Welding pipes into tube sheets: methods and flaw detection (Instructions) 

The main thing in 30 seconds

• Connection criticality:Flaring provides only mechanical fixation; the tightness and strength of the “pipe-grid” assembly is achieved solely through welding, which must withstand cyclic thermal shocks and vibration.
• Method selection:For high-pressure systems (oil and gas, energy), the priority is automatic orbital TIG welding or laser welding, which eliminates the human factor and ensures stable penetration of the weld root.
• Quality control:Visual inspection (VT) is not sufficient for critical components. A mandatory step is penetrant testing (PT) for the absence of microcracks and ultrasonic flaw detection (UT) to assess the penetration depth, especially when working with duplex steels and titanium.

Physics of the process and requirements for edge preparation

The connection of heat exchange pipes with a tube sheet (sheet board) is the most vulnerable element of a shell-and-tube apparatus. This is where maximum temperature gradients and stress concentrations arise. An error in the calculations of the welding mode or surface preparation leads not just to a leak, but to progressive stress corrosion that can damage the entire heat exchanger within 6–8 months of operation.

In our engineering practice, we have repeatedly encountered the consequences of poor-quality preparation of the gap between the pipe and the grid opening. The optimal radial clearance should be 0.2–0.4 mm. If the gap exceeds 0.5 mm, there is a risk of “failure” of the weld pool inside the pipe or the formation of lack of penetration at the root of the seam. If the gap is less than 0.1 mm, capillary suction of the filler material is difficult, which leads to the formation of pores.

Before starting welding work, the surface of the ends of the pipes and holes in the grid must be cleaned to a metallic shine. The presence of an oxide film, especially on aluminum or titanium alloys, is unacceptable. We use mechanical cleaning with abrasive materials without introducing iron particles into non-ferrous metals, followed by degreasing with acetone or isopropyl alcohol. Ignoring this stage is a common mistake of contractors who save time on pre-welding preparation.

Engineering advice:When designing joints, always provide a chamfer at the end of the pipe at an angle of 15–20 degrees to improve access of the welding arc to the root of the joint, especially if the pipe wall thickness exceeds 2 mm.

Basic welding methods: from manual arc to laser

The choice of welding technology depends on the pipe material (carbon steel, stainless steel, copper, titanium), wall thickness and performance requirements. Let's consider the most common methods used in modern mechanical engineering.

1. Manual argon arc welding (TIG/GTAW)

A traditional method that is still widely used for repairs or for the production of devices in small series. The advantage is flexibility: the welder can adapt parameters on the fly. However, for mass production this method carries high risks of human factor. The quality of the weld directly depends on the qualifications of the operator, his fatigue and visibility of the welding area.

To ensure stability during manual welding, it is necessary to use a pulse mode to control heat input and prevent burns in thin-walled pipes (1–1.5 mm thick). Argon consumption must be strictly regulated; local gas protection on the back side of the seam (blowing) is often required to prevent root oxidation.

2. Automatic orbital welding (Orbital TIG)

The gold standard for the high pressure industry. Specialized welding heads are installed on each pipe and rotate the tungsten electrode around the pipe axis at a programmable speed. This provides:

• Perfect symmetry of the seam around the entire perimeter.
• Repeatability of results: the parameter “current strength” and “rotation speed” are set in the program and do not change from pipe to pipe.
• Possibility of welding in hard-to-reach places of a dense tube bundle.

In the production cycleWuxi Kaisheng Electric Power and Petrochemical Equipment Co.,LtdWe are actively introducing automated orbital welding systems for orders where compliance with ASME Section VIII Div standards is required. 1. This allows us to guarantee the absence of defects such as uneven seam width or craters at the start/stop points of the arc.

3. Laser welding

The most high-tech method that ensures a minimal heat-affected zone (HAZ). The laser beam is focused to a point with a diameter of less than 0.5 mm, which allows welding at speeds of up to 3-5 meters per minute. This is critical for large heat exchangers containing thousands of tubes.

Laser welding is ideal for thin-walled stainless steel and titanium pipes. It provides deep penetration with a small seam width, which reduces residual deformations of the tube sheet. However, the method requires pinpoint precision in assembly: a displacement of the pipe relative to the center of the hole by more than 0.1 mm can lead to a defect.

4. Welding in shielding gases (MIG/MAG)

Used less frequently, mainly for thick-walled carbon steel pipes in non-critical environments. The high deposition rate is compensated by high heat input, which can cause warping of thin tube sheets. Requires mandatory subsequent mechanical treatment of the seam to remove metal spatter.

Engineering advice:When choosing a contractor, check what equipment is used for welding. The presence of a fleet of orbital heads indicates the manufacturer’s readiness to work with critical orders, while reliance only on manual welding is acceptable only for low-parameter equipment.

Flaw detection and quality control of welded joints

Welding pipes into tube sheets does not end when the arc is extinguished. The most important stage begins - quality verification. According to the requirements of TR CU 010/2011 and the GOST R 59971 standard (analogous to ISO 15614), control must be multi-level.

Visual and measuring control (VIK / VT)

Primary stage, performed on 100% of connections. The inspector checks:

• No external cracks, pores, sagging or undercuts.
• Seam geometry: the height of the reinforcement should not exceed 1–1.5 mm, the width of the seam should be uniform.
• Seam color (for stainless steels and titanium): golden or silver color indicates good gas protection; a blue or gray tint indicates overheating and loss of anti-corrosion properties.

Penetrant control (PVC / PT)

Mandatory for all sealed connections. A penetrant (penetrating liquid) is applied to the cleaned surface, which flows into microcracks. After removing excess, developer is applied. Even a crack a few microns wide will become visible as a bright red line. This method reveals surface defects that are invisible to the eye.

A common mistake: applying penetrant to an insufficiently degreased surface. Oil or moisture blocks the penetration of the indicator, giving a false positive result about the quality of the seam.

Ultrasonic testing (UZK / UT)

It is used to assess the depth of penetration and identify internal defects (lack of penetration, slag inclusions). For small diameter pipes (less than 20 mm), the use of standard ultrasonic flaw detectors is difficult due to the curvature of the surface. In such cases, specialized sensors with small prisms or focused transducers are used.

In the companyWuxi Kaisheng Electricity Petrochemical Equipment Co., Ltd.Quality control is carried out using certified ultrasonic equipment, which allows scanning the pipe-grid connection area from different angles. This is especially true for clad composite tubesheets, where it is important not to damage the explosive weld during welding and to ensure there is no delamination.

Hydraulic and pneumatic tests

Final test for the tightness of the entire device. The heat exchanger is filled with water (or air/nitrogen for pneumatics) at a pressure exceeding the operating pressure by 1.3–1.5 times. A drop in pressure or the appearance of water droplets in the annulus indicates the presence of fistulas in the welds. It is important to note that this method records the fact of a leak, but does not indicate its cause, so it is always combined with non-destructive testing.

Engineering advice:Require the provision of PT (PT) and ultrasonic testing (UT) protocols along with the product passport. The absence of these documents means that control was either not carried out or the results were hidden.

Typical defects and ways to prevent them

Analysis of failures of heat exchange equipment shows that 85% of problems are associated with violations of welding technology. Let's look at the main defects and methods to combat them.

1. Lack of penetration of the root of the seam.Occurs when the current is too low or the welding speed is too high. It is dangerous because it creates a gap for the accumulation of an aggressive environment, causing crevice corrosion.Solution:Calibrate the welding machine before each shift, using a pulse mode for better control of penetration.
2. Porosity.Consequence of poor gas protection (drafts, low quality argon, dirty nozzles) or damp materials. Pores reduce the strength of the connection and serve as sources of corrosion.Solution:Using high purity argon (99.99%), checking gas flow (10–15 l/min), storing electrodes and pipes in dry rooms.
3. Undercuts.Grooves along the edge of the seam, weakening the cross-section of the pipe. Often occur during manual welding due to an incorrect angle of inclination of the electrode.Solution:Transition to automatic orbital welding, where the angle and position of the electrode are fixed mechanically.
4. Oxide inclusions.Characteristic of aluminum and titanium. Lead to seam fragility.Solution:Thorough mechanical cleaning before welding, using ceramic guides to protect the welding area from air.

Particular attention should be paid to welding dissimilar metals, for example when welding copper-nickel pipes into steel gratings (or clad gratings). This requires the use of transition layers or bimetallic bushings to avoid the formation of brittle intermetallic compounds. Our production experiencecomposite tube sheetsshows that proper preparation of the transition zone increases the resource of the node by 2–3 times.

Logistics and selection of a reliable supplier

The quality of welding is determined not only by technology, but also by production culture. When choosing a partner for the supply of heat exchange equipment, pay attention to certification and production base.

CompanyWuxi Kaisheng Electric Power and Petrochemical Equipment Co.,Ltdhas a production base in the Shanghai Economic Zone, which provides unique logistics advantages. Direct access to the port and airport of Shanghai (2 hours away) allows us to quickly ship products by sea to anywhere in the world. For customers from Europe and Russia, rail routes are available, providing delivery in 15 days, which is much faster than classic sea freight.

Availability of ASME U, PED 4.3, ISO 9001/14001/45001 certificates, as well as TR CU (EAC) and BV approvals confirms that our welding procedures have been passed by an independent audience. We don't just sell metal - we provide an engineering solution, backed by strength calculations and a guarantee of the tightness of each welded joint.

Our range includes not only standard shell-and-tube heat exchangers, but also complex components: H-type finned tubes, spiral tubes, air coolers. All products undergo multi-stage testing, including spectrometric analysis of materials and hardness testing of heat-affected zones.

Conclusion: Investing in Reliability

Welding pipes into tube sheets is a process where savings at the production stage result in multiple losses during operation. Selecting the correct welding method (preferably automatic) and strict non-destructive testing are the only ways to ensure long-term plant safety.

Don't risk the integrity of your process. Trust the production of equipment to companies with a transparent history, modern welding complexes and a proven international reputation.

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