Sludge dewatering - Turning two into one: regional water company needs one less unit thanks to special, maintenance-friendly FSIP® pump design

A regional water company in Northern Italy uses two NEMO® progressing cavity pumps from NETZSCH Pumpen & Systeme GmbH to load a centrifuge for sludge dewatering. In order to ensure uninterrupted continuous operation and smooth meshing of all procedures, one of the very reliable pumps is constantly in operation, whereas the second is always on standby – so that it can step in if there is a fault or in the event of maintenance. Both standard pumps were replaced in February 2017 by the manufacturer’s new service-friendly model in “Full Service in Place” (FSIP®) design which has been available since 2016. The new service-friendly design means maintenance and repair work is so much easier, making interruptions due to repair and servicing measures so short that the standby unit is no longer needed: the new model of the NEMO® pump is characterised by a suction housing with a large inspection opening which enables all service and maintenance work to be carried out in place without dismantling the pump from the pipework. In addition, the so-called iFD® stator, which comprises a two-part re-usable metal housing with a polygonal profile and the NEMOLAST® elastomer inserted in it, ensures a lower breakaway torque, increased efficiency and service life and faster replacement, including problem-free disposal. Overall, the new design saves time for inspection, repair and cleaning work, while also reducing the space required for the pump.

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In order to continuously load the centrifuge used for sludge dewatering, two NETZSCH pumps of the NEMO® BY type were employed in block construction design.“ Due to their special design, they cope well with the abrasive medium”, says Stefano Olivotto, Head of the Italian branch of NETZSCH. “The units are based on one rotor which turns in an oscillating motion within a fixed stator that is geometrically adapted to it.” The rotor is designed as a kind of round threaded screw with extremely large pitch, large thread depth and small core diameter. The stator has an extra thread and double the pitch length of the rotor. The exact geometrical mating of both components forms conveying chambers during rotation in which the medium is transported smoothly from the inlet to the discharge side with low pulsation. The volume of these chambers remains constant and the chambers themselves are sealed in the process. This means the NEMO® progressing cavity pump offers a high level of reliability and a longer service life with a medium of this kind than other pump technologies.

This technology not only prevents backflow, but also ensures that the conveyed medium is transported at stable volume and pressure, so that virtually no shear forces or pulsation occur. Viscosity and consistency of the material are irrelevant in terms of pump output here. The size of the conveying chambers and thus the theoretical delivery rate depends on the size of the pump. A 360° rotation when the outlet is free produces the volumetric delivery rate per revolution. The pump delivery rate can therefore be modified via the speed.

Maintenance effort with previous progressing cavity pumps

For conventional progressing cavity pumps, maintenance of the conveying elements had required a relatively large amount of effort. “In order to remove the rotor and stator, you had to dismantle the whole pump from the pipework”, explains Stefano Olivotto. “Replacing the conveying elements meant removing the end connection, pulling the stator off the rotor and dismantling the sucker rod string.” When positioning the pump, this dismantling length therefore had to be taken into account and sufficient room at the front kept clear. Any major work on the pump was thus accompanied by extended downtime, lost production and personnel costs. In order to avoid this, the wastewater treatment plant therefore acquired two units, with only one of them ever being used to convey sludge into the centrifuge, while the second was permanently in standby mode in order to step in if there were faults or in the event of maintenance work, enabling uninterrupted loading to be ensured.

Since the company was very satisfied with this solution, in 2016 there was a very open-minded response to the proposal to introduce the innovative FSIP® pump. They took advantage of the opportunity to further increase the system’s cost efficiency and use only one unit in future. Because reliability, maintenance friendliness and speed of maintenance of the one remaining pump became even more crucial, NETZSCH recommended replacement with the FSIP® model of the NEMO® progressing cavity pump. In designing it, NETZSCH engineers used the design ideas of the TORNADO® rotary lobe pump. This pump allows removal of the entire front, release of the clamp-type mounting of the rotary lobes and thereby clearance of the entire interior up to the flange connections in next to no time. The pump does not need to be dismantled from the pipework; all the work can be carried out directly in place. “We are therefore talking about the “Full Service in Place” concept here, because maintenance work takes a significantly shorter time and resources and costs are reduced”, says Stefano Olivotto. “Thanks to these features, T2 TORNADO® pumps of this kind have now become the preferred solution for high-wear media or in difficult conditions, because they make the unavoidable service work on the pump considerably easier.”

Top speed rotor replacement: open, uncouple, remove

However, progressing cavity pumps are not equipped with a comparable front that could easily be removed. To enable implementation of the FSIP® principle for this pump type as well, a special inspection cover was therefore developed instead and integrated into the design. The cover is secured by just five screws that can be removed without any special tools at all in a really short time. At the same time, attention has also been paid in its design to ensuring that the cover has a reliable seal, both at high pressures and with vertical installation of the pump. The maintenance opening is located where the stator connects with the pump housing. Removing the cover therefore provides access to a two-piece clamping coupling connecting the joint on the rotor with the coupling rod. Only one screw needs to be removed here to separate the rotor-stator element from the coupling rod. The rotating unit can then easily be lifted out, and the pump interior is freely accessible from flange to flange. Because all parts are removed sideways or upwards, there is no additional space needed for removal. The dismantling length at the end of the pump, which had previously been essential, is no longer required, meaning that the necessary installation space – which had been needed until now for progressing cavity pumps – is considerably reduced. Furthermore, a cartridge design of the rotation direction-independent and wear-resistant single-acting mechanical seal MG1 is used as an option for the FSIP® model of the NEMO® pump. It can be particularly easily replaced via the inspection opening. This maintenance-friendly design is also supported by the unit’s block construction design: “The drive is flange-mounted directly onto the lantern of the pump, which not only enables compact dimensions, a low overall weight and constant shaft heights irrespective of the design and size of the drive, but also a high level of service friendliness”, explains Stefano Olivotto.

During revision of the design, in addition to convenient disassembly, NETZSCH engineers also paid attention to making handling as easy as possible when installing new components or re-inserting inspected parts. This normally requires a certain amount of skill, strength and time. Tools such as braces or bearers are even needed sometimes. In contrast, the FSIP® pump is designed so that the pump housing itself acts as both a bearer and an orientation aid. The special layout ensures that each component goes into its intended location. Positioning is practically an automatic process, which means the installer or operator does not need to readjust anything. All in all, this means any wearing part can be replaced in less than half the previous time. There is no change to the conveying geometry for "Full Service in Place" progressing cavity pumps. In this way, the customary performance is maintained in spite of easier maintenance.

Elastomer stator easily removable

To work in a particularly easy and resource-efficient way the new pump design can be combined with another modern component of the FSIP® concept, the iFD stator® 2.0. This involves a stator system where the elastomer is not vulcanised into the housing, as is otherwise common, but is fixed by axial compression. A defined limit stop provides uniform pre-tensioning and ensures the desired sealing line. By separating the steel sleeve and elastomer stator, the latter can be quite easily removed together with the rotor. This makes maintenance even easier and means the stator can be disposed of separately, if necessary, when the rubber is worn. “With conventional progressing cavity pumps, the stator tube needs to be disposed of as well, since it cannot be separated from the elastomer with proper material sorting”, explains Stefano Olivotto. “That leads to unnecessary costs”. On the other hand, the iFD Stator® allows continued use of the sleeve with a new elastomer insert, with replacement taking just a few minutes.

Furthermore, this solution also increases the cost effectiveness of the system: the whole pump runs more smoothly and with less breakaway torque. In addition, less flexing occurs between the components, which reduces the torques during operation. This means that roughly 25 percent less energy is consumed during the entire conveying process, and the drives of the pumps can have correspondingly smaller dimensioning. Furthermore, because of the lower load, the stators last approximately twice as long – in conjunction with simple maintenance thanks to the FSIP® concept, downtimes are reduced to a minimum.

Complete package ensures greater cost efficiency

This effect is supported by other components: so, a rotor, which is available in various wear and corrosion-resistant designs, can be combined with the iFD stator®.

The FSIP® pump has now been in use for four months at this wastewater treatment plant. It generally runs at a speed of 58 to 287 rpm and has a delivery rate of 10 to 55 m³/h. “The replacement was a success, the pump has no problem achieving the required performance.” As expected, the service in place for the pump is now significantly faster and easier than with the previous models. The operator is very satisfied.