In-Service

Module from the Piping Engineering Curriculum curriculum.

Module 10 — In-Service Piping Inspection (API 570) · Learning Objectives · 1. Classify piping circuits by consequence and apply API 570 Class 1/2/3 · 2. Set CML (condition monitoring location) density and inspection interval · 3. Compute corrosion rate (short-term, long-term) and remaining life · 4. Decide retirement thickness: greater of pressure design t_min or T/2 structural · 5. Prepare re-rate case per API 579 Level 1/2 when thinning approaches limit · API 570 Piping Classification & Inspection Frequency

Class	Consequence	Example	Thickness Interval	Ext Visual Interval	Notes
Class 1	High — flammable/toxic, high P/T	LPG, H2, H2S > 3% mol	5 yr or 1/2 RL	5 yr	RBI can extend
Class 2	Moderate	HC < 650 °F, amine, hot oil	10 yr or 1/2 RL	5 yr	Standard refinery
Class 3	Low — flammable off-plot, utilities	Low-P water, air, BFW	10 yr or 1/2 RL	10 yr	Lowest rigor
Class 4	Optional non-hazard	Drainage, open vent	Per owner	Per owner	Sometimes excluded
Corrosion Rate & Remaining Life Formulas
Short-term CR (in/yr) = (t_previous – t_actual) / (time between readings, yr). Long-term CR = (t_initial – t_actual) / (time since install, yr). Use the greater of short-term and long-term for remaining life: RL (yr) = (t_actual – t_min) / CR. Set next inspection at the lesser of: (a) max interval per Class and (b) RL/2.
CML Selection Guidance (API 570 §5.5)
Location	CML Rationale	Typical Count	Method	Notes	Ref
Straight runs	Baseline wall	1 per 50 ft	UT thickness	Include elbow upstream	API 570
Elbows	Erosion-corrosion hotspot	4 quadrants each	UT grid	Extrados + intrados	API 570
Tees & branches	Turbulence	1 on branch, 1 on run	UT grid	Downstream side	API 570
Reducers	Velocity change	1 upstream, 1 downstream	UT	Local dead zones	API 570
Deadlegs	NH4HS / corrosion	100% UT bank	UT grid or RT	Re-examine after TA	API 570 §7.1.5
Soil-air interface	External corrosion	1 per crossing	UT or coupon	Coatings fail	API 570 §6.4.4
Mixing points	Differential temp/chem	4 pts downstream	UT	Jet impingement	API 570
Injection points	Impingement wear	12D downstream	UT grid	Inject sleeve recommended	API 570 §5.5.2

Source: Piping_Engineering_Curriculum_v1.xlsx · Sheet: In-Service

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