A positive impact

UK interest in desalination is on the rise. Brian Hubbard of Cat Pumps discusses the benefits that small positive-displacement pumps can bring to the process

Over the past two years, there has been more interest in desalination in the UK, fuelled by debates about how climate change is affecting rainfall, and the pressure on water resources from increasing housing.

While desalination is little used here, there are small systems operating in remote areas and on small islands where demand for fresh water outstrips the existing supply infrastructure. On the Isles of Scilly, the council runs two positive-displacement reciprocating pumps, made by Cat Pumps, in small-scale desalination plants, which produce over 25% of the islands' drinking water.

Energy is a big issue for islands and small leisure complexes because local electricity generation is expensive. So it makes sense to use the most efficient pump available.

Seawater desalination has been long associated with high capital and operating costs. However, as pump and membrane manufacturers have advanced their technologies in the field of reverse osmosis (RO) desalination, this situation is changing at an unprecedented rate. Membrane prices have fallen substantially and pump manufacturers have turned their attention to developing energy recovery systems and technologies that optimise pump efficiency.

Getting the pressure right
The employment of positive-displacement pumps, as opposed to multi-stage centrifugal pumps in smaller, land-based RO desalination plants, is increasing. This is because positive-displacement pumps can provide the necessary pressure levels to make small-scale fresh water production energy-efficient. If the pump is on any kind of vessel or offshore platform, the amount of energy required to drive it, as a proportion of the total energy being used, is very small. The use of the Cat pump to feed raw water into membrane modules has secured Cat Pumps a strong market presence in RO desalination over the past 25 years.

The nature of seawater demands pumps that are made from corrosion-resistant materials which offer long leak-free performance. Cat's early pumps were manufactured in nickel aluminium bronze, known for its resistance to seawater.

However, as RO customers demanded higher levels of reliability, the company introduced 316 stainless steel for seawater contact parts. And most recently it has offered Duplex and Super Duplex alloys for their superior resistance to pitting corrosion of high-salinity seawater. The benefits of positive displacement reciprocating pumps to the manufacturer of an RO system lie in their very high energy and volumetric efficiency, whereby the pump output is highly predictable and easily controlled. The flow from a triplex positive displacement pump is almost linear to the speed of the rotation and is totally independent of the output pressure.

To minimise life-cycle costs in desalination duties, Cat pumps are typically run at around half their maximum speed because the application is essentially continuous and the pumped fluid is highly corrosive. However, even when under-running in this way, the pump returns an overall efficiency of 80-85%.

At the small end of the RO market, it is almost impossible to procure a multi-stage centrifugal pump with an efficiency anywhere close to that of a true positive-displacement pump, especially across a range of operating flows. With a positive-displacement pump, efficiency is essentially constant.

The smallest pump that Cat sells in the desalination market is the 3Frame pump. And each frame size has the option of different pump heads. For desalination systems, it is most economical to specify pump heads with the largest available plungers to achieve the maximum flow from a given frame size. And even the lowest-pressure Cat pump can deliver 70 bar, more than sufficient for most RO desalination duties.

Growing opportunities
Growth in interest in desalination systems in the UK is opening up new opportunities for Cat Pumps, particularly where applications require pressures ranging between 40 and 75 bar. This enables the operator to attain high yields, even at high water-salinity levels and across a range of flows from 5 l/min to 500 l/min.

Most importantly, Cat Pumps offers designers the chance to create desalination systems comprising a number of pumps that can be brought on stream or taken out of service when demand for fresh water changes. This is more economical than having one large plant operating at half capacity. Cat Pumps sees growing opportunities for positive-displacement pumps in the UK, especially in more remote areas producing drinking water for local needs; the development of sites in their early start-up stages; and in the commissioning and testing of modules for much larger plants that will eventually use high-pressure multi-stage pumps.

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