Strong cleaning action on all product-contact surfaces not only minimizes the risks of contamination and of system malfunction, but also enables cost-efficient cleaning. Dead legs, pockets and crevices, air pockets, and equipment surfaces are common design pitfalls, often seen in the pharmaceutical industry, that impair equipment cleanability.
It is widely understood that dead legs should be avoided or minimized in a system (see Figure 1). Some guidances state that the L/D (i.e., length/diameter) measurement for dead legs should not be more than 2—and, in some cases, not more than 3 (see Figure 1). However, the relation between the main-pipe velocity and the L/D measurement 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 1: Dead leg. (All figures are courtesy of the author).
Figure 2: Velocity and L/D measurement.
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 2).
No rule of thumb governs the depth of pockets and crevices. Figure 3 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 an astonishing 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 3: In many pharmaceutical systems, a hard-to-clean pocket is created between two metal parts and an O-ring seal.
Air pockets may be described as upside-down dead legs or crevices (see Figure 4). 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.
Figure 4: Typical air pocket.
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 it is. 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 a roughness average (Ra) of 0.8–1.0 µm, the bacteria are too large to get trapped between the surface imperfections.
However, in another study, Riedewald showed that when bacteria accumulate in a biofilm, adherence and cleanability depend on the surface finish (3). It is harder for biofilms to attach to a smooth surface, and it thus is easier 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 of 0.15–2.4 µm. Electropolished surfaces also were easier to clean than mechanically polished surfaces, which in turn were 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 that is applied on all product-contact surfaces, the easier and quicker system cleaning will be.
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).
Per-Åke Ohlsson is global manager for Alfa Laval’s Pharma and Personal Care Market Unit Box 74, Rudeboksvägen 1, SE-221 00 Lund, Sweden, tel. +46 46 36 74 18, firstname.lastname@example.org.