How to Effectively Clean Pipelines: 

The fuzz about Dead spaces and Biofilm in pipes

Any process line consists of a series of equipment components linked together by pipework. There are many equipment components used in the food industry and this should be properly cleaned at the end of the production.  One of the many process equipment are the pipelines, and the only way to clean pipelines is through an effective CIP system. 

Turbulence in the cleaning solution would probably be around 75% of the CIP regime while 25% would be attributed to the chemistry behind the cleaning detergent.  Mechanical energy (turbulence) is generated by a pump.  This energy in the form of pressure energy is dissipated in the form of friction and turbulence inside the pipe to be cleaned. For CIP, the mechanical energy required is less than for the manual cleaning. Actions of manual cleaning, including scratching and brushing, require more energy than pumping. The mechanical energy is also more constant since CIP is not subjected to human intervention.

Dead ends/dead legs/dead spots on the other hand are areas outside the main bulk product flow where product can accumulate at the beginning of production, and may remain for an extended period of time. If the environmental conditions, such as temperature, are favorable, microbial growth may occur, resulting in recontamination of the bulk flow. It may also be difficult to clean and disinfect or sterilize such an area after production has finished, owing to the lack of fluid movement within the dead space. Most such problems occur as a result of the physical assembly, installation and integration of individual equipment components with connecting pipework: examples are T sections in pipes, incorrect orientation of equipment, and installation of sensors into pipework.

It is essential that the length of the dead ends do not exceed the equivalent of 2 times the pipe diameter when measured from the centre line of the pipe.  If measuring from the outside of the pipe then a rule of 1.5 times the diameter should be used. This is depicted in the diagram:

A process pipe work dead leg, is any “T” piece where the tee pipe length is more than ½ a pipe diameter in the “non flow” direction, or 1 pipe diameter in the “flow” direction.  However, in a case that a branch pipe is installed along the main process pipe, then the branch pipe will follow the rule of >2.70 (3.0) times the pipe stem diameter of the side branch pipe is a dead leg.

If the dead leg exceeds these dimensions there will not be sufficient CIP mechanical action in the dead leg to remove soiling, and the sanitizer may not be able to contact the surfaces during the disinfection step, causing contamination in the next product processed. If the dead legs are excessively long, or configured downwards, then they may remain full of “stagnant” product.  Biofilm formation inside the pipe dead space will occur, and this we don't want to happen in our factory.  An example of a biofilm formation inside a pipe branch: 

A common error or misconception is to assume that a dead space will still be capable of being thermally disinfected or sterilized owing to heat penetrating into the stagnant zone even if the fluid cannot. A recent study by Asteriadou et al. (2006) has shown that, even with turbulent flow within a horizontal pipeline, the temperature within a vertical dead leg will fall rapidly with distance from the junction between the horizontal and vertical sections. This will potentially compromise the thermal process owing to a failure to reach the required minimum conditions for proper cleaning and disinfection, hence the required contact time and temperatures are not achieved.

Examples of uncleanable installations in the factory.  In order for these pipes to be properly cleaned and disinfected, eliminating the dead legs would only be wise or reconfiguration or reinstallation of the pipework/valve/instruments and ensure that hygienic standards are observed.

Ensuring that the CIP system is free from dead spaces or dead legs without the correct turbulence of the cleaning solution will not guarantee the cleanliness of the pipelines.  One should achieve at least the optimum flow velocity of the cleaning solution passing through the walls of the pipeline.  There are generally two types of circuit within which lengths of pipework have to be cleaned:  first, where the circuit is predominantly pipework such as product transfer lines; and second, where the pipework is only a minor part of the circuit. In either case the user will not be concerned solely with cleaning straight runs of pipe, but also with more complex geometries, including joints, bends, tees and valves. The design of the cleaning system should be based as far as possible on 1.50 m/sec (optimal), which results in substantial flowrates for larger-diameter pipework, in other words design the CIP flow requirements basing from the largest pipe in the CIP system.

Hygienically designed pipework can usually be satisfactorily cleaned at lower velocities, at the expense of an increased cleaning time.