These simple but important flow meters are often called sight flow indicators and are components of industrial process lines that require an instant and inside look of the flow inside the pipe. Unlike ultrasonic flow meters or vortex flow meters, flow indicators do not take calculations or measurements regarding mass or speed.
Flow indicators are the type of flow meters that monitor fluid clarity, level, and color through a glass or plastic observation window of varying size. The window can be horizontally or vertically oriented. The body of the flow indicator may be plastic or metal. Double windows held in position by a metal housing and gaskets are used in applications where high-pressure, corrosion, or mechanical impact may be a threat.
Flow indicators are attached to the piping system by threaded, flanged, or butt-welded connection, all of which must be waterproof. For low visibility, flow indicators may contain lights. Visual indicators such as flappers or propellers indicate the direction of the stream although the material type may interfere with such accessories. Flow indicators are used mostly in industrial and manufacturing applications where flowing liquids are part of an important process and need to be monitored.
There are various types of flowmeters and flow indicators that are used depending on the type of liquid and what is being monitored.
Plain sight indicators are the simplest option, containing only an observation window. No moving parts are used, and this type is only for easily visible fluids.
Visual indicators use floating balls, flappers, and propellers. The floating balls indicate the liquid level in hard-to-see applications as an alternative to lights. Flappers are used when the flow of the liquid could be going in two different ways. It lets the observer know the flow direction. Propellers are used for clear or transparent liquids to determine whether or not they are flowing.
Another kind of flow indicator is a drip tube where the liquid drips into the tube in systems with a downward flow.
Some flow indicators have a self-cleaning mechanism consisting of a rotor and wiper. When the observation window is twisted, wipers inside the pipe clean the plastic or glass viewing area to allow for an unobstructed view. The interfering materials on the window are wiped off and carried away with the stream. This method does not require any downtime and is often necessary when the pipes contain suspended solids, slurries or highly viscous materials.