The stop valve is mainly used to regulate and stop the fluid flowing through the pipeline. They differ from valves such as ball valves and gate valves in that they are specifically designed to control fluid flow and are not limited to closing services. The reason why the stop valve is so named is that the older design presents a certain spherical body and can be divided into two hemispheres, separated by the equator, where the flow changes direction. The actual internal elements of the closing seat are usually not spherical (e.g., ball valves) but are more typically planar, hemispherical, or plug shaped. Globe valves restrict fluid flow more when open than gate or ball valves, resulting in higher pressure drop through them. Globe valves have three main body configurations, some of which are used to reduce the pressure drop through the valve. For information on other valves, please refer to our valve buyer’s Guide.

Valve design

Stop valve is composed of three main parts: valve body and seat, valve disc and stem, packing and bonnet. In operation, rotate the threaded stem through the handwheel or valve actuator to lift the valve disc from the valve seat. The fluid passage through the valve has a Z-shaped path so that the fluid can contact the head of the valve disc. This is different from gate valves where the fluid is perpendicular to the gate. This configuration is sometimes described as a Z-shaped valve body or a T-shaped valve. The inlet and outlet are aligned with each other.

Other configurations include angles and Y-shaped patterns. In the angle stop valve, the outlet is 90 ° from the inlet, and the fluid flows along the L-shaped path. In a Y-shaped or Y-shaped valve body configuration, the valve stem enters the valve body at 45 °, while the inlet and outlet remain in line, the same as in the three-way mode. The resistance of the angular pattern to flow is smaller than that of the T-shaped pattern, and the resistance of the Y-shaped pattern is smaller. Three way valves are the most common of the three types.

The sealing disc is usually tapered to fit the valve seat, but a flat disc can also be used. When the valve is slightly opened, the fluid flows evenly around the disc, and the wear distribution on the valve seat and disc. Therefore, the valve works effectively when the flow is reduced. Generally, the flow direction is toward the valve stem side of the valve, but in high-temperature environment (steam), when the valve body cools and contracts, the flow often reverses to keep the valve disc tightly sealed. The valve can adjust the flow direction to use pressure to help close (flow above the disc) or open (flow below the disc), thus allowing the valve to fail close or fail open.

The sealing disc or plug is usually guided down to the valve seat through the cage to ensure proper contact, especially in high-pressure applications. Some designs use a valve seat, and the seal on the valve rod side of the disc press abuts against the valve seat to release the pressure on the packing when the valve is fully opened.

According to the design of the sealing element, the stop valve can be quickly opened by several turns of the valve stem to quickly start the flow (or closed to stop the flow), or gradually opened by multiple rotations of the valve stem to generate more regulated flow through the valve. Although plugs are sometimes used as sealing elements, they should not be confused with plug valves, which are quarter turn devices, similar to ball valves, which use plugs instead of balls to stop and start flow.

application

Stop valves are used for shutdown and regulation of wastewater treatment plants, power plants and process plants. They are used in steam pipes, coolant circuits, lubrication systems, etc., in which controlling the amount of fluid passing through valves plays an important role.

The material selection of the globe valve body is usually cast iron or brass / bronze in low pressure applications, and forged carbon steel or stainless steel in high pressure and temperature. The specified material of the valve body usually includes all pressure parts, and “trim” refers to parts other than the valve body, including the valve seat, disc and stem. The larger size is determined by the ASME class pressure class, and standard bolts or welding flanges are ordered. Sizing globe valves takes more effort than sizing some other types of valves because the pressure drop across the valve can be a problem.

Rising stem design is the most common in stop valves, but non rising stem valves can also be found. The bonnet is usually bolted and can be easily removed during internal inspection of the valve. The valve seat and disc are easy to replace.

Stop valves are usually automated using pneumatic piston or diaphragm actuators, which act directly on the valve stem to move the disc into position. The piston / diaphragm can be spring biased to open or close the valve upon loss of air pressure. An electric rotary actuator is also used.