# Rotating Positive Displacement (PD) Pump

Static Pressure below fluid vapor pressure or negative: This warning may occur if you have modeled a PD pump in series with a Known Flow node. By using these two nodes in series we are over constraining the model. The calculated flow rate developed by the PD pump is a function of engineering conditions (pipe diameter, length, etc.) and pump speed and cannot be manipulated by designating a Known Flow node. It is therefore best to model this scenario with known pressure nodes at the system boundaries and adjust the pump speed to reach the desired flow rate. If any of the above is unclear or you require any further information, please contact us at: support@fluidflowinfo.com.