Pipe Sizing

Chapter from the Piping Design Guide.

PIPE SIZE SELECTION METHODOLOGY

Recommended Velocity Limits by Service

Service	Min Velocity	Max Velocity	Economic Velocity	Erosion Limit	Notes	Pipe Material
Liquid — general	3 ft/s	10 ft/s	5-7 ft/s	15 ft/s for CS	Avoid low velocity (sedimentation)	Carbon steel
Liquid — pump suction	1 ft/s	5 ft/s	2-4 ft/s	N/A	Low velocity critical for NPSH	Carbon steel
Liquid — pump discharge	5 ft/s	15 ft/s	7-10 ft/s	15 ft/s	Higher velocity OK downstream	Carbon steel
Liquid — gravity drain	1 ft/s	3 ft/s	1.5-2 ft/s	N/A	Partial flow — size for self-venting	Carbon steel
Steam — saturated	50 ft/s	100 ft/s	60-80 ft/s	150 ft/s	ΔP drives sizing for long runs	Carbon steel
Steam — superheated	80 ft/s	175 ft/s	100-150 ft/s	200 ft/s	Higher velocity acceptable	Alloy per temp
Gas / vapor — general	30 ft/s	80 ft/s	40-60 ft/s	100 ft/s	Erosion limit with solids lower	Carbon steel
Gas — compressor suction	30 ft/s	60 ft/s	40-50 ft/s	N/A	Minimize ΔP for compressor	Carbon steel
Gas — compressor discharge	40 ft/s	100 ft/s	60-80 ft/s	Erosion velocity	Higher P allows higher velocity	CS or alloy
Two-phase	15 ft/s	60 ft/s	20-40 ft/s	Erosion calc req'd	API RP 14E for erosional velocity	Service-dependent
Flare header	0.2 Mach	0.5 Mach	0.3-0.4 Mach	Mach 0.7 absolute max	Sonic velocity = noise + vibration	Carbon steel
Relief valve inlet	N/A	Mach 0.5	N/A	N/A	ΔP < 3% set pressure	Carbon steel
Relief valve outlet	N/A	Mach 0.8	N/A	N/A	Reaction force consideration	Carbon steel

Pressure Drop Guidelines

Service	Max ΔP/100 ft	Total System ΔP Limit	Basis	Notes
Process liquid (general)	0.5-2.0 psi	Per system hydraulics	Economics	Balance pipe cost vs. pump energy
Pump suction	0.1-0.5 psi	Keep NPSHA adequate	NPSH	Critical — minimize friction loss
Cooling water	0.5-1.5 psi	Per pump head available	System design	Long runs may need larger pipe
Steam (100 psig)	0.5-1.0 psi	5-10% of supply P	Economics	Excessive ΔP = low delivery P
Process gas / vapor	0.1-0.5 psi	Per compressor ΔP budget	Compressor power	Every psi of ΔP = more HP
Gravity drain	Elevation-driven	Must be self-draining	Slope ≥ 1/8"/ft	Partial full, ensure venting

Source: Piping_Design_Guide_v1.xlsx · sheet “Pipe Sizing”

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