Diaphragm Pumps

The Arrival of Diaphragm Pumps as Mainstream Process Pumps

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Self-Priming Capability

Easily unloading containers from above is an easy match. However, this capability offers much more. Some processes are intermittent. A case in point is centrifuges used to separate products of different densities from fluids. Centrifuges typically need to discharge to atmosphere. Stators in this equipment rotate at more than 2,000 rpm and are heavy because of solid metal construction, which requires a considerable amount of expended energy.

Beckman and Coulter, in their safety training on small centrifuges, give the example that one kilogram at high rotation is equal to an effective equivalent force/weight of 802,000 kg, which, they say, is two jumbo jets! As such, at no time can the discharged material be permitted to back up into the centrifuge and cause an imbalance, which would cause severe damage to the equipment, not to mention the creation a safety hazard. The constant and reliable self-priming of air-operated diaphragm pumps minimizes the risk of product backup into the centrifuge that can be the result of priming issues.

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Dry-Run Capability

The air-diaphragm pump can be used to move this material reliably and safely away from the centrifuge by self-priming when the material is discharged while not having any corresponding adverse affect on the pump when no material is arriving (dry running). Add in the fact that air-diaphragm pumps can handle the loading of liquids with high solids concentrations; few other pumping solutions that are as simple and reliable are even available.

Centrifuges are one example for equipment that cannot have the discharged product back up into it. There are other critical machines that could have that requirement for best performance, reliability and safety.

Low Shear Generation

Many technologies claim to be low shear but this is relative and subjective detail. For example, many PD pumps are considered low shear. However, if operating above low pressure, with low viscosity products, then they are actually high shear.

When pumping low-concentration yeast (living cells) in a water-based solution through lobe or gear pumps, the amount of product actually being pumped can be less than 30 % of the theoretical displacement of the pump (Consider 1-cps product, 100-psi discharge pressure). That means that 70 % of the fluid is actually slipping past the clearances of the pump. This represents considerable product shear.

The amount of shear energy can be seen in the motor horsepower required to overcome the slip. Shear, such as that produced by improper pump selection or other process problems, can have a significant impact on product quality and ultimate process efficiency/profitability. Manufacturers of products that have thickeners like starch, such as pudding or light mayonnaise or tomato paste for tomato-based products, know that by reducing shear during processing, they can reduce the quantity of these expensive ingredients in their formulations. This is discovered by accident many times when processing the product in a new process that happens to be lower shear and makes a product “too thick”. Discussed previously is how diaphragm pumps operate with little slip because the fluid is either pumped forward or checked by the check valves that are used to prevent slip.

Conclusion

From specialized processes that require system-pressurization pumps, to very flexible flow regimes, to most delicate pumping of fluids/products produced, we see that the diaphragm pump is no longer in the fringes of processing but at the heart of the operation to deliver the highest quality product at the greatest yield as efficiently as possible.

* The author is the Director of Global Segments and Key Account Management for PSG, a Dover company.

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