Design of equipment flange

What is a equipment flange?

The flange on the vessel is called vessel flange, the flange on the equipment is called equipment flange, the flange on the pipe is called pipe flange, and the flange on the valve pipeline accessories is called accessory flange. This is a common term in every industry. There are no such two terms in the standard book.
Equipment flange refers to the flange connecting the equipment cylinder or equipment head on the equipment shell. The equipment flange can be HG20592 – HG20634 steel pipe flange or vessel flange.
Vessel flange refers to the flange of JB4701 – JB4703, JB/T4721 is the side flange of pipe box, and the flange of hook ring and floating head is generally only used as the flange of equipment.
Specially used in the equipment are: nozzle flange, manhole flange, handhole flange, loading and unloading hole flange, mirror flange, spectacle blind flange and so on.
Pipe flange: weld neck flange (WN), long welding neck flange (LWN), lap joint flange (LJ), blind flange / blank flange (BL), slip on flange (SO), thread flange (THR), socket weld flange (SW), reducing flange, orifice flange, anchor flange, spectacle blind flange (figue 8), spacer ring/spade flange are generally used in the pipeline. But it’s also useful on devices. When it is used as equipment flange, the pressure is generally low, and it is used when the equipment DN ≤ 400. At this time, the cylinder is mostly seamless steel pipe.

Design of equipment flange

The mainstream method of flange design is Taylor waters method. In the process of designing long neck butt welding flange, it is often difficult for novices to adjust because of many parameters. After adjustment, the drawing is often changed beyond recognition.
So how to adjust the flange of the equipment, to achieve both safe and economic, but also good redundancy.
Security: at least through calculation.
Economy: the cheapest on the basis of calculation.
Redundancy: in actual use, it can be used safely in case of slight overpressure, overheating, pipeline load and other situations.

Economy of flange

What determines the cost of flange?

The weld neck flange is generally made of long neck flange forging as blank, and then manufactured by machining.

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So the cost of flange is about:

  • A: Forging blank;
  • B: Processing fee and other expenses.

The cost of flange is decided by the basic quality of forging blank. The blank consists of flange diameter and thickness, big end thickness and cone neck height.
So the control of calculation is to make these four parameters obtain the least material.

Standard equipment flange

In the national standard design, if the flange completely conforms to Nb / T 47023, including the material of flange, temperature pressure gauge, corrosion allowance, butt cylinder thickness, matching gasket type material and stud nut material, etc., according to GB 150.3, the calculation can be exempted.

As a non-standard flange design, in order to quickly obtain the appropriate design scheme, it is often necessary to refer to the standard flange Nb / T 47023, use the data of the standard flange and slightly adjust it to obtain the final flange data.
Input data into the software, there are often some error prone places.

  • 1. The thickness of small end should be input into the smaller value of small end of cylinder and flange. For detailed analysis, see: how to select the thickness of small end of flange (Continued)
  • 2. The input of SW6 does not consider the corrosion allowance, so the corrosion allowance needs to be deducted. The modification contents include the inner diameter of flange, the thickness of big end and small end.
  • 3. In SW6, when a heat exchanger with split partition is encountered, the area of split partition is the compressed width * compressed length.

Especially for the first point, because the cylinder is relatively thin, some flange modifications are often needed when inputting the thickness of the small end. And often because the small end is too thin, resulting in a lot of stress or stiffness pass.
Many friends will have questions:
Why the standard equipment flange needs to be modified, why the standard flange can not be calculated, is there a problem in the understanding of the input?
Generally speaking, since it needs to be calculated according to GB150.3 or ASME VIII I, it is completely designed according to the code method, and there is no need to worry about whether the standard flange size can be modified.
After all, the pipe flange does not need to be calculated. If it needs to be calculated, many low pressure grades can not be calculated.

Dimensions of standard equipment flange

Standard equipment flange is an innovation of our country, which avoids many kinds of non-standard flange design, so that designers can design a more reasonable flange size more smoothly.
But is there a standard flange in the world?
What are standard parts?
For example, standard bolts, nuts, nozzles, elbows, tees, so flanges and so on. Knowing the material model, it’s easy to buy them in stock on the market. Can the flange of the equipment be bought on the market?
It can be said that it is basically impossible.
All the equipment flanges are first made of blank and then processed into the required size, which need to be customized.
Standard equipment has been very successful as a dimensional reference. The designer writes a standard number and specification, the flange processing plant or manufacturing unit describes the drawing according to the standard and specification, and then processes an equipment flange according to the drawing.
Sima Guang said: “the wealth and goods produced by heaven and earth are not in the people, but in the officials.”
It is inevitable that the drawing of equipment flange should be designed or produced by the manufacturer.
Therefore, because the flange is a standard equipment flange, the calculation is not out of date, and there is no need to worry about whether you can modify the size and whether it will bring inconvenience to the manufacture.
Because whether it’s standard or non-standard, it’s the same for manufacturers. Even if you can draw a picture, it’s more popular with manufacturers than writing the standard number.

How to input the height of the cone neck?

The standard flange acts as an anchor.
The designed flange is lighter than the standard flange, which shows that it saves the cost.
The designed flange is heavier than the standard flange, which indicates that it is too heavy.
In order to ensure the economy, according to the above instructions, if the diameter and thickness of the flange remain unchanged, the height change of the cone neck will not affect the economy of the flange. Because the same blank is used.
Increase the height of the flange neck until there is no straight edge at all. At this time, the cost is the same as that of the standard flange, but the calculation is easier to pass.
Some friends will say:
I agree that it is conservative to use the virtual h1 instead of the cone neck height h when the cylinder thickness is taken at the small end. Is that ok?
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In principle, of course, it’s OK, but the virtual cone neck needs to be calculated, which is more troublesome.
Directly modify the cone neck, leave a little straight edge, the size and cone neck height in the calculation book can match, do not need to calculate the virtual cone neck H1, more intuitive.
If not, the straight edge can be cancelled and the butt joint can be selected.
If Nb / T47020 flange and barrel joint is the exclusive invention of China, then the butt joint is universal and acceptable.
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It is very convenient to modify the height of cone neck and explain how much h is equal to.

Adjustment skill of flange

Flange adjustment skills are described in detail in the book “training course for pressure vessel design engineers”:
There are three keys to the rational design of flange connection:

  • (1) In the design of gasket, the load of gasket should be controlled as small as possible.  
  • (2) In the design of bolt configuration, the diameter of bolt center circle should be controlled as small as possible, so as to form the flange torque as small as possible and follow the “minimum load criterion”.  
  • (3) On the basis of the above, the taper neck and flange ring ratio of the flange should be designed properly, so that each part of the flange can give full play to the strength performance and ensure sufficient stiffness, that is, it tends to the full stress state and follows the “full stress criterion”.

Of course, this is the basic idea. Now the flange has a stiffness check. Some flange stress value is very low, but the stiffness control flange, also have to adjust. For the flange with split diaphragm, the gasket at the diaphragm also needs to be considered.
For a specific project, most of the time, the gasket form of the equipment and the bolt material have been determined, that is, the my value of the gasket and the allowable stress of the bolt have been determined. The adjustment of flange is equivalent to dancing with shackles, and the room for adjustment is reduced a lot.
Here are four ideas for adjusting flange, which are worth trying.

Reduce the design torque mo of flange as far as possible.

Mo is the total torque of the flange. The flange, cone neck and cylinder of the flange bear the total torque. The so-called “share happiness and share difficulties”. When the total torque is reduced, each component will bear less, and the forces on the following components will be distributed according to the rotational stiffness. This belongs to the contradictions among the people, which can be further analyzed.
The methods to reduce Mo include:

  • a) Reduce the bolt circle diameter of the flange and the force arm. When the area is the same, the smaller the diameter, the more the quantity
  • The bolt will be smaller than the big diameter, a small number of bolt center circle, arm of force, so it is better.
  • b) When Mo is determined by the preloading condition, the bolt force can be reduced. For example, reduce the number of bolts, reduce the bolt area, reduce the bolt strength, reduce the gasket my value or width.

Reasonable distribution of Mo

The total Mo is reduced. The flange, cone neck and cylinder bear the load together. The bearing ratio of the three parts is proportional to the rotational stiffness. The greater the capacity is, the greater the responsibility is. The greater the stiffness is, the more the load is. Because most of the time, the thickness of the small end is equal to the thickness of the cylinder, and thickening the cylinder is not economical.

  • a) If f is equal to 1, it means that the maximum stress is at the big end of the cone. The stress at the big end can be reduced by increasing the thickness of the big end.
  • b) When the radial stress is not out of date, the flange thickness can be increased.
  • c) When the circumferential stress is not out of date, the thickening or thinning of the big end of the cone neck will have an impact, which can be tested by inputting data. Therefore, it should be reminded that not all thicknesses have a positive effect on the strength.
  • d) When the stiffness is not out of date, in order to reduce the deflection of the flange in use, the thickness of the flange plate can be thickened or the thickness of the big end of the cone neck can be increased.

Ensure economy

As mentioned above, when adjusting the size, make the diameter and total height of the designed flange approximately equal to the matched standard equipment flange. This can ensure that no matter what parameters are finally adjusted, the cost and standard are basically the same.

Operation redundancy

In practical use, the logic and design of flange are different. For example, the bolt preload is often far greater than the design load. Usually, if the flange can not be pressed tightly, the bolts should be tightened again. Generally, although the actual preload of the calculated flange is too large, it often has no effect.
Once the bolt is pressed wrongly, there is little chance to remedy it. Therefore, the appropriate margin and put the margin in the right place will make the flange easier to seal in the actual operation.
For example, when Mo is determined by the operation condition, that is, high temperature and high pressure condition, try to add more bolts as much as possible to increase the flange torque under the pre tightening condition, but Mo at this time does not affect the flange cost. Due to the increase of bolts, the operational redundancy of additional equipment is also increased.
For another example, the thickness of the flange part of the tube plate which is also used as the flange has been calculated according to the strength, but the stiffness should not be too low. Although the overall stiffness of tubesheet and tube bundle is much greater than that of the companion flange, the local deflection here is too large, which has a great impact on the seal. “The thickness of tubesheet shall not be less than 0.6 times of the flange thickness of Tubesheet” can also be referred to as appropriate.

Source: China Flange Manufacturer – Yaang Pipe Industry (

(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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