Shutoff function and selection of regulating valve

In the application of industrial automation with higher requirements, the cutting off of the regulating valve is often required, so the cutting off function is an important use function and internal quality index of the regulating valve.In the field use, it is often bad to cut off the regulating valve, leakage and other faults.In addition to the leakage increase caused by blocking the jam, the allowable pressure difference is too small and is jacked, most of them are caused by the structural defects and improper selection of the control valve.

The following cut off the form and function of the regulating valve, the selection of a comprehensive introduction.

One, the choice of structural type

One of the main disadvantages of double-seal type regulating valve (such as double-seat valve, double-seal sleeve valve, etc.) is large leakage.However, prior to the 1980s, sleeve valves were more popular at that time. However, many manufacturers made great efforts to improve the structure of sleeve valves to reduce leakage, so that they evolved into very complex structures.Therefore, this has brought many shortcomings: more parts, poor reliability, difficult to spare parts, maintenance, cut off the effect is not ideal, and so on.In fact, this is the design ideas have fallen into error, rather than make such a complex change in the "heart" of the valve body, it is better to achieve through external adjustment.Therefore, should choose a single sealing type regulator.In this way, the problems of sealing, reliability, maintenance and spare parts have been solved one by one.

At this point, the focus of the solution to the problem of media on the valve imbalance force increase, solve it only with a strong piston actuator, bold stem can be.This external solution is obviously much easier than the internal complex improvement. It is not so much a problem of method as a problem of thinking.In the 1990s, the search began for a simpler, more reliable shut-off valve.At this time, it is found that the Angle stroke valve is more superior than the straight stroke valve (single seat valve, sleeve valve, gate valve), so the three eccentric cut off butterfly valve, full function cut off valve, hard seal cut off ball valve.

The choice of sealing surface

(1) face to face sealing:

Common plunger spool, its sealing surface is 60℃ small cone, seat is also 60℃ small cone, the width of the small cone is usually between 0.5mm ~ 2mm, to seal must ensure good contact between the two cone.But in fact, it is always affected by processing error (such as concentricity, roundness, inclination, etc.), its sealing effect is not very ideal.This type of valve leakage rate is usually 10-4, after precision grinding up to 10-6, can only achieve a better seal grade.

(2) spherical seal:

The spherical rotation of the spool is tangent to the small cone surface of the fixed seat with linear contact between them, which is better than the above face-to-face sealing effect.Hualin company launched the full function ultra-light valve, spherical seal butterfly valve is made with this idea, the leakage rate can reach 10-6 ~ 10-8, high-performance triple eccentric butterfly valve can be as high as 10-8, even zero leakage.

Three, the choice of sealing materials

(1) soft seal

In addition to the corrosion resistant lining valve, general soft sealing valve refers to the valve core or seat one of the two USES of non-metallic materials (mainly for ptfe, and rubber) sealing form of the valve.Soft seal valve sealing effect is the best, but in the pipeline installation, system cleaning are likely to leave more or less unclean sundries (such as welding slag, iron scrap, etc.), they flow through the control valve, easy to put the soft seal seat or valve core scratch, leakage increase, the reliability of the seal is poor.Therefore, the choice of soft seal structure must consider the cleaning of the medium and the strict flushing of the pipeline before operation.

(2) hard seal

Hard seal refacing welding wear-resisting alloy is the best choice for cutting off valve.This method considers the service life and reliability while considering the tightness. Although the factory index is only 10-6 ~ 10-8, which cannot reach the zero leakage effect of the soft seal, it is enough to meet the requirements of tight cutting. Moreover, it is durable and more cost-effective from the perspective of economy.

4. Consideration of sealing reliability

(1) soft seal, good cutting effect, but not suitable for media containing particles, once scratched, leakage will increase sharply.

Therefore, when conditions permit, try to choose a hard seal.Full function ultra-light valve hard seal up to 10-7 ~ 10-8, triple eccentric butterfly valve up to zero leakage.

(2) the consideration of the wear resistance of the seal: in addition to the selection of hard seal, the leakage requirements of more than 10-6 cut off type regulating valve, should (or must) surfacing welding wear resistant alloy to improve the wear resistance of the seal, erosion resistance, scratch resistance, improve the life of the valve.

(3) the cut-off control valve of the high-temperature medium is assembled at room temperature and works at a high temperature with a temperature variation of several hundred degrees.Thermal expansion will change the specific seal pressure established during assembly.Too tight, the sealing surface will be stuck, sports will be serious abrasions, pull;Too loose, not enough seal pressure, in both cases leakage increases dramatically.The best solution is to use an elastic seat to absorb the difference caused by this change.Usually there are three kinds of elastic scheme: (1) spring;(2) thin plate type flexible metal ring;(3) rigid flexible seat.The latter is the best, that is, the flexibility to overcome thermal expansion, and the stiffness and heap wear-resistant alloy to improve the reliability of the wear-resistant surface.

(4) for the erosion medium containing particles, the valve core should be considered to avoid the direct erosion of high-speed media.Straight stroke valve, butterfly valve do not have this function, and the full function of ultra-light valve eccentric rotation has this feature.Case this, should choose after two kinds of valve.

(5) for hard seal, there must be enough hardness difference, the same hardness of the material "mutual concessions", easy to scratch.

(6) the equipped actuator should increase the thrust and stiffness to make it move freely.

Five, high temperature, high pressure, large pressure difference, large caliber cut off valve selection

High temperature and high pressure, differential pressure, large diameter cut off is a common problem, the main reason is the lack of large have allowed pressure difference at the same time, cut off valve, high temperature resistant expansion CARDS, such as high pressure steam adjusting and cut off, because of the pressure difference big (1 ~ 3 mpa or greater), large diameter (DN200 ~ 300), to the use of balance type double seal structure (e.g., two-seat valves or double sealing sleeve valve), through special processing, leakage rate can reach 10-4, again raise is difficult, and fully functional ultra light regulator comprehensive performance is good, can better solve this problem, the leakage rate can reach more than 10-6 ~ 10-7.