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When a smooth weld is vital

04 November 2019

David Hughes discusses the importance of ensuring hygienic welds on equipment destined for use in food and beverage production areas. 

In an industry that faces stringent requirements when it comes to hygiene, the consequences of poor weld quality can be both costly and dangerous. So, how do you ensure that those welds – the strong joints that hold tanks, pipes and vessels together – are not the weak links in an otherwise hygienically-designed food-processing plant?

European Hygienic Equipment Design Group (EHEDG) guidelines state that ‘the design philosophy of a hygienic plant follows three central themes: Product must flow freely through the plant and not stagnate; the plant must be cleanable and must allow the destruction of microorganisms; and he contents of the plant should be protected from the external environment.’

Poor welds can contribute to a number of issues that impact this. Anything from ‘rouging’ of produce due to iron particle infiltration from welds that have oxidised to trapped product that results in potentially dangerous microbials which can cause wide-spread contamination of the plant’s products. Recent scares and listeriosis outbreaks attributed to the contamination of dairy, cooked meats and fish products at processing plants highlights that these risks are very real. 

Managing risk
Every length of weld inside a storage, process tank or vessel can be seen as a risk to be managed. And the smoother the weld, the better it is for managing this and ensuring hygiene. By necessity, welded joint quality must meet a very high standard in terms of surface finish and smoothness – for stainless steel grades 304 and 316 this usually means a surface finish of 0.8 µm Ra or better which welds should match as close as possible. 

PVSL recently worked with a food-industry client to address exactly this issue and introduce a new procedure on a viscous batter mix plant involving around 300 TIG welds on approximately 100m of 316 stainless steel pipework varying from 1 1/2in diameter to 3in. For future maintenance purposes, the team of welders (all qualified to BS EN ISO9606) photographed and certified every tenth weld. Corresponding numbers were etched onto the pipework so paperwork provided traceability on the project. Various measures using mobile devices within the pipework were also recorded including oxygen levels and ‘roughness’. The team achieved optimum results, including 0.8 µm Ra or better. 

Preparation is everything. In the food industry, there is a lot of small diameter stainless pipe, which has to be butt-welded from the outside using the TIG process. While other welding techniques, such as automated orbital welding and plasma arc welding can be used, these tend to require controlled environments which are not always applicable to many client sites. To achieve a TIG weld that is crevice free, the mating of surfaces must be flush and aligned which involves deburring the metal to ensure it is true and square.

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.

Purging using a high purity argon is also essential to remove oxygen and avoid oxidisation that could result in iron particle infiltration or a rough surface that bacteria will love. It is imperative to passivate the stainless-steel surfaces before putting food or beverage substances or cleaning liquids though the system. PVSL has developed a mobile cleaning unit that can be taken to site to passivate both new and existing plant. 

Most importantly though is the skill of the welder. For pipe welding, welders won’t be able to see the inner finish. They have to be able to achieve results 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.

After welding, both the inner and outer surfaces must be cleaned and, usually, polished smooth. Regular inspection, such as with mobile devices, can be used to ensure that the surface condition of the welds meets the expected quality.

What to do
Food facilities that have existing plant with welded contact surfaces need to carefully inspect equipment for weld quality and, if required, have an experienced welder redo joints that may present ‘weak links’. And, when building or contracting for new equipment that will be in contact with food products, specify the quality of the welding required and check the final product meets requirements.

David Hughes is director of stainless-steel fabrication company, PVSL.

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