Lesson 12 - Control of Fluid Flow 3
This lesson will consider valve technology, which is used to control fluid flow. It will cover the control functions of on-off, and modulating flow control, together with a description of the various valve types which can achieve these functions. The lesson will conclude with thoughts on the issue of managing valve leakage to avoid cross-stream contamination.
Valve Types – Butterfly Valve
A variety of valve types are used throughout the brewery, which have different flow rate characteristics.
We shall now examine each of these in more detail.
Butterfly valve
This is by far the most common valve in the brewery, whether at small or large commercial scale. The butterfly valve is simple, relatively low in cost, lightweight in construction (which reduces the need to provide additional support of the valve or pipeline), space saving, and has hygienic properties.
It consists of a circular disc which rotates on a spindle through a quarter-turn within the pipeline. The spindle is attached to an actuator, which can take the form of a simple handle or an electric motor if automated actuation is required.
The name ‘butterfly’ comes from the appearance of the circular disc on the spindle and the rotation action of the disc, which resembles the appearance and action of a butterfly’s wings.
If the disc sits parallel to the pipeline, then the valve is fully open. However, if the disc is rotated at 90° to the pipeline, i.e. a quarter-turn, then the edge of the disc seals against the inner pipe wall to shut off the flow. The figure below shows the details of the valve design and the view of the disc in various positions.
Figure: Butterfly valve
The butterfly valve is easy to install and maintain. Tight shut off can be obtained by the inclusion of a soft ring in the body, to ensure no leakage of liquids through the closed valve. However, ball valves provide a more secure isolation (these are discussed on the next page). The butterfly valve provides high flow capacities with low pressure losses through the valve (this is due to the hidden profile of the valve when it is open. The above figure shows how the open valve appears almost as a line.).
The standard butterfly valve has poor flow rate characteristics for controlling the flow rate, which can be improved by modifying the disc shape and offsetting the valve spindle from the normal central position. Because of this, the butterfly valve is often used as a shut-off valve and also as a modulating flow control valve.