Cut off the requirements of the regulating valve in the applications will require a higher cut-off function is an important control valve functions and the intrinsic quality indicators (see related information) to cut off the division level and standard. In the field, often the control valve to cut off a bad leak and large fault. Due to lay a trap, allowing the pressure is too small to open the top and other causes of increased leakage, the vast majority are caused by the regulating valve structural defects and improper selection.
Selection of structure type
In effect cut off, the structure of the rotary valve stroke than a straight valve (such as single-seat valves, sleeve valves, gate valves, etc.) is more simple, more reliable sealing effect, has more advantages. More representative of the three eccentric hard sealing butterfly valve, ball valve.
Sealing surface of choice
1) face-to-face seal:
The common plunger spool, the small conical sealing surface 60 °, the width of this cone is usually between 0.5mm ~ 2mm to seal it must ensure good contact of two cone. But in fact, it is always subject to processing errors (as the heart of not round, tilt, etc.), the sealing effect is not very satisfactory. The type of valve leakage rate is usually 10-4, to reach 10-6 after precision grinding up to a higher seal level.
2) spherical seal
The sphere of spherical rotation with fixed valve, cone tangent line of contact between them, which is better than the above face-to-face sealing effect. Ball valves, spherical sealing butterfly valve is to use this idea to make the leakage rate of up to 10-6 to 10-8, high-performance triple eccentric butterfly valve up to 10-8, and even zero leakage.
1) soft sealing
In addition to the corrosion-resistant lining valve, soft seal valve spool or the seat either of metal materials (mainly valve seal in the form of Teflon and rubber, etc.). Soft sealing valve seals the best in the pipeline installation, cleaning of the system are likely to leave more or less clean debris (such as welding slag, iron, etc.) flows through the control valve, it is easy to soft sealing valve seat or valve scratches, an increase of leakage, poor sealing reliability. Therefore, the choice of soft seal structure must be considered before the media to clean and put into operation in strict flushing the piping.
2) Hard Seal
Hard seal re-resistant alloys is the best choice of the cut-off valve. In this way also take into account while considering the tightness of the life and reliability, although the factory index 10-4 ~ 10-5, amounting to less than a soft zero leakage seal effect, but enough to meet the requirements of strict cut off, and it durable, more cost-effective from the economic point of view.
Shut-off valve reliability
1) soft sealing, cut off the effect is good, but not suitable for granular media, once scratched, leakage would increase drastically. Therefore, when conditions permit, use a hard seal. “O” type ball hard sealing up 10-5, three eccentric butterfly valves up to zero leakage.
2) consideration of the seal wear resistance: In addition to the selected hard seal leakage requirements above 10-4 cut-off control valve, in order to improve the sealing of the abrasion resistance, erosion resistance, scratch resistant alloys the ability to improve the life of the valve.
3) cut-off valve assembly at room temperature, at high temperatures, the temperature change of the high-temperature media, several hundred degrees. The thermal expansion of the established will change the assembly than the pressure seal. Too tight, the sealing surface of the card, letter, the movement will be severely severely bruised and strained; too loose, the seal pressure is not enough, in both cases the leak will be a sharp increase.
The best way to resolve the differences caused by the resilient seat to absorb the changes. There are typically three flexibility programs: (1) spring-loaded; ② The flakes flexible metal ring; (3) hardness and softness of resilient seat. The latter is best, both flexible to overcome thermal expansion, but also stiffness and resistant alloys to improve the reliability of the wear surface.
4) the erosion of media containing particles, we must consider the spool to avoid the direct erosion of the high-speed media.
5) For the hard seal must be stiff enough, the same hardness materials “to each other, it is easy to scratch caused by leakage.
Shut-off valve valve selection
According to the above, the main valve shut-off valve type as follows, select the cut-off valve should be considered in the leakage rate, media type, media, temperature, flow coefficient and other parameters and cost. In the following preferred order, the first seal, then consider the soft seal.
1) The triple eccentric metal hard seal butterfly
2) double eccentric spherical sealing butterfly valve
4) Eccentric butterfly valve
5) a single seat valve
6) sleeve valve
7) Soft sealing valve