How professional is pressure vessel welding? Old engineer: Welding joints, groove design, great knowledge!
Pressure vessels are key equipment used to hold high-temperature and high-pressure substances, and welded joints are like the "joints" that connect these equipment components, directly determining whether the equipment can work safely.
A good welding joint design should ensure that it can withstand pressure and temperature, be easy to process, and be cost-effective.
一、 What are the types of welded joints? Where are they used?
The welding joints of pressure vessels have different types, functions, appearances, and usage locations.
1. Classified by function: mainly depends on whether it is under stress or not. Weld seam: only serves as a connection, not subjected to much stress. Even if it's broken, the device won't break down immediately.
For example, the connection between the fixed bracket outside the equipment and the cylinder body, and the welding of the iron plate and shell for reinforcement.
When designing, there is no need to weld through, just spot weld, and there is no need to calculate strength, as long as it can be connected without falling off.
Bearing weld: Directly subjected to force, it needs to withstand the pressure and temperature inside the equipment. If it breaks, the device may directly have problems.
For example, the longitudinal weld seam of the cylinder body (the seam along the length of the cylinder body), the splicing seam of the circular head, and the connection seam between the pipe mouth and the shell.
When designing, it is necessary to weld through and calculate the strength. After welding, 100% inspection is required to check for defects.
2. Classified by appearance: mainly depends on how the components are connected. Joint: the end faces of the two components are flat and welded together, just like the seam of a can.
This type of joint has uniform force distribution and low degree of local force concentration (usually ≤ 1.5), making it the most commonly used.
Used at the longitudinal and circumferential welds of the cylinder body, as well as at the joints of the head. If the thickness of the components is different, the edges should be processed into a slope (bevel) to ensure welding penetration.
T-shaped/cross joint: The end of one component intersects perpendicularly with the side of another component, resembling a "T" or "X" shape. For example, the pipe mouth is vertically welded onto the cylinder body.
Incomplete welding can only be used in areas with low pressure (where local stress concentration is more obvious); Welded through can be used in areas with high pressure (reducing local stress concentration).
Overlapping joint: Two components overlap and then weld together, like two sheets of paper stacked together and stuck together.
This type of joint experiences uneven stress and severe local stress concentration, and can only be used in secondary areas with low pressure and no movement, such as auxiliary fixation of equipment bases and heads. It must never be used in areas with primary stress.
Corner joint: The ends of two components are welded at a 30 ° -135 ° angle, such as the connection between flanges and cylinder bodies, and the connection between pipe plates and shells. If subjected to high stress, the groove should be processed to ensure full penetration.
二、 What are the key considerations when designing welding joints?
When designing welding joints, the main considerations are what to choose, how to make the groove, and how to control the quality, so that the joint is both useful and easy to process.
1. The principle of selecting the shape of the joint depends on the force: priority should be given to using butt joints where the main force is applied; Secondary connections can be made using corner joints or lap joints; Do not use poorly welded T-joints for components that frequently shake, such as compressor casings.
Look at space: The joint position should be convenient for welding, such as leaving at least 5 millimeters of gap for the welding gun to extend in, and not hiding the joint in narrow corners.
Looking at materials: When welding different types of steel (such as ordinary steel and stainless steel), a layer of transition material should be added, and the joint shape should be suitable for the characteristics of the two materials (such as the difference in thermal expansion and contraction).
Why does the pressure gauge have to have a 'bend'? Old engineer: Without it, the readings are all deceiving you
2. The rules for selecting groove types in groove design: component thickness ≤ 6mm, no groove is required for direct butt welding; 6-20mm, with a V-shaped groove (one side cut into a slope); If it exceeds 20 millimeters, use an X-shaped groove (with sloping edges on both sides) to save half of the welding material;
Thick high-strength steel, with a U-shaped groove (slope with arc), easier to weld.
Size standard: The groove angle is generally 30 ° -60 ° (machine welding uses 50 ° -60 °, manual welding uses 30 ° -45 °);
The root gap (the gap in the middle during docking) is 2-4 millimeters, about the thickness of a toothpick; Blunt edge (the uncut part at the bottom of the groove) 0-2 millimeters, ensuring welding penetration without burning through.
Special situation adjustment: For containers containing low-temperature substances (≤ -20 ℃), the root gap should be slightly larger (3-5 mm) to avoid hardening and brittleness after welding;
Containers containing high-temperature substances (≥ 400 ℃) should have small groove angles (30 ° -40 °) to reduce heat input during welding.
3. Quality control requirements inspection level: The load-bearing welds must be 100% inspected by radiation or ultrasonic waves (according to the national standard AB level); Contact for a 20% spot check on the weld seam.
Surface requirements: The raised height of the butt weld should be ≤ 3 millimeters, the size error of the fillet weld should be ± 1 millimeter, and there should be no cracks or areas that are not welded properly.
Repair restriction: The same area can be welded up to 2 times, and a comprehensive inspection must be conducted again after the repair is completed.
三、 Four principles that must be followed in designing welding joints
Principle 1: Strength must be sufficient
Principle 2: Good Processing
Principle 3: Prevent Problems
Principle 4: Cost effectiveness
In actual design, adjustments should be made based on the pressure, temperature, and contents of the container (such as whether it is toxic or corrosive):
Priority should be given to fully welded butt joints for high pressure (≥ 10MPa);
Low temperature (≤ -20 ℃) should ensure that the joint is not easily brittle and cracked; When installing corrosive substances, it is important to ensure that the weld seam is corrosion-resistant.
