Seeking advice

There is another question: what is the basis for classifying weld joints? Why does Class A include welding of spherical heads and cylinders, welding of various types of convex heads and flat heads, and others?

Question: In 150, it is stated that \"when performing a hydraulic test on a vertical container in a horizontal position, the test pressure should include the hydrostatic pressure of the liquid column that would be present if the container were in a vertical position.\" Why is this necessary? Isn’t the container already in a horizontal position? Or to re-pressurize it for future inspections? Question: Why is the minimum value for hydraulic pressure 1.25 times, and the minimum value for air pressure 1.1 times?

In architectural design, the combination of various loads and design standards are intended to ensure the safety, durability, and cost-effectiveness of structures. Different types of loads—such as seismic loads and wind loads—affect structures in different ways; therefore, different handling principles apply when considering them under combined loading conditions. Earthquake loads are indeed sporadic and of short duration; they generally do not occur frequently ； Wind loads are common, but their intensity usually does not remain very high. Therefore, in design, it is generally permissible for the maximum stress that structural elements can withstand under specific combined load conditions to exceed the allowable stress under single-load conditions, so as to ensure that the structure can resist occasional extreme events without resulting in waste of materials and costs due to overly conservative design. When seismic loads and wind loads are combined with other conventional loads (such as live loads and dead loads), codes generally permit the use of higher stress levels (for example, 1.2 times the allowable stress), as it is unlikely that these loads will all reach their maximum values at the same time. Such a design allows for sufficient safety margins to handle occasional or extreme situations, while also optimizing resource utilization. .

To make it equal to the maximum pressure endured by the container under vertical operation with liquid present

Class A refers to butt welds, while Class B is essentially a version of ring butt welds with reduced internal tensile stress

Type A weld joints include weld joints of various convex and flat heads, primarily because these joint locations are on the main load-bearing paths of the vessel. From a stress perspective, the pressure inside the container generates axial stress at the junction where the head meets the cylinder. The joints connecting the convex and flat end caps to the cylinder serve as the \"critical points\" when the container is subjected to axial forces, bearing large axial tensile forces caused by internal pressure. Taking an elliptical head as an example, when there is pressure inside the vessel, the head behaves like a stretched elastomer, resulting in relatively concentrated axial tensile stress at the junction between the head and the cylinder. These stresses place high demands on the strength of the welds; should any problems occur in the joints, the overall structural integrity of the vessel will be seriously compromised. Therefore, in terms of stress characteristics and structural importance, the weld joints of convex heads and flat heads are classified as Class A.

This post was last edited by Gan Cang_aYrHY on 2024-10-25 09:18. Under combined loading conditions, when seismic loads, wind loads, and other loads act together, the design pressure of components is allowed to be no more than 1.2 times the allowable stress. There are several main reasons for this: 1. Characteristics of seismic and wind loads: Their occurrence probability is relatively low; earthquakes are accidental geological disasters that do not occur frequently in most areas, and they possess a certain degree of randomness and uncertainty. Extreme wind conditions within wind loads, such as typhoons and severe convective weather, also occur only under specific climate conditions and in certain geographical areas, and their frequency is relatively low. Therefore, when designing pressure vessels, it is reasonable to appropriately relax the relationship between the design pressure and the allowable stress, considering that such loads do not occur frequently. The duration of its effect is relatively short: even in the event of an earthquake or strong winds, the time during which it exerts pressure on the pressure vessel is relatively brief. Unlike other conventional loads such as the internal pressure of pressure vessels, which may persist over a long period of time. The short duration of load application allows the container material to withstand a certain degree of stress excess in the short term, without causing serious effects on the container’s long-term performance and safety. 2. Load-bearing capacity characteristics of materials: Materials possess a certain degree of toughness and redundancy: The materials used in pressure vessels usually have a certain level of toughness and strength redundancy. Under normal design conditions, the allowable stress of materials is determined based on a certain safety factor, to ensure the safety and reliability of the vessel under normal loads. When accidental loads such as seismic or wind loads are combined with other loads, the material can withstand stress levels slightly higher than the normal allowable stress for a short period of time without immediate failure. The properties of this material provide a certain basis for appropriately increasing the ratio of design pressure to allowable stress. 3. Economic and rational considerations in engineering design: In terms of economics, if it is required that the design pressure must be strictly lower than the allowable stress under combinations of seismic loads, wind loads, and other loads, this will result in a overly conservative design of the pressure vessel. The wall thickness and structural dimensions of such vessels will increase, thereby raising manufacturing costs and material consumption. Allowing the design pressure to be slightly higher than the conventional value within a certain range enables cost reduction and improved economic efficiency, while still ensuring the safety of the container. In terms of rationality: The design of pressure vessels requires a comprehensive consideration of various factors, such as the likelihood and characteristics of different loads, as well as the properties of materials. Reasonably combining seismic and wind loads with other types of loads, and appropriately relaxing the relationship between design pressure and allowable stress, is a sound design approach based on practical engineering conditions; it enables the design of pressure vessels to better meet actual usage requirements as well as engineering economic considerations.

1. In terms of hydraulics, the pressure in the hydraulic system is generally quite stable. Liquids are essentially incompressible, allowing pressure to be transmitted relatively evenly within a container. When the pressure increases, the force exerted by the liquid medium on the container walls remains relatively stable and constant. Setting it to 1.25x is mainly based on a higher safety margin. Because if there is a failure in the hydraulic system, such as a cracked pipe or failed seal, the leakage of fluid under pressure can lead to serious consequences, such as widespread splashing of fluid and damage to equipment. A higher safety factor ensures that, under various operating conditions including some unexpected situations, the container can withstand the corresponding pressure, thereby preventing dangerous scenarios such as container rupture. 2. In terms of air pressure, gases are compressible, unlike liquids. When the air pressure inside a container changes, the volume of the gas also changes, which makes the distribution and variation of pressure within the container relatively complex. However, under normal conditions, the pressure fluctuation range of pneumatic systems is relatively smaller than that of hydraulic systems. The safety factor is set at 1.1; this is partly because the compressible nature of gases provides a certain degree of buffer against pressure fluctuations. On the other hand, pneumatic systems generally do not result in a sudden, large-scale release of energy in the event of a failure, as hydraulic systems do. At the same time, taking into account economic efficiency and practicality in real-world engineering, using a safety factor of 1.1 ensures that the design and manufacturing of the container are not overly conservative, while still maintaining sufficient safety.

From a stress analysis perspective, when a vertical vessel is used in its normal upright position, the bottom portion bears the static pressure of the liquid column. When a container undergoes a horizontal hydraulic test, although the container is in a lying position, the purpose of the test is to verify that it can operate safely and reliably under all possible stress conditions. If the static pressure of the liquid column when the container is upright is not taken into account, it is not possible to fully verify the strength of areas such as the bottom of the container under additional stress during actual use (when it is upright). This is not to re-pressurize the container for future inspections, but rather to simulate the most adverse pressure conditions that occur during vertical use during this horizontal testing. This ensures that, whether in the testing condition or in actual vertical use, all aspects of the container such as its material strength and weld joints meet the design requirements, thereby ensuring the container’s safety and reliability.

I see. In other words, both earthquakes and wind loads are temporary or not constant, so it is more economical to use a higher allowable stress when making calculations, is that what you mean?