A CNC machined tube plate refers to a tube sheet or plate that has undergone precision machining using a Computer Numerical Control (CNC) machine.  CNC machining involves the use of computer numerical control to automate and control the movement of machining tools and equipment. This technology allows for precise and accurate shaping, cutting, and drilling of materials, including metal plates used in various applications.

 

Information on CNC machining tube plates

1. Tube Sheet in Heat Exchangers: In the context of heat exchangers, a tube sheet is a plate that separates the fluid in the tubes from the fluid in the shell of the heat exchanger. CNC machining can be used to create precise holes in the tube sheet for the tubes to pass through.

 

2. CNC Tube Cutting: This could refer to the process of using CNC machines to cut tubes into specific lengths or shapes. CNC tube cutting is commonly used in industries such as automotive, aerospace, and construction.

 

3. Tube Plate in Structural Engineering: In structural engineering, a tube plate might be a component used in the construction of steel structures, such as trusses or frames. CNC machining can be employed to create precise cuts and holes in these plates.

 

 

CNC machining tube plate

1. Hole Drilling: CNC machines can accurately drill holes in tube plates to accommodate tubes in heat exchangers or other systems. The hole patterns need to be precisely designed to ensure proper alignment and fit.

 

2. Milling and Cutting: CNC milling machines can be employed to cut and shape tube plates according to specific designs and requirements. This can include creating intricate patterns or features on the surface of the tube plate.

 

3. Surface Finishing: CNC machining can be used to achieve a smooth and precise finish on the surface of the tube plate. This is important for both functional and aesthetic reasons, depending on the application.

 

4. Customization: CNC machining allows for a high level of customization. Tube plates can be machined to exact specifications, accommodating different sizes, hole patterns, and materials based on the requirements of the specific application.

 

 

CNC machined tube plates are commonly used in the construction of heat exchangers, boilers, and similar equipment where precise alignment and secure attachment of tubes are essential for efficient heat transfer. The use of CNC machining ensures the production of high-quality, accurate, and repeatable tube plates in various industrial settings. 

 

Relying on the top processing equipment cluster, wuxi changrun can provide multiple processes from material to cutting, beveling, welding, heat treatment, vertical turning, drilling and so on; Capable of processing tube plates and folding plates made of diversified materials such as pure titanium, stainless steel composite, high-specification stainless steel and various high-strength steels.

A double tube sheet heat exchanger is a heat exchanger with two tube sheets with a certain gap at one end of the heat exchanger.

 

At the end of the heat exchange tube, there is a tube sheet called the outer tube sheet, also known as the tube side tube sheet, which serves as an equipment flange and is connected to the heat exchange tube and channel flange. There is also a tube sheet located closer to the end of the heat exchange tube, called the inner tube sheet, which is the shell side tube sheet, connected to the heat exchange tube and the shell side.

There is a certain distance between the outer and inner tube sheets, and this space can be separated from the outside by a skirt segment, forming a pressure free isolation chamber; It can also be an open structure.

 

 

Application of double tube sheet heat exchanger

In practical operation, double tube sheet heat exchangers are generally used in the following two situations:

1.One is to absolutely prevent the mixing of media between the shell and tube sides, for example, in heat exchangers where water flows through the shell side or chlorine or chloride flows through the tube side. If the water in the shell side comes into contact with chlorine or chlorides in the tube side, it will produce highly corrosive hydrochloric acid or hypochlorous acid, which will cause serious corrosion to the material of the tube side.

 

Adopting a double tube sheet structure can effectively prevent the mixing of two materials, thereby preventing the occurrence of the above-mentioned accidents.

 

2.Another scenario is when there is a large pressure difference between the medium on the tube and shell side. In this case, a medium is usually added to the cavity between the inner and outer tube sheets to reduce the pressure difference between the medium on the tube and shell side.

 

When the mixing of heat exchanger tube side and shell side media is strictly prohibited in the following situations, a double tube sheet structure is often used:

① When the two media of the tube side and shell side are mixed, it will cause serious corrosion;

② The infiltration of extremely or highly hazardous media on one side into the other can cause serious consequences;

③ When the medium on the tube side and the medium on the shell side are mixed, the two media will cause combustion or explosion;

④ When one medium mixes with another, it causes catalyst poisoning;

⑤ Mixing the tube side and shell side media can cause polymerization or the formation of resin like substances;

⑥ The mixing of the tube side and shell side media can cause the termination or restriction of chemical reactions;

⑦ The mixing of tube side and shell side media can cause product contamination or a decrease in product quality.

 

 

Comparison of double tube sheet and single tube sheet heat exchanger structures

The double tube sheet heat exchanger adopts a fixed tube sheet structure, and the tube bundle cannot be extracted for cleaning. The single tube sheet heat exchanger can adopt a variety of structural types, and the tube bundle can be extracted for cleaning. For double tube sheet heat exchangers with large temperature differences, corrugated expansion joints can be installed on the simplified structure; for single tube sheet heat exchangers, in addition to installing corrugated expansion joints on the simplified structure, floating heads or U-shaped tubes are often used to compensate.

 

There are two design concepts for double tube sheet heat exchangers: one believes that double tube sheet heat exchangers are used to absolutely prevent the mixing of media between the tube and shell sides. A drainage and backflow valve is designed to be installed on the cavity between the inner and outer tube sheets for daily observation and discharge in case of leakage of the inner tube plate, so that the medium on the tube and shell side is effectively isolated by the inner and outer layer tube sheets. This is the main purpose of using a double tube sheet structure.

 

Another view is that double tube sheet heat exchangers can be used in situations where the pressure difference between the tube and shell side media is large. A medium is designed to be added to the cavity between the inner and outer tube sheets to reduce the pressure difference between the tube and shell side media. This is similar to a typical single tube sheet heat exchanger, and it cannot be absolutely guaranteed that there will be no leakage from the pipe opening on the outer tube sheet.

 

 

Comparison of the use of double tube sheet and single tube sheet heat exchangers

Single tube sheet heat exchangers are the most common. In addition to frequent leakage of gaskets, bolts, flanges, and joint seals during use, there may also be leakage of pipe openings on the tube sheet, as well as welding cracks. Most of the pipe mouth leaks on the single tube sheet heat exchanger occur at the welding arc end. During welding, the gas was not completely discharged and there were sand holes.

 

The double tube sheet heat exchanger has inner and outer double tube sheets, and if there is a leakage at the inner tube sheet and tube ends, there is also an outer tube sheet protection.

 

Welding cracks in single tube plate heat exchangers often occur at the joint between the flange and the shell of the heat exchanger. The main reason for the problem here is that the stress at the junction between the flange and the cylinder is high; The second is the sudden change in geometric size and shape, which makes it easy to bury defects.

 

The joint between the simplified large flange and the cylinder of the double tube sheet heat exchanger is located on the outer edge of the cavity formed between the inner and outer tube sheets, and there is no medium in the cavity or the medium pressure is very low. The stress condition is better than that of a single tube sheet heat exchanger.

 

In addition, the pressure test of the double tube plate heat exchanger needs to be conducted 4 times (tube side, shell side between two inner tube plates, and cavity between inner and outer tube plates on both sides), while the pressure test of the single tube plate heat exchanger needs to be conducted 2-3 times (tube side, shell side or tube side, shell side, and small float).

 

 

Comparison of Manufacturing Double Tube Sheet and Single Tube Sheet Heat Exchangers

① Costs

Compared with a single tube sheet heat exchanger, a double tube sheet heat exchanger adds two outer tube sheets, a cavity between the two inner and outer tube sheets, and heat exchange tubes in the cavity. At present, the price of double tube sheet heat exchangers ordered domestically is about 10-20% higher than that of single tube sheet heat exchangers ordered.

If the double tube sheet structure and single tube sheet structure are used as heat exchangers respectively, the weight of the double tube sheet is increased by 10% to 20% compared to the single tube sheet, and the cost is increased by 25% to 37%. Therefore, more attention should be paid to the manufacturing quality of double tube sheet heat exchangers, so that more money can be spent to achieve good results.

 

② Expansion joint

Usually, there are roughly four forms of connection between heat exchange tubes and tube sheets, namely strength welding (commonly argon arc welding), strength expansion, strength welding+adhesive expansion, and strength expansion+sealing welding. The differences are mainly reflected in whether the tube holes are slotted, the welding groove, and the length of the tube extension. Expansion joints can be divided into non-uniform expansion joints (mechanical ball expansion joints), uniform expansion joints (hydraulic expansion joints, liquid bag expansion joints, rubber expansion joints, explosive expansion joints, etc.).

 

The design of the double tube sheet heat exchanger requires strength welding and strength expansion, and it is recommended to use the hydraulic expansion method. The general design requirement for single tube sheet heat exchangers is to use strength welding and adhesive expansion, and mechanical or manual expansion can be used.

 

At present, most domestic manufacturers do not have hydraulic expansion equipment. Even if they do, due to the high cost of purchasing hydraulic expansion heads and high losses (with an average expansion of over 100 pipe openings, a new hydraulic expansion head is required). Hydraulic expansion head is disposable and cannot be repaired.

 

Therefore, hydraulic expansion tube method is rarely used to manufacture heat exchangers.

 

Wuxi Changrun has provided high-quality tube sheets, nozzles, flanges, and customized forgings for heat exchangers, boilers, pressure vessels, etc. to many well-known petrochemical enterprises at home and abroad. Our customers include PetroChina, Sinopec, Chevron, Bayer, Shell, BASF, etc. Send your drawings to sales@wuxichangrun.com We will provide you with the best quotation and the highest quality products.

 

1. Theoretical basis for tube sheet calculation

 

The structure of shell and tube heat exchangers is complex, and there are many factors that affect the strength of the tube sheet. In particular, the tube sheet of fixed tube sheet heat exchangers is subjected to the most complex force. The design specifications of various countries basically consider the tube sheet as a circular flat plate that bears uniformly distributed loads, is placed on an elastic foundation, and is uniformly weakened by the tube holes (Figure 1).

 

Due to the many factors that affect the strength of the tube sheet, it is difficult and complex to accurately analyze the strength of the tube sheet. Therefore, various countries simplify and assume the formula for calculating the thickness of the tube sheet to obtain an approximate formula.

 

The loads that cause stress on the tube sheet include pressure (tube side pressure Pt, shell side pressure Ps), thermal expansion difference between the tube and shell, and flange torque. The mechanical model of the calculation method for the tube sheet of the heat exchanger is shown in Figure 2.

 

1.1 The design specifications of various countries consider the following factors to varying degrees for the tube sheets:

1) Simplifying the actual tube sheet into a homogeneous equivalent circular flat plate based on equivalent elasticity weakened by regular arrangement of tube holes and reinforced by tubes has been adopted by most countries' tube plate specifications today.

2) The narrow non piping area around the tube sheet is simplified as a circular solid plate based on its area.

3) The edge of the tube sheet can have various types of connection structures, which may include shell side cylinders, channel cylinders, flanges, bolts, gaskets, and other components. Calculate according to the actual elastic constraint conditions of each component on the edge of the tube sheet.

4) Consider the effect of flange torque on the tube sheet.

5) Consider the temperature difference stress caused by the thermal expansion difference between the heat exchange tube and the shell side cylinder, as well as the temperature stress caused by the temperature difference at various points on the tube sheet.

6)Calculate various equivalent elastic constants and strength parameters converted from porous plates with heat exchange tubes to equivalent solid plates.

 

 

1.2 Theoretical basis for GB151 tube sheet calculation

The mechanical model considers the tube plate as an axial symmetry structure and assumes that the tubesheets at both ends of the heat exchanger have the same material and thickness. For fixed tube sheet heat exchangers, the two tube sheets should also have the same boundary support conditions.

 

1) The supporting effect of tube bundle on tube sheet

Consider the tube sheet as an equivalent circular flat plate uniformly weakened and placed on an elastic foundation. This is because in the structure of shell and tube heat exchangers, the diameter of the majority of tubes is relatively small compared to the diameter of the tube sheet, and the number of tubes is sufficient. It is assumed that they are uniformly distributed on the tube sheet, so the support effect of each discrete heat exchange tube on the tube sheet can be considered uniform and continuous, and the load borne by the tube sheet is also considered uniformly distributed.

 

The tube bundle has a restraining effect on the deflection and rotation angle of the tube sheet under external loads. The restraining effect of the tube bundle can reduce the deflection of the tube sheet and lower the stress in the tube sheet. The tube bundle has a restraining effect on the angle of the tube sheet. Through analysis and calculation of actual parameters, it was found that the restraining effect of the tube bundle on the angle of the tube sheet has a very small impact on the strength of the tube sheet and can be completely ignored. Therefore, this

 

The specification does not consider the constraint effect of tube bundles on the corner of the tube sheet, but only considers the constraint effect of tube bundles on the deflection of the tube sheet. For fixed tube sheet heat exchangers, the tube reinforcement coefficient K is used to represent the tube sheet.

 

The bending stiffness of the perforated tube plate is η D

The elastic foundation coefficient N of the tube bundle represents the pressure load required to be applied on the surface of the tube plate to cause unit length deformation (elongation or shortening) of the tube bundle in the axial direction.

 

the pipe reinforcement coefficient K and substitute it into the expressions D and N, so that ν P=0.3:

This coefficient indicates the strength of the elastic foundation relative to the tube plate's inherent bending stiffness, reflecting the enhanced load-bearing capacity of the tube bundle on the plate. It is a crucial parameter that characterizes the strengthening effect of the tube bundle on the plate. If the elastic foundation of the plate is weak, the enhancing effect of the heat exchange tubes is minimal, resulting in a small K value. Consequently, the plate's deflection and bending moment distribution resemble those of ordinary circular plates lacking an elastic foundation. Specifically, when K equals zero, the plate becomes an ordinary circular plate. Based on the theory of elastic foundation circular plates, the plate's deflection is not solely determined by the tube's strengthening coefficient K, but also by its peripheral support and additional loads, quantitatively represented by the total bending moment coefficient m.

 

When the periphery of the tube sheet is simply supported, MR=0, then m=0; When the periphery of the tube sheet is fixed, the corner of the edge of the tube sheet φ R=0, from which a specific value of m can be obtained (the expression is omitted); When the periphery of the tube plate only bears the action of bending moment, i.e. VR=0, then m=∞.

Under certain boundary support conditions, as the K value gradually increases, the deflection and bending moment of the tubesheet exhibit a attenuation and wavy distribution from the periphery to the center. The larger the K value, the faster the attenuation and the more wave numbers. During the process of increasing K value, when passing through a certain boundary K value, new waves will appear in the distribution curve. At the center of the plate, the curve changes from concave (or concave) to concave (or concave). Solving the derivative equation of the distribution curve can obtain the K boundary value of the curve with an increase in wave number.

 

Taking the simple support around the tube sheet as an example, as the strengthening coefficient K of the tube increases, the radial bending moment distribution curve and the boundary K value when new waves appear are shown in Figure 31. At the same time, it can be seen that the radial extreme value also moves away from the center of the tube sheet towards the periphery as the K value increases.

 

For the elastic foundation plate with peripheral fixed support, the radial bending moment distribution shows a similar trend with the change of K value, as shown in Figure 3. The difference from a simply supported boundary is that the maximum radial bending moment of the elastic foundation plate supported by a fixed boundary is always located around the circular plate, while the extreme point of the second radial bending moment moves away from the center of the plate and towards the periphery as K increases.

 

For floating head and filled box heat exchanger tube sheets, the modulus K of the tube bundle is similar to the elastic foundation coefficient N of the fixed tube sheet, which also reflects the strengthening effect of the tube bundle as an elastic foundation on the tube sheet.

 

2) The weakening effect of tube holes on tube sheets

The tube sheet is densely covered with dispersed tube holes, so the tube holes have a weakening effect on the tube sheet. The weakening effect of tube holes on the tube sheet has two aspects:

 

The overall weakening effect on the tube sheet reduces both the stiffness and strength of the tube sheet, and there is local stress concentration at the edge of the tube hole, only considering peak stress.

 

This specification only considers the weakening effect of openings on the overall tube sheet, calculates the average equivalent stress as the basic design stress, that is, approximately considers the tube sheet as a uniformly and continuously weakened equivalent circular flat plate. For local stress concentration at the edge of the tube hole, only peak stress is considered. But it should be considered in fatigue design.

 

The tube hole has a weakening effect on the tube sheet, but also considers the strengthening effect of the pipe wall, so the stiffness weakening coefficient is used η And strength weakening coefficient μ。 According to elastic theory analysis and experiments, this specification stipulates η and μ＝ 0.4.

 

3) Equivalent diameter of tube sheet layout area

The calculation of the reinforcement coefficient for fixed tube sheets assumes that all pipes are uniformly distributed within the diameter range of the cylinder. In fact, under normal circumstances, there is a narrow non pipe area around the tube sheet, which reduces the stress at the edge of the tube sheet.

 

The tube layout area is generally an irregular polygon, and now the equivalent circular pipe layout area is used instead of the polygonal pipe layout area. The value of the equivalent diameter Dt should make the supporting area of the tube on the tube sheet equal. The diameter size directly affects the stress magnitude and distribution of the tube plate. In the stress calculation of the fixed tube sheet in GB151, the stress located at the junction of the annular plate and the pipe layout area is approximately taken as the stress of the full pipe layout tube plate at a radius of Dt/2. Therefore, the standard limits this calculation method to only be applicable to situations where the non pipe layout area around the tube plate is narrow, that is, when the non dimensional width k of the non pipe layout area around the tube sheet is small, k=K (1)- ρ t) ≤ 1.

 

Whether it is a fixed tube sheet heat exchanger, or a floating head or filled box heat exchanger, when calculating the area of the tube layout area, it is assumed that the tubes are uniformly covered within the range of the tube layout area.

 

Assuming there are n heat exchange tubes with a spacing of S. For a triangular arrangement of tube holes, the supporting effect of each tube on the tube sheet is the hexagonal area centered on the center of the tube hole and with S as its inner tangent diameter, i.e;

 

For tubes with square arrangement of tube holes, the supporting area of each tube on the tube sheet is a square area centered on the center of the tube hole and with S as the side length, i.e. S2.

 

The tube sheet layout area is the area enclosed by connecting the supporting area of the outermost tube of the tube sheet, including the supporting area of the outermost tube itself.

 

For a single pass heat exchanger tube sheet with uniformly distributed heat exchange tubes, the supporting area of all n heat exchange tubes on the tube sheet is the area of the tube layout area.

 

4) Consider the bending effect of the tube sheet, as well as the tensile effect of the tube sheet and flange along their central plane.

 

5) Assuming that when the flange deforms, the shape of its cross-section remains unchanged, but only the rotation and radial displacement of the center of gravity around the ring section. Due to this rotation and radial displacement, the radial displacement at the connection point between the flange and the center surface of the tube sheet should be coordinated and consistent with the radial displacement along the center surface of the tube sheet itself.

 

6) Due to temperature expansion difference γ The axial displacement of the shell wall caused by the shell side pressure ps and the tube side pressure pt should be coordinated and consistent with the axial displacement of the tube bundle and tube sheet system around the tube sheet.

 

7) The corner of the tube sheet edge is constrained by the shell, flange, channel, bolt, and gasket system, and its corner should be coordinated and consistent at the connection part.

 

8) When the tube sheet is also used as a flange, the influence of flange torque on the stress of the tube sheet is considered. In order to ensure sealing, it is stipulated that the flange stress needs to be checked for the extended part of the tube sheet that also serves as a flange. At this time, when calculating the flange torque, it is considered that the tube sheet and flange jointly bear the external force moment, so the ground force moment borne by the flange will be reduced.

 

 

About us

Wuxi Changrun has provided high-quality tube sheets, nozzles, flanges, and customized forgings for heat exchangers, boilers, pressure vessels, etc. to many well-known petrochemical enterprises at home and abroad. Our customers include PetroChina, Sinopec, Chevron, Bayer, Shell, BASF, etc. Send your drawings to sales@wuxichangrun.com We will provide you with the best quotation and the highest quality products.