There is no doubt that standard control valves can handle a wide range of control applications. Certainly, corrosiveness and viscosity of the liquid, leakage rates, and many other factors demand consideration even for standard applications. In addition to the standard ones, there are also many special kinds which can be certainly used for special applications.
High capacity control valve is one of important ones. Generally, globe-style valves larger than 12-inch, ball valves more than 24-inch and high performance butterfly valves larger than 48-inch all belong to the special valve category. As valve sizes increase arithmetically, static pressure loads at shutoff increase geometrically. Consequently, shaft strength, bearing loads, unbalance forces, and available actuator thrust all become more significant with increasing valve size.
Normally maximum allowable pressure drop is reduced on large valves to keep design and actuator requirements within reasonable limits. Even with lowered working pressure ratings, the stream capacity of some large-stream valves remains tremendous. Naturally, actuator requirements are severe, and long-stroke, double acting pneumatic pistons Van bi điều khiển khí nén is typically specified for large-stream applications. The physical size and weight of the valve and actuator components complicate installation and maintenance procedures. Installation of the valve body assembly into the pipeline and removal and replacement of major trim parts require heavy-duty hoists.
High-temperature control valves are other special valve kinds. Those valves which are used for service at temperatures above 450°F (232°C) must be designed and specified with the temperature conditions in mind. At elevated temperatures, such as may be encountered in kettle feed water systems and super-heater bypass systems, the standard materials of control valve construction might be inadequate. For instance, plastics and standard gaskets generally demonstrate unsuitable and must be replaced by more durable materials. Metal-to-metal seating materials are always used. Semi-metallic or laminated adaptable graphite packing materials are also commonly used.
Except for the above mentioned two types, low stream control valves also deserve our attention. Many applications exist where control of extremely low stream rates is required. In addition to the extremely low flows, these specialty control valves are compact and light weight because they are often used in laboratory environments where exceptionally light schedule piping/tubing is used. These types of control valves are specially designed for the accurate control of exceptionally low flowing fluid or gaseous liquid applications.