Innovation for slurry applications in all industries
This article describes a highly innovative
design of pump - the Discflo disc pump - that offers users unheard-of levels of
versatility and productivity, and exceptionally low pumping Life Cycle Costs.
The first commercial applications were over 15 years ago and since then, it has
proven successful in a wide range of slurry applications in industry, both in
the US and Europe.
The disc pump technology is based around a patented operating mechanism that minimizes contact between the pump and the product being pumped. It also gives pulsation-free pumping, while at the same time achieving the same heads and flow rates as any standard industrial pump. The benefits arising from this unique pumping mechanism include: little to no wear even in severely abrasive service; the ability to pump up to around 80% solids without breakdown; pumping viscosities over 100,000 cP without clogging; the ability to pump fluids containing large and/or stringy solids without clogging; pumping fluids with high levels of entrained air or gas; and no damage to delicate and shear sensitive products.
The first part of this article discusses the disc pump concept itself, the pump’s operating parameters, and the types of fluids pumped. The second part looks at its application in industry, focusing on the pulp & paper manufacturing, chemical, oil and petroleum products processing, municipal and industrial wastewater plants, and food processing.
History of the disc pump
The disc pump concept dates back to 1850. The first ‘disc pump’ was created in the US by Sargent. He took a series of 29 parallel discs spaced a few thousands of an inch apart, enclosed them with a metal band, and made a number of holes in the band to allow fluid to pass in and out. It was the first instance of a pump operating solely using the principles of boundary layer and viscous drag, rather than any kind of pushing mechanism. As far as pumping goes, though, it was not a great success. The idea was taken further by the Serbian-American inventor Nikola Tesla. He removed the metal band from around the discs, which improved the pump’s performance. He too insisted on keeping the spacing between the discs very small, believing that at a certain point the pump would stop pumping if the discs were spaced too far apart. This insistence on very narrow disc spacing greatly limited the pump’s capabilities and so the idea was all but forgotten.
Until an inventer in southern California, Max Gurth, came along. He did what the experts believed could not be done. He widened the disc spacing to as much as 20" [0.5m] and found that the pump still pumped. Moreover, the flow remained smooth and laminar flow, which meant that the pump was still operating via the principles of boundary layer and viscous drag (described below). Mr Gurth applied for the first patent in the late 1970s. Since then, the disc pump design has been further honed to enhance its performance in a wide range of industrial applications. Mr Gurth founded the Discflo Corporation in 1982 and the company remains the sole manufacturer worldwide of this unique pump technology, retaining ownership of all US and international patents.
From the outside, the disc pump has the appearance of a centrifugal unit but it can perform the work of centrifugals, progressive cavity pumps, lobe and gear type pumps, and in some cases, has also replaced chopper pumps. The disc pump can achieve flow rates from 2 to 10,000 GPM [2250 m³/h] and heads up to 1000 ft [300m]. The operating principle behind the disc pump is boundary layer-viscous drag. Its application in the world of pumps is new but it has been widely used in the field of fluid engineering for over 100 years. A common example of this principle, well known in industry, is the phenomenon of pressure drop or friction loss through a piping system. The resistance to flow as a liquid moves through a pipe is due to viscous shear forces within the liquid and turbulence along the walls due to roughness. It results in a loss of head or pressure, the amount of which depends on the characteristics of the liquid being handled - ie viscosity, pipe size, pipe condition and length of travel.
If you examine the cross-sectional area of a pipe under laminar (non-turbulent) flow conditions, you see numerous streams of liquid traveling at different velocities. The stationary pipe exerts a ‘drag’ force on the moving liquid, attempting to slow it down. This drag force is transmitted to all the liquid layers along their parallel ‘slip’ surfaces. The result is higher liquid velocities at the center of the pipe, with gradually lower liquid velocities as the layers approach the inner surfaces of the pipe. In fact, the layer closest to the pipe can be assumed to be stationary.
This is the phenomenon of boundary layer-viscous drag and the principle behind the operation of the disc pump. At the heart of the pump is a series of parallel discs, known as the Discpac. When a fluid initially enters the pump, its molecules adhere to the surface of these discs, providing a boundary layer on the disc surfaces. Layers of fluid molecules are then formed parallel to the discs. As the discs rotate, energy is transferred to successive layers of molecules in the fluid between the discs, generating velocity and pressure gradients across the width between the discs. This combination of boundary layer and viscous drag effectively creates a powerful dynamic force field that ‘pulls’ the product through the pump in a smooth, pulsation-free flow. (Note that the boundary layer on the Discpac is stationary relative to the discs, but relative to an external oberver, this layer is moving at the highest velocity. By contrast, the layer midway between the discs is moving fastest relative to the discs, but slowest relative to an external observer). The key point about this mechanism is that there is no ‘impingement’ by the fluid on the moving parts of the pump. This ‘non-impingement’ design is where the disc pump differs from other pumps on the market, all of which use some kind of impingement device - such as a vane, paddle, rotor, or screw - to ‘push’ product through the pump. The disc pump is the only pump to use a pulling or drag principle to move product. This principle leads to several key benefits for the disc pump over existing pump systems in the handling of ‘difficult’ fluids - severely abrasive slurries, fluids with a high solids content or containing large/stringy solids, very viscous slurries, fluids with high volumes of entrained air and/or gas, and delicate and shear sensitive fluids.
Firstly, the disc pump has a low NPSH requirement, reducings the possiblity of a reduction in head and capacity or excessive noise and vibration, caused by the vaporization of liquid prior to the low pressure area of the pump. As discussed above, the boundary layer-viscous drag operation produces smooth laminar flow within the disc pump. This gives minimal pressure drop between the pump inlet and the eye of the pump, so that the resulting NPSH required by a disc pump is about half to a third that required by a standard centrifugal pump for the same service conditions. In addition, the layers of fluid near the discs in the disc pump act as a buffer or ‘shock absorber’ to protect the discs against the effects of cavitation and implosions. Therefore, even under low NPSH conditions, the disc pump suffers little to no wear. A second benefit is the pulsation-free flow, due to the lack of an ‘impingement’ device and the laminar flow generated by viscous drag. This ensures no degradation of shear sensitive and delicate products, such as chemical crystals, emulsions and delicate foodstuffs. It also results in less wear and tear on the surrounding pipework.
No close tolerances
Another advantage of the disc pump design is the lack of close tolerances. This allows it handle large and stringy solids, as well as fluctuations in solids size and volume, without clogging. In addition, component wear has no effect on the disc pump’s performance and operating efficiency. Most pumps are engineered to close tolerances in order to operate at optimum efficiency, but as these pumps wear, their efficiency decreases and they require more frequent repairs to maintain an acceptable level of performance. The disc pump is one of the few pumps on the market that is able to run dry indefinitely. This is because there is no direct metal-to-metal contact in the pump, although the mechanical seal must still be protected under these conditions. It is also possible to deadhead the discharge and/or starve the suction for extended periods of time at normal operating speeds, without causing any damage to the pump. Radial loads on the disc pump shaft are low during operation due to the pressure being more or less equal around the casing. This feature helps ensure long pump and shaft life. The disc pump can handle a wide variety of difficult fluids, without breaking down in service. The system can pump a very wide range of viscosities, densities and solid sizes, by changing the Discpac parameters, and the same system can handle fluctuations in temperature, pressure, solids content and even produce, without breakdown.
Types of fluid pumped
One of the key differences between the disc pump and other pump designs is that the disc pump essentially takes advantage of friction (and therefore high viscosities) to move product. In fact, due to viscous drag, the higher the viscosity, the more efficient is the pump’s performance. When pumping high viscosity fluids, the disc pump’s efficiency is comparable with that of a progressive cavity type pump. And above 250 cP, the disc pump becomes more efficient than the centrifugal style pump. This also means that the disc pump is not really suitable for pumping water; it can do so, but is less efficient than other pump designs, and is therefore not marketed for these ‘easy’ applications. Slurries containing up to 80%+ solids can be pumped. These are essentially non-homogeneous slurries in which the particles easily fall out of solution because the solids have a higher specific gravity than the carrying fluid. As a non-homogenous (non-Newtonian) slurry enters the pump, the solids particles move to the point of highest velocity relative to the discs, ie, the point midway between the discs. The higher the specific gravity of the solids, the greater the ‘centrifuge’ action. The solids in the layer adjacent to the disc surfaces are near stationary and act as a molecular buffer between the discs and the other solids. The boundary layer both protects the solid from the full impact of the discs and the discs from the full impact of the solids, thus eliminating product damage in the case of delicate or shear sensitive product slurries, and reducing pump wear with an abrasive or corrosive product.
As discussed above, the disc pump is good for abrasive slurries. The abrasion resistance of a given pump design can be determined by observing the flow path of a solid as it passes through the pump. If a particle makes contact with a surface, the surface will wear. The rate of abrasion is a function of the impingement angle (angle of attack) at which contact is made. The lower the angle at which the solid impinges on the pump surface (for metal surfaces), the lower the wear. The disc pump has impingement angles close to zero, so wear rates, even in extremely abrasive service such as pumping lime sludge or sugar slurries, are low.
The corrollary to low wear on the pump is lack of damage to the product. This becomes important if the user is pumping a shear sensitive slurry or large, delicate products. The same principles that reduce abrasive wear apply here. The boundary layer formed on the disc surfaces acts as a buffer to ensure no contact between the pump and the product being pumped once the pump is running. The pump’s ability to handle delicate fluids has lead to some interesting, one-of-a-kind applications, such as pumping live plankton at the Smithsonian Institution in the US!
There are two types of performance curve that characterise the disc pump. The first is a steep sloping H-Q curve which applies to the original flat Discpac. The second is a flatter H-Q curve, which applies to the second generation, ‘high head’ Discpac design. This latter design has been shown to produce higher flow rates and discharge pressures than comparably sized flat disc designs, so the user can select a smaller pump and lower horsepower motor.
The performance curve for the ‘high head’ Discpac pump shows that the head varies only slightly with capacity from shut-off to design capacity. Therefore, when you have wide fluctuations of capacity with a near constant pressure requirement, this configuration is the better choice, and is recommended for the following applications: all delicate and shear sensitive slurries; severely abrasive fluids; fluids with high volumes of entrained air and/or gas; and situations where the pump experiences large or rapid changes in flow conditions.
Chemical and related industry applications
All types of highly viscous, high solids, abrasive slurries and shear sensitive chemicals in the chemical, oil and petroleum processing industries can be successfully pumped. In fact, the ability to handle delicate materials without damage has led to the disc pump becoming the ‘pump of choice’ for several large multinational chemical companies; one application concerns pumping crystals and another concerns pumping latex emulsions. The disc pump is also used extensively in the paint, ink and plastics industries. The following are case studies of disc pump applications in the chemical industry.
The disc pump’s no-shear operation has lead to significant benefits for a manufacturer of ammonium bromide crystals in Arkansas. The product being pumped is one of the toughest applications for any pump; it is highly abrasive, non-homogeneous, shear sensitive and has a solids content by volume of 45%. The flow rate is 300 GPM and the pump system runs for 12 hours a day.
Previously, the company used a centrifugal style pump. As well as suffering from excessive abrasion and frequent breakdown, this pump would destroy anywhere from 30% to 60% of the crystals by reducing them to a sand-like consistency. The company spent a minimum of $100,000 a year in spare parts and repairs alone, before taking into account the cost of lost production. These crystals retail at around $1.24 per lb. If we assume that the production cost was just a sixth of this figure, then at a destruction rate of 30%, the centrifugal pump was costing the company $84,000 per day in lost production.
After searching for a pump that would reduce shear to a minimum and cope with the abrasive, high solids nature of the slurry, the company decided to try the Discflo pump. The system was started up in June 1995, and since then has been operating with no downtime and no spare parts have been purchased. More importantly, the pump has reduced crystal losses to nearly zero, giving a payback time for the Discflo pump of 6 hours, or less than one day’s production.
For over 10 years, the pumps used by a motor oil and gas additive processing plant in Texas would fail frequently. The slurry being pumped was at times the consistency of peanut butter, and at other times, an oil/water two-phase liquid. Shutdowns were required once or twice a week, at an average cost of $650 for parts and labour. In addition, around four to five hours’ production would be lost due to pump failure. To resolve the problem, the plant engineers agreed to try the disc pump system. The disc pumps have operated without any problems for two and a half years, with no mechanical failures, no shutdowns to replace worn parts and no lost production. The company estimated that the savings in maintenance costs are around $65,000 annually, and the time to recover the cost of the new pumps was less than three months.
Pumping polymer emulsions
One of the world’s largest chemical producers, based in Germany, made the disc pump its worldwide standard for manufacturing a proprietary emulsion. This follows extensive testing of different pump types to find one that could handle the company’s dispersions with minimal damage. The company tested a number of pumps, including progressive cavity, rotary, lobe and vortex models, and found the disc pump gave superior performance. The tests showed the disc system could handle the full range of product viscosities, from 50 mPas to 1000 mPas, at different temperatures, and most importantly, would not shear the emulsions.
Pulp & Paper Mill Applications
The pulp and paper industry is one of the most important markets for the Discflo system. Disc pumps have been installed for virtually every hard-to-pump application encountered in pulp and paper mills, including pumping medium-to-high density stock without any change in paper freeness; pumping abrasive fluids from the chemical recovery process; and pumping viscous and high solids effluent and sludge. The non-impingement design of the disc pump is also solving the problem of pumping shear sensitive and delicate products, such as paper coating chemicals, without damage to the chemicals.
Pumping abrasive and
shear sensitive slurries
Modo Iggesund is using the disc pump system throughout its mill in northern Sweden, for some of its toughest pumping applications. The existing pumps suffered from high maintenance, excessive wear, unplanned downtime and pulsation. Disc pumps were first installed in a critical paper coatings application, pumping a highly abrasive and shear sensitive bentonite solution to the paperboard machines; unplanned downtime in this area would have catastrophic results on production.
The company recently inspected the pumps after 22,500 hours continuous operation and saw almost no signs of wear. Equally important, they have not broken down once since start-up. Modo then decided to try disc pumps in other difficult applications and have started-up pumps in the past 18 months (as of June 1997) in the chemical recovery plant (for pumping lime slurry, black liquor soap and lignin/white liquor) and the waste treatment plant (for handling coating waste with 5% dry content of fiber, clay, latex, chalk and solid sizes up to 20mm). In all these applications, the disc pumps have run with no downtime, and no spare parts have been purchased.
Modo estimates that the savings amount to SEK 50,000 to SEK 100,000 ($10,000–$20,000) per pump per year, with return on their investment between six and ten months. The company plans to purchase more disc pumps in the future for applications in lime milk, lime slurry and black liquor.
Pumping stock with an 8% density and high entrained air
A disc pump was installed at this paper mill in Connecticut in the Fall of 1995. It is being used to pump a 8% density stock at a flow rate of 450 GPM. Prior to the disc pump, the mill used a vortex type pump, which could only handle 4.5% density stock and suffered breakdown about once or twice a week. Moreover, the previous pump could not cope with the entrained air in the pulp, which at times could be as much as 12%. Installing the disc pump system has overcome the entrained air problem and allowed the mill to increase the percent stock to 8%, with savings resulting from the reduction in storage and water costs. Also, the pump has run trouble-free over the last 18 months, and maintains maximum paper freeness.
Pumping clarifier sludge
The clarifier sludge being pumped at a paper mill in Arkansas is 70% sand mixed with water and salt brine. The company previously used two self-priming centrifugal pumps in this application to move the 450 GPM of sludge from the cooling tower. These pumps broke down on average every six weeks and suffered badly from wear due to the highly abrasive nature of the sludge and the high solids content. The plant manager estimated that he was spending around $21,000 per year per pump on spare parts. The company then installed a disc pump in April 1995 to replace the two centrifugal pumps. Since start-up, they have been running with no downtime.
Pumping black liquor soap and defoamer polymer
A paper manufacturer in North Florida has made the disc pump system ‘pump of choice’ for two of its pumping applications—black liquor soap and defoamer polymer. The first pumps were installed about five years ago in the black liquor soap area, and another consignment was shipped in July 1997. The black liquor soap at this plant is highly viscous (up to 60,000 cPs when cooled),
thixotropic, and severely abrasive, with a solids content from sodium sulphate crystals and other by-products of digestion varying from 2–3% to as much as 20%. This product also contains extremely high volumes of entrained air and has a specific gravity of 0.9.
The disc pumps replace gear and lobe type pumps at the plant, both of which had problems handling the soap; on average, these previous pumps had to be rebuilt every 90 days, mainly because of problems caused by the entrained air and high solids content. The first disc pumps, which have now been running for five years, have performed well. The Pulp Mill Maintenance Supervisor comments: "In five years, we have never had to do any repairs to the pumps, and during routine inspection I have never noticed any wear to the internal parts. The first disc pump paid for itself in six months." The Pulp Mill Maintenance Supervisor has now approved the replacement of all the remaining black liquor soap pumps with disc pumps within the next couple of years, as his budget allows.
The paper mill has also made disc pumps the standard in its bleach plant. Disc pumps are being used to pump defoamer polymer to the top of the bleach tower. This is both highly viscous and abrasive, and caused the previous gear pumps to be rebuilt every 6–8 weeks. The disc pumps, the first of which started up five years ago, have run trouble-free with no spare parts required and no repairs.
Wastewater Plant Applications
There are numerous applications of disc pumps in municipal and industrial wastewater treatment and disposal plants. As well as handling all types of viscous and/or abrasive waste sludge with minimal wear and breakdown, the disc pumps have found a niche pumping waste from anaerobic digesters, where the sludge typically contains high volumes of entrained air and/or gas.
solids lime sludge
A wastewater treatment plant in Florida is using disc pumps to pump lime sludge, a severely abrasive and high solids content fluid. Prior to installing the first disc pump system, the plant operated two progressive cavity pumps to move lime sludge with a 30–60% solids content. "During the course of normal operation, the rotor/stator assembly in these pumps would start wearing out after 2–3 months" reports the Superintendent at the plant. The Discflo system was then installed to move the lime sludge. The Superintendent comments: "It has not shown any signs of wear to date.... The only maintenance required in four years of operation has been packing replacement and the solids being pumped now range from 60–80% on a daily basis." The pumps have reduced the City’s maintenance bill by several thousands of dollars yearly in maintenance costs, as well as cut downtime and improved the overall efficiency of the lime solids removal operation.
with entrained gas
Discflo pumps have solved a pumping problem in sludge recirculation for the Metro Wastewater treatment facility in Denver, Colorado. The previous centrifugal type pumps experienced air-locking when the plant installed gas mixing equipment as part of the anaerobic digestion process. The disc pumps’ ability to handle entrained gas in this viscous sludge containing 2–4.5% by weight solids was a major factor in their selection, according to the Project Manager for the Central Treatment Plant. The pumps (Fig 10) have operated with no downtime or repairs, other than routine preventative maintenance, since start-up two and a half years ago.
types of sludge
At the Ashbridges Bay Main Treatment Plant in Toronto, Canada, Discflo pumps are being used for two different sludge handling processes. Six disc pumps are installed in the disposal area pumping a 3% solids sludge from the digesters to the centrifuges, and one pump (so far) has been installed after the dissolved air flotation tank. In the first process, the disc pumps replace rotary lobe pumps, which required complete overhaul after 3000 running hours at the cost of CAN $10,000 each. As this process runs 24 hours a day, 365 days a year, "the lobe pumps were costing us too much to maintain", reports the Chief Works Supervisor (Mechanical) at the plant. It was the same problem in the sludge thickening area, only in this application, the plant used progressive cavity type pumps to move 110 dry ton/day of a 5% solids sludge. These pumps cost from CAN $15,000–CAN $20,000 to overhaul.
The first discs pumps were installed in the sludge disposal area in 1995, and in the sludge thickening facility in 1996. Since start-up, they have all run trouble-free, with no repairs and no breakdown in service. The Works Supervisor is pleased with the performance and expects the pumps to pay for themselves soon: "When you consider how much it cost us to overhaul the lobe pumps and the progressive cavity pumps, it won’t take long to recoup our investment." He has ordered four more disc pumps for the plant’s centrifuges and three more to replace the PC pumps in the thickening area.
Food and beverage processing
The disc pump’s ability to handle delicate products without destruction is a key benefit in the food and beverage industries. This advantage, together with the pump’s ability to handle large and stringy solids without clogging, has opened up markets where previously pumps could not be used—for example, pumping tomatoes, grapes, shrimp and whole chickens. The disc pump is also very widely used in the sugar industry, where its unique ‘pull’ principle ensures no damage to sugar crystals and minimal wear from abrasion.
Pumping cooked corn
A major manufacturer of corn chips in the US has standardized on the disc pump system for pumping the delicate corn in their plant. The company estimates that they are saving $10 million a year (ten million dollars every year) by switching to disc pumps. The savings have come from the reduction in corn product damage, from 17% to 0.5%. The decision to standardize on the disc pump followed extensive testing of several different types of pump for transferring the cooked corn/water slurries, all of which caused moderate to severe corn damage. The disc pump system also compared favorably with the other types of pumps in terms of capital cost, reduced maintenance and increased reliability.
In the 15 years since the disc pump was developed commercially, it has been installed in an extremely diverse range of industries. Its unique abilities in handling abrasive, viscous and high solids fluids, on the one hand, and fluids containing delicate, shear sensitive and/or large solids on the other, has lead to the disc pump being used in applications previously considered impossible to pump. It is the first genuine breakthrough in pump technology for almost 80 years.
Sarah Benson, Discflow