WET News

May 2014

Water and Effluent Treatment Magazine

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20 WET NEWS MAY 2014 One out of three check valves installed in pump systems is causing major losses due to improper selection, according to KSB The challenge: Keeping check valves in check INSIGHT PUMPS & MIXING TECHNOLOGY Looking inside a SERIE 2000 check valve C heck (or non-return) valves are needed when a pump is stopped and the liquid column acting on the dis- charge nozzle or the pressure generated by other pumps oper- ating in parallel could cause the impeller to reverse direction. Sensitive components like unidirectional mechanical seals could sustain damage as a result. If an appropriate valve is installed, any reversal of flow will close it, thus providing reli- able protection against reverse flow. O•en, however, li• check valves and dual plate check valves are installed without first having examined the pre- vailing flow conditions and resultant energy losses. Frequently, the pump is the major point of focus at the time of installation, while the check valve is selected en passant, so to speak. Depending on the equipment constellation, this can cause serious wear and tear on the valve or even keep the system from achieving good performance. The advent of speed control systems, though, has changed the situation for check valves. While pumps used to be either "on at full load" or "off ", many units are now operating at reduced speed in the part-load range. Volume flows When li• check valves are used, low flow velocities can cause constant "opening and closing", because, unlike dual plate check valves, they are unable to reliably maintain a partly open position. Consequently, they might use up their entire service life in terms of opening / closing cycles within a few weeks. High volume flows can also cause problems due to high flow velocities in the narrow gaps at the disc / seat interface. This can lead to such pro- nounced wear and vibrations as to cause the eventual failure of the valve. Indeed, resonant vibrations might even cause damage to the pump itself. Frequently, the connecting piece between the pump and the check valve is too short. As a rule of thumb, the valve should be situated at least the equivalent of five nominal diameters away from the pump – a length referred to as the flow stabilisation distance. In a study conducted by KSB, more than 30% of all check valves were found to have been incorrectly installed, poorly selected, or undersized. There were even cases in which the installed check valve consumed nearly half of the head generated by the pump. Considering the huge num- ber of check valves now in ser- vice in industry, there is an equally huge potential for cut- ting costs. THE CONCEPT • The maintenance-free valves have one-piece cast bodies • The valve disc opens stably even at very low flow velocities • This type of valve can be installed horizontally or vertically with no special tools required • Both types were therefore tested in diverse situations NEED TO KNOW • More than 30% of all check valves have been incorrectly installed, poorly selected, or undersized, says KSB • Lift check valves and dual plate check valves are installed without first having examined the prevailing flow conditions and resultant energy losses • Lift check valves can use up their entire service life in terms of opening / closing cycles within a few weeks THE VERDICT • The observed opening behaviour remained stable, even with the valve in a partially open position, and there was no chattering like in the case of the lift check valve • Depending on the application smaller pumps could be used • Test set-up for examining the behaviour of check valves in combination with variable speed pumps

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