Welding of tube sheets: technologies and quality control of seams 

Tube sheet welding technologies: from choosing a method to accepting the seam

The quality of the connection between the pipes and the tube sheet determines the tightness and service life of the entire heat exchanger. In our practice, we have repeatedly encountered a situation where a device failed after six months of operation due to microcracks in the flaring and welding zone, which led to mixing of media and a stop in production at the customer. The right technologytube sheet weldingand strict quality control of seams eliminate these risks. This article is based on real-life experience installing and servicing hundreds of heat exchangers in the oil, gas and chemical industries.

Our companyWuxi Kaisheng Electric Power and Petrochemical Equipment Co., Ltd., specializes specifically in the development and production of complex heat transfer equipment for such critical applications. We manufacture titanium shell-and-tube heat exchangers, ASME high-pressure units, and bundles in a variety of alloys, from 316 stainless steel and C46400 marine brass to N06625 nickel alloys. Our daily experience working with materials of various corrosion resistance and product certification according to PED and ASME standards allow us to deeply understand not only the theoretical aspects of GOST, but also practical nuances that are often missed in project documentation. Below we'll walk you through the key steps to creating a reliable connection.

Edge preparation and assembly: the foundation of reliability

Any mistake at the preparation stage leads to defects that cannot be corrected by subsequent welding without completely re-welding the assembly. The geometry of the cutting edges directly affects the penetration of the root of the seam and the absence of lack of penetration. In most cases, for pipes with a diameter of up to 57 mm, we use a V-shaped groove with an opening angle of 60±5 degrees. However, if the pipe wall thickness exceeds 8 mm, it is more advisable to switch to a combined groove or U-shape to reduce the volume of deposited metal and minimize warping of the lattice.

The gap between the pipe and the hole in the grill is a critical parameter. According to our internal regulations, based on the requirements of GOST R 52857 and ASME Section VIII, the optimal radial clearance should be from 0.5 to 1.5 mm. A gap of less than 0.3 mm creates a risk of lack of penetration of the root of the weld due to the impossibility of supplying an arc or filler material to the joint. A gap of more than 2.0 mm requires a significant increase in the volume of filler metal, which leads to an increase in residual stresses and deformation of a thin tube sheet. One of our clients encountered leaks precisely because, when machining the holes in the grid, there was a spread of diameters beyond the permissible limit, and the welders tried to compensate for this by increasing the current strength.

Cleaning of surfaces before welding must be carried out to a metallic shine at a distance of at least 20 mm from the edge. The presence of an oxide film, oil or moisture is guaranteed to lead to porosity of the seam. We recommend mechanical cleaning with stainless steel brushes (for austenitic steels) or abrasive wheels, avoiding the use of tools that have previously been in contact with carbon steel, to avoid carburization and corrosion. After cleaning, degreasing with acetone or alcohol is a mandatory step that cannot be ignored even when using argon protection.

Tacks are made with the same material and the same methods as the main seam. The length of the tacks should not exceed 10-15 mm, and their number depends on the diameter of the pipe (minimum 3 points for uniform stress distribution). It is important to ensure that the tacks do not have cracks or craters, as they become part of the main seam. If during the assembly process a displacement of the edges (displacement of the pipes relative to the axis of the hole) is detected, its value should not exceed 10% of the pipe wall thickness, but not more than 1 mm. Exceeding this value creates a stress concentration at the root of the weld.

Recommendation:Before starting mass welding, be sure to make a technological sample (witness) using the same grade of metal and under the same conditions in order to check the modes and the absence of macrostructure defects.

Choice of tube sheet welding technology: TIG, MIG/MAG or automation

The choice of welding method is dictated by the pipe material, wall thickness, performance requirements and the available project budget. There is no universal solution that would suit all cases. Below we will conduct a comparative analysis of the main methods used in industry.

Manual argon arc welding (TIG/GTAW)

This method remains the gold standard for critical applications, especially when working with stainless steels, titanium and thin-walled pipes. High heat concentration and excellent argon protection make it possible to obtain the highest quality welds with a minimal heat-affected zone. In our practice, TIG is indispensable when welding pipes with a wall thickness of up to 4 mm, where full penetration is required without the use of filler wire (butt welding with edge fusion) or with a minimum amount of filler.

The main advantage of TIG is visual control of the process in real time. The welder can instantly react to gap changes or defects. However, the speed of the process is low, and the qualifications of the performer play a decisive role. The human factor here is maximum: the fatigue of the welder at the end of the shift can lead to a decrease in the quality of the last seams. Therefore, for large volumes (hundreds of pipes in one grid), manual TIG welding becomes economically ineffective and risky in terms of quality stability.

Semi-automatic gas shielded welding (MIG/MAG)

The MIG/MAG method provides significantly higher deposition rates compared to TIG. This makes it preferred for thick-walled pipes and carbon steels where weld aesthetics are secondary to performance. Using the pulse mode allows you to control heat input and reduce the risk of burning through thin walls. However, metal spattering and the need for subsequent slag cleaning (when using cored wire or active gases) are significant disadvantages.

For stainless steel tube sheets, MIG is used less frequently due to the risk of contamination of the weld with silicon or other wire elements, which reduces corrosion resistance. If you choose this method, be sure to use a three-part shielding mixture (Ar+CO2+O2 or Ar+He) to stabilize the arc and improve weld formation. Control of the current source parameters is critical here: the slightest voltage deviation changes the nature of metal transfer.

Automatic orbital welding

This is the most advanced solution for mass production of heat exchangers. Orbital heads ensure consistent quality of every seam, eliminating human error. Welding parameters (current, rotation speed, additive supply) are set in the program and are strictly observed from the first to the thousandth pipe. This technology is especially effective when welding pipes with a socket or butt grid, where high repeatability is required.

The disadvantage is the high cost of the equipment and the need for careful edge preparation (tolerances must be stricter than with manual welding). If the geometry of the hole in the grid or pipe is abnormal, the automatic head may not provide a tight fit, resulting in defects. In addition, the availability of head mounting space is limited in dense tube bundles. We recommend automation for projects with more than 500 connections of the same type.

Recommendation:For one-off repairs or small-scale production, choose TIG. For the continuous production of new devices, invest in orbital welding - payback usually occurs after 3-4 large orders.

Seam quality control: non-destructive testing (NDT) methods

Welding of tube sheets does not end with extinguishing the arc. Without proper monitoring, you cannot guarantee the tightness of the device. Depending on the category of the vessel according to GOST R 52857 or ASME VIII Div.1, the set of NDT methods may vary, but the basic principle remains the same: identification of surface and volumetric defects.

Visual and measuring control (VIC)

This is the first and mandatory stage, which is carried out before using other methods. Inspection of the seam is carried out with the naked eye or using a magnifying glass with a magnification of up to 10x. The shape of the seam is checked, the presence of undercuts, fistulas, craters and sagging. The height of the seam reinforcement should not exceed standard values (usually 1-3 mm depending on the thickness), and the width of the seam should be uniform along the entire perimeter of the pipe.

We pay special attention to the transition zone from the weld to the base metal. Sharp transitions create stress concentrators. At this stage, we often discover defects such as unfilled craters at the arc start and stop points (especially during manual TIG welding without the use of backing strips or the current attenuation function). Such defects must be cleaned and welded. Measuring tools (UHS templates, calipers) must be verified.

Penetrant testing (PVK / Color flaw detection)

The method is designed to detect surface cracks, pores and lack of fusion that are not visible during visual inspection. A penetrant (colored or fluorescent) is applied to the cleaned surface, which penetrates into the defect cavities under the action of capillary forces. After removing excess and applying developer, defects become visible as bright stripes or dots.

In our practice, capillary control is mandatory for 100% of the seams of heat exchangers operating under the pressure of aggressive media. The sensitivity of the method makes it possible to detect cracks with an opening width of less than 1 micron. It is important to observe the temperature regime of the control (usually from +5 to +50 °C) and the holding time of the materials. Violation of cleaning technology before applying penetrant is the most common cause of false readings or missing real defects.

Ultrasonic testing (UT)

Ultrasonic testing allows you to evaluate the internal structure of the weld and identify hidden defects: internal pores, slag inclusions, lack of root penetration. For tube sheets, the difficulty comes from the small diameter of the tubes and the curved surface. Requires the use of specialized transducers (for example, focused or prismatic) and sensitivity adjustment on reference samples (SRP) with defect simulators (side holes, flat-bottomed reflectors).

Interpretation of echo signals from the pipe-to-grid welding zone requires a highly skilled operator. Signals from geometric features (weld fillet, hole edge) can be mistaken for defects. We recommend carrying out ultrasonic testing only after successful completion of the VIC and PVC, so that the operator is not distracted by surface imperfections. For austenitic welds, the coarse grain structure of the metal can greatly attenuate ultrasound, requiring the use of low-frequency transducers or the Time of Flight Diffraction (TOFD) method.

Hydraulic tests

The final stage of control is a hydraulic test of the entire apparatus or part of it. The test pressure is usually 1.25–1.5 times the operating pressure (depending on design standards). This test confirms the overall tightness of all connections. However, it does not indicate the location of a specific defect if a leak occurs. Therefore, carrying out hydrotests without preliminary 100% NDT of individual seams is considered a gross mistake, leading to difficulties in finding and eliminating leaks.

Recommendation:Implement a system of marking each seam with a welder's mark. This increases personal responsibility and allows you to quickly track defect statistics by performer for additional training.

Typical defects and ways to prevent them

Even with the best technology, defects occur. Understanding the reasons for their occurrence helps prevent the repetition of mistakes. Let's look at the most common problems we encountered at sites.

Weld porosity

Pores are gas cavities frozen in the weld metal. The main reason is contamination of the welded edges (oil, moisture, oxides) or insufficient protection of the welding zone with inert gas. When TIG welding, a common mistake is the premature termination of the argon supply after extinguishing the arc, when the metal is still in a plastic state and actively absorbs atmospheric air. Pores can also arise due to drafts in the workshop, which blow away the gas cloud.

Solution:Thorough cleaning, increasing gas flow (but without creating turbulence), using gas lenses for laminar flow, protecting the back of the seam with argon blowing (especially for stainless steel and titanium).

Lack of weld root penetration

This defect occurs when the metal of the edges has not melted to the full depth of the joint. Causes: too large a gap (arc does not reach the root), low current, high welding speed or incorrect torch angle. In tube sheets, lack of penetration is often hidden inside the joint and is revealed only during ultrasonic testing or hydrotesting.

Solution:Adjusting welding modes, ensuring the correct gap (0.5-1.5 mm), performing a root pass at low currents with mandatory control of penetration visually (if there is access) or using an endoscope.

Undercuts

An undercut is a groove along the edge of a weld that weakens the cross-section of the base metal. Occurs when the arc voltage is too high, the welding speed is too high or the electrode/torch position is incorrect. Undercuts are stress concentrators and are unacceptable in critical structures.

Solution:Reducing the current, reducing the length of the arc, changing the angle of insertion of the filler material (it should cover the undercut zone), stopping the arc at the edges of the weld to fill the grooves.

Tube sheet deformation

Due to uneven heating when welding a large number of pipes, the thin grid may become deformed (“go screwed”). This disrupts the geometry of the beam and makes installation of the casing difficult. We observed cases when the deviation of the lattice plane reached 5-7 mm, which made the device unsuitable for use.

Solution:The use of a symmetrical welding order (from the center to the periphery or crosswise), the use of cooling copper pads, preheating of massive elements to equalize temperature gradients, rigid fixation of the grid in the fixture.

Recommendation:Develop and validate a Welding Worksheet (WPS) for each joint type. Do not allow welders to work without familiarizing themselves with the specific parameters for a given unit.

Frequently Asked Questions

What is the minimum gap required between the pipe and the grid for high-quality welding?

The optimal radial clearance is 0.5–1.5 mm. A gap of less than 0.3 mm will not allow for high-quality penetration of the root of the weld, especially during manual welding. A gap greater than 2mm will require excessive filler metal, increasing the risk of grid warping and hot cracking. If the gap exceeds the permissible values due to a mechanical processing defect, it is necessary to weld the hole or replace the pipe with a larger diameter (if the design allows), but do not try to “clog” the gap with a large amount of electrode metal.

Is it possible to weld stainless steel pipes with carbon grate?

Yes, it is possible, but the use of special transition materials (filler wire) type 309L or 309MoL is required. These materials have a high content of chromium and nickel, which compensates for the dilution of the weld with carbon on the lattice side and prevents the formation of brittle structures and intergranular corrosion. Direct welding with materials such as 308L or 316L is prohibited in this case, as the seam will be prone to cracking. It is also important to control heat input to minimize the heat affected zone on carbon steel.

Is it necessary to carry out heat treatment after welding tube sheets?

This depends on the material and wall thickness, as well as on the requirements of design documentation and standards (GOST, ASME). For carbon steels with a thickness of more than 20-25 mm (depending on the grade), high tempering is usually required to relieve residual stresses. For austenitic stainless steels, heat treatment is most often not required and even harmful (risk of carbide precipitation), unless these are specific requirements for work in particularly aggressive environments. Решение принимается на основании расчетов напряжений и анализа свариваемости материалов.

Как проверить герметичность шва без гидравлических испытаний всего аппарата?

Для локальной проверки можно использовать метод керосиновой пробы (нанесение керосина с одной стороны и мела с другой) или вакуумный метод (камерный вакуум с нанесением мыльного раствора). Однако эти методы менее надежны, чем полноценные гидравлические или пневматические испытания под давлением. Пневматические испытания опаснее из-за накопления энергии сжатого газа и требуют соблюдения строгих мер безопасности. Капиллярный контроль (ПВК) выявляет сквозные дефекты, но не гарантирует герметичность под рабочим давлением, поэтому он является дополнением, а не заменой опрессовки.

Conclusion and next steps

Сварка трубных решеток — это процесс, где точность исполнения важнее скорости. Ошибки, допущенные на этапе сборки или выбора режимов сварки, практически невозможно исправить без серьезных затрат. Мы убедились на практике, что инвестиции в квалифицированный персонал, качественное оборудование для подготовки кромок и современный контроль (особенно автоматизированный) окупаются отсутствием рекламаций и простоев оборудования у конечного заказчика. Помните, что каждый шов — это потенциальное место утечки, и отношение к нему должно быть соответствующим.

CompanyWuxi Kaisheng LLCготова стать вашим надежным партнером в решении этих задач. Мы предоставляем индивидуальные решения для пр оектов любой сложности: от изготовления трубных пучков из экзотических сплавов (титан, монель, хастеллой) до поставки готовых теплообменников и котлов-утилизаторов, сертифицированных по международным стандартам. Наше оборудование успешно работает в нефтепереработке, химической промышленности, судостроении и системах опреснения воды по всему миру.

Если вы планируете модернизацию теплообменного оборудования или запуск нового производства, наши специалисты готовы провести аудит вашей текущей технологии сварки, предложить оптимизацию режимов и помочь с подбором комплектующих, обеспечивающих максимальную коррозионную стойкость и теплоэффективность.

Contact us todayдля консультации по вопросам сварки трубных решеток, подбора материалов и организации контроля качества на вашем предприятии. Мы поможем подобрать решение, которое обеспечит надежность вашего оборудования на десятилетия.