Interior and Exterior Design of Closed Vessels
Vessel and appurtenances must be designed to facilitate the tank cleaning process (ASME BPE committee, 2014):
• Vessels need to be designed with smooth, straight walls and curved corners that can be cleaned easily by liquid spray produced by stationary (e.g., static spray balls) or rotary cleaning devices. Corners shall be well-rounded, with a radius equal to or larger than 3 mm (Fig. 7.5).
FIGURE 7.5 Full drainage of products and cleaning solutions is required. If discharge outlets are above the lowest level of the closed vessel, self-draining will be hampered, and residual products and cleaning solutions (2) will be left in the vessel. Closed vessels must be self-draining with discharge outlets at the lowest level. Bottoms must be sloped and all corners should be well-rounded (Hauser et al., 2004b).
FIGURE 7.6 The tank-cleaning process by means of tank-cleaning devices can be facilitated by applying short-neck nozzles, which means tank head ports with reduced L/D ratios, short in length and large in diameter. During the cleaning action of the tank-cleaning devices, internal shadows in the top nozzles can be reduced to a certain extent if the lower part of these top nozzles is sloped toward the center of the vessel.
- • Junctions between vessel and piping must be smooth, flush and without crevices.
- • Closed vessels with bottom outlets must have their discharge outlet at the lowest level and their bottom shall be sloped (Fig. 7.5). Tank outlets should be flush with interior surfaces and self-draining.
- • Flat top surfaces should pitch 4% from center to sidewalls to encourage the continuous flow of cleaning solutions sprayed on these surfaces toward the side walls.
- • Death corners in the top of the vessel or tank should be eliminated. Difficult to clean areas are the annular space between the neck of the top nozzles in the tank head and agitator shafts, as well as down pipes, installed in the tank by means of an exterior tank connection. The ratio of nozzle neck length to annular space gap width should be # 2:1.
- • Short-neck ports must be used, which means tank head ports with reduced L/D ratios. To avoid a dead leg, the maximum recommended length to tank head port diameter ratio shall be two-to-one. Top nozzles should preferably be flush with the tank wall (Fig. 7.6).
FIGURE 7.7 Use sloped side ports, rather than ports perpendicular to the vessel wall.
- • The depth of manways must be reduced to avoid interior shadows, especially because they are harder to clean and a source of possible contamination.
- • Sloping the lower part of the top nozzles toward the center of the vessel may eliminate shadows and provide the tank cleaning devices good “sight” angles into the top nozzles (Fig. 7.6).
- • For maximum cleanability, side-wall sensor ports must be provided with a slope (5-degree angle) (Fig. 7.7), rather than ports perpendicular to the vessel wall.
- • Eliminate dead corners in lower tank parts.
- • To provide a reasonable flow across the tank bottom surfaces for moving suspended solids, the bottom of flat vessels should pitch no less than 2% from rear to front outlet, and 4% from side to center outlet for round bottom vessels.
- • A probe (e.g., pH sensor) in the reactor wall shall be inserted in a sloped side port, with an O-ring seal to prevent the ingress of soil into the sensor port and the probe. An elastomeric O-ring seal should be placed as close as possible to the vessel wall so that only a short crevice is formed. When this seal is placed at the entrance of the port (end opposite to the tank wall), then a long and uncleanable large crevice is formed. Where cleaning relies on a free falling film, protrusion of stationary parts like sensor probes in a vessel wall should be avoided. They may form a shadow area during cleaning (Fig. 7.8).
- • Baffles only partially fastened onto the side wall of the tank should be used instead of full-length fastened baffles. The internal support members to fasten the baffles to the tank wall must be made from solid round bar stock having a downward slope of 5 degrees (Fig. 7.9). When gaps are left between the baffle and tank wall, the flow allows the baffles and the tank wall to be cleaned more easily. Recommended gaps between the baffles and the vessel wall are equal to 1/72 of the internal vessel diameter, and 1/4 to 1 full baffle width between the bottom of the baffles and the vessel base. Instead of full-length baffles, the use of baffles can be limited to the lower part of the tank or the tank may be provided with intermittent
FIGURE 7.8 Probes shall be inserted in sloped, welded-in side ports, the weld being polished to obtain a surface finish comparable to that of the original finished metal. (A) An elastomeric seal at the entrance of the port (end opposite to the tank wall) gives rise to an uncleanable long and large crevice between the interior surface of the sensor port and the outside probe surface. (B) Protrusion of probes in a vessel wall should be avoided, as they may form a shadow area during cleaning. (C) When the elastomeric O-ring seal is placed close to the vessel wall, only a short crevice is formed.
FIGURE 7.9 The internal support members to fasten the baffles to the tank wall must be made from solid round bar stock having a downward slope of 5 degrees. When gaps are left between baffle and tank wall, the flow allows the baffles and the tank wall to be cleaned more easily (ASME BPE committee, © 2014).
baffles (underbroken baffle, resulting in two shorter baffles, one below the other), without loss in agitation efficiency. Baffles can be omitted in small tanks (<500 L), and in designs where the agitator is mounted off-center and angled at the same time. Where material can hang up or becomes trapped in stagnant regions around the baffles during drainage, profiled baffles instead of flat-plate baffles are recommended (Myers et al., 2002; ASME BPE committee, 2014).
• Suitable covers or lids must be provided and should be made of the same material as the vessel. Covers on smaller vessels should be close fitting and easily removable for cleaning. Covers on such vessels should preferably be unhinged since the hinge, especially piano hinges (Fig. 7.10), can collect dust and food debris which might fall into the product when the cover is opened. Where hinges are used they should pivot sufficiently
FIGURE 7.10 The piano hinge prohibits removal of the cover, and allows food residues to accumulate in the hinge. In the piano, microorganisms may find a niche to grow (Don Graham, Graham Sanitary Design Consulting LCC, ©2010).
outwards to avoid product contamination (Fig. 7.11). For maximum cleanability, hinges should be removable and, where possible, consist of a simple hook-on type without bolts. Larger vessels are usually fitted with manholes, either in the top or side, which preferably should be ca. 90 cm in diameter for easy access. Mandoor covers intended to protect the food products may accumulate dirt or liquids on top of the lid while in a closed/horizontal position. If these mandoor covers are not correctly designed and mounted, these liquids and dirt may slide into the vessel opening and, respectively, spill and fall into the product when the lid is opened. Policy should specify that no tank is opened during production unless absolutely necessary. Correct design and mounting of covers must prevent soil and/or liquids from dripping/falling into the product during the opening of the cover. In Fig. 7.11A, the cover protecting the vessel opening is at the back side provided with a sloped edge, draining any dirt or liquids away from the vessel opening. In Fig. 7.11B, drip of soil and liquids into the food product is prevented by the curved edges at the left and right side of the bolted flat cover plate. Seals must be of a removable type (Fig. 7.12) to allow for inspection, cleaning, and replacement.
- • Instead of a weld-on top surface, vessels, bins, etc. also can be closed with a (detachable) lid. Flat lids provide a horizontal surface (Fig. 7.13A) where dirt may accumulate. Hence, where nonremovable lids are used, preference should be given to domed lids with sloped tops that collect less dirt and allow for proper drainage of liquids (Fig. 7.13B). When the vessel is permanently covered by a lid, no sharp top corner (junction vessel wall—lid), which may hamper the cleanability, should be created.
- • Conventionally designed right-angled grooves containing O-rings invariably create gaps and crevices that are impossible to clean in-place and/or to sterilize in-line (Fig. 7.14). One cause is that the elastomer material of the O-ring has a significantly higher thermal expansion coefficient than
FIGURE 7.11 Correct design and mounting of covers must prevent any soil and/or liquids from accumulating on the cover while in a horizontal position, as they may fall into the product during the opening of the cover. (A) The cover is at the back side provided with a sloped edge (arrow), draining any dirt or liquids away from the vessel opening (Frank Moerman, © 2016). The pin hinges are less sensitive to the accumulation of debris. (B) In this example, drip of soil and liquids into the food product is prevented by the curved edges at the left and right side of the bolted flat cover plate. Pin hinges prevent the buildup of dirt. Courtesy of Fineweld Stainless Steel Pty Ltd.
FIGURE 7.12 Seals for mandoor covers must be of a removable type to allow for inspection, cleaning and replacement. Food debris may accumulate under the seal, providing nutrients for microorganisms. Courtesy of Burggraaf & Partners B.V.
FIGURE 7.13 (A) Covers are used (e.g., for process vessels, tanks, bins, etc.) to avoid contam
ination of food product (1) from the environment during processing or storage. When the vessel (2) is covered with a flat lid (3), a horizontal surface is provided where dirt may accumulate. Moreover, a sharp corner (4) is created at the top near the seal. This seal (5) is not very appropriate because overcompression may lead to protrusion of the seal in the product area, thereby impeding cleaning, while undercompression may lead to both indentations and crevices and failure to provide a reliable seal. Even when it is not visibly leaking, the seal may permit the ingress of microorganisms. (B) Preference should be given to domed lids (30) with a sloped top that collect less dirt and allow for proper drainage of liquids. The present sloped gasket groove allows for controlled compression of the gasket (50) at the product side while providing space for expansion at the nonproduct side (Lelieveld et al., 2003; Hauser et al., 2007).
steel. During heating the seal will expand to cover an increasingly larger surface of steel, protecting microorganisms trapped between the O-ring and the steel surface against contact with hot water, chemical solution or steam. Although the seal contact surface will usually reach the correct temperature during treatment with hot water or steam, the water activity in the grooves will be too low for the destruction of many microorganisms at the temperature and time applied. After cooling down and shrinkage of the seal, the surviving microorganisms may be released and will multiply and
FIGURE 7.14 (A) A conventionally designed right-angled groove (2) contains an O-ring
(3) that is compressed between the sealing faces of two stainless steel surfaces (4) to separate the product area (1) from the outside. (B) Such a rectangular groove—O-ring design invariably create gaps and crevices (5) that are impossible to clean in-place and/or to sterilize in-place. The groove provides sufficient space for microorganisms (6) to enter via the crevice. (C) During heating, due to the difference in thermal expansion between metals and elastomers, the O-ring will expand (7) to cover an increasingly larger surface of steel, protecting microorganisms (8) trapped between the O-ring and the steel surface against contact with hot water, chemical solution or steam. (D) After cooling down and shrinkage of the seal, the surviving microorganisms may be released (9) and will multiply and contaminate the product (Lelieveld et al., 2003; Hauser et al., 2007).
contaminate the product. Additionally, repeated thermal expansion of the seal into the product flow may result in damage which will not only contaminate the product but may also progressively reduce its ability to seal again upon recooling.