What does the tank welding process look like at the company – from preparation to final quality control?
The tank welding process begins with welding metal sheets and rolling them (depending on the section/shape of the tank). Next, the rolled sheets are welded together circumferentially – at this stage, we already have the ‘skeleton’ of the tank. Other tank components can be mounted on this structure and fastened together. This process involves making small welds spaced at a specific distance from each other. At this stage, the assembly is complete and a pre-welding inspection is performed. We check the correctness of the assembly, the quality of the tack welds and the preparation of the surface for welding. The tank is then transferred to a robotic station, where a welding robot performs the welds in accordance with the documentation. Hard-to-reach areas are welded manually by a welder. After welding is completed, the quality of the welds is checked.
How does welding a fuel tank differ from welding, for example, a water or gas tank?
These tanks are made of different materials and have different designs. The differences result from the welding methods themselves. For example, we use active gas for welding stainless steel tanks and inert gas for aluminium fuel tanks. This difference alone causes a change in the welding method and technique.
What standards and regulations govern the welding of pressure and transport tanks?
When designing and selecting the appropriate welding technology, we use standards such as PN-EN 13094 and PN-EN 12972 (concerning tanks for the transport of dangerous goods) and ADR. These standards and regulations refer to welding standards such as PN-EN ISO 9606 (‘Qualification testing of welders’), PN-EN ISO 15614/13 (‘Specification and qualification of welding procedures’) and PN-EN ISO 3834 (‘Quality requirements for welding of metallic materials’). Of course, these are only some of the standards we use on a daily basis. In addition to these, there are a number of other standards concerning, for example, the quality of welds or additional materials.
What welding methods are most commonly used in the production of tanks (e.g. TIG, MIG/MAG, SAW, WIG)?
We most often use the MIG method for welding aluminium tanks, also using a welding robot. We use the SAW method for welding LPG transport tanks.
When choosing a specific method, what factors do you consider?
Each method has its strengths. The choice depends on the type of joint and the desired results. For example, we use the MIG/MAG method for welding long, straight elements, while we use the TIG method for precision welding.
What materials are fuel, gas and water tanks made of, and how does this affect the welding technique?
We mainly manufacture fuel tanks from aluminium. At the moment, we manufacture gas tanks from high-strength steel, while water tanks are made of stainless steel. However, this also depends on the customer’s requirements, and STOKOTA is keen to broaden its horizons.
How important is surface preparation before welding?
Extremely important! Especially when welding aluminium, which is covered with a layer of oxides. If this layer is not removed, the weld is likely to stick.
What are the most common mistakes in preparing the material and how can they be avoided?
The most common mistake is too long a time between preparation for welding and the welding itself. As I mentioned, aluminium very quickly becomes covered with a layer of refractory oxides. Cleaning should be carried out immediately before welding, which is sometimes difficult for organisational and logistical reasons.
What does weld quality control look like – what non-destructive tests are used (e.g. RT, UT, PT, VT)?
All welds are visually inspected (VT), while some of them, in addition to VT testing, must also undergo additional tests such as RT. The scope of testing is dictated by the standard for tanks for the transport of dangerous goods and the ADR agreement. We also use non-destructive testing to inspect components – for example, we use PT testing to inspect key components for possible non-conformities in the base material.
When can a weld be considered safe?
A weld is considered safe when it complies with the required quality level and has been made in accordance with the WPS welding procedure specification. The quality level is determined by the designer on the basis of strength calculations.
What are the consequences of leaving a defective weld in a fuel or gas tank?
It all depends on the type of non-conformity, but also on the type of weld and the loads to which the joint is subjected. Operating conditions play a key role in possible failures, which is why it is so important to check for and eliminate any non-conformities as quickly as possible.
How often does a weld need to be corrected, and for what reasons?
Welding is a special process and therefore sometimes unpredictable. Inspections at various stages of production (before, during and after welding) allow us to eliminate non-conformities. If they do occur, they are most often adhesions. This is due to the fact that aluminium is inherently difficult to weld.
Do you use a welder certification system in accordance with EN ISO 9606-1?
Absolutely. Among other things, we are certified for compliance with the PN-EN ISO 3834-2 standard, which means that we are required to certify welders. Interestingly, those who perform assembly work also have basic welding qualifications. In addition, all technologies used in the production of tanks or their components are certified in accordance with PN-EN ISO 15614/5613. This gives us and our customers the certainty that the welded joints meet strength and quality requirements.
What safety measures are in place when welding pressure vessels?
Welders are equipped with modern helmets with the ADFLO system, which ensures a supply of fresh air inside the mask. In addition, we use all personal protective equipment such as protective clothing, footwear and gloves made of non-flammable materials.
How does the welding process differ for: fuel tanks, gas tanks, water transport and storage tanks?
First of all, the tanks mentioned are made of different thicknesses and types of materials. The tank shell is made by mechanised welding using a welding bench. The difference is that aluminium tanks are welded using the TIG method, while steel tanks (for waste or other liquids) are welded using plasma welding (method 15 in accordance with PN-EN ISO 4063).
What are the biggest challenges when welding aluminium?
Personally, I believe that welding itself is a challenge. The process is unpredictable, and minor changes can often affect the final result. When welding aluminium, there is an additional difficulty related to its thermal properties. Aluminium is an excellent heat conductor and also has a low density (three times less than stainless steel). The correlation between these two properties makes aluminium very susceptible to the heat we generate during welding. This results in the risk of deformation after welding.
Do water tanks also require certified welding personnel, even though they are not pressure vessels?
Of course. Regardless of whether the tank is pressurised or not, it is made by certified welding personnel (operators, welders) and checked and inspected by authorised welding supervision in the form of quality controllers and a welding engineer.
What technologies support the welding process today (e.g. automation, welding robots, positioners)?
At STOKOTA, in addition to manual welding, we also use automated welding (welding robots) and mechanised welding (welding benches, SAW welding equipment). We also use positioners, rotators and centring devices.
Is robotisation in tank welding feasible, or does most of the work still require human precision?
The role of humans in tank production is still irreplaceable. Let us remember that robotic and mechanised welding is performed by a certified operator who may not directly control the welding arc, but ensures the correct settings and welding parameters and reacts to external factors.
What modern solutions do you use to improve the quality and repeatability of welds?
The word ‘repeatability’ immediately brings to mind automation, and that is, in a way, the answer to this question. By using a robot, we eliminate the human factor during the welding process itself. However, we must not forget that assembly still plays a huge role in automated production.
Are there any plans to invest in new welding technologies in the near future?
Absolutely. The welding industry is still developing. We see this at welding seminars and symposia. We are testing new products and solutions that will save us as much time as possible and increase the efficiency and quality of our products.
How long does it take to train a welder before they can weld a tank on their own?
First and foremost, a person who can weld a tank or other structures on their own must be authorised to do so, which means they must pass a welder’s examination. I determine the scope of the examination and the assessment of the employee’s readiness in consultation with the production foreman, based on additional internal tests and the planned scope of work for the employee.
What distinguishes a good welder in the tank welding industry?
I think it is awareness of the material being welded. As I mentioned earlier, aluminium is a very difficult material to weld, so awareness and ongoing self-monitoring play a really significant role.
Are there any emergencies or mistakes from which the team has learned important lessons?
As in any company, we also have our share of ‘surprises’. Personally, I believe that mistakes allow us to develop rhetorically, but also logistically as an organisation. In the event of a failure, we always meet as representatives of different fields and break the problem down into its constituent parts. In such situations, the importance of teamwork becomes very clear.
What is it like to work in the welding department – what are the rules and atmosphere like there?
There is a lot of work to do, because as a welding engineer I have to take care of welding documentation, prepare welding instructions, keep track of welder certification deadlines (and we have a lot of them!), and search for and test new solutions to improve welding and combat non-conformities. Sometimes unexpected situations arise, so I have to be in two (or even three) places at once. The question about the atmosphere fits in perfectly with what I’m saying, because it is precisely this atmosphere that gives me the motivation and strength to act.
Why is welding a key stage in the entire tank production process?
The main reason why welding plays such an important role is that the process ensures a permanent connection between components. This is essential due to the type of medium being transported. Let us not forget that a tanker is not only a tank, but also components such as pipelines. Sometimes, in order to ensure proper liquid flow and correct placement of all necessary components, we create complex shapes, and welding allows us to tightly connect smaller parts.
What can happen if the weld does not meet the requirements – both technically and in terms of user safety?
If the weld does not meet the requirements, it may crack, for example, and thus cause the tank to leak. The hazards that may result from this depend on the medium being transported, with particular danger occurring when transporting liquefied LPG.
What would you say to young people who want to become welders?
First of all, not to get discouraged at their first attempts. Welding is a manual process, so it is most important to develop the right hand movements. Because I test employees before hiring them, I can observe their progress and see how much of an impact practice and skill consolidation have.
I can safely say that welding is the main process involved in the manufacture of tanks at STOKOTA. It is a complicated process, but every day we work to ensure the best possible quality and efficiency during our work. We do this to ensure that our products are safe, reliable and satisfactory for our customers and ourselves.
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