Two months ago, I went to a chemical factory for a return visit and encountered something bad. On a batch of newly installed pipelines, the inspection report of the size of the metal expansion joint was beautifully written, but it collapsed as soon as it was installed on the spot-the flange bolt holes didn't match, so they were screwed on hard. Within a day of test running, the bellows bulged.
Later, it was checked out that the problem was very simple: a tape measure was used when measuring the length, and the actual temperature of the pipeline on site was not considered. In addition, the pipeline itself had the installation stress, so the errors of a few millimeters were all piled on the expansion joint. The manufacturer is wronged, and the installation team is also wronged. The drawing size is there, but who cares if your drawing is accurate or not?
So, what exactly is the metal expansion joint size check looking for? It really isn't like making two gestures with a ruler.
Dimensional inspection is not just a measure of length-these parameters are the key
Interface size. The nominal diameter of the bellows must match the pipe flange, and the position, number and diameter of the flange bolt holes can't be installed if one is wrong. Generally, there will be no big mistakes in this part, and the problems are usually in the details-such as the form of flange sealing surface, protruding surface or tongue and groove surface, and whether the matched gasket can fit. We do it in our factoryUniversal corrugated expansion jointOn the drawings, the flange standard is clearly written, but the pipe flange brought by the customer may be made according to another standard. As a result, the bolts can be penetrated, but the sealing surface is not aligned.
What is easier to overlook than the interface size is the mounting length. Note that it is not the total length, it is the installation length. When the expansion joint leaves the factory, the bellows has a free-state length, which should be adjusted to the specified size according to the amount of cold tightening (pre-deformation amount) during installation. This cold tightness is deduced from the design temperature. For example, the working temperature of the steam pipeline is 300℃, and it has to be pre-stretched for a certain distance when installed at room temperature to offset the thermal expansion. The threaded tie rod does just that-adjust the nut to change the installation length. When checking the size, if you only measure the flange spacing and do not check the position of the tie rod nut, it is equivalent to a white check.
And the wavenumber and wave pitch of the bellows. The wave number directly determines the compensation amount, and the uneven wave pitch indicates that the bellows has signs of skew or local stress. Take a caliper to measure the distance from the trough to the trough. If the difference exceeds 1mm, after the expansion joint is installed, the force will be uneven, and problems will occur sooner or later. We doExternal pressure single axial expansion jointWhen, the wave pitch tolerance is strictly controlled than that of the ordinary type, because the wave trough is more prone to instability under external pressure conditions.
Don't miss the deflector size either. The function of the guide tube is to reduce the resistance of media flow and protect the inner wall of the bellows. Its diameter and insertion depth must match the inner diameter of the pipe. When inspecting, take a flashlight to see if the guide tube is eccentric and warped, which is more useful than looking at the drawings.
How to do field measurement? Tools, steps, and easy pits
Don't do those fancy tools. A calibrated steel tape measure, a vernier caliper, a gauge gauge and a laser rangefinder are enough. Measure the total length and flange spacing of a steel tape measure, measure the flange thickness and bolt hole diameter with a caliper, and check whether the flange end face is parallel with a gauge-parallelism, which many masters feel, actually comes out with a gauge.
It must be measured when the pipeline is in a natural state, and the expansion joint cannot be hard pulled or pressed. The most common mistake of on-site workers is to hoist the pipeline with an inverted chain and measure it again. At this time, the deflection caused by the gravity of the pipeline itself is all reflected in the measured value, and the measured difference can be 5~8mm. You think the size of the expansion joint is wrong.
The measurement steps are actually not complicated. First, support both ends of the pipe well, so that the expansion joint is in a free state without force. Measure the total length of the first pass and record it. Measure the wave distance in the second pass, measure at least three waves, and take the average value. Measure the parallelism between the flange surfaces in the third pass, and stick the gauge once in each of the four directions. Finally, check the position of the tie rod nut-this step has to be compared with the cold tightness marked on the drawing. Not every expansion joint has a tie rod, but for those with a tie rod, the nut position is a live indicator of the installation length.
By the way, don't forget the temperature compensation thing. The expansion joint installed at more than 30 degrees in summer shrinks by several millimeters in winter. If you measure the size in summer to the pipeline running in winter, isn't that finding fault? Therefore, the dimensional inspection before installation is best done when the pipeline temperature is close to normal temperature, and the influence of ambient temperature on the expansion of the pipeline itself must be counted.
Don't Take Drawing Dimensions as Truth: Installation Errors, Cold Tightening and Actual Working Conditions
The dimensions on the drawings are theoretical and there are always deviations in the field. The amount of shrinkage of pipe welding, the settlement of supports, and the construction error of equipment foundation will eat the colloquial language of expansion joints. Therefore, the installation length is not as much as written on the drawing, but it should be adjusted according to the nozzle spacing measured on the spot. We have aCorrugated expansion joint for power station industryThe installation length of the project is 500mm marked in the drawings, and 498mm is measured after the actual welding of the pipeline. The manufacturer produced it according to the drawings and hard-installed it on the spot. As a result, the compensation amount was reduced by half, and the bellows cracked by fatigue not long after startup.
Cold tight measurement is more specific. For pipes with design temperatures above 100℃, the cold tightness is usually half or more of the thermal compensation. For example, aHigh temperature axial expansion jointThe design compensation amount is 40mm, and the cold tightness amount may be 20mm. During installation, the 20mm is pre-stretched off, so that the bellows can approach the free state at the working temperature during operation. If you don't look at the cold tightness and the total length of light when checking the dimensions, you don't know what state the expansion joint is installed in.
The impact of working conditions should not be underestimated. For the bellows of the vibrating pipe, the installation length must avoid the resonance interval, which must be determined in combination with the natural frequency of the pipe. For expansion joints of corrosive media pipes, if there is a lining layer, the shrinkage rate of lining material must also be considered during dimensional inspection. We doPTFE-lined hoseWhen, the flange width of PTFE lining directly affects the flange seal, and this size difference of 1mm has to be reworked.
What should I do if the size inspection finds deviations? Don't be in a hurry to return to the factory, judge these situations first
The total length deviation is within ±3mm, the wave pitch is uniform, and the parallelism of flange surface is within 1.5mm. This kind of site can be installed without tossing.
If the deviation of the installation length exceeds ±5mm, first see if it is a problem with the pipeline. Loosening the pipe bracket and readjusting the position of the pipe nozzle can most likely solve the problem. If it really doesn't match, either change the pipeline or let the manufacturer adjust the length of the tie rod-note that it is to adjust the tie rod, not to hard pull the bellows.
Wave distance is uneven, look at the direction of deviation. If only the wave pitch on one side is too large, it may be that the bellows was bumped during transportation. Press it with your hand to try if you can reset it. If you can reset it, it will be no big deal; If you can't press it down or have permanent deformation, take a photo and send it to the manufacturer for judgment.
The flange bolt holes are not aligned, which is the biggest headache. Don't rush to reaming, first confirm the flange standard and the distribution of bolt holes. Some flanges used in older equipment are non-standard and inherently do not match the standard flanges of expansion joints. At this time, the conversion flange can be equipped, and the cost is not high, and it can be done in two or three days. If the bolt hole is misaligned due to installation stress, just loosen the pipe bracket and realign it.
There is also a case where the expansion joint itself is fine, but the thermal compensation design of the piping system is defective. For example, the distance between two fixed brackets exceeds the design value, resulting in the expansion joint being overstretched. In this case, it is useless to return to the factory. We have to let the design institute re-check the pipeline layout. We have encountered several so-called "expansion joint quality problems", all of which are traced back to the pipeline design.
When it comes to the root, the metal expansion joint size check is never just the expansion joint itself, but the matching degree between the expansion joint and the whole pipeline system. It takes half an hour to measure it before installing it, which saves much money than installing it and collapsing and then removing it. We have been in this business for 20 years, and we have seen too many examples of scrapping the whole pipeline because one size is not checked. It is really not worth it.