Stop valves are forced to seal the valve, so the valve is closed, the pressure must be applied to the valve flap to force the sealing surface does not leak. When the medium from below the valve into the valve, the operating force needed to overcome the resistance is the valve stem and packing friction and the thrust generated by the media pressure, off the valve force than the valve open, so the valve stem The diameter should be larger, otherwise it will happen valve stem bend. In recent years, since the self-sealing valve appears, the cut-off valve of the media flow to change over from the top of the valve into the valve cavity, then under medium pressure, the valve closing force is small, and open the valve force, the valve stem The diameter can be reduced accordingly. At the same time, in the medium, this form of valve is also more stringent. China's valve "three to" have provided that the flow of the valve, all from top to bottom. The valve should be installed horizontally when the shut-off valve is open, the valve opening height, the nominal diameter of 25% to 30%, the flow has reached its maximum, indicating that the valve has reached the fully open position. Therefore, the full-stop position of the valve, valve stroke should be determined. Stop valve works Stop valve, also called cut-off, is the most widely used type of valve, it is popular because of the friction between the sealing surface during opening and closing small, more durable, open height is not big , Easy to manufacture, easy to maintain, not only suitable for medium pressure, but also for high pressure. Closing valve closing principle is to rely on valve stem pressure, so that valve disc sealing surface and seat sealing surface close fit, to prevent the circulation of media. Globe only one-way media flow, the installation of a directional. Shut-off valve structure is greater than the gate valve length, while fluid resistance, long-term operation, the sealing reliability is not strong. Advantages 1, simple structure, manufacturing and maintenance more convenient. 2, small work schedule, opening and closing time is short. 3, good sealing, sealing surface friction, longer life expectancy. 4, cut-off valve body structure of the straight-through type, direct current and right angle. Straight-through is the most common structure, but its resistance to fluids is greatest. Direct flow fluid resistance is small, more for solid particles or viscous fluid. Right-angle valve body forgings, for smaller, higher pressure of the globe valve. Disadvantages of use 1, the fluid resistance, opening and closing the required force larger. 2, does not apply to the particles with a larger viscosity, easy to coking medium. 3, adjust the performance is poor. 4, the type of cut-off valve stem thread position by male and female thread type. By media flow points, there are straight-through, DC and angle. Globe valve by sealing form points, there are packing seal globe valve and bellows seal valve. 5, Bellows seal globe valve with bellows seal design, completely eliminate the common valve stem packing leak fast aging leak, not only improve the use of energy efficiency, increase the safety of production equipment and reduce maintenance costs and frequent Maintenance, but also provides a clean and safe working environment. Globe valve installation and maintenance should pay attention to the following: 1, handwheel, handle the operation of the globe valve can be installed in any position on the pipeline. 2, hand wheel, handle and Wei moving mechanism, do not allow lifting. 3, the flow of medium should be consistent with the arrow direction shown by the valve body.
