The round metal expansion joint can keep the pipe for ten years if it is installed correctly, but if it is installed incorrectly, it can leak and break the bellows. Two days ago, I met a buddy from a power station project, who told me that they didn't verify the model number during the on-site acceptance.External pressure single axial expansion jointIt was installed on the pipe section that needed lateral displacement, and it collapsed three days after it was put into production. So don't be too long-winded. Today, break these 5 steps apart and crush them thoroughly.
1. Receipt and acceptance: Don't let dimensional deviation ruin the whole pipeline
When the goods arrive, the first thing is not to sign, but to check. Look at the model, nominal pressure, displacement on the nameplate-say what you want isUniversal corrugated expansion jointThe pressure level is 1.6MPa, and the result is 0.6MPa, which is basically equivalent to buriing a mine. Look at the interface size and wall thickness, and take a caliper to measure the outer diameter of the flange, the bolt hole distance and the width of the sealing surface. A difference of 1mm in these three data will lead to failure to install or deviation.
guide tubeIs it in the right direction? The function of the guide tube is to guide the flow direction of the medium and reduce the scouring of the bellows. If it is installed backwards, the medium will directly scouring the corrugations, and it will be perforated in half a year. How to tell? Look at the arrow or mark on the cylinder, usually pointing to the direction of the media flow.
2. Preparation before installation: brackets, pre-stretches, and the often overlooked arrow
Get the bracket layout done first. The fixed bracket shall be arranged at both ends of the expansion joint to bear the thrust of the pipeline; The guide bracket ensures that the pipe is displaced only in the axial direction. There's a simple rule:Circular metal expansion jointThe first guide bracket on each side is not more than 4 times the tube diameter from the expansion joint, and the second one is not more than 14 times the tube diameter. Don't save money on those brackets. If you save it, you're gambling with your life.
Pre-stretching/pre-compression matter, many people just skip it to save trouble. But have you calculated steam pipes with design temperatures exceeding 100℃? When loading cold, you have toHigh temperature axial expansion jointPre-compress a section amount to compensate for thermal elongation. The calculation formula is clearly written in GB/T 12777. Don't pat your head. And the arrow- -Arrow direction of expansion joint(As specifically mentioned in Article 10 of FAQ), it indicates the flow direction of the medium and also implies the direction of stress distribution. Installation backwards will lead to the instability of the bellows.
3. Key points of installation operation: hoisting, matching, bolt screwing and tie rod positioning
When hoisting, use a soft sling, don't use wire rope to strangle the bellows. After alignment, the gap of the counter mouth should be controlled at 2~3mm, and the deviation should not exceed ±1mm. The tightening sequence of flange bolts must be divided into 3~4 steps according to the diagonal method: first pre-tighten 50% torque, then 70%, and finally 100%. Why do you do this? Because unilateral screwing will cause the flange to deflect, the sealing gasket is unevenly stressed, and leakage is a matter of time.
Let's talk moreexpansion joint tie rod— You askedHow to adjust the tie rod nut？ There are two situations: the tie rod nut for transportation and fixation must be loosened to the position corresponding to the design displacement before installation, but do not remove it; If it is with a tie rod to restrict excessive expansion and contraction of the bellows (such asLarge tie rod expansion joint), the nut should be positioned at the calculated cold tight value position, and the nut spacing on both sides should be consistent. Tsk, tsk, I have seen a project in which the installer locked the tie rod nut directly because it was troublesome. As a result, the bellows couldn't be expanded and contracted, and the pipe deformed and the bracket was crooked.
4. 4 Details That Are Easy to Roll Over
First, the direction of the guide tube is reversed.I just said it, and I won't repeat it. But one more question: Have you confirmed that the flow direction marked on the drawings is consistent with the site? Many times the process is changed, and the direction of the guide tube has not been changed.
Second, the transportation fixtures were not dismantled.Round metal expansion joints usually come with several sets of angle steel or screws to fix the bellows when they leave the factory to prevent transport jitter. This thing must be removed after installation and before the system is tested for pressure. I have a customer who tested the pressure after installation and directly pressurized it to 1.0MPa. As a result, the fixing parts were not disassembled, the bellows was held without displacement, and the weld was torn. It is recommended that the action of removing the fixing parts be done before pressing after the matching and bolt tightening is completed.
Third, the tie rod nut is locked.Whether it isCompound hinge transverse expansion jointStillStraight pipe pressure balanced expansion jointThe function of the tie rod nut is to limit rather than lock. When locked, the expansion joint becomes a rigid part, and the thermal expansion and contraction force of the pipe is all suppressed to the bracket and equipment. Correct practice: After screwing the nut to the contact end plate, retreat by 1~2mm, leaving a small gap.
Fourth, the cold tight value is skipped.Especially for steam and high-temperature flue gas pipelines, the cold tightness value is an unsavable step. The cold tightening amount is usually half of the compensation amount, see the design documents for details. Consequences of not cold tightness: After commissioning, the bellows is stretched or compressed to the limit, and its life is directly cut in half.
5. Trial operation inspection: Displacement and leakage detection after heating up
People can't leave during the process of heating and pressurizing the system. Take a marker to draw a straight line on the shell of the expansion joint as a displacement mark. After the temperature stabilizes, measure whether the actual displacement is within the design range. If the displacement exceeds the design value, check whether the bracket slips first, and then check whether the fixed bracket fails.
Use soapy water or ultrasonic leak detector for leak detection, focusing on the flange sealing surface, the weld at the end of the tie rod and the trough of the bellows. If there is a leakage point, tighten the bolts again after pressure relief. Note:Metal expansion jointThe bolt re-tightening must be carried out in the cold state, and the bolt tightening in the hot state is easy to cause overload. In addition, if the bellows itself leaks, don't think about welding and repair, just change the parts-repair welding will change the metallographic structure of the material, and it won't last long.
Circular metal expansion joint installationIn this matter, the details are bigger than the sky. It takes an extra 10 minutes to check during acceptance, which can save the next 10 days of emergency repair. Don't believe it? You try.