Flow Control Valves

Flow control valves forms an important component of any industrial instrumentation system and control system. When a change in a measured variable with respect to a reference has been sensed, it is necessary to apply a control signal to an actuator to make the corrections to an input controlled variable for example via a valve bringing the measured variable back to its preset value.

In most cases, any change in the variables e.g. Temperature, Pressure, and Level) can be corrected by controlling the flow rates. The actuators used to control valves for flow rate control can be electrically, pneumatically or hydraulically controlled.

Actuators can be self-operating in local feedback loops. The two most common types of variable devices used for flow control are the globe valves and the butterfly valve.

Globe Valve

Let’s consider the following diagram:

The actuator controlling the valve can be controlled or driven electrically, pneumatically or hydraulically.

The actuator determines the speed of travel and the distance that the valve shaft travels.

We have various Globe valve configurations. The globe valve can be designed for quick opening operation, for equal percentage operation or with a linear relationship between flow and lift or any combination of these. We also have other configurations like two-way valve (diverging type) and three-way valve.

The selection of the type of control plug depends on a careful analysis of the process characteristics. If the load changes are linear, then a linear plug should be used; conversely, if the load changes are nonlinear, then a plug with the appropriate nonlinear characteristics should be use.

Other types of Globe valves include: Needle valve, which has a diameter of 1/8 to 1 inch size. The balanced cage-guided valve and the split body valve.

Note: Globe valves aren’t suitable for use with slurries.

You can also read: 

• The Ultimate Guide to Industrial Flow Instruments
• Key Elements that make up a Control Loop

Butterfly Valve

A butterfly valve consists of a cylindrical body with a disk the same size as the internal diameter of the valve body, mounted on a shaft that rotates perpendicular to the axis of the body.

The relationship between flow and lift is approximately 50 % open, after which it is linear.

Butterfly valves offer high capacity at low cost. They are also simple in design, are easy to install and have tight closure. The torsion force on the shaft increases until the valve is open 70° then reverses.

Butterfly valves have a limited pressure range, and are not used for slurries.

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