Hygienic Design and Installation of Agitators in Closed Vessels
The traditional arrangement of agitators is a driveshaft with an overhead drive unit and impeller blades mounted on the shaft. A wide variety of blade designs are used and typically the blades cover about two-thirds of the diameter of the vessel.
Hygienic Design of Permanently Installed Agitators
Top entering agitators with shaft seals are typically mounted to a vessel using a flanged or hygienic clamp connection, with hygienic O-rings or gaskets to seal between the mating surfaces. Agitators and agitator shaft assemblies passing through the seals shall be designed and constructed to be smooth, with all surfaces meeting all the hygienic design criteria applicable to a product contact area. Agitator ends shall have surfaces of minimum area immediately adjacent to the recipient ends and no longer than necessary to ensure proper incorporation of ingredients into a mix. The design of agitator product contact parts should minimize the occurrence of crevices and sharp corners, and be free of pockets, screw threads, void spaces, and dead spaces in grooves (Fig. 7.15).
FIGURE 7.15 Install hygienically designed agitators, free of pockets, sharp corners, crevices, screw threads, etc. Courtesy of Post Mixing Optimizations and Solutions, LLC.
FIGURE 7.16 (A) The hub (2) is secured to the shaft (1) by means of a screw (3), which is
exposed to product that may collect in and around the screw head. The hub-to-shaft connection gives rise to a metal-to-metal joint (60) that may permit the ingress of product and bacteria. Agitator blades (4) should be welded to the hub, although screw connections are sometimes observed. These exposed screw heads (even bolts with dome head nuts and washers of suitable food grade material) again will create a food safety hazard, and the blade-to-hub connection gives rise to a new metal-to-metal joint (6). To avoid the latter problem, the joint between the blade and the lug on the hub can be sealed by a thin gasket. Keyways (5) exposed to product are not recommended, because product and microorganisms may be retained in the keyway. Keyways may require additional design and/or cleaning practice to ensure drainage and cleanability, e.g., spray ball and wand additions, increased CIP flow and adjusted spray coverage. (B) Once the hub (2) is secured to the shaft (1), an end cap (impeller nut, 7) is screwed on the interior male thread end of the shaft. The nonwelded impeller hub-to-shaft and hub-to-end cap connections give rise to crevices and metal-to-metal joints (respectively 6 and 6") that may allow the ingress of product and bacteria. In that way, the keyway (5) may retain product and microorganisms. The sharp corners of the spanner flats (8) on the end cap may be difficult to clean (Hauser et al., 2004b; Moerman and Kastelein, 2014).
Less suitable, not recommended designs are those in which hub, impeller blades and end nut are assembled together by screw joints (bolting) (Figs. 7.16A and 7.17A). Debris may collect on exposed screw threads, even if bolts with dome head nuts and washers of suitable food grade material are
FIGURE 7.17 (A) The hub is secured to the shaft by means of bolts with dome head nuts.
However, product may collect in and around the screw head (red arrow). (B) This nonwelded hub-to-shaft joint also lacks a food grade gasket that could seal the dead spaces in the groove and avoid crevices at points of metal-to-metal contact. Ingress and accumulation of product and/ or microorganisms at the inside (red arrow) are shown. Welds also have a high degree of roughness. Courtesy of Burggraaf & Partners B.V.
FIGURE 7.18 The hub (2) with blades (3) is secured to the shaft (1). All voids should be closed by either fabrication (welding) or approved sealing techniques (O-rings, seals, etc.) to give surfaces ground flush and free of crevices at points of metal-to-metal contact. (A) Food quality gaskets under controlled compression respectively may seal the propeller hub to the shaft (4) and end cap (40). Keyways (5), where employed due to mechanical design considerations, shall have edge radii not less than 3 mm. The corners of the spanner flats (6) on the end cap have been radiused. (B) An all-welded impeller assembly (e.g., hub, blades, end cap) is still preferred. Impeller hubs welded to the shaft are preferred over removable hubs. Although the designer may omit the hub, and immediately can attach the blades to the shaft by welding (30). Finally, also the end cap (70) can be welded to the shaft (Hauser et al., 2004b; Moerman and Kastelein, 2014).
used. Metal-to-metal joints (e.g., keyways, hub-to-shaft joints, hub-to-end cap joints, etc.) may allow ingress and accumulation of product and/or microorganisms (Figs. 7.16B and 7.17B). Food quality gaskets under controlled compression may seal the propeller hub to the shaft and to the impeller nut (end cap) that secures the end of the agitator shaft (Fig. 7.18A). Alternatively, the hub should be welded to the shaft and the end cap (Fig. 7.18B), with the blades of appendages (stirrers, homogenizers, mixers, etc.) welded to the hub. As an alternative to impeller blade-to-hub attachment, blades can be immediately attached to shafts by welding (without hub) (Fig. 7.19). In this all-welded one-piece design, all welds must be ground flush and polished.
Permanently joined metal surfaces with a total included internal angle less than 135 degrees on agitators (e.g., at hubs and nuts) shall have a radius of not less than 3 mm tangential to both adjacent surfaces. Corners (e.g., at hubs, nuts, spanner flats, etc.) must be radiused to facilitate cleaning, and horizontal areas must be sloped to prevent debris from becoming lodged on the surfaces and to allow for maximum drainabilty. Machined transitions such as shaft steps, coupling surfaces, spanner flats, etc. should have 15- to 45-degree sloped surfaces. Impellers with flat, horizontal surfaces (e.g., flat-blade disc turbines, concave-blade disc turbines) may require additional design and/or cleaning practice to ensure drainage and cleanability, e.g., drain holes, spray ball and/or wand additions, increased CIP flow, adjusted spray coverage and faster impeller rotation. Where permanently installed agitators are equipped with an outer frame to which rubber, plastic or other similar scraping edges (Fig. 7.20) are attached, these scrapers shall be readily removable from the agitator. They should be regularly checked for integrity. Cases are known where plastic scrapers were broken and pieces lost in the product as a foreign-body contaminant.
Welded in-tank shaft connections are preferred, although in-tank shaft couplings (Figs. 7.21A—C and 7.22A—B) and in-tank threaded shaft connections (Fig. 7.22C) are allowed if they are of acceptable hygienic design. Threaded shaft connections are preferred over in-tank shaft couplings, although shaft rotation of the first is limited to a single direction to avoid the shaft
FIGURE 7.19 All-welded impeller assembly (e.g., hubs, blades, end cap). The agitator paddle blades being attached to the shaft by welding must have their welds ground and polished. Photo right, courtesy of Intechwell/Alfa Laval AB.
FIGURE 7.20 This agitator may cause hygiene problems because of the bolts and plastic scrapers. Cases are known where plastic scrappers were broken and pieces lost in the product as a foreign body contaminant.
FIGURE 7.21 (A) Bolted agitator couplings with flat hexagon head screws without elastomer gasket under the bolt head and the nut give rise to metal-to-metal crevices (1) that may allow the ingress of food product and bacteria. Moreover, debris may lodge in and around the bolt thread (2). The absence of a circumferential O-ring or flat gasket gives rise to another metal-to-metal crevice, and product and microorganisms may be retained in the cavity (3). (B, C) Agitator couplings made by means of domed hexagon bolt heads and nuts (4) provided with an elastomer gasket (5) under the bolt head and the nut allow for a crevice free joint without metal-to-metal contact. Due to the presence of a circumferential O-ring (6) or flat gasket (7), no product and microorganisms can enter inside the agitator coupling. Corners are radiused (8). However, there is still a horizontal flat surface at the upper side of the agitator coupling where debris may lodge (Hauser et al., 2004b; ASME BPE committee, 2014; Moerman and Kastelein, 2014).
sections separating. The designer must ensure that the use of a threaded shaft connection is appropriate for the selected shaft diameter and design loads. To avoid exposure of the threads to the product, O-rings or flat gaskets (preference for the first mentioned) should be used to seal mating surfaces (Fig. 7.22C). Hygienic bolted coupling construction may be used where appropriate for the
FIGURE 7.22 (A, B) Aseptic applications require the fastening of hardware at the bottom side
of the agitator coupling. Agitator couplings made by means of domed hexagon bolt heads and nuts (1) provided with an elastomer gasket (2) under the bolt head and the nut allow for a crevice-free joint without metal-to-metal contact. Due to the presence of a circumferential O-ring (3), no product and microorganisms can enter the inside of the agitator coupling. The upper parts of the coupling should be sloped to a minimum 15—45 degrees (4) to prevent debris from collecting at these places and to allow for maximum drainability. (c) The optimal agitator coupling in an aseptic environment is a threaded shaft connection with O-rings or flat gasket (preference for the first mentioned) (3) to seal the mating surfaces to avoid exposure of the interior thread. The corners of the spanner flats on the end cap have been radiused (5) (ASME BPE committee, 2014; Moerman and Kastelein, 2014).
particular application. The preferred location for fastening hardware is on the underside of couplings, and the fasteners typically used should be hex-head cap screws, acorn-head cap screws and threaded studs with acorn nuts (Fig. 7.22A). These fastener heads shall be free of raised or engraved markings that might inhibit cleanability. Again, O-rings or flat gaskets (preference for the first mentioned) should be used to seal coupling mating surfaces. Elastomer seal washers (Figs. 7.21B—C and 7.22A) must avoid metal-to-metal contact.