This flange bolt thing, it looks unremarkable, but it is the last line of defense for the sealing and safety of the expansion joint. I have seen too many cases-the designer spent a lot of energy choosing the material of the corrugated pipe and calculating the compensation amount. As a result, the bolt was randomly matched with carbon steel, and it leaked after half a year of operation, and even the whole expansion joint was pulled off. Today, we will start from the actual project, break up the selection logic and common misunderstandings of metal expansion joint flange bolts.
1. Material and strength grade: not the harder the better, but the more "right" the better
Some buddies come up and ask "Is level 8.8 enough? Is level 10.9 OK?" – come on, strength levels are not a master key. High temperature conditions are the real touchstone. For example, with corrugated expansion joints in the power station industry, the temperature of steam pipelines always goes to 500℃, and ordinary carbon steel bolts are directly softened above 400℃. As soon as the preloading force drops, the flange surface leaks. In this case, 35CrMoA or 25Cr2MoVA should be used, and the creep strength at high temperature can be withstood after quenching and tempering treatment.
Let's talk about corroding the environment. The expansion joint matched with the desulfurization flue gas baffle door contains SO₂ and H₂S in the medium, and the pH value is as low as 1-2. Some people want to use galvanized bolts cheaply, thinking that they can prevent rust. As a result, the zinc layer peels off after three months in the acidic smoke, and the bolt body begins to pit. Listen to my advice: this working condition directly on 304 or 316L stainless steel bolts. Don't feel bad about the difference, a parking loss from a leak accident is enough for you to buy bolts for ten years.
2. Specifications and dimensions: Don't just look at the nominal diameter, but calculate the load
Bolt specifications (M16/M20/M24) and number depend on nominal expansion joint pressure, temperature, and pipe thrust. For example: DN600's general-purpose corrugated expansion joint is usually equipped with M20×8 pieces below 1.0MPa. Does it seem enough? But if you have a deflector (see our stationSpecific Function of Expansion Joint Guide TubeThat article), the deflector will increase the medium flow resistance, causing the axial thrust to become larger. Recalculated according to EJMA standards, the bolt load may have to be added by 20%.
How does that count? Actually, we haveMetal expansion joint weight tableYou can reverse the bolt force-but to be honest, that's just a reference. The most reliable way is to directly provide the working condition parameters (temperature, pressure, medium, pipeline layout diagram), and we will give you a calculation book. Especially for complex structures such as compound hinge transverse expansion joint and straight pipe pressure balance expansion joint, the calculation of bolt load must be checked item by item, so don't blindly estimate it yourself.
3. The most neglected pit during installation-bolt preloading force
Who can't screw bolts? But how tight is the right fit? Many on-site workers are polarized by their hand feeling: either they are too tight, and the flange sealing surface is deformed. Especially for flexible end faces such as PTFE-lined metal hoses and rubber compensators, the flange surface is soft seal, and excessive preloading force directly cracks the PTFE layer or rubber pad, leaking on the spot. Either it is screwed too loosely, and the medium pressure will spread the sealing surface open after running for a period of time. Tsk, not at either end.
Symmetrically tighten with torque wrench in 2-3 times, and the final torque is calculated according to GB/T 196. Note that the adjustment of the tie rod nut of the expansion joint is not the same thing as the flange bolt-the tie rod nut is responsible for limiting the position to prevent the expansion joint from over-stretching or compressing, and it adjusts the axial displacement range; While the flange bolt tube is sealed. Don't be confused. I have seen people screw the tie rod nut as a bolt, and as a result, the tie rod is deformed and the expansion joint is stuck. Detailed adjustment methods can be found in our stationHow to adjust the tie rod nut of expansion jointThat Q&A.
4. What do you see in daily inspections? Two key points
Any noticeable corrosion or loosening of the bolts. Especially for high-temperature axial expansion joints and directly buried expansion joints, the expansion joints of pipelines will expand and contract repeatedly after thermal cycling, and the bolts are prone to stress relaxation. If you think about it, the elastic modulus of the bolt material drops at high temperature, and coupled with vibration, the preload force is lost a little bit. It is recommended to sample a few wrenches with torque wrenches every quarter, and if the torque is less than 80% of the initial value, make it up.
Whether the flange clearance is even. If the clearance on one side becomes larger, it may be that the pipe is skewed, and you must check whether the expansion joint has exceeded the displacement. The service life of the expansion joint often depends on the maintenance of the bolt. Don't wait until it leaks before replacing it-at that time, the flange surface may have been eroded by the medium, and it can't be saved even by replacing the bolt.
V. Summary: There is no general formula, but there is a shortcut
When it comes to choosing bolts, each item has to be determined in combination with temperature, pressure, media and pipeline layout. Want to save trouble? Report the working condition parameters directly to us, and match the flange bolts according to the expansion joint model you selected (such as compound hinge transverse type or straight pipe pressure balance type). Pre-assemble them before leaving the factory, and just tighten them on the spot. Our station still hasCorrect installation method of expansion joint of large tie rodWait for practical articles, it is recommended to read them together.
Metal expansion joint flange bolts are not consumables, they are safety pieces. Every penny you save on the bolts, you end up having to get it back in a leak accident. Is that the truth?