Water. desalination + reuse

TECHNOLOGY liner can easily be replaced. On high-quality butterfly valves the shaft has no contact at all with the fluid handled. To ensure the upper and lower shaft passages are uniformly leak-tight across the entire 90-degree disc rotation, the liners are spherically machined at these points (figure 2). Since the valve disc is also specially machined at the shaft passages, it fits perfectly into the spherically shaped liner. The result is a reliably airtight seal restricting any contact with the atmosphere. Tight sealing at the flanged line connections is achieved by compressing the liner between the body and the pipe flanges. A range of elastomers which have been developed for the liner provides optimum chemical resistance to seawater or any corrosive influent or effluent. Ethylene propylene diene monomer rubber (EPDM) is generally employed for seawater desalination applications. The manufacture of high-quality liners requires a great deal of experience with regard to the formulation of the rubber and its further processing stages. In the closed position, the disc seals the valve by pressing its edge into the liner. This compression uses the elasticity of the liner to avoid damage during the penetration of the disc. The disc's penetration also ensures that butterfly valves remain tight even if the differential pressure is relatively high. Particles which have deposited on the disc's edge are stripped off during closure or penetrate into the elastomer liner (figure 3). The liner's taper towards the sealing line helps minimise both flexing and deformation resulting in increased service life and reduced actuating torque. ENErGY-saviNG HYdrOdYNamiC disC sHapE With regard to the direction of the fluid flow, the valve disc moves in a perpendicular fashion - it rotates around its own axis by 90 degrees. The pivot is in the valve seat's centre and therefore also in the pipe's centre. In the opened position, the fluid flows on both sides of the valve disc. Pressure losses are considerably lower than those encountered with other valve designs such as offset-disc or double-offset disc valves. This is a product of the hydrodynamic disc shape in the shut-off butterfly valve. Since a plant will often feature more than 1,000 shut-off butterfly valves, the energy saved is remarkable, and far smaller pumps can be employed. Thanks to their short face-to-face length, shut-off butterfly Figure 2. Cutaway section of a centred-disc butterfly valve. Figure 1. Design and section of centred-disc butterfly valves. | 42 | Desalination & Water Reuse | November-December 2014

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