In high demand applications, cut-off requirements are often required for regulating valves, so the cut-off function is an important functional and internal quality indicator of regulating valves (see relevant information for the classification and standards of cut-off levels). In on-site use, faults such as poor shut-off of the regulating valve and large leakage often occur. Except for the increase in leakage caused by blockage and being pushed open due to a small allowable pressure difference, the vast majority of it is due to structural defects and improper selection of the regulating valve.

1、 Selection of structural types

One of the main drawbacks of double sealed regulating valves (such as double seat valves, double sealed sleeve valves, etc.) is large leakage. But before the 1980s, due to the prevalence of sleeve valves at that time, many manufacturers put a lot of effort into improving the structure of sleeve valves to reduce leakage, making them evolve into very complex structures.

So, this has brought many shortcomings: too many parts, poor reliability, difficult spare parts, difficult maintenance, unsatisfactory cutting effect, and so on.

In fact, this is a design concept that has fallen into a misunderstanding. Instead of making such complex changes within the "heart" of the valve body, it is better to achieve it through external adjustments.

Therefore, a single sealed regulating valve should be selected. In this way, sealing issues, reliability issues, maintenance and spare parts issues have all been resolved one by one.

At this point, the focus of resolving the contradiction is shifted to the problem of increasing the unbalanced force of the medium on the valve. Solving it only requires a strong piston actuator and a thickened valve stem. This method of solving problems through external means is obviously much easier than complex internal improvements. Rather than being a problem of methods, it is more of a problem of ideas.

In the 1990s, people began to search for simpler and more reliable shut-off valves. At this point, it was found that the angle stroke valve had more advantages than the straight stroke valve (single seat valve, sleeve valve, gate valve), so three eccentric cut-off butterfly valves, full function cut-off valves, and hard sealed cut-off ball valves were produced.

2、 Selection of sealing surfaces

1) Face to face sealing:

The common plunger valve core has a sealing surface of a small conical surface at 60 ℃, and the valve seat is also a small conical surface at 60 ℃. The width of this small conical surface is usually between 0.5mm and 2mm. To seal it well, it is necessary to ensure good contact between the two conical surfaces. However, in fact, it is always affected by machining errors (such as eccentricity, out of roundness, inclination, etc.), and its sealing effect is not very ideal. The leakage rate of this type of valve is usually 10-4, and after precise grinding, it can reach 10-6, which can only achieve a good sealing level.

2) Spherical sealing:

The spherical rotation of the valve core is tangent to the small conical surface of the fixed valve seat, and they are in line contact, which is better than the face-to-face sealing effect mentioned above. The fully functional ultra light valves and spherical sealed butterfly valves launched by Hualin Company are made using this idea, with a leakage rate of 10-6-10-8, and high-performance triple eccentric butterfly valves can reach 10-8, or even zero leakage.

3、 Selection of sealing materials

1) Soft sealing

Except for corrosion-resistant lined valves, general soft sealed valves refer to valves that use non-metallic materials (mainly polytetrafluoroethylene, as well as rubber, etc.) to seal the valve core or valve seat. The soft seal valve has the best sealing effect, but it may leave more or less unclean debris (such as welding slag, iron filings, etc.) during pipeline installation and system cleaning. When they pass through the regulating valve, they are easy to scratch the soft seal valve seat or valve core, increasing the leakage amount and poor sealing reliability. Therefore, when choosing a soft sealing structure, consideration must be given to the cleanliness of the medium and strict flushing of the pipeline before operation.


2) Hard sealing

Hard sealing and then welding wear-resistant alloys are the best choices for shut-off valves. This method not only considers sealing performance but also service life and reliability. Although the factory indicators are only 10-6-10-8, which cannot achieve the effect of zero leakage in soft sealing, it is sufficient to meet the requirements of tight cutting. Moreover, it is durable and more cost-effective from an economic perspective.

4、 Considerations for sealing reliability

1) Soft sealing has a good cutting effect, but it is not suitable for use in media containing particles. Once scratched, leakage will increase sharply. Therefore, if conditions permit, hard seals should be used as much as possible. The hard seal of the fully functional ultra light valve can reach 10-7-10-8, and the three eccentric butterfly valve can achieve zero leakage.

2) Consideration of sealing wear resistance: In addition to selecting hard seals, for cut-off type control valves with leakage requirements of 10-6 or more, wear-resistant alloys should (or must) be welded to improve the sealing wear resistance, erosion resistance, scratch resistance, and extend the valve's lifespan.

3) The cut-off regulating valve for high-temperature media is assembled at room temperature and operates at high temperatures, with temperature changes of several hundred degrees. Thermal expansion can alter the sealing pressure established during assembly. If it is too tight, the sealing surface will get stuck, and there will be serious scratches and strains during movement; Excessive looseness and insufficient sealing pressure can lead to a sharp increase in leakage in both cases. The best solution is to use an elastic valve seat to absorb the difference caused by this change. There are usually three types of elastic schemes: ① spring type; ② Thin sheet flexible metal ring; ③ Elastic valve seat with a balance of rigidity and flexibility. The latter is the best, which has flexibility to overcome thermal expansion and rigidity to stack wear-resistant alloys to improve the reliability of the wear-resistant surface.

4) For erosive media containing particles, the valve core should be considered to avoid direct erosion by high-speed media. Straight stroke valves and butterfly valves do not have this function, while fully functional ultra light valves have this characteristic of eccentric rotation. In this case, the latter two types of valves should be selected.

5) For hard seals, there must be sufficient hardness difference, and materials with the same hardness are not mutually exclusive, which can easily cause scratches.

6) The thrust and stiffness of the equipped actuator should be increased to make it move freely.

5、 Main valve types of shut-off valves

According to the above analysis, the main types of shut-off valves are as follows. When selecting a shut-off valve, various parameters such as leakage rate, medium type, medium temperature, flow coefficient, etc., and cost-effectiveness should be comprehensively considered. In the following preferred order, consider hard sealing first and then soft sealing.

1) Fully functional ultra light regulating valve

2) Triple eccentric metal hard seal butterfly valve

3) Double eccentric spherical sealed butterfly valve

4) Ball valve

5) Eccentric rotary valve

6) Single seat valve

7) Single seat sleeve valve

8) Soft seal valve

6、 Selection of high temperature, high pressure, large pressure difference, and large diameter shut-off valves

High temperature, high pressure, large pressure difference, and large diameter cut-off are common difficulties, mainly due to the lack of valves that allow for large pressure difference, good cut-off, and high-temperature expansion card, such as high-pressure steam regulation and cut-off. Due to large pressure difference (1-3MPa or more) and large diameter (DN200-300), balanced double sealing structures (such as double seat valves or double sealed sleeve valves) have to be adopted. After special treatment, the leakage rate can reach 10-4, Further improvement will be difficult, and the comprehensive performance of the fully functional ultra light regulating valve is good, which can better solve this problem, with a leakage rate of 10-6 to 10-7 or above