A graphical method is used to specify the volume of an equalization vessel. A plot of flow rate versus time gets a horizontal line for the average flow rate. All flows above this line are integrated to get the fluctuating volume for the vessel. This is shown in the next sketch.
The vessel volume should equal the average flow multiplied by the time period, and then add on the fluctuating volume. Note that when the flow variation is small, the fluctuating volume is small so that the required vessel size is close to that based on average flow. When the variation is great, the equalization vessel is quite a bit larger than that based on the average flow.