Defects and hazards of natural gas pipeline valves

1. Process defects during valve casting

•   (1) Hole defects, such as pores, shrinkage holes, porosity, slag holes, blisters and iron beans. They may expand into hidden dangers in use. Most of the pores are distributed on the surface of cast iron or near the surface, which are spherical or pear-shaped; the slag pores are irregular in shape, rough in surface and in various colors; shrinkage and looseness are inverted cones, and the surface of the pores is rough, mostly distributed on the casting body shaft In the heart, trachoma and iron beans are mostly scattered on the surface of cast iron. Perforated defects are caused by irregular sand molds or a large amount of slag mixed casting.

•   (2) Crack-type defects, including hot cracks and cold cracks. Hot cracks are formed at high temperatures, and cold cracks are formed at low temperatures. Therefore, the surface of the crack in the former is oxidized, and the crack shape is tortuous and irregular; the crack in the latter is slightly oxidized, the crack is straight, and there is no branch. They may be caused by segregation of casting chemical composition or metallographic structure that does not meet the minimum requirements specified by the CB7216-87 standard. The part is too hard and it is easy to cause valve rupture accidents when the operating pressure is not high. Therefore, this situation should arouse special attention.

•   (3) Defects in structural shape and size refer to whether there are fleshy, insufficient pouring, sand falling, deformation and damage defects in the valve body casting.

　　The existence of the above-mentioned various defects makes the actual strength of the valve body far lower than the theoretical value, especially the crack type casting defect is an important reason for the valve body to break and blast when the pressure is not high.

2. Defects that are easy to produce when encountering steam

Valves are mostly made of HT200, which is a typical brittle material. When encountering a steam medium, the valve bears the same internal pressure as the pressure component of the boiler, and the pressure component of the boiler is mostly made of low carbon steel or low alloy steel, which is a tough material. Under the same internal steam pressure, the steam valve The possibility of fracture blasting is higher.

3. Defects caused by improper installation and use

For example, when installing the steam pipeline, the problem of thermal expansion and contraction of the pipeline is ignored. For example, the position of the fixed bracket is not correct. It is easy that the pipeline cannot expand freely after being heated, and stress is generated on the pipeline. With stress, the valve body is cracked. For the material of the valve body, the fracture is a brittle fracture. The shoulder crack of the crack and the rapid destabilization propagation occur almost at the same time. Therefore, once a macro crack occurs on the surface of the valve body, the valve will easily be broken.

4. Destructive defects caused by water hammer

Water hammer is also known as water hammer. There is water hammer produced by the gravity of water and water hammer produced by steam operation. The water hammer caused by the gravity of water mostly occurs when the pump is suddenly stopped. The water hammer produced by steam operation is a phenomenon in which the sudden impact of condensate water causes sound and vibration in the steam pipe during steam operation. When the output steam meets the water in the pipeline, part of the heat is quickly absorbed by the water, causing a small amount of steam to immediately condense into water, and the volume is suddenly reduced, resulting in a partial vacuum, which causes the high-speed impact of the surrounding steam medium and causes the water to hit the pipeline The pressure value increases or decreases sharply, and this process of pressurization and decompression changes rapidly and frequently.

　　The pressurization value generated when water hammer occurs can reach dozens of times the normal pressure value in the pipeline, and the alternating frequency of pressurization and decompression is very high, which is very harmful.