The Three Factors that Affect the Flow through a Flow Control

There is more to consider than the adjustment of an orifice when determining flow. There are actually three factors that affect the flow through a flow control:

  1. The size of the orifice
  2. The pressure difference between the inlet and the outlet of the flow control
  3. The temperature of the fluid

Size of the Orifice


Figure 1

Some flow controls have a fixed orifice size.  Figure 1 shows the symbol of a fixed orifice flow control.  This may be a proper component that can be removed and replaced or it may be simply a drop in line size or a smaller flow path drilled through a manifold.  If it is removable, it should never be removed and replaced by a variable orifice flow control because the designer put it there with something specific in mind.


Figure 2

The variable orifice flow control has an angled arrow drawn through the symbol to indicate that it is variable or adjustable in some fashion (Figure 2).  The amount of flow through the orifice can be varied by opening or closing the adjustment.  The larger the orifice size, the more flow will pass.

Pressure Difference across the Flow Control


Figure 3

The second variable is the pressure drop across the flow control.  In Figure 3 two gauges are added – one upstream and one downstream of the flow control.  The greater the difference between the two gauges, the more fluid is forced through the orifice.

Temperature of the Fluid

The viscosity of the fluid is inversely proportional to the temperature.  Thus, the higher the temperature the thinner the fluid and the more will pass through the orifice.

An increase in any of these variables will result in an increase in flow and therefore an increase in the speed of the actuator that is moved by the fluid.