Ball valves are the most common type of valve used in the process industry. While frequently used for small line flow control and commodity applications, ball valves can also be large in size as well. Ball valves characterized by a spherical shell of metal that is rotated in and out of the flow path. In this article, you will learn about the different types of ball valves, common applications, and repair and maintenance methods.
Ball Valve Types
Ball valves are categorized by the following attributes: material of construction, housing, port size, mounting type, number of ports, materials, and actuation method.
Material of Construction
When specifying a ball valve for service, the material of construction is an important consideration. The body of the valve will typically be made from either iron, steel, bronze, or plastic. Iron is compatible with low pressures, but the added ductility of steel becomes important for higher pressures. Bronze ball valve bodies are seen in light industrial applications such as HVAC, air, and water service. Plastic bodies are typical for utility applications as well as low pressure processes that demand plastic’s compatibility features.
The ball and seats are called the trim of a ball valve. The trim is typically either hard metal or soft plastic. The proper trim for the application depends on the chemical compatibility with the process media as well as the pressure and temperature experienced by the media.
For general purpose applications with clean fluid, soft plastic trims are suitable. Dirty process fluids are not recommended for soft plastic trims as the plastic can be damaged by entrained debris. If the application has a high erosion or corrosion rate, a metal seated ball valve is recommended.
Soft plastic seats possess superior shut-off capabilities versus their hard metal seat counterparts. Soft seat ball valves have a lower operating torque requirement than their metal seated counterparts, which influences actuator sizing and power requirements.
This superior performance of soft seats in typical process fluids as well as their lower cost, and superior shut-off capacity makes soft trims the preferred material of construction. Metal trim should only be specified in high temperature, corrosive, or dirty media service.
There are three types of housing assemblies: one-piece, two-piece, and three-piece. The type of housing that is chosen depends on the application’s demands. A one-piece housing is the most economical variant as the valve is welded together and cannot be opened after placed into service. Two-piece valves can be taken apart for cleaning and inspection. The valve must be completely removed from the pipe. The most expensive valves are three-piece and connected and clamped together via bolts.
Port size is either reduced bore or full bore, also referred to as reduced port and full port. Reduced bore is the standard and a more economical way to produce a ball valve. Reduced bore construction results in a slightly reduced bore size compared to the pipe to which it is connected.
Full bore valves ensure the pipe system bore size remains constant throughout a pipe run. This design is a good application for the following situations: a pigging device is required, flow turbulence must be minimized, or pressure drop is a concern. Full bore valves are more expensive on the basis of their larger size than their reduced bore counterparts.
The mounting type refers to the construction details of the valve. The ball design can either be trunnion or a floating ball valve.
The floating ball valve is secured by an upper stem only. Thus, the ball can move slightly in a vertical direction. A trunnion ball valve ball has a stem at the bottom which impedes ball movement completely and blocks flow in both directions. Floating ball valves, due to their simplicity of construction, are more common. This is particularly in low and medium pressure applications. The additional ruggedness of trunnion valves is a fit for high-pressure applications and when control of bi-directional flow is required.
Number of Ports
The number of ports (openings that can accept flow) can be two, three, or four. The most common amount of ports is two. This equates to a two-way valve, which fits into a pipe with a singular flow path.
Three-way ball valves can consolidate, divert, or block flows in several ways. The ball valve can either be in a T or L flow path, depending on how they are configured in the piping.
Four-way valves allow for flows to be diverted, each in an L type path. The possibility of a three or four-way mixing of the fluids does not exist due to the flow design. Valve shut-off is accomplished by a 45-degree turn of the handle/actuator.
Turning of the ball may be a done through a physical handle that is manually turned, or pneumatically/electrically actuated. A mechanical handle valve allows for direct operator control. Pneumatic or electrical actuators are a good fit for when large opening torque is needed or manual control is not needed or practical.
Ball valves are easy to operate and tend to have a long service life. Their production cost is low and they can withstand high pressure, velocity, and temperature flows. The force required for valve actuation is lower than gate and globe valves. These characteristics make ball valves a common process control choice in standard industrial applications.
Ball valves are not able to be cleaned easily. This reduces their use in medical, and food and beverage applications. They are also not a good choice for sustained throttling applications.
Maintenance and Repair Considerations
Ball valves are frequently replaced instead of repaired. This depends on the housing construction method. Single-piece housing are not able to be maintenance and are thus considered disposable.
In the instance where the two or three-piece housing of a ball valve can be repaired, a kit containing the ball, seats, bearings, and o-rings is typically administered by a technician directly. Depending on process conditions encountered by the ball valve, occasional retightening of the seat packing may be required.