Plastic-lined magnetic drive pumps: A concept with a future
The better is the enemy of the good: Thanks to design refinements and targeted optimization of individual material properties, plastic-lined magnetic drive pumps have evolved to become a technical alternative to pumps made of expensive special materials and are conquering new application possibilities. This report deals, by way of example, with the world's largest plastic-lined magnetic drive pump which permits the conveyance of highly corrosive media.
Hermetic magnetic drive pumps: Are there limits as regards the lining materials, performance and size? – It can be repeatedly heard that the pump industry is a "mature" sector – which generally means that no more major leaps in development can be expected. In fact, pumps are some of the oldest machines in industrial history. The development was and still is dominated by improvements and innovations in small and very small steps. They were largely aimed at improving the reliability and service life of the machines. Improvements were achieved successively, above all in the areas of production, materials, the hydraulics and design details of the bearing systems and the guidance and sealing of the rotating components. One major step was the introduction of hermetic designs (magnetic drive and canned motor pumps).
The use of other or improved materials resulted again and again in new application possibilities for time-tested pump designs. Taking the example of the world's largest plastic-lined magnetic drive pump, MNK 200-150-315, the following describes how innovations in materials and material know-how gained over many years can help to capture new markets.
When emissions are not wanted
The main engines behind the development of hermetic pump designs were the requirements of nuclear power plants and the ever more stringent environmental provisions (the German Clean Air Code, the Clean Air and Clean Water Acts in the USA).
Emission-free pumps are now mandatory for many applications in the chemical industry. The operator can only choose between complicated mechanical seal systems, canned motor and magnetic drive pumps in these cases. The general consensus is that complex mechanical seal systems are quite complicated to operate, the canned motor pump is only compulsory and advisable for special cases and magnetic drive pumps cover the widest spectrum of requirements.
Pumps are some of the oldest machines in industrial history. Their development is mainly marked by improvements and innovations in terms of reliability and service life. For example, in the areas of production, materials, design details of the bearing systems, the guidance and sealing of rotating components.
For many operators magnetic drive pumps are the system of choice for technical considerations and from the aspect of the cost of ownership (total costs over the service life). Rising safety standards and the need to reduce maintenance costs contribute to this just as much as the high reliability of these pumps. This applies in particular to plastic-lined magnetic drive centrifugal pumps. Their main field of application is conveying corrosive chemicals and other critical media in the fine chemical and pharmaceutical industries. The rivalry between the "optimum" materials is solved here in a quite pragmatic fashion: Such pumps offer a combination of he best materials – plastic as a wetted lining material, metal as a support material, ceramic for the plain bearing etc. Thanks to the combination of the favorable properties which could not be obtained with any one of the materials on its own, plastic-lined pumps are generally less expensive than "pure" plastic pumps or "pure" metal pumps, especially when they have to be made of very expensive special metals and, in addition, are to be more reliable than "pure" plastic pumps.
Plastic-lined magnetic drive centrifugal pumps, specially designed for conveying highly corrosive media, have practical advantages, for example robustness and reliability. All wetted components either have a thick perfluoroalkoxy fluoroplastic shell, are made of solid polytetrafluoroethylene fluoroplastic or are made of sintered silicon carbide.
MNK: Pumps to convey highly corrosive media
Back in 1983 the first plastic-lined magnetic drive pump was launched with the type MCK: It developed into the MNK series which is specially designed to convey highly corrosive media (Fig. 1). Robustness, reliability and a high degree of interchangeability of components among the different sizes are some of the practical benefits of this series. All the wetted components either have a thick shell of perfluoroalkoxy (PFA) fluoroplastic (housing, plain bearing pedestal, impeller, shaft and inner magnet assembly), are made of solid polytetrafluoroethylene (PTFE) fluoroplastic (can insert) or of sintered silicon carbide (SSiC) (plain bearings).
Freedom from leakage is guaranteed by the double can system between the inner and outer magnet assemblies in that the fluoroplastic can insert is highly corrosion-resistant and the outer can made of carbon-fiber composite bears the pressure loads. This principle has major advantages:
- The can system does not absorb any additional bearing forces, it merely performs a pressure vessel function with a high safety margin.
- The can system is not brittle or sensitive to rupture (like ceramic designs for example).
- The can offers double safety as the outer carbon-fiber can is also very corrosion-resistant.
- No performance losses arise as a result of eddy currents as are known from metallic can designs; this also prevents the introduction of heat into the medium.
The problem of eddy currents is often a key criterion for the use of plastic pumps as these may account for more than one tenth of the power consumption in metallic magnetic drive pumps.
Owing to its high reliability, low life-cycle costs and low maintenance compared with mechanical seal pumps, the MNK was quickly accepted by the market. Initially in the processing chemical industry (fine chemicals) but later also in the pharmaceutical and basic chemical industries, FDA applications, and ultrapure media (semiconductor industry). The MNK finally developed into the alternative to high-alloy metallic magnetic drive pumps – especially as it does not create any eddy current losses and in view of economic factors. The applications sometimes resulted in product refinements and special series (Fig. 2):
- Peripheral pumps MPB convey smaller volumes to great delivery heads. These pumps offer accurate adjustment of the impeller/ring channel.
- Vortex pumps MNK-X with an extended housing chamber and special impeller geometry are predestined for deployment when, for example, very large or long-fiber particles or high solids contents also have to be conveyed or the medium has a gas content higher than 5% by vol.
- Self-priming pumps MNK-S prove successful in the overhead emptying of chemical vessels and sumps, also at elevated back pressures.
Corrosion-resistant linings for pumps in the chemical industry
What experience was gained by pump designers together with operators as regards the plastics available over the years (see Fig. 3, table 1)?
Polypropylene (PP) and polyethylene (PE) can be considered as pump lining materials for fairly low thermal and chemical resistances. PE is considerably more resistant to abrasion than PP. Polyvinylidenfluoride (PVDF) has relatively good chemical resistance. Polytetrafluoroethylene (PTFE) and perfluoroalkoxy (PFA) exhibit virtually universal chemical resistance. These materials can also be made to be electrically conductive with additives and are FDA-compliant.
Linings made of fluoroplastics have been used since about 1970 as alternatives to high-alloy metals for pumps, valves, control valves, vessels etc. PFA generally exhibits significantly lower permeation rates than PTFE owing to its microstructure given the same wall thicknesses (Fig. 4) but has the same chemical and thermal resistance. Even the good barrier effect of pure PFA is occasionally insufficient to achieve satisfactory service lives with particularly highly permeating media such as chlorine or bromine compounds. With the compound PFA-P (the 'P' stands for 'permeation'), a PFA variant which can be thermoplastically processed is available with a much higher resistance to permeation but with the same chemical and temperature resistance. Long-time tests in the laboratory and in the field show: Permeation through the compound PFA-P is reduced by at least a half over all operating ranges.
Wear-resistant engineering ceramics
As a so-called engineering ceramic, silicon carbide (SiC) is an important material for plain bearings of pumps in view of its hardness and temperature resistance. Shaft sleeves made of SiC serve, as the name implies, to protect rotating shafts against chemical attack. Sintered silicon carbide (SSiC) exhibits extremely high hardness and comprehensive chemical resistance. Here, too, the saying applies: The better is the enemy of the good: No dry-running can be accepted with magnetic drive pumps with conventional, uncoated SSiC plain bearings. Even a check of the direction of rotation without the medium may result in a sudden rise in the plain bearing temperature to several hundred °C and to subsequent damage in the pump. SAFEGLIDE®Plus bearings are considerably more stable: they have, in addition, an amorphous carbon layer just a few microns thick which takes on a diamond-crystal-like microstructure. The properties of the coating:
- amorphous, isotropic, good adherence
- high hardness (HV0.05 = 4000 to 6000)
- good elasticity
- high wear resistance
- optimum surface quality
- good thermal conductivity
- thermal resistance up to 300°C
- universal chemical resistance, cannot be destroyed wet chemically
SAFEGLIDE®Plus reduces the coefficient of friction of the plain bearing and, as a result, the plain bearing friction and thus the temperature rise are minimized. Thanks to this effect, pump damage due to brief dry-running can be avoided.
High-performance materials for the double can system
Magnetic drive pumps from Richter exhibit a metal-free double can system (the plastic can insert made of plastic offers corrosion resistance, the outer can made of CFRP absorbs the pressure load). A carbon-fiber reinforced plastic (CFRP is the term for a fiber plastic composite in which carbon fibers are embedded into a matrix of plastic artificial resin for reinforcement. The CFRP cans used in these pumps (Fig. 5) are characterized by the fact that the rotating magnets do not induce any eddy currents in the metal-free can and therefore do not produce any heat. As a result, no drive energy is uselessly wasted by introducing heat into the medium. The drive power can be increased by approx. 15 to 20% compared with metallic cans. In this way, more than € 1,000 of energy costs can already be saved per year with, for example, a size 50-32-160 in 3-shift operations. A further advantage is that media with temperatures close to the evaporation point can also be safely conveyed.
The outer magnet assembly rotates about the double can, which represents the barrier between the medium and the surrounding area, the inner magnetic assembly rotates within the can. Permanent magnets with maximum energy densities are available as materials (Fig. 6, Fig. 7), such as neodymium-cobalt magnets (NdFeB) and samarium-cobalt magnets (SmCo). SmCo magnets belong to the group of rare earths. They offer a high energy density and, at the same time, a very high coercive field strength Hc. – Hc is a measure of how well the magnet retains its magnetism. This permits miniaturization of the design of magnet systems as well as their use at high temperatures of up to +300°C and in strong opposing magnetic fields. NdFeB thanks its outstanding properties to the combination of the rare-earth element neodymium with iron. Nowadays, NdFeB magnets are used everywhere where strong magnetic fields are needed with a small volume.
The latest development: Magnetic drive pumps for up to 600 m³/h
The many years of experience and know-how with MNK pumps have led to the development of the world's largest plastic-lined magnetic drive centrifugal pump: the MNK 200-150-315 (Figs 9 and 10). It is designed as a single-stage, sealless, chemical centrifugal pump of heavy-duty, horizontal design according to EN 22858/ISO 2858/ISO 5199 (Fig. 8). The pump is therefore qualified for use in a number of processes, e.g. for chlorine electrolysis (anolyte and catholyte sector, precipitation and clear brine, bleaching solution), in the production and treatment of sulfuric acid and is equally suitable for so-called world-scale plants and major multi-purpose facilities. The design of the pump was optimized in line with the experience gained and adapted to suit the special needs of this pump size.
The MNK 200-150-315 can be used for flow rates Q of up to 600 m³/h at delivery heads H of up to 60 m LC and is rated for operating pressures of up to PN 25. Depending on the flow rate required, it can be operated at speeds of between 750 and 2000 rpm. The operating temperatures are between -60 and +150°C. The closed PFA impeller with flow-optimized vane channels contributes to the high efficiency and low NPSH value (Net Positive Suction Head). The PTFE housing lining (optionally: PE-UHMW) has a wall thickness d of between 12 and 20 mm. The full-surface shell made of ductile cast iron EN-JS 1049 bears the system pressure. A housing drain is a standard feature, a heating jacket is optional.
The pumps have this double can system
- on the wetted side: modified TFM-PTFE, 4 mm thick, considerably more resistant to permeation than standard PTFE
- on the pressure-bearing side: carbon-fiber reinforced plastic CFRP (eddy-current-free, pressure-resistant, metal-free, fracture-proof, with high safety reserves).
The particularly sturdy plain bearing system is made of pure silicon carbide.
"SAFEGLIDE®Plus" prevents damage in the event of brief dry-running. The high-performance permanent magnets are rated for torques M of up to 800 Nm and transmit 120 kW at 1450 rpm and 165 kW at 2000 rpm respectively. With this performance the MNK 200-150-315 is an alternative for users in the basic chemical industry to the mechanical seal pumps or metallic magnetic drive pumps made of special metals predominantly used today. It is interesting to users when, in view of a highly corrosive medium, only pumps made of Hastelloy, titanium, tantalum, nickel, model, Alloy20 or ceramic materials were previously considered as such designs are cost-intensive.
Plastic-lined magnetic drive centrifugal pumps are an alternative for users in the basic chemical industry to mechanical seal or metallic magnetic drive pumps made of special metals. This is the case when, in view of a highly corrosive medium, only pumps made of Hastelloy, titanium, tantalum, nickel, model, Alloy20 or ceramic materials were previously considered.
Summary and outlook
When highly corrosive media have to be conveyed at a flow rate of over 300 m³/h, only plastic pumps with a mechanical seal were previously considered or – if a leakage-free version is required – only metal pumps made of special metals or ceramics. However, these materials are very expensive and procurement is extremely tedious. The world's largest plastic-lined magnetic drive pump MNK 200-150-315 doubles the flow rate previously possible up to 600 m3/h and, as a result, flow rates, such as for example are required for applications in world-scale plants in the basic chemical industry, are also available.
To answer the question at the outset about the performance and size limits of plastic-lined magnetic drive pumps: A combination of experience, the use of advanced materials and innovative solutions will also be the prevailing yardstick for developments in the future. In this way progress is also possible in a market which is said to be "mature".