Industrial Instrumentation and Control: Variable Area Flowmeters

Monday, October 15, 2018

Variable Area Flowmeters

Variable area flowmeters typically feature a vertically positioned measuring cone through which the medium flows from bottom to top, lifting against the weight. We also have the horizontal and inverted (top to bottom) versions that are used where the installation structure doesn't allow for vertical version.

We have two main types of variable area flowmeters:

Float type (Rotameter)
Tapered plug type

Float Type

The float is inside a tapered tube as shown below:

The fluid flows through the annular gap around the edge of the float. The friction causes a pressure drop over the float and the pressure forces the float upwards. Because the tube is tapered, the restriction is decreased as the float moves up. Eventually a level is reached where the restriction is just right to produce a pressure force that counteracts the weight of the float. The level of the float indicates the flow rate. If the flow changes, the float moves up or down to find a new balance position.

In case dangerous fluids are used, protection is needed against the tube fracturing. The tube may be made of non-magnetic metal. The float has a magnet on it, as it moves up and down; the magnet moves a follower and pointer on the outside. The position of the float may be measured electrically by building a movement transducer into the float.

You can also read:

Tapered Plug Type

This type has a tapered plug aligned inside a hole or orifice.

A spring holds it in place. The flow is restricted as it passes through the gap and a force is produced which moves the plug. Because it is tapered, the restriction changes and the plug takes a position where the pressure force just balances the spring force. The movement of the plug is transmitted with a magnet to an indicator on the outside.

Industrial Applications of Variable Area Flowmeters

Variable flowmeters are used in various industrial flow measurement applications, e.g.