Sizing

Module from the Piping Engineering Curriculum curriculum.

Module 3 — Pipe Sizing & Pressure Drop · Learning Objectives · 1. Apply rule-of-thumb velocity limits for liquid, gas, and two-phase service · 2. Calculate friction pressure drop using Darcy-Weisbach with Moody friction factor · 3. Avoid erosion-velocity issues per API RP 14E for two-phase flow (ρm·v² < 10,000) · 4. Select line size based on allowable ΔP, not just velocity · 5. Recognize slug-flow risk in horizontal two-phase lines and mitigation · Typical Velocity Limits (ft/s)

Service	Min	Economic	Max	Notes	Reference
Pump suction (liquid)	3	4–7	8	Avoid cavitation, NPSH	API 610
Pump discharge (liquid)	5	6–10	15	Economic diameter	IPE-EP-5-1-1
Gravity flow liquid	1	2–4	6	Siphon breaker at high point	—
Steam (saturated)	60	80–120	180	Avoid erosion of elbows	—
Steam (superheated)	80	120–180	250	Higher specific volume	—
Compressor suction (gas)	30	40–60	80	Low ΔP, protect compressor	API 617
Compressor discharge (gas)	50	60–100	150	Higher density	API 617
Flare header	—	0.5 Mach	0.7 Mach	Sonic velocity limit	API 521
Two-phase	—	ρm·v² ≤ 10000	API RP 14E	Erosion criterion	API RP 14E
Amine service	—	3–5	6	Prevent alkaline SCC	API RP 945
Economic Line Sizing — Typical Optimal Velocity
Total cost = Capital (pipe, fittings) + Operating (pumping ΔP). Economic optimum minimizes the sum. For most refinery services v_opt ≈ 5–10 ft/s for liquids and 60–100 ft/s for gas. Revalidate when: pumping power > 1 hp/1000 ft, or velocity > maximum limit.
Darcy-Weisbach Equation
ΔP (psi) = f · (L/D) · (ρ·v²/2gc) · 1/144 where f = Moody friction factor (from Re and ε/D), L = equivalent length (ft), D = ID (ft), ρ = density (lb/ft³), v = velocity (ft/s), gc = 32.174 lbm·ft/(lbf·s²). Use Churchill or Colebrook for f at turbulent Re. Add equivalent length L/D for fittings (90° ell = 30, gate valve open = 13).

Source: Piping_Engineering_Curriculum_v1.xlsx · Sheet: Sizing

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