KEEPING IT CLEAN by better design

Many different pumping methods are used in hygienic applications, from centrifugal pumps to positive displacement pumps such as flexible impeller types. Particularly popular is the rotary lobe pump, which produces a gentle pumping action that allows fragile fluids to be pumped with no shear damage, yet allows high pressures to be generated.

Lobe pumps are also ideally suited for pumping abrasive fluids, since the pump head components are not easily worn - unlike gear pumps, where the contact between the gears can wear away material, not only reducing efficiency, but also risking contamination of the pumped fluid by wear particles.

Design Considerations

A number of factors affect the performance and characteristics of lobe pumps. These include rotor clearances, which affect volumetric efficiency; rotor lengths, which affect the maximum working pressure; and rotor shape, the choice of which is dependent on the type of fluid to be pumped.

Although most lobe pumps still use 3-lobe rotors, there are other shapes available which offer particular benefits. Commonly used are 2, 3 and 5 lobes, as well as the `Scimitar' type. Three-lobe rotors give a combination of gentle handling of viscous and shear sensitive liquids, plus the ability to pass smaller soft solids with minimal damage. When used in a conventional `double figure eight' rotor case shape (see picture on this page) they give good volumetric efficiency even with thinner liquids.

Five-lobe rotors, on the other hand, are designed for use with a straight-sided self-draining rotor case shape (also illustrated) and give exceptionally smooth flow and low noise.

Two-lobe rotors are used primarily for very delicate viscous fluids and those containing large soft solids. Scimitar, or two wing rotors, are also known as hammer-head rotors (pictured overleaf). They pump viscous liquids without pulsation and can achieve high efficiencies on thin liquids.

With such a variety of designs available just within the rotary lobe pump sector, it is hardly surprising that as new hygiene standards evolve, manufacturers are tempted to make modifications to existing designs rather than starting from scratch.

However, consideration of some of the criteria required for hygienic and ultra-hygienic operation shows that sometimes it is best to return to the drawing board (or CAD workstation).

For general hygienic applications, the main requirements are that the materials of construction that come in contact with the pumped fluid must themselves not introduce any contamination, and that the pump can be completely and thoroughly cleaned between batches or at product changeover.

To clean a pump, it must either be dismantled or cleaned in place (CIP). The latter is naturally preferable since it is quicker and easier than taking the pump apart. For ultra-hygienic applications, however, the requirements are more rigorous, with the pump needing to be cleaned in place to a microbiologically-clean standard and also be sterilisable and bacteria-tight.

Two widely recognised hygienic standards exist, specified by the 3A Sanitary Standards Administrative Council in the USA and the European Hygienic Equipment Design Group (EHEDG). These either define the types of materials and design criteria required (the dos and don'ts of pump design), or in the case of the EHEDG, lay down specific, reproducible, scientific tests which offer an actual measure of hygiene.

HYGIENE TESTING

The EHEDG clean-in-place protocol compares the cleanliness of the pump to the cleanliness of a piece of reference pipe connected to the pump and subjected to the same test procedure. This involves first dismantling, cleaning and sterilising the pump and reference pipe. After reassembly, the system is soiled under pressure with a soured milk solution containing spores of a bacterium strain. After draining and drying with filtered air, the assembly is subjected to a clean-in-place routine, involving a cold water rinse, followed by circulating a 1% {w/v) detergent solution at 63iC and a further cold water rinse.

The inner surfaces are then covered with an agar and, after incubation, the degree of discoloration of agar (resulting from residual bacteria after cleaning) covering the various components inside the pump is compared to that from the reference pipe.

The sterilise-in-place (SIP) protocol requires that the pump is first dismantled and cleaned as for the start of the CIP test. The inside surfaces of the dismantled pump are wetted with a spore suspension of bacteria, the components are allowed to dry, reassembled and then steam-sterilised in line. After sterilisation, the pump is connected to a sterile test circuit and a sterile, spore-sensitive broth is circulated through the pump for a two-hour period every day for five days. If the broth remains clear throughout that time, the pump is classed as steam sterilisable in line.

Assuming the SIP protocol has been satisfied, the pump can then be tested for bacteria tightness. Using the same test circuit, but with a freshly prepared culture of a particularly small and highly motile bacterium, the pump is coated twice a day for five days and the broth is circulated for two hours each day as before. A further five-day detection period follows, in which the broth is again circulated for two hours per day. If the broth remains clear at the end of all this, the pump is deemed to be bacteria-tight.

GETTING IT RIGHT

These tests obviously place great demands on pump design. In particular, there should be no crevices in which product and hence bacteria could accumulate. The process undertaken by ITT Jabsco in the design of its Series 55 ultra-hygienic lobe pump was to address from scratch every dynamic and static seal and the entire draining process for CIP and SIP.

Areas investigated included the rotor retainers, the shaft seals, end cover joints and the draining properties of the rotors themselves. The solutions resulted in a new approach to pump construction. Firstly, only US FDA-approved materials were used in areas of fluid contact. To enhance self-draining properties, a straight-sided rotor case shape was used. For low flow applications, a 5-lobe rotor design was chosen, giving smooth pumping for product ranging from thin liquids such as protein suspensions to viscous liquids like dermatological creams.

But, for the larger flow rates required for full-scale process plant applications, a simple scaling up of the 5-lobe design was not appropriate. This is why in the newly launched 55600 series of pumps, Scimitar-type rotors are used to reduce shear damage to delicate, low-viscosity liquids.

For both high and low flow rate pumps, however, external rotor retainers were designed to hold the rotors onto their shafts outside the pump chamber. This eliminates bolts and recesses where pumped fluid can collect, which in turn makes CIP faster and more effective. Special attention was paid to the shaft seals and it was here that the fresh approach to design was of particular significance.

In most rotary lobe pumps the fluid contacts the inside diameter of the shaft seals, an area from which it can be very difficult to clean out product and contaminants. Attempts to upgrade existing lobe pumps will always be constrained by this conventional seal design. However, ITT Jabsco's design team was able to work closely with its seal supplier to ensure that new seal technology, in which fluid only contacts the outside diameter of the seal, was designed into the 55 Series pump (see diagram).

Single crevice-free shaft seals, using carbon or silicon carbide face materials, were selected to provide bacteria-tight joints, while double seals offer extra security by providing steam or sterile fluid secondary barriers at all entry points.

The use of gasket type static joints eliminated the crevices normally associated with O-ring seals in the fluid contact areas of the pump.

These design features were all introduced through a radical approach to the problem. For example, the new shaft seal technology contributed more than other feature to the 55 Series's high standard of hygiene.

The end result is a series of pumps that have been specifically designed to meet the CIP, SIP and bacteria tightness protocols specified by EHEDG.h

Brian Hubbard is marketing manager with ITTJabsco, Hoddesdon, Herts.