Pump Up the Volume
A BREAKTHROUGH in pump design is helping a leading specialty chemicals producer save thousands of euros spent on replacing broken equipment which was not up to the job of conveying natural waxes.
Wax work is now a model of efficiency thanks to a special pump design, finds Reinhold Bimüller.
Clariant Produkte (Deutschland) produces and processes, among other things, synthetic and natural waxes at its Gersthofen site in Germany and has long struggled with the transfer of montan wax, also known as lignite wax or OP wax. Conventional pumps were prone to failing within a day and only had a maximum lifespan of two weeks.
Natural montan waxes are mainly used in the care product sector, such as car and shoe polishes, as they improve scuff resistance, increase water repellence and impart high gloss. They are obtained by solvent extraction of certain types of lignite or brown coal, commercially viable deposits of which exist in only a few locations, including Amsdorf, also in Germany.
Its properties make it a superb raw material for polishes and as a lubricant for moulding paper and plastics, however, these same properties present unique challenges for engineers conveying the wax within a process.
Franz Kovanetz, production engineer at Clariant, explained: “Montan wax is recovered from bitumen-containing lignite and processed in several stages to obtain the end product. We have operating conditions here under which some pump designs would fail very quickly!
“Firstly, we need pumps which can convey a foaming, corrosive medium at a pressure of over 4 bar. Centrifugal pumps frequently fail in this task; if the media starts to foam, conventional centrifugal pumps cavitate, causing damaging vibrations. In addition, as we are dealing with molten wax, the pumps must be able to run at temperatures of up to 130°C with heating provided by steam at 145°C. They must also be able to handle varying volumes, from 300-2,000 l/h.
“Finally, in the process stage of oxidation, pumps have to convey on the one hand, the oxidant itself, and on the other hand, the oxidation product still contaminated with oxidant. In this step of the process, legislation demands the use of hermetically tight pumps. In the past, all conventional magnetic drive pumps caused problems in this application. That meant shorter service lives due to sedimentation and damage due to cavitations.”
Pictured below (left to right): The cutaway diagram of an MPB pump permits a view of the narrow impeller; performance curve of the MPB peripheral pump from Richter: a straight line with maximum delivery head at zero conveyance and zero head at maximum conveyance. This curve shows the special feature of a peripheral pump: a centrifugal pump with displacement characteristic.
Peripheral pumps, also known as regenerative or regenerative turbine pumps, have an impeller which has short vanes at the periphery which pass through an annular channel.
These challenging requirements had created severe problems for the plant engineers as Kovanetz continues: “For a long time there was not one single pump manufacturer who could offer a corrosionresistant centrifugal pump with this broad range of flow rates – 2,000 l/h was not the problem, but rather the low flow rate of 300 l/h. Some manufacturers even declined to offer, given the required pressure of over 4 bar. The centrifugal pumps with mechanical seals available 20 years ago had a service life of between one day and two weeks – a dream for every pump manufacturer, but a nightmare for ourselves!”
Kovanetz and his team eventually achieved a service life of around two years using a specially installed mechanical seal to comply with the challenge of statutory demands regarding freedom from leaks.
A crucial breakthrough was only finally made by using Richter Pumps’ MPB range. These peripheral pumps are specially tailored to the delivery of low flow rates at high delivery heads – a range not economically covered by standard centrifugal pumps. The pumps are hermetically sealed and have a magnetic power coupling which equates to no seals and virtually maintenance-free operation. Varying volumes can be coped with efficiently by means of frequency control.
Peripheral pumps, also known as regenerative or regenerative turbine pumps, have an impeller which has short vanes at the periphery which pass through an annular channel. The fluid enters between two impeller vanes and is set into a circular motion – this adds energy to the fluid particles, which travel in a spiral-like path from the inlet to the outlet. Each set of vanes continuously adds energy to the fluid particles.
These pumps are more efficient at these low flow, high head conditions than centrifugal pumps. They also require much less net positive suction head available (NPSHA) than an equivalent centrifugal pump. What is more, they can also handle liquids with up to 20% entrained gases.
The Richter MPB pump conveys volumes between 0.1 and 5 m³/h and achieves delivery heads of up to 100 m. It covers a temperature range from -60–150°C and operates from a vacuum (at standstill) up to 16 bar, depending on the accessories. It is hermetically tight and has a magnetic drive rating of 6 kW at 2,900 rpm or 7.2 kW at 3,500 rpm on request. The MPB pump virtually has universal corrosion resistance and can handle media with a gas content of up to 30% by volume.
From Infernal to Invaluable
The reduced life cycle costs of the peripheral pumps give Clariant two major advantages as Kovanetz explained: “Firstly, we have a considerably longer service life and the maintenance costs are substantially lower. Given the expected service life of four years, we are assuming a cost advantage of several thousand euros for each pump.
“Secondly, and equally important, is that the plant has to cope with fewer disruptions. Every interruption in a continuous process always means affecting product quality. Even if one continuous plant is run constantly, the product quality remains at a high level without fluctuations. Moreover, all pumps are integrated into the process control system – for example, there is also an automatic routine for cleaning the pump.”
The long service life with low maintenance costs is mainly due to the design of the impeller seal which produces minimum friction and minimum wear.
Another crucial advantage is the simple and quick assembly: the resilient sealing lips dispense with the need for adjustment of the impeller ring channel to an accuracy of 0.1 mm. Such close but hydraulically important gaps frequently result in premature wear in conventional peripheral pumps – especially with fluctuating temperatures – and require a large amount of time for their precise assembly and maintenance. However, the sealing lips of the MPB impeller compensate for the dimensional changes caused by thermal action.
This is particularly important for Clariant: both the oxidant and the oxidation product still containing acid have temperatures of well over 100°C. “The Richter peripheral pump offers very good temperature resistance and it easily withstands even very small temperature shock,” says Kovanetz.
Today, the Richter peripheral pumps are now standard at Clariant for conveying highly oxidising substances and other corrosive media. These pumps are proving successful and have the longest service lives – an MPB pump installed for test purposes in 2004 is still running without any problems in 2011.
Reinhold Bimüller is sales manager of Richter Chemie-Technik based in Kempen, Germany.