Why do tie rods and screws have to be "released" when transported? — — Not releasing is equal to white clothing
When the metal expansion joint leaves the factory, the manufacturer will install temporary fixings-tie rods, nuts and screws on the bellows in order to make you worry-free in transportation and installation. The purpose is simple: forcibly restrict the deformation of the bellows and prevent it from being damaged by bumps on bumpy roads. But when you think about it, these fixings are like putting a "shackle" on the expansion joint. Once the pipeline system is put into operation, the temperature rises and the medium flows, and the pipeline will expand, contract and vibrate. If the shackles are not unlocked, the bellows cannot move, and all thermal displacements and mechanical vibrations can only be hardened. And the result? The stress is concentrated in the trough and weld of the bellows, and the fatigue life drops directly. When it is serious, it will crack and leak in a few months, and the maintenance cycle will be cut off in half. Is that the truth? Therefore, release is not something that can be done or not done, it is something that must be done and done right.
Timing of Release: Before or After Hydraulic Test? A lot of people are rolling over here
All transport fixings must be released after installation and prior to hydraulic testing of the pipeline. Why release before pressure test? Because the internal pressure of the pipe is very high during hydraulic test (usually 1.5 times the design pressure), if the expansion joint is still locked, the bellows cannot expand and contract freely, and the internal stress will be superimposed on the weld and the corrugated root. That pressure shock directly causes permanent damage-plastic deformation or even micro-cracks in the bellows, and it is too late for you to find out.
Expansion joints designed with pre-tension or pre-compression requirements. For example, for high-temperature axial expansion joints, some projects will let manufacturers do pre-stretching to compensate for cold tightness. At this time, it is necessary to adjust the pre-deformation amount according to the drawings and then fix it again, and then release it after the pressure test is completed. However, this kind of working condition is rare, and most projects are directly released. After you get the equipment, look at the nameplate or design drawings first. If you don't write pre-stretch, without saying anything, loosen the nut before pressure test.
Three-Step Release Operation: A Case Study of Universal Corrugated Expansion Joint and Direct Buried Expansion Joint
Release sounds easy, just screw it. However, there is a particular practice, and it is divided into three steps:
Step 1:Locate the locking nuts at both ends of the tie rod and loosen them with a wrench. Attention! Do not remove the nut completely to prevent the tie rod from falling off into the pipe or injuring someone. Just loosen enough that the nut can rotate freely.
Step 2:The tie rod is loose enough to slide freely axially. Usually, a gap of 2-3mm is reserved, and you can push and pull the pull rod with your hand, and you don't feel stuck. For directly buried (fully buried) expansion joints, since insulation material is filled between the outer sleeve and the bellows, check whether the guide sleeve is stuck simultaneously when releasing. How to check? Turn the guide sleeve by hand, or probe the gap with a gauge to ensure that the insulation layer is not squeezing the bellows.
Step 3:If it is a high-temperature axial expansion joint or an external pressure single axial expansion joint (the kind with the bellows on the outside), push and pull the bellows with your hand after releasing to confirm that there is no jamming. The cylinder of external pressure single type is delicate, so be careful not to scratch the surface of external pressure cylinder when operating the wrench. Also remember: the screw cannot be removed directly, just loosen the nut, because the screw itself also plays a limiting role to prevent the bellows from being over-stretched and causing instability.
What happens if the release is not in place? — Three real cases tell you the cost
Steam pipeline of a thermal power plant, after installation, forgot to remove the expansion joint tie rod nut. After three months of operation, penetrating cracks appeared at the trough of the bellows, which cost 80,000 yuan for maintenance and replacement, and more than 100,000 yuan for production shutdown. Guess what the reason is? That is, the bellows is locked, thermal stress is nowhere to release, and fatigue cracks.
Case 2: A chemical plant directly buried the pipeline. When it was released, the worker only loosened one nut, and the other side was stuck and left alone. Results After the pipeline heated up, one side of the expansion joint could move and the other side could not move, the offset was insufficient, and the pipeline support was hard-pulled and desoldered.
Case 3: Compound hinge transverse expansion joint, the hinge flexibility was not checked after release. During operation, the hinge jammed, and the bellows was torsed and torn-the hinge structure was originally used to absorb lateral displacement. When it jammed, it forced the bellows to bear torque, which it couldn't bear.
Release is not as simple as a screw, and the degree of freedom of each component must be confirmed item by item. Alas, sometimes it is just to save trouble for that minute, and then you have to spend dozens of times the cost to make up.
Differences in release requirements for different product models-do not apply general-purpose experience to special models
The general corrugated expansion joint and the directly buried expansion joint were mentioned earlier, but other models have their own temper.
For directly buried (fully buried) expansion joints, confirm whether the insulation layer presses the bellows before releasing. Some insulation materials are swollen by moisture, which can jam the bellows. External pressure single type axial type expansion joint, bellows on the outside, pay attention to protect external pressure cylinder when releasing to avoid tool damage. Compound hinge transverse type expansion joints and compound straight tube bypass pressure balance type expansion joints have complex tie rod structures-usually with primary tie rods and secondary tie rods. The order of release is particular: first loosen the main tie rod, and then loosen the auxiliary tie rod. Why? If the secondary tie rod is loosened first, the hinge will be offset, causing the bellows to skew. Special vacuum hose and PTFE-lined metal hose, the internal lining is brittle material such as PTFE or rubber, and it is strictly forbidden to operate barbaric when released to avoid twisting or excessive bending, otherwise it will be unusable if the lining is cracked.
By the way, there are also corrugated expansion joints for power station industry and metal corrugated expansion joints for cement industry. These expansion joints in high temperature conditions usually have heat insulation layer. Don't break the heat insulation layer when released, otherwise the heat will be directly transferred to the bellows, and the life will be discounted.
Inspection and maintenance after release: Once in place does not mean once and for all
The release is complete, and you think it's all right? It's still early. It is recommended to stop and check whether the actual displacement of the expansion joint meets the design value after the pipeline heats up to the design temperature for the first time. Specific method: Measure the position change of the marking points at both ends of the tie rod, or use the displacement indicator to read. If the displacement is found to be insufficient or too large, it may be that the release is not complete (for example, a nut is not loose in place), or it may be that the pipe bracket is blocked from sliding-the bracket is stuck, and the expansion joint has to be carried by itself.
During daily inspection, pay attention to whether there are loose marks of the tie rod nut. Vibration in operation can cause the nut to slowly retreat, especially with some older expansion joints that don't have a loose washer. It is recommended to tighten the nut once in half a year, but don't screw it to death-or keep a gap of 2-3mm. For high-temperature working conditions (expansion joints used in power stations and cement industries), the bellows surface should be checked for wear, corrosion points or fatigue cracks during annual maintenance. These hidden dangers often start to accumulate from improper release, are not visible for the first few months, and the problem is exposed after a year.
Therefore, the release requirements of metal expansion joints look simple, and there are many details in practice. Don't bother, release when you need to release, check when you need to check. After all, the length of the pipeline maintenance cycle depends on whether this step is done correctly.