Material selection of pressure vessel
Factors to be considered in material selection of pressure vessels
Service conditions of equipment
① Medium composition and concentration.
Generally, the corrosion rate increases with the increase of medium concentration, but it is not the same. For example, the corrosion of carbon steel and stainless steel is the most serious in 50% sulfuric acid, but the corrosion rate decreases sharply when the concentration increases to more than 60%. On the contrary, the lower the concentration of sulfuric acid, the smaller the corrosion rate, but when the concentration exceeds 96%, the corrosion rate increases. The corrosion rate of stainless steel in acetic acid is 80% ～ 90%. The corrosion rate of aluminum and aluminum alloy is the highest in 10% ～ 15% sodium thiocyanate solution.
For the concentration of the medium, we should pay attention to not only the overall concentration, but also the uneven concentration. The concentration of a corrosive medium contained in the mixed materials (such as organic compounds) may not be high, but it may be high concentration medium relative to the water in the materials; The corrosive medium (impurity) in the liquid film on the surface will be concentrated due to evaporation in the alternate dry and wet environment, resulting in the local concentration much higher than that of the bulk solution.
When selecting materials, if the range of concentration variation includes the edge of corrosion rate mutation, special attention should be paid to the selection of materials that can withstand any concentration.
② Dissolved oxygen, oxidant and reducing agent.
When selecting materials, we should first determine whether there is oxidant or reductant in the environment, which plays a decisive role in corrosion in many cases. For example, the acetic acid concentration tower made of deoxidized copper accelerates the corrosion due to the mixing of oxygen. The same is true for DMSO recovery tanks made of carbon steel. If it is a strong oxidizing medium, it can be considered to use materials that are easy to passivate under the action of oxidant, such as stainless steel, titanium, aluminum, etc; If it is a reducing acid, some non-metallic materials can be considered.
③ The content of mixed acid, mixed liquid and impurity.
The corrosivity of mixed acid and mixed liquid is different from that of single component. When selecting materials, we often only pay attention to the important components in the process medium, and ignore the impurities. But the trace impurity is the important factor of corrosion. For example, because of the increase of formic acid content in acetic acid and the mixing of Cl -, the corrosion rate of Cr Ni austenitic stainless steel column containing Mo and Cu increases sharply. Trace amount of ammonia or NH4Ci can cause stress corrosion cracking of copper and copper alloy, and CI – can cause stress corrosion cracking of austenitic stainless steel. Sulfur is also a kind of harmful impurity. The high sulfur content in crude oil will directly affect the service life of refining equipment. If we make some changes from the perspective of technology, such as purifying the materials and removing the impurities that have a great impact on the corrosion, the corrosion problem may be solved easily and get twice the result with half the effort.
④ Temperature and temperature distribution.
When selecting materials, we should know whether the equipment is at high temperature or low temperature, the temperature gradient distribution, and whether there is thermal shock caused by rapid cooling or rapid heating. There is a critical temperature for uniform or localized corrosion. For example, aluminum material can be selected for acetic acid at room temperature, 18-10 stainless steel for 40-70 ℃, ultra-low carbon Molybdenum Stainless Steel for 80-135 ℃, and titanium material for 135 ℃. High silicon cast iron, ceramics and enamel should not be used for the equipment with drastic temperature change. For the equipment with dew point corrosion, such as the boiler with high sulfur fuel, the exhaust temperature should be raised above the dew point (about 150 ℃), so as to avoid or slow down the corrosion of the economizer. The environmental temperature should also be considered. The outdoor temperature in the north is low, so it is not suitable to use PVC and other plastics which are easy to be brittle. Besides corrosion resistance, the effect of temperature on strength and stiffness should also be considered. For example, aluminum is used below 150 ℃, polyvinyl chloride and polyethylene are used below 60 ℃, chlorinated polyether and nylon are used below 120 ℃. Polytetrafluoroethylene can be used to 260 ℃, natural rubber can only be used to 65 ℃, synthetic rubber can be used to 110 – 120 ℃.
Attention should also be paid to the local temperature distribution and fluctuation of the equipment. For example, the “inlet side” temperature of heating steam is higher than the medium temperature“ The surface temperature of heating coil is higher than medium temperature; If there is no vent at the top or the vent is not activated, there will be a “dead gas layer” at the top, which is affected by heat transfer. The temperature of the heat exchange tube here is higher than the average temperature of the shell side; Some intermittent operation conditions may also lead to temperature rise, such as the exothermic reaction when the residual concentrated sulfuric acid is replaced. Corresponding to the above local overheating, low local temperature will also cause the so-called “cold spot corrosion”. It is characterized by the high temperature of the medium in the equipment or pipeline, which is in the gas phase state and has little corrosiveness to the material. However, when the surface temperature of some parts of the equipment is lower than the dew point of the gas medium, the gas medium will form a strong corrosive condensate, and the strong corrosive components may also be concentrated in the condensate. It is easy to form heat dissipation points at the supporting steel frame, connecting pipe, leg and stiffener of the vessel or pipeline, resulting in low local temperature and cold spot corrosion on the corresponding inner wall.
⑤ PH value.
The size of pH not only affects the corrosion rate of the material, the nature of the corrosion process, but also has an important influence on the solubility of the metal surface film or corrosion product. Some metal materials are resistant to corrosion in a certain medium because they are in a specific pH range.
⑥ Water content
For non-aqueous corrosive environment, such as corrosive gas, solid materials, organic matter, atmosphere, etc., the content of water often has an important impact on the selection of materials. In some environments, the more dry it is, the more corrosive it is to some metals. For example, the corrosion of alcohol to aluminum and phenol to low carbon steel hardly corrodes when there is a little water, and the corrosion intensifies when there is no water; Titanium is resistant to corrosion in wet chlorine (moisture content > 0.5%), but not in dry chlorine (moisture content < 0.5%). It can not only resist corrosion, but also cause fire. On the contrary, some non-aqueous substances do not corrode metals when they are dry, and once they absorb water, they become strongly corrosive substances. For example, once magnesium chloride and aluminum chloride absorb water, they will produce strong corrosive hydrochloric acid, which is very corrosive to many metals.
The fluctuation of water content also affects the corrosion rate. The so-called “dry for a thousand years, wet for a hundred years, not dry and not wet for half a year” refers to this phenomenon.
⑦ Stress condition
Even the atmospheric pressure vessel also bears many external mechanical loads, and the pressure vessel bears more internal and external pressure loads. The local residual stress of different degrees will be produced in the process of equipment processing. These stresses will lead to stress corrosion cracking and corrosion fatigue cracking. The combination of such materials and environment should be avoided when selecting materials. For the unavoidable situation, effective measures should be taken to eliminate the stress.
⑧ Stress conditions
First of all, different pressure vessels have different requirements on the strength and corrosion resistance of materials. Secondly, the saturated vapor pressure of liquid is related to the pressure, so the selection of material should consider whether the equipment is in high pressure, low pressure or vacuum state, and whether there is pressure fluctuation or pressure shock. For example, decompression and reduced dissolved oxygen are beneficial to improve the corrosion resistance of many metal materials, but for stainless steel, in addition to stress corrosion cracking, the opposite effect will be produced. Thirdly, due to the change of pressure, the gas escaping from the saturated solution often becomes the cause of serious erosion, which can not be ignored in the pipeline or pressure reducing valve with the change of cross section.
⑨ Flow rate and agitation
The flow state of the medium will change the dissolution rate of the metal, the stability of the surface protective film and the form and mechanism of corrosion, so the effect of the flow rate of the process medium on the corrosion rate should be considered when selecting the material. In diffusion controlled corrosion, the corrosion rate of carbon and copper increases with the increase of flow rate due to the supply of oxygen and hydrogen sulfide. On the contrary, when the inhibitor is used, the inhibition effect is enhanced and the corrosion rate is reduced with the increase of flow rate. In the case of high-speed flow, the corrosion is also aggravated due to the damage of the protective film on the metal surface. In order to prevent deposition corrosion, a certain flow rate is often required. When solid particles are mixed into the fluid, attention should be paid to avoid causing wear and corrosion.
⑩ Side effects and pollution
The material selection should also consider whether there are other harmful side reactions in the process reaction, whether it will affect the process flow, pollute the product quality, cause catalyst poisoning or endanger the use of materials.
⑪ Natural environment.
In addition to paying attention to the medium conditions in the equipment and pipelines listed above, the influence of natural environment on the equipment and materials should also be considered. Such as the influence of various industrial atmosphere and marine atmosphere on the outside and even the inside of equipment, the influence of temperature and humidity of natural environment, etc. The corrosivity of atmosphere in industrial area is 50-100 times higher than that in desert area; The corrosion rate of steel in the atmosphere near the coast is 400-500 times higher than that in the desert.
① Corrosion resistance
Not only uniform corrosion resistance but also local corrosion resistance should be considered.
② Mechanical properties
Comprehensive comparison of various mechanical properties. Sometimes it is more reasonable to choose metal materials with lower strength but not sensitive to corrosion than those with ineffective heat treatment, because the latter is often sensitive to stress corrosion and corrosion fatigue.
③ Physical properties
Density, melting point, thermal conductivity and coefficient of linear expansion should also be considered. For example, the thermal conductivity of materials has a great influence on the efficiency of heat exchange equipment, and the thermal conductivity of non-metallic materials (except graphite) is poor. The difference of linear expansion coefficient between base material and lining material should be considered when selecting anticorrosive lining of equipment.
In order to improve the corrosion resistance, special treatment methods can be considered, such as special welding, stress relief measures, surface hardening, surface spraying, seal welding, etc. However, it should be noted that any manufacturing method may aggravate the corrosion damage of materials if it is not handled properly.
Material prices and sources
Sometimes, the price and source of materials may become the decisive factors in material selection, but they can only be determined through specific accounting and comparison, and some common factors are easy to be ignored.
For example: the corrosion of a heat exchanger is serious, and duplex stainless steel or titanium heat exchanger tube can be used. The market price of duplex stainless steel and titanium changes, but the corrosion resistance and density of the material remain unchanged. The specification of titanium heat exchange tube is Φ twenty-five × The specification of duplex stainless steel heat exchange tube is 1.5 Φ twenty-five × 2.5； The density of titanium is 4.51kg/cm3, and that of duplex stainless steel is 7.70kg/cm3. The material is priced according to the quality, and the mass ratio of titanium to duplex stainless steel is: (1.5 / 2.5) ×( 4. 51/7. 70)≈1/3。 Referring to the market prices of the two materials, we can estimate the price ratio of the materials, and then determine the selection scheme. The difference of wall thickness and density can not be ignored.
Another example: when comparing single-layer stainless steel plate with corresponding composite steel plate, the corrosion allowance, equipment specification, structural complexity, processing difficulty and risk, importance and other factors should be considered comprehensively. When the cost of various composite steel plates is obviously lower than that of stainless steel (or non-ferrous metal) plates, it is reasonable to choose composite steel plates. At this time, the structural characteristics of composite steel plate can not be ignored.
Source: China Stainless Steel Plate 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.)
If you want to have more information about the article or you want to share your opinion with us, contact us at firstname.lastname@example.org