HYGIENIC DESIGN OF PROCESS AND UTILITY PIPING
Drainable Process and Utility Lines Without Dead Ends
Pipes must ensure minimum resistance to flow, and therefore there should be no sudden changes in cross-sectional area or obstructions that are likely to hinder the flow. However, valves and flowmeters may restrict the flow. Pipes also must be hygienically designed and meet standards that are required nationally and internationally. Consideration of CIP should be integrated into the mechanical and process design at an early stage, rather then being incorporated into an already fully specified plant. Hence, pipework must be so designed that as far as possible a minimum velocity of 1.5 m/s is achieved over the whole trajection of pipes, unless data for the specific soil indicate otherwise. As a consequence, substantial flowrates for larger- diameter pipework are required. Failure to achieve 1.5 m/s does not necessarily mean that effective cleaning cannot be achieved, but the process is likely to be nonoptimal.
To avoid the formation of standing “pools” of liquid that can support the growth of microorganisms, process and utility piping runs should be sloped to at least 3% in the direction of flow and should be properly supported to prevent sagging (Fig. 7.27). Nondrainable pipe sections are not allowed (Fig. 7.28). Where valves are fitted, additional support must be given. Where plastic piping is installed, special care should be taken to avoid sagging by increasing the frequency of support. Pipelines and valves should be supported independently of other equipment to reduce the chance of strain and damage to the equipment, pipework, and joints.
A properly designed food processing line must not have dead legs, as blanked-off tees constitute a hazard. A dead space, being an area outside the product flow where liquid or gas can become stagnant and where water is not exchanged during flushing, is formed. An air pocket may be present if the branch of a blanked-off tee is pointing vertically upwards (Fig. 7.29A). Hence it will prevent liquids (cleaning solutions, disinfectant solutions, or hot water) from reaching all surfaces to be treated, with the result that CIP and
FIGURE 7.27 Pipes must be completely self-draining. (A) Sagging of piping must be avoided because standing “pools” of liquid can support the growth of microorganisms. Changes in the level of horizontal runs of pipelines should be avoided; otherwise, there will be an undrainable section. Horizontal runs of pipe which are routed vertically up and then down to bypass beams, doorways or other obstructions will allow air to collect in the raised section. (B) Process and utility piping runs should be pitched at least 3% in the direction of flow. Piping must be installed in a way that air doesn’t collect in the raised section. While automatic air release valves can be installed (on top of elevated horizontal pipe sections) to remove trapped air, the resulting dead leg may cause contamination and/or cleaning problems. Where liquid collects in a lower horizontal pipe section, fitting a valve in a shortened tee allows liquid to be drained (CFCRA, 1997). Courtesy of Campden BRI.
FIGURE 7.28 Nondrainable pipe section. Courtesy of Mondelez International, © 2016.
FIGURE 7.29 (A) When cleaning and disinfection solutions (1) flow through the piping, an air
pocket (2) will be formed if the branch of a blanked-off tee is pointing vertically upwards. This will prevent the solutions from wetting the surface in the dead leg. (B) Drain points pointing downwards (3) again act as a dead leg, providing an area of entrapment that may not be reached by cleaning or sterilizing procedures, and hence they may lead to contamination of the product. Moreover, during a hot water treatment, the hot water also will stagnate in the downwards pointing pocket, so that the temperature of the surfaces in the dead area may be lower than required as the consequence of heat loss (4). (C) A downwards pointing dead area also will collect condensate (6) due to heat loss (4) during steam sterilization (5), with the result that again the temperature of the surfaces in the dead area may be lower than required. Blanked-off tees should be positioned such that they are a few degrees above the horizontal (Lelieveld et al., 2003; Hauser et al., 2007).
decontamination processes will be unsatisfactory. Drain points pointing downwards act as a dead leg (Figs. 7.29B and 7.30) and are not acceptable because they provide an area of entrapment that may not be reached by cleaning or sterilization procedures. During a hot water treatment, the hot water also will stagnate in the downwards pointing pocket, so that the temperature of the surfaces in the dead area as a consequence of heat loss may be lower than required. A downwards pointing dead area also will collect condensate during steam sterilization (Fig. 7.29C), with the result that again the temperature of the surfaces in the dead area may be lower than required. As a consequence, the thermal disinfection or sterilization of the dead space is compromised.
FIGURE 7.30 This drain point pointing downwards acts as a dead leg (organization Sanitary Design Workshop, © 2016).
FIGURE 7.31 (A) Impact of the flow velocity and leg geometry on the cleanability of the
dead zone. For an L/D of 6, it is possible to remove food residues adequately even when the main-pipe velocity is higher than 1.5 m/s. When the main pipe velocity is lower than 0.7 m/s, then it is impossible to remove food residues in a T-section with L/D of 3. (b) Moreover, thermal disinfection processes may be compromised due to a failure to reach the required minimum temperature conditions. (B) Dead end inner pipe surface shear stress (Haga et al., 1997).
Even with turbulent flow within the horizontal pipeline, the shear rate and temperature within the dead leg will fall rapidly with distance from the junction between the horizontal and vertical sections (Fig. 7.31).
The direction of the flow of food product has a significant influence on the residence time in the dead leg. When the food product flows in the direction as indicated in Fig. 7.32A, B and C, part of the product will stand still in the dead leg, especially if the length or depth of the T-section is too long. If the length of the T-section is equivalent to the diameter of the main pipe, a flow velocity of 2 m/s in the main pipe will already result in a reduced velocity of
FIGURE 7.32 When the food product flows in the direction as indicated in (A, B, and C), part of the product will stand still in the dead leg, especially if the length or depth of the T-section is too long. Long T-sections outside of the main flow of cleaning solutions are also very difficult to clean. For most liquids, the dead leg should be positioned as shown in (D, E, and F). In particular, the configuration in (F) is quite acceptable if l # d, because the flow directed into the short dead leg provides sufficiently high velocities for proper cleaning. If the dead leg is very short (l # d), configuration (D) is acceptable, although flow across a dead leg results in much lower velocities within it and thus only provides moderate cleaning. Configuration (E) may not be suitable, if products contain any particulate matter that may accumulate in the dead leg (CFCRA, 1997; Lelieveld et al., 2003; Hauser et al., 2007).
0.3 m/s in the T-section. This decrease in flow velocity provides a relatively stable pocket or dead leg in which product residues can accumulate and microorganisms begin to multiply. Long T-sections outside of the main flow of cleaning solutions are also very difficult to clean. During cleaning there is much less transfer of thermal (heat), chemical (detergent and disinfectant chemicals) and mechanical energy (action of turbulent flow) to the food residues in the T-sections (outside the main flow of cleaning liquids) than to the soil in the main flow. Notice that flow away from the dead leg such as in Fig. 7.32A and C further gives rise to more contamination problems and worse cleaning, as velocities in these dead legs are even much lower.
For most liquids, the dead leg should be positioned as shown in Fig. 7.32D, E and F. If the dead leg is very short, configuration Fig. 7.32D is acceptable, although flow across a dead leg results in much lower velocities within it and thus prolongs the time for cleaning. Configuration Fig. 7.32E may not be suitable, if products contain any particulate matter, which may accumulate in the dead leg. The configuration in Fig. 7.32F is the most acceptable, because the flow directed into the short dead leg provides sufficiently high velocities for proper cleaning
For pipe diameters of 25 mm or larger, T-sections should have a depth/ length of preferably under 28 mm, while for smaller pipe diameters this length should be smaller than the diameter. Blanked-off tees should be positioned such that they are a few degrees above the horizontal. The dead leg will then be drainable but not necessarily cleanable even if made as short as possible. If a sensor must be installed in a process line, it should be installed in a bend on a shortened tee in a position that the flow of cleaning fluid should be directed into the tee (Fig. 7.32E and F). Where an angle valve is installed in the process piping circuit, this valve also must be mounted in a shortened tee so that no or a minimum of annular space above the side branch is formed. Again, the flow of cleaning solution must be directed into the tee. In all cases, the cleaning procedure must take the presence of the dead leg into account.
Flow diversion should not be done in a way that would cause part of the product to stand still in a dead leg. The two-valve system for flow diversion (Fig. 7.33A) creates a dead leg toward the closed valve. The correct type of valve is shown in Fig. 7.33B.
For horizontal piping, eccentric reducers should be used instead of concentric reducers, because the latter provide a dead spot where condensate and dirt may collect (Fig. 7.34).
FIGURE 7.33 (A) Flow diversion should not cause part of the product (1) to stagnate in a
dead area (2). The system of two butterfly valves (3) for flow diversion creates a dead area (2) toward the closed valve. (B) The correct type of valve is shown on the right (Lelieveld et al., 2003; Hauser et al., 2007).
FIGURE 7.34 Changes in pipe diameter should be made by the use of reducers to ensure a smooth transition of the product flow. (A) In vertical piping, a concentric reducer is fully acceptable for food product to flow. However, this is not the case for horizontal piping, where the concentric reducer prevents full drainage if product flow is in the wrong direction. A dead spot is created where condensate and dirt may collect. (B) For horizontal piping, eccentric reducers are preferred. The reducers should be long enough (2) to avoid shadow zones during product flow (1). If a short eccentric reducer (3) is applied, a potential shadow zone (4) will be created (Lelieveld et al., 2003; Hauser et al., 2007).