The term fluid system as herein considered refers to any part of a building or unit or series of units of equipment which is related to fluid mechanics.
It may be a complete system such a water or ventilating system for a processing plant or a single unit such as a valve, filter, pump, or a length of pipe.
It must have definite boundaries. Consider Fig. 2.1. This hydraulic system consists of a pump, filter, valve, elbow, and connecting pipe and is only a small part of a complete system.
Points A and B are the boundaries that define the system under consideration. The simplest case is based on the assumption that all conditions are constant with time at each point in the system.
Fluids frequently flow With an irregular rate, that is, surge, under certain conditions. Situations where this condition must be recognized are few.
If the rate oi flow is constant at any point and there is no accumulation or depletion of fluid within the system, the mass rate of flow at any number of points Within the system must be constant since matter can be neither created nor destroyed.