When calculating the flange of equipment, should the thickness of small end of flange or cylinder be input?
This issue has been debated for a long time. It is difficult to convince each other that the thickness of the input flange and the thickness of the cylinder are both different. Because Nb and GB150 are different.
The author suggests that the thinner part (the thickness of the cylinder) should be input in the software.
1. The calculation of flange is based on waters method in many countries. In terms of calculation principle, flange is divided into three parts: ring plate, cone neck and cylinder (straight side section). The height of cone neck is h, and the height of cylinder is H0 = sqrt (Bg0). Generally speaking, the height of straight edge is less than H0. If the thickness of the small end of the input flange is too thick, it is inclined to run forward, which is equivalent to a long section of material missing. From the calculation model, the thinner part (the thickness of the cylinder) should be input.
For example, water method is also adopted in codap specification, and G0 and l0 = sqrt (D g0) of integral flange are described in detail. It can be seen from the diagram that the thickness of the cylinder should be taken.
2. Although GB150 says “effective thickness of small end of flange neck” for the thickness of small end, we can see how other specifications describe it. ASME has not mentioned that the thickness of big end is “G1 = thickness of hub at back of flange” and that of small end is “G0 = thickness of hub at small end”. EN13445 is consistent with the English description of ASME. Therefore, from the description of the specification, the small end should take the thinner part thickness, rather than the flange small end thickness.
3. Flange stiffness is inversely proportional to G0 square. Because the thickness of the small end of the flange increases (the thickness of the cylinder does not), the whole stiffness of the flange system will not increase greatly. If the thickness of butt joint between flange and cylinder is very thin, how should flange be deflected and how to deflect? The whole stiffness will not be improved by leaps and bounds because a little material is added to the small end. Therefore, from the angle of controlling flange stiffness, the thickness of thinner parts should be input.
4. On the other hand, for ASME / EN design, there is no NB flange standard, and their equipment flanges are made with the same height of flange neck and the same thickness of cylinder. Can we explain from the side that the thickness of thinner parts should be input.
However, if H0 is defined as a length that must be guaranteed, it is difficult to explain why the integral flange and head can be directly welded without guaranteeing the length of H0. The figure below is taken from the standard gb150.3-2011 figure 5-26 flanged convex head.
Of course, it can also be understood from another aspect that the head has a strengthening effect on the flange. The corner of the head is subjected to compressive stress, and the membrane stress on the small end of the flange is tensile stress. The effect of superposition with the flange is to make both stresses better. Therefore, it is considered that this kind of structure can be calculated separately by national standards.
The circumferential membrane stress of small end of flange is not considered in the flange calculation of GB150. For example, when ASME calculates flange, the thickness of small end of flange should be calculated according to internal pressure. If not considered in GB150, the thickness of the straight edge may be very thin, even the membrane stress can not pass, the flange calculation is passed. There are also risks.
But does it mean that the thickness of the small end of the flange must be the same as that of the cylinder? I don’t think the answer is certain. The reason is to interpret the waters law from another perspective.
The water method requires that the cylinder and flange have the same bearing capacity. However, the values of G0 and H0 only consider the geometric dimension. In fact, their strength is controlled by the allowable stress and the geometric dimension. It is not comprehensive to only talk about the geometric dimension (equivalent to only calculating the bending stiffness of the flange), especially when the allowable stress of the flange is lower than that of the cylinder. The flange stress * the thickness of the small end of the flange shall not be less than the cylinder stress * the cylinder thickness.
For example, the allowable stress of flange (stainless steel) is 80MPa, and that of cylinder (carbon steel) is 160MPa. If the thickness of the cylinder is 10 mm, it will be very difficult to pass the calculation. Because the small end of the flange is 10 mm, the local membrane stress may not pass. In this case, the geometry and strength should be considered at the same time. The small end of flange should be 10 * 160 / 80 = 20mm. The model of waters method assumes that their strength is continuous. The butt joint of the flange and the cylinder body can be cut at a ratio of 1:3.
This is described in ASME B16.5.
Where (c) is written in detail: when the minimum yield strength of the two is inconsistent, the thickness of the small end of the flange shall be at least the thickness of the pipe t times the stress ratio of the pipe and flange.
To sum up, in general, when calculating the flange of equipment, the thickness of the small end should be taken as the thickness of the cylinder, and the influence of internal pressure membrane stress should also be considered for the thickness of the small end of the flange, which should not be too small. When the allowable stress difference between flange and cylinder is large (generally occurs when the flange is stainless steel and the cylinder is carbon steel clad plate), the thickness of small end of flange should reach the equivalent strength of cylinder.
People often ask:
The thickness of the small end of the flange is input the thickness go of the small end of the flange, or the thickness TN of the cylinder welded with the flange should be input.
Because in general, according to Nb / T 47023 equipment flange selection calculation, the flange small end thickness is generally greater than the cylinder thickness.
In order to save materials in many units, the input thickness go is the thickness of the small end of the flange. Even some units regard this method as a summary of engineering experience.
So is this right?
In the mandatory Appendix 2 of ASME VIII i-2019, for the value of the small end thickness, in the case of the above figure, the thickness of the thinner part should be taken, that is tn.
This has been updated in the 2017 version.
The standard compilers should have noticed the similar arguments, and in order to avoid misunderstanding, they wrote them in the main body of the standard.
What to do after that?
I know that many units often input the thickness of go as the small end thickness in order to reduce the cost.
This avoids increasing the thickness of the cylinder, and the margin of the flange will look better.
But as designers, they should know:
This kind of input method is wrong, it is the behavior of playing edge ball, and with the specification gradually clear, this value will be gradually prohibited.
According to the ASME Design, the small end thickness of flange is always taken as a small value, no matter in the code or in the supporting software. If the flange small end thickness input is greater than the cylinder thickness, the software will have a red font prompt. There’s no more edge ball.
The equipment designed according to GB 150 may be controversial, because there is no similar explicit regulation in GB after all. On the contrary, Nb flange still supports that the thickness of small end of flange is greater than that of cylinder. But designers should know which is right. The boss thinks that the thickness of the cylinder will not be wasted in the future.
Safety and compliance
Many friends will say:
I have been inputting the small end thickness of flange before, and there is no safety problem, which indicates that this kind of input is correct.
I don’t think that the input method of go will have security problems.
But no safety problems, does not mean to meet the requirements and spirit of the specification.
Designers try their best to be compliant and safe, which may be a kind of protection for themselves.
Compliance and safety are the best conditions.
It’s dangerous to break the rules. It’s easy to understand why we should not do it.
Safe but illegal, the following conditions:
1. For example, go is used for small end, but pay attention to a certain margin. A lot of practice has proved that most of them are safe.
2. For example, a heat exchanger originally met with creep fatigue condition, which was designed in the 1970s without considering fatigue, but it has been in safe operation for 40 years. Considering the actual use situation, the owner cancelled the creep fatigue design condition, and designed according to the original drawing.
- It is difficult to understand that it is in line with the specifications, but it is still dangerous. In fact, there are many similar cases, which are often due to the blind spots of the specifications. They mistakenly think that only the specifications can be considered, without considering the particularity of the equipment. The water pressure test of a heat exchanger is qualified, but it leaks as soon as it runs on site. It is impossible to stop the leakage of bolts. Check the calculation sheet and it is completely correct. The preload of diaphragm groove gasket is also considered. Later, it was found that the temperature difference of each cavity of the heat exchanger was too large, and the deformation was too large under the operating temperature.
- For example, the reinforcing ring of the original vacuum tower designed according to the specifications was broken. When the temperature is high, the stress produced by the temperature field of the external accessories with large rigidity, such as saddle and rigid ring, will be relatively large. Although the conventional calculation is over, it is actually dangerous.
- When the nozzle load is large and the wall thickness is thin, the equipment is easy to cause deformation and even crack. It is also a kind of protection for equipment design to write the nozzle load table. After all, the damage caused by overload is the problem of operators.
- The empty tower was shaken by the sea for half a year, and cracks appeared.
Source: China Flange Manufacturer – Yaang Pipe Industry (www.steeljrv.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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