Mechanical Technology June 2016

⎪ Pump systems, pipes, valves and seals ⎪

Metso’s MD pumps are double adjustable: the front and the back impeller gap can both be restored at same time. making the pump significantly cheaper, easier to handle and to maintain,” Sedgwick explains. “Hence the importance of selecting the appropriate aspect ratio for the slurry being pumped. It is the starting point in selecting the most cost effective pump,” he advises. Metso’s slurr y pump selection chart and its pump selector software, Pumpdim, use the aspect ratio as the pri- mary variable in organising its range for different slurry requirements. “The pink colour (HM and HR range), for example, represents an aspect ratio of around 2.5. Our slurry pumps are organised on the selection chart in order of reducing aspect ratio, that is, from most abrasive (3) to least abrasive (2), with Metso’s MD pumps being the most suited to cope with the highest abrasion levels,” Sedgwick notes. Another differentiator for Metso is its focus on limiting the inlet fluid velocity of its pumps. “For heavy slurries we keep our inlet velocity down to below 5.5 m/s to minimise impact damage to the impel- ler from sharp, coarse and heavy solids. From a hydraulic design perspective, we also try to make sure that the pump always operates just to the left of the best efficiency point (BEP), whilst maintain- ing a relatively low inlet velocity. This results in the highest possible efficiency, minimum internal turbulence and the longest wear life. Wasted energy has to go somewhere, into vibration or turbulence, for example, which may cause bearing failure and accelerated wear,” Sedgwick points out.

A typical MDM series pump section summarising some of the pump’s key features.

Metso’s EnviroSet seal has a built-in lantern ring and a helical grooving system that imparts a centrifugal flow to the flush water, which throws the particles in the fluid to the outer bore of the separation chamber. Gland flush water savings of over 50% can be achieved.

“The more high-energy material that is flowing around the front of the impeller, the higher the wear rate on the liner – and the larger the gap the higher the recircu- lating flow. So this gap must be kept to a minimum at all times,” says Sedgwick. “Recirculation also reduces the spe- cific energy of the pump, ie, the energy used per m 3 of flow, since some of the flow is being recirculated,” he adds. “So this front liner gap has to be adjustable. The conventional way is to push the impeller forward on its shaft until it hits the front liner, then to shift it back a little for clearance. “On our MD pumps, however, we have a front liner that can be adjusted independently of the impeller shaft. The front adjustment can be made by mov- ing the suction wear plate liner forward, without disturbing any of the shaft components,” he explains, adding: “Big

Addressing the design innovations that improve the hydraulics for abrasive, applications, Sedgwick says: “A pump is always going to wear when pumping slurry. On some applications, the best products might only last for 400 to 600 hours, which can be less than a month. This makes it important for operators to choose pump designs that extend wear life and make it easier to maintain hydraulic efficiency.” As an example, he says that a slurry pump does not operate well with a big gap between the impeller and the front casing liner. “Pumps accelerate fluid under centrifugal action and convert this kinetic energy into pressure energy. If there is a gap between the suction wear plate liner and the impeller, fluid under pressure at the outlet is forced back to the inlet through this gap. This process is known as recirculation.

Mechanical Technology — June 2016

11