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Installation knowledge of tubular heat exchangers

Tubular heat exchanger is one of the most widely used heat exchangers in chemical industry. It has simple structure, high heat transfer efficiency and strong adaptability (it can be used at high or low temperature, high pressure or low pressure).

The main structure of tubular heat exchanger is a cylindrical shell, which consists of a bundle of parallel tubes.

Flanges are installed at both ends of the shell and the end cover, and bolts are used to connect the end cover with the shell.

The shell and the end cover are respectively provided with small flanges at the entrance and exit of two kinds of media, so as to facilitate the corresponding pipeline connection.

The commonly used tubular heat exchangers in chemical production include fixed tubesheet type, floating head type, U-shaped tubesheet type and shell type with thermal compensation function.

The main structure of fixed tubesheet shell-and-tube heat exchanger is as follows:

Head; Heat Exchange Tube; Baffle Plate; Shell; Support; Tube Plate

The tubesheet (also known as the flower board) forms an irremovable fixed connection with both ends of the shell.

Expansion welding is used to connect the two ends of the tube to the tube sheet. In order to change the flow direction of the medium between the tubes and improve the heat transfer efficiency, several baffles are arranged on the inner wall of the shell.

The end caps at both ends are connected with the shell in the form of flanges.

There are two different media flowing in and between the tubes.

In order to improve the heat transfer efficiency of heat exchangers, sometimes a baffle is set up in the head to form a fluid distribution chamber, that is to say, a so-called double-tube heat exchanger or a four-way tube heat exchanger is formed.

1. Pre-installation preparation

Full preparation before installation of heat exchanger can make the installation work go smoothly, meet the installation technical indicators and ensure the installation quality.

(1) On-site preparation for construction

According to the layout of the construction site, the other aspects of the site are surveyed, and the transportation route, parking location, unloading location and surrounding environment are measured to determine whether the transportation and installation of the equipment are affected, so as to meet the working requirements of hoisting in coordination with the relevant parties. In order to dredge the transportation road, it is necessary to ensure that the road is smooth and solid so that the vehicle can pass smoothly and the heat exchanger can be safely transported to the site. Installation width should meet installation requirements.

(2) Acceptance and acceptance of heat exchange equipment

The drawings of safety equipment are carefully checked, including the type, quality, geometric size, orifice orientation and technical characteristics of the equipment. Look up the technical documents such as the factory certificate, specifications, quality assurance certificate and so on. Check whether the equipment is damaged or missing (including gaskets, bolts, gaskets, accessories, etc.). Make a good record of inspection and acceptance.

(3) Acceptance of Foundation

Before installing the heat exchanger, the foundation must be carefully checked and checked. Foundation construction units shall submit quality certificates, survey records and relevant construction technology information. There should be obvious elevation line and vertical and horizontal center line on the basis, the foundation should be cleaned up, and if there are defects, it should be dealt with.

(4) Preparations for hoisting

The hoisting department should prepare all the rigging equipment, such as crane, holding rod, wire rope, pulley block, chains and clasps, and do a good job of inspection according to safety regulations. For large heat exchangers, because of its large diameter, many heating tubes and large lifting weight, the lifting and binding part should be located at the reinforced cushion plate of the shell support, and wooden square should be set on both sides of the shell to protect the shell, so as to avoid the deformation of the shell when the steel wire rope is crushed during lifting.

(5) Compiling construction plan

In order to make the installation work proceed in an orderly way, the construction plan should be compiled before installation. The contents of the construction plan should include compilation instructions, compilation basis, general situation of the project, construction preparation, construction methods and measures, technical measures and requirements, construction tools, construction materials, allocation of construction personnel, construction plan schedule and so on.

2. Installation of Heat Exchanger

Installation is generally carried out in the following order and requirements:

1) Check whether the deviation of heat exchanger parts size meets the requirement of the standard. The standard of heat exchanger stipulated by national standard GB151: training materials of allowable deviation of heat exchanger.

2) The sliding board should be embedded on one side of the movable support on the foundation, and the pad iron should be placed on both sides of the anchor bolt. After leveling, the oblique cushion iron can be welded with the base plate of the equipment, but can not be welded with the underlying flat cushion iron or slide plate, and the cushion iron must be smooth and flat to ensure the free expansion of the movable support.

3) Two tightening nuts should be installed on the foot bolts of the movable supports. The clearance between the nuts and the floor should be 1-3MM, so that the floor can slide freely.

4) The allowable deviation of heat exchanger after installation shall meet the following requirements:

A. Elevation < 3MM

B. Verticality (vertical) less than 1/1000 and less than 5MM,

Horizontal (horizontal) displacement is less than 1/1000 and less than 5MM C center displacement is less than 5MM.

5) Pipelines connected with heat exchangers should be connected without force in order to avoid strong assembly, and should not hinder the thermal expansion of heat exchangers.

3. Pressure test

Pressure test of heat exchanger (for example, fixed tubesheet type), different types of heat exchanger, the test methods are slightly different.

Firstly, the head of the heat exchanger is removed and the shell side is tested by hydraulic pressure (usually water).

When the test pressure is reached, besides checking the shell of the heat exchanger, the joint between the heat exchanger tube and the tube sheet (hereinafter referred to as the joint) should be checked to see whether there is leakage in the expansion joint or the welding joint.

If a few joints are leaking, they can be marked well. After pressure relief, they can be re-expanded and welded, and then pressure test can be done until they are qualified.

After the test pressure is qualified, the gasket is added to install the head, and then the test of the test pressure is carried out.

4. Installation of accessories

The heat exchanger can be installed with pipes, valves and safety accessories after installation, alignment and fixing. The installation method can refer to the installation of pipelines.

In the process of using tubular heat exchanger, the most prone part to failure is the tube. Flushing and corrosion of the medium on the tube may cause the damage of the tube. Therefore, the heat exchanger is often inspected in order to find out the fault in time and take appropriate measures to repair it.

The most common faults of tubular heat exchangers are fouling, leakage and vibration.

The tube bundles of tubular heat exchangers are arranged by many tubes. The main reason for tube leakage is the erosion and corrosion of medium. Before repairing pipes, it is necessary to check the leakage of pipes.

The commonly used method of checking pipe leakage is to set up sampling nozzle at the low-pressure outlet of cooling water and to conduct sampling and analysis of cooling water regularly. If the cooling water contains the composition of the cooling medium, it indicates that there is leakage in the bundle. Then the pressure test method is used to check which pipes in the bundle are leaking.

During the inspection, one end of the bundle is added with blind plate, and the bundle is immersed in the pool. Then compressed air with pressure less than 1 *100000 Pa is used to pass into each pipe orifice for test.

When compressed air enters a pipe orifice, if there are bubbles in the pool, it means that the official has leaked, it can be marked on the pipe orifice. This method checks all the pipes. Finally, according to the number of damaged pipes, different repair methods are used.

1. Repair methods for leakage of a small number of pipes

If only one or more tubes in the bundle leak, considering that the heat transfer efficiency of the heat exchanger has little effect, plugging method can be used to repair the leaked tubes. Use tapered metal plug to tighten the welding at both ends of the pipe to stop up the damaged pipe.

The taper of conical metal plug should be between 3 and 5 C. The diameter of the large end of the plug should be slightly larger than the inner diameter of the expanding pipe.

2. Repair methods for leakage of more pipes

If there are more leaking pipes, the heat transfer efficiency of heat exchangers will be greatly reduced by using plugging method (usually, the number of plugged pipes can not exceed 10% of the total number of pipes).

At this time, the method of replacing pipes is adopted for repairing. The operation steps are as follows:

(1) Removal of leaked pipes

The removal of the tube is the process of pulling the tube out of the tube sheet. When dismantling the pipe, the thin-walled non-ferrous metal pipe can be dismantled by drilling or winching, or by cutting the pipe orifice with a sharp spoon.

When removing a pipe by drilling or twisting, the diameter of the drill bit or cutter shall be equal to the inner diameter of the orifice on the tube sheet.

When drilling or winching holes, cutting off the basic metal in the expanding part of the tube in the orifice of the tube sheet, the tube can be removed from the tube sheet. When removing the tube by chipping, the pipe orifice of the expanding part can be contracted inward by a sharp chisel, so that the tube and the tube sheet can be detached and the tube can be removed from the orifice of the tube sheet.

For pipes with large wall thickness, the oxygen-acetylene flame cutting method can be used to dismantle the pipes. First, 2-4 notches are cut out in the expansion part of the pipes, and then the pipes are knocked and contracted inward, so that the holes on the pipes and the pipes are opened, and then the pipes are jacked out by a screw jack or pulled out by a pulling tool.

Whatever method is used to dismantle the pipe, both ends of the pipe should be dismantled, and attention should be paid not to damage the orifice of the tube sheet during dismantling, so that when replacing the new pipe, the pipe and the tube sheet can be tightly connected.

(2) Replacement and expansion of new pipes

After the damaged tube is removed from the tube sheet, the new tube can be inserted into the hole of the tube sheet. Replace the new pipe with the same specification as the original pipe.

When penetrating the tube, the corresponding orifice of the tube sheet should be positioned in the correct position. Then you can connect the pipe to the tubesheet.

The connection methods of pipe and tube sheet are expansion joint method and welding method.

1) Expansion method

Expansion method is to use the expander to roll the end part of the tube which extends into the hole of the tube sheet, so that the tube wall expands and the diameter increases. The outer diameter of the tube is completely adhered to the inner wall of the hole of the tube sheet, and the end of the tube is held tightly by the hole of the tube sheet, so that the two parts are tightly leak-free and firmly connected.

When repairing expansion joints of heat exchanger pipes, inclined column manual expansion pipes are usually used.

Its main parts are expansion rod, shell and roller.

Sometimes, in order to make the tube expand more firmly, the end part of the tube extending out of the tube sheet is flanged. At this time, the expander with flanging roller can be used. The expander with flanging roller has three more flanging rollers than the oblique column manual expander.

Before expanding the tube, the inner wall of the tube plate hole and the outside of the tube end should be cleaned up to reveal the metallic luster, and the wall end should be annealed to check the outside diameter of the tube and the inside diameter of the tube plate hole. The size of the tube should conform to the numerical range shown in the table below.

Corresponding Value Range of Outer Diameter of Pipe and Inner Orifice of Tube Plate

When expanding the pipe, the working temperature should be no less than - 10 C. When expanding the pipe, first apply a little oil on the inner wall of the pipe end, put the expander into the pipe port, and insert the conical expanding rod between the three rollers, so that the roller and the pipe can be stepped tightly. The handle with square holes is sleeved in the direction of the head of the expanding rod, while the hands are pressed lightly, the handle is rotated clockwise, and the roller is driven to rotate with expanding. When the rod is pushed forward, the roller strengthens the rolling of the inner wall of the pipe and enlarges the wall of the pipe.

Close to the inner wall of the orifice of the tubesheet, after the expansion operation is completed, it is necessary to check whether the orifice is skewed, whether the transition part is smooth, and whether there are cracks at the flange, etc.

If there are serious defects, the pipe should be replaced and re-expanded.

Finally, the water pressure test method is used for comprehensive inspection. During the water pressure test, the tube bundles should be packed into the shell. After filling the tubes with water, the pressure should be charged to check whether there are any pitfalls in the expansion joints. If there is leakage or sweating, the expansion joints should be re-expanded.

Expansion method is usually used when the pressure is below 25 *100 000 Pa and the temperature is not more than 300 C.

2) Welding method

When the medium temperature and pressure are higher or the requirements are stricter, the pipe can be fixed on the tube sheet by welding method. The welding structure of pipe and tubesheet has the following several structures: see the training textbook for details.

In the case of strict connection requirements between pipe and tube sheet, the method of expanding first and then welding both can be adopted.

The gap between the tube and the inner wall of the orifice of the tube sheet is reduced by expanding the tube, so the gap corrosion can be reduced or eliminated, the welding quality can be improved and the service life can be prolonged.

3) Explosive Expansion

Explosive expanding pipe is a new technology for connecting pipe with orifice of tube sheet. The principle is to use high-energy explosives to explode in extreme time and produce high pressure. Under the action of high-pressure gas and shock wave, the end of the tube quickly undergoes plastic deformation and firmly adheres to the orifice of the tube sheet.

During operation, the outer circle of the end of the tube and the inner circle of the hole of the tube plate are cleaned up to make it leak out the metallic luster, and the tube is passed into the hole of the tube plate. Then the explosive bag with detonator is put into the end of the tube. The position of the explosive bag and the detonator must be coaxial with the tube to ensure the uniform expansion relay during the explosion.

The axial position of the explosive cartridge at the end of the tube, such as training materials. It is possible to rupture the orifice when it is in front, or to rupture the middle of the pipe or close the end of the pipe when it is in back.

Finally, the explosion is initiated by electrification. Under the action of great pressure during the explosion, the end of the tube is tightly expanded on the orifice of the tube sheet. The explosion expansion tube can expand several tubes at the same time. Each time, the same batch of detonators must be used, and the wire material of each detonator should be the same, so as to unify the initiation time of the end of the tube and ensure the quality of the explosion expansion joint.

The two ends of a pipe can not explode at the same time, so one end should be exploded before the other end. The quantity of explosive used is the key to the quality of expansion joint. It is easy to burst the tube when explosive is used more, and the quality of expansion joint can not be guaranteed when explosive is used less.

Before construction, it is best to determine the amount of explosive by simulation test.

The method of explosive tube expansion has the advantages of low cost, simple operation and small workload, but it must be very careful and safe in operation.

Vibration of tubes in tubular heat exchangers is another form of common faults. Vibration of the tube will loosen the expansion joint at the end of the tube, cause wear and tear at the contact between the tube and the baffle plate, and reduce the service life of the tube bundle.

Therefore, the vibration of pipes should be repaired in time to avoid unnecessary accidents.

The vibration of the tube is caused by the excessive gap between the tube and the baffle and the impulsive flow of the medium.

For the loosening of the expansion joint at the end of the pipe, it can be repaired by welding. In order to reduce welding stress, low current arc welding should be used.

For the vibration caused by the excessive gap between the tube and the baffle plate, the wear between the tube and the baffle plate should be repaired in different ways depending on the degree of wear. If the pipe wall is seriously worn out, it can be repaired by replacing the new pipe.

If the tube wall wears slightly, some measures should be taken to reduce the gap between the tube and the baffle, such as increasing the number of baffle plates, or using wood between the tube and the tube, in order to reduce and eliminate the vibration of the tube.

In order to reduce the possibility of pipe vibration caused by impulsive flow of medium, it is necessary to eliminate the impulsive flow of medium. For example, setting buffer in the pipeline can make the medium flow more stable and eliminate the vibration of the pipe.