, a small disc centrifuge has a higher RPM than a bigger size centrifuge. The RPM differs between the centrifuge models based on the size of the bowl; however, the centrifugal force is consistent.
A disc stack centrifuge (self-cleaning type) requires water in addition to electricity to operate. Centrifuge equipped with diverter valves or sludge pumps also requires compressed air to operate.
Disc stack centrifuge manufactured by a quality manufacturer (like Alfa Lava) has most bowl parts made out of 316 stainless steel. Some bowl parts are made of marine-grade bronze in mineral oil separator centrifuges.
A standard disc stack centrifuge constitutes a stainless steel bowl with some parts that are not stainless steel. A food-grade disc centrifuge is made entirely of stainless steel, specifically all wetted parts of 316 stainless steel. A food-grade centrifuge also has some design features that allow for ease of cleanup.
The smallest metal particle size a disc stack centrifuge can separate is 0.5 microns. Non-metallic or organic particles separation efficiency depends on the particles’ specific gravity and can be in the 1 to the 5-micron range.
The largest particle size allowable for separation in a disc stack centrifuge is defined by the spacing between the discs in the centrifuge bowl. Typically, the largest particles size is around 250 microns because the disc spacing in most centrifuges is in the 500 microns range.
There is a particle size limit on a disc stack centrifuge because the discs in the bowl are closely spaced, and any particles larger than the intra-disc space get stuck between the discs. These particles block the fluid flow passage rendering the centrifuge ineffective.
A disc stack centrifuge can break some kinds of emulsions. The ability of the centrifuge to separate emulsion is dependent on the type of emulsion. A chemically bonded emulsion is impossible to separate without chemical assistance.
A typical disc stack centrifuge uses 50 weight gear oil in the transmission gearbox. The operator can use a lighter oil (30 weight) in the centrifuge gearbox in warmer climates.
The disc stack centrifuge operator should change the centrifuge transmission oil every 1500 to 2000 operating hours. This oil change frequency may be different for some special centrifuges, and the centrifuge manual can provide an accurate time interval.
Nitrogen blanketing of a disc stack centrifuge is the process of purging the internal space of the centrifuge with a non-flammable gas such as nitrogen. Nitrogen displaces the ambient air, which carries oxygen from the centrifuge cavities. This purging is essential for processing flammable liquids and other fluids affected by the presence of oxygen.
Disc stack centrifuges are purgeable with inert gases such as nitrogen and carbon dioxide for special applications. Special purge fittings are installed on the centrifuge frame to allow the inert gas to flow into the centrifuge.
A direct drive adapter on a disc stack centrifuge replaces the friction clutch assembly to directly couple the drive motor to the centrifuge transmission shaft. It is essential to eliminate any possible sources of heat or sparks in specific applications, such as a friction clutch, to ensure safety.
If you did not find the answer above - feel free to contact a Dolphin Centrifuge engineer!
by Sanjay Prabhu MSME
Engineering Manager, Dolphin Centrifuge