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日期: 2019-03-20

Valve Setting and Type Selection Method of Desulfurization System

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Although there are many references for the design and selection of valves for chemical equipment, there are few references for the selection of valves for limestone wet desulfurization system. Combining with the case of 2x130t/h environmentally friendly thermoelectric boiler flue gas desulfurization system, the methods of valve setting and selection in FGD system of small boilers are summarized.

Valve Setting and Type Selection Method of Desulfurization System

Flue gas desulfurization (FGD) is the main measure to control SO2 emission in coal-fired power plants. Among them, wet flue gas desulfurization with limestone gypsum is the most widely used and mature technology in the world. Although there are many references for the design and selection of valves for chemical equipment, there are few references for the selection of valves for limestone wet desulfurization system. Combining with the case of 2x130t/h environmentally friendly thermoelectric boiler flue gas desulfurization system, the methods of valve setting and selection in FGD system of small boilers are summarized.

Brief introduction of desulfurization system valve

Valves are the control devices used to deal with fluid in pipeline system. Before selecting valves, we must first understand the functions of various valves. The main valves used in FGD system are cut-off valves, control valves, check valves, pressure relief valves and safety valves. The first two kinds of slurry systems are widely used. The cut-off valve is used to control the passage or cut-off of the medium in the pipeline and isolate the pipeline system. Regulating valves are used to regulate the medium flow and pressure in the pipeline so as to stabilize the process parameters within the desired range. Throttle should be avoided as far as possible in the slurry pipeline system with severe wear. Speed regulating pumps and switching valves or other operation modes can be used instead of regulating valves to avoid valve wear.

There are many kinds of valves used in FGD system because the pipeline in FGD project contacts with the medium in bad conditions. Scaling, blockage, corrosion and wear are common problems in FGD system, which seriously affect the equipment operation rate of the system. Therefore, it is necessary to know the properties of the medium in which the valve is contacted to select its type correctly.

FGD slurry system can be divided into three parts: limestone slurry, limestone gypsum slurry and gypsum slurry according to different media composition. Its main characteristics are as follows:

(1) It is granular and abrasive. The mass fraction of limestone slurry can reach 55% and the density can reach 1500kg/m3.

(2) pH=4~7, inorganic chemical weak acid corrosivity;

(3) Organic solutions containing inert substances are corrosive;

(4) The slurry flow rate is fast. In general design, the slurry flow rate is controlled below 2.5 m/s. In actual operation, the maximum is 3.5 m/s. There is particle jet phenomenon, and the maximum of circulating water and process water is 5 m/s.

(5) High Cl-concentration: Desulfurization engineering companies generally design according to 20,000 mg/L, the actual tower up to about 180,000 mg/L, waste water about 40,000 mg/L, in addition to acid corrosion, there are strong oxidation and reduction.

Generally speaking, the abrasion of limestone slurry is more serious, and the gypsum slurry is more corrosive. Therefore, in addition to the conventional requirements for valve selection, such as excellent product design, small torque, flexible movement, reliable operation, long service life, low maintenance costs, it also requires anti-corrosion, wear resistance, the use of high-quality anti-corrosion materials. For the actuator of valve matching, it is also necessary to be familiar with the function of each valve in FGD system, the frequency and difficulty of operation.

Valve Selection Example and Performance Brief Introduction

Considering the reliability and economy of FGD system, butterfly valves and adjusting valves are mainly used in slurry pipeline in FGD project, and diaphragm valves, globe valves and ball valves are used in instrument-controlled primary valves. The setting of each valve and the characteristics of the valve are described below.

According to the process conditions, consider the status of the valve, whether it is pneumatic, electric or manual. What is the fault position of the valve?

According to environmental considerations, it is necessary to use explosion-proof or non-explosion-proof, and the level of explosion-proof. According to the installation location, the protection level of the valve should also be considered. There are several types of valve in desulfurization process.

Butterfly valves, ball valves, needle valves, diaphragm valves, globe valves, self-regulating valves, check valves, traps, safety valves, etc.    

Butterfly valve:

According to the type, it can be divided into middle line butterfly valve, which is commonly used in desulfurization process, single eccentricity, double eccentricity (medium pressure is used as switch, regulator), three eccentricity (high pressure is used as switch, regulator) and middle line butterfly valve. The medium of desulfurization process is divided into process water, slurry, wastewater, circulating water, tap water, compressed air, etc.

Valve body material is generally divided into the following categories: cast iron, ductile iron, carbon steel and so on. Cast iron and nodular iron are commonly used in consideration of cost. Carbon steel valves can be considered if the weather in the north is bad. The seat of butterfly valve is usually lined with glue. Now EPDM is widely used, that is, EPDM. Valve plate is generally considered according to the medium: in desulfurization, the material of valve plate is generally considered according to the concentration of Cl ion contained in the medium. Valve plate material is generally divided into: nodular iron chromium plating, nodular iron lining nylon, lining fluorine rubber, 304, 316, 316L, 2205, 2507, 1.4462, 1.4529, C276 and so on.

Process water, circulating water, valve plate material is generally chromium plated with ductile iron, 304 can. Ductile iron chromium plating, 304 for air or oxidized air. However, the medium temperature should be paid attention to when used in oxidation air seats, and heat-resistant EPDM is used in general seats. The high concentration medium containing CL ion such as slurry and wastewater is generally 2205, 2507, 1.4462, 1.4529 and C276. The higher the Cl ion concentration, the higher the material quality of valve plate.

Butterfly valve connection can be divided into: pair clamp, single clamp, flange and so on.

Small caliber butterfly valves are usually connected by pair clamp or single clamp, and the diameter is DN25-DN450. Flange connection is generally recommended for diameters above DN500. The minimum butterfly valve is DN25, and the valve body is made of aluminium alloy, which can replace the diaphragm valve for sampling. The position of the liquid level gauge is usually connected by a single clamp.

About manual butterfly valve: DN200 and above should choose worm wheel butterfly valve, now our company also produces low torque butterfly valve, DN300 can also use handle mode.

According to industry regulations DN250 and above, butterfly valves are usually connected by special flanges for butterfly valves. 、

Diaphragm valve: According to the structure, it can be divided into weir type diaphragm valve (A-TXXXX) and through type diaphragm valve (KB-TXXXXXX), all-through type diaphragm valve (K-TXXXXXX). The connection mode is flange type.

Conventional valve body is usually cast iron liner, non-liner valve is less applied.

Weir diaphragm valve: mainly used for process water, waste water, acid, alkali and other media 

Direct Diaphragm Valve:

It is mainly used in slurry medium (slurry medium contains solid granular substances, which is not easy to plug). Direct diaphragm valves routinely maximize DN80. Where possible, when the through diaphragm valve is larger than DN80, the appropriate deviation can be butterfly valve.



 


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