Equipment design pitfalls
Strong cleaning action on all product-contact surfaces minimizes the risks of contamination and of system malfunction and
also enables cost-efficient cleaning. Common design pitfalls, however, impair equipment cleanability. Dead legs, pockets and
crevices, air pockets, and improper equipment surfaces are pitfalls too often seen in the pharmaceutical industry.
Figure 4: Dead leg.
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Dead legs.
It is widely understood that dead legs should be avoided or minimized in a system (see Figure 4). Some guidance states that
the length to diameter (L/D) measurement for dead legs should not be more than 2, and, in some cases, not more than 3. The
relation between the main-pipe velocity and the L/D measurement, however, is often overlooked. High main-pipe velocity makes
the turbulence go deeper into the dead leg, and if the turbulence or action is strong enough, it will remove the residues
at the bottom of the dead leg.
Figure 5: Velocity and length/diameter measurement.
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In a 1997 article, Haga et al. presented results from tests with various velocities in the main pipe in various L/D measurements
(1). They found that for an L/D of 6, it is possible to clean the residue adequately if the main-pipe velocity is higher than
1.5 m/s. They also found that for an L/D of 3, it is impossible to remove the residue if the main-pipe velocity is lower than
0.7 m/s (see Figure 5).
Figure 6: A hard-to-clean pocket can be created between two metal parts an an O-ring seal.
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Pockets and crevices.
No rule of thumb governs the depth of pockets and crevices. Figure 6 shows a typical crevice found in pharmaceutical systems.
Many guidances state that crevices should be avoided or eliminated when possible, a statement that seems weak considering
that a crevice could be likened to a dead leg with an L/D measurement of 50–100, compared with the normal 2–3. Following Haga
et al., it would be impossible to achieve the velocity required to clean the bottom of a crevice. Thus, pockets and crevices
should not exist in pharmaceutical systems because they will always pose a major contamination risk.
Figure 7: Typical air pocket.
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Air pockets.
Air pockets may be described as upside-down dead legs or crevices (see Figure 7). Although residues do not collect in an air
pocket, they stick to its surface. It is difficult to evacuate the air from these pockets during the cleaning process, which
means that the cleaning liquid will not reach the top of the air pocket and, accordingly, will not clean it. Air pockets,
therefore, must be eliminated, or they will introduce a high risk of contamination.
Surface finish.
Surface finish is often considered a measurement of hygienic design. The maxim is that the smoother the surface, the more
hygienic and easy to clean. But this principle is, in fact, open to debate. A 2003 study by Hilbert et al. tested the adherence
of bacteria to several surfaces and the cleanability of these surfaces (2). The surfaces, from 0.1 μm electro-polished to
0.8 μm mechanically polished, showed no differences in adherence or cleanability. The main reason was the relatively large
size of the individual bacteria compared with the small size of the surface imperfections. As long as the surface finish is
below Ra 0.8–1.0 μm, the bacteria are too large to get trapped between the surface imperfections.
In another study, however, Riedewald showed that when bacteria accumulate in a biofilm, adherence and cleanability depend
on the surface finish (3). It is hard for biofilm to attach to a smooth surface, and thus it is easy to detach them from such
a surface.
The same is true for other sticky residues. A study at the Institute of Technology in Kolding, Denmark, tested the cleanability
of surfaces spiked with a yogurt solution that had been oven-dried (4). This study clearly showed that a surface with a low
Ra value was easier to clean than one with a high Ra value. The tested surfaces ranged from Ra 0.15 to 2.4 μm. Electropolished
surfaces also were easier to clean than mechanically polished surfaces, which, in turn, are easier to clean than pickled surfaces.
Equipment designed correctly will avoid the above pitfalls, thus facilitating safe and cost-efficient cleaning. The more cleaning
action is applied on all product-contact surfaces, the easier, safer, and quicker system cleaning will be.
Per-Åke Ohlsson is the global manager for Alfa Laval's Market Unit Pharma & Personal Care, Alfa Laval Lund AB, Box 74, Rudeboksvägen 1, SE-221
00 Lund, Sweden, tel. 46 46 36 74 18, perake.ohlsson@alfalaval.com
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References
1. R. Haga et al., Pharm. Eng.
17 (5), 8–21 (1997).
2. L.R. Hilbert et al., Int. Biodeterior. Biodegradation
52 (3), 175–185 (2003).
3. F. Riedewald, PDA J. Pharm. Sci. Technol.
60 (3), 164–171 (2006).
4. D. Bagge-Rawn, Microbial Adhesion and Biofilm Formation in the Food Processing Industry (Technical University of Denmark, Kolding, Denmark, 2007).
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