The rotary lobe PD pump efficiently moves food products gently with minimum slippage. Source: Fristam Pumps.
While standard centrifugal pumps are typically used for mixing, blending and transporting product from points A to B, moving product with minimal shearing requires a positive-displacement (PD) pump. PD pumps can move food and beverages with a minimum of shear because the product is moved, in the case of a rotary PD pump, within the rotor pockets from the inlet to the outlet.
Shear, the relative motion between adjacent layers of a moving liquid, affects different liquids in different manners. Paint thins in viscosity as it is stirred; but cornstarch and water thicken when sheared. With a low-shear pump, delicate soups can be transported from the kettle to the filler, keeping food particles intact, and yogurt can be moved in a dairy without subjecting it to shear that may cause it to separate later after packaging.
Peristaltic pumps use rollers that pinch a tube, providing pumping action. Pumped product does not come in contact with the pump body, and the tube can be cleaned in place like piping. Source: Watson-Marlow.
“If you want a pump for high-shear applications, the best pump technology would be centrifugal,” says Eric Nofziger, Cole-Parmer product manager. “There are specific designs that can accommodate the chopping and grinding of particulates in the system water,” he adds. The key item to take into consideration is the size of the suspended particles. Depending on the size constraint, the pump’s impeller can be either a traditional closed or an open one-and if open, the geometry is adjusted to take the size into consideration.
Two pumps are compared for slip. The pump on the left has a tight performance band and shows a slip of 4.2 gpm, while the pump on the right has a wide or loose performance band and produces a slip of 28.2 gpm. Source: Pump Solutions Group.
A gentler, kinder pump
When processors have a viscous product or a product that needs to be transferred gently, PD pumps are the pumps of choice, says Chuck Treutel, Watson-Marlow food & beverage division sales manager. Typically, PD pumps are used in applications where product integrity is paramount. They run at slower speeds and handle higher viscosities than centrifugal or rotary pumps. The most common type of PD pump found in the food industry is the rotary lobe or circumferential piston. These pumps have two shafts and two rotors that turn opposite each other where the meshing rotors create a vacuum at the inlet pulling product in, carrying the product through and discharging it.Lobe pumps are not great for metering duties because fluid slips between the lobes and the case, explains Treutel. While lobe pumps are low-shear devices, peristaltic pumps are better PD devices as they use a pinched-tube principle to move product, resulting in lower slippage and shear. Another option, says Treutel, is the sinusoidal pump, which has a single sinusoidal rotor that’s capable of powerful suction with low shear, low pulsation and gentle handling.
Treutel points to a leading poultry processor that used two sinusoidal pumps in its marinated boneless breast application, replacing vacuum and air pumps that were previously used to transfer the product into a drop hopper to meter product onto a conveyor. The drop hopper was also replaced, and the sinusoidal pumps were set up to meter boneless products through end-user-supplied spreader horns. Two pumps and two horns were arranged to feed two 16-in.-wide, half-inch sheets of boneless breast meat onto the conveyor, creating an almost-labor-free roasting line.
While perhaps similar in concept to a peristaltic or sinusoidal pump, Mouvex’s eccentric movement technology produces a peristaltic effect with a disk moving in an eccentric (non-rotating) motion inside an annular cylinder, says Wallace Wittkoff, Pump Solutions Group global hygienic director. Because the pumping elements don’t rotate, but instead move in an eccentric motion, there is no need for a mechanical seal. A rubber boot or metal bellows is used to accommodate this eccentric motion. For very low- or almost no-shear applications, the eccentric pump and an old standby, the air-operated diaphragm pump, both are good choices for delicate food products, adds Wittkoff.
This winery pump with a progressing cavity design moves up to 20 tons of must or whole clusters of grapes per hour and includes a built-in auger-feed mechanism. Source: Moyno.
Shear, slip, efficiency and saving energy
“Since the initial price of a pump is only about 5% of its life cycle costs, I advise customers to look at the total cost of ownership of the equipment,” says Sam Raimond, Fristam Pumps customer service supervisor. “This includes maintenance costs (reliability, CIP-ability), more efficient motors and pump efficiency (energy costs),” he adds.Processors want to save energy, and one way is to use efficient pumps. Shear and slip are inter-related and can affect pump efficiency. Pumps that are best for low-shear applications typically run at slow speeds, have small-diameter rotating parts, and produce minimal internal slip, says Mike Dillon, president of seepex Inc. By definition, PD pumps have the lowest shear rates, he adds.
In pump design, the higher the pump efficiency, the tighter the performance and the greater the energy savings. “When looking at centrifugal pumps, you want a design that utilizes most of its energy for moving liquid instead of being converted into thermal energy,” says Jim LeClair, SPX Flow Technology global product manager. In the design of rotary PD pumps, the minimization of slip within the rotor and case design maximizes the pump’s efficiency, he adds. Finding the optimum pump size based on pumping efficiency is important in maximizing energy savings.
For PD pumps, it’s assumed that no slip equals tight performance. But in reality, most rotary PD pumps do not attain true positive displacement because of slip. The actual flow produced is subject to product viscosity, back pressure, temperature (large clearances are needed) and wear, which is the most troubling, says Wittkoff. Since the Mouvex eccentric technology has very low slip, it can compress air, thus purging much of the product from the line when the supply tank runs dry, Wittkoff adds.
Fewer moving parts in the design of a pump can save energy. Sinusoidal pumps have only one set of bearings, one shaft and one rotor, compared to two of everything for the rotary lobe design, says Treutel.
“The result is less energy required to drive our pumps. Also peristaltic pumps are unique [because] they are 100% volumetrically efficient. This means zero slip,” he adds. No slip means very low shear is imparted on the product, and if the product is not slipping back from the discharge to the inlet, it means the product doesn’t have to be pumped twice, saving time and energy.
While the physical appearance of PD pumps has changed little over the years, better materials and improved technology have afforded internal pump changes that save energy, says Lewis. Now smaller drive ends can handle larger elements than in the past, which is both a cost and energy saver. Still the best way to save energy is to apply the right pump to the application. The correctly-sized pump can overcome friction loss, avoid excessive back pressure and still handle the upset condition.
For maximum overall efficiency, a rotary lobe pump can be teamed up with a variable-frequency drive to find the optimum spot on the pump’s performance curves. Source: Cole-Parmer.
"CIP-ability"
The term CIP-ability implies the ability for a device to be compatible with clean-in-place methods-to be able to withstand harsh chemicals and extreme wash temperatures. But several criteria raise concern for processors. For example, how quickly and easily can a CIP-able pump be taken apart to verify cleaning? How can a CIP process in a pump be validated? Do seals and pumps exist that can withstand the CIP process? What approvals show that a pump is CIP-able?“It seems as though the most desirable trait is ease and simplicity to manually clean,” says Dillon. Even though CIP has gained acceptance, many plants still disassemble pumps and piping for frequent inspection, he adds. This explains the persistence of lobe and centrifugal pumps in some plants when other designs are less expensive, are more CIP-able, have lower shear, and are more efficient, says Dillon.
Lewis says his company’s progressive cavity pumps take about an extra 5 to 10 minutes to disassemble and reassemble. The connections are made through a tri-clamp system that makes disassembly easy. More importantly, he adds, the pump should have regulatory approvals in place such as 3-A Sanitary Standards Inc., which indicate the pump has been rigorously tested to meet sanitary conditions.
Quick disassembly and repairs to CIP-able pumps depend on the design from the supplier, says Nofziger. Some designs have a wing-nut-type release bolt to remove the cover plate, while others require more extensive tools. As for repairs, front seals are easier to replace than rear seals, and working in the gearbox can be problematic, he adds. Gear-box repairs on lobe pumps can be tricky because of the timing issues. And with lobe pumps, Nofziger recommends not turning them on until the head temperature has stabilized to the fluid or product. Otherwise spalling could occur inside, causing slippage.
“Temperature variations, which are common in many food applications, can be very hard on pumps,” says Dillon. Dairy products are pumped cold, yet CIP or SIP (steam-in-place) is really hot. Tolerances can change and excessive wear can take place during CIP. Using thinner cross sections of temperature-sensitive materials-like elastomers-leads to less sensitivity to temperature fluctuations, he adds. seepex introduced a new line of 3-A pumps with thinner cross sections of elastomers in the stators to increase longevity in CIP applications.
Doug Silvey, CSI Designs solution expert, believes processors should spend the time normally spent on disassembling and reassembling pumps on validating the efficacy of CIP process on the pump. Adjusting the CIP time, cleaning chemicals, backpressure and pump speed when cleaning, and validating the pump is free of bacteria, eliminates repeated disassembly/re-assembly steps that can cause rotor alignment problems and damage.
The design of a CIP-able pump should meet all the requirements of the CIP definition as stated in the PMO and 3-A standards, says LeClair. When all equipment in the process line-including the pump-meets these standards, the processor can be assured that CIP will be effective for the entire line. Furthermore, if the pump has been tested to the European EHEDG standard, the processor can be assured that the pump will be clean according to the hygienic standards as outlined in the European Union, he adds.
One thing to keep in mind is that pump costs can be much higher if they have to comply with certain standards like 3-A.
“When a pump is rated 3-A, the pump manufacturer not only ensures the correct material of construction [is used], but also pays special attention to O-ring design (how the pump is connected to the rest of the system) as well as making sure there are no sharp corners in the fluid path,” says Nofziger. This detail ensures that the pump can be cleaned to eliminate the chance of bacteria growth. Additional costs accumulate in the third-party verification step where a 3-A member visits the pump manufacturer’s plant and reviews the engineering drawings, he adds.
Seal or no seal
Seals can be a sticky issue, especially with harsh CIP fluids and temperatures. Lewis suggests it may not be that mechanical seals fail due to undergoing CIP. Rather, it could be that the type of seal is incorrect for the process. Double mechanical seals may need to be considered, or the use of a magnetic drive can replace mechanical seals.The materials used in seals have improved and the standard material provided has been optimized to fit 80% of applications that are currently on the market, says LeClair. The materials most used for rotary seals are silicon carbide against a carbon face. As new elastomeric technologies have been introduced, the use of new compounds has helped seals hold up against many of the new cleaning agents used today as well as improved their heat resistance, he adds.
“There are many options available for pumps to withstand chemical attack, high temperature and high pressures,” says Raimond. “We offer perfluoroelastomers (ASTM designation: FFKM), which are designed to withstand high temperatures and are compatible with many aggressive chemicals. Hastelloy and AL6XN metals have high chemical resistance,” he adds.
Pump manufacturers have addressed seal problems with exotic materials and designs, seal flushing systems and recommended plant process changes, says Treutel. “But the best way of preventing seal problems, such as leaks and premature wear, is to eliminate the seal altogether.
“If a [processor] is having seal problems and cannot find a solution, try a peristaltic pump…they do not have product seals,” he advises.
In the case of peristaltic pumps, during CIP the rollers or shoes that compress the tube are retracted so the pump becomes nothing more than an extension of the piping, says Treutel. If the piping becomes clean during CIP, then the pump will as well because the product and CIP fluid is contained within the tube at all times. Although the pump has no seals, there is some mechanical wear on the tube itself, but replacing it is generally easy.
Hygienic diaphragm pumps such as Wilden’s typically don’t have a mechanical seal; their diaphragm serves as isolation between product and the outside environment. According to Wittkoff, his company’s 3-A and EHEDG-approved pumps have no counter-moving surfaces, so there is no issue with material buildup or abrasive wear.
Trends and challenges
Most processors ultimately want CIP-able PD pumps, and that has been a focus for many suppliers. In creating a CIP-able PD pump, Fristam designed it such that not only the pump remains in place during CIP, but also all of its parts-covers and rotors, says Raimond. The challenge was to make the pump without modifying or increasing its clearances to accommodate CIP.Suppliers have also dealt with some interesting applications. For example, Lewis describes a sanitary pump system for cold sausage with a viscosity topping 1,000,000 cp. The pump had to be made of stainless steel and designed for quick disassembly. It has also been used in icing, batter and dough applications.
LeClair describes pumping a mixture of fruit skins and grain husks. The low moisture content of this product made for a very abrasive application that required a rubber lobe pump.
Nofziger suggests that a real challenge is educating the customer in properly specifying a pumping system. Choosing a pump that is operating too close to either end of its performance curve tends to happen when a processor is looking for a pump mostly with a price point in mind.
While it is important to find something that will work within the budget, it is also important to review the performance curve of the pump. If a pump costs a bit more and provides an operating point closer to the middle of the curve, it would be a better choice than one closer to the end of its curve. When choosing a pump with an operational point that is close to the end of its curve, the end user will also be spending more on the operation of the pump versus one that is in the middle of the curve (which should have a better efficiency). If the pump is operating 24/7, this operational inefficiency could add up to many times more than the cost of the original equipment.
For more information:
Chuck Lewis, Moyno Inc., 937-327-3111, chuck.lewis@robn.com
Eric Nofziger, Cole-Parmer, 800-323-4340, enofziger@coleparmer.com
Chuck Treutel, Watson-Marlow, 608-883-6851, chuck.treutel@wmpg.com
Wallace Wittkoff, Pump Solutions Group, 502-905-9169,
wallace.wittkoff@pumpsg.com
Jim LeClair, SPX Flow Technology, 262-728-4912, jim.leclair@apv.com
Mike Dillon, seepex Inc., 937-864-7150, mdillon@seepex.net
Sam Raimond, Fristam Pumps, 608-831-5001
Doug Silvey, CSI Designs, 800-654-5635, dougs@csidesigns.com
