Pumps Handle Manufacturing’s Challenging Liquids

Aug. 3, 2020
Certain kinds of liquids and slurries are challenging to move, but they can be handled by judiciously chosen pumps.

Pumps, you might say, are the hearts of a food plant: They circulate vital fluids.

The problem is, unlike human hearts, the material they have to handle varies considerably.

“The food and beverage industry asks pump manufacturers to offer solutions for pumping a variety of food/beverage products,” says Ron Crutchfield, senior engineer and liquid handling product specialist with Motion Industries. “Every application is unique, and there must be a variety of pump options capable of offering smooth processes.”

The wide variety of foods and beverages and other fluids associated with their processing presents a basic challenge for pump specification: how to move fluids with as little transformative force as possible, while maintaining efficiency.

That depends, of course, on the makeup of a given fluid: how hard it is to pump, how sensitive it is to stress, and other factors. And there are certain substances that just present ongoing challenges in pumping, for various reasons.

Pumps come with all different kinds of operational principles; which kind to specify is the most basic choice. There is usually just one option – but not always.

Applications that involve “large solids and passing product without damage tend toward circumferential piston, progressing cavity and flap check diaphragm pumps,” Crutchfield says.

“Moving clean, less viscous products (juices, oils, gravies, etc.) tend toward centrifugal pumps.” Crutchfield lists the advantages, and disadvantages, of the major types of pumps to be found in a food plant:

  • Conventional centrifugal pumps move products the fastest but cause the most churn, with potential damage to the product.
  • Twin- and triple-screw pumps can move product with minimal damage and are good for high-viscosity products, but tend to be expensive, both to buy and to repair.
  • Diaphragm pumps with flap checks can handle solids without damage, have no seals to leak and are simple to maintain, but are limited in the volume they can handle.
  • Lobe or circumferential piston pumps are also limited in volume, but handle solids and slurries well.

There are applications where a choice between pump types is possible. In addition, even when the pump type is more or less determined by the application, certain applications demand variations in a pump’s parts and tolerances, pretreatment of the product and other factors.

Some of the characteristics of a product, or other substance, that can prove especially challenging for pumps include:

Rotary-lobe pumps are a good option for handling slurries and flows containing solids.
Photo: Unibloc

Viscosity. When something doesn’t flow readily, it takes more force to move it – but that’s not the whole picture. Certain products, viscous as they are, have to be handled as gently as possible. Some others, like ketchup and molten chocolate, can change viscosity as they’re pumped.

The biggest challenges in pumping highly viscous products are feeding them into the pump in the first place and dealing with the high downstream pressures they often require to move, says Sean Dolyk, sales engineer for Unibloc. Feeding, which is usually done from a tank or hopper above the pump, will be helped by the pump’s suction – but highly viscous products can’t reliably be fed using that suction alone, Dolyk says. Learn more about Pumps with this video from our Fundamental Video series

“The system flow should be vertical to let gravity best feed the pump,” he says. “The distance between the tank and the pump should be as minimal as possible and the opening as big as possible in order to create the least amount of obstruction to flow. It is also best to increase the level of the fluid within the tank.”

Other options include a pressurized tank or one with an auger, says Bob Garner, engineering manager for Ampco Pumps. “Once it’s in the pump, we can move a million centipoise,” Garner says. “The problem is getting it into the pump.”

Positive displacement pumps, of various kinds, are the most common option for more viscous products.
Photo: Ampco

As for the high pressures downstream that high-viscosity products cause when pumped, that has to be addressed in the design of the pump and/or the entire system that carries the product, Dolyk says.

“Increasing the diameter of the line, reducing downstream obstructions, reducing the distance the fluid is traveling and reducing the flow rate are all system design solutions for reducing pressure,” he says. “Increasing the clearances on the pump and the use of high-pressure seals can typically accommodate most reasonable production system design pressures.”

Particulates. Solid objects in a pumped stream always need attention. Moving them along with a pump can be problematic, in terms of both keeping a steady flow and protecting the particulates from damage. That damage can occur either from the pump mechanism or the pressure of the fluid carrying the particulates, especially delicate ones.

“Blueberries are as likely to be crushed by the pressure of the process fluid as they are by the pump itself,” says Michael David, manager of pump technology for Central States Industrial (CSI).

Centrifugal pumps, usually used for less viscous products, can handle small solids if the impeller vanes are spaces far enough apart.
Photo: Ampco

Gently handling particulates like blueberries is largely a matter of sizing the pump’s parts, such as the gaps between the impeller vanes of a centrifugal pump. “If we have a yogurt with blueberries in there, we know what size pump will handle this three-eighths-inch blueberry, and we’ll size the pump so that it won’t damage too many of the blueberries,” Garner says.

Centrifugal pumps work best when speed and high flow are important; if gentle handling is more of a priority, twin-screw pumps are good options, depending on the screws’ size and pitch (angle of the threads).

In any case, David says, an important key to gentle handling is efficiency, which means the product spends a minimal amount of time in the pump. “If you have low internal circulation, meaning the pump is efficient, then any of the product that enters the pump is spending less time in the pump, and so is abused less,” he says.

Abrasiveness. Certain naturally abrasive substances can cause long-term wear on the product-contact surfaces of a pump. Examples include slurries containing sugar, mustard with seed shells, tomatoes and other produce with dirt and sand, and bentonite, a clay-like substance that’s added to wine to remove impurities.

“Many products are somewhat abrasive while also being ‘sticky’ – these would include any product using sweeteners/sugar in their product,” Crutchfield says. “Sugar acts as an abrasive and gum that can cause seal failures – think of a soft drink when it spills and you let it dry; how difficult it is to clean off the surface of the spill. Many products use sugars and have these properties that pump manufacturers must deal with.”

Abrasive fluids can be dealt with in two basic ways: by opening up clearances to reduce shear, and by using harder materials of construction. Ampco deals with some abrasive product applications by sending the product-contact parts of the pump to a specialized vendor, Garner says. This vendor treats the parts with a proprietary process, involving heat and nitrogen, to make them harder and more resistant to abrasion.

Chemical attack. Certain liquids or slurries, due to their chemical properties, can attack pump components as surely as abrasive ones. This can happen in food products with high levels of salt or acid, but is more likely to occur in cleaning solutions.

“We have to make sure our elastomers and our stainless steel are compatible with the chemicals at the concentrations and temperatures they want to pump,” Garner says. For flexible components like seals, this is mostly a matter of specifying elastomers that can resist chemical attack; steel components can use a more resistant alloy.

Another problem with chemicals is what might be called indirect attack through gassing, says Jared Gabel, business development manager for water treatment at Grundfos Americas Corp.

“Disinfectants and acids such as sodium hypochlorite, sodium bisulfite and peracetic acid are all commonly used in food processing for disinfection purposes, in the beef and poultry industry for example,” Gabel says. “All of these chemistries off-gas readily and can create air bubbles in the dosing head.” Grundfos makes pumps that can compensate for these situations by sensing the percentage of the volume in the pump’s dosing head and making up for any variance on the next suction-and-discharge stroke, Gabel says.

Grundfos uses variable frequency drives and motors that can adapt to changes in flow volume, using only the energy needed at any given moment.
Photo: Grundfos

Variable flow. Pumping is always challenging when there are wide variances in the amount of fluid to be pumped. That’s nearly always the case when it comes to cleaning and sanitation, which is usually done intermittently.

“Food processing plants of all types deal with some of the same challenges when it comes to the varying need for water in the plant,” says Colin Cummings, senior district sales manager for industry at Grundfos. “During plant downtime there is minimal water usage; however, during peak production time, the water use ramps up, with some plants consuming over 3 million gallons of water per day.”

One problem is that sizing the pump and pipes to handle peak flow means they’re oversized for non-peak flow, which can lead to “deadheading” or running dry. Grundfos handles this problem by using variable frequency drives and motors that can adapt to changes in flow volume, using only the energy needed at any given moment.

Pumps can be found for just about any given food or beverage application, no matter how challenging, with the right selection and modification.

“When the food and beverage industry asks for a pump to move their product, the breadth of options from pump manufacturers is wide and varied,” Crutchfield says. “But it allows for the industry to choose pumps to meet their needs, as required for their product.”

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