Valve repair and installation-1

Release time:

2024-03-20

How to select valves? Answer: the valve in the pipeline is selected according to medium type, pressure, temperature and flow (or pipe diameter) and other factors. Gate valve and stop valve are only used to fully open and close the medium of various pipelines and equipment. Throttling is not allowed to avoid quick wear of closing parts and seat rings. With the change of temperature, the pressure, concentration and corrosion of the same medium are also changing. Code of maximum working pressure at a certain temperature: the number added at the lower right corner of P word is the integer divided by 10 for medium temperature, and the value in the table is the maximum working pressure at a certain temperature. If the medium temperature is 240 ℃, the maximum working pressure is expressed as P. Carbon steel valves below 200 ℃ can be selected according to the nominal pressure; when the temperature increases, the distance between the maximum working pressure and the nominal pressure becomes larger and larger, to 450 ℃, the maximum working pressure is half or more smaller than the nominal pressure. Such a large gap must cause the attention of valve users. When the temperature is high, do not only look at the nominal pressure, but also understand the maximum working pressure at this temperature. What are the key points of valve selection?

1) Lift horizontal disc check valve can only be installed horizontally, and the disc is vertical upward. Lift type vertical disc check valve can only be installed vertically, and the flow direction of medium must be upward. When the swing check valve is installed on the vertical pipeline, the medium flow direction must be upward.

2) The pressure reducing valve and drain valve shall be installed on the horizontal pipeline; the safety valve must be installed vertically upward

3) The valve of gas pipeline shall have good air tightness, quick switch, flexible action, convenient maintenance and good corrosion resistance. The valve with copper sealing ring shall not be used to prevent the corrosion of sulfide impurities in the gas. Special oil seal cock valve for gas pipeline, special oil seal gate valve and seal butterfly valve can be used.

4) Stop valve or ball valve can be used for oxygen pipeline, and special oxygen valve should be used. It is forbidden to use gate valve for oxygen pipeline with working pressure P > 0.1MPa. When p < 1.6Mpa, malleable iron, ductile iron or steel valve can be used; when 1.6Mpa < ps3.0mpa, all stainless steel, all copper alloy or combination of stainless steel and copper base alloy shall be used. It is strictly prohibited to use oil-containing or combustible materials at the parts contacting with oxygen in the valve. Valves shall be inspected and handled according to relevant construction and acceptance specifications before installation.

5) It is forbidden to use gate valve for hydrogen pipeline with working pressure P > 0.1MPa, and it is better to use plug valve, ball valve and stop valve. For the hydrogen pipeline with P < 1.6Mpa, cast steel valve can be used; for the hydrogen pipeline with P < 0.1MPa, ductile iron can also be used. When there is alkali in electrolytic hydrogen, valves with copper or copper alloy shall not be used.

6) Malleable cast iron, nodular cast iron or steel valves can be used for acetylene pipes; gate valves are not allowed for pipes > DN50. The relationship between the nominal pressure and pipe diameter of the valve is as follows: < DN20, nominal pressure > 1.0MPa; < DN50, nominal pressure > 1.6Mpa; DN50 - DN80, arej > 2.5MPa