The steps of replacing the metal expansion joint look simple. Isn't it just dismantling the old one and installing the new one? But anyone who has worked in this business knows that the expansion joint is a thing, if it is not installed properly, it will happen to great things. Torn bellows, leaky flanges, sinking pipes, none of which are trivial. Today, break it open and crumble it, and go through the steps of replacing the metal expansion joint from beginning to end. Seven steps, one step is not in place, if you change it, it will be in vain.
Step 1: Find out how the old expansion joint died
The old expansion joint you removed, is it a fatigue crack? Is it corrosion perforation? Or was it pulled out during the installation phase? These three reasons correspond to completely different ways of handling. Fatigue crack indicates that there is a problem in the calculation of thermal displacement of the pipe system or the layout of the bracket. You can directly replace it with a new one, and it will still crack before long. Corrosion perforation depends on whether the media composition matches the material of the bellows. If the material is chosen wrong, it will be useless to change it many times. As for what was pulled out during installation, it was purely the pot of the construction link-the bellows withstood stretching or twisting outside of the design, and this damage is especially common on newly installed pipes. Therefore, before dismantling the old parts, spend half an hour to see the cause of failure clearly. If this step is saved, the rest is all useless work.
Step 2: Shutdown, pressure relief, replacement and isolation, none of them can be missed
To put it bluntly, some people find it troublesome and dismantle it directly. When the remaining hundreds of degrees Celsius medium in the bellows is sprayed out, it is too late to regret it. Steam pipeline, heat medium pipeline, acid-alkali medium pipeline, each working condition has corresponding pressure relief and replacement process. You must confirm that the valve is completely closed, the pressure gauge is zeroed, the medium is emptied, and if necessary, a blind plate is installed for isolation. Especially the high-temperature medium remaining inside the bellows, the temperature can reach several hundred degrees, and it is no joke to burn it. Safety measures are in place, and the work is worth doing.
Step 3: Measure the actual installation length and displacement direction of the old expansion joint
This step directly determines whether the new expansion joint is pre-stretched or pre-compressed. The length difference between the pipeline in the cold state and the hot state ranges from a few centimeters to more than ten centimeters. If you only load it according to the cold size, the expansion joint may die directly in the hot state; According to the size of the hot state, the bellows bears extra tension in the cold state. The correct method is: first measure the actual distance between the two flanges of the old expansion joint, and then calculate the cold tightness according to the design temperature of the pipeline and the type of bracket. By the way, take a look at the displacement direction-use a universal corrugated expansion joint for axial displacement, and choose a compound hinge transverse expansion joint or a universal hinge type for transverse displacement. If the direction is reversed, the bellows will wait to be twisted.
Step 4: Fix the pipe before dismantling the old parts
Many people ignore this step. As a result, once the expansion joint is disassembled, the pipeline collapses with a "clang", and the adjacent valves and brackets are all stressed, ranging from deformation to cracking. Before dismantling, support both sides of the pipeline with inverted chains or temporary brackets to confirm that the pipeline will not be displaced. Especially for large-diameter thick-walled expansion joints, their own weight plus pipeline stress, the impact force in that instant can hurt people. Once the pipe is secured, cut or loosen the connecting bolts. Don't move too fiercely. While dismantling, observe whether there is any abnormal displacement of the pipeline, and stop immediately.
Step 5: Install new parts, model, flow direction, flange surface, each must be checked
Bring the new expansion joint. Don't rush it. First, check whether the model, pressure level and compensation amount are consistent with the design drawings. These parameters are on the nameplate and instructions. Don't be too troublesome, just take a look. Second, look at the flow direction sign. For general-purpose corrugated expansion joints with guide tubes, the direction of the guide tubes must be consistent with the flow direction of the medium. If it is installed backwards, the guide tubes will be washed away by the medium in minutes. Third, the flange faces are aligned, and the bolts are tightened in diagonal order. It is strictly forbidden to screw to death on one side. When the flange is tilted and the bolts are screwed, the sealing surface is unevenly stressed, and the pressure test will leak. When tightening, divide it two to three times, and gradually apply force to make the flange fit evenly.
Step 6: Tie rod and screw rod, loosen when it should be loose, and adjust when it should be adjusted
Did you notice? When the new expansion joint leaves the factory, the tie rod is locked, which is used for transportation and fixation to prevent the bellows from shaking and deforming during transportation. After installation in place, the transportation rod must be loosened, otherwise the expansion joint will not be able to absorb the displacement of the pipeline at all, which is equivalent to white installation. However, there is one exception-tie rods with limit function, such as external pressure single axial expansion joint or straight pipe pressure balance expansion joint. This type of tie rod needs to be set according to the design drawings, and the value is clearly marked in the installation instructions. You just remember two sentences: the transport rod must be loose, and the limit rod must be adjusted according to the diagram. Don't get mixed up.
Step 7: Reset, check, test pressure, the last line of defense
After installation, turn around the expansion joint to see if the bellows are bumped and twisted, whether the flange sealing surface is flat, and whether the insulation layer has squeezed the bellows. After confirmation, make a hydraulic test according to 1.5 times of the system pressure, or make an air tightness test according to the design requirements. During the pressure holding period, don't hide far away, just look around the expansion joint in a circle, look at the weld, the bellows trough and the flange joint surface, and see if there is any leakage, bulge and deformation. After the pressure test is qualified, the insulation layer is restored. If you skip this step, the first six steps are all in vain.
At the end of the day, these are the seven steps to replace metal expansion joints, but it tests your overall understanding of the plumbing system. The expansion joint is not a matter of screwing a few bolts. It is the last line of defense for pipe safety. If you follow these seven steps, you don't dare to say that it's foolproof, but at least you won't make low-level mistakes.