To avoid burn through, there has been different values listed for the piping thickness before an online welding is performed, based upon piping content state.
What's the reason of having these two different values?
Online welding of piping - Burn through thickness
Re: Online welding of piping - Burn through thickness
The contents of the pipe act as a cooling medium. The rate at which they carry heat away from the weld zone dictates the risk of the pipe wall reaching its melting point.
Gas/Vapor (Higher Thickness Requirement): Gases are poor conductors of heat and have low heat capacity. They do not "quench" the weld zone effectively. Because the heat stays concentrated in the pipe wall for longer, there is a much higher risk of the inner surface reaching the melting point. Therefore, a thicker "buffer" of metal is required to ensure the inner wall stays structural.
Liquids (Lower Thickness Requirement): Liquids (especially moving liquids) have high thermal conductivity and heat capacity. They act as an efficient heat sink, rapidly pulling heat away from the internal surface. This cooling effect prevents the inner wall from reaching the melting temperature as easily, allowing for a slightly thinner minimum wall thickness.
While state (liquid vs. gas) is the primary divider, the velocity of the content also plays a role.
Static or Low Flow: Increases burn-through risk because the heat isn't being "carried away" by new, cool molecules.
High Flow: Reduces burn-through risk but increases the risk of hydrogen cracking (cold cracking). Because the liquid cools the weld so fast, it can create a brittle grain structure (martensite) in the heat-affected zone (HAZ).
Gas/Vapor (Higher Thickness Requirement): Gases are poor conductors of heat and have low heat capacity. They do not "quench" the weld zone effectively. Because the heat stays concentrated in the pipe wall for longer, there is a much higher risk of the inner surface reaching the melting point. Therefore, a thicker "buffer" of metal is required to ensure the inner wall stays structural.
Liquids (Lower Thickness Requirement): Liquids (especially moving liquids) have high thermal conductivity and heat capacity. They act as an efficient heat sink, rapidly pulling heat away from the internal surface. This cooling effect prevents the inner wall from reaching the melting temperature as easily, allowing for a slightly thinner minimum wall thickness.
While state (liquid vs. gas) is the primary divider, the velocity of the content also plays a role.
Static or Low Flow: Increases burn-through risk because the heat isn't being "carried away" by new, cool molecules.
High Flow: Reduces burn-through risk but increases the risk of hydrogen cracking (cold cracking). Because the liquid cools the weld so fast, it can create a brittle grain structure (martensite) in the heat-affected zone (HAZ).
Re: Online welding of piping - Burn through thickness
The engineering challenge is that with too Thin: The inner wall melts, causing a catastrophic release (Burn-through), and with too much cooling (Liquid): The weld cools too fast, causing cracks (Hydrogen Cracking)
Re: Online welding of piping - Burn through thickness
Thanks.
mechcolor wrote: 21 Mar 2026, 09:37 The contents of the pipe act as a cooling medium. The rate at which they carry heat away from the weld zone dictates the risk of the pipe wall reaching its melting point.
Gas/Vapor (Higher Thickness Requirement): Gases are poor conductors of heat and have low heat capacity. They do not "quench" the weld zone effectively. Because the heat stays concentrated in the pipe wall for longer, there is a much higher risk of the inner surface reaching the melting point. Therefore, a thicker "buffer" of metal is required to ensure the inner wall stays structural.
Liquids (Lower Thickness Requirement): Liquids (especially moving liquids) have high thermal conductivity and heat capacity. They act as an efficient heat sink, rapidly pulling heat away from the internal surface. This cooling effect prevents the inner wall from reaching the melting temperature as easily, allowing for a slightly thinner minimum wall thickness.
While state (liquid vs. gas) is the primary divider, the velocity of the content also plays a role.
Static or Low Flow: Increases burn-through risk because the heat isn't being "carried away" by new, cool molecules.
High Flow: Reduces burn-through risk but increases the risk of hydrogen cracking (cold cracking). Because the liquid cools the weld so fast, it can create a brittle grain structure (martensite) in the heat-affected zone (HAZ).