**Pipe flow**is studied under hydraulics and fluid mechanics which is a type of liquid or water flow takes place within a closed conduit. The type of flow in an opened conduit is called open channel flow.

Pipe flow does not have a free surface so that there is no direct effect of atmospheric pressure and other aspects. But open channel flow is directly affected by the atmosphere. Due to closeness, pipe flow exerts hydraulic pressure on the conduit.

All flow within a closed conduit cannot be considered as pipe flow. Storm sewers or Storm Drain are closed conduits, but it is considered as open channel flow. It is because of free surface present in flow and influenced by atmospheric pressure.If a Storm Sewer runs with full capacity, then it can become a pipe flow.

The Energy of liquid flow in the pipe is expressed in the head. Head can be represented by using the Bernoulli's equation.

Potential head = Z

Kinetic Head= V^2/2g

Pressure Head= H = P/Y

The sum of these heads always be conserved but the small head loss may occur.

i.e. Z1+V1^2/2g + P1/Y = Z2+ V2^2/2g + P2/Y + Headloss

Head losses in pipe flow are of different types, the major one is the frictional loss which is defined by Darcy-Weisbach formula.

Head can be represented in diagrams by Hydraulic Grade Line (HGL) or Total Energy Line (TEL).

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