Dead zones

Dead zones are places in an installation where product collects and does not flow with the rest, and which water and cleaning agents do not reach during cleaning.

In a well-designed installation, product flows continuously, and once production ends it can be completely removed and every part of the pipes and tanks thoroughly cleaned. A dead zone is a place that “drops out” of this flow. Product flows into it but does not flow out again, so it stays there for a long time.

Dead zones most often occur:

• in pipe branches closed off with a blind cap or a closed valve (known as dead legs): the flow runs through the main pipe, while the short side branch remains full of standing product,
• in places left over from old modifications: sections of pipe that once served a purpose but were cut off and capped after changes to the installation,
• in poorly selected valves, fittings and connectors that have crevices or gaps inside,
• in pipes without a slope, from which product does not drain completely when emptied,
• in tanks: in corners, around agitator supports or in places that the spray from the cleaning head does not reach.

Why this is a problem:

• Microbiological risk: bacteria, yeasts and moulds grow in the standing product and then get into subsequent batches and contaminate them. The product may spoil faster or change in taste or smell, and in extreme cases it has to be recalled.
• Cross-contamination between products: residues of the previous product can end up in the next one, such as a different colour, taste or smell, and in the food industry also an allergen.
• Ineffective cleaning: even the best cleaning system cannot clean a place that the cleaning solution does not flow through.
• Product losses: some of the raw material stays in the installation and ends up as waste.

That is why installations in the food, pet food and cosmetics industries are designed to avoid dead zones: pipe branches are kept as short as possible (hygienic design guidelines such as ASME BPE recommend that a side branch be no longer than two pipe diameters), hygienic valves and fittings without crevices are used, pipes are installed with a slope, and redundant sections of the installation are removed rather than capped.

Example: In a sweetened beverage plant, a short branch leads off the main sugar syrup line to a sampling valve. The valve is closed most of the time, so the syrup in the branch stands still. During cleaning, the cleaning solution flows through the main pipe and does not reach the end of the branch. Sweet syrup is an ideal environment for yeast, so after a few days it starts to multiply there and gets into subsequent batches of the beverage. The drink begins to ferment in the bottles, and the manufacturer has to recall the batch from shops. The solution is to use a valve mounted directly in the pipe wall, without a branch, so that the flow washes over it during every production run and every cleaning cycle.