What role does the deflector play in the expansion joint? Why diameter deviation is more critical than you think
The guide tube inside the expansion joint looked like a cylinder, and many people treated it as an ordinary bushing. However, after working in this line for a long time, you will understand that the diameter deviation of the guide tube directly determines that the expansion energy saving can't run a maintenance cycle safely. The primary task of the guide tube is to divert the flow-allowing the medium to pass smoothly through the inside of the bellows, reducing the impact of turbulence and vortices on the corrugations. Secondly, it separates the high-temperature medium from the bellows to avoid the bellows from being directly heated and overheated. Another function is easy to overlook: the guide tube can hold the impurities and welding slag washed down by the tube wall, preventing these things from getting stuck in the gaps of the corrugations.
Therefore, once the diameter of the deflector is ridiculously deviated, trouble comes. The diameter is large, and the gap between the bellows and the inner wall is not enough. As soon as the bellows are compressed, the guide tube directly presses against the corrugated valley, and it will wear out in a few cycles. The diameter is small, the gap is too large, the medium forms a step at the entrance of the bellows, the scour is intensified, and a trench is cut out of the root of the bellows. Do you say this deviation value is critical or not?
Are there any deflector diameter deviations specified in ASME and EJMA standards? Where do the reference values come from
Looking through ASME Volume VIII directly, it only specifies the wall thickness, fatigue life calculation and pressure test requirements of bellows, and does not give specific table-format values for the diameter tolerance of deflectors. The EJMA (American Association of Expansion Joint Manufacturers) standard also lacks a direct provision for baffle tolerances. When many buyers get the drawings, they ask: What is the diameter deviation of ASME expansion joint guide tube? It is indeed the norm that a single citable number cannot be found on standard text.
But don't rush to conclusions, although EJMA does not give tolerance values, it gives the recommended clearance range between the deflector and the bellows inner diameter in the design chapter. According to the empirical data of EJMA, the unilateral gap between the outer diameter of the guide tube and the inner diameter of the bellows is usually 6mm to 12mm, depending on the nominal diameter. The larger the diameter, the clearance is appropriately enlarged. This gap is pushed back, which is the lower deviation range of the outer diameter of the guide tube. The domestic supporting GB/T 12777 and HG/T 20205 also follow a similar idea, and the design institute and the manufacturer follow this interval by default.
Therefore, in actual engineering, the diameter deviation of the guide tube is usually determined by the way of "inner diameter as the benchmark and controlling negative deviation". For example: DN1000 universal corrugated expansion joint, the inner diameter of the bellows is 1016mm, the outer diameter of the guide tube is about 1000mm, the deviation is controlled from 0 to-3mm, and the bilateral clearance is about 16mm. For the large diameter thick wall expansion joint of DN2000, the gap must be enlarged to more than 20mm, and the deviation of the outer diameter of the guide tube should be relaxed to 0 to-5mm.
How to control the deflector diameter deviation at the manufacturing end? Measurement location, method, and common deviation ranges
The most common process for rolling guide tubes in manufacturing plants is longitudinal welding of coil plates, and the circumferential direction is inevitably ellipticity. Controlling the diameter deviation does not rely on the accuracy of the plate coiling machine alone-the welding deformation is often greater than the coiling error. It is common that the longitudinal seam shrinks after welding, and the radius of the guide tube shrinks by 2-3mm near the weld.
To control the deviation, the measurement position must be standardized. According to the recommendation of EJMA, the diameter of the guide tube shall be measured at the end and the section not less than 150mm from the end, and the average value shall be taken in two mutually perpendicular directions for each section. This method can effectively avoid the interference of ellipticity. In actual production, inner diameter micrometer and auxiliary support rod are commonly used to measure large diameter guide tubes, and attention should be paid to avoid the weld area when measuring.
The common deviation range is generally classified into three grades in the industry: ordinary grade 0 to-5mm, which is used for general flue ducts and low-pressure pipes; Precision grade 0 to-3mm for high temperature axial type expansion joints and corrugated expansion joints for power station industry; Special Grade (Matching Machining) 0 to-1mm for use in rotary compensators, vacuum-specific and other clearance-sensitive occasions. You will know by comparison, what kind of precision equipment and what inspection methods are used, and the corresponding cost is quite different.
If the deviation is too small, the bellows will be stuck, and if the deviation is too large, it will accelerate the erosion and wear-the field case tells you how to choose the range
The high-temperature axial expansion joint of a power plant, DN1400, with a design temperature of 560℃, has obvious wear on the bellows after more than one month of operation. Disassembly and inspection found that the inner diameter of the guide tube was too large by about 8mm, the medium formed a sudden expansion at the outlet of the guide tube, and the dusty flue gas directly washed the corrugated root. The deflector was later remade with a deviation of 0 to-3mm, and the wear problem disappeared.
There is also a case in a chemical plant. The medium is a slurry with particles, and the expansion joint model is a general-purpose corrugated expansion joint. The outer diameter of the guide tube is enlarged by 1.5mm, and it can't rotate when installed at room temperature. When the corrugated tube is compressed during pressure test, the end face of the guide tube directly bends the corrugated valley. After rework and removal, the ellipticity of the guide tube exceeded the standard, and the long axis direction was nearly 5mm larger. The manufacturer re-rolls it and strictly controls the deviation before it is solved. When the two cases are put together, the truth is clear: the deviation should leave enough gaps, but it should not be large enough to form a step scouring. What about that? Select the deviation gear according to the medium properties and working conditions, take the intermediate value for dusty medium, take the small gap for pure medium, and make the inlet chamfer of the expansion joint of high-pressure and high-speed fluid.
When signing the technical agreement with the supplier, the deviation of the guide tube diameter must be written in to avoid discord in acceptance
The following four items are clearly written in the technical agreement, which can save ten quarrels during acceptance.
First, give the benchmark size. Make clear whether the inner diameter or outer diameter of the guide tube is the reference, and indicate the design nominal size and deviation range. Don't just write "conforming to EJMA". EJMA doesn't give a value, and everyone will hold their own opinion when the time comes. Write directly: the outer diameter of the guide tube is D =1000mm, the deviation is 0/ -4mm, and the ellipticity is not more than 1/2 of the diameter tolerance.
Second, specify the measurement method. Note that the measurement position is 150mm from the end face, and each section is measured in two vertical directions, except for the weld area. If it is not written clearly, the supplier will take a caliper on the end face and hand it in for inspection, and the measured data can't reflect the true ellipticity at all.
Third, the gap range is determined. According to the one-sided clearance of 6-12mm recommended by EJMA as the acceptance basis, and specify the maximum clearance limit. This is particularly important-the design time gap is 8mm, and as a result, the guide tube is made 13mm smaller, and the scene looks fine. In fact, the risk of scour has doubled.
Fourth, clarify the end treatment requirements. The inlet end of the guide pipe must be chamfered or rounded, and the outlet end should be burred smoothly. The chamfer radius R shall not be less than 2mm. If conditions permit, the hard surface treatment of surfacing welding shall be done. The reason for writing this one is simple, the outlet end burrs and steps are the starting point of bellows wear.
These items are written into the agreement, so suppliers dare not fool them, and inspectors are worried. If you have signed the contract but have not written these, the factory inspection report requires that the measured data be filled in, and then the gap mark on the final assembly drawing is checked. After this set of actions is completed, you will have a bottom in your heart.