Optimal design and research on lap joint flange of heat exchanger

The design of titanium lap joint flange (loose flange) is the key and difficult point in the design of seawater heat exchangers for offshore oil platforms. In order to make the design of loose flanges more reasonable, the article takes titanium loose flanges as the research object, and builds physical models based on an example of a platform heat exchanger, and establishes mathematical models based on relevant standards and mechanical knowledge; using EDR, SW6 and FORTRAN Language and other tools are used to optimize the solution, and the final solution results meet the requirements of on-site working conditions, design strength and material saving. Compared with the pure titanium lap joint flange, the solution not only saves space but also saves a lot of money; at the same time, compared with non-optimized loose flange, the design not only saves money, but also increases the safety of use.

What is heat exchanger?

The heat exchanger is a commonly used heating equipment on offshore oil platforms. It can not only be used as a heater and condenser alone, but also an important accessory equipment for the operation of some chemical units. Therefore, it has an important position in the production of offshore oil technology. With the rapid development of the chemical industry and the increase in energy prices, the investment ratio of heat exchangers will further increase. Among them, the shell and tube heat exchanger is the most widely used. It has a wide range of materials, easy to clean the heat exchange surface, and strong adaptability. , Large processing capacity, can withstand high temperature and high pressure.
Due to the special mining environment of offshore oil platforms, seawater has become the most convenient and cheap cooling medium, but the high corrosiveness of seawater also puts forward higher requirements for its usability; titanium and titanium alloy materials have good seawater corrosion resistance Properties, mechanical properties, welding properties, forming properties and other process properties make it and physical properties widely used on offshore oil platforms; however, due to the high price of titanium and the limited space of offshore oil platforms, the structural dimensions are optimized. It has the dual meaning of saving energy and saving space.

What is a lap joint flange?

Lap Joint Flange is a two piece device that is much like a weld-neck flange but also like a loose slip-on flange. One piece is a sleeve called a ‘Stub-end” and is shaped like a short piece of pipe with a weld bevel on one end and a narrow shoulder on the other end called the hub. The hub is the same outside diameter as the raised face (gasket contact surface) of a weld neck flange. The thickness of the hub is normally about ¼” to 3/8″. The back face of the hub has a rounded transition (or inside fillet) that joins the hub to the pipe sleeve.

When to use a lap joint flange?

lap joint flange 2 - Optimal design and research on lap joint flange of heat exchanger

1. Because of the structure of a Lap joing flange,  it can swivel around the stub end and pipe lining. 

When the piping system is assembe and disassemble frequently, it is better to use a Lap joint flange. It means the flange can work even the two flanges bolt holes are misalignment.
2. In a corrosive situation, the flange joints need to be exchange very soon.

To a lap joint flange, only the stub end is touch with the pipe and fluid, the backing flange no need to touch it. It means you could only replace the stub end , no need to replace the backing flange, so the lap joint flange can decrease the cost of the piping systems.
3. The backing flange and the stub end is seperated, so we can use two different materials for the two pieces. It can work for more complicated application. 

Uses of Lap Joint Flange
Economy
Because a lap joint flange has a two piece configuration, it offers a way to cut cost when piping systems requires a high cost alloy for all “wetted” parts to reduce corrosion. In this situation, it is only required for the stub-end to be can be made of the higher cost corrosion-resistant material, where the flange itself can be the produced from lower cost steel.

Ease of Work
By using lap joint flanges, work can be simplified in situations that require frequent and rapid disassemble and assembly during the operation of a plant. The ability to spin that backing flange compensates for misalignment of the bolt holes during assembly.

Physical Features
Flange
1.  The backside, has a slight shoulder that is square cut at the center or pipe hole
2.  The front side has a flat face with a filleted (rounded) center hole to match the filleted back face of the stub end. Here the stub end will wrap tightly around the center hole of the flange.

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Stub End
1.  Shaped like a short piece of pipe with a weld bevel on one.

This portion of the stub end is also called the sleeve.
2.  Narrow shoulder on the flange facing end called is the hub.

The back face of the hub has a rounded transition (or inside fillet) that joins the hub to the sleeve.

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Design physical model of titanium loose flange

Selection of instance model

There are many seawater cooling equipment on offshore oil platforms. The article takes an example of a seawater cooler on a platform as the calculation object, and conducts design analysis and research on the model. The simple design data is shown in Table 1.
Table.1 Design basic parameters of sea water cooler on a certain platform

Design data Tube pass Shell side
Working medium Seawater Natural gas
Characteristics of the medium Easy to corrode Flammable and explosive
Design pressure/MPaG 1.3 1.3
Design temperature/·c 11. 110

Generally speaking, considering the relevant constraints of the standards “HG/T 20615-2009” and “NB/T47020~47027-2012” and the manufacturer’s actual manufacturing capacity, loose sleeves can be selected for equipment flanges with a pressure of less than 10MPa and an inner diameter of less than 800mm Flange, but detailed calculation and calculation report are required. According to the design basis shown in Table 1, considering that the pipe side fluid is seawater, the design is suitable for loose flanges, and 16Mn is selected as the base material and TA2 as the cladding material. Using 16Mn as a base material can reduce costs and increase strength, and is beneficial to save platform space; using TA2 as a cladding material can meet the requirements of seawater corrosion. However, this also increases the difficulty of flange design. Since the current equipment flange standard “NB/T 47020~47023-2012” does not have a suitable standard flange for this design, non-standardized design is required. It gives designers enough space for design optimization, and needs to meet the requirements of process strength calculation and site space layout according to the actual working conditions of the site. This requires the designer to perform optimized design and optimization on the basis of fully familiar with the process strength design theory. Choose to save resources and energy for the company and the country.

Material selection

Taking into account the anti-corrosion requirements of this design, the design material selection and some physical parameters are shown in Table 2.

Article Parameter Value
Front tube box (material TA2) Allowable stress [σ]f/Mpa 117
Inside diameter/mm 500
Thickness of cylinder/mm 12
Flange (material 16Mn+TA2) Allowable stress [σ]f/Mpa 178
Allowable stress [σ]f/Mpa 175.8
Allowable stress [σ]b/Mpa 210
Allowable stress [σ]b/Mpa 189
Bolt (material 35CrMoA) Nominal diameter d b/mm 20
Bolt root diameter d1/mm 17.3
Number n 24
Gasket (Material Ti+Graphite) Gasket inner diameter/mm 522
Outer diameter of gasket/mm  554
Gasket factor m 3
Specific pressure y/MPa  69

Physical model

Using the process software EDR and the strength calculation software SW6 to calculate repeatedly, the calculation result of the heat exchanger is: BIUS00-1.3-39-3/19 Ti-2. For flanges of the same nominal diameter and nominal pressure, regardless of the structure type, the connection size They are all the same; the standard flange is selected by the shell side. The pipe box flange and the pipe box side flange need to be installed together. The connection size of the flange is a fixed value that cannot be optimized. The connection size is shown in Figure 1.

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Fig.1 Connection dimensions of non-standard titanium loose flange

The loose flange is not directly fixed to the container or the pipe, but the flange ring is loosely sleeved on the flange of the container. The flange can be without or with a tapered neck. This article chooses a flange with a tapered neck to establish Physical model. On the basis of the known connection dimensions in Fig. 1, the physical model of the loose flange made by the pipe process as shown in Fig. 2 is established. As can be seen from Figure 2, the equipment flange strength dimensions (flange neck height h, flange thickness δr, neck large end height δ1, neck small end height δ0. and flange diameter Dch, total 5 indicators) are available Optimal design variables.

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Fig.2 Physical model of titanium loose flange

Mathematical model of titanium loose flange design

Flange, gasket, and bolt connection are a whole detachable connection piece. The true stress and deformation of each piece should be considered, and “leakage” shall be used as the basis for judging whether it fails. At present, the relevant pressure vessel specifications are still based on the elastic analysis proposed in the 1940s, and the method that controls the stress of each part of the flange as the criterion is the essence of controlling the deformation of each part of the flange to control leakage. Based on the above-mentioned physical model, this paper relies on the basic theoretical foundation of Water to establish the mathematical model required for calculation.

Basic assumptions

Although the physical model established in Fig. 2 has a relatively simple structure, the force situation is very complicated, and there are many stress analysis factors that affect each part of the flange. According to the theory of elasticity, this paper calculates the stress of each part of the flange of the narrow face seal, and uses the physical model established above to make the following basic assumptions:

  • (1) The flange ring and cylinder are both in an elastic state, and no yield or creep deformation occurs.
  • (2) Whether it is the flange ring or the shell connected to it, it is only affected by the moment formed by the bolt force, omitting the membrane stress directly caused by the medium pressure on the flange ring. Under operating conditions, the influence of medium pressure has been included in the bolt load.
  • (3) Regarding the flange ring and the taper neck as a pair of edge connection problems formed by two elements, the two parts of the flange ring on the flange ring under the action of the external torque caused by the bolt force are calculated accordingly. Bending stress and edge stress on the cone neck.

Torque calculation of flange

Calculation of bolts for gaskets

The focus of this article is on the optimization design of flanges. According to the calculation process of “GB 150.1-150.l—2011, Pressure Vessel [S]”, the calculation and selection results of gasket bolts are directly given:

  • (1) Effective sealing width b/mm: 7.6
  • (2) The diameter of the center circle of the gasket action DG/mm: 539.5
  • (3) The minimum bolt load Wa/kN required in the preloaded state: 836.9
  • (4) The minimum bolt load Wp/kN required in the operating state: 391.8

Torque calculation of flange

According to the aforementioned physical model and basic assumptions, the force diagram of the loose flange in this example is shown in Figure 3.

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Figure.3 Force and moment diagram of titanium pipe sleeve

(1) Calculation of flange moment Ma under preloaded state:
Ma = WxLG = 30. 56 MN·mm
(2) Calculation of flange torque Mp under operating conditions: The torque caused by gasket pressing force FG is:
MG= FGxLG=2.86 MN·mm)
The moment caused by the axial force FD caused by the internal pressure acting on the inner diameter section of the flange is:
MD= FDxLD=10.64MN·mm
The moment caused by the difference FT between the total axial force F caused by internal pressure and the axial force FD on the inner diameter section:
MT=FTxLT=0.51MN·mm
The flange design torque takes the larger value of formula (1), M0 = 30.56MN·mm

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Mathematical model of stress calculation for loose flange
Axial bending stress on cone neck:

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In the formula, σH is the axial stress of the flange neck, MPa; f is the stress correction coefficient of the flange neck; λ is the coefficient, which is obtained by looking up the table or calculating.
Radial bending stress on the flange ring:

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In the formula, σR is the radial stress of the flange neck, MPa; δf is the effective thickness of the flange, mm; e is the coefficient, which is obtained by looking up a table or calculating.
The hoop bending stress on the flange ring:

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In the formula, σT is the circumferential bending stress of the flange neck, MPa; Y is the coefficient, obtained by looking up the table or calculated; Z is the coefficient, obtained by looking up the table or calculated.

Mathematical Model of Shear Stress Calculation of Loose Flange

The research object of this paper is a loose flange, whose flanging part needs to be checked for shear stress. The shear stress τ is calculated in two states of preload and operation.
Under preload state:

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In the formula, τ1 is the shear stress in the preloaded state, MPa; ι is the calculated height of the shear surface, mm.
In operating state:

20200806014513 50452 - Optimal design and research on lap joint flange of heat exchanger

In the formula, Dτ is the calculated diameter of the shear surface, and the outer diameter is taken as mm.

Computational Research on Optimal Design

Optimal design selection of titanium flange lining ring

Due to the special working conditions of offshore oil platforms, the maintenance cost of the heat exchanger is very high, and the loose flange is a weak link in the design of the entire heat exchanger. The flange lining ring must meet the requirements of shear strength and anti-corrosion; , Leakage due to unreasonable gasket design will cause great losses to platform production. After years of actual operation of the titanium loose flanges of offshore oil platforms, it has been found that the traditional liner ring design can no longer meet the actual working conditions of the offshore oil platforms. Once the design service life is not reached due to leakage and corrosion, the platform production will be affected. The enterprise has brought great losses. Therefore, an optimized design is required (as shown in Figure 4). The advantages are as follows:

  • (1) The slotted countersunk head screw design greatly enhances the overall strength and rigidity of the loose flange;
  • (2) Increase the outer diameter of the liner ring. The outer diameter of the flange and the liner ring are consistent to ensure the strength and anti-corrosion requirements.

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Fig.4 Force and moment diagram of titanium loose flange

Research on Optimal Design of Pipe Box Loose Flange
Pipe box loose flanges mainly meet the requirements of equipment strength. This article combines the formulas in the above mathematical models. On the basis of satisfying the design margins and structural size requirements, the weight of the pipe box loose flanges is taken as the optimization object , Use the FORTARN language to compile a related calculation program to solve, and optimize the design of the flange neck height h, flange thickness δr;, the large end height of the neck δ1, the small end height of the neck δ0, and a total of 4 strength dimensions.
In order to facilitate the design and research, ensure that the other three parameters of the four parameters h, δr, δ1 and δ0 remain unchanged during the calculation process. The relationship between one of the variables and the weight of 16Mn in the loose flange of the pipe box is simulated. The simulation results are as follows: As shown in Figure 5-Figure 8. The relationship between the neck height h and the weight of the pipe box loose flange 16Mn is shown in Figure 5. It can be seen from the figure that as the neck height increases, the weight of the g pipe box flange 16Mn presents a parabolic-like distribution. When the neck height is 30mm, the material consumption is the least and the cost is the lowest.

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Fig.5 The relationship between the neck height h and the weight of the pipe box loose flange 16Mn

The relationship between the flange thickness δr and the weight of the pipe box loose flange 16Mn is shown in Figure 6. It can be seen from the figure that as the flange thickness increases, the weight of the pipe box loose flange 16Mn shows a parabolic distribution. When the flange thickness is 42mm, the material consumption is the least and the cost is the lowest.

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Fig.6 Relationship between flange thickness δr and weight of pipe box loose flange 16Mn

The relationship between the thickness of the large and small ends δ1 and δ0 and the weight of the pipe box loose flange 16Mn is shown in Figure 7 and Figure 8. It can be seen from the figure:

  • (1) As the flange thickness increases, the weight of 16Mn in the pipe box loose flanges of the same thickness at the large and small ends shows a parabolic distribution. When the flange thickness is 16mm, the material consumption is the least and the cost is the lowest.
  • (2) With the increase of flange thickness, the weight of 16Mn in the loose flanges of different thickness pipe boxes at the large and small ends shows a parabolic distribution; when the thickness of the large and small ends are 18mm and 12mm respectively, the material consumption is the least and the cost is the lowest.
  • (3) The comparative analysis of flanges with the same thickness on the large and small ends and flanges with different thicknesses shows that the material for the equal thickness of the large and small ends is less than that of the unequal thickness on the large and small ends.

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Fig.7 The relationship between the thickness of the large and small ends (the same thickness) and the weight of the 16Mn pipe box loose flange

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Fig.8 The relationship between the thickness of the large and small ends (different thicknesses) and the weight of the 16Mn pipe box loose flange

Through the above comparison and analysis, the simulation optimization results (h=30mm, δr=42mm, δ1=δ0=16mm) are calculated for process and strength; after calculation, the calculation results meet the requirements of process strength design margin, the result shown is optimize design results.

Conclusion

The article uses a certain platform example to study the research object, combined with the actual working conditions of the offshore oil platform, and uses tools such as EDR, SW6 and FORTRAN to conduct a research on the optimization design of loose flanges. The conclusions are as follows:

  • (1) In this paper, the flange lining ring has been optimized for the structure design. After years of comparative operation results of the optimized heat exchanger, it is shown that the optimized design is safer and more reasonable than the traditional design.
  • (2) By optimizing the design and solving of 4 strength dimensions of the loose flange neck height h, flange thickness δr, neck large end height δ1, and neck small end height δ0, an optimal design solution is obtained. The calculation results of the, make the structure reasonable and save materials.
  • (3) Comparing this design with the traditional non-optimized design flange, the design in this paper can save about 20Kg of 16Mn material and greatly improve the safety of the flange.
  • (4) Comparing this design with the whole titanium flange design, although the overall titanium flange reduces the design difficulty, the cost will increase greatly; only one heat exchanger with an inner diameter of 500mm in the article can save money. The material is about 50kg, and the cost is about 50,000 RMB. Take one offshore oil platform calculations can save enterprises and the country about 300,000 RMB in cost through optimized design.

Author: Guo Tao, Zhang Bin, Wang Wei, Yang Qian

Source: China Lap Joint 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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Summary
optimal design and research on lap joint flange of heat exchanger - Optimal design and research on lap joint flange of heat exchanger
Article Name
Optimal design and research on lap joint flange of heat exchanger
Description
The design of titanium lap joint flange (loose flange) is the key and difficult point in the design of seawater heat exchangers for offshore oil platforms. In order to make the design of loose flanges more reasonable, the article takes titanium loose flanges as the research object, and builds physical models based on an example of a platform heat exchanger, and establishes mathematical models based on relevant standards and mechanical knowledge.
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www.steeljrv.com
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