Choosing the Right Valve: Gate Valves vs. Ball Valves vs. Globe Valves
Gate, ball, and globe valves are used extensively in isolation (on/off) and throttling (flow control) applications across residential, commercial, municipal, and industrial settings. Selecting the right valve for the intended use is oftentimes critical, as the results of misapplication can have catastrophic impacts in terms of safety, budget, and the environment.
Numerous variables require careful consideration when choosing the right valve for a given application, but generally, the selection process seeks to match valve functionality with its intended purpose. For basic on/off applications, the selection process can often be relatively simple, but the process becomes increasingly complicated as valve function becomes more complex and applications increase in severity.
An array of conditions exist that may affect the choice of a valve for a given application, including:
- System pressure
- Flow conditions (Cv/flow rate)
- Media (liquid, gas, steam)
- Media properties (corrosive fluids, lethal chemicals, costly fluids, etc.)
The intended purpose for the valve is also a key variable. For instance, the selection process for the valve used to flow water into a municipal building differs significantly from that for the valve used to control high-pressure steam in a power plant, or a valve used in a highly corrosive chemical application. Consider the following during valve selection:
- Is the valve intended to isolate flow?
- Is it intended to control flow?
- Is it required to be multi-functional (e.g. throttle and isolate, isolate and prevent backflow)?
- What degree of seat tightness (sealing integrity) is required for the application?
Valves are designed to perform specific functions, including:
- Isolation (on/off)
- Controlling flow (throttling)
- Preventing reverse flow
- Diverting flow
- Overpressure protection
Most isolation and throttling valves will consist of a valve body that houses the sealing surfaces (seats) and provides end connections for mating pipes or vessels. The body will have a mating bonnet (cover) and a yoke to house the stem drive mechanism, though in some designs, the bonnet and yoke are incorporated into one piece. Valves are supplied with an obturator designed to be placed in the flow path (driven by the stem) for sealing or regulating flow, such as a:
Valves may be automated utilizing electric, pneumatic, or hydraulic actuators to effect valve stroking, both for sealing and for regulating flow.
Gate valves are utilized to isolate flow. They use an obturator inserted into the flow path via a drive mechanism, employing a threaded stem and a handwheel designed to provide a minimum torque required to seal the obturator against the adjoining seats. In addition to manual operation, a host of options exist featuring electric, pneumatic, or hydraulic actuation to perform the closing function.
Gate valves are often referred to as multi-turn valves, as the handwheel must be repeatedly turned 360 degrees as the valve is opened or closed. They are provided in reduced and full port configurations, and in wedge gate and parallel slide designs. While they seal well, Wedge gate valves are susceptible to thermal binding in applications exceeding 800°F. Above this temperature, parallel slide gate valves are recommended. Gate valves 3 inches and under are typically supplied with forged bodies, while larger diameter valves are typically supplied with cast bodies. A number of manufacturers also supply large diameter, high pressure forged gates.
Low-pressure (ASME 150-600#), cast steel gate valves are readily available through valve distribution organizations, and are used in a wide variety of general purpose applications across industries, including process applications, pipelines, water distribution, and steam distribution.
Ball valves are generally used to isolate flow. They employ a ball as an obturator with a flow path through the ball matching the bore diameter at the seating surfaces. Available in reduced, full, and “V” port (for controlling flow) configurations, ball valves are supplied in a wide variety of styles, including one piece, two piece, three piece, top entry, and welded. Ball types include a floating ball design (a ball held in place by the seats and stem) or a trunnion-mounted design which mechanically holds the ball in place at its top and bottom.
Considered quarter-turn valves, ball valves go from fully open to fully closed with a 90 degree movement of the handwheel/lever connected to a shaft, which makes them easier to operate and typically less expensive to actuate. A wide range of electric, pneumatic, and hydraulic actuators may be incorporated to automate ball valves.
Low-pressure ball valves are readily available and used in a wide variety of industries and applications, including
pipeline, water distribution, steam distribution, chemical, refineries, and petrochemical. Standard ball valves may be used only for very light throttling, except those intended for control and supplied with a “V” port style ball.
Globe valves are utilized to isolate flow or to control (throttle) flow in a piping system. They feature a globe or disc obturator driven into the single body seat via a drive mechanism. This mechanism employs a threaded stem and a handwheel that are designed to provide a minimum torque required for sealing.
The single-seated globe valve is capable of both throttling and isolation services. As the valve closes, flow is evenly distributed around the full circumference of the sealing surfaces, resulting in even wear. This is not the case in gate valves or ball valves. The flow path through the globe valve is often referred to as tortuous, as flow must come up (or down) through the single seat. This flow path creates a higher pressure drop and lower Cv compared to gate or ball valves.
Like gate valves, globe valves are multi-turn valves. There are a wide range of options for electric, pneumatic, or hydraulic actuation to perform the closing function. Globe valve designs include y-pattern, T-Pattern, and angle pattern, based on the orientation of the stem to the flow path. Each of these may be used for on/off and throttling services, but the angle-pattern globe is typically utilized where right angles exist in the piping and where higher degrees of throttling are required (i.e., continuous blowdown).
These valves are typically sized for the flow conditions found within their intended critical application. The flow path for Y-pattern valves is less tortuous than in T- and angle-pattern valves, providing for a marginally higher Cv. Y-pattern valves are used extensively in vent and drain applications found in power plants and other similar applications.
Ball Valves vs. Gate and Globe Valves
Valve manufacturers and process owners/users can now design and specify ball valves for non-throttling applications typically reserved for gate and globe valves thanks to the tighter tolerance capabilities of modern CNC machining equipment, as well as advances in the development of extremely hard coating materials and the processes to apply them. The smaller relative footprint and weight of ball valves versus gate and globe valves reduces stresses in piping systems, while quarter-turn stroking facilitates easier operation and reduces automation costs. These factors may also provide longer service life in some severe-service applications. Ball valves can sometimes be more costly than their gate and globe counterparts, but in many instances, these advantages may compensate for the difference.
Conval is a premier manufacturer of high-quality forged steel valves used in high-pressure, high-temperature, and severe-service applications. We offer fast and on-time delivery of severe-service flow control products that exceed expectations. Valve solutions from Conval offer numerous benefits to users, including:
- Excellent life cycle value
- Easy maintenance
- High performance
- Reduced plant downtime
- In-line renewability/repairability
To see how long-lasting, severe-service valves from Conval can improve your operation, please don’t hesitate to contact us today.