# Allowable external load calculation of pipe flange

There is a set of equipment with design pressure of 4MPa and design temperature of 230 ℃. The material of American standard pipe flange is 16Mn, class300, and the maximum allowable working pressure of flange is 4.266Mpa.

Due to the large nozzle load, the equivalent design pressure of flange is much higher than the design pressure according to the calculation formula of flange equivalent design pressure:

For example:

Design pressure P = 1.6Mpa, design temperature T = 320 ℃, shell material 20, size Φ 325 × 8. The standard of equipment flange is HG / T20592-2009 wn300 (b) – 25 FM, material is 16Mn Ⅱ. The total height of the equipment is H0 = 4913mm, the seismic fortification intensity is less than 7 degrees, the equipment is installed indoors, and the eccentric load Ge = 0. Total mass of equipment Mo = net weight of equipment 685 + net weight of material 335 = 1020kg
The calculation is as follows:
The total mass of the equipment under operation: Mo = 685 + 335 = 1020kg
Acceleration of gravity: g = 9.81m/s2
The seismic fortification intensity is 7 degrees (the user condition map is lower than 7 degrees, so it is conservative design according to 7 degrees). When the basic seismic acceleration is 0.15g, the seismic influence coefficient is as follows: α= 0.12
Seismic load: Pe= α mo g=0.12×1020×9.81=1200.7 N
Since the equipment is installed indoors, PW = 0
Pe= 1200.7 n
Horizontal bending moment: M = Pe ٠ h=1200.7 × 1296=1556107.2 N ٠ mm
Where: H is the height of flange sealing surface from the center of mass, H = 1296mm
Axial force: Fz = MOG = 1020 × 9.81=10006.2 N
After checking HG / T20610-2009 and HG / T20592-2009, Dg = 351.5mm is determined according to GB 150
Pc because the total height of the tower is 5.2m, it is not very high, so the static pressure of the liquid column is not considered, taking Pc = P = 1.6Mpa
Flange equivalent design pressure:

Through investigation: the maximum allowable working pressure of HG / T20592-2009 WN300 (b) – 25 FM 16Mn Ⅱ flange at 320 ℃ is 2.054Mpa, so Pe < 2.054mpa, so the flange is qualified.

It’s very dangerous. Do you need to raise the pressure level of flange?
I asked: do you want to be promoted or not?
My friend said: “from the perspective of drawing, I don’t want to improve. After all, the design has been completed and I don’t want to modify it.
From the perspective of safety, if you want to improve, you can’t leave hidden dangers.”

This is what designers say.

## The use of UG-44

In fact, in addition to the equivalent design pressure method, other methods can also deal with this problem.
Open ASME VIII I 2021, UG-44:

Sorry, I forgot that the cover of 2019 is the same as that of 2021.

When ASME was updated every two years, there was no time to redesign the cover, not to mention the UG-44 content related to external load.

This formula can be used to calculate whether the flange can bear the current external load, so as to know well.
Of course, if conditions permit, pvelite can be directly used to check the influence of nozzle load on flange bearing capacity.
However, the report of pvelite is compiled according to the formula of the specification, which does not look very intuitive and cannot understand the contribution of bending moment and axial force.

## The realization of Excel

The composition of the formula was analyzed, which was basically the same as the calculation formula of flange equivalent design pressure, but the FM value was added.

In order to conveniently measure the contribution of bending moment and axial tension to equivalent pressure, I decided to make an excel table, and its idea was evaluated according to the above formula after deformation.
The table interface is as follows:

Table features:
1. Automatically query the bending moment factor FM.

2. Calculate the contribution PME of bending moment to equivalent pressure.
3. Calculate the contribution PFE of axial force to equivalent pressure.
4. The calculation is completed and printed into a page of A4 paper for filing.

## The bending moment dominates the equivalent pressure

According to UG-44 calculation, we found that the flange is safe under external load.
Generally speaking, the contribution of bending moment to equivalent pressure is much greater than that of axial force.

Why?
In fact, most of the nozzle load tables are basically:
The bending moment is proportional to the square of D, and the axial force is proportional to d.
Let m = b * 164 * D * D * 1000, FA = b * 2000 * D
PME / FME = 328d / g, D is inch, so G is generally 25.4 * D
So PME / FME = 12.9 times.
The contribution of bending moment is more than 10 times larger than that of axial force.
The bending moment dominates the equivalent pressure.

## Pay attention to some details

UG-44 is applicable

• 1. Because the current specification only studies wn flange with high strength bolts, this method is not suitable for so and other kinds of flange.
• 2. In addition, the bolt pre tightening force of flange should be large, and it should be tightened according to the bolt torque of ASME pcc-1 non mandatory appendix o. σ B refer to appendix o of ASME pcc-1, the maximum value is 40% ~ 70% of the yield strength of bolts at room temperature, and the minimum value is 20% ~ 40% of the yield strength of bolts at room temperature.
• 3. If there is no FM value in UG44, extrapolation and interpolation can not be carried out.
• 4. If the continuous primary load and the temperature of bolt flange make the gasket creep or relax, the FM value should be reduced. The specific reduction is not specified in the specification.

## Conclusion

• 1. The flange equivalent design pressure method is very conservative. In the national standard design, if the conditions of UG-44 (American standard flange, under creep temperature, etc.) are met, UG-44 can be used to judge the damage degree of external load.
• 2. The contribution value of bending moment to equivalent pressure is greater than that of axial force, so if adjustment is needed, priority should be given to the reduction of bending moment.
• 3. If UG-44 is used, it is better to provide the following bolt torque according to appendix o of ASME PCC    -1. Try to avoid using the pre tightening force of GB150 to calculate flange to calculate bolt torque. Large pre tightening force can better ensure the sealing.

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.)