How to correctly select nuclear power plant valves

How to correctly select nuclear power plant valves

Select according to the safety level of nuclear power valves

The selection of valves for nuclear power plant is mainly based on the installation position and nuclear safety level of the valves. Nuclear class 1 valves, especially nuclear class 1 valves in the containment, are obviously characterized by welded connection, low-carbon austenitic stainless steel, Stellite cemented carbide surfacing on the sealing surface, sealing welding on the middle flange, disc spring pre tightening or bellows sealing on the filler, so as to ensure that the valve has no external leakage. This type of valve mainly adopts wedge gate valve, stop valve, check valve, fully enclosed safety valve and other structural forms, which can work reliably under high temperature. Ball valve, butterfly valve, diaphragm valve, etc. are generally used for nuclear level 2 and 3.

The contents related to nuclear safety level include: specification level requirements, seismic category requirements, quality assurance requirements, electrical classification requirements, etc.

Specification level refers to the level of equipment design and manufacturing, which corresponds to nuclear safety level 1, 2 and 3.

The seismic category indicates the ability of the equipment to withstand seismic loads. It is generally divided into class 1a and class 1E.

The electrical classification is based on the four grades of KL, K2, K3 and NC of rcc-e. Level K1 refers to the level that can maintain the operability under the ionizing radiation environment for 40 years and earthquake accidents and the level that can maintain the operation under loca (loss of coolant accident), level K2 refers to the level that can maintain the operability under the normal ionizing radiation environment for 40 years and earthquake accidents, level K3 refers to the level that can maintain the operation under the normal environment for 40 years and earthquake accidents, and level NC refers to the level that can maintain the operation under the normal environment for 40 years, Applicable to non nuclear valves.

Select valves according to flow characteristics

The shape of the valve opening and closing parts and the valve flow passage make the valve have certain flow characteristics. This must be taken into account when selecting valves.

Switch type valve is a valve with quick opening and closing characteristics. Generally, valves with small flow resistance and straight through flow passage are selected. Such valves include gate valve, globe valve, ball valve and butterfly valve.

The flow control valve usually selects the valve that is easy to adjust the flow, such as control valve and throttle valve. The valve seat size is nearly proportional to the stroke of the hoist. Rotary (e.g. ball valve, butterfly valve) and flex body (diaphragm valve) valves can also be used for throttling control, but are usually only applicable to a limited range of valve diameters. In most cases, people usually change the disc shape of the stop valve for throttling. ③ the valve for reversing and shunting can have three or more channels according to the needs of reversing and shunting. It is suitable to select multi-way stop valve or ball valve. Most of the valves used for reversing and shunting use this type of valve. In some cases, other types of valves, which are only properly connected with each other by two or more valves, can also be used as reversing and shunting of media.

Valve for medium with suspended particles if the medium has suspended particles, Zui is suitable for the valve with wiping effect by sliding its hoist along the sealing surface. Such as flat gate valve.

Select the valve according to the connection form

There are many types of connection between valves and pipelines, the most important of which are thread, flange and welding connection.

Threaded connection this connection is usually to process the inlet and outlet ends of the valve into taper pipe or straight pipe threads, so that they can be screwed onto the pipe of the threaded joint. Since this connection may have large leakage channels, sealant, sealing tape or gasket can be used to block these channels. If the material of the valve body can be welded, seal welding can also be carried out after threaded connection. If the materials of the connecting parts are allowed to be welded, but the linear expansion coefficient is very different, or the variation range of the working temperature is large, the threaded connection must be sealed welded. The nominal size of valves with threaded connection shall not be greater than DN50. If the drift diameter is too large, it is very difficult to install and seal the connecting part.

The installation and disassembly of flange connected valves are convenient. But it is heavier than the threaded connection, and the corresponding price is also higher. Therefore, it can be applied to the connection of pipes with various diameters and pressures. However, when the temperature exceeds 350 ° C, due to the creep relaxation of bolts, gaskets and flanges, the load of bolts will be significantly reduced, and leakage may occur to flange connections with great stress.

Welding connection is applicable to various pressures and temperatures, and is more reliable than flange connection when used under severe conditions. However, the valve connected by welding is difficult to disassemble and reinstall, so its use is limited to the occasions where it can normally operate reliably for a long time, or where the service conditions are harsh and the temperature and pressure are high. Welding valves with nominal size not greater than DN50 usually have welding sockets to accept pipes with flat ends. As the socket welding forms a gap between the socket and the pipe, it may cause the gap to be corroded by some media, and the vibration of the pipe will fatigue the joint, so the use of socket welding is limited to a certain extent. In the occasions with large nominal size, harsh service conditions and high temperature, the valve body is often connected by groove butt welding.

Select valves according to medium performance

Many media have certain corrosivity. The same media has different corrosivity with the change of temperature, pressure and concentration. In addition, nuclear power valves must have good radiation resistance, impact resistance and intergranular corrosion resistance. Therefore, valves suitable for nuclear power operation shall be selected according to the corrosion resistance and radiation resistance of materials.

Cast iron valves are suitable for working in water, steam, air and other media with low temperature and pressure. It is rarely used in nuclear power plants. Ductile iron valves have strong corrosion resistance and can be used in corrosive media with a certain concentration. Nickel cast iron (austenitic stainless steel cast iron) valve has better alkali resistance than gray cast iron and nodular cast iron valve, and has better corrosion resistance in seawater. It is an ideal material for seawater valve.

Cast iron valves are not allowed to be used on the equipment and pipelines of the nuclear power plant, and can only be used in the manufacture of non critical valves in the auxiliary system.

The corrosion resistance of carbon steel valves is similar to that of gray cast iron, but slightly inferior to that of gray cast iron. Carbon steel is widely used in non nuclear grade, nuclear grade 3 and some nuclear grade 2 valves in nuclear power plants. Carbon steel can also be used on non pressure parts of nuclear grade 1 and 2 valves, such as supports, hand wheels, etc.

The stainless steel valve has excellent atmospheric resistance, can resist the corrosion of most alkali, water, salt, organic acid and other organic compounds, and has more distinctive radiation resistance. It is the preferred material for class 1 and 2 valves in nuclear power plants. In general, stainless steel pressure bearing parts must adopt the materials specified in tables 2a and 2b of ASME BPVC FL EHL or the materials specified in French RCC-M m3301 and m3306.

Valve stems and pressure retaining bolts are often made of precipitation hardened steel. The filler is usually graphite fiber or expanded graphite.

Molybdenum 2%? 4% stainless steel, such as 0crl7nil2mo3 (RCC-M z5cnd17-12, z6cnd17-12) z2cnd18-12 nitrogen control, has better corrosion resistance than chromium nickel stainless steel (Type 304).

Stainless steel containing titanium or niobium has strong resistance to intergranular corrosion.

Copper valves have good corrosion resistance to water, seawater, various salt solutions and organic substances. In nuclear power plants, it is mainly used for valves or valve components that are often in contact with seawater.

Titanium Valve titanium is an active metal, which can form an oxide film with good corrosion resistance at room temperature. It can resist the corrosion of seawater, various chlorides, hypochlorite, wet chlorine, oxidizing acid, organic acid, alkali, etc. In addition, titanium alloy valves are also used in some important places.

Zirconium valve zirconium is also an active metal, which can generate a tight oxide film. It has good corrosion resistance to nitric acid, chromic acid, alkali liquor, molten alkali, salt liquor, urea, seawater, etc., but it is not resistant to hydrofluoric acid, concentrated sulfuric acid, aqua regia, wet chlorine and oxidizing metal chloride.

In addition, ceramic valves, FRP valves, plastic valves and various lined valves are also used in nuclear power plants.