Technical description of the gas valve
Natural gas is a very promising clean energy quality. With the continuous expansion of natural gas production continues to enrich and amount of gas collection, transmission will account for an important position.
For added convenience the collection, transmission and natural gas, people use cooling and compression methods gaseous natural gas into liquefied natural gas, namely liquefied natural gas. Liquefied natural gas (LNG) as a pollution-free energy, has been widely used as fuel in power stations and city gas and other construction. Currently, in addition to the use of liquefied natural gas and liquefied natural gas car in two-vehicle compressed natural gas delivery more flexible way, the more will be the use of transmission pipeline, to transport liquefied natural gas from the base to the liquefied natural gas, by accepting stored in liquefied natural gas pipeline tank (there are two kinds of ground and underground style), and then through the pump to the required boost pressure, into the carburetor, the carburetor is forced vaporization of liquefied natural gas, and then into the user (power plants, factories and households) by aspiration device . As a fuel, liquefied natural gas increase in the proportion of thermal power plants, the construction of urban gas, liquefied natural gas demand along the valve is also increased. As a valve manufacturer, seize the opportunity to size up the situation, research and development of liquefied natural gas valves, the prospects promising.
Because of liquefied natural gas is flammable (at atmospheric pressure is -162 ℃ low temperature), the size of a large source of their supply base, combined with a variety of machines and equipment is large, is a large-diameter pipeline. Therefore, the valve valve and other large-diameter pipeline from the receiver to send piping and other small-bore valve diameter, pressure, and other types are varied. Enterprise development, the development of liquefied natural gas valves, should follow three principles: First, to understand the characteristics of liquefied natural gas; second is to understand the characteristics of liquefied natural gas valves and material requirements; third is to understand the direction of development of liquefied natural gas valve (decision-development, development of specific products).
1. LNG characteristics:
① liquefied natural gas at ambient temperature, is a colorless, odorless gas at atmospheric pressure, if liquefied, it becomes under the atmospheric pressure with a boiling point of -162 ℃ colorless transparent liquid;
② is the main component of natural gas, liquefied methane, its critical temperature and critical pressure were -82.1 ℃ and 45.5kg / cm2, and the pressure increases as the temperature rises;
③ gasification of LNG, which is about 600 times the original;
④ Since LNG small molecular weight, low viscosity, and thus the permeability, easy to leak;
⑤ liquefied natural gas produced water will freeze and form a shock Ling-shaped lumps.
2. Liquefied natural gas valve characteristics and material requirements:
1) of liquefied natural gas at atmospheric pressure due to a low temperature -162 ℃, so, the use of a common feature of the valve is "cold." Second:
① Due to the long-term liquefied natural gas valve body withstand repeated temperature changes in the media the additional stress of the total load temperature change and connecting piping stress generated due to its higher strength requirements;
② liquefied natural gas valve, globe valve, ball valve, butterfly valve cover designed according to different cold temperatures to facilitate the long-necked bonnet structure to ensure that the temperature is maintained at the bottom of the stuffing box above 0 ℃; (Note: long-necked valve the long-necked portion of the cover should be sufficient thickness; gate valves, globe valves and other valves require a low temperature sealing; body and bonnet joints require the use of bolts, welding or pipe fittings, does not allow the use of threaded connections).
③ and out of liquefied natural gas valve can seal the ends, required to set to prevent abnormal pressure in the cavity buck hole, down the channel (or take other relief mode), the requirements set down in the inlet end of the hole;
Sealing requirements ④ liquefied natural gas valve is designed for metal-to-metal or metal soft sealing surface. Such as the use of soft sealing surface is required by the metal seat support to prevent cold flow deformation of soft seal valve seat;
⑤ liquefied natural gas valve, the valve (the valve shut-off valve designed to seal the cone or spherical structure) and the sealing surface of the valve body surfacing alloy requirements in line JB / T6438 standards. When using temperatures below -100 ℃, surfacing after the required cryogenic treatment (ie, immersed in liquid nitrogen before grinding mediator -196 ℃ cold 2 to 6 hours after the removal of natural treatment, then grinding assembly);
⑥ liquefied natural gas valve stuffing, requires the use of two heavy metal packing structure with intermediate isolation ring;
⑦ liquefied natural gas valve packing, its working conditions are very complex, once the loss of elasticity, media leaks, the stem is frozen, it will lead to stuck phenomenon. It requires a combination of fillers;
⑧ liquefied natural gas valves require anti-static structure designed to ensure that the valve between the various components can be conductive.
2) liquefied natural gas valve materials used by the operating temperature and the material properties required in accordance with:
① at operating temperature, do not produce low-temperature brittle fracture;
② at operating temperature, its organizational structural stability; (for liquefied natural gas valve -100 ℃ below its body, valve cover, valve, valve seat, stem and other parts before finishing requirements for cryogenic treatment.)
③ When welded structure, welding performance and reliability at low temperatures welds;
④ frequently operate at low temperature conditions, within which piece of material without jamming, bite and scratch and so on.
3. Understanding of the development direction of the liquefied natural gas valve (decision-development, development of specific products). From an economic and practical point of view, liquefied natural gas valves:
1) to fully open fully closed based, mainly valve, butterfly and ball valves. Since the gate valve is flexible, and the pressure discharge hole, it is from the outlet side of the valve, with the increase of pressure, the leakage increases (which is the fate of the liquefied natural gas valve), the high pressure natural gas liquefaction system (300 lbs class) usually still use gate. Butterfly, due to the increasing usage, users are familiar with, eliminating insecurity initial use. But only for the low pressure system valve (150 pound class) pipeline. Ball valve, since the operation time is short, resistance is small media, typically used at room temperature can effectively play the advantages of the pipeline system.
2) to adjust the flow rate based mainly butterfly valves, globe valves and needle. In general, large-diameter liquefied natural gas, low-pressure pipeline, usually using butterfly. Globe valve, easy to operate, the pipeline for small caliber, but before the valve after the pressure great. Needle valve needs to be done in the case of small flow regulation.
3) mainly in order to prevent backflow of the main check valve. Usually nominal diameter <DN40mm, use the lift check valves; nominal diameter> DN50mm, use swing check valves; larger diameter in the pipeline, the use of special doors separate check valves. LNG valve, allowing the use in many cases at the medium temperature without losing the intensity of the general structure, but it must require the use of those materials comprising at medium temperature to make the necessary strength characteristics.
With the expansion of natural gas extraction, collection, transportation, supply source base, the demand for liquefied natural gas valve will be increasing. Among the many varieties of liquefied natural gas valves, the ball valve (operating time is short, small medium resistance) and the shut-off valve (applicable to the general flow adjustment, cut off a good performance) but also a better performance, use the most. For this feature, the development of liquefied natural gas ball valves and globe valves most optimistic about their prospects. (Suggestion: development, development nominal PN1.6 ~ 6.4Mpa, DN DN25mm ~ 400mm of liquefied natural gas ball valves and shut-off valves.)