Metallic circular expansion joint cast steel: From material selection to engineering application, this matter is not as simple as you think
When many people hear "metal circular expansion joint cast steel", the picture that pops up in their minds is a heavy cast steel flange. This reaction is understandable, but let's correct a misunderstanding first: the core component really responsible for "expansion and contraction" in the expansion joint is the bellows, and the cast steel is mostly made of flanges, connectors and end structures. In other words, cast steel is to "set up a platform" for the bellows-the platform can't stand, and the bellows can be deformed in vain.
1. What role do steel castings play?
Two days ago, I met a customer, took the drawings and asked, and when he opened his mouth, he reported "cast steel expansion joint". As a result, it was discovered later that the drawing was actually marked with cast steel flange connected to stainless steel bellows. This kind of vagueness in the name is particularly common in the industry, but if you choose the wrong material, there will be trouble behind it.
In circular expansion joints, steel castings are basically used on flanges, termination pipes or intermediate barrel joints. Cast steel is selected for large-diameter pipelines. The picture shows that it is suitable for complex structures and has controllable cost. But be aware that cast steel is not a panacea. If there are chloride ions or sulfide in the medium, the surface of the cast steel flange is not coated with anti-corrosion coating, or the material grade is not upgraded, then the bellows will be useless no matter how long it lasts-the flange is perforated first, and the whole set of equipment will still be scrapped.
Second, cast steel, forged steel, stainless steel flange, what is the difference?
Steel castings are suitable for large-diameter flue gas pipes above DN600, and are cast and molded in one piece, which saves materials and processing time. However, the casting process determines that there may be shrinkage cavities and pores inside it. If the flaw detection standard is fooled, problems will occur sooner or later under high pressure conditions.
Forged steel has a dense structure, good strength and impact resistance, high processable cost, and long lead time for large-diameter forgings. For power station, cement, desulfurization and other working conditions, cast steel flange and stainless steel bellows are common combinations. However, as soon as the corrosion comes up, the materials have to go to the grades of 316L and 254SMO, and the surface of cast steel has to be treated with anti-corrosion. Otherwise? It will corrode and peel off in less than half a year, so you can wait for the shutdown for maintenance.
Third, why do power stations, cement and desulfurization all prefer cast steel?
In these places, the pipeline diameter is large, the temperature fluctuates violently, and the installation space is limited. Taking the tail of cement kiln as an example, the flue gas temperature jumps back and forth from 300℃ to 900℃, and the displacement of thermal expansion and contraction of pipeline is quite large. For the circular expansion joint to absorb axial displacement, the overall rigidity must be sufficient. The cast steel structure has sufficient rigidity, and with the guide tube, it can also reduce the direct erosion of the bellows by air flow.
The thicker the wall thickness is not better. The wall thickness and fillet transition of steel castings directly affect the fatigue life. As the wall thickness increases, the rigidity increases, and the thermal stress also increases. This balance is checked by finite element analysis and fatigue test, not by slapping the head. You look at the simple structure, and every transitional fillet is calculated.
4. The most common pits in installation and maintenance
The pits at the installation site are turned over and over:The bolts are tightened too tightly, the direction of the guide tube is installed backwards, and the pre-deformation of cold drawing is not done。
If the bolts are tightened too tightly, the sealing surface of the flange will be directly deformed, and air will leak instead. The guide tube is installed backwards, and the airflow blows violently at the bellows, which will wear out in a few months. Cold drawing pre-deformation is to compensate the thermal displacement of the pipeline. If it is not pre-stretched according to the design value during installation, the actual compensation amount of the expansion joint will be greatly reduced.
Do steel castings need heat treatment after welding? For materials such as ZG230-450, stress relief treatment is recommended for thick-walled parts after welding. If it is not done, the residual stress after welding is superimposed with the running stress, and the weld area is prone to cracks.
When must lining be added? When the medium contains particulate matter and the flow rate is higher than 15m/s, neither the guide tube nor the inner liner can be saved. Wear and corrosion come together, and bellows bear the brunt. Saving lining at this time? It's like pushing a bellows into a fire pit.
5. Ask the manufacturer these questions when selecting the model
First,What is the grade of steel casting？ ZG230-450, ZG270-500, or low alloy cast steel? The price of different materials is quite different, and the performance is quite different.
Second,How to implement flaw detection standards？ By GB/T 7233 or JB/T 6402? What is the proportion of spot checks? Whether flaw detection is strict or not directly determines whether internal defects will leak through the net.
Third,Connection Forms of Bellows and Cast SteelIs it welding or bolting? The welding sealing is good but the replacement is troublesome, and the bolt connection is convenient to maintain but the flange flatness requirements are high.
Fourth,Can the fatigue life test data be taken out？ The verbal promise that "it can last for ten years" doesn't count, and the fatigue test report is the hard basis. Ask these points clearly, and you will basically not be cheated.
In the final analysis, the selection of cast steel with circular expansion joint is not solved by a parameter table. The material is not matched, the structural design is not reasonable, the installation is not in place, and the ring is interlocked. Ask these questions thoroughly, and your plumbing system can save years of tossing.