Welding and hygiene in the food and beverage industry
By FoodProcessing Staff
Monday, 15 August, 2016
The hygienic requirements of the food and beverage industry place high demands on the welds that hold tanks, pipes and vessels together.
The requirements for a high-quality weld and weld surface finish are paramount in the dairy and other food and beverage industries, as the consequences of poor surface and weld quality can be costly and dangerous.
Recent contamination scares in the dairy sector provide some examples of the consequence of not getting things right. In effect, every metre of weld inside a storage or process tank or vessel represents a risk to be managed. Fabricators must make significant efforts to ensure that both the weld integrity is adequate and that the surface finish meets the specified requirement for hygiene.
Automatic keyhole plasma arc welding (PAW)
In the early 2000s, after a number of in-house implementations of semiautomatic welding, Furphy Engineering began searching for automated welding equipment to produce higher quality welds with improved efficiency. A global search resulted in the selection of plasma arc welding (PAW) as the desired welding method.
PAW enables an excellent weld quality to be produced, with the introduction of minimal heat and with no removal of parent material required for weld preparation. Unlike the TIG process, which is susceptible to tungsten inclusions from the exposed electrode, PAW has no exposed electrode and, consequently, significantly reduced risk of inclusions.
Weld quality is evidenced by the superior radiography test performance that results from the PAW method. Butt welds up to 8 mm thickness can be completed in a single pass with only a gas backing shield required. Weld reinforcement is minimised and this assists in the subsequent surface treatment to obtain a smooth weld finish suitable for sanitary applications.
Early PAW equipment didn’t come preloaded with the variety of programs that modern-day plasmas do and, when compared to other processes of the day, the systems were at first intolerant of all but the most precise preparation. This fostered a strong focus on stringently consistent weld preparation and the development of a robust welding R&D program became essential to realising the full potential of the process.
The need to tightly control preparation and understand the technicalities of this process in turn led Furphy Engineering to focus on welding system control. The company expanded its AS 1796 welding supervisors program and today has three WTIA accredited AS 1796 Certificate 10 Welding Supervisors, with a fourth in the pipeline. Welding supervisors oversaw the development of the PAW process and the writing of the welding programs, still in use today.
Furphy Engineering is now operating four PAW machines, with another to be commissioned with the completion of a significant workshop extension in September, which together perform 100% of the seam welds in the hundreds of tanks that are produced in its Shepparton workshops. The consistency now delivered by the PAW process has now dovetailed with the automated planishing and polishing systems to deliver the highest quality and consistency in sanitary finishing essential for clients in the food, beverage and pharmaceutical industries.
These automated planishing and polishing systems not only reduce fabrication time and costs, but result in a consistently smooth and high-quality finish to weld seams that is integral to achieving a sanitary finish. The surface roughness of the internal weld seams is specified in ‘Ra’ — a measurable roughness, in micrometres, which is obtained using an Ra meter. Traditionally, surface roughness has been specified in ‘grit’, which is not practical in a sanitary application. Grit refers to how course the polishing medium is. The finished surface is not measurable in grit and may have a varied roughness when completed by different operators. This is not ideal when working within hygienic environments.
Other tank design considerations for sanitary applications
There are many design considerations that must be made when manufacturing tanks for sanitary applications, including the design for and subsequent selection of clean-in-place (CIP) devices, nozzle configuration and associated fittings, which all affect internal surface finishing.
When selecting CIP devices, whether it be spray balls or the higher pressured rotary spray heads, it is important to design in such a way to ensure that the entire internal surface of the vessel is adequately wetted during the CIP process and that no shadowing is present — either as a result of internal components or poorly designed and located fittings — to ensure a sanitary clean is achieved. It is also important to not overdesign either, as CIP devices with higher flow rates will result in excess use of caustics and other fluids, ultimately creating excess cost every time the cleaning process is run. In various process tank applications, multiple spray devices are required to ensure wetting of the entire surface is achieved.
When designing and fabricating tanks for sanitary applications, Furphy Engineering will form a knuckle radius to all tank ends, heads or cones, as well as flare openings where tubes and nozzles are to be welded. This enables all welded connections to be butt welds, which provides a flush connection with rounded edges. Eliminating these sharp corners is necessary to ensure a sanitary finish without areas prone to product and bacteria build-up.
Advanced thermal exchange plate
Technical expertise and process development in the welding area enabled the development of Furphy Engineering’s Laser Beam Welded (LBW) ATEX (Advanced Thermal Exchange) plate. The use of cooling (or heating) cavity plates — an outer skin on a tank shell separated by a cavity to enable liquid to be passed through and across the surface of the tank to cool or heat — is one of a number of heat transfer options available when designing a process or storage tank.
The term used often in the industry for such equipment is ‘dimple plate’, owing to the dimpled appearance of the welds which attach the outer skin to the shell. The manufacture of this plate was originally very labour-intensive, with the pressed and punched plate being placed over the outer skin of the formed tank and then manually welded at the punched holes and outer seams. Modern methods of producing a dimple plate involve fusing the flat, thinner outer skin to the shell of the tank prior to rolling the tank and then using hydraulic (Furphy Engineering inflates dimple plates to 4000 kPa) pressure to deform the thinner outer skin and therefore create the cavity.
An ATEX plate is essentially pressure equipment. Operating pressures within the cavity can be as high as 1000 kPa. The result of poor welding in the manufacture of a dimple plate can be disastrous. Leaking of coolant (dimple plates can be used to heat but are predominantly used to cool) through the shell and into the tank can contaminate and destroy the stored product. Leaking of coolant externally can damage insulation. Repair can be difficult as the dimple plate is usually covered by insulation and a hard cladding, both of which must be removed. Weld tint from repair operations is likely and must be removed by gaining access to the inside of the tank.
Single embossed dimple (lap welded without through penetration) using the LBW process isn’t captured in Australian standards. The designer/manufacturer is instead referred to ASME VIII (and IX) to establish the necessary design verification testing and subsequent manufacturing requirements, such as weld procedure qualification.
Furphy Engineering utilised a hybrid test regime incorporating all key elements, in consultation with experts such as the WTIA, and also obtained third-party witnessing and approval as is a requirement of the test standard. As a result, Furphy Engineering’s ATEX plate is Worksafe design registrable (if required), independently verified and manufactured in compliance with the pressure equipment standard for maximum reliability — a key element in any process plant environment.
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