Classification and Features of Ball Valves
Add time：2018/5/8 13:53:50 Page views：4456
Ball valves came into being in the 1950s. With the rapid development of science and technology, as well as continuous improvements in manufacturing processes and product structures, ball valves have rapidly evolved into one of the major valve types within merely 40 years. In industrially‑developed countries, the application of ball valves keeps increasing year by year.
In China, ball valves are widely used in petroleum refining, long‑distance pipelines, chemical industry, papermaking, pharmaceutical industry, water conservancy, electric power, municipal engineering, iron and steel and other industries, occupying a pivotal position in the national economy. It performs a 90‑degree rotary motion; its plug body is a sphere with a circular through‑hole or passage running through its axis.
In pipelines, ball valves are mainly used for shut‑off, flow diversion and changing the flow direction of media. Tight shut‑off can be achieved only by a 90‑degree rotation with small operating torque. Ball valves are most suitable for on‑off and shut‑off service.
Classification and Features
Ball valves include: floating‑ball ball valves, fixed‑ball ball valves, orbit ball valves, V‑port ball valves, three‑way ball valves, stainless‑steel ball valves, cast‑steel ball valves, forged‑steel ball valves, ash‑discharge ball valves, sulphur‑resistant ball valves, pneumatic ball valves, electric ball valves, ferrule‑type ball valves and welded ball valves.
1. Floating‑ball Ball Valve
The sphere of the floating‑ball valve is floating. Under medium pressure, the sphere produces certain displacement and presses tightly against the sealing surface at the outlet side to realize outlet‑side sealing. The floating‑ball valve features simple structure and good sealing performance. Nevertheless, the full load exerted by the working medium on the sphere is transferred to the outlet sealing ring. Therefore, it is necessary to verify whether the sealing‑ring material can withstand the working load from the sphere and medium. This structure is widely adopted for medium‑ and low‑pressure ball valves.
2. Fixed‑ball Ball Valve
The sphere of the fixed‑ball valve is fixed and produces no displacement under pressure. Fixed‑ball valves are equipped with floating seats. Subjected to medium pressure, the valve seats move and press the sealing rings tightly against the sphere to ensure sealing. Bearings are normally fitted on the upper and lower shafts of the sphere, delivering low operating torque. This type is suitable for high‑pressure and large‑bore valves.
To reduce operating torque and improve sealing reliability of ball valves, oil‑sealed ball valves have been developed in recent years. Special lubricant is injected between sealing surfaces to form an oil film, which improves sealing performance and reduces operating torque. It is more applicable for high‑pressure, large‑bore ball valves.
3. Elastic‑ball Ball Valve
The sphere of the elastic‑ball valve is elastic. Both the sphere and valve‑seat sealing rings are made of metallic materials with high sealing specific pressure. Sealing cannot be achieved merely by medium pressure, and external force must be applied. Such valves are suitable for high‑temperature and high‑pressure media.
Elasticity of the elastic sphere is obtained by cutting an elastic groove at the lower part of the sphere’s inner wall. When closing the passage, the wedge‑shaped head of the valve stem expands the sphere and presses it against the valve seat for sealing. Before rotating the sphere, the wedge‑shaped head is released, and the sphere restores its original shape, creating a tiny gap between the sphere and the valve seat. This reduces friction on sealing surfaces and lowers operating torque.
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