1.What is the core reason why galvanized elbows are prohibited in high-pressure pipelines?
The galvanizing process weakens the material: Hot-dip galvanizing requires immersing the workpiece in 450℃ molten zinc, which causes an annealing effect on the carbon steel substrate, reducing the yield strength by 5% to 8% and the high-pressure bearing capacity.
Risk of zinc layer peeling: High-pressure fluid (especially medium containing particulates) will erode the zinc layer, and the peeled zinc chips may clog the valve/instrument or enter the precision equipment and cause wear.
Hydrogen embrittlement risk: Acid pickling pretreatment may cause hydrogen atoms to penetrate into the steel matrix, accelerate crack propagation under high pressure, and cause brittle fracture (>4MPa systems need to be vigilant).
Sealing surface damage: The galvanized layer covers the flange sealing surface or threaded interface, resulting in poor sealing and easy leakage under high-pressure conditions (additional machining is required to remove the zinc layer, increasing costs).
Temperature limit: The adhesion of the zinc layer drops sharply in an environment of ＞60℃. High-pressure systems are often accompanied by temperature rise (such as steam pipes), which accelerates the shedding and failure of the zinc layer.
2.If it is necessary to change the direction of a pipe in a high-pressure system, what options are available?
Made by forging or hot-pressing process, no risk of weld defects
Choose wall thickness grade SCH80, SCH160 or XXS (extra thick wall)
Stainless steel elbows (austenitic/duplex steel): suitable for corrosive high-pressure media (such as chemical pipelines).
The inner wall of the carbon steel elbow is sprayed with PTFE (polytetrafluoroethylene) or epoxy ceramic coating, which takes into account both corrosion resistance and pressure resistance.
The exterior uses heavy anti-corrosion paint instead of galvanizing
3.When what conditions are met at the same time, can galvanized elbows be considered?
Low-pressure fire water pipe: pressure <=1.0MPa, and the zinc layer status needs to be checked regularly (ultrasonic thickness measurement every 2 years).
Air conditioning cooling water system: pressure <=1.6MPa, temperature <50℃, medium pH=6.5~9.5
Temporary low-pressure pipeline: Before use, conduct a water pressure test of 1.5 times the working pressure for 6 hours to check whether the zinc layer is bubbling/cracked
4.What are the selection specifications and standards for high-pressure pipe elbows?
Complies with ASME B16.9 (factory elbows) or ASME B16.28 (forged elbows)
Pressure-temperature ratings refer to ASME B16.34 valve standard (equivalent for fittings)
5.What are some typical accident cases that serve as warnings?
Steam pipe burst in a chemical plant (pressure 4.2MPa):
The zinc layer of a galvanized 90℃ elbow peeled off under 120℃ steam, causing local corrosion perforation and jet leakage.
Zinc scraps from the galvanized elbow blocked the servo valve, causing an emergency stop of the equipment and a loss of 3.7 million yuan.