The Selection and Inspection of Valves

There is a variety of valves, which are important parts in the pipeline. A valve is one of the main sources of leakages in equipment, so the requirements for leakages of valves are of great importance. The main sealing parts of the valve include the surface between the opening and closing parts and the valve seat, the matching among packing, valve stem and packing box, and the connection between the valve body and valve bonnet. The leakage at the first part is the internal leakage, which directly affects the ability of the valve to cut off the medium and the normal operation of the equipment. The leakage at later two parts is the external leakage, which means the medium leaks from the inside to the outside of the valve. It directly affects safe production, causing loss of working medium, business economic losses, environmental pollution, and production accidents in severe cases. External leakages of valves for high-temperature and high-pressure, inflammable, explosive, toxic and corrosive media are not allowed, because the consequences caused by them are more serious than the internal leakage. Therefore, a valve is required to have good sealing performance so that it can meet the requirement for the amount of leakage for the operating condition.
 
The selection of valves without leakages
A bellows sealed valve is one of the valves with little leakage. In the past, when there was a requirement for leakages of valves, bellows-sealed valves were generally selected. However, the material for bellows sealed valves can't be popularized in China because of the difficult process and high technical requirements. The too high cost makes the bellows sealed valve difficult to be popularized in the petrochemical industry. Valves with little leakage that meets standards have a simple structure and can be processed easily, which have a cost about 10% to 20% higher than that of general-purpose valves. They can replace some of the bellows sealed valves. At present, valves with little leakage are of more practical significance for oil and gas field purification systems with high hydrogen sulfide content, because hydrogen sulfide is a highly toxic and combustible gas, heavier than air, and can accumulate in low-lying areas. Inhaling hydrogen sulfide with a certain concentration can harm the body and even cause death. Therefore, the leakage requirements for purification facilities of such natural gas are higher.
 
Leakages of the medium between the sealing surfaces in the high-pressure valve will cause surface erosion. If a corrosive medium leaks, the metal at the leaked part will be corroded. As the leakage gap increases, the amount of leakage will increase quickly, and the valve will be scrapped. Therefore, for valves with high-pressure or corrosive working conditions, higher requirements should be put forward for sealing. The leakage of the media between the valve's sealing surfaces may cause personal hazards, economic losses and even accidents in pipelines that transport flammable, explosive and toxic media. Therefore, the requirements for sealing should be reasonably put forward for valves that deliver inflammable, explosive and toxic media according to the danger level of the media.
 
Grind the sealing surface well and increase the specific pressure of the sealing surface can ensure good sealing performance of the valve, but it should be lower than the allowable specific pressure of the sealing surface's material. Meanwhile, the rigidity of the structure must be improved. The structural design, manufacturing and processing of the valve itself have the most obvious impact on its external leakages, and there are higher requirements for valves with little leakage concerning the design, manufacturing and processing of key components such as the valve body, stem and packing box, which are as follows:

(1) The valve body and bonnet

Prevent defects such as folding, slag inclusion, gas holes, loose organization, hidden cracks and uneven composition especially in the forging or casting process.

(2) The parts connecting the valve stem and valve body

Pay attention to the roughness of the valve stem and packing box, straightness of the valve stem, verticality and processing accuracy of the packing box hole of the bonnet, and processing accuracy.

(3) The structural selection of the valve's packing box

Wear easily happens for the valve stem in the rotating or sliding process because the valve stem has a dynamic seal. It is necessary to select a special packing seal with little leakage and packing seal combination, and strictly control the gas between the packing and stem as well as the packing and packing box.
 
2. Testing standards
There are three commonly used standards for testing little leakage, including the U.S. Environmental Protection Agency EPA method 21, International Organization for Standardization ISO/CD 15848 and Shell Oil Company SHELL MWSC SPE 77/312. These standards or regulations have different detection methods and leakage standards for valves. The test medium for EPA method 21 is VOC, and the instrument is a VOC detector. The leakage rate is specified as 0.05%. This method is used for flammable methane, which is dangerous. The test medium of SO/CD 15848 is 97% helium or 97% methane, and the test instrument is a helium mass meter or a VOC monitor.