Choosing the best pump for handling corrosives

Pumps are a crucial "component" in most process systems and as a consequence of these tougher demands, pump design and operation is increasingly a focus for development and innovation.

The combination of stricter environmental requirements, increasing explosion safety regulations (ATEX), stringent emission reduction targets and specific operational requirements of end users inevitably demands measures to be taken to prevent leakage of corrosive fluids and other hazardous media.

Pumps are a crucial "component" in most process systems and as a consequence of these tougher demands, pump design and operation is increasingly a focus for development and innovation.

For example, particular emphasis has been on eliminating seals, an inherent weakness in certain pump types and clearly a factor when considering the areas of potential risk leakage.

Developments in seal technology, along with innovations such as the inclusion of a double mechanical seals, combined with safety and signaling devices are some of the measures designed to improve safety and reliability.

However, in many cases, these measures do not meet the increasingly tougher requirements, or they are too complicated or impractical.

For these reasons, seal-less pumps, specifically magnetically driven pumps, have become a preferred option because they offer the fundamental design advantage of removing the shaft passage from pump to atmospheric area.

In particular, these hermetically sealed pumps guarantee a completely closed pumping system.

This ensures a more reliable production process by eliminating a major weakness because the process fluid is safely contained inside the system.

The main choices, as far as seal less centrifugal pumps are concerned, are magnetically coupled pumps or canned motor pumps.

A good example is the CombiMag range of magnetically driven centrifugal pumps designed and manufactured by Johnson Pumps.

The principle of the CombiMag pump is based upon a Hastelloy C or stainless steel separation can, which separates the pumped liquid entirely from the environment ensuring that the pump is 100% leakproof.

The power from the driven pump shaft is transmitted to the impeller shaft by means of strong samarium cobalt magnets.

The pump shaft is provided with an outer rotor and this rotor is internally covered with magnets.

When the driven shaft rotates the opposite magnet poles on inner and outer rotor attract each other which causes the inner rotor to move, without slip.

The slide bearings of the inner rotor are lubricated by the liquid in the pump.

Also, to meet the strictest requirements the radial and thrust plain bearings are made of highly abrasion resistant silicone carbide (SiC), which guarantees an extended bearing life time.

This material has a universal chemical resistance, a limited heat generation and is insensitive to temperature changes.

The standard bearings design allows maximum working temperatures up to 300C and they are capable of handling significant radial loads.

These bearings are axially balanced by the impeller back vanes and lubricated by the pumped fluid.

With canned motor pumps the rotor of the motor and the pump shaft are integrated with one of the simplest examples being the circulation pump found in domestic central heating systems.

The rotor and stator of the motor are separated from each other by a can that also hermetically seals off the pump part.

Part of the liquid to be conveyed passes the rotor bearings and these bearings have to comply with the same requirements as those of magnetically coupled pumps.

Although magnetic drive pumps and canned motor pumps are recognised as being particularly effective in handling hazardous and corrosive fluids, like any other type of pump, they do have disadvantages.

For example, although they are a good solution for transferring corrosive, toxic, chemically expensive or vulnerable liquids they may not suit applications involving viscous liquids, or those containing particulates or solids.

Also, because of their design they provide a less efficient performance, resulting in higher energy consumption compared to other types of pump.

Another disadvantage of magnetically driven pumps is the potential for "flexible" coupling or bearing frame misalignment by virtue of their shaft/coupling design.

However, the introduction of close-coupled magnetically driven pumps eliminates this problem.

A good example is the recently introduced CombiMagBloc range of close-coupled pumps from Johnson Pumps.

These pumps incorporate a self-centering close coupling feature which enables the outer rotor to be directly mounted onto the motor shaft and this feature effectively eliminates any potential for flexible coupling or misalignment.

The CombiMagBloc is designed according to a strongly implemented modular construction method, where most parts are interchangeable with parts from other pumps in the Combi range.

This interchangeability is characterised by its compact build.

A standard IEC electric motor (model IM3001 (B5) to 112M, model IM2001 (B3/B5) for larger types) is fitted to the pump by means of a lantern piece, while the intermediate cover is fitted directly to the pump casing.

In construction terms the most important parts of the pump are the casing and the impeller and for every pump type the casing and impeller in the various material types, are structurally similar and interchangeable.

A replacement casing wear ring is fitted in the pump casing at the location of the impeller inlet.

The rear side of the impeller is fitted with back vanes which provide a partial "balancing" of the axial forces acting on the impeller.

At the same time, the back vanes support the circulation of liquid through the slide bearings.

An important feature of CombiMag pumps is their back pull out construction which facilitates simple drive end maintenance.

This means the pump does not need to be drained and the process system remains pressurized.

The intermediate cover acts as the connecting piece between the pump section and the magnetic coupling, while the stationary part of the slide bearings and the containment can are both fitted to the intermediate cover.

The intermediate cover is connected to the pump casing as a separate element and is provided with apertures to ensure that the pumped medium can circulate around the magnets of the inner rotor and the slide bearings.

This circulation is maintained by the pressure difference between the external circumference of the impeller and the impeller hub.

The intermediate cover is also provided with a connection to fit a temperature sensor to the containment can, while the bottom of the intermediate cover is fitted with a connection to fit a pressure gauge, but which can also serve as a drain for the lantern piece.

The maximum torque which can be transferred by the magnetic coupling is 168Nm which is comparable to a force of 45kW at a speed of 3000rev/min.

The CombiMagBloc range includes three magnetic coupling sizes of MAG 75, 110 and MAG 135, the selection of which depends upon the torque that needs to be transferred.

Each coupling size can transfer a number of different torques by varying the magnet length in steps of 20mm.

The magnets of the inner rotor are encapsulated by a thin stainless steel jacket which prevents exposure to the liquid.

During operation all axial and radial forces generated by the impeller are absorbed by the liquid lubricated bearings.

These bearings contain grooves which ensure optimum lubrication and cooling.

However, to ensure a constant lubrication and cooling of the bearing, solid, non- abrasive particles may not be larger than 0.25mm (the groove cross-section).

The slide bearings are shrunk fit into a stainless steel holder and fitted to a silicon carbide shaft sleeve, which is centered in the axial bearing construction.

Finally, the containment can which separates the liquid to be pumped from the atmosphere is a deep drawn metal can supplied in either stainless steel or Hastelloy and is designed for system pressures up to 2500kPa (25bar).

The wall thickness is such that torque loss, caused by the occurring, is minimal.

Clearly, seal-less pumps and in particular magnetically driven pumps offer important advantages where applications involve the need to transfer corrosive, toxic or vulnerable liquids.

A pump design which eliminates the need for seals and offers a closed pumping system to ensure containment of dangerous liquids will benefit users who need to meet tough regulations and strict environmental controls.

The range of CombiMagBloc offers these benefits, offering a capacity of up to 280m3/h, maximum heads to 140m and maximum system pressure to 16bar.

They are designed to operate in a temperature range of -50 to +200C and can handle liquids with a viscosity of 0,3mPas to 150 mPas, slurries with a maximum weight of 5%, maximum size of 0,25mm while solids with a maximum diameter of 0.1mm and a hardness of 700 HV are suitable.

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