The elliptical head is an indispensable component of the boiler pressure vessel. Because the elliptical head has a better stress distribution than the dish and flat head, and its depth is shallow, the overall punching is easier than the spherical head, so it is a commonly used head for boiler pressure vessels. Under the same operating conditions, the stress distribution and level are different for the shape of the head. The shape of the head is too poor, causing an increase in stress, which may cause its stress to exceed the allowable stress value, resulting in failure of the head.
Sample design and production
According to the requirements of the ASME specification, the shape deviation of the head is between the maximum deviation of 1.25% Di (outward convex, the radius of curvature is small) and the minimum deviation of 0.625% Di (inward concave, large radius of curvature), and no sudden The shape of the elliptical head is compliant.
When manufacturing the head template of the ASME product, the specified elliptical curve is translated along the normal direction (in the direction of increasing curvature) by an equal distance (this distance must be greater than 0.625% Di, and 0.625% Di) +5mm is more suitable, and the curve formed is used as the outline of the template. Such a template can more accurately reflect the shape deviation of each position of the elliptical head. In addition, we mark the straight edge of the sample on the straight edge of the sample, so that the same sample can be used to check the elliptical head of any straight edge of different thicknesses with the same nominal diameter Dn. Usually the elliptical head die used in the manufacturing plant is an approximate ellipse composed of R (R = 0.9 Di) and r (r = 0.17 Di). For such a head pressed with an approximately elliptical mold, it can be on the sample plate. Draw the scale shown in the following figure (for example, the nominal diameter 800mm elliptical head, the translation distance in the normal direction is 10mm, the same below), with such a full model can more accurately measure the elliptical head each The shape of the position is deviated.
The model of the elliptical head can be made of 1-3mm aluminum plate or galvanized steel plate. To ensure the rigidity of the sample plate, the head sample with larger nominal diameter should be relatively thicker.
Elliptical head check
Place the head to be inspected horizontally upwards, and place the template into the elliptical head. The length of the straight edge of the pattern required to be aligned with the scale on the template is shown in the figure below. Select a position with a large gap and measure the gap between the inner surface of the template and the elliptical head with a ruler or feeler gauge. Note that it must be measured along the normal direction of the template. When the head is convex, the measured gap is less than 1.25% Di after subtracting 10mm; when the head is concave, the absolute value after measuring the gap minus 10mm is less than 0.625% Di, and the shape deviation of the elliptical head is The scope of the specification is acceptable. On the other hand, if the local shape is abrupt, the elliptical head is unqualified.
The ASME specification does not stipulate that the over-standard head cannot be used. For a head with a shape deviation exceeding the standard, if it is safe, it can be used as it is. Specifically, if the shape deviation of the elliptical head exceeds the requirements of the specification, but does not change abruptly, the ellipsoidal head can be regarded as a dish-shaped head, and the dish-shaped seal is determined by the actual measured value. The large and small radius of the head is then calculated for its strength. If the stress is less than the allowable stress value, the head is safe and can be used on a pressure vessel in accordance with the dish head. This treatment can save costs and guarantee the construction period.