Don't ask the model in a hurry. Tell me what your working condition is like first
Two days ago, I met a customer. When I came up, I threw a screenshot and asked, "Is this model of new metal expansion joint in stock?" I looked at the parameter table. The temperature was 450℃, the pressure was 2.5MPa, and the medium was superheated steam. In this working condition, with the general corrugated expansion joint he asked, the corrugated pipe would have to crack before three months.
This matter of model selection is really not copying homework against the model table. Temperature, pressure, medium, none of these three can leak. Temperature determines what grade of material to choose, pressure determines the number of layers and thickness of bellows, and medium determines whether to add lining or choose special alloy. It is not too late to find out these three parameters first, and then talk about the model. Especially the medium-there is sulfur and no sulfur in the flue gas, which is the difference between stainless steel 316L and 321. If you make a mistake, it will be an unplanned shutdown.
Where exactly are the structural differences? Can the general-purpose type and the external-pressure single-type axial type each work
Many customers regard "new metal expansion joint" and "corrugated compensator" as two things. In fact, they are the same thing, but they are called differently. The real concern is the type of structure.
Universal corrugated expansion joint is suitable for ordinary pipelines with small displacement and low pressure, such as circulating water pipelines in the factory area, which is cheap and sufficient. But it wouldn't make sense if you used it to carry a large displacement of the main steam pipe.
The external pressure single axial expansion joint is different. Its bellows bears external pressure and has good stability. Under the same pressure, it can be made thinner and compensate more. This kind of pipeline is basically used in the main steam pipeline and reheating hot section pipeline of power station boilers. There is also a high-temperature axial expansion joint, which is specially designed to deal with the working conditions above 600℃. The design of heat insulation layer and guide tube is completely another idea.
If you choose the wrong structure, no matter how careful you are, it will be useless. For example, in a large-section and low-pressure scene such as desulfurization flue, don't use a circular pipe compensator to make it hard. You have to use a metal rectangular expansion joint to match the flue section, otherwise the flow resistance and dust accumulation problems will be enough for you to have a headache.
Material and Manufacturing Process: The money saved here will be returned to you as maintenance fees
Corrugated pipe, guide tube and take-over, the matching of these three materials is particular.
Corrugated pipe is the core, and austenitic stainless steel, such as 304, 316L and 321, is commonly used. If the temperature exceeds 550℃, you have to consider superalloys. The function of the guide tube is to protect the inner wall of the bellows and avoid high-speed fluid erosion and particle wear. Its thickness and the gap size between the bellows are directly related to how long to run for expansion energy saving. Some people made the guide tube thin in order to save cost, but it was washed through in half a year, and the bellows was directly exposed to the dusty airflow. The picture was too beautiful to look at.
Taking over this piece, many people tend to overlook it. The pipe material must be welded with the pipe, and the strength must be matched. Don't underestimate these details. No matter how beautiful the bellows is, if the welding seam of the joint is cracked, it will still leak.
Why are low-priced goods low? Either the material grade is shoddy, or the number of bellows layers is reduced, or the deflector is cut corners. In the case of industrial equipment procurement, the principle that you get what you pay for is always true.
Industry application comparison: power station, cement, desulfurization, each has its own door
The working conditions of power station boilers are characterized by high temperature and high pressure. The main steam pipeline and reheating pipeline need external pressure single axial expansion joint, or large diameter thick wall expansion joint to carry large displacement and high pressure. All the way to the air-cooled island vacuum pipeline, you have to use a special vacuum hose or double-hinged expansion joint, because the instability problem under vacuum conditions is more difficult than that of pressure pipelines.
In the waste heat power generation system of cement industry, the waste gas temperature is high and the dust content is large. The expansion joint of air duct is the first choice of high temperature and wear resistant type, and the guide tube must be thickened. A friend in the cement factory told me that it took three days to repair an accident in which the deflector was worn out and the bellows was damaged, and the lost electricity was enough to buy dozens of expansion joints.
The medium has strong corrosion, large cross section and low pressure. Metal rectangular expansion joint with reasonable flow diversion design is the mainstream scheme in this condition. Moreover, the desulfurization system is often used in conjunction with the flue gas baffle door. How to isolate and bypass during maintenance, and the layout position of the expansion joint must be planned in advance.
Installation checksum pre-stretching: Details determine the actual compensation amount
The expansion joint is not welded as soon as it is unboxed to the site. Look at the tie rod nut first-the tie rod is locked under transportation, and it should be loosened after installation, otherwise the expansion joint can't move. But how much looser? Some expansion joints need to be pre-stretched, while others need to be pre-compressed, depending on the difference between the ambient temperature during installation and the design temperature of the pipeline.
The installation direction of the guide tube is also prone to problems. The gap of the guide tube should be facing the opposite direction of the flow direction of the medium. Once it is installed backwards, the medium directly washes the bellows, which is equal to chronic suicide. There is also the position of the fixed bracket. There must be reliable fixed brackets on both sides of the expansion joint, otherwise the pipe pressure thrust will be uncontrolled and the bellows will be pulled over the head.
These details are written in the specification, but the site construction team does not necessarily do them according to the specification. If the engineer of Party A doesn't know it in his mind, it will be expensive to remember to look at the drawings when the bellows are bared when the pipeline is swept by water pressure or steam.
Installation is not the end, operation monitoring and life assessment should keep up
With the expansion joints installed and the system running, this is not over.
Bellows are flexible elements, and every stretch or contraction is a fatigue cycle. The fatigue life is clearly stipulated in the national standard, but the actual working conditions are often harsher than the design working conditions. Therefore, during the operation, it is necessary to regularly check whether there are corrosion points on the surface of the bellows and whether there are micro-cracks in the heat-affected zone of the weld. For key locations, such as the expansion joint on the main steam pipe of the power station boiler, it is recommended to measure the wall thickness and corrugation depth regularly.
Some people will ask, when exactly should I change it? There's no fixed number of years. Gas bellows will be changed once in a few years, and industrial metal expansion joints completely depend on the operating conditions and monitoring data. When the corrugated pipe has stress corrosion cracks, the corrugation depth becomes obviously shallow, or the compensation amount can't meet the design requirements, it is time to retire.
In the final analysis, choosing a new metal expansion joint is not high-tech, it means talking about the working condition data, guaranteeing the manufacturing process and covering the installation details. After all three things are done, this expansion joint is the right choice.