control valve often has some problems when used, such as can not shut-off well and large leakage. Besides for the result of leakage caused by blockage, too small allowable pressure difference and jacking, most of them are caused by the structural defects and improper selection of control valves. In modern industry, the requirement of control valves is more higher and higher. In high demand application occasions, the cut-off function is an important function and intrinsic quality index of control valves (see relevant information for the classification and standard of cut-off grade). So we should be pay special attention.
一． Selection of Structural
One of the main defect is large leakage for double-seal type control valves (such as double-seat valves, double-seal cage guide valves, etc.) . However, before the 1980s, cage guide control valves were prevalent. However, cage guide control valves have many shortcomings: many parts, poor reliability, difficult spare parts, difficult maintenance, unsatisfactory cut-off effect and so on. In fact, this is a misunderstanding of the design ideas. We would rather making such a complex change in the "trim" of the valve body, Than achieve it through external adjustment. Therefore, the single seal type control valve should be selected. In this way, the problems of sealing, reliability, maintenance and spare parts have been solved one by one. At this time, the key to solve the contradiction is to increase the unbalanced force between the medium and the valve. To solve the problem, These can be need only a strong piston actuator and a thicker valve stem . This external solution is obviously much easier than the internal complex improvement.. By the 1990s, people began to look for more simple and reliable shutoff valves. At this time, it was found that the angular stroke valve has more advantages than the direct stroke valve (single seat valve, cage guide valve, gate valve). Thus, Triple eccentric butterfly valve, full-function cut-off valve and metal seal cut-off ball valve were produced.
二．SELECTION OF SEALING SURFACE
(1) Face-to-face sealing:
The common plunger spool has a small conical surface,sealing surface and seat are also for 60 degrees C . The width of the small conical surface is usually between 0.5 mm and 2 mm. It should be contact tightly between the two conical surfaces in order to seal well. But in fact, it is always affected by processing errors (such as centricity, non-roundness, inclination, etc.), and its sealing effect is not very ideal. The leakage rate of this kind of valve is usually 10-4. After precise grinding, it can reach 10-6, which can only achieve better sealing grade.
(2)valve ball seal:
The valve ball rotation of the spool is tangent to the small conical surface of the fixed seat, and the line contact between them .So it is better than the face-to-face seal .
三．Selection of Sealing Materials
(1) Soft seal
Except for lined corrosion-resistant valves, the general soft-seal valve refers to either disc or seat with non-metallic material (mainly PTFE, rubber, etc.)
(2) Metal Sealing
Metals seals and surfacing wear-resistant alloys are good choices for cut-off valves.
(1) Soft seal,Have a good cutting effect, but not suitable for granular media, once scratched, leakage will increase sharply. Therefore, Metal seals should be chosen as far as possible when conditions permit.
(3) The cut-off control valve for high temperature medium is assembled at room temperature and operates at high temperature. The temperature changes several hundred degrees. Thermal expansion will change the specific pressure of the seal established during assembly. Excessive tightness, sealing surface will be stuck, serious scratches and strains will occur during movement; excessive looseness and insufficient sealing pressure will lead to a sharp increase in leakage in both cases. A good solution is to use an elastic seat to absorb the difference caused by this change. Usually there are three elastic schemes: spring type, sheet type flexible metal ring and rigid flexible elastic seat. The latter is good in that it has flexibility to overcome thermal expansion and rigidity to stack wear-resistant alloys to improve the reliability of wear-resistant surfaces.
(1) Soft seal
In addition to corrosion-resistant liner valves, the general soft-seal valve refers to a valve with non-metallic material (mainly PTFE, rubber, etc.) as one of the two types of valve core or seat.
(2) Hard Sealing
Hard seals and surfacing wear-resistant alloys are good choices for cut-off valves.
Consideration of wear resistance of seals: In addition to selecting hard seals, If regulators requiring higher leakage grade ,The surfacing should be weld stellite to improve the wear resistance, erosion resistance, scratch resistance of seals and the life of valves。
(3) For erosion particles medium, the spool should be considered to avoid the direct erosion of high-speed media. Direct stroke valves and butterfly valves do not have this function,But eccentric rotation of full-function ultra-light valves has this feature. In this case, the latter two valves should be selected.
(4) For metal seals, there must be enough hardness difference . The same hardness material "does not agree with each other" and easy to scratch.
(5) The actuator should be increased to make the actuator move freely.
