Two days ago, a customer called and said that the general-purpose corrugated expansion joint arrived, but the flange screw hole didn't match. After asking, he found out that the caliber he measured was the outer diameter of the wave peak, not the diameter. Anyone who has done this kind of thing on the spot knows it-welding the wrong flange, buying the wrong model, the construction period is delayed for a week, the boss scolds and the workers complain. The measurement of the diameter of the expansion joint looks simple, but many of them overturn. Exactly where to measure? Use what tool? What is the difference between different kinds of expansion joints? This article makes the doorway clear.
Find out first: What exactly does the "caliber" of the expansion joint mean?
The caliber in the pipeline industry usually refers to the nominal diameter (DN), that is, the inner diameter of the pipe. However, the structure of the expansion joint determines that it can't directly measure the inner wall-the bellows has peaks and valleys, the flange has stops, and the guide tube extends a section. You take a tape measure and put a card on the crest of the wave, and the data is at least 10-20mm larger than the diameter. That's the margin left for deformation, not the caliber.
Circular metal corrugated expansion jointFor example, general-purpose corrugated expansion joint and corrugated expansion joint for power station industry, the diameter is equal to the nominal diameter of the pipeline connected to it. For physical measurement, measure the diameter between the sealing surfaces of the flange-that is, the diameter of the inner hole after removing the flange boss. Or measure the inner diameter of the connector directly. Remember: Don't measure the crest, and don't measure the outer diameter of the flange.
Rectangular expansion joints and non-metallic expansion jointsHow to measure? Metal rectangular expansion joints and rectangular non-metallic expansion joints, the caliber is actually the length × width of the rectangular channel. Take a tape measure to measure the long and short sides of the inner mouth of the flange, and pay attention to whether the four corners are right or chamfered. The frame of non-metallic expansion joints (fabric fiber expansion joints) is metallic and the caliber is also the inner frame size. Don't count the thickness of the outer skin when measuring, otherwise it will be too large. In addition, such as PTFE-lined hoses, lined with PTFE may affect the inner diameter. It is necessary to confirm whether to measure the diameter of the metal shell or the lining-usually calculated according to the actual diameter after the lining.
Measuring tools and hands-on details
Choose the right tool and save half the detour. The vernier caliper is the most accurate and suitable for small calibers (below DN200). Use a tape measure for large diameter (DN300 or above), but it must be straightened and fit the end face. Measurement position: Select the inner circle of the flange end face to avoid welds and burrs. Measure two to three directions and take the average value-the pipeline may have ellipticity, especially large-diameter thick-walled expansion joints or directly buried expansion joints. Excessive ellipticity will lead to lax seal.
Does the expansion joint come with a deflector? The inner diameter of the guide tube is generally consistent with the diameter, but the front end of the guide tube may shrink. When measuring, it should be measured from the flange surface to the inner wall of the guide tube. Don't be lazy and only measure the outer port, otherwise install it and find that the guide tube holds up against the pipeline, so rework it as well.
So what to do? If conditions permit, it is best to actually remove and measure, or check the original equipment nameplate-the model and size of the expansion joint are generally marked DNxx on the nameplate. Before buying, take a photo of the nameplate and send it to the manufacturer for confirmation, which is more reliable than your own quantity.
It is not enough to have the caliber alone, but to give all these 4 data in the selection
Caliber is just the first step. When buying an expansion joint, you must also report the connection mode (flange or take-over), nominal pressure (PN), working temperature, medium, and compensated displacement (axial, transverse and angular). For example, high-temperature axial expansion joint is used in steam pipeline, and the temperature is not reported, so the consequences of choosing the wrong material are serious. Another example is the expansion joint matched by the desulfurization flue gas baffle door. The medium is corrosive, so the sulfur content, temperature and pressure must be indicated. Also, the number of flange bolt holes, the diameter of the distribution circle, and the form of the sealing surface (convex, flat) must be provided-this directly affects the ability to match your existing pipe flange.
Do you find it troublesome? The price of trouble is that the arrival can't be loaded, the return freight is paid out of pocket, and the construction period is delayed by yourself. Thinking back, it is much more cost-effective to spend 10 more minutes writing all the parameters than tossing for a month.
Finally, let's talk about a few places that are prone to fall into pits
FirstDon't take the mounting size of the non-metallic expansion joint as the caliber. The mounting size is typically larger than the through diameter because the outer edge of the flange is covered. You report the installation size in the past, and the manufacturer produces it according to the diameter. When you install it back, the flange edge is exposed outside, and the sealing gasket is not compacted.
Second, rubber compensator or rubber PTFE compensator is soft, gently press when measuring, do not deadlift, otherwise the data is on the big side. Soft materials deform as soon as they are pulled. You measure 300mm, but in fact, the diameter is only 250mm. It is best to gently lean against it with a caliper, not hard.
ThirdWhen the old equipment is replaced, it is the most hassle-free to check the original nameplate. If you can't see the nameplate clearly, remove it to measure the inner diameter of the flange, or look for the design drawings. If it doesn't work, take the outer diameter of the pipe minus the wall thickness to estimate it-but this method has a large error and is only suitable for emergencies.
Place an order after the quantity is accurate, so as to save returning and exchanging back and forth and delaying the construction period. Remember: the diameter of the expansion joint measures the diameter, not the crest; The inner diameter is measured, not the outer diameter of the flange; Clearance is measured, not hemming size.How to measure the expansion joint diameter?The answer is one: measure the diameter of the inner hole between the end faces of the flange, the diameter of the circle, the length and width of the rectangle, and the attention of the guide tube to avoid the shrinkage. Choose a caliper or tape measure as a tool, and measure three directions to take the average. When the data is complete, the rest will be smooth.