Linear and Quarter-turn Industrial Valves

Various valves are designed and used for multiple roles in process control. Linear and quarter-turn valves are two types of valves used to regulate and control fluid flow in the industry. Their design and construction reflect the intended use of the valves, with each suited to a different class of service. 

All valves work by controlling the position of an internal structure that obstructs fluid passage to some extent. In general, fluid flow at the valve classifies as unrestricted (valve fully open), stopped (valve fully closed), or throttled (valve partially open). The operational requirements of the process will determine whether only two of those conditions (fully open and fully closed) or all three are required. When choosing an appropriate valve, the fluid, the process, and the surrounding environment must be considered. It is not always a simple task. 

Linear valves distinguish themselves using straight-line motion to position the valve plug, disc, diaphragm, or other flow controlling elements. The linear valve trim's shape, size, and arrangement provide the operator with a flow range through the valve. The linear valve's positioning allows it to regulate fluid flow slower but more accurately. Linear motion valves include gate and fixed cone valves—linear valves best suit flow control.

Quarter turn valves move from fully open to closed by rotating a shaft connected to the controlling element 90 degrees. Their relatively simple operation allows for a rugged and compact design. The ability of quarter-turn valves to quickly reposition from open to closed positions is one of their distinguishing features. The torque required to operate the valves is typically low to moderate. Quarter turn valves include ball and butterfly valves. 

Depending on the situation, linear valves and quarter-turn valves are the best choices for specific process environments. The linear valve's accuracy and ability to move in a linear fashion rather than a quarter-turn come with easy maintenance and a lower likelihood of cavitation. Both valve types are widely used and are not competing for the same application. Each excels in a specific set of applications.

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Quarter Turn vs Linear Valves

This lined ball valve is an example of a
quarter turn valve.
Image courtesy Flowserve - Atomac

Different types of valves are designed and applied for different roles in the process control. Linear valves and quarter-turn valves are two different types of valves utilized throughout industry to regulate and control fluid flow. Their design and construction reflect the intent of the valves’ application, with each being suited for a different class of use.

All valves operate by providing control of the position of an internal structure that impedes fluid passage to some degree. Generally, fluid flow at the valve can be characterized as one of three conditions, unrestricted (valve fully open), stopped (valve fully closed), and throttled (valve partially open). Process operational requirements will dictate whether just two (fully open and fully closed) or all three of those conditions will be needed. Many aspects of the fluid, the process, and the surrounding environment come into play when making an appropriate valve selection. Not always an easy task.

Linear valves are generally characterized by their straight line motion that is used to position the valve plug, disc, diaphragm or other flow controlling element. The shape, size, and arrangement of the linear valve trim is generally intended to empower the operator with a range of flow through the valve. Through its positioning, the linear valve is able to regulate fluid flow at a slower, but more accurate rate. The valves can move a disk or a plug into an orifice, or push a flexible material, such as a diaphragm, into the flow passage. Gate valves and fixed cone valves are common examples of linear motion valves. Linear valves are best applied as flow controllers, and are often suited for frequent operation and repositioning.

Quarter turn valves traverse from fully open to fully closed by a 90 degree rotation of a shaft connected to the controlling element. Their comparatively simple operation allows for a design that is rugged and compact. One distinction of the quarter turn valves is their ability to quickly reposition from open to closed positions. Torque requirements to operate the valves are generally low to moderate. Ball and butterfly valves are examples of quarter turn valves.

Depending on the specific scenario, linear valves and quarter-turn valves are optimal choices for particular process environments. The accuracy of the linear valve and its ability to move in a linear fashion as opposed to a quarter-turn comes coupled with easy maintenance and decreased likelihood of cavitation. Both valve types enjoy widespread use and should generally not be viewed as competing designs for the same application. Each has a range of applications where it excels.

Share your fluid flow control challenges of all types with valve specialists, leveraging your own knowledge and experience with their product application expertise to develop effective solutions.

Electric Actuator for Linear and Quarter Turn Control Valves

Many process control valve installations present the option of selecting either electric or pneumatic actuators as part of the control component train. Pneumatic actuators have been in use for many years, but advances in electric motor design that delivered greater torque and more precise operation have brought electric valve actuators into a prominent market position.

Electric actuators are compact and comparatively self contained, requiring only cable connections and none of the additional devices sometimes needed for a pneumatic installation. There are some points of advantage to consider with electric actuators. Rotork introduced their CVA line of electric actuators almost ten years ago, making it something of a mature product now. Here are some advantageous points about the CVA actuators that likely apply generically as well.

• Setup is accomplished with a Bluetooth enabled device which provides quick calibration of open and closed positions, as well as establishment of valve setup parameters.
• A separately sealed electrical connection compartment keeps motor and mechanical compartment isolated from the environment while electrical connection section cover is removed.
• An on board datalogger records thrust and position data over time for use in asset management and service functions. Data can be downloaded by Bluetooth or transmitted by common protocol to another station.
• Change in setpoint produces a rapid and precise change in valve position with high resolution accuracy and repeatability.
• Actuator can be programmed to move to a preset condition in the event of a loss of electric power. The energy to achieve the failsafe position is stored in the actuator.
• Force balance positioning used in pneumatic valves, with spring force vs. air pressure, has resilience that can result in a change in position of the valve trim in response to a bump in system pressure. Resistance from the gear train on electric drives prevents this movement.
• Static friction of the valve packing and other parts increases the amount of force to intially get the valve moving toward a new position. The additional time required to build air pressure and force to overcome static friction results in delayed valve response, then overshoot of the new setpoint. A combination of a sensor system and the mechanical drive section of an electric actuator eliminates overshoot and delayed response.

Electric actuators can be had in quarter turn and linear versions, with torque ranges suitable for a broad range of process control applications. The datasheet below, from Rotork, provides useful illustrations of the actuator interior, along with additional detail about electric actuators. Share your process control valve requirements and challenges with product application specialists, combining your own process knowledge and experience with their product application expertise to develop the best solutions.

Electric Control Valve Actuators

CVQ Electric Valve Actuator
For quarter turn valves
Courtesy Rotork

Many process control valve installations present the option of selecting either electric or pneumatic actuators as part of the control component train. Pneumatic actuators have been in use for many years, but advances in electric motor design that delivered greater torque and more precise operation have brought electric valve actuators into a prominent market position.

Electric actuators are compact and comparatively self contained, requiring only cable connections and none of the additional devices sometimes needed for a pneumatic installation. There are some points of advantage to consider with electric actuators. Rotork introduced their CVA line of electric actuators almost ten years ago, making it something of a mature product now. Here are some advantageous points about the CVA actuators that likely apply generically as well.

• Setup is accomplished with a Bluetooth enabled device which provides quick calibration of open and closed positions, as well as establishment of valve setup parameters.
• A separately sealed electrical connection compartment keeps motor and mechanical compartment isolated from the environment while electrical connection section cover is removed.
• An on board datalogger records thrust and position data over time for use in asset management and service functions. Data can be downloaded by Bluetooth or transmitted by common protocol to another station.
• Change in setpoint produces a rapid and precise change in valve position with high resolution accuracy and repeatability.
• Actuator can be programmed to move to a preset condition in the event of a loss of electric power. The energy to achieve the failsafe position is stored in the actuator.
• Force balance positioning used in pneumatic valves, with spring force vs. air pressure, has resilience that can result in a change in position of the valve trim in response to a bump in system pressure. Resistance from the gear train on electric drives prevents this movement.
• Static friction of the valve packing and other parts increases the amount of force to intially get the valve moving toward a new position. The additional time required to build air pressure and force to overcome static friction results in delayed valve response, then overshoot of the new setpoint. A combination of a sensor system and the mechanical drive section of an electric actuator eliminates overshoot and delayed response.

Electric actuators can be had in quarter turn and linear versions, with torque ranges suitable for a broad range of process control applications. The datasheet below, from Rotork, provides useful illustrations of the actuator interior, along with additional detail about electric actuators. Share your process control valve requirements and challenges with product application specialists, combining your own process knowledge and experience with their product application expertise to develop the best solutions.

Electric Control Valve Actuators for Industrial Process Applications from CTi Controltech