• Categories:Industry News
• Author:
• Origin:
• Time of issue:2019-11-26
• Views:2

(Summary description)In modern industrial control systems, the requirements for regulating valves are getting higher and higher, and shut-off requirements are imposed on regulating valves in applications with higher requirements. Therefore, the shut-off function is an important use function and intrinsic quality index of regulating valves For the classification and standard of cut-off grade, please refer to the relevant information). In field use, faults such as poor shutoff of the regulating valve and large leakage often occur. Except for the increase in leakage caused by jamming, being allowed to open due to too small allowable differential pressure, most of them are caused by structural defects and improper selection of the regulating valve.

First, the choice of structure type

One of the main disadvantages of double-sealing control valves (such as double-seat valves, double-sealing sleeve valves, etc.) is large leakage. But before the 1980s, due to the prevalence of sleeve valves at that time, however, many manufacturers worked very hard on sleeve valves to improve the structure to reduce leakage and evolved into very complex structures. As a result, this has brought many shortcomings: many parts, poor reliability, difficult spare parts, difficult maintenance, unsatisfactory cutting effect, and so on. In fact, this is because the design ideas have fallen into a misunderstanding. Instead of making such a complicated change in the "heart" of the valve body, it is better to achieve it through external adjustment. Therefore, a single-seal type control valve should be selected. In this way, sealing problems, reliability problems, maintenance and spare parts problems are all solved one by one. At this time, the focus of solving the contradiction is transformed into the problem of increasing the unbalanced force of the medium on the valve, and it only needs to be equipped with a powerful piston actuator and a thicker valve stem. This external solution is obviously much easier than complex internal improvements. It is not so much a question of method as a question of ideas. In the 1990s, people began to look for cut-off valves with simpler structure and more reliable sealing. At this time, it was found that the angular stroke valve was more superior than the straight stroke valve (single seat valve, sleeve valve, gate valve), so three-eccentric shut-off butterfly valves, full-function shut-off valves, and hard-sealed shut-off ball valves were produced.

Selection of sealing surface

(1) Face to face seal:
For common plunger spools, the sealing surface is a small tapered surface at 60 ° C, and the valve seat is also a small tapered surface at 60 ° C. The width of this small tapered surface is usually between 0.5mm and 2mm. The tapered surfaces make good contact. But in fact, it is always affected by processing errors (such as heart rate, out-of-roundness, tilt, etc.), and its sealing effect is not very satisfactory. The leakage rate of this type of valve is usually 10-4, and after precision grinding, it can reach 10-6, which can only reach a better sealing level.

(2) Spherical seal:
The spherical rotation of the valve core is tangent to the small cone surface of the fixed valve seat, and they are in line contact, which is better than the above-mentioned face-to-face sealing. The full-featured ultra-light valve and spherically sealed butterfly valve introduced by Hualin Company are manufactured using this idea. Its leakage rate can reach 10-6 to 10-8, and the high-performance three-eccentric butterfly valve can reach 10-8, or Zero leakage.

Selection of sealing materials

(1) Soft seal
In addition to corrosion-resistant lined valves, general soft-sealed valves refer to valves in which either the spool or the seat is sealed with a non-metallic material (mainly polytetrafluoroethylene, but also rubber, etc.). The soft sealing valve has the best sealing effect, but may leave more or less dirty debris (such as welding slag, iron filings, etc.) in the pipeline installation and system cleaning. When they pass through the regulating valve, it is easy to remove Scratches on soft-sealed valve seats or spools increase leakage and poor reliability of the seal. Therefore, when choosing a soft seal structure, consideration must be given to the cleaning of the medium and strict rinsing of the pipeline before commissioning.

(2) Hard seal
Hard sealing and surfacing welding of wear-resistant alloys are the best choice for shut-off valves. This method also takes into account the service life and reliability while considering the tightness. Although the factory index is only 10-6 to 10-8, it can not achieve the effect of zero leakage of the soft seal, but it is sufficient to meet the strict and tight requirements, and it Durable, more economical from an economic point of view.

Fourth, the consideration of sealing reliability

(1) The soft seal has good cutting effect, but it is not suit