Industry Standards to Help Improve Hygienic Design and Installation Practices of Process Systems
Industry Standards to Help Improve Hygienic Design and Installation Practices of Process Systems
Barbara K. Henon emphasises the importance of Standards in improving hygiene levels in processing equipment, with special reference to weld criteria
The construction site of a modern biopharmaceutical, pharmaceutical or even some dairy plants today would present a very different picture than a similar site 20 years ago. Improvements in both hygienic design and installation practices have made enormous strides in this time period and much of this is the result of volunteer work on several industrial Standard Committees.
The ASME Bioprocessing Equipment (BPE) Standard
In 1988, the Standard used for installation of process piping systems in both the Dairy Industry and the Pharmaceutical Industry in the USA was the 3-A Sanitary Standard. The 3-A ‘program’ was introduced by the US Public Health Service in 1944, and in 1954, the use of the 3-A Symbol was initiated to show that processing equipment marked with the symbol met certain material, design and fabrication standards for cleanability and inspection.
The increasingly stringent hygienic requirements of the evolving biotechnology sector led to the writing of a new Standard under the auspices of the American Society of Mechanical Engineers, namely the ‘ASME Bioprocessing Equipment (BPE) Standard’. The first edition was published in 1997, with revisions in 2002, 2005 and 2007. A 2009 edition will be released later this year.
The BPE Standard deals with the requirements of the bioprocessing and pharmaceutical industries, as well as other applications with relatively high levels of hygienic requirements or bioburden control. BPE covers directly or indirectly the subjects of design of process equipment for cleanability and sterility, component manufacture, materials of construction, fabrication including welding, pressure systems (vessels and piping), examinations, inspections, testing and certifications. The BPE Standard applies only to those systems and components that are in direct or indirect contact with the product. It does not apply to those components that are not in contact with the finished product or part of the intermediate manufacturing stages. The BPE is an American National Standard that, by 2002, had become an adopted International Standard referenced in 29 countries.
Achieving hygienic design by improved joining technology
The stated purpose of Part SD Design for Sterility and Cleanability of the BPE Standard is “to create a design framework using proven practices, for maintaining clean and sterile process systems.” It applies only to new construction. Part SD states that the design of equipment should minimise the number of connections and, to ensure the highest degree of hygienic design, piping systems should utilise welded connections except where make-break connections are necessary. Achieving welds of consistent high quality compatible with the principles of hygienic design was an initial goal of the BPE Committee. Butt welded connections should be used minimising lap joint welds and eliminating stitch welding. From the beginning, orbital GTA welding was recommended by the BPE for joining of piping and process components. Welds were often discoloured due to poor inert gas purging. These conditions were addressed by the Subcommittee on Material Joining (MJSC) and the Subcommittee on Dimensions and Tolerances (DTSC).
Weld criteria facilitate hygienic design
The MJSC developed a set of weld criteria that, in addition to code criteria for adequate structural integrity, was designed to minimise the growth of microorganisms and promote cleanability. Orbital welds are smoother than manual welds and, when done properly, are free from crevices, pits and other defects that could harbour microorganisms. All welds must have complete penetration to the inside surface of the weld which, in bioprocess applications, is usually the surface that has contact with the product. Unpenetrated welds are entrapment sites for product and are difficult to clean. Process piping lines are sloped for drainability as gravity is the most efficient means of draining fluid from a system. Limits on inside diameter (ID) and outside diameter (OD) concavity and misalignment are set to promote drainability and cleanability of weld surfaces.
Control of material chemistry and weld end dimensions
In the 1950s, the 3-A Sanitary Standard specified 300 series stainless steel for process piping in food and dairy applications because it was cleanable and has reasonable resistance to corrosion.
This was a good choice but, prior to the introduction of the BPE, piping and component materials in biopharmaceutical applications could be 304 or 316 or 304L or 316L. There is considerable heat-to-heat variation in the weldability of stainless steel. The BPE recommended the use of 316L for weld ends but, even when only 316L is used, variations in sulphur content can cause problems. The BPE has specified a range of 0.005 to 0.017 wt.% sulphur for weld ends of fittings, valves and other components.
If the tube or fitting wall thickness is variable, the weld consistency will suffer. If either the chemistry or the dimensions of tubing and fittings is not consistent, this can and did delay piping system installation until the Dimensions and Tolerances Subcommittee specified limits in variability to weld end dimensions. Tubing dimensions are established by ASTM.
AWS D18.1 Specification for Welding of Austenitic Stainless Steel Tube and Pipe Systems in Sanitary (Hygienic)
The American Welding Society D18 Committee was formed in 1995 to respond to the request of the 3-A Sanitary Standards Committee for help in preparing welding standards for use in the manufacture and construction of dairy and food product processing plants. The Committee has maintained liaison with 3-A. The specification includes welding of tube and pipe for the transportation of sanitary (hygienic) system products and cleaning or sanitising solutions. Both manual and mechanised welding such as orbital welding are covered. One of the contributions of this Standard (and AWS D18.2) to achieving high quality hygienic welds was the publication of a colourphoto showing the amount of weld discolouration on the ID surface of a series of orbital welds (Fig. 1).
Inspection and examination of welds
Weld inspection and examination has become more systematic in recent years. The use of daily test coupons or sample welds was initiated by 3-A in the 1950s. For manual welding this demonstrated that a welder had the skill and capability of making a good weld before being allowed to make a production weld. For orbital welding a test coupon demonstrates that the welding equipment is working properly and that the purge is satisfactory. In the 1950s, 3-A required that a borescope be available on a jobsite but did not specify a number of welds to be examined. BPE requires that a minimum of 20% of the welds be examined with a borescope, which covers the entire 360 degrees of the ID of the weld. This in-process borescopic examination is done in lieu of radiography that would otherwise be required.
Installation of equipment in food and biopharmaceutical applications have been made more efficient by the use of skid-mounted equipment modules so that installation and fabrication can be telescoped into a shorter time-scale. Fabrication and, in some cases, installation and commissioning of the skids can occur at the same time as plant construction. This means that all of the materials and components must be inspected prior to installation and provision for weld inspection and documentation must happen during the manufacture of the skid.
Field welds in hygienic piping systems are typically left in the as-welded condition; the only post-weld treatment being chemical passivation with nitric or citric acid solutions following installation. Passivation restores, at least in part elements in the passive surface layer that are disturbed by welding and the concomitant loss of corrosion resistance. The BPE Surface Finish Subcommittee has added a new non-mandatory appendix to the 2009 Edition of BPE that offers guidelines to owners for qualification of their passivation procedures.
All Standards cited in this article operate on the assumption that quality must be built in -it cannot be added after the fact. These Standards are consensus standards with work done by volunteers. They are continously being updated to represent a consensus of current good manufacturing practices (cGMP) for the industry(s) they serve. A new edition of AWS D18.1 was published in 2009 and a new edition of ASME BPE is due to be published later this year. Other standards such as the 3-A Sanitary Standards, Pharmaceutical-3A (P-3-a) Standards for equipment used in the production of active pharmaceutical ingredients, International Society of Pharmaceutical Engineers (ISPE) Baseline Guides, and EHEDG, European Hygienic Equipment Design Group have all been advancing the concept of building hygienic design into equipment and systems for various hygienic applications.
All of these Standards groups are in agreement that improvements in welding contribute to improvement in hygienic design and installation practices. Orbital welding has played a key role in improving hygienic design in the most critical applications because of the superior cleanability of orbital welds and, is being used more and more in less demanding applications because of its practicality and ease of use.
A full list of References used in compiling this article is available from the author.
Dr. Barbara K. Henon is a contract employee at Arc Machines, Inc., 10500 Orbital Way, Pacoima, CA 91331, USA. She is also a former Vice Chair of the ASME BPE.
This article appeared in Food Science & Technology, vol 23 (3), 50-51 (2009), the quarterly journal of the UK's Institute of Food Science and Technology (www.ifst.org)