How To Long Heat Exchanger Tube Welding?

Here are the steps and key points for long time welding of heat exchanger tube:

Preparation before welding

1. Tube and tube sheet cleaning:

Thoroughly clean the surfaces of the heat exchanger tubes and the tube sheet to be welded. Remove any oxides, oils and dirt. For example, you can use a wire brush or a chemical cleaner suitable for the material. For stainless steel tubes and tube sheets, an acid pickling solution can be used to clean and passivate the surface.

2. Tube alignment:

Make sure the tube is properly aligned with the hole in the tube sheet. The fit should be precise to ensure good weld penetration and a proper joint. Use alignment tools such as tube expanders or clamps to hold the tube in the correct position. This helps to maintain a consistent gap between the tube and the tube sheet while welding.

Welding method selection

1. TIG (Tungsten Inert Gas) welding:

TIG welding is often used for heat exchanger tubes because it allows precise control of heat input and produces high-quality, clean welds. It is suitable for a variety of tube materials, including stainless steel and titanium.

The welding gun is held at an appropriate angle (usually around 70 - 80 degrees to the tube sheet surface) to direct the arc and filler metal (if used) into the joint. Pure argon shielding gas is used to protect the weld pool from atmospheric contamination, which is essential to achieve good weld quality and corrosion resistance.

2. MIG (Metal Inert Gas) welding:

MIG welding can also be used for thick-walled tubes or when higher welding speeds are required. However, it may produce more spatter than TIG welding.

The correct wire feed speed and welding voltage need to be set according to the tube material and thickness. For example, for carbon steel tubes, the appropriate wire feed speed and voltage combination can be determined through trial welds to achieve proper penetration and weld bead formation.

Welding process

1. Weld penetration:

Achieve proper weld penetration to ensure a strong joint. Penetration should generally reach the full thickness of the tube sheet without excessive burn-through. This requires careful control of welding current and speed. For example, when TIG welding a thin-wall stainless steel tube to a tube sheet, a welding current of about 80 - 120 amps may be used, depending on the diameter and thickness of the tube.

2. Use of filler metal:

If a filler metal is required (such as when welding dissimilar materials or establishing a weld bead), select a filler metal that is compatible with the tube and tube sheet materials. Filler metals should have similar mechanical and chemical properties to ensure good weld integrity. For example, when welding a nickel alloy tube to a carbon steel tube sheet, a nickel-based filler metal may be selected to prevent galvanic corrosion and ensure a strong joint.

3. Weld bead formation:

The weld bead should be smooth and free of cracks, pores, and undercuts. The welder needs to move the welding gun at a constant speed to form a uniform weld bead. In TIG welding, filler metal (if used) should be added in a controlled manner to form a well-shaped weld bead. The width and height of the weld bead should meet the design and quality requirements of the heat exchanger.

Post-weld inspection and quality control

1. Visual inspection:

After welding, perform a visual inspection of the weld. Check for any surface defects such as cracks, porosity, lack of fusion or spatter. The weld should have a consistent color and appearance. Any visible defects should be repaired or re-welded.

2. Nondestructive testing:

Internal defects in welds are detected using nondestructive testing methods such as dye penetrant testing, magnetic particle testing (for ferromagnetic materials) or ultrasonic testing. These methods can identify subsurface cracks, voids or lack of fusion that may not be visible to the naked eye. For example, in dye penetrant testing, a colored penetrant is applied to the surface of the weld and after a certain dwell time, the excess penetrant is removed. A developer is then applied and any penetrant that has penetrated the defect is drawn out and becomes visible, allowing cracks or porosity to be detected.

3. Hydrostatic or pneumatic testing:

Hydrostatic or pneumatic testing is performed to check the integrity of welded pipes. In a hydrostatic test, the heat exchanger is filled with water (or a suitable liquid) and pressurized to a specified pressure (usually higher than the operating pressure). The pipe and welds are then checked for any leaks. Pneumatic testing can also be performed using air or inert gas, but requires greater caution due to the potential energy stored in the pressurized gas.