What is double tubesheet heat exchanger?
Double tubesheet heat exchanger is used to absolutely prevent the mixing of media between the tube and shell. For example, if the water in the shell side contacts with the chlorine or chloride in the tube side of the heat exchanger, strong corrosive hydrochloric acid or hypochlorite will be produced and serious corrosion will be caused to the tube side material. The double tube plate structure can effectively prevent the mixing of the two materials, so as to prevent the occurrence of the above accidents.
Working principle of double tubesheet heat exchanger
- 1. The material flows through the stainless steel tube bundle, and the refrigerant or heat medium flows through the external pipeline in the opposite direction. The end of the heat exchange tube bundle is fastened with double tube plates, and serves as the leakage monitoring point to prevent the two-way cross contamination of the material and the heat exchange medium.
- 2. Double tubesheet structure design is adopted for the heat exchanger, so that the tube side and shell side are connected by their own tubesheet respectively, breaking the traditional tube side and shell side of the tube heat exchanger share a joint tubesheet, minimizing the risk of cross pollution, facilitating the timely detection of leakage potential, and ensuring the safe production of users.
- 3. The heat exchange tube is 316L tube, with smooth surface, straight through structure, no dead angle, convenient and thorough cleaning. The lowest point of the equipment has an evacuation valve, which is conducive to the evacuation of materials, cleaning water and condensate after online sterilization. The full emptying design can prevent the dead angle of the product contact part, prevent the growth of microorganisms, and is easy to clean and sterilize.
- 4. The heat exchanger is made of 316L stainless steel or the material required by the customer. The roughness coefficient of the inner surface of the pipe in contact with the material ranges from 0.25 μ m to 0.4 μ m, which is suitable for CIP / SIP online high-temperature cleaning and sterilization.
Structure of double tubesheet heat exchanger
There are generally two types of structure of double tubesheet heat exchanger. One is a fixed tubesheet heat exchanger. There are four tubesheets in one heat exchanger, as shown in Figure 1 (no polymer shell). The flow direction of the two mediums in the shell side and the tube side of the heat exchanger is counter current. The heat transfer coefficient is high and the heat transfer effect is good. The other is U-tube heat exchanger. One heat exchanger has two tube plates in total, as shown in Figure 2 (with liquid accumulation shell). In this kind of heat exchanger, the flow direction of medium inside and outside of half of the tube bundle is parallel flow, and the other half of the tube bundle is parallel flow
The flow direction of the medium inside and outside the tube is counter current, so its heat transfer system
The number is low.
The characteristics of two structures: the heat transfer coefficient of fixed tubesheet heat exchanger is higher than that of U-tube heat exchanger, but one more tubesheet has one more leakage point. However, the number of Tubesheet in U-tube heat exchanger is less than that in fixed tubesheet heat exchanger. In addition, it is easy to dry after the shell side hydrostatic test. If the heat exchange area of the heat exchanger is small, the temperature difference between the shell side and the tube side is large, or the medium in the shell side is dirty, and the surface of the tube bundle needs to be cleaned frequently, the U-tube heat exchanger is generally used.
- When the fixed tubesheet heat exchanger is used as a cooler or heater, it is better to let water or steam flow from the shell side, so that no liquid accumulation shell can be set, and at the same time, the trouble of drying after the shell side hydrostatic test can be avoided.
- U the double Tubesheets of the tubular heat exchanger are generally connected with each other by the polymer shell. The liquid gathering shell can be used to adjust the distance between tube plates and ensure that the two tube plates are parallel to each other. At the same time, the liquid gathering shell is used to seal the gas (liquid) leaked between two adjacent tubesheets to prevent the overflow of toxic gas (liquid). The highest and lowest part of the polymer shell shall be provided with vent and drain holes respectively, which are used to guide the gas (liquid) leakage body in time. If the temperature difference between shell side and tube side is large, in order to reduce the stress at the joint between shell side and tube side tube plate and heat exchange tube, the wall thickness of the short joint should be reduced as much as possible, and an expansion joint can be added if necessary.
- In the design of shell side tubesheet, it is recommended to use the fixed tubesheet with the extension part of tubesheet as flange. If the fixed tubesheet without flange is used, when welding the joint between Tubesheet and shell shell shell, the welding deformation will cause the pipe hole and tubesheet not vertical, so that the strength expansion can not achieve the expected effect.
- Strength expansion is adopted for the connection of shell side tube plate and heat exchange tube. Generally, the connection between tube side tube plate and heat exchange tube adopts strength welding plus stick expansion. If the working conditions are harsh, strength welding plus strength expansion can be used.
In the design, it should be noted that there must be a certain hardness difference between tube sheet and heat exchange tube. Generally, the hardness of tube sheet is higher than that of heat exchange tube (hb20-hb30). It is better to use the material with higher strength grade as tube sheet (such as 16Mn forging), and the material with lower strength grade as heat exchange tube (such as No. 10 seamless steel tube). When the hardness is similar, the tube end can be annealed to reduce the hardness of the heat exchange tube.
Manufacture of double tubesheet heat exchanger
Tube plate processing
To ensure the diameter, perpendicularity and spacing of the tube holes, CNC drilling machine is used to process the tube plate holes. During drilling, the cutting speed and tool withdrawal speed shall be as slow as possible to ensure the surface roughness of the pipe hole is ra3.2-ra6.4, and a reaming allowance of 0.1mm shall be reserved during drilling. Ream the tube sheet hole after drilling to eliminate the longitudinal scratch on the tube hole and ensure the surface roughness of the tube hole is ra1.6-ra3.2. After reaming, the pipe holes of each tube plate shall be inspected with stop gauge and through gauge according to the drawing and the pipe hole tolerance requirements specified in GB151-1999. Special attention shall be paid to the fact that no through spiral or longitudinal striation is allowed in the pipe hole.
Spot weld and fix the tube plate and baffle plate, mark the orientation, take the tube plate hole as the guide, and drill the tube hole of baffle plate. In order to facilitate the threading, the drilling direction shall be consistent with the threading direction. The pipe holes on the front and back sides of each baffle plate shall be chamfered carefully to remove burrs, so as to prevent the external surface of the heat exchange tube from being damaged when passing through the tube.
Check the outer diameter of heat exchange tube
The allowable deviation of the outer diameter of the heat exchange tube for the double tube sheet heat exchanger shall be ± 0.10 mm.
Strength expansion of shell side tubesheet and heat exchange tube
Hydraulic expansion shall be adopted for strength expansion of shell side tube plate and heat exchange tube. The hydraulic expansion has good reliability, the heat exchange tube is not prone to over expansion, the expanded parts do not generate movement, and the stress distribution of the joint between the heat exchange tube and the tube sheet is uniform in the whole length.
In order to ensure the quality of expansion joint, expansion joint evaluation test shall be carried out before expansion.
First, the simulated heat exchanger is made, as shown in Figure 3. The thickness of the tubesheet shall be determined according to the actual tubesheet thickness of the product, and the spacing of tubesheet and the arrangement of tubesheet holes shall be determined as shown in Figure The distance between tube and shell, tube hole size, heat exchange tube size and tube spacing are consistent with the products. The thickness of the shell side shell of the simulated heat exchanger shall be able to bear the shell side test pressure of the double tube plate heat exchanger of the product. The thickness and the cutting position of the test plate can refer to Appendix C of GB151-1999. Take the hydraulic tube expansion as an example, set the hydraulic value of the tube expander controller according to the past experience, and then expand the tube.
Fig.3 Simulated heat exchanger
Before piercing, measure the diameter D of the shell side tube plate, the outer diameter do of the heat exchange tube before expanding, the inner diameter Di of the heat exchange tube and the wall thickness t of the heat exchange tube, so as to calculate the range of the inner diameter D of the heat exchange tube after expanding according to the required expansion rate and expansion rate formula. For carbon steel or stainless steel heat exchange tube, the expansion rate of strength KD should be 10% – 20%.
The expansion rate formula is:
According to Fig. 3, strength welding and sticking expansion shall be carried out for tube side tube plate first, and then strength expansion shall be carried out for shell side tube plate according to D value. During expansion joint, the expansion relay n of tube expander shall be recorded.
According to the requirements of the product drawings, carry out the shell side hydraulic test and ammonia leakage test on the simulated heat exchanger to check whether there is any leakage. If the hydraulic test and ammonia leakage test are qualified, it shows that the expansion process can meet the strength expansion requirements of the simulated double tubesheet heat exchanger.
According to the expansion relay n obtained in the expansion joint evaluation test, conduct strength expansion on the shell side tube plate. In order to prevent leakage, the expansion joints shall be marked one by one.
Hydrostatic test and air tightness test
Firstly, the shell side hydrostatic test shall be carried out according to the pressure required by the drawing, and the connection quality between the heat exchange tube and the inner tube plate shall be checked from the space of the liquid gathering chamber. After the shell side water pressure test is qualified, assemble and weld the short joint of the isolation chamber to make it a closed chamber. Conduct air tightness test according to the pressure required by the drawing. Install transparent U-tube inspection tooling at the two drain holes under the liquid gathering chamber, and add water into the U-tube to maintain a certain level. If there is micro leakage of test gas, the horizontal liquid level in the U-shaped tube will change, and it is qualified that the liquid level in the U-shaped tube is still maintained during the air tightness test. Carry out ammonia penetration test on the shell side according to the requirements of the drawing, and stick test paper on the discharge hole of the isolation chamber. If the test paper does not change color, it is qualified. Then according to the requirements of the drawing, the air tightness test is carried out for the polymer chamber, and finally the water pressure test and air tightness test are carried out for the tube side.
Application of double tubesheet heat exchanger
Double tubesheet heat exchanger is a heat exchanger with two tubesheets or two tubesheets equivalent to a certain gap at one end of the heat exchanger. In practical operation, double tubesheet heat exchanger is generally used in the following two situations: one is to absolutely prevent the mixing of media between the tube and shell, for example, the heat exchanger with water passing through the shell, chlorine gas or chloride passing through the tube, if the water in the shell and When chlorine or chloride contacts in the tube side, strong corrosive hydrochloric acid or hypochlorite will be produced, which will cause serious corrosion to the tube side material. The double tubesheet structure can effectively prevent the mixing of two kinds of materials, so as to prevent the occurrence of the above accidents; the other is the case where the pressure difference of the medium between the tube and shell is very large, at this time, a medium is usually added into the cavity between the inner and outer tubesheet to reduce the pressure difference of the medium between the tube and shell.
In the following cases, the medium in the tube side and shell side of the heat exchanger is strictly prohibited to mix, so the double tube plate structure is often used:
- ① When the two media in tube side and shell side are mixed, it will cause serious corrosion;
- ② One side is extremely or highly hazardous medium, which will cause serious consequences if it infiltrates into the other side;
- ③ When the tube side and shell side media are mixed, the two media will cause combustion or explosion;
- ④ When one medium is mixed with another, the catalyst is poisoned;
- ⑤ The mixture of tube side and shell side medium will cause polymerization or resin like substance;
- ⑥ The mixing of tube side and shell side medium will cause the termination or limitation of chemical reaction;
- ⑦ The mixing of tube side and shell side media causes product pollution or product quality degradation.
What are the differences between double tubesheet heat exchanger and single tubesheet heat exchanger?
The double tubesheet heat exchanger and single tubesheet heat exchanger are compared in terms of structure, application and manufacture. Compared with single tubesheet heat exchanger, double tubesheet heat exchanger has much lower leakage probability and better stress condition. In terms of structure, the double tubesheet heat exchanger adopts fixed tubesheet structure, and the tube bundle cannot be drawn out for cleaning. The practical application shows that the double tubesheet heat exchanger manufactured by mechanical expansion method can meet the use requirements.
Comparison of the structure of double tubesheet and single tubesheet heat exchanger
The double tubesheet heat exchanger adopts the fixed tubesheet structure, and the tube bundle can not be drawn out for cleaning. The single tubesheet heat exchanger can adopt a variety of structural types, and the tube bundle can be drawn out for cleaning. For the double tubesheet heat exchanger with large temperature difference, corrugated expansion joint can be added on the shell; while for the single tubesheet heat exchanger, besides corrugated expansion joint on the shell, floating head or U-tube type is often used to compensate.
For double tubesheet heat exchanger, there are two design concepts: one is that the double tubesheet heat exchanger is used to absolutely prevent the mixing of media between the tube and shell side. The design is to install a drain valve on the cavity between the inner and outer tubesheet for daily observation and discharge in case of leakage of the inner tubesheet, so that the medium in the tube and shell side is effectively isolated by the inner and outer two-layer tubesheet. This is the main purpose of adopting double tubesheet structure. Another is that the double tubesheet heat exchanger can be used in the case of large pressure difference between the tube and shell. A medium is designed to be added into the cavity between the inner and outer tubesheet to reduce the pressure difference between the tube and shell. This is the same as the general single tubesheet heat exchanger, which can not absolutely guarantee that the pipe orifice on the outer tubesheet will not leak.
Comparison of the use of double tubesheet and single tubesheet changers
Single tubesheet heat exchanger is the most common. In addition to sealing leakage of gasket bolt flange joint, leakage of pipe orifice on pipe plate and welding crack will also occur in use. Most of the nozzle leakage on the tubesheet of the single tubesheet heat exchanger occurs in the welding arc. The gas is not discharged completely and there are sand holes during arc stopping.
The double tubesheet heat exchanger has inner and outer double tubesheet. If the inner tubesheet nozzle leaks, the outer tubesheet protection is also provided.
The welding crack of single tubesheet heat exchanger often occurs at the junction of the small cone end of the large flange plate and the shell. The main reasons for the problems are as follows: first, the stress at the junction of the small end of the cone of the large flange and the simplified body is large; second, the geometric size and shape are abrupt and the defects are easy to be buried.
Example 1 in November 2001, the first cycle hydrogen preheater f ~ 04A (diameter 610 mm) imported from abroad in the benzene plant of Beijing Yanshan Petrochemical Company. The thickness is 30mm, the length is 3780mm, and the material is 13crm04). There are several circumferential cracks on the weld bead between the small end of the cone of the large flange and the simplified body. The heat exchanger has been used for 6 years, and the front process is pre vulcanization process, which results in H2S remaining in the environment of heat exchanger, producing fine-grained corrosive film that is easy to crack and peel off; when the temperature exceeds 300 ℃, the fine-grained in the film becomes large crystal structure, resulting in cracks.
Example 219 in December 1996, a new heat exchanger E108 (diameter 1000 mm, thickness 28 mm, material 13crmo4) was installed in the reforming and hydrogenation unit of Beijing Yanhua Petrochemical Company. During the pressure test, there was a crack at the joint weld between the small cone end of the large flange plate on one side of the heat exchanger and the cylinder. Do metallographic analysis. It can be seen that there are relatively dense iron oxide inclusions in and near the crack, which are perpendicular to the axis of the heat exchanger; under the test pressure, they are pulled and cracked.
The joint of the small cone end of the large flange of the double tube plate heat exchanger and the cylinder is located on the outer edge of the cavity formed between the inner and outer tube plates. There is no medium or the medium pressure in the cavity is very small. The stress condition is better than that of single tube plate heat exchanger.
In addition, pressure test of double tubesheet heat exchanger shall be conducted four times (tube side, shell side between two inner tubesheets, cavity between inner and outer tubesheets on both sides), and pressure test of single tubesheet heat exchanger shall be conducted two to three times (tube side, shell side or tube side, shell side, small floating head).
Comparison between the manufacture of double tubesheet heat exchanger and single tubesheet heat exchanger
Compared with single tubesheet heat exchanger, double tubesheet heat exchanger consists of two outer tubesheets, the cavity between the two inner tubesheets and the heat exchange tube in the cavity. At present, the price of double tubesheet heat exchanger ordered in China is about 1.6 × 104 RMB / T, and the price of single tubesheet heat exchanger ordered is about 1.4 × 104 RMB / T. There is little difference in the unit price. According to three double tubesheet heat exchangers in the benzene plant of Beijing Yanshan Petrochemical Company, if the double tubesheet structure and single tubesheet structure are used as heat exchangers respectively, the double tubesheet will increase 10% – 20% in weight and 25% – 37% in cost compared with the single tubesheet. Therefore, more attention should be paid to the manufacturing quality of double tubesheet heat exchanger, so as to spend more money and achieve good results.
(2) Expansion joint
Generally, there are four types of connection between heat exchange tube and tube sheet, i.e. strength welding (common argon arc welding), strength expansion, strength welding + Stick expansion, strength expansion + seal welding. The difference is mainly reflected in whether the tube hole is slotted, the welding groove and the extension length of the tube. Expansion joint can be divided into non-uniform expansion joint (mechanical ball expansion joint), uniform expansion joint (hydraulic expansion joint, liquid bag expansion joint, rubber expansion joint, explosive expansion joint, etc.).
The design of double tubesheet heat exchanger requires strength welding + strength expansion, and the hydraulic expansion method is recommended. The general design requirements of single tube sheet heat exchanger are strength welding + Stick expansion, mechanical expansion or manual expansion. At present, most domestic manufacturers do not have hydraulic expansion equipment, even if there is, because the purchase cost of hydraulic expansion head is high and the loss is large (with an average expansion of more than 100 nozzles, a hydraulic expansion head should be replaced). The hydraulic expansion head is disposable and cannot be repaired. Therefore, it is seldom to use hydraulic expansion method to manufacture heat exchanger. The three double tubesheet heat exchangers are manufactured by mechanical expansion method. After field installation, the pressure test and more than one year’s use show that the double tubesheet heat exchanger manufactured by mechanical expansion method can meet the requirements.
Double tubesheet heat exchanger, as a more reliable heat exchanger structure than single tubesheet heat exchanger, can be better promoted and applied; but there is a first premise that the manufacturing quality of double tubesheet heat exchanger is better, especially the quality of heat exchanger tube, to ensure that there is no cracking of heat exchanger tube wall in use.
As a double tubesheet heat exchanger mainly used in high temperature, high pressure, inflammable, explosive, toxic or strong corrosive medium, if it is to be popularized and applied better, it is necessary to master all aspects of design, manufacturing, operation and maintenance, of which the most important is that the manufacturing quality of the double tube plate heat exchanger is good, requiring high manufacturing accuracy, and the quality of the heat exchange tube is good, so as to ensure that There is no rupture of the heat exchange tube wall in use.
Source: China Tube Sheet Manufacturer – Yaang Pipe Industry Co., Limited (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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