Quality assurance is a critical factor for any pressure vessel welding job. Pressure vessels pose a significant danger to everyone working or living around them, as they can explode if they aren’t fabricated to meet code criteria. Quality assurance involves weld testing to ensure the welded seams meet the requirements for a safe and long pressure vessel service life.
Pressure Vessel Fabrication and Inspection Codes
Pressure vessels, boilers, and heat exchangers are subject to welding codes and standards. All US states have legislated the requirement to manufacture pressure vessels according to the ASME code. But rules can vary from state to state, depending on many factors like climate. High/low temperatures, moisture, and marine environments influence the design and fabrication requirements, so you should always ensure you are fabricating and testing the pressure vessels to meet your local law requirements on top of meeting your client’s expectations.
ASME Boiler and Pressure Vessel Code (BPVC)
“ASME” is the name in the pressure vessel industry. But, when people say “ASME” in the pressure vessel context, they are actually referring to the American Society of Mechanical Engineers (ASME) and their Boiler and Pressure Vessel Code (BPVC). This massive code consists of over 30 books with over 17,000 pages, but the following are the most important for welding and inspecting the pressure vessel welds:
These standards contain rules for the design, fabrication, welding, and testing of welded pressure vessel assemblies. Likewise, they contain rules for the qualification of personnel and the responsibilities and duties of authorized inspectors.
Being an “ASME code shop,” or an “authorized shop” as many refer to, simply means that you are certified to perform pressure vessel welding according to the ASME code. You would need to enter into a contract with an authorized inspection agency and have a dedicated authorized inspector. The inspector would often interact with your shop as you conduct your business producing ASME pressure vessels.
Being an authorized shop brings many benefits, including better access to the market and allowing you to stamp the ASME mark on your pressure vessels. However, this requires rigorous quality assurance for every pressure vessel to ensure a high weld quality for safety and compliance with the code.
The National Board of Pressure Vessel Inspectors
Another critical organization for pressure vessel safety is The National Board of Pressure Vessel Inspectors. They develop standards, training programs, and certifications for the construction, installation, repair, inspection, and repair of pressure vessels and boilers.
There are also other bodies, including the American Petroleum Institute, that develop codes for pressure vessel fabrication and testing. But, the ASME is the most critical code for most applications.
Quality Assurance From The Get Go
“Staying vigilant in the pre-welding phase can save significant resources and prevent rework or penalties”
Quality Assurance Plan (QAP) forms the framework for activities before, during, and after the fabrication process. So, quality is ensured from the very beginning. Some of the most critical pre-welding quality control steps are:
- Verifying welder qualifications for specific weld types, materials, and positions.
- Ensuring that the welding equipment is set for meeting the WPS requirements.
- Reviewing WPS documents to ensure accuracy and completeness.
- Conducting tests on weld coupons as needed to verify WPS qualification.
- Ensuring compliance with industry standards and requirements for base metals and welding consumables.
- Checking joint cleanliness and proper fit-up during the inspection process.
- Verifying that joint design and weld preparation meet WPS and applicable welding standards (ASME, AWS, API).
- Reviewing clearance dimensions of rings, consumable inserts, or backing strips as needed.
Staying vigilant in the pre-welding phase can save significant resources and prevent rework or penalties, depending on the situation and the job. Missing a critical error can mean a complete profit loss or worse. That’s why spotting irregularities early on is so important for long-term welding shop success, especially in the pressure vessel industry.
Welding pressure vessels involves many people: metallurgists, engineers, authorized inspectors, welding personnel, production managers, and the sales department that stands between you and the customer. So, if a critical step is missed or done incorrectly, you might get into a situation where lots of people have to re-do their jobs, inspect the newly present situation, and decide on how to proceed. This can lead to delays and profit loss, which is why quality assurance early in the process not only ensures pressure vessel safety, but hitting the deadlines and achieving the projected profitability.
Non Destructive Testing (NDT) For Pressure Vessel Welds
Welding the pressure vessel is only part of the job. Making sure those welds will actually hold is another thing entirely.
While quality assurance includes welder, welding procedure, and material qualifications, the produced welds must also be inspected.
All welds are first inspected visually. An experienced certified welding inspector (CWI) can easily spot many discontinuities and weld flaws from the visual inspection alone, which can save the effort and time of performing the NDT if the welds aren’t up to the visual standards. However, once the welds pass the visual test, they also must pass NDT tests like X-ray, ultrasound, and other NDTs appropriate for the pressure vessel at hand.
All NDT testing should be done with a turning roll system to ensure operator safety and convenience, especially when dealing with very large pressure vessels. The operator must apply radiographic imaging, ultrasound probe, or other equipment over the circumference of the pressure vessel. But, this can require the operator to get underneath the pressure vessel or work at an awkward angle, which can be extremely dangerous with heavy equipment. Welding turning rolls allow the rotation of the pressure vessels so that the relevant part of the wall always faces the operator, ensuring easy and safe welding and testing.
Radiographic testing is applied to all critical welds on pressure vessels, like butt and seam welds, to ensure weld penetration and quality. Radiography sends X-rays or gamma rays into the weld, creating an image that captures even the tiniest of weld defects. RT can spot voids, dents, cracks, porosity, changes in material thickness, and other weld discontinuities and defects.
Radiation is a safety hazard, and RT requires skilled technicians, but this is the most widely adopted weld NDT method that’s especially useful for pressure vessel testing. RT provides hard evidence in the form of a film rather quickly, while digital radiography can give you results on screen in seconds.
Ultrasonic testing detects weld discontinuities and defects such as cracks, inclusions, and thickness variations in a pressure vessel’s material. It involves sending high-frequency sound waves into the material and measuring the time it takes for the echo to return. As these waves travel through the weld, they will reflect back some energy if they hit a weld discontinuity, which is what the operator is looking for.
UT is more challenging to perform and requires a highly skilled operator. It’s also slower than RT, but it can be a preferred method, depending on various inspection and weld factors.
Magnetic Particle Testing
Magnetic particle testing is only suitable for evaluating ferromagnetic materials. But, the equipment is inexpensive and portable. By subjecting the surface to a magnetic field and introducing magnetic particles, any defects or fissures will attract the magnetic particles. Under proper lighting conditions, even the most minuscule imperfections become visible and easily identifiable. However, operators can use fluorescent magnetic inks to improve visibility and flaw detection.
MT testing can only detect surface weld discontinuities. It’s not the best way to inspect the weld quality deep in the material.
Liquid Penetrant Testing
Liquid penetrant testing is also applied to test surface-level weld discontinuities. It’s a great way to quickly inspect for cracks, laps, cold shuts, laminations, porosity, and other weld defects. Results are very easy to read, so less operator training is required.
PT works by applying a liquid penetrant to the surface, waiting for it to penetrate into discontinuities, and removing it from the surface. Next, another substance is applied, which draws out the penetrant trapped in the cracks and other discontinuities, making them easy to spot visually.
Pressure vessels, boilers, storage tanks, and piping systems are also subject to hydrostatic testing after the vessel is completed or repaired. Hydrostatic testing is performed by filling the vessel with water and pressurizing the system up to 1.5 times the design pressure limit. The water can also be dyed to help spot any leaks.
Critical Weld Defects
The NDT methods are used as a part of a quality assurance protocol to prevent critical weld defects from jeopardizing the safety of pressure vessels. Some of the most detrimental weld defects for pressure vessels are described below with their associated dangers.
- Cracks – It doesn’t get worse than weld cracking for pressure vessels. Transverse, longitudinal, or crater weld cracks can propagate and lead to catastrophic weld failure and pressure vessel explosion.
- Burn-through – Excessive welding current or inadequate welding speed can cause the burn-through and deterioration of the welding joint. As a result, the weld can get compromised as not enough root material is left.
- Inclusions – Tungsten, flux, slag, and oxides can get trapped in the weld as inclusions and negatively affect the weld strength. They are usually easy to spot in NDT.
- Excessive weld reinforcement – A highly problematic weld discontinuity because it can cause stress concentration at the toe of the weld, leading to an increased chance of weld rupture.
- Incomplete fusion and incomplete penetration – Both are severe weld defects that can lead to pressure vessels being unable to withstand operating pressure, causing failure and explosion.
- Porosity – Usually caused by inadequate shielding from the gas or flux or from the shielding agent contamination. Weld porosity manifests as trapped bubbles deep in the weld or on the surface and weakens the weld’s integrity.
- Overlap – A condition where a weld metal extends beyond the weld toe or root without fusing and creates a mechanical notch parallel to the weld. This is a serious issue that usually calls for weld rejection. Overlap usually means weak fusion, which is a big problem for vessels experiencing pressure and temperature swings.
- Undercut – Occurs when the weld face or root surface is below the adjacent base metal surface, which can significantly weaken the joint.
Red-D-Arc – Your Trusted Partner For Industrial Welding
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Contact us today, and our team of experts will help you choose the most appropriate weld automation equipment and power sources for the job you are looking to bid or take on.
Cast metals have a lot of benefits over more common versions of those same metals. Cast iron has better thermal conductivity than standard iron. Cast steel is stronger than mild steel. The secret – increased carbon content – increases durability and changes the material properties of the metal, but it comes with a cost. Cast metals are wear-resistant, strong, and easy to produce. As a trade-off, they’re more brittle, and when they break, they crack and fracture, rather than bending or warping. This also makes them much harder to repair via welding, and it’s generally very difficult to weld a piece of cast metal, either to another piece of cast metal or to another kind of metal entirely.
Difficult, but not impossible!
Welding cast metals is one of the more significant challenges that a welding operator is likely to face in their career. It’s difficult enough that simply applying your normal techniques and hoping to wing it on the fly is going to leave you with a worse mess than where you started.
So, how can you weld cast metals? What considerations do you need to keep in mind? Let’s talk about it.
Underwater welding is an extremely challenging, technical, and complex task. It’s also extremely dangerous. Welding has many dangers of its own, from fumes to UV rays to the fire and shock hazards of sparks and arcs; add to that doing it all underwater and you can see why it’s an extremely specialized task.
Yet there are always people who will seek it out as a profession. Why? The $200-$300k per year take-home is certainly a draw.
The reality is that those numbers are on the high end, for a skilled and lucky underwater welder. Since underwater welding pays by the project, not as a steady salary, it’s very inconsistent. A single project could be worth $30,000 and take a few days or weeks, but it might be months in between projects. Aggressive and skilled welders can find more frequent work, but that’s not always easy.
And, of course, there’s a lot of difference between rookies and veterans, inland and offshore, and other variations in the career. There are even bonuses based on depth; the deeper the project is underwater, the more dangerous it is, so the more pay is involved. Still, it’s a significant draw for many who love both welding and diving.