Spring loaded variable area Flowmeter

 

In describing this type of flowmeter there are two fundamental principles of flow metering which must be remembered

1. The differential pressure flowmeter produces a pressure difference which varies as the rate of flow through the flowmeter. As the rate of flow increases, so does the differential pressure. By measuring this pressure difference it is possible to calculate the rate of flow through the flowmeter. The area of constriction remains constant.
2. In the variable area flowmeter the reverse effect occurs because it is balanced by the weight of the float, the differential pressure remains constant as the flowrate varies. The area of the aperture through which the flow passes is allowed to increase.

The spring loaded variable area principle is in effect a hybrid between these two. A spring is used to press together the two members forming the variable aperture so that, as the flow increases both the pass area and the force tending to close the aperture- and hence differential pressure across it must increase together. In mathematical terms it means that such a flowmeter has two degrees of freedom, resulting in a most important benefit or very good turndown

Advantages:

The general advantage of spring loaded variable area flowmeter that they are robust, provide a high turndown with good accuracy and less sensitive to viscosity changes.

Disadvantages:

The only disadvantage of this type of flowmeter is that it can be susceptible to wear after exceptionally long periods when used with extremely wet dirty steam. Proper installation techniques utilizing strainers and separators will correct this.