Slurry pipeline material selection and layout design

At present, the very mature limestone - gypsum wet flue gas desulfurization in China has the characteristics of high desulfurization efficiency, wide adaptability to coal types, low price of desulfurizer, convenient procurement, mature technology and stable operation. The wet process is suitable for different types. Different types of thermal power plant boilers and other coal-fired boilers are also widely used in domestic and foreign applications for desulfurization processes ( accounting for about 80% of all desulfurization processes ) . The pipelines involved in the wet process are mainly divided into the following categories: flue, slurry pipeline, soda water pipeline, and air pipeline. Among them, the slurry pipeline is not available in the previous power engineering design. It is a pipeline of two kinds of medium flow of water and solid particles. It has almost all the characteristics of common fluid pipelines, and at the same time has ordinary fluids that are easy to wear, easy to corrode and easy to block. The characteristics that the pipeline does not have, especially when the pipeline is started, operated, stopped, etc., if the design is unreasonable, it will cause deposition or even blockage. It is precisely because of the latter features that the slurry pipeline is designed to differ greatly from ordinary fluid pipelines.

2 , the characteristics of the slurry pipeline medium
     The wet flue gas desulfurization slurry pipeline has almost all the characteristics of the ordinary fluid pipeline, and the medium in the slurry pipeline is a mixture of fine particles such as limestone powder or gypsum powder and water, and is mixed with a part of chloride ions ( within 20,000 ppm ) and heavy metal ions. The slurry pipeline is characterized by being easy to wear, easy to corrode and easy to block.

2.1 wearability

     The abrasiveness of the slurry refers to the impact and destruction of solid particles ( especially silicates ) in the slurry against the material being worn. The wet flue gas desulfurization slurry medium is mainly composed of limestone (CaCO3) particles ( containing a small amount of SiO2) , gypsum (CaSO4•2H2O) particles and water, and the solid content of the slurry is generally 4.0% to 50% . The diameter of the limestone slurry particles depends on the number of limestone powders. The diameter of the limestone slurry particles is generally less than 60 μm , and the gypsum particle size is mostly less than 100 μm , as measured by the low standard of 250 mesh . At higher flow rates ( above 3 m/s ) , these particles can cause severe wear or erosion on the inner wall of the pipe.

2.2 Corrosive

     Because the slurry is weakly acidic, and also contains some chloride ions and fluoride ions, these materials will chemically react with the carbon steel pipe wall to corrode the steel pipe until it wears out, which affects the service life of the desulfurization device. The main reaction formula is: 4Fe+SO42-+4H2O=FeS+3Fe(OH)2+2OH-.

     In addition, Cl- is more easily adsorbed on the metal surface than oxygen, and the oxygen is squeezed out, so that the passivation state of the metal is partially damaged and pitting corrosion occurs, and some stainless steel materials are also difficult to avoid. The corrosion forms of the slurry on the metal pipe are: pitting corrosion, crevice corrosion, stress corrosion, fatigue corrosion, galvanic corrosion, and the like. The choice of lime slurry regulating valve to prevent corrosion is to prevent the slurry from contacting the metal surface, such as rubber ( currently widely used as butyl rubber ) or lining plastic.

2.3 easy to block

     The wet flue gas desulfurization slurry pipeline is a two-phase flow. The characteristic of the two-phase flow is that the flow rate must be controlled within a suitable range. High flow rates tend to cause wear and greatly increase pipe resistance, while low flow rates produce deposits that shrink the flow surface of the pipe until it blocks the entire pipe. The easy clogging of the slurry pipeline is also manifested in the fact that the long-term non-cleaning of the sediment will lead to hardening and agglomeration, and the entire pipeline is scrapped.

     For pipes containing weakly alkaline ( Ca2- containing ) , there is also the tendency to scale. Regardless of the flow rate, long-term operation will cause blockage of the pipe. This is also one of the important factors for maintaining the pH of the wet flue gas desulfurization absorption tower slurry at 5-6 ( weak acidity ) .

3 , the design of the slurry pipeline

     In view of the medium characteristics of the wet flue gas desulfurization slurry pipeline, the design and requirements of the ordinary low-pressure fluid pipeline design must be met when designing the slurry pipeline, and the particularity of the slurry pipeline should be considered. The following are introduced from the general requirements of the design of the slurry pipeline, pipe selection, pipe diameter calculation, slope requirements, pipe gallery layout, valve selection and arrangement, and support frame arrangement:

3.1 General requirements

     General requirements for the design of slurry pipeline process: full attention should be paid to the use of advanced technology, rational use of energy in the plant, proper treatment of waste gas and waste liquid ( due to the high content of soluble salts and chloride ions in the desulfurization wastewater, for the reuse of users The system material and product will cause adverse effects, so the desulfurization wastewater is good for boiler slag, ash ash, slag replenishment or coal yard watering, etc .; it must meet the requirements of normal production, start-stop, safety and accident handling. And should consider maintenance requirements and certain operational flexibility ;

     A shut-off valve shall be provided at the pipe inlet and outlet device ; when the device is overhauled due to an accident or periodic shutdown, measures shall be taken to discharge all materials in the device to the accident slurry tank.

     General requirements for slurry pipeline layout design: should meet the process design requirements ; try to arrange " step by step " or " step by step " to avoid liquid bag or " cecal " , otherwise you need to set the shower point, or at least by manual removal ; It is difficult to make it difficult first. For example, arrange important, large-diameter, and slurry pipes, and then arrange small, minor, and light pipes . The pipes should be arranged neatly and orderly, horizontally and vertically, to form rows, which are easy to support. The vertical and horizontal elevations should be staggered, generally changing the elevation while changing the direction ; the ducts are laid overhead or laid on the ground, and the slurry pipes are generally not allowed to be laid or directly buried ; they do not interfere with the operation of equipment, pumps, instruments and valves . Maintenance ; meet the special requirements of pipelines such as flowmeters, density meters and PH meters ; the reducing pipe fittings should be close to the position where the diameter is needed to save the pipe ; the pipe should be properly supported ; the bottom elevation of the pedestrian passage should not be less than 2.2m . vehicle traffic bottom elevation of the tube of not less than 4.5m; spacing disposed side by side spacing between the outer edge of the pipe flanges not less than 25mm (flangeless pipe of 50mm), or between the insulation layer not less than 50mm; wear F The hole should be left when provided along the retaining plate, the aperture and out of the flange should satisfy; no flow in the pipe plates to be arranged on the riser to facilitate drain; position of the flange provided space to meet the operator mounting bolt; conduit Generally, the slope should be set (the slope requirement will be explained in detail later ); the slurry pipeline should be away from the electrical equipment and cable tray. If it is unavoidable, the pipeline should be walked under as far as possible to prevent the dripping from corroding the electrical equipment ; the pump suction section should be effective. The cavitation allowance is generally at least 1.2 times the required cavitation allowance of the pump. The pump suction section should be as short and straight as possible. The size of the pump inlet should be as close as possible to the slurry pump.

3.2 Pipe selection

     At present, domestic slurry pipelines generally use materials such as rubber-lined carbon steel (RL) , plastic-lined carbon steel (PL) , glass steel (FRP) , polypropylene (PPR , PPH) , stainless steel (304 , 316) , etc. .   The rubber-lined carbon steel pipe (RL) is made of ordinary carbon steel pipe (Q235-A) or high-quality steel pipe (20#) as a steel frame material, and rubber ( generally butyl rubber ) is used as a lining layer to combine metal properties and rubber properties. one. The rubber-lined carbon steel pipe has the characteristics of wear resistance, impermeability, corrosion resistance, heat resistance (120 °C ) , and the flange connection between the pipe and the pipe. The carbon steel-lined rubber hose is currently widely used in the flue gas desulfurization slurry pipeline. The carbon steel lining should be thorough, not only the inner wall of all the pipes, but also all the parts that may contact the slurry, such as the flange surface (the rubber should cover the flange surface without the gasket ) , the pipe fittings, the valve, the slurry Pumps and the like are lined. Otherwise, as long as one is corroded, the bad spots will spread until the entire part is affected.

    The plastic-lined steel pipe (PL) is a pipe made of ordinary carbon steel pipe or high-quality steel pipe lined with plastic. Lining-plastic steel pipe (PL) steel-lined polypropylene (PP) , steel-lined polyethylene (PE) , steel-lined polyvinyl chloride (PVC) , steel-lined polytetrafluoroethylene (PTFE), etc. Flange connections are also used. Since the price is more expensive than the rubber-lined steel pipe, the plastic-lined steel pipe (PL) is generally only used in a small-diameter slurry pipe in the flue gas desulfurization project .

      Glass-reinforced steel pipe (FRP) is a composite material made of two basic components of a base material and a reinforcing material and added with various adjuvants. Commonly used substrates are various resins. Commonly used reinforcing materials are mainly carbon fiber, glass fiber, organic fiber and the like. Glass steel pipe (FRP) has the characteristics of corrosion resistance, heat resistance, wear resistance and light weight. There are many types of FRP pipe joints, including: socket bonding, flat end butting, flange connection, etc. The nominal pressure ranges from atmospheric pressure to 4.0 MPa , and the temperature range is -40 to 100 °C . The disadvantage is that compared with the metal pipe, the strength is low, the rigidity is poor, and it is easy to be aged by ultraviolet radiation for a long time, and it is used in a small part of the wet flue gas desulfurization project, such as a spray pipe and an oxidizing air pipe.

      Polypropylene (PPR, PPH) is the use of random copolymer polypropylene tube extruded from the tube (injection molded into the tube), the early 1990s development and application of new plastic pipe products in Europe. PPR (PPH) having a light pipe in addition to the general plastic pipe weight, corrosion resistance, no scaling and other characteristics, as well as good heat resistance, PPR (PPH) tube zui high operating temperatures up to 95 ℃. PPR (PPH) pipes and fittings can be connected by hot melt and electrofusion, which is easy to install. The disadvantage of PPR (PPH) tube is that it has lower strength and lower rigidity than metal tube, and there is a certain low temperature brittleness below 5 °C . PPR (PPH) tube is easily aged and degraded by ultraviolet radiation for a long time ; in addition, the linear expansion coefficient of PPR (PPH) tube Larger (0.15mm/m °C ) , measures to absorb thermal expansion during layout design. A few wet desulfurization projects use PPR (PPH) pipes as slurry pipelines, mainly because they are cheap and easy to install.

      Stainless steel (304 , 316) tubes have anti-corrosion, heat resistance, high strength and aesthetic properties. The disadvantages are high prices and rust when exposed to chloride ions. Stainless steel (304 , 316) tubes are generally used in small, incapable rubber-lined slurry lines, such as DN10~DN40 .

3.3 Valve selection and arrangement

     The main function of the valve is to switch on or off the fluid path, regulate and throttle, regulate the pressure and release excess pressure and prevent backflow.

     The butterfly valve has the characteristics of small size, light weight and rapid opening and closing, and has certain adjustment functions. Due to the wiping nature of the butterfly valve plate, most butterfly valves can be used for media with suspended solid particles, and butterfly valves used in slurry pipes are lined with butterfly valves and alloy steel butterfly valves. The butterfly valve has a small structural length and overall height, and has a fast opening and closing speed. When fully opened, it has a small fluid resistance and is suitable for a desulfurization slurry ( low pressure fluid ) system. According to the connection type butterfly valve, it can be divided into a flange connection and a clip type connection.

In addition, the butterfly valve has poor sealing performance, and the use pressure and working temperature range are small. In some cases where strict requirements are not recommended, butterfly valves are not recommended.

     The diaphragm valve ( in the slurry system, the use of the through-type valve ) is a kind of rubber or plastic diaphragm which is flexible in the valve body as a closing member, and the valve body cavity is separated from the inner membrane of the valve cover. valve. Diaphragm valves have the advantages of good sealing, low flow resistance and low price, but the diaphragm valve has a short mechanical life. Diaphragm valves can be used with corrosive media containing hard suspensions and can be installed anywhere in the slurry line.

     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 the sealing ring is isolated from the medium, and the medium passing through the valve at high speed does not erode the sealing surface, thereby increasing the service life of the ball valve. Ball valves can be used in the slurry line. An electric regulating valve is also used on the gypsum discharge to the first-stage cyclone feed pipe and the limestone slurry into the absorption tower pipe , and the regulating valve must have an anti-corrosion function.

     Check valves are used only in water and air lines, and check valves should not be used in slurry systems.
     Flushing and venting valves and slurry loop feed / isolation valves should be as close as possible to the main piping to avoid clogging ; in general, pneumatic actuators are used where it is difficult to operate or where valves are often used ; All valve installation positions should be easy to operate, maintain and overhaul. Valves with too high installation position should be equipped with operating platform ; when the valves are adjacently arranged, the clearance between hand wheels should not be less than 100mm; for horizontally arranged valves, actuators and The valve stem should not be lower than the horizontal pipe, the valve stem of the valve should not face downward ; the installation height of the valve handwheel on the riser should be 1.2~1.5m; when the butterfly valve is installed, the valve flap should be stopped at the closed position, and Care should be taken to rotate the lower part of the valve plate toward the flow of the medium while the valve is open.

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