Tough stuff

When it comes to moving fluids, the centrifugal pump is the most commonly specified and by far outstrips all other types of pumps in terms of capabilities for handling the widest range of applications.

Taking on many different forms and configurations, its fundamental characteristic is that it takes in fluid and pushes it through the pump casing in a circular manner by the use of a rotating impeller.

Where centrifugal pump types and designs differ is in their ability to handle many different types of media, ranging from raw water to challenging fluids containing a high solids content, having high levels of viscosity, are corrosive or have poor levels of lubricity.

Where challenging fluids are encountered, it is necessary to consult the experts. While virtually every pump manufacturer will now produce their own guide to selecting the optimum model from their range for moving a particular medium, there are few basic guides that advise what kind of pump, never mind what make or what model, would best be suited for a particular application.

This is where the pump distributor steps in. Tomlinson Hall & Co. business development director Colin Simpson says there is a huge range of criteria to be considered when advising a client on the pumping of fluids that may be hazardous, high temperature, high solid content, corrosive or highly aerated.

“There’s no magic panacea to pump selection – it comes down to product knowledge and experience but generally the principles involved for challenging fluids are exactly the same as for any other medium,” says Simpson.

“These break down into four main groups: full research into type, viscosity of medium, COSH and data sheets if applicable, is the medium corrosive, what solid content is involved, is it aerated and what temperature has to be maintained; surrounding environment and system details; pressure and flow requirements; and maintenance and cost indicators.”

Amarinth business development director Alex Brigginshaw says that it is often easy to identify early in the engagement process a manufacturer or supplier who is interested in a customer’s overall challenge and willing to work with the customer in partnership to develop a complete solution, rather than simply supplying a pump.

“Our advice is to work with the supplier, share as much details relating to the application and process to allow the pump supplier to really understand all the process, piping and application arrangements,” says Brigginshaw.

“It is better to be seeking out suppliers with previous experience and the engineering capabilities to tackle bespoke designs and applications for difficult processes.”

By way of example, Brigginshaw highlights sugar as a notoriously difficult product to handle. At British Sugar’s factory at Newark, Nottinghamshire, Amarinth centrifugal C-Series chemical process bare shaft pumps have recently been installed on the sugar beet production line.

During the extraction of sugar from the beet, the thin juice sugary water produced during purification is pumped through a falling film juice evaporation process.

This boils the water off to produce syrup which then passes on to the crystallisation process. The Amarinth C-Series pump was supplied with a purpose-designed stainless steel impeller to move this sticky syrup and an AES CDSA mechanical seal was added to contain the fluid.

In stark contrast to this application is the application of Wernert NE pumps for a wet scrubber process in flue gas desulphurisation (FGD) plants at one of the UK’s largest coal-fired power stations.

Supplied by AxFlow, the Wernert pumps are subjected to a very high work rate, typically handling aggressive, corrosive and erosive slurries.

The wet scrubber process uses wet limestone slurry, through which flue gas containing SO2 is passed in absorber spray towers.

A total of 40 Wernert NE pumps of varying sizes and delivery outputs pump the limestone slurry into the four FGD plants, together with the removal of the gypsum slurry and the back-end wastewater removal duties.

Certified to ATEX 100a, the Wernert NE pumps have robust thickwalled metal casings and parts made from corrosion and abrasion resistant high performance plastics.

These ensure that each pump is safe for pressure rating to 16bar. For the most aggressive duties, the pumps are fitted with highly abrasion and corrosion resistant liners.

The Wernert pumps have been designed to provide extremely high levels of reliability and plant availability on such duties and include simple but extremely robust Wernert single mechanical seals, having silicon carbide faces which are lubricated and cooled by the pumped fluid, without the need for external flushing.

Meeting ever more challenging pumping duties means that pump manufacturers are constantly developing and introducing new materials to counter difficult fluids.

ARBO has recently introduced a range of thermoplastic mechanically sealed centrifugal pumps available in a choice of polypropylene, polyethylene, PVDF and PTFE in order to resist the effects of aggressive chemicals which can cause corrosion problems for metallic pumps.

Introduced in the UK by Michael Smith Engineers, these compact, yet robust pumps are designed to handle flows up to 500m³ /hr, heads to 90 metres and are available in close, or long coupled designs.

Manufacturing all wetted parts out of a solid block without the need for injection moulding eliminates the potential for stress-cracking.

The combination of tough, non-metallic materials and stress relieving production methods also provides outstanding chemical and temperature resistance to ensure safe and reliable pumping, even in the most challenging applications.

Typical applications for the new ARBO centrifugal pumps include pumping acids in anodising plants, circulating aggressive chemicals in pickling lines, tanker unloading of ferric salts, pumping chemicals and untreated effluent in electronics manufacturing, demineralised water circulation, and any application where metallic pumps will either suffer from corrosion or can cause contamination.

Leak free

Pump leakage is to be avoided at all costs, but there are applications where even the slightest degree of leakage cannot be tolerated.

This is where hermetically sealed and canned motor pumps come to the fore. The double containment design, free of shaft sealing, makes the canned motor pump 100% leak free.

Even in the unlikely event of a stator liner rupturing there is still no leakage outside of the pump.

Without there being any shaft penetration, torque is transmitted electro-magnetically by a stationary motor stator.

Both parts are inherently separated by a thin stator liner as the sealing element, this typically being manufactured from a Hastelloy material for minimum electrical losses during operation.

Simpson points to the challenge of pumping molten wax as a production engineer’s nightmare that calls for the use of hermetically sealed pumps.

Citing one application involving transferring molten wax at 130°C in a polish production process, Simpson says the requirement was for maintenance free operation and hermetically sealed pumps.

The solution involved installation of Richter MPB peripheral pumps (Fig.4) which were specially tailored to the delivery of low flow rates at high delivery heads.

The pumps are hermetically sealed with magnetic power couplings offering a virtually maintenance-free operation.

Another nightmarish challenge for engineers is the rather delicate matter of pumping CO².

This highly versatile material is 1.5 times heavier than air, is almost odour-free and potentially life-threatening.

The danger of gas leakage to the atmosphere, the low viscosity of CO² in liquid form and the requirement for it to be pumped at high pressures due to its weight means that the pumps used must provide total containment.

Speciality pumps

Because CO² in a liquid form is a low viscosity product, conventional centrifugal pumps have to be ruled out as leakage can occur across the faces of the mechanical seal resulting in gas escaping into the atmosphere.

Ice can also form on the pump components, causing the mechanical seals to freeze, resulting in pump failure.

For these reasons the option should be to select a seal-less or canned pump. Even when completely encased in ice, the canned pump will continue to operate.

Hermetic Pumps specialises in the design and manufacture of hermetically sealed centrifugal pumps for difficult pumping applications and offers three pumps specifically for CO² duties, all with nominal pressures up to 40 bar.

The CNF, CAM and CAMR models are of mono-bloc design (pump and motor share a common shaft) and as such are quieter, more compact and easier to install than conventional pumps.

They do not require a base plate, couple or coupling guard and there is no need for alignment of motor and pump to ensure totally leak-free operation.

These examples of centrifugal pump types for challenging fluids and examples of applications represent just a snapshot of what is available to the process industries.

As there are so many options from which to choose, it makes sense to talk to those manufacturers and suppliers that have proven experience in providing effective solutions.

“Sometimes the customer can merely look at the perceived needs and requirements from the pump and omit key application elements of the wider arrangement,” says Brigginshaw.

“The more information supplied to an experienced manufacturer, the greater opportunity they have to consider previously proven designs or new solution options. Year after year market demands and customer expectations become more challenging, demands increase, solutions often become more technical and market influencers change. We try to respond to this by keeping abreast of the individual customer influencers and try to determine what is important to the customer early in the engagement.”