Introduction of Thread Ring Locking Heat Exchanger for High Pressure Hydrogenation

Yuan Rongting

(Material and Equipment Department, Sinopec Corp., Beijing 100728, China)

Abstract: The design, structural characteristics, manufacturing process of the threaded ring lock-up heat exchanger and the main technical parameters of the typical threaded ring lock heat exchanger are introduced. The quality control of the product manufacturing process is expounded.

Key words: high pressure heat exchanger; hydrogenation; threaded ring locking; design; manufacturing; quality

CLC number: TQ 050.6; TQ 051.3 Document code: B

The hydrocracking unit is an important device in petroleum refining. The heat exchangers for high temperature and high pressure, such as reactants and feeds, new hydrogen hydrogen, and non-condensable gas, both sides of the phase change are generally used in threaded rings. Locked heat exchanger. The heat transfer technology of this kind of heat exchanger is patented by ABB Lums Heat Transfer Company of the Netherlands. In order to achieve the requirements of localization and improve the design and manufacturing level of China's threaded ring lock-type heat exchangers, based on the experience of designing and manufacturing such heat exchangers abroad, Sinopec Beijing Design Institute and Lanzhou Petrochemical Machinery Factory have successfully cooperated. Designed and manufactured three high-pressure double-shell thread-locking type heat exchangers for Tianjin Petrochemical. This article takes this as an example to introduce the structure and manufacturing of high-pressure hydrogenation threaded ring lock-up heat exchanger.

1 Equipment technical parameters

Tianjin Petrochemical's three high-pressure hydrogenation threaded ring lock-type heat exchangers are double-shell double-tube U-tube heat exchangers, which are made up of pipe boxes, shells, tube bundles, threaded locking rings, and pipe box glands. It consists of a split partition and a pressure ring. The structure is shown in Figure 1. The design parameters are: shell design pressure 15.02 MPa, working pressure 14. 3 MPa, test pressure 20.08 MPa, design temperature 370 ° C, working temperature 334 ° C; inner diameter Φ1 200 mm, wall thickness (72 mm) +6.5 mm) / (225 mm + 6. 5 mm); the medium is a mixed oil. The tube design pressure is 13.8 MPa, the working pressure is 13.0 MPa, the test pressure is 19.0 MPa, the design temperature is 420 ° C, the working temperature is 391 ° C, the inner diameter is Φ1 276 mm, and the thickness is different; the medium is the reaction product. The heat exchange area of ​​a single unit is 750m2, the volume is 11.7 m3, the equipment quality is 79.25 t, the outer diameter of the U-shaped heat exchange tube is Φ19 mm, the wall thickness is 2.2~2.7 mm, and the length of the straight section tube is 7500 mm, and the number is 861.

2 Structural features

(1) The heat transfer efficiency of the double-shell and double-tube heat exchangers with double shell is higher than that of the single shell. The medium in the double shell process can achieve full countercurrent heat transfer. Divided into a single shell process by a single shell, the cross-sectional area of ​​the heat exchanger is reduced by half, the flow rate of the medium can be doubled, and the heat transfer efficiency is improved. Data show that the heat transfer efficiency of the double-shell heat exchanger is about 20% higher than that of the single-shell heat exchanger. The heat exchanger structure adopts a double tube process, and a partition chamber of the tube inlet and outlet is arranged in the inner sleeve, and the inlet and outlet are insulated by a semicircular cover at the chamber end. The double tube can make the heat transfer inside and outside the heat transfer tube close to pure countercurrent heat transfer, and the temperature correction coefficient is close to 1.0. Therefore, the heat transfer area can be greatly reduced.

(2) The double-sealed threaded ring lock-type heat exchanger has one inner tube seal structure than the general heat exchanger, and the sealing force is transmitted piece by piece through the split ring, the split box and the tube plate. The seal between the tube and the shell is achieved by sealing the wound gasket by pressing the bolt through the tube sleeve, the packing and the tube sheet. The tube box seal is achieved by pressing the outer seal ring with an external compression bolt and an outer pressure rod. A sealing partition and a gasket are provided between the casing and the process to seal. In order to avoid the leakage caused by the deformation of the stainless steel separator under the use of long-term high temperature conditions, the tube process is flat sealed, and a sealing tape or packing is provided between the pipe and the process.

(3) Replace the main bolt with a large thread. The shell of the high-pressure double-shell threaded ring-locked heat exchanger is forged and welded into one, so there is no large equipment flange and corresponding large bolt-nut connection. The main bolt structure of the heat exchanger is changed to the large diameter of the pipe box, the pressure ring and the small-sized compression bolt structure, wherein the pipe box and the thread locking ring adopt short-toothed trapezoidal threads. From the sealing point of view, the structure is reasonable, and the bolts for pressing the gasket only bear the pressing force of the gasket, which has almost nothing to do with the internal pressure. Therefore, the seal is guaranteed, the possibility of external leakage is reduced, and the operation is safe and reliable. Once the tube seal is leaked during the operation or the leak occurs between the shell and the shell, the equipment can be stopped, and the outer ring of the end face is used to compress the bolt. Tightening with the inner ring compression bolts eliminates leakage and meets sealing requirements [1]. The above structure enables the heat exchanger shell-side outlet nozzle to be disposed as close as possible to the tube sheet, so that the length direction of the heat exchange tube is fully utilized, and the metal consumption per unit heat exchange area is reduced.

(4) The structure of the machined part multi-thread ring lock type heat exchanger is more complicated, and there are many machining parts and large processing amount.

(5) The assembly difficulty is complicated, the number of parts is large, which makes assembly difficult. It requires special tooling for installation and disassembly.

(6) Others of this heat exchanger have the characteristics of small volume, small floor space, compact structure, material saving and less leakage points.

3 materials

The high pressure heat exchanger tube and shell side medium contain hydrogen + hydrogen sulfide gas, and the design and manufacturing requirements are similar to those of the hydrogenation reactor. The selection of materials should consider the corrosion of hydrogen sulfide, and choose different materials according to the temperature. The tube and shell shell are all made of 2.25Cr-1Mo (SA-336) plus internal surfacing E309L+E347. The pipe is generally selected from TP321 (0Cr18Ni10Ti).

For high temperature parts with hydrogen sulfide corrosion, austenitic stainless steel should be deposited on the base metal.

All internal parts in contact with the medium are made of SA-240 321 austenitic stainless steel. The inner sealing winding pad is made of graphite strip and 321 stainless steel strip. The inner pressing bolt is made of austenitic high alloy steel SA-453 660B.

The inner and outer compression bolts and the inner and outer pressure rods are made of 25Cr2MoVA, and the heat exchange tubes are made of SA-213 TP321.

4 Manufacturing quality control points

In addition to all the features of high temperature and high pressure hydrogen storage equipment, the threaded ring lock type heat exchanger has various design and structural features as described above. Therefore, in the manufacturing process, corresponding control measures should be formulated according to these characteristics to ensure that the manufacturing quality meets the design requirements.

Due to the special structure, complicated manufacturing process and high process requirements, the whole process monitoring must be carried out in the manufacture of equipment for welding, heat treatment and machining. The main links that have a major impact on the quality of the equipment and those dangerous points that are prone to problems should be formulated. Corresponding measures, efforts to eliminate quality hazards.

4.1 Materials

2.25Cr-1Mo should be smelted by electric furnace and vacuum deoxidation process to minimize impurities and trace elements to obtain fine-grained pure killed steel. Control of trace elements in chemical composition is the most effective way to reduce temper brittleness.

In addition to witnessing smelting, degassing, forging, performance heat treatment, reviewing material warranty, checking the appearance quality and checking the material marking, on-site supervisors must also witness various re-inspections according to technical conditions or drawings. In the re-inspection, attention should be paid to the sampling position, the number, the simulated heat treatment state of the sample, and the calculation of the J and X coefficients.

4.2 Welding

Before the product is officially welded, the validity and coverage of the welding procedure qualification of the factory, the qualification of the welder, the grade specification of the welding material and the warranty book shall be checked. During welding, attention should be paid to the execution of welding process disciplines to avoid repairs. After welding, the chemical composition of the weld metal, the chemical composition of the weld overlay and the determination of the ferrite number should be included in the inspection.

In order to prevent the formation of cold cracks in the 2.25Cr-1Mo material during the welding process, the supervisor should always pay attention to whether the welding conditions on site meet the welding procedure, especially the preheating before welding and the hydrogen elimination after welding. (or stress relief) implementation. When welding stainless steel, pay attention to check current, voltage, speed, interlayer temperature, length of the strip and the amount of overlap between the welds, and strictly control the interlayer temperature during surfacing to avoid thermal cracks due to excessive temperature. .

Due to the particularity of the structure, in order to ensure the reliability of the shell-side separator seal, the shell-shaped cylinder of the plate-welded structure should be controlled to have a roundness and the amount of misalignment of the girth weld is not more than 1 mm. To control the roundness, it is necessary to strictly control the size of the blank and the angle of the weld bead. For this purpose, the upper and lower parts of the inner wall and the seal can be machined after the barrel is surfacing.

The deformation of the tube caused by the welding of the tube box and the tube barrel and the internal welding of the barrel can only be solved by machining. In addition to controlling the heat input during welding, it is necessary to take measures to prevent deformation, such as preparing tooling to reduce deformation. Secondly, the short axis is used as the machining reference during machining to avoid uneven or insufficient thickness of the weld overlay.

When the double structure is used, it is unfavorable to observe the welding of the process connecting pipe due to the influence of the welding angle. The weld is a nickel-based welding material, and the line energy should be controlled during welding, and the interlayer temperature should not be strictly controlled.

4.3 Heat treatment

In the manufacturing process of the hydrogenation equipment, heat treatment is indispensable and the number of repetitions is large. Therefore, it should be noted whether the intermediate heat treatment should be heat treated without heat treatment, that is, the heat treatment process is performed. For the final heat treatment, the inner and outer wall arrangement of the thermocouple should be checked. For the local final heat treatment, in addition to the inspection of the inner and outer wall couples, the heat treatment equipment and insulation measures should also be checked. For stainless steel welded parts, the stabilization process should strictly enforce the requirements of drawings and engineering standards.

Since the trapezoidal thread of the threaded ring lock heat exchanger tube box must be processed after the final heat treatment, in order to prevent the machined tube box thread from being heated any more, the circumferential joint heat treatment of the tube box cylinder circumferential welded joint should be performed. However, the wall thickness of each part of the pipe box is not equal and the shape is irregular. Therefore, it is necessary to pay attention to the slow rise and fall temperature during the heat treatment. The equipment for local final heat treatment is ideal for medium frequency induction heating.

The stainless steel U-shaped heat exchange tube is solution treated to avoid stress corrosion caused by the bent portion of the pipe during use. Therefore, when performing heat treatment, it is necessary to check the tools for heat treatment, especially the electric clamps, etc., and check whether there is damage to the clamping part. Also pay attention to whether there is any abnormality in the bending water pressure test.

4.4 Nondestructive testing

In the manufacturing process of hydrogenation equipment, the non-destructive testing methods of different parts are not the same. The main body welds are even required to be tested in a variety of ways and are repeated at different stages of manufacture. Therefore, in addition to the quality inspection and quality control personnel of the manufacturing unit must strictly inspect the manufacturing process, the supervisors should always supervise and check the factory to implement these inspections in stages, and require the factory to provide inspection reports in a timely manner.

For X-ray inspection, the supervisor should confirm the film one by one, and the ultrasonic inspection of the important parts, the supervisor should witness the witness (such as the take-over and the head or the weld with the cylinder), and the factory should be required for the suspected part. Inspection, non-destructive testing of the surface should be witnessed as far as possible. There is a certain blind zone in the ray inspection of the double-structured process joint pipe weld, which cannot be fully achieved. The welding quality of the blind zone can only be guaranteed by the welding process and the skill level of the welder, but 100% PT test should be carried out.

4.5 Machining

The machining accuracy and fit size of the internal thread of the pipe box, the external thread of the thread locking ring and the seal of the pipe box will directly affect the reliability of the heat exchanger seal and the safety of the product. Therefore, the relevant machining accuracy and fit dimensions need to be measured. The sealing surface must be inspected after processing and before pressure testing.

The pipe box has the characteristics of large diameter, long length, many internal seals and high processing precision. When processing, the benchmark must be determined first, and the positioning is performed once. In order to ensure the dimensional accuracy of the inner and outer thread processing of the pipe box, the CNC boring and milling machining center is generally used for machining.

The two sealing surfaces on both sides of the tube sheet (tube side, shell side) should be completely parallel, and the same CNC drilling machine and the same programming hole should be used for the tube plate, baffle plate and support plate to ensure the precision of the hole spacing and Related parts coaxiality. When the pipe hole is machined by the numerical control drilling machine, the through hole is drilled after drilling the hole of the tube plate hole by the numerical control drilling machine.

There are many types of baffles, and the number of sheets is large. The whole board should be cut off, and the board surface should be drilled after drilling.

Shell-shell stainless steel splitter partitions are large in size, small in thickness, easy to deform in processing, and difficult to install. The manufacturer shall establish a reasonable processing technology to ensure the processing accuracy of the separator. When the partition is cut, the diagonal length should be controlled to be no more than 1.5 mm. Please pay attention to the tear hole when processing.

4.6 Component Assembly

(1) Tube bundle assembly 1 baffle plate, support plate tube hole should be free of burrs to prevent scratching the pipe wall. 2 When the pipe is worn, it is not allowed to be beaten forcibly. The anti-vibration bar should be in close contact with the heat exchange tube and the two ends should be welded and fixed to the connecting plate. The connecting plate and the tube bundle supporting plate should also be welded and fixed. 3 Note that the tube bundle separator seal band should be bent toward the inlet side of the housing. 4 The height of the baffle plate extending from the baffle and the support plate and the height of the leg should conform to the drawings. 5 The tube bundle must be pickled and passivated before loading into the housing and check that the positioning tube retaining nut is tightened. 6 Check the size of the inner slide and the outer diameter of the bundle and the size of the partition. The edge of the slide should be smooth and free of burrs.

(2) Assembly of internal parts In order to make the inner parts of the equipment accurately fit into the casing, a certain assembly of the tires, such as threaded ring fittings, dismantling tools and thread protection tires, should be prepared, and the fittings should be assembled one by one.

4.7 Inspection

The overall inspection includes visual inspection, assembly inspection, pressure test, non-destructive testing after hydraulic testing, and review of paint, equipment nameplate, packaging and shipping (and factory documentation). In addition to the orientation of the nozzle before the opening, the visual inspection (including the pre-weld inspection) must be carried out before the final heat treatment of the equipment. After the inspection, the equipment can be finally heat treated.

When performing the shell-and-tube hydraulic pressure test (ie, the differential pressure test), since the tube sheet is designed according to the differential pressure, the pressure test procedure must be strictly in accordance with the technical requirements, and the pressure difference between the tube and the shell is always less than the pressure specified in the pattern. During the pressure test, pay attention to observe whether there is leakage of the signal hole, and pay attention to whether the pressure gauge changes.

Before the painting, the outer surface of the shell should be sandblasted and rusted, and the relevant standards for sandblasting should be met. Paint grades, layers, and film thickness should meet the requirements of the drawings or technical conditions.

Equipment orientation and center of gravity markings should be clearly visible and comply with the requirements of the technical agreement. All the nozzles are covered with a waterproof material to cover the sealed package, and the blind bolts are pre-tensioned. The model and quantity of the packing parts should be consistent with the order list.

After the equipment is fully dried, it is protected by nitrogen filled with a certain pressure by the open nozzle.

5 Conclusion

The high-pressure hydrocracking unit adopts double-shell and double-tube thread-locking type heat exchangers, and the double-shell and double-tube setting can achieve the purpose of reducing heat exchange area and improving heat exchange efficiency. Investment 20%~30%. Therefore, the localization of the threaded ring lock-up heat exchanger of the high pressure hydrocracking unit and its use of double shell and double tube have great economic significance. At present, the largest thread-locked heat exchanger manufactured in China has a diameter of Φ1 700 mm and has been used in the Qingdao large refinery project. Therefore, mastering the manufacturing technology of high-pressure spiral-number ring-locked heat exchanger and improving its manufacturing quality are important contents to improve the manufacturing level of China's petrochemical equipment industry.

references:

[1] Wu Wei, Zhang Yongmei. Manufacture of Thread Locking Ring Heat Exchanger [J]. Petrochemical Equipment, 2007, 36 (increase): 50-52.

[2] Wang Jinguang. Design of Large High Temperature and High Pressure Thread Locking Ring Double Shell Heat Exchanger[J]. Pressure Vessel, 2002,19(4):8-10.