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Hard casing – robust core

Plastic centrifugal pumps for the chemical industry
Hard casing – robust core

After years of intensive discussion and input on the costs that arise during the lifecycle of a pump, managers from all business areas have become more aware of the subject. The central issue here is the optimisation of energy consumption and maintenance work. Basically, however, cost optimisation begins with the selection of an appropriate pump. Crucial factors are the right size, a suitable material and an adequate sealing concept.

Frank Pauly

Although there is an increased awareness of the cost issue, the pump selection is exactly where potential cost reductions are often ignored. This article provides an informative insight into why an objective evaluation of the pump material and the sealing concept are so important and how plastic centrifugal pumps with plastic linings help to keep industrial processes running efficiently and economically.
Centrifugal pumps with PFA linings have long been established for demanding chemical applications while pumps made from other materials, such as PP, PE or PVDF are often still branded with the negative image of a “plastic pump” even though the pumps are mostly manufactured of solid material and are in no way inferior to their lined counterparts. In fact, these pumps offer a great deal of advantages and often substantial savings. Nor-mally designed as standardised chemical pumps, the polypropylene (PP) and polyethylene (PE) centrifugal pumps are used at moderate temperatures of up to 100 °C for transporting, for example, inorganic acids and acidic effluent. Compared to nickel-based metal pumps they offer a high degree of flexibility for fluctuating acid lev-els and temperatures.
For a long time plastic pumps only played a minor role in the chemical industry, and were only to be found in marginal areas, such as for waste water treatment. They only really became interesting for applications such as the demanding transport requirements in chlorine production when used in combination with fluorine plastics like PVDF, PFA, PTFE and ETFE. The permitted temperature range and the chemical resistance to organic and inorganic fluids mostly rise with increasing fluorination. PE1000, PVDF or PTFE pumps are often selected in preference to titanium, titanium-palladium or nickel pumps when fluids that contain chlorine are to be transported at temperatures of up to 190 °C. In this way, companies not only make considerable savings on the initial investment, but it is also far easier to obtain spare parts.
Wear-resistant plastics
When comparing plastic pumps to conventional metal pumps, it is mainly the dura-bility of plastics which is often totally underestimated. Plastics particularly come into their own where there are solid impurities in aggressive solutions, while metal alloys often only provide a balance between premature wear and corrosion. In contrast, polyolefin plastics, especially highly cross-linked polyethylene (also known as RCH1000, PE1000 or PE-UHMW) are exceptionally resilient when it comes to wear, and in many cases can bear comparison with cast chromium or rubber linings.
Although, at first the plastic pumps produced by a lot of the manufacturers were easily recognisable as plastic, these days it is usually only noticeable after a second inspection as the centimetre thick plastic is normally completely encased by a metal covering. This covering prevents pipe pressures from deforming the pump unit and ensures that the material is not dam-aged by UV rays. The high safety requirements set out by the chemical industry are all met by these pumps and are in fact sometimes more reliable than some other materials.
Friatec extended the range of pump sizes to meet the increasing industry requirements and now caters for flow rates of up to 2800 m³/h. The growing demand and relatively straightforward work processes required for plastic parts have also led to the development of new designs, in particular vortex and self-priming pumps. Plastic pumps with free-flow hydraulics are capable of transporting aggressive fluids with a solid content of up to 50 % and have no difficulties with media containing gas or low inflow rates.
Seal selection
In addition to the importance of the material of the pump, the selection of a suitable and reliable shaft seal is essential for a long, economical lifecycle. Needless to say, there is no universal solution which can meet all industrial requirements equally well. Du-ring operation or once the pump has been switched off, solid particles collect on the sensitive surfaces of the slide ring seals and form deposits that can very quickly lead to premature wear or seal failure. It is impor-tant to keep solid matter away from these areas during operation and plastic pumps offer excellent design possibilities to ensure this. The seal area on the product side can be modified so that particles are centrifugally catapulted out of the critical area. The careful positioning of flushing systems ensures that even the toughest dirt can either be flushed out with clean fluid during operation or after the pump has been switched off, thus extending the service life of slide ring seals even when faced with the toughest of fluids.
Fuelled by environmental regulations and technological advances, the development of magnetic drive pumps has advanced considerably and these pumps are now available in practically all materials and sizes. General statements that magnetic drive pumps are only suitable for pure media have been proved to be false. Whether a magnetic drive pump is the right solution for contaminated, corrosive or toxic media largely depends on the type and properties of the contamination. Many products can handle a solid content of 5 %. Nearly all types of pumps are governed by design restrictions regarding particle size and abrasion that are dependent on the design. In this case, external flushing systems for the sensitive slide bearing of the magnetic drive are one way of greatly increasing the service life of a pump.
To ensure economic efficiency, it is recommended to compare all the different sealing systems, as generalisations lead to ineffi-ciency. Depending on the requirements determined from the properties of the fluid to be transported and previously gained experience, systems, for example, with slide ring seals and drives of more than 30 kW can provide considerable investment and maintenance savings. When considering lifecycle costs for most applications in which non-critical fluids are to be pumped the most important factor is rightly seen as energy efficiency.
In niche applications, however, in which corrosion, wear, toxicity and occasional high temperatures occur together, wear and subsequent damage must be taken into account, so the pump material and the selection of the seal are of the utmost impor-tance.
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