THE BENEFITS OF ELECTRIC DIAPHRAGM PUMPS OVER CENTRIFUGAL PUMPS

Centrifugal pumps are incompatible with a large number of fluids. The relatively simple design is limiting when there is any variation in a processes’ parameters such as flow rate and pressure. Graco’s QUANTM electric operated double diaphragm (EODD) pumps address all of these issues.

Centrifugal pumps

Centrifugal pumps are by far the most commonly used pumps on the market because they are reasonably priced and offer a good price/performance ratio for high flow applications. However, centrifugal pumps have a number of limitations that make them less than ideal for a range of industrial and hygienic applications.

• Narrow optimum operating range and efficiency point
  Centrifugal pumps have a specific impeller diameter, which means they only operate at optimum efficiency at a specific flow. The preferred flow range below or above this best efficiency point (BEP) is quite narrow. Not only does working outside the preset range significantly reduce the pump’s efficiency, but eventually, it also leads to cavitation, vibration, impeller damage, suction and discharge recirculation, or reduced bearing and seal life forcing complete rebuilds of the whole unit.

• Impact of changes in viscosity
  Some liquids can vary in viscosity due to temperature, chemical reaction or separation (varied layers or solids). The performance of a centrifugal pump is especially impacted when pumping more viscous liquids, because of the increased resistance to flow as the impeller rotates. The volume pumped by a centrifugal pump depends on head (back pressures) viscosity and the homogeneity of the material. If these values vary dynamically in a certain application, it is difficult to keep a centrifugal pump operating close to its best efficiency and also without prematurely wearing out the pump.

• Damage caused by deadheading
  When a discharge valve is closed or a line blockage occurs, it causes the fluid to recirculate in the pump, and pressure to continue to build resulting in a build-up of motor temperatures. When a pump is run in a deadheaded condition for too long, the excess heat damages the expensive seals and reduces pump life. Without purchasing additional sensors and controllers, a centrifugal pump will continue to run until something fails. The extra cost of these additional sensors and controllers needs to be included in the total cost evaluation.

• Not self-priming
  Most centrifugal pumps are not self-priming. For the pump to work properly, its casing must be filled with liquid before start-up. When the casing fills with vapors or gases, the pump impeller becomes gas-bound and incapable of pumping. To make sure the pump remains primed and does not become gas-bound, centrifugal pumps need to be installed below the fluid level, from which the pump takes its suction. Alternatively, the pump can be primed by supplying liquid under pressure through another pump placed in the suction line.

• Unsuitable for abrasives and solids
  Particularly in the chemical industry, selecting a centrifugal pump for pumping solids is quite a challenge, as process owners need a pump that is not only compatible with the chemicals being processed, but also capable of handling solids without choking the impeller and thus causing increased pump wear and potential blockages. The centrifugal pump types that are suited to this specific set of applications tend to have lower performance areas and be more expensive. 

• Unsuitable for shear-sensitive liquids
  High-speed systems, such as the centrifugal pump, tend to shear liquids which is why this technology is not the best choice for shear-sensitive liquids.

• Unable to run dry
  A centrifugal pump cannot run dry without causing damage to the system. The system needs to have resistance to dissipate rotation speed of the impeller.