Design and selection of steel pipe and valve for desulfurization system in power plant
At present, limestone gypsum wet flue gas desulfurization (FGD) is widely used in China, which has the characteristics of high desulfurization efficiency, wide range of coal, low price of desulfurizer, convenient procurement, mature and reliable technology and stable operation of the device. The wet flue gas desulfurization process is suitable for different types and specifications of thermal power plant boilers and other coal-fired boilers, and is also the most widely used desulfurization process at home and abroad About 80% of all desulfurization processes). The steel pipelines involved in wet process are mainly divided into the following categories: flue, slurry pipeline, steam water pipeline and air pipeline. The slurry pipeline is not in the previous electric power engineering design. It is the pipeline of water and solid particle flow. It has almost all the characteristics of ordinary fluid pipeline, but also has the characteristics that ordinary fluid pipeline does not have, such as easy to wear, easy to corrode and easy to block. Especially when the pipeline starts, runs and stops, if the design is unreasonable, it will It causes deposition and even blockage. It is because of the following characteristics that determine the great difference between slurry pipeline and ordinary fluid pipeline in design.
Medium characteristics of slurry pipeline
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The slurry pipeline of wet flue gas desulfurization has almost all the characteristics of ordinary fluid pipeline. At the same time, because the medium in the slurry pipeline is a mixture of limestone powder or gypsum powder and other fine particles and water, and is mixed with some chloride ions (within 20000 ppm) and heavy metal ions, the slurry pipeline is easy to wear, corrosion and blockage.
The abrasiveness of slurry refers to the impact and damage of solid particles (especially silicate) on the worn materials. The slurry medium of wet FGD is mainly composed of limestone (CaCO3) particles (containing a small amount of SiO2), gypsum (CaSO4 · 2H2O) particles and water, and the solid content of slurry is generally 4.0% ~ 50%. The diameter of limestone slurry particle depends on the mesh number of limestone powder. According to the requirements of low standard 250 mesh, the diameter of limestone slurry particle is generally less than 60 μ m, and the particle size of gypsum is mostly less than 100 μ M. At a high flow rate (above 3m / s), these particles will cause serious wear or erosion on the inner wall of the pipeline.
Due to the weak acidity of the slurry, and some chlorine ions and fluorine ions are mixed, these substances will react with the carbon steel pipe wall and cause the steel pipe to corrode and wear out, which will affect the service life of the desulfurization device. The main reaction formula is as follows:
In addition, Cl – is more easily adsorbed on the surface of metal than oxygen, and the oxygen is pushed out, so that the passivation state of metal is destroyed locally and pitting corrosion occurs. Some stainless steel materials are also difficult to avoid. The corrosion forms of slurry to metal pipe include pitting corrosion, crevice corrosion, stress corrosion, fatigue corrosion, electrochemical corrosion, etc. The best way to prevent corrosion is to prevent slurry from contacting with metal surface, such as rubber lining (butyl rubber is widely used at present) or plastic lining.
The slurry pipeline of wet flue gas desulfurization is a two-phase flow. The characteristic of two-phase flow is that the velocity must be controlled in a proper range. When the flow rate is high, it is easy to cause wear and greatly increase the pipe resistance, while when the flow rate is low, the deposition will occur, which will reduce the flow surface of the pipeline until the whole pipe is blocked. The slurry pipeline is easy to block because the sediment is not cleaned for a long time, which will lead to hardening and caking, and finally the whole pipeline will be scrapped.
The pipeline containing weak alkali (containing Ca2 -) is easy to scale. No matter what the flow rate is, long-term operation will lead to scaling and blocking. This is also one of the important factors for the pH value of the slurry in the absorption tower of wet flue gas desulfurization to keep 5 ~ 6 (weak acidity).
Design of slurry pipeline
According to the medium characteristics of wet flue gas desulfurization slurry pipeline, the design of slurry pipeline should not only meet the requirements of common low-pressure fluid pipeline design, but also consider the particularity of slurry pipeline. The following are the general requirements for slurry pipeline design, pipeline material selection, pipe diameter calculation, slope requirements, pipe gallery layout, valve selection and layout, support and hanger layout, etc
The general requirements of slurry pipeline process design are as follows: full attention should be paid to the use of advanced technology, rational utilization of energy in the device, and proper treatment of waste gas and waste liquid (due to the high content of soluble salts and chloride ions in desulfurization wastewater, which will cause adverse impact on the system materials and products of the reuse users, so the desulfurization wastewater is best used as the supplement of slag bailing, ash flushing and slag flushing of boilers It must meet the requirements of normal production, start-up and shutdown, safety and accident treatment, and maintenance requirements and certain operation flexibility should be considered; cut off valve should be set at the inlet and outlet of the pipeline; when the device needs to be overhauled due to accident or regular shutdown, measures should be taken to discharge all materials in the device to the accident slurry tank.
General requirements for the layout design of slurry pipeline: it shall meet the process design requirements; it shall be arranged as “step by step high” or “step by step low” to avoid liquid bag or “cecum”, otherwise, it is necessary to set up a drainage point, or at least manually remove it; first, the difficulty shall be followed by the easy, for example, the important, large diameter and slurry pipelines shall be arranged first, and then the small, secondary and light pipelines shall be arranged; the pipeline layout shall be in accordance with the requirements of process design The installation shall be orderly, horizontal and vertical, forming a row for easy support; the longitudinal and horizontal elevation shall be staggered, and the elevation shall be changed at the same time of changing direction; the pipeline shall be laid overhead or on the ground, and the slurry pipe shall not be laid in trench or buried directly; it shall not hinder the operation and maintenance of equipment, pumps, instruments and valves; it shall meet the requirements of flowmeter, densitometer and pH meter for pipeline special requirements: the reducing pipe fittings should be close to the position where the diameter needs to be changed to save the pipe materials; the pipe should be properly supported; the pipe bottom elevation at pedestrian passage should not be less than 2.2 m, and the pipe bottom elevation at the vehicle passing place should not be less than 4.5 m; the clear distance between the outer edges of the pipe flanges arranged side by side should not be less than 25 mm (50 mm for non flange pipes), or the clear distance between insulation layers should not be less than 50 mm; holes should be reserved when passing through the floor Water retaining edge shall be set, and the hole diameter shall meet the requirements of flange in and out; the orifice plate of non pressure flow pipeline shall be arranged on the riser to facilitate the drainage; the position of flange shall meet the operation space for installing bolts; the pipeline shall be generally provided with slope (the slope requirements will be described in detail later); the slurry pipeline shall be far away from electrical equipment and cable tray, and if unavoidable, the pipeline shall go below as far as possible to prevent dripping Liquid corrodes electrical equipment; effective cavitation allowance should be reserved in the suction section of the pump, which is generally at least 1.2 times of the cavitation allowance required by the pump. The suction section of the pump should be as short and straight as possible, and the size head of the pump inlet should be as close to the slurry pump as possible.
Pipeline material selection
At present, the common materials used in domestic slurry pipeline are: rubber lined carbon steel pipe (RL), plastic lined carbon steel pipe (PL), glass fiber reinforced plastic pipe (FRP), polypropylene (PPR, PPH) pipe, stainless steel (304, 316) pipe, etc. Rubber lined carbon steel pipe (RL) uses ordinary carbon steel pipe (Q235-A) or high-quality steel pipe (20 ා) as steel frame material, and rubber (generally butyl rubber) as lining layer, which combines metal characteristics and rubber characteristics into one. The rubber lined carbon steel pipe has the properties of wear resistance, impermeability, corrosion resistance and heat resistance (120 ℃), and the pipes are connected by flange. Carbon steel rubber lined pipe is the most widely used flue gas desulfurization slurry pipeline. Carbon steel rubber lining should be thorough, not only for the inner wall of all pipelines, but also for all parts that may contact the slurry, such as flange surface (the rubber lining should cover the flange surface without gasket), pipe internals, valves, slurry pump, etc. Otherwise, as long as there is corrosion, the rotten spot will spread until the whole part is affected.
Plastic lined steel pipe (PL) is a kind of common carbon steel pipe or high quality steel pipe lined with plastic. Plastic lined steel pipe (PL) is divided into steel lined polypropylene (PP), steel lined polyethylene (PE), steel lined polyvinyl chloride (PVC), steel lined polytetrafluoroethylene (PTFE), etc., and flange connection is also used between pipes. Due to its higher price than rubber lined steel pipe, plastic lined steel pipe (PL) is usually only used in slurry pipe with small diameter in flue gas desulfurization project.
Glass fiber reinforced plastics (FRP) is a kind of composite material, which is made of two basic components: matrix material and reinforcement material, and adding various auxiliary agents. The commonly used matrix is various resins, and the commonly used reinforcing materials are carbon fiber, glass fiber, organic fiber, etc. FRP has the characteristics of corrosion resistance, heat resistance, wear resistance and light weight. There are many kinds of joints for FRP pipes, mainly including socket and socket joint, flat end butt joint, flange connection, etc. the nominal pressure ranges from normal pressure to 4.0Mpa, and the temperature range is – 40 ~ 100 ℃. The disadvantage is that compared with the metal pipe, the strength is low, the rigidity is poor, and the long-term exposure to ultraviolet radiation is easy to aging, which is rarely used in wet flue gas desulfurization project, such as spray pipe, oxidation air pipe, etc.
Polypropylene (PPR, PPH) pipe is a kind of pipe which is extruded by random copolymerization polypropylene (PP) by injection molding. It is a new type of plastic pipe product developed and applied in Europe in the early 1990s. PPR (PPH) pipe has the characteristics of light weight, corrosion resistance and not easy to scale, but also has good heat resistance. The maximum working temperature of PPR (PPH) pipe can reach 95 ℃. PPR (PPH) pipes and fittings can be connected by hot melt and electric fusion, which is convenient for installation. The disadvantage of PPR (PPH) pipe is that it has low strength and poor rigidity compared with metal pipe, and there is a certain low temperature brittleness below 5 ℃. PPR (PPH) pipe is easy to be aged and degraded by ultraviolet radiation for a long time; in addition, the linear expansion coefficient of PPR (PPH) pipe is large (0.15mm / m ℃), so the measures of absorbing heat expansion should be taken in the layout design. A few wet FGD projects use PPR (PPH) pipe as slurry pipeline, mainly because of its low price and convenient installation.
Stainless steel (304, 316) pipes have the properties of corrosion resistance, heat resistance, high strength and beauty. The disadvantage is that the price is high, and it will rust when encountering chloride ions. Stainless steel (304, 316) pipes are generally used in small slurry pipes, such as DN 10 ~ Dn40, etc.
Valve selection and layout
The main functions of the valve are to connect or cut off the fluid path, regulate and throttle, adjust the pressure, release the excess pressure and prevent backflow.
Butterfly valve has the characteristics of small size, light weight, quick opening and closing, and has certain adjustment function. Since the movement of butterfly valve plate has wiping property, most butterfly valves can be used for medium with suspended solid particles. Butterfly valves used in slurry pipeline include rubber lined butterfly valve and alloy steel butterfly valve. The structure length and overall height of butterfly valve are small, and the opening and closing speed are fast. When the butterfly valve is fully opened, it has small fluid resistance, which is more suitable for desulfurization slurry (low pressure fluid) system. According to the connection type, butterfly valve can be divided into flange connection and clamp connection. In addition, the butterfly valve has poor sealing performance and small operating pressure and temperature range. Butterfly valve is not recommended for some occasions with strict requirements.
Diaphragm valve (straight through valve is best used in slurry system) is a kind of valve with diaphragm made of rubber or plastic which can produce flexibility in the valve body as the closing part, and separates the inner cavity of valve body and the inner membrane of valve cover. Diaphragm valve has the advantages of good sealing, small flow resistance and low price, but the mechanical life of diaphragm valve is short. Diaphragm valve can be used in corrosive medium containing hard suspended solids, and can be installed in any position of slurry pipeline.
Ball valve is widely used in recent years. It has the advantages of small fluid resistance, simple structure, reliable sealing, wide application range and flexible operation. At the same time, when the ball valve is fully open and fully closed, the sealing surface of the ball and sealing ring is isolated from the medium, and the medium passing through the valve at high speed will not erode the sealing surface, thus increasing the service life of the ball valve. Ball valve can be used in slurry pipeline. The electric control valve is also used on the feed pipe of the first stage cyclone for gypsum discharge and the pipeline for limestone slurry entering the absorption tower, which must have anti-corrosion function.
Check valve is only used in water and air pipeline, check valve should not be used in slurry system.
The flushing and discharging valves and the feeding / isolating valves of the slurry loop should be as close to the main pipeline as possible to avoid blockage; in general, pneumatic actuators are preferred in places where it is difficult for people to operate or frequently used; the installation positions of all valves shall be convenient for operation, maintenance and overhaul; the valves with too high installation position shall be provided with operation platform; the adjacent layout of valves shall be arranged When installing, the clear distance between hand wheels should not be less than 100 mm; for horizontally arranged valves, the actuator and valve stem should not be lower than the horizontal pipeline, and the valve stem should not face down; the installation height of valve handwheel on the riser should be 1.2 ~ 1.5m; when installing butterfly valve, the valve disc should stop at the closed position, and attention should be paid to make the lower part of valve plate rotate towards the direction of medium flow when the valve is opened.
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.epowermetals.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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