Tie rod is not a display: first understand whether it tubes axial force or lateral displacement
When many people get a metal expansion joint, their first reaction is "Tie rod, isn't it just to prevent the bellows from being pushed open by pressure?" This is half right. In fact, the duties of the tie rod are divided into two kinds: one is to restrain the axial internal pressure thrust and prevent the bellows from over-stretching; The other is to limit the lateral displacement so that the compensation occurs in the direction the design allows. For example, the double hinge transverse expansion joint and the straight tube pressure balance expansion joint, their tie rod (or hinge) system undertakes completely different tasks. If you install it backwards or incorrectly, the bellows may bulge directly when pressing.
Is the tie rod of this compensator in your hand pulled or only guided? Look at the drawings, don't guess.
Must do before installation: check the model, check the transport rod, confirm the initial position of the nut
When the manufacturer delivers goods, the bellows usually have several thick transportation rods at both ends, which lock the expansion joint to a fixed length. That thing is meant to prevent shipping bumps from damaging the bellows, not for you to keep it as a work tie rod. All must be removed before installation, and none remains. And guess what? We really encountered people who had a work map to save trouble, and directly welded the pipeline with the transportation tie rod. As a result, after the temperature rose, the compensation amount was all eaten to death by the tie rod, and the bellows was forcibly pressed to become unstable.
Whether the model is the one set by the design institute, whether the tube caliber is correct, and the initial position of the tie rod nut. There is a common misunderstanding here-the tie rod nut is not tightened casually when leaving the factory. For some preloaded expansion joints, the pre-deformation amount has been reserved at the nut position when leaving the factory. If you loosen it all, it is equivalent to losing the pre-deformation in vain.
Three steps of on-site installation: positioning, alignment and adjusting the pre-deformation amount of tie rod nut
The first step is positioning. The pipe supports and hangers on both sides of the expansion joint must be put in place first and adjusted, so that the expansion joint is at the installation length specified in the design. It is self-deception to forcibly pull the expansion joint to a certain position with a tie rod and then weld it to death-the tie rod is pre-deformed, not used to correct the pipeline installation error.
The second step is alignment. Check whether the center line of the expansion joint coincides with the center line of the pipe and whether the flange surface is parallel. If the deviation is large, the bellows will generate additional shear stress, which will not be reflected in the tie rod. Do not exceed the coaxiality tolerance indicated on the product sample, usually within 1% of the outer diameter of the connector.
The third step is to adjust the pull rod. If it is a working condition that requires cold tightening, screw the tie rod nut to the designated scale according to the design value, and measure the exposed thread length at both ends of the tie rod with a vernier caliper to ensure symmetry on both sides. Once the adjustment is complete, lock the back mother.
Three details that are most prone to rollover: whether the screw is removed or not, the direction of the lock nut, and the over-stroke protection
Detail 1, the screw of the working tie rod cannot be disassembled. Some masters think that the pull rod is in the way, so they remove it after installation. This is a joke about the life of the bellows. For axial expansion joints, the tie rod is the direct force bearing part of internal pressure thrust. If the tie rod is disassembled, the bellows is equivalent to streaking. As soon as the pressure comes up, it will directly extend excessively, and the fatigue life will drop like a cliff.
Detail two, locknut direction. The lock nuts at both ends of the tie rod must be facing toward the force-bearing side (i.e., the bellows side), not outward. If facing outward, the lock nut is easy to loosen under vibration conditions, and the tie rod loses its restraining effect. The inspection method is very simple: twist the lock nut in the reverse direction with a wrench. If it can be twisted, it will be installed backwards.
Detail three, over-stroke protection. Many tie rod nuts have a scale mark or limit ring at the end of their stroke, which limits the maximum displacement of the bellows. During adjustment, it is prohibited to screw the nut out of the limit range, otherwise the bellows may be stretched to plastic deformation. Especially for high temperature axial expansion joints, the length difference between cold state and hot state is very large, so it is necessary to leave sufficient allowable value when installing.
Not finished after installation: Secondary tightening and re-inspection of tie rod status after hot operation
After the pipe heats up to operating temperature for the first time, the force on the tie rod is redistributed. After shutting down and cooling down, don't rush to accept-re-check the locking state of each tie rod nut, and whether the tie rod is bent or abnormal plastic deformation. If the tie rod is found to be slightly bent, it means that the pre-deformation amount is too large during installation or the pipe support fails, which needs to be treated immediately and cannot be dragged to the next overhaul.
Another point is that after the expansion joint is running for a period of time, rust and dust build up on the tie rod surface, which can affect the flexibility of subsequent adjustments. It is recommended to apply molybdenum disulfide grease to the thread of the tie rod, but do not wrap the nut in the grease. During regular inspection, use a marker to draw an alignment line on the nut and the tie rod, and you can see at a glance whether the nut has moved.
So you see, the metal expansion joint tie rod installation is not the iron rods, but the life of the bellows. Spend more than ten minutes to understand the drawings and screw the correct positions, and the operation and maintenance cost saved later can be more than ten minutes.