The application with higher requirements will put forward requirements to the shut-off valve, so the shutting off function is an important function and intrinsic quality index of the shut-off valve. Being used in a working situation, faults such as poor shutoff of the regulating valve and large leakage often occur.

#1 Selection of structure type

One of the main disadvantages of double-sealed control valves is the large leakage.

Before the 1980s, due to the prevalence of sleeve valves at that time, many manufacturers worked hard on improving the structure of the sleeve valve to reduce leakage and evolved it into very complex structures. As a result, this has brought many deficiencies: more  parts, poor reliability, difficulty in parts preparation, difficulty in maintenance, unsatisfactory cutting effects, etc.

In fact, it’s 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 adjustments. Therefore, a single sealed type control valve should be selected. In this way, sealing problems, reliability problems, maintenance and spare parts problems can be all solved one by one. At this time, the key to solve 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. In the 1990s, people began to look for shut-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.

#2 Selection of sealing surface

Face to face sealing：

For common plunger spools, it is necessary to ensure good contact between the two tapered surfaces in order to seal well. However, in fact, it is always affected by processing errors (such as coaxiality, out-of-roundness, inclination, etc.), and its sealing effect is not very satisfactory. The leakage rate of this type of valve can only achieve a better sealing level.

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 linear contact, which is better than the face-to-face sealing. The full featured ultra-light valve and spherically sealed butterfly valve are introduced by Hualin Company are manufactured by using this idea. Its leakage rate can reach 10-6 to 10-8,High performance triple eccentric butterfly valve can up to 10-8 so that the achievement of zero leakage.

#3 Selection of sealing materials

Soft sealing

Except for corrosion-resistant lined valves, general soft-sealed valves refer to valves in which the valve core or valve seat is sealed with a non-metallic material. The soft sealing valve has the best sealing effect. But there may leave more or less dirty debris (such as welding slag, iron filings, etc.) during the process of pipe installation and system cleaning. When they flow through the regulating valve, it is easy to scratch the soft-sealed valve seat or valve core, which increases the amount of leakage and reduces the reliability of the seal.Therefore, when choosing a soft seal structure, consideration must be given to the cleaning of the medium and the strict rinsing of the pipeline before operating.

Metal-metal sealing

This method also considering the service life and reliability while considering the leakproofness . Although the factory index can not achieve the effect of zero leakage of the soft seal, it is sufficient to meet the strict and tight requirements, and it is durable, so it is more economical from an economic perspective

#4 Considerations of Seal reliability

Soft seal has good cutting effect, but it’s not suitable for granular media. Once scratched, leakage will increase sharply. Therefore, try to choose hard seals when the condition allows.

Consideration of seal wear resistance: In addition to selecting hard seals, wear-resistant alloys should be (or must be) welded for shut-off control valves with leakage requirements of 10-6 or more to improve the seal wear resistance, erosion resistance, Scratch resistance, which can improve valve life.

The high temperature medium cut-off control valve is assembled at room temperature, and the temperature changes by several hundred degrees when working at high temperature. Thermal expansion will change the seal pressure established during assembly. The sealing surface will be stuck when it is too tight, and the sealing pressure will not be enough when it is too loose, and the leakage will increase sharply in both cases. The best solution is to use elastic valve seats to absorb the differences caused by this change.

There are usually three kinds of elastic solutions: ① spring type; ② sheet type metal flexible ring; ③  rigid and flexible elastic valve seat. The latter is the best, it not only has the flexibility to overcome thermal expansion, but also has the rigidity to stack wear-resistant alloys to improve the reliability of the wear surface.

For particle-containing erosion media, the valve core should be considered to avoid the direct erosion of high-speed media. Neither the straight-stroke valve nor the butterfly valve has this function, while the full – function ultra-light valve with eccentric rotation has this characteristic. In this case, the latter two valves should be selected.

For the hard seals, there must be enough hardness difference, otherwise, it is easy to scratch.

The thrust and stiffness of the assigned actuator must be increased to make it move freely

#5 Main valve types

When selecting a shut-off valve, comprehensive consideration should be given to various parameters and cost-effectivenesses such as leakage rate, medium type, medium temperature, and flow coefficient. The main steel valve types of shut-off valves are as follows.

  • The full-function ultra-light regulating valve
  • Three eccentric metal hard seal butterfly valve
  • Double eccentric spherical seal butterfly valve
  • Ball valve
  • Eccentric rotary valve
  • Single seat valve
  • Single seat sleeve valve
  • Soft sealing valve