Which stress should be used for strength verification?
Thread Content
The last edit to this post was made by WANGJIABAO on 2017-7-13 at 15:24. When performing strength verification of pressure vessels subjected to uniform corrosion, should the allowable stress at the design temperature be determined based on the nominal thickness of the vessel or based on its actual minimum thickness? This is because if there is a large difference in thicknesses, the stress levels will fall into different ranges.
This post was last edited by Pinocchio on 2017-5-19 07:51. The allowable stress of the material should be determined based on the thickness supplied by the steel mill (without considering the allowable thickness tolerances). This is because the allowable stress of a material is based on its mechanical properties, which are determined under the conditions in which the material is supplied. During the manufacturing or use of pressure vessels, the material may thin out, and the allowable stress of the material should not be increased as a result.
From a safety perspective, the allowable stress should be determined based on the nominal thickness...
This post was last edited by Pinocchio on 2017-5-19 08:41. If it is a design from a design institute, the allowable stress can only be determined based on the nominal thickness, as the reduction in thickness due to shaping is not taken into account. When selecting thicker steel plates for procurement, manufacturers need to consider whether there will be an issue with a shift in allowable stress levels. If it is a design from the manufacturer, the nominal thickness on the drawing should be equal to the purchase thickness. In my opinion, it is inappropriate for the design not to take into account the amount of thinning that occurs during forming; the design should take the manufacturing process into account in order to establish appropriate manufacturing acceptance criteria. The actual reduction in thickness achieved by the manufacturing plant does not exceed 3 percentage points, and this occurs mainly at the head. A 3% difference in the head thickness has a minimal impact on the overall cost of the pressure vessel, so it is acceptable to be somewhat more conservative. Just as a maximum allowable deviation can be set for steel plates, a maximum allowable thinning amount can also be specified for head forming. The design can use this maximum allowable thinning amount to determine the purchase thickness, which is then used as the nominal thickness for determining the allowable stress, and this value is indicated in the construction drawings.
Mm. . The allowable stress value, the minimum operating temperature, and so on should all be selected based on the nominal thickness – this is a simple example. Due to limitations in the precision of the sealing surface and the thickness of the raw material, 32mm thick steel plates are processed into 22mm thick flanges. The stress value of your flange material and the operating temperature range should be selected in accordance with 32mm steel plates. Actually, there is a problem with this. When calculating the flange in SW6, the nominal thickness of the flange was entered as 22 mm; therefore, the stress values and temperature ranges used in the calculations were also based on 22 mm. Q345R steel plate (22mm – the minimum operating temperature is -10°C, while for 32mm steel plates, the minimum operating temperature is 0℃ ； (The raw materials are not subjected to impact testing.)
The allowable stress of the material shall be determined based on the thickness specified in the quality certificate of the selected material; it cannot be determined based on the thickness of the material after it has been processed or on the actual measured thickness of the material. In cases where there is no thinning or only minimal thinning during the forming process, the allowable stress of the material is determined based on the nominal thickness specified in the design drawings. When the amount of thinning is such that using the nominal thickness would result in a minimum thickness after forming that does not meet the minimum thickness requirements specified in the design, and if this leads to an issue with changing the allowable stress, strength calculations should be performed using the allowable stress corresponding to the actual thickness of the material, in order to determine the minimum thickness required to ensure sufficient strength throughout the entire service life.