Solution to the difficulty of opening and closing large diameter valve
In the daily use of large-diameter globe valve users, we often reflect a problem, that is, when the large-diameter stop valve is used in the medium with large pressure difference, such as steam, high-pressure water and other pressure work, it is often difficult to close, no matter how hard to close, there will always be leakage phenomenon, it is difficult to close, The reason for this problem is due to the structural design of the valve and the insufficient output torque of human limit level.
Cause analysis of difficult opening and closing of large diameter valve
Table of Contents
The average adult’s horizontal ultimate output force is 60-90kg, depending on the constitution.
Generally, the flow direction of the stop valve is designed as low in and high out. When people close the valve, the human body horizontally pushes the hand wheel to rotate to make the valve disc move downward to realize closing. At this time, it is necessary to overcome the combination of three forces, namely:
- (1) Axial jacking force FA;
- (2) Friction force between packing and valve stem FB;
- (3) Contact friction between valve stem and disc core FC.
The total moment is ∑ M = (FA + FB + FC) r
It can be seen that the larger the diameter is, the larger the axial jacking force is. When it is close to the closed state, the axial jacking force is almost close to the actual pressure of the pipe network (because P1-P2 ≈ P1, P2 = 0 when it is closed)
For example, if a DN200 diameter stop valve is used on a 10 bar steam pipe, only the first item is closed, and the axial jacking force FA = 10 ×π R2 = 3140kg, and the horizontal circumferential force required for closing is close to the limit of the horizontal circumferential force output by normal human body, so it is very difficult for one person to completely close the valve under this working condition.
Of course, some factories suggest reverse installation of this kind of valve, which solves the problem that it is difficult to close, but it is difficult to open after closing.
Analysis of internal leakage of large diameter globe valve
Large diameter stop valve is generally used in boiler outlet, main cylinder, main steam pipe and so on:
- (1) Generally, the pressure difference at the outlet of the boiler is relatively large, so the steam flow rate is also greater, and the erosion damage to the sealing surface is also greater. In addition, the combustion efficiency of the boiler can not be 100%, which will result in a large water content in the steam at the outlet of the boiler, which is easy to cause cavitation and cavitation damage to the valve sealing surface.
- (2) For the stop valve at the boiler outlet and near the sub cylinder, because the steam just coming out of the boiler has intermittent overheating phenomenon, in the process of saturation, if the boiler water softening treatment is not too good, some acid and alkali substances will be precipitated, which will cause corrosion and erosion to the sealing surface; There are also some crystallizable substances may adhere to the valve sealing surface crystallization, resulting in the valve can not be tightly sealed.
- (3) The steam consumption of the inlet and outlet valves of the sub cylinder is large or small due to the production requirements and other reasons. In the case of large flow rate change, it is easy to produce flash, cavitation and other phenomena, thus causing erosion, cavitation and other damage to the valve sealing surface.
- (4) Generally, when the large diameter pipeline is opened, the pipeline needs to be preheated, and the preheating process generally requires a small flow of steam to pass through, so that the pipeline can be heated slowly and evenly to a certain extent before the stop valve can be fully opened, so as to avoid excessive expansion caused by rapid heating of the pipeline and damage to some connecting parts. But in this process, the valve opening is often very small, resulting in the erosion rate is far greater than the normal use effect, seriously reducing the service life of the valve sealing surface.
Solution to the difficulty of opening and closing large diameter stop valve
- (1) First of all, it is suggested to choose bellows seal globe valve to avoid the influence of friction resistance of plunger valve and packing valve.
- (2) The valve core and seat must be made of materials with good erosion resistance and wear performance, such as stellite cemented carbide;
- (3) It is suggested that double disc structure should be adopted to avoid excessive erosion due to small opening, which will affect service life and sealing effect.
Source: Network Arrangement – China Valve Supplier: www.epowermetals.com
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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