Rotary lobe pump design enables six times higher service life

A robust rotary lobe pump design enables a six times higher service life of pumps handling abrasive ceramic slip

Until 2013, a German ceramics manufacturer used a progressing cavity pump for filling and recirculation of the storage tanks for grog and ceramic slip. The wear parts of this pump, however, had to be replaced every three months due to the highly viscous and abrasive substance. To minimise service times and enable more economical production, Netzsch suggested replacing the unit with a special rotary lobe pump from the Tornado T2 range.

The T.Proc is designed for a long service life with minimum wear, so that it can also convey highly abrasive compounds without problems. This is made possible by the combination of the operating principle and a special material pairing: instead of conventional elastomer lobes in a metal housing, this pump comprises two steel lobes rotating in a replaceable rubber housing insert. This achieves a higher durability of the moving components because steel is less susceptible to material fatigue from dynamic forces than plastic elastomer. In addition, the interior is designed without dead space so no slip residue can adhere. Due to the clearly reduced wear and the longer service life of one and 1.5 years, respectively, the ceramics manufacturer has now changed over its entire production to the Tornado T.Proc.

The ceramic production process offers a range of forming processes that can be categorised into the basic types of pressing, plastic forming and casting. For ceramic slip casting, for example, a watery suspension is poured into a porous mould that drains liquid from the slip. This produces a solid layer, the so-called cullet. Slip and grog are initially kept in storage tanks. Due to the viscous and abrasive medium, the German ceramics manufacturer used a Nemo progressing cavity pump from Netzsch for filling and recirculating ceramic slip until 2013. "We initially recommended this model to the ceramics manufacturer as we have had very positive experiences with mineral suspensions, such as plaster, mortar or concrete primer," Egon Grainer, team leader Paper & Chemicals, explains.

Due to the high level of abrasiveness of the conveyed grog slip, however, it turned out that the wear parts – rotor and stator – still had to be replaced every three months and the shaft sealing every six to eight months. This caused very long service periods and accordingly high costs. To increase the efficiency of the system, Netzsch therefore suggested to replace the unit with a different operating principle, which had not been available in its product range when the Nemo pump was installed. The manufacturer now uses the T.Proc rotary lobe, which is designed to be highly robust and have a long service life. The "Full Service in Place" design of the pump also minimises the maintenance-related downtimes.

"At the heart of the system are two straight bi-lobe rotors that rotate in opposite directions, moving the medium smoothly but continuously from the inlet to the discharge side," Grainer says. A characteristic feature of this type of pump is the special material pairing. Whereas other pump manufacturers normally have elastomer lobes rotating in a metal housing and exposed to a high dynamic load, this unit uses steel rotary lobes while the inside of the pump is lined with a rubber insert.

This is prevented on Tornado T2 pumps via the innovative materials mix: the lobes are made from a particularly robust hardened steel and are not affected by the rotating motion, which clearly increases their service life. In addition, the metal is subject to less deformation under changing temperatures, which means smaller tolerances can be used in production. This is reflected in accordingly lower wear and higher efficiency. The protective housing liner made from acrylonitrile butadiene rubber (NBR) can easily be removed as a separate part and replaced cost-efficiently. Rubber pads vulcanised onto the faces of the metal lobes ensure a continuous hard-soft contact, which also protects components and increases service life. Overall, Netzsch states that this achieves six times longer service life and therefore substantial cost savings for abrasive media.

Because the pump action is based on displacement, as in the previously used progressing cavity pump, the viscosity of the medium is irrelevant. The pump can handle substances from 1 mPas to 1 million mPas. At the same time, pressure fluctuations and shearing forces are excluded: "This type of pump does not generate any pressure on the medium, but merely conveys against the existing backpressure – in this case 4 or even 5 bar," Grainer explains. The simple shape of the vanes allows a large free ball passage to prevent blockages.

Furthermore, this design has a positive influence on the efficiency of the pump. Thanks to the long sealing line, only a very low backflow is generated. Additionally, the specially designed internal geometry of the delivery chamber keeps any pulsation at a very low level, ensuring a gentle, continuous product flow without turbulences. This allows the desired properties of the respective slip to be maintained. At the same time, the special housing geometry in connection with the large ball passages additionally minimises wear.

The rotary lobes are attached outside of the housing with quick-release elements. As a result, they are completely smooth, have no surfaces for deposits and can also be removed and reinstalled conveniently and independently of each other. They can be released or tightened with just a few steps, without special tools. A positioning and installation gauge integrated into the front cover makes correct positioning easier in this process. The shaft seals are also designed so their glide surfaces seal flush against the rear side of the lobes. This prevents dead spaces where abrasive particle residue could adhere. The seals are preconfigured and easily slide onto the shaft with the rotary lobes during assembly. Thanks to the cartridge design, different types of seals fit into the same installation space to suit changing requirements. "For pumping slip, the shaft seal is especially designed as an I-seal with quench. The silicon-carbide glide surfaces are highly robust and durable, even with this highly abrasive medium," Grainer explains.

The innovative drive concept of the range is also characterised by a long service life and long maintenance intervals. A belt drive, toothed on both sides, replaces the timing gear that was previously common for rotary lobe pumps. Due to its complex design, this was prone to malfunctions and defects. In contrast to this, the new drive technology can even survive damage or blockages without major damage. In the worst case, the belt will come off or tear. If this situation occurs, only two screws must be released to remove the belt and replace it. The robustness is also supported by the smooth running of the belt drive and the low vibrations during operation, because a hard-soft contact is always maintained between elastomer belts and metal gears. This provides gentle operation for all pump components. In addition, the drive works completely without oil, so that time-consuming oil changes and the risk of leaks are no longer a problem.

The double-sided belt is not only used for transmission of forces from the motor to the shafts, however, it also synchronises the rotating motion of the lobes. Thisdual function has the great advantage of simplifying the overall drive design considerably, which increases operational reliability while also saving installation space. In conjunction with the motor being flanged onto the top of the pump chamber, this provides one of the most compact pumps on the market. This allows the pump to still be accommodated even in very confined spaces. Furthermore, there is no need for an extensive spare part stock for the belt drive, which helps reduce storage costs and downtimes to a minimum.