Device and operation of shut-off valves

In the process of selecting the shut-off valve size, the method of moving the stem, the nominal diameter, the material and the method of connecting the valve to the pipeline are important. Technical requirements for the production of these units are regulated by GOST 5762-2002.

Construction

In a valve with a rising stem, the upper part of the rod has a thread, and the nut is firmly attached to the flywheel, held in the housing by thrust clamps. When the flywheel turns, the rod moves up or down. In non-return valve rods, the lower end of the rod is threaded and screwed into the disc, and the vertical movement of the rod is restrained by a thrust ring.

The valve with a rising stem requires more space when opening. Despite this, such valves are used more often in practice, since the position of the stem immediately visually shows whether the valve is open or closed. For this purpose, shut-off valves sometimes have a built-in rod position indicator.

Some two-disc valves with a parallel seat have a design that allows you to move the locking disc depending on the fluid pressure. Usually it is applied to one of the sides of the disk to ensure its tightness. The main advantage of this type is the elimination of the danger of jamming the disc in the housing, which is especially important for drive shut-off valves.

Management features

They are determined by the type of mobile flow control system. She can be:

• With a rising rod;
• Single-disc with parallel saddle;
• Two-disc with parallel saddle

Unlike a valve with a wedge gate, which comes into contact only in the closed position, each disc in the valve with a parallel seat slides to its seat when the valve opens or closes. Therefore, these components must be made of metals that do not rust and do not break down during sliding contact with each other. A two-disc valve with a parallel seat is preferable for pipelines transporting high-temperature steam: in this case, with any temperature change, sticking of the valve in the closed position is practically excluded.

The choice of the shut-off valve size is determined by the internal diameter of the pipeline: it must be equal to the internal diameter of the connecting part of the valve.

The systems in question are unsuitable for pipelines transporting untreated water or wastewater when there is a significant amount of suspended solids, as well as other contaminants. For low and medium temperatures (up to 230...2400 C), valves with Teflon seats can be used. Valves with metal seats can be used for temperatures up to 500...5300 C.