Oil Pipeline Selection Guide: Three Key Decisions for Customizing Metal Expansion Joints
The oil pipeline you are responsible for is about to have expansion joints. Looking through the samples, you find a lot of models-general-purpose, axial, pressure-balanced... Then the purchasing department threw a sentence, "Can't you buy a standard one?"
Oh, no. I can't do it. Any one of the three mountains of oil conditions-medium, temperature and pressure-can rub the "standard" on the ground. Let's talk about it today,Customization of petroleum metal expansion jointsHow the hell should we start.
1. Why must the oil working conditions be customized? — — The three mountains of medium, temperature and pressure
Let's start with media. Crude oil, refined oil, associated gas, which is not corrosive? Some contain hydrogen sulfide, some contain chloride ions, and ordinary 304 stainless steel bellows may be perforated in a few months. You have to go with 316L, duplex steel or even nickel-based alloys. This is not a slap on the head. You have to check the corrosion rate table according to the composition, concentration and temperature of the medium, and then determine the material. Standards? How does it know if you're running light oil or high-sulfur crude oil in your pipeline?
Let's talk about the temperature. In the petroleum refining unit, the high-temperature section can reach above 600℃, and the low-temperature section has LNG of-196℃. The common carbon steel expansion joint may creep at high temperature and brittle fracture at low temperature. When customizing, you must calculate the thermal expansion amount clearly, and select the correct number of bellows layers and wall thickness. Such asHigh temperature axial expansion jointSpecifically for steam and hot oil pipelines, the temperature resistance grade and fatigue life are completely different.
Final pressure. Oil pipelines are often dozens of MPa, and low-pressure ones are five or six kilograms. The higher the pressure, the wall thickness, wave height and wave pitch of the bellows have to be recalculated. If the general-purpose type is used, the wall thickness is not enough to blasting directly-this is no joke.
Therefore, petroleum metal expansion joint customization is not optional, it is a must. Not customized? The pipeline is waiting for repair.
2. The first step of type selection: first determine the structure, then talk about the parameters-how to choose from axial type to pressure balance type
A lot of people ask "What caliber? What displacement?" as soon as they come up – the direction is wrong. First determine the structure type, and then talk about the dimensional parameters.
The simplest single axial displacement, such as the thermal elongation of a straight pipe section, is usedUniversal corrugated expansion jointOrAxial expansion jointThat's enough. But if the pipe is displaced in multiple directions, such as an L-shaped pipe section, you needCompound hinge transverse expansion jointOrCurved tube pressure balance expansion joint。 The pressure balance type is a good thing, which can cancel out the blind plate force and avoid the pipe support being made like a bunker. LikeStraight pipe pressure balanced expansion jointAndCompound straight pipe bypass pressure balanced expansion jointHe often appears in high-pressure and large-diameter oil pipelines.
Also, something likeExternal pressure single axial expansion jointSuitable for buried or space-limited scenes, the bellows is on the outside, the medium is not directly washed, and the life is longer. AndSleeve type pipe expansion jointAlthough it is also a compensator, it has a simple structure and high pressure, and is also used in oil pipelines.
After the structure is settled, the parameters such as caliber, wave number, total compensation amount and stiffness are analyzed. The pit of stiffness will be detailed later.
3. Pits in customization: deflectors, tie rods, stiffness-these details determine whether the equipment can hold up for ten years
Two days ago, I met a customer, and I bought an expansion joint without a deflector for cheap. As a result, it took half a year, and the inner wall of the bellows was washed like a washboard by the medium. The function of the guide tube is not only to guide the airflow, but also to protect the bellows from being directly washed by high-speed media. In particular, there are solid particles (such as catalyst powder) in oil pipelines, and there is no guide tube, so the life of bellows is directly discounted by 30%. Regarding the specific function of the guide tube, there are detailed diagrams in the FAQ of our station.
Let's talk about the pull rod. Many people think that the pull rod looks good when installed. Wrong.Function of expansion joint tie rodIs to absorb and balance the internal pressure thrust and prevent the bellows from excessive elongation or compression. After installation, how to adjust the tie rod nut? Note that after the installation is completed, the locking nut on the tie rod should be loosened a little, so that the bellows can deform freely, but it can't be completely disassembled-the internal pressure thrust is fully pressed on the fixed bracket, and the bracket can't bear it. We also have FAQ for specific adjustment methods.
Stiffness is even bigger.Stiffness and Calculation Formula of BellowsIt directly affects the reaction force of the equipment on the pipeline. If the stiffness is too large, the force on the pipe support will be greater; If the stiffness is too small, the bellows may become unstable. When customizing, be sure to let the manufacturer provide a calculation book, including single-wave stiffness, overall stiffness and fatigue life estimation. Don't just look at the price, bargains are often substandard.
4. Manufacturing and inspection: welds, bellows layers, factory hydraulic tests-don't let low price orders ruin your pipelines
After the custom contract is signed, do you think it's done? Have to keep an eye on the manufacturing process.
The weld is the weak link. Especially the circumferential weld between the bellows and the end pipe and the guide tube must be inspected by 100% radiation or ultrasonic. Standards may only be sampled, but the customization of petroleum metal expansion joints must be fully inspected.
The number of bellows layers is also particular. Single layer has high withstand voltage but poor flexibility, while multi-layer (such as 2~4 layers) has good flexibility and long fatigue life, which is suitable for scenarios with frequent displacement. However, the more layers, the more difficult it is to manufacture, and the poor fit between layers is easy to bulge. Is there any interlayer helium leak detection in the factory? Ask clearly.
Factory hydraulic test, don't just look at how much pressure hits, but also look at the pressure holding time and leakage. The test pressure is generally 1.5 times of the design pressure, and the pressure is kept for at least 10 minutes. Only when there is no leakage is it qualified. In addition, the expansion joint should be tested for stiffness and pre-stretched for fatigue before leaving the factory? Many small factories directly omit it. How dare you pretend to be this kind of thing?
Oh, by the way, andMetal expansion joint weight table— Don't think heavy is good, but too light is definitely fishy. Material thickness, wave height and wall thickness must match the design drawings.
5. Installation and maintenance: Can the screw be removed or not? How to adjust the tie rod nut? — 90% of the faults at the site are here
Finally, talk about on-site installation. I dare say that 90% of expansion joint failures are not a product issue, they are installed wrong.
Do you need to remove the screw of the expansion joint？ This is the most common problem in the field. Both ends of the expansion joint will be locked with transport screws for transportation protection when leaving the factory. These screws must be removed before installation! Otherwise, the bellows won't be able to stretch or contract at all, and it's a fart to compensate. Some workers try to save trouble and don't dismantle it. When the pipeline is hot, the bellows blows up.
How to adjust the tie rod nut of expansion joint？ Simply put: After installation in place, loosen the locking nut on the pull rod so that the pull rod can slide freely. However, the nuts at both ends of the pull rod can't be removed, only loosen a little, so as to ensure that the pull rod can move when the bellows is stressed. There are pictures and texts in our station for specific operation.
In addition,Directional arrow of expansion jointIt refers to the flow direction of the medium. If the guide tube is installed opposite, the direction is wrong, and the bellows is also washed. Check the drawings and nameplate before installing.
In terms of maintenance, check every six months whether there are cracks and corrosion points on the surface of the bellows, and whether the tie rod nut is loose. If so, don't screw it yourself, contact the manufacturer.
Don't believe the "standard expansion joints also work" nonsense. Oil pipeline is a place to exchange your life for money. Save this customization fee. If there is an accident back, who will carry it? Choose the right structure, keep an eye on manufacturing, and standardize installation. Only when these three points are achieved can your pipeline system run safely for ten years.