# Difference between ASME flange and GB flange design

ASME flange calculation is in mandatory Appendix 2, and its calculation principle and method are consistent with those in GB150, and the calculation results are basically the same. But what are the differences between the design details of the two flanges?

The equipment flange in GB generally refers to the data in Nb / T47021-47023-2012, and the corresponding gasket generally adopts the corresponding specifications in Nb / T 47024 ~ 47026-2012. Generally according to the standard nominal diameter and pressure rating, select the corresponding specifications of gasket, fill in the standard number.
ASME has no standard gasket for equipment flange. Generally, the gasket width can be selected according to the corresponding pipe flange or appendix table 2-4, and the gasket diagram can be drawn.

It can be seen from the table that the width of gasket recommended by ASME is wide. The maximum width of spiral wound gasket in NB is 25, and the maximum width of cushion is 30. If the gasket width is too large, the preload will be too large, so as to obtain a thicker flange, which is not particularly economical. In actual design, Nb flange and gasket size can be used to check.

The basic parameters of the gasket in table 7.2-2 are the same as those in table 7.2-2 of ASME.

For example, if O-ring gasket is used, it can refer to ASME gasket value, m and y are all 0.

## Calculation of flange stiffness

In GB150, the flange stiffness only needs to be checked for integral flange.
In addition to the successful experience of the same project or not too harsh cases, the stiffness calculation can be exempted. In other cases, the stiffness of integral flange, neck loose flange and arbitrary flange shall be checked according to table 2-14 of ASME VIII I appendix.

According to the ASME definition of stiffness coefficient, Ki and KL refer to the maximum allowable deflection angle of flange sealing surface. As the deflection angle KL of loose flange is 0.2 °, generally speaking, the loose flange designed according to ASME is thicker than that calculated in GB.

According to GB, it is unnecessary to check the stiffness of loose flange.
The case I met before, large diameter A-type flange, passed the calculation, but it couldn’t be sealed in the process of use. The manufacturer called to ask us if the calculation was wrong. After careful check, the calculation is really no problem. But the stiffness is calculated according to ASME, which is not enough. Of course, at that time, even the overall flange did not need to calculate the stiffness, let alone the type a flange. Therefore, the reply from the technology department is that the design is completely in accordance with the specifications, and there is no problem with the design. In the end, I don’t know how they solved it.
This place is really open to discussion. You can’t abandon it as a whole just because the rigidity of loose / looper flange increases the thickness of flange. It’s OK to increase the stiffness coefficient for accounting. You can’t stop eating because of choking and avoid medical treatment.

## Calculation of pipe box flange

When calculating the flange of the pipe box, the influence of the gasket on the preload is not considered in GB151.
In ASME VIII I, bolt load should be considered for split diaphragm gasket.

## Thickness value of small end of WN flange

Theoretically speaking, the calculation model of flange is to consider the same thickness of cylinder and small end of flange. If the thickness is not the same, the flange needs to use the straight side section with attenuation length H = sqrt (bg0).
Wn flange of ASME, small end thickness = min (flange small end thickness, butt cylinder thickness).

When the taper is 1:3 trimmed, no straight edge segment is left. Of course, in order to ensure the welding and filming effect, the straight edge length that can make groove can also be reserved.

NB/T47023 allows the small end thickness to be greater than the cylinder thickness.

## Correction factor of bolt spacing

In ASME, for highly toxic vessels or when specified by the user or his designated representative, the maximum bolt spacing shall conform to the following formula.

When the distance between two adjacent bolts exceeds 2A + T, when calculating the flange stress, M0 shall be multiplied by the bolt distance correction factor BSC, which is equal to the root of the distance ratio.

BSC is greater than 1, so when the bolt spacing is too large, larger flange torque Mo should be used as compensation.
In GB, this is a pity. For example, in SW6, if the design bolt spacing exceeds the maximum bolt spacing, the program does not deal with it. Many designers only look at whether the final stress and stiffness are qualified. It doesn’t matter if the bolt spacing exceeds. This will leave a leak.
In fact, it is easy to understand that excessive bolt spacing will affect the uniformity of the seal. The compression force of the gasket in the middle of the two bolts must be less than that of the bolt.
How to deal with this situation in GB design?
I suggest that BSC should be calculated manually at this time, and then the stress calculation results should be expanded by BSC times. If the calculation is passed, then there is basically no problem. But it is suggested that SW6 should take the initiative to consider this, which is convenient and reasonable.

## Type a flange and type b flange

In GB, NB/T47021 type a flat welding flange is often used.

or type b flange:

Generally, NB/T47021 / 47022 type A or B flange is not used in ASME, mainly because this kind of welding structure cannot be found in ASME.

Figure 2-4 (11) seems to be similar to type flange.

(m) (n) of figure uw-13.2 is as follows:

A is three times the thickness of the cylinder, which is basically impossible. Compared with type A and type B flange, the height of welding leg is much smaller.
If flat welding flange must be used, similar welding structure can be adopted. See Figure 2-4 (4a) in Appendix 2.

I think, since the requirements are so high, type a and b should not be used.

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.)