Material selection for steam pipes
Thread Content
The steam in our factory comes from a steam generator; heat is applied inside the generator using heat transfer oil. The water quality is treated to remove impurities, and only after its hardness meets the required standards is it fed into the steam generator. The main steam pipes as well as the branch pipes are welded from carbon steel seamless pipes, while the condensate pipes are made by welding ordinary galvanized pipes and carbon steel stamped elbows. It has been in use for ten years now, and the main steam pipes (carbon steel seamless pipes) have not leaked. However, leaks have recently occurred at the elbows of the branch pipes, usually those that are 2 inches in diameter. The condensate pipes (galvanized pipes) suffer from severe corrosion both in the branch and main sections; especially the main pipes, which have been replaced with stainless steel ones. Upon inspection, it seems that the elbow has been corroded and punctured by steam; as a result, carbon steel can no longer be used in the areas where leaks occur. Is it related to water quality?
The definite answer is yes. However, in daily management, such inspection and supervision tasks must be carried out on a daily basis; by identifying abnormalities promptly and taking corrective actions, corrosion can be reduced. It is a management issue. If this problem isn’t resolved, there will likely be issues even without replacing it with stainless steel, as stainless steel can also corrode under certain conditions.
Yes, moreover, erosion is most severe at the elbows; it is recommended to replace all of them.
1. Users should be aware of the service life of pipelines; for example, for low-pressure steam pipelines, a corrosion allowance of 1.5 mm is typically specified in the design, meaning that they cannot be used indefinitely. 2. Corrosion in steam pipelines occurs both on the interior and exterior surfaces. For main steam pipes with large diameters, the high temperature keeps the pipe walls dry, so external corrosion is relatively mild. However, in the case of steam pipes with smaller diameters, the heat loss can sometimes exceed the amount of heat added, resulting in a drop in the temperature of the outer wall of the pipe and the formation of condensation. An environment that is humid and contains oxygen facilitates corrosion. 3 When the fluid passes through elbows, its flow pattern changes, leading to uneven flow velocities across the pipe cross-section; as a result, erosion becomes more pronounced. Additionally, a circumferential secondary flow downstream of the elbow exerts stress on the side of the elbow with a small radius, which can also cause damage to the pipe. 4 When deoxygenation of the water supply is inadequate, steam will contain some oxygen, which inevitably leads to corrosion of the metal. 5 By selecting carbon steel or CrMo steel for use in pipes based on the steam parameters, especially temperature, it is possible to increase the wall thickness appropriately to enhance the corrosion resistance.
Without considering daily monitoring and maintenance, from a design perspective, corrosion in steam pipes is inevitable. There are differences in the corrosion rates between straight pipes and elbows of the same specification within the same system; generally, erosion is more pronounced at elbows than in straight pipe sections. Then, any component has a service life, which may be extended or shortened depending on the specific circumstances.
For low-temperature steam, 20# steel is generally suitable; for high-temperature steam, CrMo steel can be considered. Leaks at the elbows may be related to water hammer, or stress corrosion may occur in those areas due to the fact that the elbows have not been replaced for a long time
Carbon steel can be used, as well as specialized boiler pipes. At elbows, whatever the medium may be, corrosion occurs most easily; therefore, elbows with a large radius or those with increased wall thickness can be chosen； Another scenario is that the water quality is substandard
Why is galvanized steel used for condensate water? Condensate water is usually made of the same material as steam pipes, thank you!
Galvanized pipes contain zinc filings; try to use pipes without such filings