Page 1 2
Next: Radially split case versus axially split case

- 26 September 2007 -

High pressure pumps for energy-efficient desal

As a favourite tourist destination, Cyprus faces its greatest demands for water when supplies are at their lowest. With energy efficiency central to the operation of the recent 17 million refurbishment of the Dhekelia desalination plant, KSB Aktiengesellschaft has been able to show how its radially split HGM-RO multistage high pressure feed pumps can help. Bryan Orchard reports on a project where the emphasis is on energy efficiency and maintenance costs rather than the overall price of the plant refurbishment.

For many years Cyprus has been a highly popular destination for millions of tourists. During the summer months the resident population of around 800,000 can soar past the two million mark and this at a time when rainfall is almost non-existent it. This places greater pressure on the availability of potable water because over the past three years, reservoirs and ground water resources have been substantially depleted.

Back in 1997, the Cyprus Government made its first commitment to using reverse osmosis desalination as a means of increasing the availability of potable water through the opening of the Dhekelia plant. Built and operated by Caramondani Desalination Plants Ltd under a 10 year BOOT contract the plant had an initial capacity of 20,000 m³/d, but this was soon increased to 40,000 m³/d. Subsequent to the commissioning of Dhekelia, a further plant at Larnaca was built, bringing the Country's total desalination capability up to 92,000 m³/d.

In November 2005, the Cyprus Government decided to call in competitive tenders for the complete refurbishment of the Dhekelia plant, giving it a 20 year lifespan and the potential to produce up to 40,000 m³/d. The tender also laid down conditions that imposed stricter water quality specifications and major improvements to the plant's power consumption. The successful tender came from Caramondani Desalination Plants Ltd and one of the platforms of its bid was the high level of energy savings that it could offer.

The Caramondani design uses a high pressure system comprising two equal streams of pre-treated conditioned seawater. One stream employs three radially split case KSB HGM-RO 8/3 multistage water pumps (two duty and one standby) to pressurise the water to 70 bar before entering the first reverse osmosis (RO) pass in the RO trains (Figure 1). The other stream feeds eight pressure exchangers and eight booster pumps, which bring the pressure up to 70 bar, from where the water joins with the first stream and enters the RO trains. It is this arrangement which contributes to the high degree of energy efficiency levels required by the Cyprian government's contract and early indicators reveal that the pumps are operating above the required energy efficiency level.

In May 2007, the completely refurbished desalination plant was commissioned, the refurbishment programme taking just seven months to complete. Not only did this involve stripping out existing RO racks and pumps, it was necessary to construct new skids for the pressure vessels and piping, install a second pass permeate RO system along with a Boron Removal Systems and building completely new seawater intake and treated seawater pump stations.

Focusing on energy efficiency

For Caramondani, as the plant operator, Osmo Sisterni Srl and Cadagua, the engineering contractors, the over riding decision in designing and equipping the new plant was energy efficiency, followed by the costs of maintenance, rather than the overall costs of the plant refurbishment. The process of equipment selection was jointly taken by the three companies responsible for the design and construction of the new plant. Several major pump manufacturers with RO experience were invited to present quotations, the emphasis being on producing solutions based on energy efficiency.

"From the outset our tender was based on energy efficiency, so we decided to design the system around the use of pressure exchangers, with which we had previous successful experience, and very high efficiency pumps," says Olga Sallangos, Dhekelia plant manager. "Our objective was to reduce energy costs as much as possible over the full operating life of the plant. The decision as to which pump manufacturer we would select for the high pressure pumping duty was taken almost entirely on pump efficiency. Having looked at a number of pump manufacturers with proven experience in RO, it was KSB with its radially split case pump that convinced us that they had the most energy efficient and practical solution."

Under test conditions at their Frankenthal plant in Germany , each of the three pumps achieved an efficiency of 87% pumping water at 20°C. At its best efficiency point the HGM-RO 8/3 can handle 1100 m³/h at a discharge head of 615 m and a pump input power of approximately 2.1MW.

A further influential factor in the choice of the HGM-RO pump was its compact size. Because the design of the new plant had to be confined to the existing premises, all available space had to be optimised. The three KSB pumps and their motors occupy less space than the previous high pressure pumping units, resulting in a freeing-up of a considerable amount of space within in the main production hall.

"Rather than just looking at the original investment costs, our priorities were efficiency, maintenance, spares availability and of course construction materials," adds Sallangos. "The water, when it gets to the RO pump, does contain salt so its material construction must be from very high grade stainless steel with very high PREN (Pitting Resistant Equivalence Number) values. In fact all the pumps up to the membrane stage must be robust, as the water is not typical of the Mediterranean . Dhekelia works on very high water temperatures and salinity values, so it is untypical of the Mediterranean ."

The seawater is taken into the plant through a 1200 mm diameter 600 m offshore pipeline. The seawater has a high saline content of 42TDS (42000 mg/l) which is much higher than other areas in the Mediterranean . This places higher demand on the pumps. Also the water temperature presents a problem because the plant is located close to the power station which discharges cooling water close by. The seawater temperature can be as high as 34°C, so the combination of high salinity and high temperatures require the water intake pumps to be made from a high grade of Stainless Steel. All the pumps from the raw seawater to the membranes have to be manufactured from very high alloys. The horizontal split case seawater intake pumps and filtered seawater pumps are manufactured from Duplex and were supplied by KSB Halle. For these pumps KSB Halle took time selecting the impeller for and diameters in order to keep to the best efficiency point to the operation point, keeping the speed at a low level of<1500 rpm.

Page 1 2
Next: Radially split case versus axially split case

To receive news and features like this direct to your inbox sign-up for the World Pumps enewsletter.

Simply register your details to receive a fortnightly roundup of the latest news from the pump and sealing industries direct to your inbox.

If you would like to advertise in the World Pumps enewsletter please contact our sales team.

If you have some news for the pump industry or would like to comment on any of the articles on this site, contact our editorial team.

You can also access the full list of contacts here.