How to determine the minimum wall thickness of low pressure vessel?

DataTime:[2021-5-27]

For micro internal pressure or low pressure vessels, the wall thickness obtained by rounding is generally smaller after considering corrosion allowance and negative deviation according to the internal pressure pitch diameter formula. If the thickness is directly used as the nominal thickness, it will lead to a series of problems in manufacturing, transportation and installation. It includes:

a. In the manufacturing process of thin-walled cylinder, in order to ensure the roundness and stiffness, a lot of auxiliary measures are needed, which costs manpower and material resources. For example, it is necessary to use internal strengthening tooling to round the tube section, especially at the butt edge of two tube sections.
b. Thin walled cylinder wall, especially when the diameter of cylinder is small, usually adopts the welding method of single side welding and double side forming. However, the single side welding of thin-walled vessels is prone to defects such as incomplete penetration, burn through and poor back forming. Even if the backing plate is added at the back, it is easy to produce welding defects at the root because the backing plate is not easy to stick tightly. At the same time, the influence of misalignment and angular deformation on the welding quality of thin-walled cylinder is much greater than that of thick-walled cylinder.
c. Due to the poor rigidity and easy deformation of thin-walled vessels, it is usually necessary to temporarily support or reinforce the shell during transportation and hoisting.
If the cost of manufacturing, transportation and installation is too high, it is better to increase the thickness of the cylinder to ensure the quality. Therefore, the minimum thickness of pressure vessel cylinder is also specified in GB 150 and other standards.

GB 150-1989 has strict requirements on the minimum wall thickness of carbon steel and low alloy steel pressure vessels.

The minimum wall thickness of carbon steel and low alloy steel specified in GB 150-98 is: when DN < 3800, 2DI / 1000, not less than 3mm, without corrosion allowance. Dn3800 above, beyond the railway transportation limit, on-site production, no regulations. For high alloy steel, due to the material price, the minimum thickness requirement is low, so the strengthening in transportation and hoisting needs special consideration.

GB / T 150-2011 only requires the minimum wall thickness of 3 mm for carbon steel and low alloy steel.

The minimum wall thickness of high alloy steel cylinder in each version is 2 mm.

The minimum wall thickness requirements of GB 150-1989 refer to the formula previously recommended by American Society of mechanical engineers

δ min=0.001D+2.54 mm

And the former Soviet Union "oil production machinery and equipment" in the formula:

When DN ≤ 1 200 mm, δ min=0.001D +2.54 mm；
When DN > 1 200 mm, δ min=0.001D +4 mm.

GB 150-1989 comprehensively considers the minimum wall thickness requirements of carbon steel and low alloy steel pressure vessels in transportation, manufacture and hoisting.

The requirements for minimum wall thickness of carbon steel in GB 150-1998 and GB / T 150-2011, as well as the requirements for minimum wall thickness of high alloy steel in various versions, are more from the perspective of welding. As for transportation, installation and other aspects, designers need to consider according to the actual situation.

On the whole, GB 150-1989 has strong operability, which should be liked by designers. However, such detailed and rigid standards will hinder technological progress. However, the later version of GB 150 only stipulates in general terms that considering the force during transportation or hoisting, it is obviously not easy to operate under the background of current design and manufacturing division in China. Of course, there are also successful examples, such as the hoisting of large towers. The hoisting scheme (including the required reinforcement measures) is proposed by the hoisting unit and confirmed by the design.

Some things should not be specified too carefully in national standards and industry standards, but enterprise standards and group standards can.

For example, a wealthy and prudent owner made the following provisions on the minimum thickness of the equipment:

DN<900, tmin=6；
DN: 900~1500, tmin=8;
DN: 1500~2100, tmin=8;
DN: 2100~3600, tmin=10;
DN>900, tmin=11.

The above thickness includes a corrosion allowance of 3 mm. Of course, the owner also stipulates that if the quality can be guaranteed, the minimum thickness can be discussed.

Source: China Flanges Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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