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Application Of Mixed Flow Pump In Flue Gas Desulfurization(FGD), Also Named Desulfurization Pump (Part One)

Learn - Sep. 23, 2022

question about mixed flow pump

Why do pump manufacturers call it a mixed flow pump?  But in the thermal power industry, it has a new name -Desulfurization pump.

 In five minutes, you will find the answer.

1.Brief Introduction of Mixed Flow Pump   

2.Application Of Mixed Flow Pump In Flue Gas Desulfurization - Desulfurization Pump

3. Two major technical difficulties of desulfurization pump:Mechanical seal

                                                                                              Wear resistance

1.Brief Introduction of Mixed Flow Pump:

Pump is a kind of fluid machinery that converts the mechanical energy of the motor into kinetic energy and pressure energy of the fluid to be transported, that is, it gives energy to the fluid to be transported. Pumps are used in everything. ① Pumps are needed for agricultural irrigation and drainage, as well as for urban water supply and drainage. ② In various sectors of industry, pump is more indispensable equipment. Among the various types of pumps, centrifugal pumps are the most widely used. Mixed flow pump is a kind of pump between centrifugal pump and axial flow pump. It has the characteristics of large flow rate, medium head, high efficiency and wide application range. Therefore, it is widely used. Volute mixed flow pump, also known as horizontal mixed flow pump has the advantages of short axial size, high operation stability and reliability of the unit.

The flow rate of the mixed flow pump is 12.5 ~ 20,00L /s; The head is H =3 ~ 30m, some can reach 60m; Caliber is 100 ~ 1500mm, customized large mixed flow pump caliber up to 5000mm; The specific speed range is usually NS =300-500, and the highest speed is NS =1110 at present; Speed n= 980-2900r /min.

Mixed flow pump can be used in Agricultural Drainage and Irrigation, Flood Prevention and Drainage, Water Conservancy Projects, Sewage Treatment, Power Plant (Thermal Power Unit, Nuclear Power Plant, Tidal Power Plant ) cooling system and desulfurization process. Mixed flow pump is suitable for solid-liquid two-phase flow of complex processes. The slurry pump containing CaCO3 solid particles is a typical mixed flow pump.

2.Application Of Mixed Flow Pump In Flue Gas Desulfurization - Desulfurization Pump

mixed flow pump desulfurization pump

The mixed flow pump has the characteristics of large flow, medium head, and high efficiency, which is suitable for desulfurization process in thermal power plant. Desulfurization pumps are used to pump a corrosive slurry containing lime, limestone and gypsum solid particles.

Worldwide, thermal power generation accounts for 60% of the total electricity generation, and the thermal power industry is a major part of the world's energy industry. The main pollutants of thermal power plants include soot, sulfur dioxide and nitrogen oxides, etc., among which sulfur dioxide emissions are the largest. Therefore, it is a very important task to effectively control sulfur dioxide flue gas pollution in thermal power plants. The main ways to control sulfur dioxide include: ①desulphurization method before combustion, ②desulphurization method in combustion, ③desulphurization method after combustion. Desulphurization after combustion (Flue Gas Desulphurization) is the main technology to control flue gas pollution and related acid rain. After combustion desulfurization mainly have a wet limestone gypsum method, spray drying method, electron beam irradiation, seawater desulfurization method. the limestone-gypsum wet flue gas desulfurization technology ( for short wet flue gas desulfurization) is the world's only large-scale commercial application way of desulfurization, desulfurization efficiency is 90% ~ 95%, reliable technology, process system is complete, All the desulfurization methods mentioned in this paper are limestone-gypsum wet flue gas desulfurization method (WFGD), and desulfurization pump is one of the key equipment of wet desulfurization device, its performance is directly related to whether the desulfurization device can run safely and efficiently.

In limestone-gypsum wet flue gas desulfurization method, the medium transported by desulfurization pump is generally lime, limestone and gypsum slurry. The slurry is a corrosive medium containing limestone and gypsum particles with a weight concentration of 20%~35% (up to 60%), pH is 4.5~7, and chloride content of 2000 ~ 6000× 10-7. Mixed flow pump with large flow rate, moderate head, high efficiency characteristics, the choice of wear and corrosion resistant material, to meet the desulfurization pump wear resistance, corrosion resistance, smooth operation requirements.

FGD technology for desulfurization pump design requirements and technical indicators:

  • 1.The flow rate of desulfurization pump is required to be large enough to reduce the number of pumps and reduce the operation and maintenance costs of the power plant;

large flow rate mixed flow pump
  • 2.The flow-head characteristics of the desulfurization pump must adapt to parallel operation and idling;

  • 3.The desulfurization pump should have a higher suction head, need to take into account , in the process requires the introduction of a certain amount of air or oxygen, to ensure the complete oxidation of SO2 into sulfate;

  • 4.The material of the wetted parts of desulfurization pump should have good corrosion resistance and wear resistance ability;

  • 5.Single mechanical seal or double mechanical seal with higher reliability is required;

  • 6.Desulfurization pump must be durable, stable operation, under the specified working conditions, 24 hours a day continuous operation, and the continuous fault-free operation time is more than 24 000h;

  • 7.The size of the pump shaft and bearing assembly of the desulfurization pump should be large enough to prevent slurry or other impurities from entering;

  • 8.The design of the pump structure must be easy to disassemble and assemble.

3. Two major technical difficulties of desulfurization pump:

(1) Desulfurization pump adopt for Mechanical seal to ensure no leakage

Desulfurization pump transport medium is generally lime, limestone and gypsum slurry, containing a large number of chloride ions and solid particles, when the medium flows through the pump equipment, the pump and mechanical seal parts of the surface has a certain corrosion and wear. The damage of solid particles to the seal end face is particularly serious. The solid particles entering the seal end face will accelerate the wear of the seal end face, and the deposition of particles will make the compensation ring unable to work normally, resulting in seal failure. The shaft seal of the horizontal desulfurization pump adopts double end mechanical seal, which is more reliable, ensures no leakage.

  • ● The structural design of the mechanical seal should meet the overall design requirements of the pump, that is, to ensure that in the process of pump operation, the slurry sealing of the pump shaft should be reliable and leak-free. Considering the axial adjustment requirements of the rotor of the pump equipment, the mechanical seal shall allow axial adjustment of the rotor.

  • ● Due to the pressure of the pump chamber and the temperature of the medium, the mechanical seal should form a liquid film of sufficient thickness between the end faces of the friction pair, which can not only resist the pressure of the pump chamber, but also lubricate and cool the end faces of the friction pair, and effectively prevent dry friction of the friction pair.

  • ● Desulfurization pump transport medium contains a large number of solid particles, and has a certain concentration, in the mechanical seal parts of the peripheral pump cavity, affected by the impeller blade disturbance, resulting in the cavity medium pressure and flow rate prone to turbulence phenomenon.  The structural design of the mechanical seal should effectively prevent the precipitation of slurry around the mechanical seal, damage and prevent the formation of friction pair, which also puts forward requirements for the structural design of the pump chamber.

  • ● The structural parts of mechanical seals such as the base of dynamic sing and static ring, O-rings, friction pairs should have the same requirements of wear resistance and anti-corrosion as the wetted parts of the desulfurization pump.

  • ● Under the influence of pump starting and stopping, a small amount of slurry will pass through the end face of the friction pair and enter between the sealing friction pair and the shaft sleeve before the stable liquid film is formed between the friction pair and the stable liquid film is destroyed. If the slurry is not excluded in time, it will accumulate at the root of the friction pair, and even enter the spring seat body. The accumulation of excessive slurry affects the elasticity of the spring, and then affects the specific pressure of the friction pair. Therefore, the mechanical seal should be designed with a reasonable flushing device to eliminate this part of slurry in time.

  • ● Because the slurry delivered by the pump has a certain temperature (generally in 60-75 ℃), at the same time, the friction pair of the end face of the static ring relative friction friction heat, if the friction pair temperature is too high, will accelerate the aging of the surrounding sealant, comprehensive this and the former factors, should be calculated to determine the reasonable flush plan and flush flow rate.

  • ● In addition to meeting the static pressure requirements, mechanical seals should also meet the requirements of sufficient manufacturing accuracy, sufficient operation accuracy and certain operation reliability.

  • ● It is indicated in the order contract that the service life of the mechanical seal shall not be less than 7000 hours.

(2) Wear resistance of desulfurization pump