Bernouilli's Equation relates the velocity, height and pressure of an ideal fluid in a pipe.
P is the absolute pressure of the fluid in the pipe.
Whenever a real fluid flows in a pipe there is a viscous drag opposing motion. The force per unit area needed to move the fluid through the pipe is directly proportional to the velocity gradient produced in the fluid. The nature of the flow depends on the density, viscosity and the diameter of the pipe. If too great laminar flow turns into turulence. The flow is laminar only if Reynolds' number is less than 2000.