Pipe Friction and Pressure Drop

Natural gas traveling through a 1,000-mile pipeline loses about 10-15% of its pressure to friction with the pipe walls. Compressor stations every 50-100 miles re-pressurize the gas to keep it moving. Without them, the gas would stop. Friction is the tax that every pipeline pays on every cubic foot delivered.

When a fluid (gas or liquid) moves through a pipe, it rubs against the inner walls. This friction converts kinetic energy into heat, reducing the fluid's pressure. The pressure drop depends on four factors: pipe length (longer = more friction), pipe diameter (smaller = more friction per unit of flow), flow velocity (faster = more friction), and the roughness of the inner wall surface.

For energy systems, pipe friction has direct economic consequences. Oil pipelines lose about 1-3% of throughput energy to pumping (overcoming friction). Natural gas pipelines use 2-5% of the gas they carry as fuel for compressor stations. Longer pipelines and smaller diameters cost more to operate because they require more compression energy.

Worked Example

A natural gas pipeline carries 1 billion cubic feet per day. Compressor stations consume 3% of the gas as fuel for re-pressurization.

• Calculate gas consumed. 1,000,000,000 x 0.03 = 30,000,000 cubic feet/day consumed by compressors.

Which of the four factors gives engineers the most leverage over friction losses?

Diameter. Length is fixed by geography, roughness is fixed by pipe material, and velocity is capped by erosion limits. Diameter is the one variable engineers can still choose, and its effect on friction is nonlinear enough to reshape an entire project's economics, a relationship the next lesson works out in full.

Pipe friction is the physical constraint that determines pipeline routing, compressor spacing, and the economics of energy transport.

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