The problem of pipeline stress in petroleum refining and chemical units is not big or small. But if the pipeline stress really stops the unit, the loss is not small. The expansion joint, a seemingly unremarkable component, plays a key role in it. Today, let's talk about the supply of petroleum metal expansion joints.
How harsh are oil conditions?
Think about it, what are the pipes running in the oil refining unit? High-temperature crude oil, high-pressure steam, hydrogen sulfide, chloride... the temperature always rises to 400℃, and the pressure of tens of MPa is also common. Coupled with the corrosive components in the medium, if the expansion joint of ordinary carbon steel is put on it, it will give you a look if it can't last for three months.
Before, I encountered a project, the high-temperature pipeline of catalytic cracking unit. In the early stage, ordinary stainless steel expansion joints were used for cheap. As a result, the bellows cracked less than half a year after it was put into use, and the unit was stopped for emergency repair. The loss that day was enough to buy several batches of qualified expansion joints. Therefore, the oil working condition can't be applied to the standard of ordinary industrial pipelines.
Material selection: corrosion-resistant alloy, doorway of multi-layer corrugated pipe
Material selection is the first step in the design of expansion joints, and it is also a key step in determining the life. The medium composition in oil pipeline is complex, including sulfide, chloride and naphthenic acid, so it is difficult for a single material to take it all.
It is common practice to choose Inconel 625 or 825 alloy for heavy sulfur-containing media. However, 316L can only be regarded as entry-level, and it is prone to pitting and stress corrosion cracking in high-temperature chlorine-containing environment. In terms of corrugated pipe structure, multi-layer corrugated pipe is more pressure-resistant and flexible than single-layer corrugated pipe. For example, the high-temperature axial expansion joint we often do, the high-temperature section has to use multi-layer structure to disperse thermal stress.
How to select materials according to media? The core logic is to look at the concentration and temperature of corrosive components in the medium. It is much more reliable for you to find the manufacturer with the composition analysis table and operating temperature and pressure of the medium than to report the model.
Axial, lateral, angular displacement, have you figured it out?
The most common mistake in the selection of expansion joints is to confuse the displacement types. The axial displacement is the expansion and contraction of the pipe along the axis direction, the transverse displacement is the misalignment in the vertical direction, and the angular displacement is the bending of the pipe. Three displacement types correspond to three different expansion joint structures.
The universal corrugated expansion joint is suitable for pipe sections with axial displacement as the main part; If the pipeline space is limited and the transverse displacement is large, the compound hinge transverse expansion joint should be selected; When the device pipeline vibrates greatly and needs to absorb pressure thrust, it is necessary to consider straight pipe pressure balance expansion joint or curved pipe pressure balance expansion joint.
Can't I buy all the generic models? Alas, the general-purpose type is not a panacea. If the lateral displacement is too large, the general-purpose type is used hard, and the bellows will quickly fatigue and crack. When it comes to model selection, only when the structure is right can we talk about longevity.
Parameters that are most likely to be missed in a contract
Many purchase orders are thrown at the supplier and done. Something went wrong when I turned back. After checking, the deflector was not added. The function of the guide tube is to protect the inner wall of the bellows and prevent the erosion and corrosion of high-speed media. Especially for pipelines with particles or high flow rate in the medium, no guide tube is equivalent to letting the bellows streak naked. However, the guide tube is also divided into one-side and two-side, and the flow direction of the medium should be clearly marked, otherwise the guide tube will become a flow dead angle.
Let's talk about pre-stretching. When the cold pipeline is installed, the ambient temperature is low, but the operating temperature is high. The expansion joint should be pre-stretched during installation to reserve the amount of thermal expansion. This amount is not calculated correctly. Either the bellows is compressed to the limit, or it is overstretched and fails directly.
The calculation of fatigue life and stiffness is more technical. According to the EJMA standard, there should be a clear calculation book for the fatigue life of corrugated pipes. The stiffness directly affects the pipeline stress analysis and the bearing stress. These two parameters are not written out in black and white. What does the supplier do to ensure that your device can run safely for a full overhaul cycle?
Real Lessons from Installation and Maintenance
An atmospheric and vacuum distillation unit uses external pressure single axial expansion joint in the pressure reduction pipeline, and the transportation fixed tie rod is not removed during installation. After the device is started, the pipeline heats up, the expansion joint cannot expand and contract freely, and the bellows is forcibly stretched and unstable. When the inspection found it, the trough had already appeared bulging, so it had to be stopped and replaced. The manufacturer's installation guidance makes it very clear that the transport tie rod must be removed after passing the pressure test-this step of the process has been missed.
Non-metallic expansion joints (fabric fiber expansion joints) are used in the outlet flue gas pipeline of the catalytic cracking regenerator. After two years of operation, the guide tube was worn out, and the high-temperature flue gas was blown directly onto the heat insulation layer, burning through the skin. When disassembled, the material of the guide tube is thin, and the wear-resistant layer is not strengthened.
For the high-temperature and high-pressure hydrogen pipeline of a hydrocracking unit, the model of expansion joint was selected correctly and the material was correct. However, during installation, it was found that the deviation of the pipeline counterpart was too large, and the workers just used the expansion joint to compensate for the installation deviation. The consequence is that the bellows is subjected to additional bending moments outside the design range, and microcracks appear after less than a year of operation. Install this piece, align the deviation of a few millimeters, and the expansion joint can cover the bottom for you, but you can't use the expansion joint as a universal joint.
What do these cases say? For expansion joints, design accounts for 30%, installation accounts for 30%, and use and maintenance account for 40%. You can't just buy it, you have to use it.
Don't just look at quotes when choosing suppliers
The supply market of petroleum metal expansion joints is mixed. Some small factories sell the general-purpose type as the high-temperature type, with shoddy materials and many welding defects. When choosing a supplier, start with the design qualification. Whether there is a manufacturing license for pressure pipe components is the key threshold. Then look at the production standards, EJMA, ASME, GB/T 12777, and whether the model covers the general-purpose type, straight-pipe pressure balance type, double hinge transverse type and other structural types you need.
Non-standard customization ability is the real test. The operating conditions of petroleum plants are varied, and the standard models can't cover them at all. Dare to accept non-standard, be able to produce calculation books, and service providers to cooperate with on-site installation guidance, which are much more important than the quotation of a few more points and a few less points. You bought the device to run steadily, not to buy back a time bomb for cheap.
In the final analysis, the supply of petroleum metal expansion joints means that every link from materials, structure, parameters, installation and after-sales is not relaxed. Only when the early effort is done can the device be stable.