Water technology: Using valves to provide water for drinking

6 min read

The subject of butterfly valve technology and its suitability for desalination plants raises a number of issues in respect of providing adequate protection against corrosion. Bryan Orchard visited the KSB Amri Water Valve Division Competence Centre in France to learn more about their technologies.

Figure 1: Halar coated Isoria centred disc butterfly valve with Actair pneumatic actuator and Smartronic positioner.
Figure 1: Halar coated Isoria centred disc butterfly valve with Actair pneumatic actuator and Smartronic positioner.
Figure 2: Leak-tightness is obtained by compression of the elastomer between the body and the spherical edge of the disc.
Figure 2: Leak-tightness is obtained by compression of the elastomer between the body and the spherical edge of the disc.
Figure 3: Manually operated Isoria centred disc butterfly valve with Halar coating.
Figure 3: Manually operated Isoria centred disc butterfly valve with Halar coating.
Figure 4: KSB Mammouth valve.
Figure 4: KSB Mammouth valve.

Water has always been a precious commodity, but as global demand increases at an unprecedented rate the pressure is on to realise new resources and technologies. In those parts of the world where water resources have always been in short supply, the emergence of desalination technology and its continuing development has been taken up enthusiastically.

Techniques and technologies are being developed that are now making desalination a more cost efficient and attractive solution to meeting demand for potable water. Not surprisingly, much media attention has been given to those technical developments and new products that are contributing to low operating costs and higher levels of plant efficiency. However, there are other technologies that must not be overlooked and the valve is one such example. Across the entire water industry spectrum, valves are an essential element in the treatment of water and in its supply, but they are not always afforded the attention that they deserve.

KSB Amri is considered to be at the leading edge of centred butterfly valve technology when it comes to supplying the global water industry. With manufacturing plants in France, Spain and China the Company is well positioned to meet the growing demand for valves not only from the desalination industry but also for water treatment and transmission. KSB Amri’s water butterfly valves are available in three different models, the Boax B, Isoria (see Figure 1) and Mammouth and range in size from 40 mm to 4000 mm. All are of the centred disc design.

The international desalination industry is a particularly important industrial sector for KSB Amri and the Company sees SWRO and MED as its core markets, although it has an impressive record in MSF going back over the past 30 years. Currently, KSB is supplying butterfly valves for desalination, water treatment and water transmission projects in Europe, Japan, the Middle East, Australia and South East Asia.

Butterfly valves

The butterfly valve has a long history in the water industry, so it comes as no surprise to find it in widespread use in SWRO, MED and MSF desalination plants. It is a simple yet effective design principle whereby the disc rotates around an axis at right angles to the flow. When open, the fluid passes around both sides of the disc.

Butterfly valves are installed in the low pressure areas of a desalination plant, typically in the early stages of production upstream of the membranes and then at the end on the permeate lines. By keeping the butterfly valves open each valve creates a pressure loss and with the centred disc the differential pressure is much lower than an offset disc valves. Where a high number of valves, say 1,000 units, are installed in a desalination plant the accumulation of pressure saved on each valve will result in energy savings.

 “Centred disc butterfly valves are particularly suited to the requirements of the SWRO and MED desalination processes,” says Pascal Viaud Desalination Leader, “because they are more energy efficient when compared with off-set discs. This is because head losses are minimised due to the elimination of hydraulic turbulence and the hydrodynamic shape of the disc.”

For KSB galvanic corrosion is the all-important consideration when it comes to valve selection. To be wholly effective, valves need to be resistant to the potentially aggressive or corrosive nature of the water being handled and it is a subject that has been at the centre of KSB AMRI’s product development programme for some considerable time. In order to minimise the effects of corrosion, all KSB’s butterfly valves contain a thick rubber liner encapsulated within the valve body and discs are offered in a choice of Halar and hardened rubber surface finishes. It is only the disc and liner that come into contact with the fluid.

The liner is the most important component as it ensures the integrity and tightness of the pipe flange sealing faces, positive sealing of the disc and 100% sealing of the shaft. In the case of KSB’s valves the leak-tightness is obtained by compression of the elastomer between the body and the spherical edge of the disc (see Figure 2). This compression uses the elasticity of the liner in order to avoid damage during the penetration of the disc. To ensure a leak-free design this area needs a special spherical molding of the liner at the upper and lower shaft passages. The disc and the liner are inscribed in a spherical shape and the spherical machining of the disc at the shaft passage automatically centers it on the liner so a perfect contact is obtained between the disc and the liner. Leak tightness at the flange passage is achieved by the compression of the elastomer liner between the valve body and the piping flanges. This overall design means that the liner is mechanically within the valve body.

KSB’s investment in component design and coating surfaces that come into contact with water is enabling it to establish new standards and levels in corrosion resistance. Far from being just a manufacturer of valves, KSB Amri has its own plant for the research, development and manufacturer of rubber compounds. The company has been supplying the water industry with hard rubber coated discs for more than 25 years and these have proved to be equally as successful as bronze aluminium and duplex stainless steel discs and according to KSB cost less. The rubber coating is applied using vulcanisation in an autoclave and this ensures an even coating of 5mm in thickness.

Halar coated discs

However, it is the application of Halar® ECTFE, a melt processable fluoropolymer, to the valve disc that is creating considerable interest within the desalination community as this material offers optimum resistance to chemical attack. The material is well known and proven within the chemical processing industries and was first taken up by KSB Amri over two decades ago. Degremont was the first desalination company to accept KSB’s Halar coated valves and since they were first introduced all subsequent contracts have specified these valves.

The liner is the most important component as it ensures the integrity and tightness of the pipe flange sealing faces, positive sealing of the disc and 100% sealing of the shaft.

“Our discs are manufactured in ductile iron and sent to a specialist coatings application company where we have our own manufacturing standards in place,” continues Pascal Viaud. “The surface of the discs must be very smooth and clean and are specially prepared in order for the Halar powder to adhere to the surface. The Halar coating is applied by fusion processing and this procedure is followed up by each disc being tested by high voltage sensors to check that there are no defects in the coating. The objective is to ensure an even coating of between 600 and 800 microns in thickness across the surfaces of the disc.” The result is a butterfly valve that is more than competent at withstanding the corrosive nature of raw seawater and treated water (see Figure 3).

Although having proved popular with certain desalination plant contractors and operators in Europe, and KSB can point to many reference sites where its Halar coated valves have demonstrated excellent long service life, global acceptance of Halar coated discs has been rather muted. In the Far East KSB has experienced reluctance to change from bronze aluminium, which is perceived there as the industry standard. “For many years Halar was only seen as another option to austenitic SS316 or 316L or Duplex and possibly only suited to the seawater conditions experienced in Europe,” comments Pascal Viaud. “We are now starting to change this perception as we can demonstrate to potential customers that Halar is suitable for all water conditions (see Figure 4).”

Historically, KSB Amri has enjoyed a good working relationship with desalination contractors working in France and Spain. This is now paying considerable benefits for the Company and its products as contractors in these countries are starting to work well beyond their geographical boundaries, securing major international projects both in the northern and southern hemispheres.

With the price of metals such as nickel and chromium increasing substantially in recent years, KSB with its Halar coated discs is in a position to create greater awareness of the technology as it is both cost competitive and equally as robust as accepted alternatives. As a consequence historical and possibly illogical prejudices, together with a reluctance to move away from solid materials to a coating, may have to be abandoned. Certainly the rise in SWRO projects is starting to provide greater opportunities as is the move to BOOT contract operations where long lifetime guarantees are involved.

Another notable development that will also help KSB to maintain its position in this area of valve technology is the introduction of a new Halar coating from Solvay Solexis SpA that is suitable for potable water. This development has come about through a request from the Spanish water industry regulators who wanted all products, even those in desalination plants, to use products that avoid contamination of potable water. It is why today, as a standard, the Halar coating supplied by KSB has FDA approval.

Summary

“Low pressure valves are rarely considered as a critical item,” says Pascal Viaud, “but they can create a lot of trouble if not correctly specified at the outset of a project. We know that there is a gap between the importance of the valves at the purchasing stage compared with the real importance of the valves at the operational stage. Wrong selection of valves can be the cause of up to 40% all operational problems, so the customer must consider the real importance of valves at the specification stage. Even though valves may only constitute 1% of the plant investment costs, it only takes a single valve to create problems and seriously affect plant production.” 

Contact:

Christoph P. Pauly

KSB Aktiengesellschaft

Johann-Klein Strasse 9

D-67227 Frankenthal

Germany

Tel: +49 6233 86-3702

Email: Christoph.pauly@ksb.com

www.ksb.com