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Preparations before Welding in Pressure Vessel Fabrication

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 plate rolling machine
pressure vessel fabrication equipment: plate rolling machine Image source: Plate and Angle Rolling

Certain gases and liquids are housed in closed containers, known as pressure vessels, under high pressure. These containers are held together with welds, instead of bolts, due to the high-pressure levels and a need for air tightness. Because of the critical nature of pressure vessel applications, their welds undergo rigorous quality checks, which can include X-ray inspection and certification.

If a weld fails to meet the requirements of a nondestructive test, costs escalate. The weld must be removed, and the area must be appropriately patched. This is an expensive process that can cause a project to miss its expected outcomes for cost and deadline.

Common Concerns for Inappropriate Surface Cleaning

When an inspection uncovers a weld defect, it is usually one of several varieties. Here is a look at some common defects that can cause welds in pressure vessels to fail the inspection and some best practices for preventing them.

Molten weld pool

Porosity is caused when gas is trapped in the molten weld pool. As the weld cools and solidifies, the gas forms bubbles that appear as voids upon inspection. Numerous issues can cause porosity in a weld. It’s important to check that proper welding techniques are followed and appropriate consumables are being used.

Proper preparation and cleaning of the weld surface before welding and between welding passes are also important to help prevent porosity. This is particularly true in applications involving aluminum. During aluminum welding, any liquid hydrocarbons in the weld area are a potential source of hydrogen, which will readily dissolve into an aluminum weld pool. When the weld solidifies, the hydrogen atoms will collect and form small pockets of trapped gas, or porosity. Ensuring that the weld area is clean and dry is the best way to prevent this problem.

After-rust

After-rust is light rust-colored surface contamination that appears in a stainless steel weld zone several hours or days after welding. It is the nemesis of shops and contractors that work with stainless steel.
The most common source of after-rust is cross-contamination. This typically occurs when a grinder with a steel brush is used to clean a stainless steel weld.

However, it can also be caused by cleaning the surface with a stainless steel brush previously used to clean steel. The magnetic characteristic of a stainless steel brush will cause it to collect steel particles, which will be deposited on a stainless steel surface and can result in after-rust. The best way to address this issue is to keep stainless steel brushes in tightly closed containers and never use them for steel applications.

The third source of after-rust is a metallurgical phenomenon that results from severe cold-working of a surface during cleaning with a brush. If after-rust is encountered and cross contamination can categorically be eliminated as the source of the problem, a potential solution is to switch to a brush with finer wire and use it with light pressure. This will eliminate the deleterious effects of cold-working that can be caused by very aggressive brushing.

Nitrides

Nitrides are highly adherent contaminants created when plasma cutting with compressed air or nitrogen. They make the edges brittle and create porosity in some welding processes, especially gas metal arc welding. Because nitrides can exist at 0.005 to 0.010 inches. below the surface of the material, you cannot remove them with brushes. Also, nitrides often cause bonded abrasive grinding wheels to load and subsequently smear. Flap discs or flap wheels work well for this issue because they are aggressive enough to remove a small amount of base material, but the grain and cloth of the accessories wear down at a rate that resists loading.

Inclusions often result from surface contaminants that become mixed into the weld pool and are trapped during solidification. In multipass welding applications, slag that is not completely removed can be a source of inclusions. Thorough cleaning with a suitable wire brush before welding and between passes is a very effective means of eliminating this type of defect.

Clean Pressure Vessels Before Welding

A pressure vessel needs to be strong. To prevent potential leaks and hazards, the workman must carefully prep the container pieces before welding them. Without proper preparation, there is a good chance that the high-pressure gases or liquids could leak out.

  • The edges of the pressure vessel where the welding will happen need to be clean of paint and dirt. For this type of cleaning, an angle grinder will do the job. Add a grinding wheel to the grinder and secure it according to the manufacturer’s directions.
  • Wearing safety gear is important when grinding metal due to the sparks and flying metal bits. One layer of protection is long pants and a long-sleeved shirt, which will help protect the skin of the legs and arms. Wearing a welder’s mask and gloves provides the front line of protection. The mask protects the face and eyes while the gloves offer protection for the hands, wrists, and forearms.
  • Use the angle grinder to clean the surfaces of the pressure vessel you are going to weld. When grinding, it is critical that you take it down to the bare metal. There should be no dirt or paint on the surface, which could weaken the weld. Flatten down any sharp edges before welding.
  • Once you have bare metal on the container pieces, use a vacuum on the pieces, floor, and surrounding surfaces to pick up dust and bits of metal. You do not want any particulates to contaminate the weld.
  • Take a moist rag and wipe down all the surfaces of the pressure vessel you will be welding. Wiping down these surfaces will get rid of any loose bits that the vacuum did not pick up. Then the pressure vessel surface is ready for welding.

Pressure Weld Inspection Processes

Two common types of inspection for pressure welds are X-ray inspection and ultrasonic inspection.

X-ray inspection

With X-ray inspection, subsurface cracks and inclusions can be detected. This is an expensive process, but to ensure safety in critical weld joints (such as those found in submarines and nuclear power plants), a 100 percent X-ray examination is typically conducted.

X-ray testing is used to create a detailed image of an object’s internal structure. X-ray is a suitable method for weld inspections because technicians do not have to alter or destroy the object to thoroughly analyze it. X-ray is also a flexible method that can be used on almost any material.
X-rays require access to two opposite sides of the weld. On one side, the source transmits radiation into the object, while on the other side a detector film records the differences in absorption to develop a clear image. The film or digital image serves as a permanent record of the inspection.
Due to the safety concerns whenever radiation is involved, X-ray inspections should be performed by experienced radiography experts such as those at ATS. Additionally, our technicians accurately interpret resulting X-ray images and provide detailed weld inspection reporting for our clients.

Ultrasonic inspection

The ultrasonic inspection also can be used to detect surface and subsurface defects. This process involves directing a high-frequency sound beam through the base metal and the weld on a predictable path. When the beam strikes a discontinuity, some of the sound beams is reflected back. This reflected beam is received, amplified, and processed. When the time delay is used, the location of the flaw is estimated.

Two basic methods

Ultrasonic testing can be performed using two basic methods – pulse-echo and through-transmission.

With pulse-echo testing, the same transducer emits and receives the sound wave energy. This method uses echo signals at an interface, such as the back of the object or an imperfection, to reflect the waves back to the probe. Results are shown as a line plot, with an amplitude on the y-axis representing the reflection’s intensity and distance or time on the x-axis, showing the depth of the signal through the material.

Through-transmission testing uses an emitter to send the ultrasound waves from one surface and a separate receiver to receive the sound energy that has reached the opposite side of the object. Imperfections in the material reduce the amount of sound that is received, allowing the location of flaws to be detected.

Selection and Operating Tips

When it comes to selecting and using the right products for an application, a few general tips should be kept in mind:

  • Industrial-grade, heavy-duty wire brushes quickly and efficiently remove surface contamination that causes defects and is ideal for many pressure vessel applications. Be sure to match the type of brush to the type of material being welded. A stainless steel brush should be used to prepare stainless steel materials, for example.
  • Use the largest brush that the space will allow. A larger brush reduces the cost of a cleaning operation by reducing cleaning time and increasing consumable life. Choosing a knotted- or crimped-style brush depends on the adherence of the surface contaminant. Knotted brushes are best for applications requiring maximum aggression. Crimped brushes provide superior conformability to irregular surfaces.
  • Applying the right amount of pressure when using the wire brush is also important to maximize cleaning action. With a wire brush, the tips of the wires are designed to do the work, and the appropriate amount of pressure is just the weight of the tool itself. You should not need to push down hard to get a wire brush to work efficiently. If you must apply significant pressure to accomplish the job, you probably need a different brush.