Fixed tube sheet: welding and installation features 

What is a fixed tube sheet and why does the service life of a heat exchanger depend on it?

The fixed tube sheet is a critical component of a shell-and-tube heat exchanger, where the quality of welding directly determines the tightness of the circuit under pressure up to 10 MPa. In our practice, we have seen how microcracks in the fusion zone led to mixing of media and the shutdown of entire oil refining lines after 6 months of operation. The main feature of the installation is the impossibility of compensating for temperature expansion without special compensators, which requires pinpoint precision during assembly. If you are planning to purchase equipment for aggressive environments, ignoring the nuances of attaching the grille to the body will be the most expensive mistake in the project estimate.

Technological features of welding fixed grids: from edge preparation to seam control

The process of welding the tube sheet to the heat exchanger body is radically different from standard butt welding due to the difference in metal thickness and requirements for the absence of gaps. We use automatic submerged arc welding for long seams, but the final fillet joints are always manually Tig welded to ensure root penetration. An error in the choice of filler material is unacceptable here: the use of Sv-08G2S wire instead of alloyed Sv-10KhGNM wire when working with acids leads to corrosion cracking in the first year of operation.

Edge preparation takes up to 40% of the time of the entire welding process. The surface must be cleaned to a metallic shine to a width of at least 20 mm from the edge, and the presence of moisture or oil causes pores that cannot be detected by visual inspection. In one case, our client tried to save time on drying the electrodes, which led to the saturation of the seam with hydrogen and subsequent brittle fracture during hydrotesting with a pressure of 1.5 from the working pressure. This cost them two weeks of shop downtime and a complete rework of the unit.

Key parameters of the welding process

• Current and polarity:For austenitic steels, we use direct current with reverse polarity to minimize penetration of the base metal and avoid overheating.
• Welding area protection:It is obligatory to purge the internal cavity of the pipes with argon from the grille side, otherwise the return roller will oxidize and become a source of corrosion.
• Temperature:Preheating to 150-200°C is necessary for low-alloy steels with a thickness of over 20 mm to relieve residual stresses.
• Layering:The seam is performed in several passes, each of which is carefully cleaned of slag before applying the next one.

Quality control of welded joints to a fixed grid is carried out in three stages: visual inspection (VIK), ultrasonic testing (UT) and capillary method (PVC). According to GOST R 58907-2020, any surface cracks, unsealed craters and pores with a diameter of more than 0.5 mm are unacceptable. We recommend that customers require radiographic inspection (RA) protocols for at least 10% of joints, even if the standard does not require it, since this is the only way to see internal defects.

Step-by-step guide to installing a heat exchanger with fixed grilles

Installation of heat exchange equipment with rigidly fixed tube sheets requires a strict sequence of actions, violation of which leads to bundle distortion and depressurization. Below is the algorithm we use at all our facilities, from unloading to commissioning. Following these steps will avoid 90% of the common problems that installation organizations face.

1. Reception and storage of equipment.Before starting work, check the product passport and the compliance of the markings with the design data. Pay special attention to the condition of the sealing surfaces of the flanges and tube sheets: any scratches or dents more than 0.1 mm deep require mechanical treatment. The device should be stored only in a horizontal position on wooden supports to prevent metal-to-ground contact in order to prevent corrosion of the support legs.
2. Foundation and installation of supports.The base must be strictly horizontal, height tolerance no more than 2 mm per 1 meter of length. We use laser levels to check the plane, since the skew of the foundation creates a bending moment on the body of the device when filled with coolant. Anchor bolts are tightened only after the final alignment of the position; one nut must protrude above the other by at least 2-3 threads.
3. Slinging and lifting.Lifting of the device is carried out only using special load-handling devices (eyes) provided for by the design. The use of slings by holding pipes or covers is strictly prohibited - this will deform the flange connections and disrupt the geometry of the pipe bundle inside the housing. When lifting, use soft pads where the slings contact the hull to avoid damaging the insulation or anti-corrosion coating.
4. Connection of pipelines.This is the most critical stage. The heat exchanger pipes should not take forces from the weight of the pipes and thermal expansion of the line. We require the installation of the first pipeline supports no further than 1-1.5 meters from the flange of the apparatus. The bolted connections of the flanges are tightened crosswise with a torque wrench in three steps (30%, 70%, 100% force) to ensure uniform compression of the gasket.
5. Hydraulic testing and delivery.After installation, a test pressure with water is carried out, exceeding the working pressure by 1.25-1.5 times (according to PB 03-576-03). The pressure exposure is at least 30 minutes, during which all welds and flange connections are inspected for leaks. A decrease in pressure on the pressure gauge, even by 0.05 MPa, indicates the presence of a hidden leak, which must be eliminated before signing the acceptance certificate.

A common mistake installers make is ignoring temperature gaps when connecting to hot pipelines. If the line is brought in under tension, then when heated to 200°C, the resulting stresses can tear the pipe from the body or deform the tube sheet. Always allow room for free expansion of the pipe system.

Problems of thermal expansion and methods for solving them in structures with a fixed lattice

The main limitation of devices with fixed tube sheets is that the temperature difference between the casing and pipes should not exceed 50°C for carbon steels and 30°C for stainless steels. If this limit is exceeded, colossal thermal stresses arise that can tear pipes out of the boards or deform the body. In our practice, there was a case at a chemical plant, where ignoring this rule led to the fact that a bundle of pipes bent like an “arc” into the casing after the first start, blocking the flow of the medium.

To compensate for expansion, such devices use lens compensators built into the body. This is a corrugated insert that works like a spring, taking on the elongation of the pipes. However, installing a compensator reduces the maximum operating pressure of the device and complicates its manufacture. An alternative option is to use floating heads or U-shaped pipes, but if the project requires a fixed grid (for example, for easy cleaning of the inter-tube space using a chemical method), then stress calculation becomes a mandatory design stage.

If your technological process involves contamination of the interpipe space with scale or sediment, a fixed screen remains the only choice, as it allows you to remove the casing cover and gain direct access to all pipes. In this case, you have to compromise on temperatures or install expensive expansion joints, but this pays off in reduced maintenance costs.

The choice of materials and the influence of the environment on the durability of the fastening unit

The tube sheet material often differs from the body material, which creates a galvanic couple problem during welding. For example, when using titanium pipes and a steel body, a bimetallic insert or surfacing of the contact surface of the grid with titanium is required. We encountered a situation where the customer saved on surfacing, and after a year the contact zone became a zone of intense electrochemical corrosion, leading to through damage to the metal.

For aggressive environments (acids, alkalis, sea water), we recommend using gratings made of austenitic steel grades 08Х18Н10Т or 10Х17Н13М2Т. It is important to understand that the steel grade itself does not guarantee durability: the structure of the metal after welding must be restored by heat treatment (hardening). Without tempering, chromium carbides precipitate in the heat-affected zone, and the steel loses its anti-corrosion properties, becoming vulnerable to intergranular corrosion.

The thickness of the tube sheet is calculated based not only on the pressure, but also on the force of pulling out the pipes. When pipes are flared, tension is created in the grid holes, which holds the pipe. If the board is not thick enough, it may warp (“wave”), weakening the connection. Our engineers always carry out strength calculations according to GOST 34233.1-2017, taking into account real loads, and not just tabular values.

Typical operating mistakes and how to avoid them

Even a perfectly welded and installed heat exchanger can fail due to the fault of operating personnel. The most common problem is water hammer. A sudden opening of the shut-off valves at the inlet creates a pressure surge that can knock the pipes out of the gratings, especially if they are held in place only by flaring without additional welding. We strongly recommend installing pulsation dampers and training personnel in soft start-up procedures.

Another hidden threat is stagnant zones. If the device is not operating at full capacity, some of the pipes may be unused. In these pipes, the environment stagnates, local overheating occurs or, conversely, freezing in winter when stopped. Frozen water expands and literally bursts the pipe or deforms the grate. Be sure to provide drainage taps at the lowest points of the device to completely drain the water in the winter.

Stress corrosion is another enemy of fixed grids. The combination of an aggressive environment and residual welding stresses leads to the rapid development of cracks. To mitigate this risk, we carry out a stress-relieving operation (high tempering) of the entire apparatus in the furnace after all welding work has been completed. This increases the production cost by 10-15%, but extends the service life significantly.

Compliance with international standards and certification

The production of fixed tube sheet heat exchangers is subject to strict regulations. In Russia and the EAEU countries, the main document is TR CU 032/2013 “On the safety of equipment operating under excess pressure.” A certificate of compliance with this regulation is required for legal operation. Our devices also comply with ASME Section VIII Div. 1 for export to other regions, which confirms the high level of quality control.

An important aspect is labeling. Each tube sheet must have a welder's mark and the date of inspection. This ensures traceability: in the event of an accident, it is possible to determine exactly who welded the weld and what materials were used. We keep an archive of quality certificates for each batch of electrodes and wire used in the manufacture of your order for 10 years.

When choosing a supplier, pay attention to the presence of its own certified NDT (non-destructive testing) laboratory. Factories that outsource control often cannot guarantee prompt correction of defects, which delays shipment deadlines. Our laboratory is equipped with modern flaw detectors and allows us to carry out 100% inspection of critical seams directly on the shop floor.

Why you should order production from us: experience, technology, guarantee

We don't just sell metal structures, we provide an engineering solution for your task. Our experience shows that the correct design of a fixed tube sheet saves up to 30% of maintenance costs over the life cycle of the device. We use CNC machines to drill holes in the grids with an accuracy of 0.05 mm, which ensures a perfect fit of the pipes and eliminates leaks during the flaring stage.

Wuxi Kaisheng Electric Power and Petrochemical Equipment LLC specializes in the development and production of high-tech solutions specifically for such complex tasks. Our portfolio includes titanium shell-and-tube heat exchangers, ASME high-pressure units, and specialty tube bundles in C46400 marine brass, copper-nickel alloys, and N06625 nickel alloys. We manufacture components including 321 stainless steel and C70600 alloy tube sheets using carbon steels, alloy steels, titanium and copper alloys. Products are PED and ASME certified, allowing us to supply equipment for the petroleum refining, chemical, seawater desalination and shipbuilding industries to customers around the world. By choosing us, you get not just a product, but a stable solution with high corrosion resistance and thermal efficiency, adapted to your specific operating conditions.

The production time for a standard heat exchanger is 4-6 weeks, for non-standard projects - up to 10 weeks. We work with metal of any grade, from St3 to VT1-0 titanium, and are ready to conduct test welding to approve the technology before launching the main series. The guarantee for the tightness of welded seams is 24 months, but the actual service life of our devices exceeds 15 years, subject to operating conditions.

Do not risk the safety of your production by purchasing cheap equipment without documentation. The fixed tube sheet is the heart of the heat exchanger, and saving on its quality is tantamount to saving on the foundation of a skyscraper. Entrust the production to professionals who understand the physics of the processes and are responsible for the result.

Frequently Asked Questions

What is the maximum length of the tube bundle for a device with fixed grids?

Technically, the length is limited by the possibilities of transportation and installation, usually up to 6-12 meters. However, from the point of view of thermal stresses, the longer the bundle, the greater the absolute elongation of the tubes. For long devices (> 4 meters), the installation of a lens compensator becomes mandatory even with small temperature differences. We recommend breaking long sections into several series-connected devices of shorter length, if technology allows.

Is it possible to replace pipes in a heat exchanger with a fixed grille without dismantling the entire apparatus?

Yes, this is one of the main advantages of this design. By removing the casing cover, you gain access to all pipes. Plugging damaged pipes is allowed in an amount of up to 10% of the total without recalculating heat transfer. Complete replacement of the beam is possible, but requires special equipment for rolling out and new flaring. In our practice, we supply repair kits with spare pipes and replacement instructions.

Which tubesheet welding inspection method is most effective?

A combination of ultrasonic testing (UT) and capillary method (color flaw detection). Ultrasonic testing reveals internal lack of penetration and pores, and the capillary method shows microcracks on the surface that ultrasound cannot see. Radiographic testing (X-ray) for grille fastening fillet welds is ineffective due to the geometry of the assembly. We use automated ultrasonic testing to record the results digitally.

Is it possible to repair the welds of a fixed grid under operating conditions?

Local repairs are possible only after the device has been completely stopped, the media have been drained, steamed and degassed. Cooking “on the fly” or under pressure is strictly prohibited. Repairs must be carried out using approved technology with preliminary chipping of the defective area. After repair, repeated hydraulic press is required. We do not recommend carrying out more than two repairs to the same section of the seam, as this deteriorates the metal structure.

If you still have questions about the selection of heat exchange equipment or need to calculate the cost of a device with fixed tube sheets based on your parameters, contact us today. Our engineers are ready to prepare a commercial proposal taking into account all the nuances of your technological process.

Read more about our capabilities for the production of capacitive equipment in the sectionproduction of pressure vesselsor studywelding control methods, used in our factory.