Definition of high pressure valve:
(1) The valve with absolute pressure lower than 1 atmospheric pressure is called vacuum valve;
(2) Pressure ≤ 1.6Mpa is called low pressure valve;
(3) The pressure of 2.5 ~ 6.4Mpa is called medium pressure valve;
(4) The pressure of 10.0 ~ 80.0mpa is called high pressure valve;
(5) Pressure ≥ 100.0mpa is called ultra-high pressure valve.
There are two working conditions for high-pressure valves. One is that the medium is at room temperature but the pressure is very high, such as valves for oil production and water injection in oil fields; The other is that the medium is at high temperature and high pressure, such as the main steam system valve used in thermal power plant. Although these two types are also "high pressure valves", the design criteria and material selection are different. The former only considers the strength and stiffness, while the latter should also consider the strength and creep of materials at high temperature.
Material selection of high pressure valve
1. Pressure bearing parts and pressure control parts
Valve parts can be divided into "pressure bearing parts" and "pressure control parts" according to their role in the whole valve. The following statements are made in American API6D "pipeline valve" standard:
Pressure bearing parts refer to parts designed to bear pipeline medium (pressure), such as valve body, bonnet, packing gland, valve stem, gasket and bolt;
Pressure control parts (pressure control parts) refer to parts used to block or allow medium flow, such as closing parts, valve seats, etc.
2. Shell material
The shell includes valve body, valve cover and other parts, which directly bear the medium pressure. It is the pressure bearing part of the valve. Its material must have the due mechanical properties and chemical composition under the specified medium, temperature and pressure, and have good cold and hot processing technology.
3. Valve stem material
The valve stem is one of the pressure bearing parts of the valve. During the opening and closing of the valve, it bears the tensile, compressive, shear and torsional forces and is in direct contact with the medium. At the same time, there is relative friction movement with the filler. Therefore, the valve stem material must ensure sufficient strength and impact toughness under the specified medium, temperature and pressure, and have corrosion resistance and scratch resistance that can meet the requirements, And good workmanship.
4. Sealing surface material
The sealing surface is the key part of the valve. The materials used for the sealing surface shall be corrosion-resistant, erosion resistant, scratch resistant, have sufficient extrusion strength (specific pressure), have sufficient oxidation resistance and thermal fatigue resistance at high temperature, and shall be close to the linear expansion coefficient of the base material and have good processing and welding processability. For the sealing surface of high-pressure valve, cemented carbide is usually used.
5. Materials of pressure bearing fasteners
Pressure bearing fasteners mainly refer to the connecting bolts and nuts of the middle flange, which directly bear the pressure of the medium on the valve and play an important role in preventing medium leakage. Therefore, the selected materials must ensure sufficient strength and impact toughness under the service pressure and temperature.
According to the provisions of GB / t150-1998, JB / t5300-1991, ASME b16.34a and other standards, the commonly used fastener materials of high-pressure valves include alloy steel and stainless steel.
★ 1. Alloy steel
Chinese standard: GB / T 3077 30crmoa, 35CrMoA, 40Cr, 25Cr2MoVA, 25cr2mo1va;
American standards: ASTM a193-b16, ASTM a194-7.
★ 2. Stainless steel
Chinese standards: gb1220 2Cr13, 0Cr18Ni9, 0Cr18Ni10Ti, 0Cr17Ni12Mo2
1Cr17Ni2、1Cr18Ni9、1Cr18Ni9Ti；
American standards: ASTM a193-b8, b8m, ASTM A194-8, 8m.
Packing and gasket materials
The packing and gasket used for the valve are the filler to prevent the leakage of medium. The packing is used to fill the space between the packing chamber and the valve stem to prevent the medium from leaking through the space between the valve stem and the packing chamber; The gasket is used to fill all irregularities between the two joint surfaces (such as the sealing surface between the valve body and the valve cover) to prevent medium leakage from the joint surface. Therefore, the requirements for filler materials are corrosion resistance, good sealing and low friction coefficient; The requirements for gasket materials are corrosion resistance, certain elasticity, plasticity and sufficient strength at working temperature.