The guidelines are written with Europeanconditions, standards and laws in mind.Therefore valves for hygienic or aseptic useshould conform to EHEDG guidelines as follows:

Doc. 8: Hygienic Equipment Design Criteria Doc. 9: Welding Stainless Steel to meet

Hygienic Requirements Doc.10: Hygienic Design of closed Equipment

for the Processing of Liquid Food Doc.13: Hygienic Design of Equipment for

Open Processing Doc.14: Hygienic Design of Valves for Food

Processing Doc.16: Hygienic Pipe Couplings Doc. 20: Hygienic Design and Safe Use of

Double-Seat Mix proof Valves

This will ensure, amongst other things, adequatesurface finish, cleanability and absence of crevicesand sharp corners and absence of metal-to-metalcontact surfaces. Where the valves are powered,they must also conform to the Machinery Directiveand be CE-marked.

Origins of valve-types used in food manufactureHistorically, there has been widespread adoptionand adaptation of existing types of equipmentcomponents, including valves, for food applications.In some cases, usually where the product is resistantto spoilage, this has been relatively successful eventhough the design may be relatively weak inhygienic terms. However, given that such valveswere originally designed neither for hygienicapplications nor with the benefit of modernhygienic design knowledge, variable levels of hygienic success and failure have resulted.

Sliding seal valvesBall, gate, plug and other sliding-seal valveshave been used, but these rely on seat shut-offand, in some cases, body sealing on comparativelylarge sliding seals that may easily become scoredand/or fouled. The sliding seals commonlycontain a thin, stagnant film of product and theremay also be stagnant product in spaces behind thesliding seals. They usually need to be dismantledeach time for cleaning, except perhaps when usedwith very clean products such as water or ethanol.Some manufacturers have adapted them byproviding drainage to these cavities, but it isup to the users to satisfy themselves as to whetherthis really works successfully with their ownproducts and conditions. Generally there arebetter options.

In a special case, sliding seal configurations haveoften been chosen for integral aseptic block andbleed sampling designs, primarily in order to permitsingle-step manual operation of the block-and-bleed, together with the product seat opening.However, the sliding seals are not bacteria-tight,even though this is usually a requirement for asepticperformance. Generally, butt sealing is preferablefor shut off applications.

Metal-to-metal seated valvesAlthough some suppliers have offered thesetypes for hygienic duties, a clear category that isnormally unsafe for hygienic or aseptic dutiesis that based on metal-to-metal seat seals.Such seals are not bacteria-tight and thereforeshould be used only where bacteria-tightness isnot required.

Butterfly valvesButterfly types can be hygienic and cleanable inplace if designed according to EHEDG guidelines.They are useful where a low-cost single or doubleseat solution is required and have the advantage ofoccupying a very short length of pipe-run.However, as they are not hermetically sealed, theyare not suitable for aseptic duties.

new food

8

F O O D P R O C E S S I N G V A L V E S

Hygienic design andassessment Dr. Roland Cocker, Member, EHEDG Valves subgroup

When assessing valves for use in safe foodproduction there are a number of considerations tobe made. This article discusses the implication ofEHEDG guidelines.

new food

10

Weir-type valvesWeir-type diaphragm valves were originallydesigned as an alternative where hermetic sealingwas required. They have been widely adopted foraseptic duties because the diaphragm seal providesa permanent hermetic body seal as well as a seatseal. However, as they were not designed specificallyfor hygienic and aseptic application, care needs tobe exercised with respect to a number of points:

There is usually no fixed compression-stop onthe bonnet fastening screws, so these are commonlyover-tightened. This may lead to shrinkage andhardening (compression-set) of the membrane, withconsequent crevice and/or leak formation. Thecommon fix of re-tightening the fittings to combatleaks leaves the risk of unseen crevices andaggravates the compression-set. In the worst cases, acircular disc is cut out of the rectangularmembrane.

This configuration of valve is often suppliedwith an adjustable compression-stop on the sealtravel to allow for interchangeable fitting ofmembranes of different thickness. This providessignificant opportunity for error that is oftenrealised in practice. In the worst case, themembrane may be punctured by the valve-stem. Itcan be safer to opt for valve tops with fixed stops ifavailable, even though this would mean a change oftop if the membrane were changed to a differentthickness.

In vertical piping, this type of valve is normallyfree draining, but when the pipe axis isapproximately horizontal, there is a criticaltolerance on orientation for drainage past the weir.Typically the valve-stem has to be inclined from thevertical within a narrow arc of tolerance. The exactangle is in the range 60-70 degrees from thehorizontal. Unfortunately, this angle can differaccording to the valve-size, giving furtheropportunity for error where multiple valve-sizes areused in the same installation.

Blocks of steel can be machined-out to providemulti-axis, multi-seat valve blocks, but care needs tobe taken to ensure that all of these seats are freedraining.

Weir-type diaphragm valves are, more than anyother, often supplied and installed with couplingson both sides. Apart from increasing the number ofvulnerable seals, this carries a very high risk that thevalve will be re-installed in the wrong orientation,because the couplings allow rotation outside thesafe angle of inclination.

Pinch valvesPinch-type valves, where a reinforced flexibleelastomeric tube is externally compressed to close it

off, are intrinsically hermetic and thereforepotentially hygienic and aseptic. This type iscompact, can give full-bore flow and has the widestrange of orientations for self-draining. It iscurrently available in a hygienic implementationonly in small sizes. It is not found in multi-seatversions.

Circular seal valvesThe introduction of flush circular seals has allowedvalves to be purpose-designed for hygienic andaseptic applications. The entry of a number ofcompetitors, principally based in Europe, has led toa family of improvements and advantages for thistype and they are now used widely wheredemanding hygienic performance is required, suchas in food, (bio) pharmaceutical and even paintprocessing. Figure One gives an example of adouble-seat variant (the most important variant) ofthis configuration of valve:

Characteristics include: They are self-draining in a wide range of

orientations This is the most flexible design for multiple seat

versions, with unique integration capabilitiesthat permit highly efficient and hygienically safeplants

Because of their flush-fitting seals, multiple seatversions permit the elimination of stagnantzones in a way not possible with single-seatvalves

It is possible to integrate several valve functionsinto a single, factory-made multiple-seat valvemodule, eliminating the vagaries of site-assembled equivalents. An example would be anaseptic block-and-bleed module such as an aseptic barrier

Figure One: Schematic of a Double-Seat Circular Seal Valve

Hermetically sealed aseptic versionsincorporating flexible polymeric diaphragms orstainless steel/polymeric bellows are nowcommonplace

Balanced pressure versions are available thatprevent the valve being forced open by hydraulicshocks. The combination of powerful return springswith fixed compression-stops provides fail-safeoperation without over compression of theelastomeric seals. The use of sensors on the valve-top permits positional feedback, criticallyconfirming when the seat is properly closed.

With double-seat mix-proof types, there must beatmospheric leakage detection. However, this meansthat a small amount of product can be lost at eachchangeover unless the design incorporates axialseals. These are circular seals that maintain a slidingseal during movement, followed by a butt seal atclosure (See EHEDG Doc. 20 Hygienic Design andSafe Use of Double-Seat Mix proof Valves).

Note however the emphasis has been on larger-size piping (>DN50) and that most of thepneumatic actuators currently available are notcompact, relative to the smaller sized versions. Thismay be problematical in confined spaces or wheresmall valves are required. Some more compact non-pneumatic actuators are available which may allowthem to be employed.

Assessment and selection Inadequate selection practiceIn real life, the choice of valves for hygienicapplications may often be made on the basis ofcustom and practice, tradition or even dogmarather than on a systematic technical and hygienicappraisal. This can result in designs that arerelatively expensive to run and maintain and havefailure-rates well below best practice.

To counter this and to make assessment andselection more objective and scientific, the EHEDG

Figure Two: Aseptic Mix proof Valve

new food

12

has pioneered the provision of guidance on hygienicdesign and the establishment of standardisedchallenge tests and corresponding certificationfacilities.

Concurrent designHygienic considerations need to be madeconcurrently with all other legal, design andperformance requirements when assessing valves forhygienic use, so that the valves fulfil the global userspecification as comprehensively as possible.

Valves for dry materialsValves for use with dry materials may follow amuch more relaxed set of design rules. For examplethey often have extensive sliding contact surfaces.They will not be considered here, but note that ifany valve is expected to be wetted at any time, forexample cleaning between successive batches toremove a strong flavour or to Kosherize, then allthe considerations of this article apply.

Defining the required hygienic performance levelAn early consideration determining theperformance specification is whether aseptic orhygienic performance is needed. (It may be that avalve for use in food production etc. does not evenneed to conform to a full hygienic specification, forexample if the product is edible oil, vinegar or ricegrains). The documented risk assessment phase asdescribed in EN 1672-2 and ISO 14159 or theHazard Analysis and Critical Control Points(HACCP) study of the process/product design canbe an opportunity to justify a relaxation of someprescriptive requirements of these standards. Forexample many potable water valves are not made ofstainless steel, but instead brass, copper andchrome-plated brass.

Durability and reproducibility of hygienicperformanceA major factor is the durability and reliability ofhygienic performance for the application inquestion. This is covered to a large extent by theguidance in the above EHEDG guidelines.However, issues such as materials compatibility areoften poorly considered, especially at designchanges such as the introduction of newproducts/formulations or expansions of heat- andtemperature -exposure limits.

In this context, the availability of a goodselection of seal elastomers can be very importantin allowing such variation to be accommodated. Arestricted range can mean that a branded valverange is unable to perform to its full potential withcertain products and/or conditions. Elastomers do

age and are relatively perishable, therefore theirhandling, composition, packaging, labelling andsurface finish are critical for reproducibleperformance. It has also been known for criticalattributes of original seal parts to be varied by themoulding company without the knowledge of thevalve manufacturer. Therefore it is important thatthe valve supplier can provide documented evidenceand control of composition, traceability, storageand handling. Seals should also be suppliedindividually labelled and have an expiry date.Colour-coding via elastomer pigmentation can be ahelpful fail-safe, but only if it does not impairperformance of the elastomer compound. Non-original supplies of non-proprietary seal shapessuch as o-rings can cause serious problems unlessthey comply exactly with the valve manufacturer'sspecifications and all the above quality assuranceprovisions are implemented.

Seals made of fluoroplastics have beenintroduced for use in general applications wheremuch higher chemical and heat resistance isrequired. In some cases, these are also offered forhygienic applications. Bacteria-tight seals for liquidsrequire that at least one part of the seal is elasticand can flow into the microscopic imperfections ofthe seal face and maintain this seal duringexpansion, contraction and vibration of theequipment. It is questionable whether a plastic seal-face contacting a metal seat can do this, even whenbacked by an elastomer for resilience. A morerecent adaptation has been to use a fluoroplastic-on-elastomer seat, which is likely to be moresuccessful. However, careful designers and users willsatisfy themselves by challenge testing that any sealsrequired to be bacteria-tight still give a bacteria-tight seal after a simulated normal lifetime of use.

MaintenanceEase, speed and reliability of maintenance areimportant issues for users. It is important in aninstallation with many valves that each valve can beoverhauled reliably and quickly to avoid expensivedowntime. Useful enhancements here are theprovision of single-armature multi-seat valvemodules (you can replace several seat seals at once),single-fastener body clamps and fail-safe aids suchas fixed compression-stops on all seals. For arealistic appraisal of such features it is wise toinvolve the maintenance technicians and even forthem to conduct replacement tests to estimate thereplacement time per seal and the frequency ofcorrect body and seal compression.

Life cycle costsAs with all equipment purchases, the buying

strategy can vary between an emphasis on lowinitial cost to one on low life-cycle costs. Life-cyclecosts can be strongly dependent on maintenancefrequency, because a major component of valvechange-out costs stems from the loss of productiontime and output. It is important therefore to takethis into account when considering maintenancecosts. Comparison of replacement costs will alsoshow that for the larger pipe-size, a comparablemembrane for a weir-type valve is considerablymore expensive than the seal kit for a circular sealvalve. In my experience, the seals of circular sealvalves also last far longer under the sameconditions than those in weir-type valves. This canprobably be attributed to the minimisation of thetension forces on the elastomeric seals, assisted bythe use of non-adjustable metal compression stops.(In the past, the single-diaphragm circular sealvalve was modified so that the body seal wasseparate from the seat seal. This eliminated thecycle of simultaneous stretching and compressionof the elastomer, and was found to increase the seallifetime. Such a modification is not at presentavailable for the weir-type, to my knowledge).

It is therefore important to ensure that thecost/benefit assessment is based on a comprehensiveidentification of all costs rather than just the cost ofthe valve and its service parts.

Body stiffnessValve-body stiffness may be a key defence againsttorsion, compression and pipe-stress, for examplefrom thermal expansion and contraction or frompoor installation practices. Instances are knownwhere the valves distorted and there was anintermittent failure to close fully. Although the useof pipe-stress software calculations can minimisethis risk, variations from plan do occur duringinstallation and a strong, stiff valve-bodyconstruction is therefore less likely to run intotrouble.

Materials compatibility under non-routineconditions Assuring materials compatibility is a requirement ofEHEDG guidelines, but in practice this is notalways as simple as it may seem. It is important toconsider all operations such as fail-safe modes,shutdown, cleaning and maintenance. For example,where high temperatures and/or halides are present,drying-out can lead to very corrosive conditions,even though normal operational conditions aremuch less severe. Intelligent changes to operationalprocedures and the integrated design can mitigatethese kinds of stresses but in other cases the cost ofusing higher-grade materials may have to be

new food

14

balanced against the cost of failure duringproduction.

Design principlesApart from the requirements for conformance tothe more generic EHEDG guidelines 8 and 10,valves have some specific hygienic designrequirements. Whilst it is not intended to duplicatethe EHEDG guidelines Doc.14: Hygienic design ofvalves for food processing and Doc. 20: HygienicDesign and Safe Use of Double-Seat Mix proofValves, some examples of specific requirements are:

Leak detectionContinuous leak detection for the seat seal isnormally required for assurance of safe operationand should be an integral part of the design. As aminimum, this takes the form of an atmosphericleakage port, though some automatic detection andfeedback systems are in evidence, especially foraseptic barrier modules, where the barrier zone hasto be pressurised. Note that in the case of plastic-faced elastomeric membranes, an unacceptablecrack in the plastic face will usually not bedetectable because it will still be sealed from theleak-detection port by the elastomeric component.Note that if such a penetration is accompanied bydelamination, then a hazardous crevice will formbetween the plastic and elastomeric layers.

Seat-lift actuationThe ability to clean the face of each seal seat has tobe facilitated in multiple seat and mixproof valvesby the provision of seat-lift actuation during CIPoperations, independently of concurrent productoperation.

Safe closureValve seats are usually sprung closed and opened bypowered actuators, so that in the event of powerfailure, the valve fails in the closed position.

ValidationChallenge-testing where and whenIt is not unknown for organisations to test or evenimplement new valve designs, brands orcomponents in their production equipment underworking conditions after only a cursory inspectionof the design. This can be a hazardous andpotentially expensive strategy. In the majority ofcases it is safer to challenge-test the equipment off-line (following simulation of a required cycle andconditions of use) before testing a sample in theproduction environment. A useful aid to this can bewhere the manufacturer has had the valve type-tested and certified by an EHEDG-accredited

laboratory, to the EHEDG guidelines. SeeEHEDG certification below.

Appropriate specification and selectionSuppliers are regularly confronted with complaintsabout their valves from users who have madeinappropriate selections from their range of modelsand components. For example use of EthylenePropylene Diene Terpolymer Rubber (EPDM) withoily or fatty material such as antifoam or use of lowtemperature EPDM with steam sterilisation. It isimportant prior to design changes or validation ofany equipment to define and document the exactrequirements and conditions of use (temperatures,chemical exposure, maintenance availability,available space, temperatures, pressures, etc). It isalso necessary to make the optimal selection fromthe supplier's available body and seal materials forthese requirements.

Legal complianceAny power-actuated valve is subject to the EUMachinery Directive and CE-marking. EN 1672-2was produced in support of the food safety aspectof the Machinery Directive and provides guidanceon prescriptive equipment design, augmented andpotentially modifiable by a documented hygienicrisk assessment. (ISO 14159 is effectively a morerecent update of EN 1672-2 and could be usedinstead if desired). Users should ask for a copy of amanufacturer's risk assessment to support theirinitial appraisal and validation of the design. A keyfeature of such documentary evidence could be theprovision of a documented EHEDG or comparablechallenge-test or EHEDG certificate.

EHEDG certificationA considerable aid to validation and selection ofvalves is testing and certification by EHEDG-approved laboratories to EHEDG guidelines. TheEHEDG tests are reference tests that challenge theequipment for cleanability, sterilisability andbacteria-tightness under defined test conditions.The certification qualifies compliance to EHEDGguideline number 8, supplemented by thecleanability test. Note that certification does notcover bacteria-tightness of the body orsterilisability, although there is no reason why amanufacturer cannot request this EHEDG test andprovide a test-report. There is not yet an EHEDGtest for bacteria-tightness of the valve seat, nor isthere a period of simulated use prior to testing aspecified requirement. The test and/or certificationare for reference only and users should request bothtest reports and certifications to confirm exactlywhat was done. User's conditions may in some

respects be more challenging than the testconditions and this should be taken into account.As described above, it is suggested that a period ofsimulated use is performed prior to challenge-testing and users with critical applications may bewise to have this done prior to testing in processeswhose results or products are costly.

Integration characteristicsThe manufacturers instructionsIt is essential that everyone involved in design andinstallation understands the manufacturer'sinstallation instructions. For example, a frequentcause of hygienic failure of valves is the incorrectorientation of the valve, such that the valve bodydoes not drain freely. (It is of no use to haveselected and validated equipment, for example to asurface finish of 0.8 micron Ra on everycomponent of a plant, only to have comparativelyenormous sumps or weirs as a result of incorrectvalve orientation. This is especially true wheresteam sterilisation is needed, as the collection ofcondensate will result in cold spots and anincreased risk of microbial survival). Consequentlyit can be safer where feasible to select valves withweld-stubs rather than couplings and to preferdesigns that have a greater range of orientations inwhich free draining is assured. Similarly, it is

essential that valves can be and are operated withinthe manufacturers' recommended operatingconditions. Excessive temperatures, pressures, andhydraulic shocks are common causes of hygienicfailure and it can be frustrating for suppliers toreceive complaints about their designs, sometimesvia third parties, only to find that a sub-optimalselection or easily-avoided abuse was the root cause.

Integration flexibilityThe flexibility and suitability of a valve design forintegration into relatively compact precisionfabricated modules/manifolds can be veryimportant. Such modular constructions caneliminate dead-legs and eliminate vessel wait-times.This capability is most evident with circular sealtype valves, particularly double-seat mix-proofvalves, in which fluids that may not safely beallowed to contact each other can be isolated withina single valve-body.

ConclusionThis article has focused on some practicalimplications of implementing EHEDG guidelineswith respect to the assessment of valves for hygienicand aseptic applications. For more specific anddetailed information, readers should refer to therelevant EHEDG guidelines.