Static Equipment QC Inspector Interview Questions: API 510 Guide

Static equipment — pressure vessels, tanks, heat exchangers, and columns — is the backbone of oil and gas facilities. Static equipment QC inspectors are in high demand, and interviewers test your knowledge of API 510, API 653, ASME Section VIII, NDT, and in-service inspection. This guide covers 40 questions with detailed answers.

---

API 510 Fundamentals

1. What is API 510, and what does it govern?

API 510 is the Pressure Vessel Inspection Code — it governs the in-service inspection, repair, alteration, and rerating of pressure vessels. It covers:

• Inspection intervals — external, internal, on-stream
• Corrosion monitoring — thickness measurement, CMLs
• NDT requirements — UT, MT, PT, RT
• Repair procedures — weld repair, plate replacement
• Rerating — changing design pressure/temperature
• Pressure relief devices — inspection and testing

2. What is the difference between API 510 and ASME Section VIII?

• ASME Section VIII: Governs the design and construction (fabrication) of new pressure vessels.
• API 510: Governs the in-service inspection, repair, and alteration of vessels already in operation.

Section VIII is for the manufacturer; API 510 is for the inspector in the operating facility.

3. What are the API 510 inspection types?

Inspection	Frequency	What's Checked
External	Every 5 years (or RBI)	Visual, leaks, corrosion, insulation, supports, grounding
Internal	Every 10 years (or RBI)	Shell thickness, welds, internals, corrosion, deposits
On-stream	Per RBI or spec	UT thickness, CMLs, while vessel is in service
Pressure relief	Every 5 years (or per spec)	Pop test, set pressure, seal

4. What is a CML (Condition Monitoring Location)?

A CML is a specific point on a vessel where thickness is measured periodically to monitor corrosion. Per API 510:

• CMLs are selected by the inspector based on service and corrosion rate
• UT thickness is taken at each CML during internal inspections
• Corrosion rate is calculated from the thickness history
• Remaining life = (current thickness − minimum) / corrosion rate

CMLs are the backbone of corrosion monitoring — without them, you can't predict remaining life.

---

Thickness Measurement and Corrosion

5. How do you calculate the remaining life of a vessel?

Remaining Life = (t_actual − t_minimum) / corrosion_rate

Where:

• t_actual = current measured thickness (UT)
• t_minimum = minimum required thickness (per ASME VIII or API 510)
• corrosion_rate = short-term or long-term rate (mm/year)

If remaining life < 2 years, the next inspection is scheduled sooner.

6. What is the minimum thickness for a pressure vessel shell?

Per ASME Section VIII, UG-27, the minimum thickness for internal pressure:

t = (P × R) / (S × E − 0.6 × P)

Where:

• P = design pressure
• R = inside radius
• S = allowable stress (from ASME II, material, temperature)
• E = joint efficiency (1.0 for full RT, 0.85 for spot, 0.7 for none)

If the measured thickness is below t_minimum, the vessel must be rerated, repaired, or retired.

7. What is corrosion under insulation (CUI), and how do you detect it?

CUI is corrosion on the exterior of an insulated vessel, hidden under the insulation. It's caused by moisture trapped under insulation — common at 0–150°C. Detection:

• Remove insulation at suspect areas (low points, penetrations)
• UT thickness on the exposed shell
• Infrared thermography — wet insulation shows cooler
• Neutron backscatter — detects moisture under insulation

CUI is one of the leading causes of vessel failure — inspectors must look for it.

---

NDT for Static Equipment

8. What NDT methods are used on pressure vessels?

Method	What It Finds	Typical Use
UT	Thickness, internal flaws	CML thickness, weld inspection
MT	Surface/near-surface (ferromagnetic)	Carbon steel welds
PT	Surface-breaking (any metal)	Stainless welds, nozzles
RT	Internal volumetric	Fabrication welds, critical repairs
WFMT	Wet fluorescent MT	Enhanced surface crack detection
Hardness	HAZ hardness	PWHT verification, sour service

9. What is a hydrotest, and when is it required on a vessel?

A hydrotest proves the vessel is leak-tight and structurally sound. Per ASME Section VIII (new) and API 510 (repair):

• New vessel: Test at 1.3–1.5× design pressure (corrected for temperature)
• After repair/alteration: Test per API 510 — often 1.3× MAWP
• After rerating: Test if the new rating is higher
• Medium: Water (hydrostatic) — gas (pneumatic) is rare and dangerous

10. What is PWHT, and when is it required on a vessel?

PWHT (Post Weld Heat Treatment) relieves residual stresses. Required when:

• Thickness exceeds the ASME VIII limit (e.g., >32 mm for P-No. 1)
• Service environment causes stress corrosion (caustic, sour)
• Client spec mandates it (Aramco SAES-W-011, ADNOC AGES-W-001)

Typical PWHT: 595–620°C for carbon steel, held for 1 hour per inch of thickness.

---

API 653: Tank Inspection

11. What is API 653, and what does it govern?

API 653 is the Tank Inspection Code — it governs the in-service inspection, repair, and alteration of above-ground storage tanks (ASTs). It covers:

• Inspection intervals — external, internal
• Shell thickness — UT measurement, remaining life
• Bottom inspection — corrosion, MFL scan
• Roof inspection — corrosion, settlement
• Repair procedures — plate replacement, weld repair

12. What is the inspection interval for a storage tank per API 653?

Inspection	Frequency
External	Every 5 years (or RBI)
Internal	Every 10 years (or per RBI, max 20)
UT thickness	Every 5 years (external, at CMLs)
Bottom MFL	At each internal inspection

13. What is MFL (Magnetic Flux Leakage)?

MFL is an NDT method for tank floor inspection. A magnetic scanner magnetizes the floor plate; sensors detect flux leakage at corrosion pits and thinning. It finds floor corrosion without removing the tank bottom — the standard for API 653 internal inspections.

---

Scenario-Based Questions

14. You find a vessel shell thickness below the minimum. What do you do?

1. Raise an NCR and notify the engineer
2. Verify the measurement — re-UT to confirm
3. Calculate the remaining life — (t_actual − t_min) / corrosion rate
4. Determine the action:
   • Remaining life > 2 years: Schedule next inspection sooner
   • Remaining life < 2 years: Rerate (lower MAWP), repair, or retire
5. If rerating: Engineering analysis per API 510, update the nameplate
6. If repairing: Weld repair per ASME IX and API 510, PWHT if required, re-UT
7. Document — NCR, calculation, rerating/repair record, new inspection plan

15. A vessel has CUI (corrosion under insulation) at a nozzle. What do you do?

1. Remove insulation at the suspect area and beyond (at least 500 mm each way)
2. Clean and inspect — visual, MT/PT for cracks
3. UT thickness on the affected shell and nozzle
4. Raise an NCR if thickness is below minimum or cracks are found
5. Repair — grind out cracks, weld repair, PWHT if required
6. Re-insulate with proper weatherproofing to prevent recurrence
7. Document — NCR, repair record, new CML for monitoring

---

Final Tips for Static Equipment QC Interviews

1. Know API 510 and API 653 — the in-service inspection codes
2. Understand CMLs and remaining life calculation — #1 question
3. Practice UT thickness interpretation — common practical test
4. Mention ASME Section VIII — the fabrication code
5. Carry your API 510 / API 653 / CSWIP certificate
6. Be ready for CUI and corrosion scenarios

---

Practice Static Equipment QC Questions with SkillJet.co

SkillJet.co's QC Inspector Simulator includes:

• ✅ Static Equipment discipline with 200+ code-based scenarios
• ✅ API 510 and API 653 inspection scenarios
• ✅ Aramco SAES-M-001 and ADNOC AGES-M-01 overlays
• ✅ Real-time readiness scoring

Start your free QC diagnostic →