Thickness calculation and use of the hottest trial

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Try to describe the thickness calculation and use of blind plates


in the safety isolation of production system and maintenance system in chemical enterprises, the method of adding blind plates is usually adopted. Now the stress analysis of blind plates is done, and the calculation method of blind plate thickness is given. Combined with an example, the result is the thinnest thickness of the blind plate. Corresponding precautions are also put forward for the use of blind plates

in chemical enterprises, the situation of maintenance while production is often encountered. In order to ensure the effective isolation of production system and maintenance system, the method of adding blind plates between the flanges of associated pipelines is usually adopted. At present, Changhong high tech has launched product utilization in many primary schools in the province through its downstream enterprise Zhejiang dingyang new materials Co., Ltd., which involves the selection of blind plates. For high-pressure blind plates - perforated through diameter pads, there are a series of products for purchase, which will not be introduced here. For the medium and low pressure blind plates, each unit uses a circular iron plate made by gas cutting as the blind plate, and the diameter and thickness of the blind plate are mostly selected by experience, without scientific basis. If the blind plate is thin and deformed during use, it is difficult to pull it out from between the flanges. If the blind plate is thick and the spacing between flanges is not wide enough, it is difficult to insert it between flanges. In addition, this is also a waste of materials. Whether the blind plate is too thick or too thin, these two situations will bring inconvenience to the construction. In order to use the blind plate as thin as possible and ensure that the blind plate will not deform in the process of use, this paper makes a stress analysis on the stress condition of the blind plate and gives the calculation method of the thickness of the blind plate with an example. The calculation result is the thinnest thickness of the blind plate. For the non pressurized blind plate, it only plays the role of isolation and sealing, which is not discussed here

thickness calculation of the second blind plate

because both sides of the blind plate are compressed by the flange, and the pressure on the blind plate is evenly distributed on the blind plate, the result of the force on the blind plate is to make the blind plate bend and deform. It can be seen from this that the stress analysis of the blind plate can be carried out according to the "flat plate head with uniformly distributed load fixed on the periphery"; There is no welding between the blind plate and the pipe, and there is a flange at the pipe end, so the blind plate will basically not cause boundary stress to the pipe; The main consideration of the stress analysis of the blind plate is the bending deformation of the blind plate after compression

1 thickness calculation of flat plate head

the maximum bending stress of flat plate head is obtained by internal force analysis of circular plate:

where: σ Max - maximum bending stress of flat head, MPa

k - coefficient related to the peripheral support mode of the plate, dimensionless

p - pressure on head, MPa

d - pipe diameter, mm

s - plate thickness, mm

1.2 calculation of plate seal thickness

by σ max≤[ σ]Φ And formula (1) to obtain the thickness calculation formula of flat head:

where:[ σ]—— Allowable stress of steel plate at design temperature, MPa

Φ—— Weld coefficient

c - additional amount of wall thickness, which is composed of negative deviation C1 of steel plate and corrosion allowance C2, that is, C = C1 + C2

for the blind plate, (2) the specific selection of various parameters in the formula:

① the support method of the blind plate is peripheral fixation, and the value of K is 0.188

② there will be no weld when cutting the blind plate on the whole iron plate, Φ The value is 1

③ take the highest possible use pressure of the blind plate

a. for the convenience of calculation, P and[ σ] Take the same unit of measurement mpa

b. when the blind plate used is heavy, the dead weight of the blind plate should be considered, and a dead weight correction value PG should be added to the pressure = weight of the blind plate/pipe flow area

c. when the isolated medium is liquid, consider whether there is static pressure. Calculation of static pressure value:

p = ρ gh/1 × 106

where: P - static pressure, MPa

ρ—— Medium density, kg/m3

g - gravitational acceleration, 9.8m/s2

h - liquid level height, M

④ the unit of blind plate thickness SC is the same as the nominal diameter DC of the pipe. When the calculated blind plate thickness is small, this rule is only applicable to single 1 fault of 3mm, take 3mm. That is, the minimum thickness of the compression blind is 3mm

⑤ since the blind plate is mainly subject to bending stress, the allowable stress value adopts the allowable bending stress of the steel plate, and its determination method is as follows:

a. the primary bending stress in the blind plate is distributed in a straight line along the plate thickness. With the increase of internal pressure, the upper and lower surfaces first enter the yield, and the other parts are still in the elastic state. Although the maximum bending stress at this time has reached the yield limit of the material, the whole blind plate is still in an elastic state except the upper and lower surfaces, so it will not lead to the failure of the blind plate. According to the principle of limit design method, the structure is considered invalid only when the stresses at all points on the whole section of the plate reach the yield limit. The load on the plate at this time is called the ultimate load, and the stress corresponding to this load is called the ultimate stress:

σ max=1.5 σ S (according to 3)

where σ B is the yield limit of the material, which can be obtained by checking the table of "allowable stress value of steel plate"

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c. in addition, the allowable bending stress of the steel plate can be obtained from (3) and (4) as

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where ns is the safety factor. In practice, in order to make[ σ b] Not more than σ s. Generally, the selected safety factor ns ≥ 1.5. Substitute formula (5) into formula (2) to obtain:

the allowable stress of steel plate at design temperature in the formula[ σ] T refer to the table of "allowable stress value of steel plate"

d. due to the specific service conditions of the blind plate, it is allowed to have a small amount of permanent deformation in use. Therefore, ns can take a lower value than that selected in the design of pressure vessels. Accordingly, formula (6) has two special cases

when the diameter s = 1.0,


when the diameter ≥ 400mm and the temperature s = 1.5,


from the applicable conditions and practical results, formula () is most used in production practice

⑥ C. generally, the thickness of blind plate will not exceed 30mm, so the negative deviation value of steel plate is 0 ~ 1mm; Due to the short service time of the blind plate, the corrosion allowance can be ignored. Therefore, the rounding of C value combined with the calculation results can be selected within the range of 0 ~ 1mm as appropriate

after calculating the thickness of the blind plate, check the pipe flange specification for the diameter of the blind plate, and take the diameter of the flange sealing surface (D2 value of the general flange specification)

for example, add a blind plate on the pipe flanges of DN200, PN16 and normal temperature, and calculate the blind plate specification when the pressure on one side is p = 0.686mpa and the steel plate material is 20g

check the table, the diameter of the sealing surface of the pipe flange of DN200 and PN16 is 268mm, and the yield limit of 20g steel plate is 245MPa, then calculate according to the formula ():

take C = 0.3mm, then SC = 3.7+0.3 = 4.0mm

that is, the specification of the blind plate: 4mm thick, 268mm in diameter. After using this calculation result in practice for a period of time, remove the blind plate and check that the blind plate is not deformed

problems needing attention in the use of three blind plates

1 selection of seals and gaskets. Pipe flanges are generally sealed with asbestos plate gaskets, but the surface of the blind plate is not processed twice, so the surface of the blind plate is not smooth. If ordinary asbestos plate gaskets are used [as shown in figure (B)] it is easy to cause medium leakage after compression. Therefore, it is necessary to change the form of asbestos board gasket, and the gasket between the blind plate and the flange adopts the same shape as the blind plate [as shown in figure (a)], so as to avoid leakage after use

2 the construction of extracting and adding blind plates shall strictly comply with the provisions of "safety requirements for blind plate extraction and blocking operation" in the "safety regulations for blind plate extraction and blocking operation in plant area" (chemical industry standard Hg of the people's Republic of China). It is better for qualified factories to establish a special blind plate account to uniformly manage the blind plates used in the production area of the factory

3 it is not allowed to isolate the system by closing the valve or adding a asbestos blind plate behind the valve for sealing. Once the valve leaks or the workers misoperate, the medium of the production system will soon burst the asbestos blind plate and enter the maintenance system, and the consequences are unimaginable. (Wang Jinping)

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