Hello everyone, I'm new to this forum and this is my first post so go easy! I have an engineering problem which I am uncertain as to how to solve. I am trying to design a basic ballast system for pumping seawater. The idea is to use compressed air to evacuate the water through a series of pipes. By controlling the compressed air the pressure will be known at "inlet". So the following information is known: Inlet - Pressure = 6 bar - Flow Rate = 0.783 l/s Outlet - Pressure = 1 bar - Flow Rate = 0.783 l/s - Pipe Diameter = D What I desire to know is what diameter pipe I will require at outlet to permit the above flow rate. I realise the answer may not be simple but any suggestions of which approaches or formulas would be much appreciated! I have tried using Bernoulli's equation which hasn't come back with sensible results. One of the problems is that the Inlet surface area is unknown (and constantly changing). Thanks in advance for any input! Ian PS, I have attached an image to explain a little further.