What are the causes of explosion and leakage of heat exchanger
Cause analysis of explosion of heat exchanger
- 1. The self-made heat exchanger blindly makes major changes in the structure and material of the heat exchanger, the manufacturing quality is poor, does not meet the pressure vessel specifications, and the equipment strength is greatly reduced.
- 2. The welding quality of the heat exchanger is poor, especially the welded joint is not fully welded, and the weld flaw detection and blasting test are not carried out, resulting in leakage or fatigue fracture of the welded joint, and then a large number of flammable and explosive fluids overflow and explosion.
- 3. Due to corrosion (including stress corrosion and intergranular corrosion), the compressive strength decreases, resulting in tube bundle failure or serious leakage, and explosion in case of open fire.
- 4. During the air tightness test of the heat exchanger, oxygen pressure supplement or combustible refined gas leakage test shall be used to cause physical and chemical explosion.
- 5. Operation violation, operation error, valve closing, resulting in overpressure explosion.
- 6. Long term non blowdown, excessive accumulation of flammable and explosive substances (such as nitrogen trichloride) and high operating temperature lead to violent explosion of heat exchanger (such as liquid chlorine heat exchanger).
- 7. Peroxide explosion.
Cause analysis of leakage of heat exchanger
The combustion explosion, suffocation, poisoning and burn accidents of heat exchanger are mostly caused by leakage.
Flammable and explosive liquid or gas overflows due to leakage. In case of open fire, it will cause combustion and explosion accidents. The leakage of toxic gas will cause suffocation and poisoning. The leakage of strong corrosive fluid will lead to burns.
The parts most prone to leakage are the welded joint, the connection between head and tube sheet, the connection between tube bundle and tubesheet and flange.
Safety accident caused by leakage of heat exchanger
The main causes of tube leakage of heat exchanger are corrosion, frequent startup and shutdown, excessive temperature change, leakage at the expansion of flower plate caused by rapid expansion and contraction of heat exchanger, and manufacturing defects of equipment itself.
Serious corrosion (such as steam droplets, hydrogen sulfide and carbon dioxide) causes pipe lining.
Due to frequent startup and shutdown, excessive temperature change and rapid expansion or contraction of the equipment, the expansion pipe of the flower plate leaks.
The heat exchanger itself has manufacturing defects and the welded joint leaks.
Flange leakage is caused by the increase of operating temperature, elongation of bolts and loosening of fastening parts.
Leakage is caused by loose assembly parts of heat exchanger tube bundle, pipe vibration, thermal shock caused by startup and shutdown and emergency shutdown, as well as mechanical shock caused by improper operation during regular maintenance.
Cause analysis of tube bundle failure of heat exchanger
The tube bundles of shell and tube heat exchanger, synthetic tower and waste heat boiler are weak links and are most likely to fail. The main failure forms of tube bundles are corrosion cracking. Rapid decline of heat transfer capacity, collision damage, pipe cutting, tube bundle leakage and so on. Common reasons are as follows:
Heat exchangers are mostly made of carbon steel. The oxygen polarization corrosion caused by dissolved oxygen in cooling water is very serious. The service life of tube bundles is often only a few months or one or two years. In addition, many working media are corrosive, such as the cooling water tank of carbonization tower of small nitrogen fertilizer, which is under the dual action of corrosion of high concentration ammonia carbide and crystallization corrosion of ammonia bicarbonate, Sometimes the carbon steel cooling water tank leaks after only two or three months of use.
During the operation of the heat exchanger, the inner and outer walls of the tube bundle may scale, and the thermal resistance of the dirt layer is much larger than that of the metal pipe, resulting in the rapid decline of the heat exchange capacity, and in serious cases, the flow channel of the heat exchange medium will be blocked.
Fluid flow induced vibration
In order to enhance heat transfer and reduce fouling layer, the method of increasing shell side fluid velocity is usually used. With the increase of fluid velocity in the shell side, the possibility of induced vibration will be greatly increased, resulting in the vibration of the tubes in the tube bundle and eventually the destruction of the tube bundle. Common failure forms are as follows:
1. Collision damage
When the amplitude of the tubes is large enough, the tubes will collide with each other, and the tubes located around the tube bundle may collide with the inner wall of the heat exchanger shell. In the collision, the pipe wall is worn and thinned, and finally cracking occurs.
2. Pipe cut at baffle
There is a radial gap between the baffle hole and the pipe. When the amplitude of transverse vibration of the pipe is large, it will cause repeated collision between the pipe wall and the inner surface of the baffle hole. Because the thickness of the baffle is small and the pipe wall contacts it many times and frequently, it will bear a great impact load. Therefore, the local failure of the pipe may occur in a short time.
3. The connection between pipe and tubesheet is damaged.
The connection structure can be regarded as a fixed end constraint, when the pipe vibration produces transverse deflection; The stress at the connection is the largest, so it is one of the areas most prone to tube bundle failure.
4. Failure caused by the expansion of material defects.
If there are defects in the pipe material itself (including defects caused by corrosion and abrasion), under the action of alternating stress caused by vibration, the defect crack in the direction of main stress will expand rapidly and eventually lead to pipe failure.
5. The tensile stress in the vibration alternating stress field will also become the stress source of stress corrosion.
Flow induced vibration causes pipe damage, which is easy to occur in the area with relatively large deflection and high transverse velocity on the shell side. This area is usually U-bend, shell side inlet and outlet nozzle area, tubesheet area, baffle notch area and pipe under compressive stress.
Causes and treatment of scale inhibition in heat exchanger
- 1. Particle fouling: the accumulation of solid particles suspended in the fluid on the heat exchange surface. This kind of fouling also includes the precipitation layer of large solid particles due to gravity on the horizontal heat exchange surface, that is, the deposition of so-called precipitated fouling and other colloidal particles.
- 2. Crystalline fouling: deposit formed by the crystallization of inorganic salts dissolved in the fluid on the heat exchange surface, usually during supersaturation or cooling. Typical fouling, such as calcium carbonate, calcium sulfate and silica scaling layer on the cooling water side.
- 3. Chemical reaction dirt: dirt generated by chemical reaction on the heat transfer surface. The heat transfer surface material does not participate in the reaction, but can be used as a catalyst for chemical reaction.
- 4. Corrosive dirt: dirt caused by corrosive fluid or corrosive impurities in the fluid corroding the heat exchange surface. Generally, the degree of corrosion depends on the composition of the fluid, the temperature and the pH value of the fluid being treated.
- 5. Biological fouling: except for seawater cooling device, general biological fouling refers to microbial fouling. It may produce slime, which in turn provides conditions for the reproduction of biological dirt. This kind of dirt is very sensitive to temperature. Under appropriate temperature conditions, biological dirt can form a dirt layer with considerable thickness.
- 6. Solidified dirt: the dirt formed by the solidification of fluid on the supercooled heat exchange surface. For example, when water is below the freezing point, it solidifies into ice on the heat exchange surface. The uniformity of temperature distribution has a great influence on this kind of dirt.
Anti corrosion measures for heat exchanger
In the design, the steam is placed on the side of the tube side to avoid high-speed gas flowing through the shell side. When there is a large flow medium on the shell side, multiple shell side inlets can be designed to buffer the pressure. In addition, anti scour plates shall be set to reduce the scouring and corrosion of equipment caused by high-speed fluid.
In order to avoid the retention of residual liquid and sediment, double-sided butt welding and continuous welding shall be adopted as far as possible during welding, and lap welding and spot welding shall be avoided. In the welding process, according to practical experience, the stress causing stress corrosion cracking is mainly residual stress, and the residual stress is mainly composed of internal stress caused by cold working and welding.
Heat treatment of cold worked and welded parts is helpful to eliminate residual stress and prevent stress corrosion. Stress annealing heat treatment is often used to eliminate residual stress or other methods to eliminate residual stress, such as hydrostatic test, vibration aging and hammering.
In addition, nylon belt must be used for lifting the tube bundle to ensure that the metal surface is flat, free of scratches and can enter the shell smoothly.
Corrosion resistant materials (such as binocular stainless steel, Hastelloy, titanium, titanium alloy, copper, etc.) are used. These materials have strong corrosion resistance and can improve the service life of the heat exchanger. However, these high corrosion resistant materials are expensive, high manufacturing cost, large one-time investment cost, generally unacceptable to enterprises and difficult to promote.
Electrochemical protection can not only prevent stress corrosion cracking, but also stop the propagation of cracks even if they occur under the condition of appropriate protection parameters. Sacrificial cathodic / anodic protection or surface spraying of corrosion-resistant metal can be used.
The anode on the metal surface is changed into cathode and protected by external DC power supply. This method consumes a large amount of electricity, has high cost and is rarely used.
Anodic protection method:
Connect the protected equipment to the anode of external power supply to form a passive film on the metal surface, so as to achieve protection. Carbon steel heat exchanger has low cost but poor corrosion resistance.
The service life of the heat exchanger can be improved by using the sacrificial anode protection technology, but the protection effect of this technology is limited to the limited length at the inlet of the pipe, and it is difficult to realize cathodic protection at the depth of the pipe. Therefore, the application of sacrificial anode protection method in the heat exchanger is greatly limited.
In corrosive media, a small amount of certain substances are added, which can greatly reduce or even stop the corrosion of metals. Such substances are called corrosion inhibitors. Figure 6 shows the comparison before and after the use of corrosion inhibitor. The addition of corrosion inhibitor shall not affect the production process and product quality.
Stress corrosion can be controlled by removing dissolved oxygen and oxidant from the medium. Reducing the mass concentration of Cl – in the medium and strictly controlling the mass concentration of sulfur in the medium are also effective measures to control stress corrosion.
The metal surface shall be covered with a corrosion-resistant coating protective layer through certain coating methods to avoid direct contact between the metal surface and the corrosive medium.
This technical method is the most economical and effective. It was originally used to prevent gas medium corrosion. Most of the coatings used are organic polymer mixture solutions. Now people are gradually developing towards oil and solvent resistant coatings, high-temperature coatings, heavy-duty anti-corrosion coatings and coatings for special environment.
When the heat exchanger is started up, fill the container with cold fluid, close the inlet, and then slowly inject the hot fluid to minimize the thermal expansion difference between the pipe and shell formed by the introduced fluid.
After shutdown, drain all fluids in the heat exchanger with dry compressed air, so as to minimize the stress and avoid stress corrosion. During start-up, the upper and lower water valves shall be kept fully open to avoid slow flow and corrosion after scaling caused by impurities in the medium settling on the tubular surface.
Source: China Tube Sheet 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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