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How to Select Pumps for Food and Beverage Applications
Time: 2016-07-16
A pump system in food and beverage applications is used to transfer product from one place to another,
from unloading rail cars filled with corn syrup for a cola beverage; to dewatering devices and high
temperature cooking processes or heat exchangers.
Depending on the size of the facility, there could be hundreds of pumps at a food or beverage
processing plant. Pump types could range from centrifugal pumps to positive displacement (PD) pumps,
including rotary lobe (RL) and progressing cavity (PC). The pumps used for these processes are
subject to a variety of nationally and internationally manufacturing and hygiene standards and
guidelines. The most significant – and the baseline for all pumps in the food market – is
the US Food and Drug Administration’s requirement calling for use of “approved” materials.
Use of FDA-approved materials like 300-grade stainless steel is required for the pump
housing Elastomer materials used in pump seals must also be submitted and approved. FDA-approved
food-grade lubricants are required for pump lubrication. Beyond the basic FDA requirements, there are
additional national and state requirements. For example, many companies in the US follow
standards set by 3-A Sanitary Standards, Inc. (3-A SSI), an independent, not-for-profit corporation
dedicated to advancing hygienic equipment design for the food and beverage industries.
Those exporting internationally follow guidelines issued by the European Hygienic
Engineering & Design Group (EHEDG), which supports European legislation requiring that
handling, preparation, processing and packaging of food is done hygienically using hygienic
machinery and in hygienic premises. Both 3-A SSI and EHEDG provide a third party evaluation
of a company’s equipment by running it and testing it to make sure it can be cleaned properly.
Advantages and Disadvantages of Pumps
The types of pumps typically used in food and beverage applications include centrifugal pumps
and positive displacement (PD) pumps, including progressing cavity (CP) pumps, and
rotary lobe (RL) pumps. Centrifugal pumps are the most commonly used kinetic-energy pump.
Centrifugal force pushes the liquid outward from the eye of the impeller where it enters the casing.
Differential head can be increased by turning the impeller faster, using a larger impeller,
or by increasing the number of impellers. The impeller and the fluid being pumped are isolated
from the outside by packing or mechanical seals.
A progressive cavity pump transfers fluid by means of the progress, through the pump, of a
sequence of small, fixed shape, discrete cavities, as its rotor is turned. This leads to the volumetric
flow rate being proportional to the rotation rate (bi-directionally) and to low levels of shearing being
applied to the pumped fluid. A rotary lobe pump has two counter-rotating pumping elements (rotors),
each with two or more lobes. The drive shaft counter-rotates the driven shaft using oil-or
grease-lubricated timing gears located outside of the pumping chamber. With rotary lobe pumps,
fluid flows around the interior of the casing.
The main advantage of a PC pump is it does not produce shear, which could change the
product consistency. Shear is defined as a strain in the structure of a substance produced by
pressure, when its layers are laterally shifted in relation to each other. The PC pump operates
quite gently, so it does not impart shear and does not change product consistency. RL pumps
are also a good choice for similar applications. Both are gentle, impart little shear, and can
handle shear sensitive products. In addition, they can pass solids through the pump.
The main advantage of an RL pump is its footprint is smaller than a PC pump, so it takes up
less real estate on the processing line. It also offers good clean-in-place and
steam-in-place (CIP/SIP) characteristics. The food industry has been leaning toward RL pumps
in recent years, but there are limits to its use, mainly based on the discharge pressures under
which it can operate. A PC pump can handle much higher head pressure than those of a RL pump.
Therefore, one would employ a PC for higher discharge pressures, for example if you are running
a long distance with a very viscous product. The pump type needed depends on how the plant
is laid out. If there is a storage tank outside the plant, say 500-600 feet away from where the
product must be delivered, the pump needs to create a higher pressure due to line loss than if
it was 20 feet away. Also, a PC pump can handle higher viscosities than an RL pump. Finally,
with a progressing cavity pump, designs are available for pumping material very high in chunky
solids quite long distances.
Cleanability, serviceability and durability are the most important features to look for when
purchasing a pump for this industry. A good starting point is to make sure the pump has been
certified by 3-A SSI. With regard to cleanability, find out if the pump can be cleaned in place (CIP)
and/or steamed in place (SIP). All pumps are CIP-able; but RL pumps are also SIP-able. This gives
RL pumps an advantage over PC pumps; with the PC, one has to disassemble piping to steam-clean it.
A factor affecting durability is the speed at which the pump runs. For example, centrifugal pumps
run at 1800 or 3600 revolutions per minute (rpm), whereas many PC pumps run at 300-400 rpm.
The PC, running at the slower speed, will not wear as quickly. Centrifugal pumps are designed for
moving water or other thin fluids. When working with more viscous products, one would lean
toward one of the PD pumps. For metering applications, PD pumps are also a better option.
Another factor to consider is a centrifugal pump cannot handle any changes in viscosity or
discharge head that may occur. Once designed for an application, the centrifugal pump can
best be used for those conditions only. If these conditions change, so does the performance.
By contrast, a PD pump is a lot more forgiving with changes of viscosity and changes in discharge head.
The net positive suction head (NPSH) requirements are typically much lower on an RL and PC pump
compared to those of a centrifugal pump. A pump requires a positive head to be able to draw; there is
a minimum positive head required on any pump. If there is not enough positive pressure feeding the
pump, the material to be pumped will not get through. Centrifugal pumps may need 32 feet of positive
head, compared to a PC pump, which may only need five feet of positive head. This different can be
critical, depending on pump placement in relationship to the head. Also, both the PC and RL pumps
feature self-priming suction lift, but this is not the case with centrifugal pumps.
Pump accessory features, including variable speed drives, load cells, and monitoring systems,
must be examined, too. Most pump systems include a variable speed control system that allows
users to change pump speed, which will vary the flow rate. Some pumps are mounted on a frame
or bed plate, with load cells under the tank to regulate flow going into the system based on weight.
Source: www.foodproductiondaily.com