What exactly are metal expansion joints carrying in petrochemical plants?
In the device you have, does the temperature break through the bellows first, or does the corrosion eat it first? Many engineers report the temperature and pressure as soon as they come up. These figures are indeed important, but what really decides the selection is often the medium-those process media containing sulfur, chlorine and acidic components.
High temperature determines the creep limit of the material, and high pressure determines the pressure bearing capacity of the corrugation, but the corrosiveness of the medium determines the life and death of the corrugated pipe. If the temperature is higher, you can change it to a material with a higher temperature resistance grade; With a little more pressure, you can increase the number of layers or adjust the wave distance. But if you choose the wrong material, cracks and pitting will come to your door before you have finished a major overhaul cycle. None of the media running in petrochemical plants are good-natured, and they can't match the media when selecting the type. Later rework is a trivial matter, and unplanned parking is really fatal.
How to choose the material and structure of corrugated pipe?
For the material, the most commonly used in the petrochemical industry is austenitic stainless steel, with 304 and 316L base, high temperature and high pressure or strong corrosive medium, directly go to Inconel 625 or Hastelloy C-276. Don't save money on this-the expansion joint is the flexible joint of the pipe system. If it breaks, the stress of the entire pipe system goes to the flange and bracket of the equipment.
Structurally, the universal corrugated expansion joint is suitable for the pipeline with small displacement and limited space. External pressure single axial type expansion joint is different. Its bellows bears external pressure, better stability and larger stroke. It also absorbs axial displacement, and the performance of external pressure single type is obviously more stable under high pressure and large displacement conditions. When choosing a structure, look not only at the displacement on the sample, but also at the actual fatigue life after the superposition of pressure and temperature.
Guides, pull rods, hinges: these accessories are not furnishings
Many Party A think the diversion tube is dispensable. Would you really try to tear it apart? The medium washes the bellows trough at high speed, and the corrosion rate is several times faster than static corrosion. The function of the guide tube is to let the medium flow smoothly and separate the bellows from the high-speed fluid, especially in gas-liquid two-phase flow or pipelines containing particulate media. Without the expansion joint of the guide tube, it is polite to discount the life by 30%.
The action of the tie rod and hinge is more direct-constraining the blind plate force. For high-pressure pipelines, the larger the cross-sectional area of the expansion joint, the more terrible the blind plate force. The tie rod is subjected to internal pressure thrust, and the hinge constrains lateral and angular displacement. Without these things, the expansion joint will disengage the top of the pipe. Two days ago, I encountered a case in a chemical plant. During the pipeline pressure test, the expansion joint tie rod was not adjusted properly, and the flange gasket was directly broken. You have to say that attachments are not important, and speak for safety incidents.
Pre-deformation, arrow direction and tie rod nut during installation, if mistaken, they will be scrapped directly
There are three things to install. If one is wrong, the expansion joint will basically be wasted.
Let's start with the arrow direction. The arrow on the expansion joint indicates the flow direction of the medium. If it is installed backwards, the guide tube can't play a protective role, and the bellows is directly washed by the medium, so early leakage can't escape. The direction of the arrow should be checked clearly before the pipeline pressure test. If this step is wrong, it will be all wrong.
Pre-deformation, also known as cold tightening, is to make the expansion joint in the best stress state when working in the hot state. How to calculate the amount of cold tightness? According to the difference between the installation temperature and the working temperature of the pipeline, multiplied by the thermal expansion coefficient of the pipeline, this data will be given by every design institute. If cold tightening is not done during on-site construction, the bellows may not be able to reach the design compensation amount during hot operation, and the displacement is exceeded, and the fatigue life will collapse.
What about tie rod nuts? Before installation, adjust the tie rod to the specified pre-tension or pre-compression value, and loosen it after installation. Yes, this set of nuts must be relaxed to the designated position before the pipeline is officially put into operation, so that the expansion energy saving can deform freely. Many construction teams don't know this detail. The tie rod is locked tightly, and the expansion joint becomes a rigid short joint. Where does the stress of thermal expansion and contraction of the pipeline go? — Looking for the weakest flange or weld.
How many years of use is considered normal? How to judge from the appearance that it is time to change
The design life of metal expansion joints for petrochemical use is generally calculated according to the number of fatigue times, and 15 to 20 years is a common range. But to be honest, there will always be differences between the actual working conditions and the design conditions. The fluctuation of medium temperature, the number of starts and stops, and the impact of operating pressure are all variables that affect the actual life.
How do you judge that it's time to change? Appearance inspection can show a lot of signs. Pitting pits or rust spots appear on the bellows surface, especially in the trough position, which is a precursor of chloride ion corrosion. The corrugation spacing is obviously uneven, indicating that plastic deformation or instability has occurred. Micro-cracks appear in the heat-affected zone of the weld, even if you discover them only by magnetic particle detection, it is a scrap signal.
The wear marks at the outlet of the guide tube should also be paid attention to. After wearing out, the bellows directly contacts the medium flow beam, followed by leakage. There is also a hidden signal-the insulation layer on the surface of the expansion joint is abnormally wet, or the insulation aluminum skin has corrosion marks, which is highly probable that the bellows has a tiny penetration point.
What about inspection cycles? Daily inspection is carried out to see the appearance once a month, comprehensive inspection is carried out when work is stopped, and PT/MT flaw detection, wall thickness measurement and stiffness test are all carried out once. If you find any item unqualified, don't hesitate to change it. An expansion joint can sustain tens of thousands to hundreds of thousands of dollars, and an unplanned parking loses millions of output. Anyone can settle this account.