Why is it difficult to open and close large-diameter valve
In the daily use of large-diameter stop valve users, we often reflect a problem, that is, when the large-diameter globe valve is used for media with large pressure difference, such as steam, high-pressure water and other pressurized work, it is often difficult to close. No matter how hard it is closed, it is always found that there will be leakage and it is difficult to close it tightly, The reason for this problem is caused by the insufficient structural design of the valve and people’s ultimate horizontal output torque.
Cause analysis of difficult opening and closing of large diameter valve
Generally, the horizontal limit output force of adults is 60-90kg, depending on different physique.
Generally, the flow direction of the stop valve is designed to be low in and high out. When people close the valve, the human body horizontally pushes the handwheel to rotate to make the valve disc move downward to close. 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 torque is ∑M=(Fa+Fb+Fc).R.
It can be seen that the larger the diameter, the greater the axial jacking force. 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 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 that can be 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 such valves, which solves the problem that it is difficult to close, but it is difficult to open after closing.
Cause analysis of internal leakage of large diameter stop valve
Large diameter stop valves are generally used at boiler outlet, main and sub cylinders, main steam pipes, etc. these positions have the following problems:
- 1) Generally, the pressure difference at the boiler outlet 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 boiler combustion efficiency cannot be 100%, which will cause large steam water content at the boiler outlet and easy to cause cavitation and cavitation damage to the valve sealing surface.
- 2) For the globe valve near the boiler outlet and sub cylinder, because the steam just coming out of the boiler has intermittent overheating, in the process of saturation, if the boiler water softening treatment is not very good, some acid and alkali substances will be precipitated, which will cause corrosion and erosion to the sealing surface; Some crystallizable substances may also adhere to the sealing surface of the valve, causing the valve to be unable to seal tightly.
- 3) For the inlet and outlet valves of the sub cylinder, the steam consumption behind the valve is large and small due to production requirements. When the flow rate changes greatly, it is easy to produce phenomena such as flash and cavitation, resulting in 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 stop valve can be fully opened only after the pipeline is heated slowly and evenly to a certain extent, so as to avoid excessive expansion caused by rapid temperature rise of the pipeline and damage to some connecting parts. However, in this process, the valve opening is often very small, resulting in the erosion rate is far greater than the normal use effect, which seriously reduces 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 recommended to select bellows sealed stop valve to avoid the influence of friction resistance of plunger valve and packing valve and make it easier to open and close.
- 2) The valve core and valve seat must be made of materials with good erosion resistance and wear performance, such as stellite cemented carbide;
- 3) It is recommended to adopt double disc structure, which will not cause excessive erosion due to small opening, affecting service life and sealing effect.
Source: Network Arrangement – China Globe Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.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.)
If you want to have more information about the article or you want to share your opinion with us, contact us at email@example.com
Please notice that you might be interested in the other technical articles we’ve published: