contact surfaces thereforeshouldbe free of crevices,pitting,pinholes and any
hairline cracking that can causematerial penetrationand cleaningdifficulties.
A roughness standard ofRa 0 : 8 m is recommended where thereis a risk
of microbial growthassociatedwithhighmoisture contentin the dry material or
wet cleaning.As the surfaceroughness of cast materials and carbonsteelsdoes
not meetthe recommended figureabove,the cleanability of the components
made withthese materials requirefurtherinvestigation in relationto the actual
dry material beinghandled.
In order to carry out a dry cleaning operation,contact surfacesshouldbe fully
accessiblefor safemanual cleaning and inspection.In orderto carryout a
hygienic wet cleaningoperation,contactsurfacesshould not be horizontal,but
havea slightslope to facilitatedrainability of cleaningsolutions.
The possibilityfor product contacton sharp internalcorners(r6 mm) and
recesses,etc., wheredry material can accumulate, shouldbe avoided.Windows
and inspectionportsmounted in productcontact surfaces should be flushwith
the surrounding surfacesto minimise dry material build-up.Whenusingnon-
metallic materials as contactsurfaces, the porosity of the materials should be
investigatedwithregardto theireaseof cleanability.
13.4.3 Staticseals(gaskets) for ductand flange connections
Staticsealsshouldbe of an elasticmaterial, havea non-poroussurface and be
cleanable.Theyshould be mounted to create a flush surfacewithoutany crevice
withthe surrounding metallic body(Fig. 13.1a).The seal material should be
abrasion-resistant to the dry material being handled. In the case of dry
processingand dry cleaning only,closedcell-foamednon-absorbingmaterials
for gaskets or sealscan be applied. Openfoammaterialis not allowed.Static
sealsshouldbe clean beforeassemblyand the possibilityfor penetrationof dry
material into the gasket or seal duringequipment operationshould be avoided.
Misalignmentof ducts shouldbe avoided as dry materialscan be trappedon
the misaligned ridges(Fig. 13.1b). Assemblyof sealsand gaskets for vessels of
large diameter require special attention to prevent operational problems,
especiallyair and liquid(washing)leakageand material dustemissionsto
atmosphere.
PTFEcan be usedas a staticseal in combinationwithan elastomer (food
grade, FDAor EFSAapproved or nationalequivalent).The PTFEshouldbe of
high-density resilient quality. Metal-to-metal contact duct assemblies (Fig.
13.1c)and paper-typegaskets betweenflangescan be appliedwherea plant
operates at atmospheric pressureand requires no wet cleaning.
13.4.4 Flexibleconnections
One of the biggesthygienicdesignconcerns in the dry materials handlingarea is
that of the flexibleconnectionsin process lines.Flexible connectionsbetween
ductendsare alwaysliableto causedry material build-up between the flexible
224 Handbookof hygiene controlin the foodindustry