Cut-off valve selection skills

In industrial automation control engineering, the higher demanding applications will place blocking requirements on the regulating valve, which requires fast blocking valves. Therefore, the cut-off function is an important application function and intrinsic quality index of the regulating valve (for the classification and specification of the blocking level, see the relevant information). In field applications, problems such as poor blocking of the control valve and large leakage are often present. Except for the leakage added due to jamming, and the pressure difference is too small to be pushed open, most of them are due to the structural disadvantages of the regulating valve and improper selection.

1.Selection of structure type

In terms of blocking effect, the angle travel valve has a simpler structure, more reliable sealing effect and more superiority than the straight travel valve (such as single seat valve, sleeve valve, gate valve, etc.). The most representative ones are three-part pain hard seal butterfly valves and ball valves.

2.Selection of sealing surface

(1) Face to face seal

For common plunger spools, the sealing surface is a small taper surface at 60 °. The width of this small taper surface is generally between 0.5mm and 2mm. To seal well, the two taper surfaces must be in good contact. But in fact, it is always affected by machining errors (such as heartness, out-of-roundness, tilt, etc.), and its sealing effect is not very ambitious. The leakage rate of this type of valve is generally 10-4. After fine grinding, it can reach 10-6, which can reach a higher sealing level.

(2) Spherical seal

The spherical rotation of a spherical surface is tangent to a fixed valve fixed small cone surface, and there is a line contact between them, which is better than the above-mentioned face-to-face sealing effect. Ball valves, spherically sealed butterfly valves, etc. are produced using this idea. Its leakage rate can reach 10-6 to 10-8, and high-performance triple-bias butterfly valves can reach 10-8, or even zero leakage.

3.Selection of sealed materials

(1) Except for corrosion-resistant fabric valves, general soft-sealed valves refer to valves in which the valve core or valve seat is made of metal materials (primarily polytetrafluoroethylene and rubber, etc.). The soft sealing valve has excellent sealing effect, but may leave more or less unclean impurities (such as welding slag, iron filings, etc.) in the cleaning of pipeline equipment and systems. The soft seal valve seat or valve core is scratched, so that leakage is added, and the sealing is poor. Therefore, the selection of the soft seal structure must consider the cleaning of the medium and the rigorous rinsing of the pipeline before commissioning.

(2) Hard seal

Hard seal and surfacing welding wear-resistant alloys are an excellent choice for stop valves. This method also considers the use of life and reliability when considering the tightness. Although the factory index is only 10-4 to 10-5, it can not achieve the effect of zero leakage of the soft seal, but it is sufficient to meet the requirements of tight blocking It is durable and more cost effective from an economic point of view.

4. Reliability of globe valve

(1) Soft seal, good blocking effect, but not suitable for granular media. Once scratched, leakage will be added sharply. Therefore, hard seals should be used whenever possible. "O" ball valve hard seal can reach 10-5, three partial pain butterfly valve can reach zero leakage.

(2) Considerations on seal abrasion resistance: In addition to hard seals, plug-type control valves that require leakage of 10-4 or more must be welded with abrasion-resistant alloys to improve seal wear resistance, erosion resistance, and scratch resistance. Injured talents increase valve life.

(3) The blocking and regulating valve for high temperature media is assembled at room temperature, and the temperature is changed by several hundred degrees when operating at high temperature. Thermal expansion will alter the seal specific pressure established during installation. If it is too tight, the sealing surface will be stuck, and when it is in motion, it will be severely scratched and strained; if it is too loose, the seal will not work, and leakage in both cases will be sharply added. A good way to deal with this is to use elastic valve seats to absorb the differences created by this change. Generally there are three kinds of elastic plans: ① spring type; ② sheet metal ring; ③ rigid and flexible elastic valve seat. The latter is good. It has both flexibility to overcome thermal expansion, and stiffness and surfacing welding of wear-resistant alloys to improve the reliability of the wear-resistant surface.

(4) For particle-containing erosion media, it is necessary to consider that the valve core avoids direct erosion of high-speed media.

(5) With regard to hard seals, there must be sufficient hardness difference. The same hardness data is "tip-to-tip", which is easy to scratch and cause leakage.

5.Selection of primary valve type of globe valve

According to the above, the primary valve type of the shut-off valve is as follows. When selecting the blocking valve, various parameters such as leakage rate, medium type, medium temperature, flow coefficient and cost-effectiveness should be considered. In the following preferred sequence, a hard seal is considered first, followed by a soft seal.

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