Water & Wastewater Treatment

38 | OCTOBER 2014 | WWT | www.wwtonline.co.uk In the know Getting to grips with pumps inlet and outlet losses in the suction pipework of the pumping system. During design it is therefore neces- sary to calculate all the friction losses, inlet and outlet losses and process unit losses in the suction pipework and then deduct those losses from the suction head available to the pump. Therefore, at the point where the pump is installed, we are le with a net pres- sure remaining and available for the pump. There is no alternative to getting pump system design right at the design stage. During design the value of NPSHa (which is independent of the pump to be selected) can be deter- mined quite easily. Having determined the NPSHa it can then be compared to the NPSHr (the suction head required by the pump) for the types of pumps being considered. If there is insuffi- cient NPSHa it is much easier to make changes to the system at design stage rather than aer construction and installation. It is strongly recommended that any changes necessary are made at design stage as any additional costs incurred may pale into insignificance compared to the costs of rectifying an installation with cavitation problems. How do you fix it? In the event that pump cavitation is a problem on an existing installation there are essentially only two routes that can be followed to rectify the prob- lem. These are to increase the NPSHa to the pump or decrease NPSHr by the pump. Options available to increase the NPSHa will depend upon the nature of the system in question. This can include increasing the pressure on the suction end of the pump or reducing the friction losses in the pipework, thereby making more pressure avail- able to the pump. Increasing supply pressure can be achieved by raising the static head of the supply, applying pressure to the supply vessel, using a booster pump, or reducing friction losses in the pipework by using larger diameter pipes or fewer components and fittings. Pressure could be supplied to the supply vessel with the use of a booster pump. However, these are rarely viable options for an existing installation and nearly always impractical due to space issues, cost and potential disruption. Similarly, it is rarely practical to replace the suction system pipework with a larger diameter. A second option is to replace the existing pumps with pumps that have a lower NPSHr or install par- allel pumping using multiple pumps. In many cases the above options may not be viable, and in all cases they may in- volve considerable cost and disruption. Top tips 1: Get it right at design stage There is no alternative to getting it right at design stage, regardless of the pressure to reduce costs. If it is neces- sary to take out two metres of concrete in order to lower the pump suction connection and achieve the correct NPSHa, then my strong advice is to do it or look for an alternative pump loca- tion. Good design to avoid cavitation is always the best option. 2: Don't automatically blame the pump All too oen the pump itself is unfairly blamed for pumping system problems. In actual fact, problems such as cavita- tion oen manifest themselves at the pump but are rarely caused by it. In fact, nine out of ten pump problems are not caused by the pump itself but by issues such poor system design and lack of maintenance. sponsored by Smart liquid lime www.neutralac.co.uk Figure 2. Cavitation damage to impellers Figure 3. Cavitation damage to pump impeller Figure 4. Hole in pump impeller vane caused by cavitation

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