How to calculate the thickness of blind flange
How to calculate the thickness of blind flange?
In chemical enterprises, it is often encountered that production and maintenance are carried out at the same time. In order to ensure the effective isolation of production system and maintenance system, blind flanges are usually added between the flanges of associated pipelines. This involves the selection of blind flanges. For high pressure blind flanges, there are a series of products available for purchase, which will not be introduced here. For the medium and low pressure blind flange, each unit makes a round iron plate as blind flange by gas cutting, and the diameter and thickness of the blind flange are mostly selected by experience, without scientific basis. If the blind flange is thin, it will deform in the process of use, and it is difficult to pull out from the flange. If the blind flange is thick enough and the spacing between the flanges is not wide enough, it is difficult to insert it between the flanges. In addition, it is also a waste of materials. Whether the blind flange is too thick or too thin, these two situations will bring inconvenience to the construction. In order to use as thin as possible blind flange, and ensure that the blind flange does not deform in the process of use, this paper makes stress analysis on the stress of blind flange, and gives the calculation method of blind flange thickness with examples. The calculation result is the thinnest thickness of the blind flange. For the blind flange without pressure, it only plays the role of isolation and sealing, which is not discussed here.
Because both sides of the blind flange are pressed by the flange, and the pressure on the blind flange is evenly distributed on the blind flange, the result of the force on the blind flange is the bending deformation of the blind flange. It can be seen that the stress analysis of blind flange can be carried out according to the principle of “plate head with fixed periphery and bearing uniform load”; there is no welding between blind flange and pipeline, and there is flange at pipe end, so the blind flange will not cause boundary stress to pipeline; the main consideration of stress analysis of blind flange is the bending deformation of blind flange after compression.
Thickness calculation of flat head
By analyzing the internal force of circular plate, the maximum bending stress of plate head is obtained as follows:
In the formula:
- – maximum bending stress of flat plate head, MPa;
- K — the coefficient related to the supporting mode around the plate, dimensionless;
- P — pressure on head, MPa;
- D – pipe diameter, mm;
- S — plate thickness, mm.
Thickness calculation of flat head
According to and formula (1), the calculation formula of flat head thickness is obtained
In the formula:
- — allowable stress of steel plate under design temperature, MPa;
- Φ – weld coefficient;
- C – additional wall thickness, which is composed of negative deviation C1 and corrosion allowance C2, i.e. C =C1+C2.
For the blind flange, (2) the specific selection of parameters in the formula:
① The support mode of blind flange is peripheral fixation, K value is 0.188;
② There will be no weld on the whole sheet of iron, and the value of Φ is taken as 1;
③ Take the highest possible pressure of blind flange.
- a. For the convenience of calculation, P and take the same unit of measurement MPa.
- b. When the blind flange used is heavy, the dead weight of the blind flange shall be considered, and the pressure shall be added with a dead weight correction value PG= weight of the blind flange / pipe flow area.
- c. When the isolated medium is liquid, the existence of static pressure should be considered.
In the formula:
- P — static pressure, MPa;
- — medium density, kg / m3;
- — acceleration of gravity, 9.8m/s2;
- — liquid level height, M.
④ The unit of blind flange thickness SC is the same as the nominal diameter DC of the pipeline. When the calculated blind flange thickness is less than 3mm, take 3mm. That is, the minimum thickness of the blind flange is 3 mm;
⑤ As the blind flange is mainly subjected to bending stress, the allowable bending stress of steel plate is adopted as the allowable stress
a. The primary bending stress in the blind flange is linearly distributed along the thickness of the plate. With the increase of internal pressure, the upper and lower surfaces yield first, and the other parts are still in elastic state. Although the maximum bending stress at this time has reached the yield limit of the material, the whole blind flange is still in an elastic state except for the upper and lower surfaces, so it will not lead to the failure of the blind flange. According to the principle of limit design, only when the stress of each point on the whole section of the plate reaches the yield limit, the structure will be considered as failure. The load on the plate at this time is the limit load, and the stress corresponding to this load is called the limit stress.
In the formula, is the yield limit of the material, which can be obtained by referring to the table of “allowable stress value of steel plate”.
b. According to the elastic failure criterion, the working pressure multiplied by a safety factor should not be greater than the yield limit of the material. or (4)
c. Combined with equations (3) and (4), the allowable bending stress of steel plate can be obtained as
In the formula, is the safety factor. In practice, in order to make not more than , the safety factor of is generally selected. Substituting equation (5) into equation (2) yields:
of the allowable stress of the steel plate under the design temperature in the formula can be obtained from the table of “allowable stress value of steel plate”.
d. Due to the special service conditions of the blind flange, it is allowed to have a small amount of permanent deformation in use. Therefore, the value of is lower than that selected in the design of pressure vessel. Correspondingly, there are two special cases in equation (6).
When the diameter is less than 400mm and the temperature is less than 300 ℃, it can take =1.0,
When the diameter is more than or equal to 400mm and the temperature is less than 300 ℃, = 1.5 can be taken.
In terms of applicable conditions and practical results, formula (6-1) is most frequently used in production practice.
⑥ In general, the thickness of the blind flange will not exceed 30mm, so the negative deviation of the steel plate is 0 ~ 1mm; because the blind flange is used for a short time, the corrosion allowance can not be considered. Therefore, the rounding of C value combined with the calculation results can be selected in the range of 0-1 mm.
After calculating the thickness of the blind flange, the diameter of the blind flange, check the flange specification, and take the flange sealing surface diameter (D2 value of the general flange specification).
For example: add blind flange on DN200, PN16 and normal temperature pipe flange, single side pressure P = 0.686mpa, steel plate material 20g, calculate blind flange specification.
According to the table, the sealing surface diameter of DN200 and PN16 pipe flange is 268mm, and the yield limit of 20g steel plate is 245MPa.
Take C = 0.3mm, then Sc＝3.7+0.3＝4.0mm
That is, the specification of the blind flange is 4mm thick and 268mm in diameter. After this calculation result has been used in practice for a period of time, the blind flange is removed for inspection, and the blind flange is not deformed.
Problems in blind flange application
Selection of seal and gasket: asbestos sheet gasket is generally used for pipe flange sealing, but the surface of blind flange is not subject to secondary processing, so the surface of blind flange is not smooth. If ordinary asbestos sheet gasket is 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 gasket, and the gasket between blind flange and flange should be in the same shape as blind flange [as shown in figure (a)], so as to avoid leakage after use.
The construction of blind flange installation shall strictly comply with the provisions of “safety requirements for blind flange removal and blocking operation” in “safety regulations for blind flange removal and blocking operation in plant area”. It’s better to set up a special account for blind flange in a factory where conditions permit, and carry out unified management for blind flange used in the production area of the whole factory.
It is not allowed to isolate the piping system by closing the valve or sealing the asbestos blind behind the valve. Once the valve leaks or workers misoperate, the medium of the production system will soon burst the asbestos blind into the maintenance system, and the consequences are unimaginable.
Source: China Blind Flange Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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