Design of lap joint flange

Design of lap joint flange

Lap joint flange has many applications in chemical industry.
The size and maximum allowable pressure of standard lap joint flange can be obtained by consulting the temperature pressure gauge of flange in corresponding specification.
Non standard lap joint flange is designed as loose flange.

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The main reasons for using loopers are as follows:

  • 1. The metal in contact with the medium is relatively precious (such as zirconium, titanium, aluminum). If all precious metals are used, the cost of the equipment will be greatly increased.
  • 2. The materials included in the flange pressure temperature table are limited, some materials can not be found in the table, or the allowable stress is very low (such as non-metal), so there is no suitable flange size to choose from.
  • 3. The body needs special processing and installation process, and the flange must be installed later. Such as glass lined equipment, floating end flange and hook ring of floating head heat exchanger.

So what should we pay attention to in the design of loose flange?

Welding structure of looper ring

Generally, the welding joint figure 2-4 (1a) of ASME VIII I will be used for the looper flange, and the thickness of the lining ring shall be greater than that of the nozzle.

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However, the waist height of fillet weld in the welding structure is greater than 0.7tn.
For the standard loose flange, it is generally difficult to do.

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In HG/T 20592 butt welding loose flange, from DN300 to DN600, the g value is 8mm. At this time, if the wall thickness of connecting pipe is greater than 11.4mm, the standard size of loose flange can not be used, and the g value needs to be increased.

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ASME VIII Figure 2-4(1a):

  • On the one hand, fillet welds can be processed to meet the standard loose flange;
  • On the one hand, the minimum waist height of fillet weld is 0.7tn.

The two are even contradictory in the norms.
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In this case, there are three methods:

  • One is that the size of flange G is not modified, and the flange is not calculated, so it does not comply with the requirement of 0.7 times of nozzle wall thickness in the specification. The former is implemented.
  • One is to consider the welding height of 0.7 times the wall thickness of the nozzle, and modify the size of the loose flange. That is, to carry out the latter.

The third is the use of integral looper, no welding structure, so the cost will be higher.
Which one is suitable?

Dialogue with French designers

Former unit a French designer, designed a special material equipment, which used a lot of liner flange, sometimes used the node on the specification, sometimes used the following node.
As the French labor is more expensive, I was called to a remote town in France to help them break down the general layout into detailed drawings.
When I disassembled the drawing, I felt very strange. One kind of lining ring structure is as follows:
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As a rookie, of course, I don’t understand, so I ask:
Why use two different lining ring structures on one drawing?
His explanation is:“
Because the nozzle is thick, the insert type fillet weld can not reach 0.7 times the thickness of nozzle. Therefore, it is replaced by the placement type, which is welded with the lining ring. The benchmark is the node in Figure 2-4 (1), which does not need to meet the requirements of fillet weld waist height, so that the standard loose flange can be used.

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2. For the welding of connecting pipe and lining ring, if the insertion type is used, the stress of fillet weld is shear stress, which is relatively poor, so the waist height of 0.7 times is required in the specification; if the placement type is used, the stress of weld is tensile stress, which is better than shear stress, so the height of fillet weld is not required. “
I asked again:
Since the effect of placement is good, why not use this kind of structure?”
He replied:
“Both of the structures in my drawing are useful, but you find that there is no rule. Small size, basically plug-in type, large flange, and placement type.
It is not convenient to weld the small size in the nozzle, and the thickness of the general nozzle is relatively thin, so it is easy to meet the requirements of the specification.
The inner diameter of large-size nozzle is relatively large, and the thickness of nozzle and lining ring is relatively thick. It is convenient to weld inside the nozzle, and the deformation of lining ring will not be too large. “
I nodded and realized:
“NICE!”

Slip-on flanges Vs lap joint flanges?

Currently we are using lap-joint flanges and welding them directly to the piping.  Is this an appropriate way to use lapped flanges, or should I change to using slip-on flanges.
I understand that lapped flanges are generally used with but ends, so as to make it possible to rotate the flange when needed, but do not understand wether or not a lapped flange should be welded or not.
Please advise as to the correct use of flanges.
lap joint flanges are very similar to a slip-on flange, with the main difference being that it has a curved radius at the bore and face to house a lap joint stub-end. Lap joint flanges and stub-end assemblies are typically used in situations where frequent dismantling is required for for inspection.

Shear stress of lining ring

The calculation of the liner flange needs to check the shear stress of the liner.
The allowable stress of shear stress is 0.8 times of the allowable stress.
General standard liner pipe flange does not need to check the shear stress, such as liner material is zirconium, titanium, etc.
However, if the allowable stress of the lining ring material is much lower than that of the flange material, it is usually necessary to check the shear stress. For example, the lining ring material is aluminum, non-metal, etc.
When checking the shear stress of pipe flange, the calculation of pipe flange itself can not pass, but the lining ring can pass. At this time, only the liner ring can be considered to pass, and the safety of the movable sleeve flange body can be ensured by checking the maximum allowable working pressure of the flange.

Stress analysis of lining ring

In the design of liner flange of ASME VIII I, it is also required to check the strength of liner.
In the actual design, many manufacturers have paid attention to this problem. Many times, some non-metallic looper rings are thickened. Sometimes, some improved structure will be used.
For example, as shown in the figure below, the looper liner ring is processed as a whole, and the stress of the liner ring is better.
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How to check this kind of lining ring? Generally speaking, the shear force can be calculated by preliminary check according to gb150.3.
If the requirements are strict, the actual stress distribution can be checked by analysis.
The boundary conditions are as follows:

  1. Apply internal pressure pi from the inner surface of the liner ring to the inner diameter of the equivalent gasket.
  2. Constraints are set at the contact between the looper flange and the liner ring, UX = 0, uy = 0.
  3. The lower end of the looper adopts internal pressure equivalent force, FA.
  4. The equivalent stress FB is applied at the gasket.

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The calculation of FB is as follows:
a) The bolt loads, WM1 and W, under operation and pre tightening state are extracted from conventional calculation.
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In most cases, the shear force is determined by the pre tightening state.
b) Calculate the equivalent force h caused by internal pressure to the center circle of gasket.
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c) Since the internal pressure has been applied, the axial force h caused by the internal pressure needs to be removed from WB, so FB = wb-h.
Apply the load in this way:
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The stress distribution obtained after operation is as follows:
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The shear force distribution is as follows:
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The shear stress obtained by FEA is 26.76mpa.
According to the shear force formula in GB 150.3, the theoretical shear force is 23.6mpa.
The results are quite close.

Split loop flange

At present, only one integral looper flange can be calculated in GB 150.3.
In some cases, the looper flange will be divided into two pieces for installation.
In a project, it is found that the thickness of the split looper flange is 86mm and that of the matched overall flange is 102mm according to the calculation sheet provided by the manufacturer.
The first impression of this thickness is that there must be something wrong, because the split loop flange is generally thicker than the integral flange. According to the previous experience, considering the stiffness of the looper piece flange, it is about twice thicker than the overall looper flange.
Looking at the calculation sheet, it is found that the main problem is that the value of number of splits in ring in the software is wrongly entered.
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Here, it is easy to take a literal meaning, that is, 1 is the looper flange into one piece, 2 is the looper flange cut into two pieces.
Sure enough, the manufacturer’s understanding is that 1 is the whole ring, and it is difficult to dismantle it later.
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In fact, the software help has a very detailed explanation. There are three values here:

  • 0: the looper flange is not cut and is integral
  • 1: The looper flange is cut into two pieces.
  • 2: The looper flange was cut two times and divided into four pieces.

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What is the difference between these three types? Please refer to ASME VIII I appendix 2-9.
Assume that the overall thickness of the looper flange is t.
When the looper flange is divided into two pieces, the flange design torque needs to be doubled, that is, the flange thickness will be increased to 1.414 * t.
When the looper flange is divided into four pieces, the flange design torque is 75%, so the total thickness of the flange (assuming that the thickness of the two pieces in the thickness direction is equal) will increase to 2 * sqrt (0.75) = 1.73 * t. 866t for each looper flange.
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Although there is no mention of flange in gb150.3, the actual design is in accordance with the requirements of ASME.
For example, the design torque of the hook ring in GB151-2014 is also multiplied by 2.
You don’t think norms are taken into consideration. In fact, most of the time you disdain expression.

Stiffness of looper flange

The manufacturer was informed to modify the number of pieces in the software when calculating the looper flange. The manufacturer updated the calculation sheet and changed the thickness from 86 to 130mm.
When checking the calculation sheet again, the rigidity calculation of the flange could not be found.
The manufacturer sent a screenshot. It’s true that the stiffness is checked, but it’s not in the calculation sheet!
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I feel very strange, so I also input the size of the looper flange into pvelite 2018 to see the effect. It is found that the thickness of the flange will reach 180mm if the stiffness check is checked.
Since the pvelite software version needs to be replaced with the latest version of win10 system, it has not been updated to the latest version. For the lack of stiffness in the calculation, it is uncertain whether it is the manufacturer’s designer’s operation error or the specification’s cancellation of stiffness calculation.
Whatever it is, ask questions first and work together.
If we believe that the manufacturer is honest and the software is reliable, then it must be the code that cancels the stiffness calculation.
After looking for it, as expected, in the article 2-9 (d) of the code, the stiffness check was cancelled.
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I checked the 2017 version of the specification and found that (d) is not included.
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So I’m relieved.
Think about the original looper flange check stiffness, wasted how much material.
ASME update is too fast, so it is very important to use the latest version of software in calculation. Individuals may be busy with the project, and it is difficult to pay attention to these changes. At this time, the software gives a good aid to reduce the probability of error.

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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