Monday, November 17, 2014

Control Valve Flow Characteristics

control valve
Valtek ControlValve
Flow characteristics, the relationship between flow coefficient and valve stroke, has been a subject of considerable debate. Many valve types, such as butterfly, eccentric disk and ball valves, have an inherent characteristic which cannot be changed (except with characterizable positioner cams). Flow characteristics of globe valves can be determined by the shape of the plug head.
control valve flow
Control Valve Flow
Characteristics

The three most common types of flow characteristics are quick opening, equal percentage and linear. The adjacent figure shows the ideal characteristic curve for each. These characteristics can be approximated by contouring the plug. However, inasmuch as there are body effects and other uncontrollable factors, plus the need for maximizing the flow capacity for a particular valve, the real curves often deviate considerably from these ideals. When a constant pressure drop is maintained across the valve, the characteristic of the valve alone controls the flow; this characteristic is referred to as the “inherent flow characteristic.” “Installed characteristics” include both the valve and pipeline effects. The difference can best be understood by examining an entire system.

Equal Percentage

Equal percentage is the characteristic most commonly used in process control. The change in flow per unit of valve stroke is directly proportional to the flow occurring just before the change is made. While the flow characteristic of the valve itself may be equal percentage, most control loops will produce an installed characteristic approaching linear when the overall system pressure drop is large relative to that across the valve.

Linear

An inherently linear characteristic produces equal changes in flow per unit of valve stroke regardless of plug position. Linear plugs are used on those systems where the valve pressure drop is a major portion of the total system pressure drop.

Quick Open

Quick open plugs are used for on-off applications designed to produce maximum flow quickly.

This information provided courtesy of Flowserve Valtek. Further technical information can be found at this link.

Thursday, November 13, 2014

Unique Design Rotary Ball Control Valve Provides Superior Control

This rotary ball control valve video (courtesy of MOGAS) provides an informative look at the flow conditions inside a valve designed with a tortuous flow path and specialized characterization.

The Flexstream rotary control valve uses a standard ball valve body with a very specialized ball. As the ball opens, the video shows the tortuous path. The design allows for volumetric expansion when controlling gas. Velocity, noise and cavitation are controlled by repeated 90 degree turns in the flow path. This design is excellent for applications that require wide range-ability.

Friday, November 7, 2014

Torque and Limit Switch Considerations When Applying Electric Valve Acuators

When selecting an electric actuator for industrial valves, an understanding of the valve torque requirements, both at start of travel, and end of travel, is critical for proper operation and optimal service life. Additionally, the ability to accurately set, or limit, disk travel is required.

Many of today's industrial electric valve actuators provide easy setting or programming of travel and torque sensing.

This short video provides a brief visual understanding of the reasons why valve torque and travel limits are important.