Two days ago, I met a customer, and I called in a hurry: the old expansion joint was burned out, and the newly ordered goods couldn't be installed-a full 20mm difference. 20mm, the welder on the spot scolded his mother, and the construction period was delayed for two days. Finally, it was found that the wave number was calculated wrong when measuring the size. This kind of thing happens countless times a year in power plants, steel mills and chemical plants.
How to measure flue expansion joint specifications? Six words to put it bluntly:Recognize the shape first, then lower the ruler。 But there are all pits between the recognition and the lower ruler. Instead of theory today, let's talk with a caliper and a tape measure.
1. Life and death line before measurement-clarify the type and installation condition of expansion joint
When you stand in front of the flue, the first thing you do is not take out the ruler, but look up. What's to watch? See it isSingle typeStilldouble？ YesAxial typeStillAngular type？
There was a project last year, and a photo was taken on the spot and sent: an expansion joint was installed on the vertical flue, flanged connection, and the nodes were evenly spaced. Purchase according to the reported axial type, as a result, when the goods arrived, it was found that they were curved tube pressure balance type-these two are similar in shape, but the length of the end tube is completely different from the structure of the guide tube. Finally, the order was changed in a hurry, and the freight cost was 8,000 yuan more.
1. Installation position (horizontal/vertical/elbow)
2. Working medium (flue gas, sulfur gas or air? Temperature range?)
3. Is there a limit tie rod or guide tube (which direction does the guide tube face?)
If you don't figure these out, the numbers you measure are waste paper.
2. Disassemble the core parameters one by one: how to measure the diameter, length, wave number and wave distance
The most basic four parameters, let's talk about them one by one.
Diameter (DN): Don't measure the outer ring of the flange, that's a deception. Find the annular surface on the inside of the flange sealing surface and measure the inner diameter with a tape measure. If the flange is welded to the pipe, measure the inner diameter of the pipe. The error is controlled within ±2mm. There's a lazy trick: measure the diameter of the center circle of the flange bolt hole, and reverse the diameter according to the standard-but only if the flange standard is clear (such as HG/T 20623 or GB/T 9119).
Total length (L): Distance from one end flange sealing surface to the other end flange sealing surface. Note that if the expansion joint has connections at both ends, measure port to port, not to the outer wall of the bellows. Some people at the scene took a tape measure on the ripples to measure it. When they let go, the tape measure bent and read out 520mm, but in fact it was only 495-this error directly caused it to not fit in.
Wave number (n): Count how many full crests the bellows has. Single or multi-layer? In multiple layers, the wavenumber of each layer is usually the same. Note when disassembling an old piece: Some nodes have been crushed to death, and it looks like one is missing. Take a flashlight and shine on the bottom of the trough.
Wave pitch (W): The distance between the centers of two adjacent crests. Measure with a caliper and average at least three different locations-because there is an error of about ±1mm when the bellows is manufactured. The wave distance directly determines the compensation ability. If the difference is 3mm, the compensation amount will be 10% different.
Third, do you think it's over after measuring the size? Compensation measurement cannot be skipped
Many brothers issued purchase orders after measuring the length, width and height. As a result, as soon as the equipment ran, the expansion joint cracked. Why? He didn't look at the nameplate or the design documents at all.
How to measure the amount of compensation (Δ L)? It depends on whether the installation is cold or hot. If the site is already operating temperature (such as hot flue), you have to measure the current compression/tension state of the expansion joint. Most axial expansion joints are at free length in the cold state and compressed in the hot state. Take a tape measure to measure the current length, and then compare it with the free length on the design drawing. The difference is the amount of displacement that has been generated. But note: If the expansion joint is pre-stretched at the factory (for example, pre-stretched by 10mm), you have to add this 10mm back to calculate the total compensation requirement.
4. Five pits that are easy to step on at the measured site (with real case data)
Pit 1:Calculate the length of the guide tube into the total length。
In a cement plant, the guide tube extends out of the end pipe by 15mm. When the total length is measured on site, it is measured together with the guide tube, and the reported length is 15mm. After installing it, the flange didn't match. Finally, a section of the guide tube was cut off and re-welded-the node was full of welding slag, which leaked after 3 months.
Pit 2:Ignore flange deflection angle。
When the flue is installed for a long time, the flanges at both ends may not be parallel. Take a level ruler and hit it. If the deviation exceeds 3°, you must measure the spacing in four directions (0°, 90°, 180°, 270°), and take the minimum value as the total length. Otherwise, you order according to the maximum spacing, and it is inclined when installed, and the side stress will burst the pipe directly.
Pit 3:Confusion of cold and hot。
One waste heat boiler, design temperature 400℃, direct order after on-site cold measurement. As a result, the expansion joint was compressed by 60 mm after the equipment was started, exceeding the design limit, and cracks appeared after 2 cycles. The right thing to do: Check the thermal displacement in the design file and add this displacement to the cold free length as the order length-or just tell the supplier the cold free length and thermal displacement and let them calculate.
Pit 4:Wrong number of bellows layers。
Thermal insulation layer may be sandwiched between the outer layer and the inner layer of multi-layer corrugated pipe, which is easy to leak with the eye. Take a magnet and suck it? Stainless steel is not magnetically conductive, but the magnetic permeability is different from grade to grade. The most reliable method: cut a sample of scrapped corrugated pipe, with the number of layers in cross section. If it is not easy to cut, clamp the outside of the bellows with a caliper, and the reading is subtracted from the estimated wall thickness of the single layer. The error is controlled at ±1 layer.
Pit 5:Ignore installation space constraints。
After the expansion joint is installed, it is necessary to leave an inspection space on one side of the bellows. Some site pipes are close to the wall, and you measure the right size, but the wrench can't fit in when the worker disassembles and assembles it. Take a video in advance and send it to the supplier to confirm whether the end tube length and flange bolt position are easy to operate.
5. How to sort out the measurement data? Key points of drawing review and supplier communication
It is not enough to keep the data in a notebook-you have to draw a simplified diagram. Even if it is painted by hand, the total length, diameter, wave number, wave pitch, wall thickness, number of layers, flange standard, number of bolt holes and diameter of distribution circle in four directions should be standardized. The graph indicates the temperature at the time of measurement (if hot) and the state of the expansion joint (free/compressed/stretched).
Before sending it to the supplier, do an arithmetic problem:
Diameter × pressure grade × compensation amount ≈ effective area of bellows × material yield strength × safety factor. If you find that the number is beyond the norm (for example, the expansion joint compensation amount of DN1000 is marked with 200mm but the wave number is only 4), call directly to question.
-Bellows material (304/316L/254SMO or superalloy?)
-Guide material and thickness (usually 2-3 times thicker than bellows)
-With or without lining rings, limit rods, transport supports (these are to be removed upon arrival)
-Whether it is necessary to pre-stretch/pre-compress before leaving the factory (tell the supplier the actual displacement amount on site)
When buying a new piece, be sure to keep a photo of the nameplate on the old piece。 The nameplate is printed with the factory number, pressure rating, temperature range, compensation amount-these data are more accurate than you can measure yourself. If the nameplate burns out, you can only test it step by step according to the above method. Don't be troublesome, it's even more troublesome if you can't fit it.
How to measure flue expansion joint specifications? Don't think of it as a simple measurement. It is a comprehensive judgment of field working conditions, mechanical parameters and manufacturing tolerances. By completing the above six steps, at least 80% of docking accidents can be avoided. The remaining 20%, by experience, and by luck – but luck never takes care of the sloppy.