African Fusion talks to Air Products’ welding gas specialist, Sean Young, about the products and services offered for the fabrication of surfaces and piping for the hygienic handling of food and beverage products.
Stainless steel has become a standard material for the construction of handling systems for food-grade produce. “Typically the austenitic grades, 304 and 316 are preferred, but for joining these materials to suit this environment, the preparation work and the welder’s skills are critical,” Young begins.
Not only do the welds have to be defect free but also all the surfaces in contact with food or beverage products must be smooth. “Since welding is the usual method of joining the various pipes, elbows and conveyor component of a food handling plant, it is important to en- sure that weldments reflect the smooth hygienic qualities of the parent plate or pipe as closely as possible,” says Young.
He cites three central design themes for sanitary plant:
Product must flow freely through the pipe or over the chutes in a plant to avoid stagnation or entrapment at any point.
The plant must be easy to clean, allowing access to cleaning materials that can efficiently destroy micro-organisms and prevent bacterial growth.
The food and beverage produced at the plant should be protected from contamination from dirt, moisture and bacteria in the external environment at all times.
“From a welding perspective, this means that welded joint quality must meet a very high standard in terms of surface finish and smoothness,” he says, adding: “Poor welds contribute to a number of hygiene problems, such as the retention of product in crevices, dead areas or on rough surfaces, all of which may be difficult or impossible to clean using CIP (clean-in-place) wash down cycles.
“Trapped food allows time for potentially dangerous microorganisms and bacteria to multiply. These regions can then contaminate fresh produce as it moves past, potentially causing wide-spread contamination of the plant’s products. The recent Listeriosis outbreak attributed to the contamination of cooked meats at processing plants highlights these risks as very real,” Young suggests.
In addition to areas of poor weld quality being favourable environments for microbial growth, the associated biofilms impede the effectiveness of the sanitising solutions used for CIP. “These biofilms are difficult to remove without some form of physical contact such as a brush,” Young tells African Fusion.
“The size and orientation of voids in many of the welds I have seen makes physical cleaning impossible, which will result in these sites becoming persistent sources of microbial contamination. Inadequate welding can, therefore, com- promise product quality in an otherwise hygienically designed and structurally sound plant,” he adds.
“The primary purpose of any weld is to provide a joint of sufficient mechanical strength to function according to the design. Consequently, a weld must meet all the mechanical strength and legislated requirements – the pressure vessel codes for piping and containers under pressure, for example.
“In addition, though, increasingly more stringent sanitary welding requirements are coming into play, which can o en be more demanding than the mechanical considerations that are usually prioritised,” Young tells African Fusion.
“Welding is an art that involves a great deal of science. It is a skill requiring knowledge along with ‘feel’ that can only be acquired through practice and experience. I believe that there is considerable variation in the welding skills of those constructing and repairing food-handling equipment. Many welds are rough and have excessive pitting along the joints, all of which result in areas that are very difficult, if not impossible, to clean.
The techniques of sanitary welding are well documented and clearly described in a number of resources: such as the ASME Bioprocessing Equipment Standard (BPE-97); the AWS D18.1/DI8.2 Specification for welding of austenitic stainless steel tube and pipe systems in sanitary (hygienic) applications; and the ISPE series of Pharmaceutical Engineering Baseline Guides, written to help end user comply with FDA regulations.
“I encourage food facilities that have equipment with welded contact surfaces to carefully inspect that equipment for weld quality and, where needed, have an experienced welder redo joints that may be providing uncleanable microbial entrapment. And when building or contracting for new equipment that will be in contact with food products, there is a need to specify the quality of the welding required and to then check to ensure that the final product is, in fact, smooth and cleanable,” advises Young.
Simple guidelines for achieving hygienic weld quality
At the starting point of producing good stainless steel welds for hygienic applications is the preparation work. “A lot of the critical work in this industry involves small diameter stainless pipe, which has to be butt-welded from the outside using the TIG process,” says Young.
To achieve a TIG weld that is crevice free, the piping fit-up has to be flush and free of misalignment or ovality. “Mating surfaces of joints need to be carefully and accurately cut square and polished so that they can be mated to each other at all points around or along the joint,” Young advises.
The second important preparation issue is cleanliness. As with all welding, but especially for food-grade stainless joints, the inside and the outside of the joints should be properly cleaned and dried before welding begins.
“Back purging using a high purity argon is also essential to keep oxygen away from the heat-affected zone on the inside surfaces of joints. Poor purging will result in oxide formation on the inner weld surface, which can result in the rough ‘cauliflower’ inner surface that bacteria will love,” Young says, adding that when purging a pipe, it is advisable to wait until all of the oxygen has been pushed out of the a pipe before beginning to weld.
Most importantly, though, Young suggests that skilled TIG welders who have adapted well to stainless steel welding carry out the welding. “It takes time for operators to develop the skills necessary to produce good TIG welds. For pipe welding, for example, welders won’t be able to see whether the inner root profile is fully penetrated and smooth. They have to be able to produce this smooth root through a combination of keeping all the welding variables inside the welding procedure specifications, and from experience of how the weld pool is responding on the top surface,” he explains.
As an alternative to manual TIG welding, Young says that some fabricators have moved across to automated orbital welding systems to improve the quality of the welded joints for sanitary applications. “An orbital welding procedure can offer much better consistency where many identical joints have to be completed, but the procedure must be very carefully developed, after which, none of the essential variables can be changed,” he notes.
After welding, both the inner and outer surfaces must be cleaned and, preferably, polished smooth. Regular inspection, using systems such as borescopes, for example, can be used to ensure that the surface condition of the welds meets the expected quality.
“It is also important to re-passivate the stainless steel surfaces before put- ting food or beverage substances or CIP liquids though the system,” Young advises.
Services to fabricators
“Most fabrication houses for the food industry have their own project engineers and welding technologists who have a very good idea of how to produce good results. We at Air Products liaise with the onsite specialists to ensure that their choices best meet the job requirements,” Young continues.
“Many of the welding procedures (WPSs) used are developed overseas for use with standard gases available there, but not all of these gases are locally available. The shielding gas for welding is an essential variable, so in such cases, we will o en make the exact specified mixture to avoid the need for clients to prequalify a new welding procedure,” he advises African Fusion.
And as well as offering expertise, Air Products offers several cost-saving initiatives to help fabricators to product quality welds. “Our CryoEase solutions, for example, can reduce downtime associated with changing over gas cylinders and contamination risks due to a cylinder running empty while welding,” he points out.
CryoEase consists of one liquid bulk tank, usually argon for stainless welding applications, coupled to gas-cylinder manifolds of mixing gases, typically oxy- gen for MIG welding stainless steel and CO2 and O2 for welding mild or carbon steel. “The solution is ideal for any user who consumes more than 50 cylinders of gas per month,” Young suggests.
A mixing panel is used to create a mixed gas that can be piped to 50 different drop points/welding bays in the facility. In addition, though, for TIG welding and back purging, the pure argon can also be piped to these points, giving the flexibility to use both processes from the same bay – and the exact mix required for a WPS can be preset via the mixing panel.
To ensure continuity of supply, CryoEase has telemetry built in that will communicate directly to Air Products as soon as the tank volume drops below 30%. “This allows a tanker to be scheduled to refill the mini or maxi tank long before the welders run out of gas,” he says.
“CryoEase enables stainless fabricators to use the more productive MIG process for stainless structures that are not in direct contact with food, and sanitary TIG welding for the piping and contact surfaces, all from the same welding bays,” Young notes.
“We have the expertise to help fabricators reduce their total costs. We offer comprehensive consultancy and after-market services and a range of different supply options to suit sanitary welding and purging needs and to help customers to maximise welding productivity,” he concludes.