The sanitary design of centrifugal pumps By Lars Sorensen, product manager, Alfa Laval Flow

Increasing demands placed on the sanitary process in industry mean that pumps must now come with long-life shaft seals, low noise levels and a sanitary design of the highest standards. The pump must be thoroughly cleanable by the circulation of cleaning liquid - by cleaning-in-place (CIP).

T he centrifugal pump is the most commonly found pump type within the food and

drinks industry. It is mainly used for transport of low viscosity products (efficiency deteriorates at higher viscosities). The principle of the centrifugal pump is that during rotation of the impeller vanes, some of the liquid is collected and is then transferred into the impeller channel. Centrifugal force then pushes the liquid out of the impeller with increased pressure and velocity.

Figure 1. Typical design of open impeller

I m p e l l e r d e s i g n For hygiene and cleanliness, pumps

should be fitted with an open type pump impeller - the vanes must be open in front. This allows for visual

inspection of the vanes and the area between them, ensuring that possible holes in cast surfaces are discovered

(see Figure 1). Furthermore, the open impeller design enables surfaces to be polished if required.

Closed impeller designs made of cast iron can have casing flaws, which makes efficient cleaning impossible. If the impeller is made of pressed steel

sheets and is welded, it is inevitable that there will be splitting resulting in bacteria traps. In order to obtain high

efficiency, the open impeller requires a small gap of about 0.5 mm between

the impeller and back plate. Gaps smaller than this are difficult to obtain due to the limitations of bearings and thermal expansion.

The open impeller generates more axial power than the closed impeller.

.............................................................................................. There are balancing holes on the

a s

Figure 2. Open impeller with balancing holes in the backplate.

back plate of the impeller to com-

pensate. Besides the balancing effect they provide, the holes give an increased circulation behind the

impeller, which ensures optimum cleaning of the pumps during CIP (see Figure 2).

The impeller may be fastened onto the shaft without an impeller screw by having a threaded shaft, giving an easy clean impeller front without slots

(Figure 3). This solution provides the best possible hygienic conditions and

a further benefit, the inlet conditions are optimised with very low NPSH requirements.

If the impeller is secured by means of an impeller screw, it is important that

the required stationary seal is made correctly without any slots. An

incorrect seal can result in difficult cleaning and therefore unhygienic

conditions may occur (Figures 4 & 5). Additionally, it is important to limit the size of the impeller screw to

reduce the restriction of the inlet conditions.

Seal t ypes The mechanical shaft seal of a sanitary pump must be cleaned correctly in a normal CIP procedure.

The shaft seal has to be made of the correct materials (approved for food

use) to avoid product contamination. It must also have as few slots as possible, where product might settle, and the surface finish needs to be of a

high standard.

The mechanical shaft seal consists of the following main parts (Figures 6,

7 & 8):

• A stationary seal ring, fitted (fixed) in the pump casing or the back plate and sealed with an O-ring.

• A rotating seal ring, sealing off the shaft with an O-ring.

• Spring(s) including the driving

ring/part.

Pumps can be fitted with either an internal or an external mechanical shaft seal, the difference being that

that the rotating part of the shaft seal is situated inside (internally) or outside

(externally) of the product area.

Figure 6 shows a commonly used internal shaft seal. Having the

2 4 WORLD P U M P S November 2000 0262 1762/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved

rotating parts in the product area guarantees a strong

circulation of the pumped media around the sealed parts. The disadvantage of the design, from a sanitary point of view, are

the slots situated where the spring contacts the shaft, the rotating seal ring and the O-ring groove. Product residues may become trapped in the slots and be difficult to remove during cleaning, resulting in the risk of infection. Another type of internal shaft seal that is regarded as sanitary is shown in Figure 7. This has a caged spring system, where the spring(s)

and drive arrangement is situated inside the rotating seal housing. This is sealed (}ffwith two O-rings and guarantees that the media does not contact the springs.

Figure 8 shows a typical external shaft seal. This has the advantage of the spring and the drive ring being placed outside

the product area, thus avoiding slots and possible bacteria traps in the product zone. The external shaft seal is easily cleaned if it is correctly designed, with sufficient flow through the shaft seal. This flow is obtained by the balancing holes in the impeller which, from a sanitary point of view, are the optimal solution.

Options fi~r single shaft seals, as described above, are flushed shaft seals and double mechanical shaft seals. These types of

seals are recommended fi)r demanding applications such as:

• Pumping viscous or sticky fluids or fluids which crystallise ie.

sugar solutions • Pumping abrasive fluids • Where cooling of the seals is required to prevent product

deposits burning due to high temperatures • When a water barrier is needed to avoid air entering the

fluid when pumping with very low inlet pressures ie. from a

v a c u u m tank • Providing steam barriers when pumping sterile products

The flushed seal consists of the same stationary and rotating parts as the single shaft seal (primary seal). In addition, a lip seal, a seal housing and two flushing tubes for the flushing media, form

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Figure 6. Internal shaft seal

the flushing chamber. The lip seal is

fitted in the seal housing and seals against the rotating drive ring. This design forms a flushing chamber where

the flushing fluids (tap water for example) flow through in order to clean or cool the seals or to create a

barrier between the atmosphere and the fluid to be pumped.

The double mechanical shaft seal consists of the same stationary and rotating parts as the single shaft seal (primary seal). In addition, the seal has a seal housing, two flushing tubes for the flushing media, a stationary seal ring and a rotating seal ring,

similar to the primary rotating seal ring. The stationary seal ring is fitted in the seal housing and seals against the secondary rotating seal ring, which

is fixed together with the primary rotating seal ring. This design forms the flushing chamber where the cleaning fluid flows through. It is only the primary seal that is in contact with the product to be pumped, the secondary parts are only in contact with the flushing media, giving the same optimal hygienic conditions as the single shaft seal.

The Alfa Laval LKH (heavy duty) centrifl~gal pump range has the optimal hygienic solution; external single shaft seal as standard. The entire range has only one universal shaft seal that fits all the sizes.

This approach limits the need for stocks of spare parts.

product-wetted parts

All product-wetted surfaces should be in a condit ion that enables efficient cleaning, hence the

production and treatment methods must be carefully considered. A pump casing is normally cast or pressed. This basic finish may, in some cases,

be sufficient. However there will be a need for some machining and final blasting to obtain an even

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i

Z .. . . . . . . . . . 2 ~L 12_Z . . . . . . . . . . . . . . . . . . . . . .

Figure 7. Internal shaft seal with caged spring system

Figure 8. External mechanical shaft seal

finish. A minimum roughness of Ra 0.8 micron, will usually meet the requirement for an "easy-clean" surface. A pressed pump casing is the optimal solution due to the fact that cast pump casings may have cavities (casing flaws) in the surface or even just below the surface, and will leave holes in the surface, if treated any further.

For other product-wetted parts, there may be a need for manual polishing/grinding if the design makes machine operation (turning) impossible. A turned surface has a more even structure than a manuaUy polished/ground surface, and

experience shows that the character of the irregularities on the material

surface is normally more important to deanability than their size.

Design for CIP-ability When choosing a centrifugal pump for a sanitary (food and drink) application, it is very important to

look at the details and the features that provide optimal cleanability. It is of extreme

importance that the pump is designed for CIP cleaning thus ensuring a completely clean pump after the standard CIP procedure. Having considered the cleanliness of the pump and other aspects such as product handling, operating & mainten-

ance features, efficiency (power consumption) and design charac- teristics, these should influence the

final choice of the centrifugal

pump. •

Lars Sorensen has a B.S. in mechanical

engineering, with a major in flow dynamics, and a Bachelor of Commerce in sales and marketing. He has held

numerous engineering and R&D positions for Alfa Laval Flow, where he

has worked for the past 11 years, most recently as product manager for

centrifugal pumps.

CONTACT Lars Sorensen, product manager, Alfa Laval Flow, PO Box 73, S-221 00 Lund, Sweden. Tel: +46 4636 7109; Fax: +46 4636 7150, www.alfalaval.com

2 6 W O R L D P U M P S November 2000 www.worldpumps.com