System Sizing

To ensure that a priming system provides the desired performance and trouble free operation, it must be appropriately selected based on specific project conditions and goals.  The primary sizing factors that affect performance are described below, but rest assured that an experienced Q-VAC engineer is available to review each project and ensure proper sizing.

Suction Volume

In order to calculate the vacuum pump performance required to prime a system, we must first know the total air volume that needs to be filled with water.  This is determined by calculating the total non-flooded suction piping volume, plus adding in the displacement volume of each pump that will be connected.

To calculate the pipe volume (ft3) you can use the formula below:

V = R2 x L x 3.14

R = Pipe Radius (inches)

L = Total inlet pipe length (feet)

Lift Requirement

To determine the minimum vacuum level required to create and maintain prime, we must know the difference in elevation from the water source to the top of the pump’s volute.  For sizing purposes, the worst-case scenario (e.g. lowest predicted water level) should be provided.

The maximum theoretical height that water can be lifted is 33.89 feet at sea level.  In practice, the lift will be somewhat lower.  Vacuum lifting capability is also affected by the local barometric pressure, the specific gravity of the fluid being lifted, and the maximum vacuum rating of the vacuum pump.

Fact:  It requires 0.8826” HgV to lift water 1 foot.


How quickly do you need your pumps primed?  Most customers specify an initial prime time of 1 minute to 10 minutes, however faster or slower prime times can often be accommodated.  Once primed, a Q-VAC automatic priming system ensures that every pump stays fully primed and ready to run.

Optimum Operating Range

Every vacuum pump technology is designed to operate within a specific pressure range.  Routine use outside of this intended range can lead to poor performance, increased maintenance, and shortened life.  Regardless of the vacuum technology that is chosen, every Q-VAC system is designed to operate in an ideal performance range to optimize performance, reduce maintenance, and ensure the longest life.

Priming Valves

The shape and size of an orifice determines the amount of air that can flow through it at any given pressure level.  Like all valves, priming valves have flow limitations due to internal orifices and connection sizes that can negatively impact priming if they are restrictive.

When determining which priming valve should be selected for an application, the size and number of priming valves that are needed will impact the valve selection.