How to Do Supplier On-Site Audits: A Practical Checklist for EPC Teams

Why On-Site Audits Matter for EPC Projects

When your project depends on high-risk equipment — pressure vessels, heat exchangers, compressor skids, industrial refrigeration units — a failed supplier is not just an inconvenience. It can mean redesigns, liquidated damages, offshore rework, or even safety incidents.

On paper, most vendors look similar: ISO certificates, a glossy capability presentation, a few reference photos. The real difference shows up only when you walk their shop floor and start asking detailed questions.

This article gives EPC and Owner teams a practical, engineering-driven checklist for doing supplier on-site audits, with a focus on manufacturers of:

• Pressure vessels and heat exchangers
• Gas compression and process skids
• Industrial refrigeration units
• Modular skid packages

We'll keep it pragmatic: what to look at, who to talk to, which documents to request, and how to turn your visit into an objective risk picture.

1. Industry Context & Why On-Site Audits Matter

Modern EPC projects face tighter schedules, stricter regulations, and complex multi-vendor interfaces. At the same time, ISO 9001:2015 and similar standards put explicit emphasis on control of externally provided processes and products (clause 8.4). You're no longer just "buying equipment"; you're outsourcing part of your quality system.

Standards like ISO 9001 require you to establish criteria for selection, evaluation, and re-evaluation of suppliers — and keep records. For pressure equipment, you may also need compliance with ASME Boiler & Pressure Vessel Code (BPVC), PED, DNV, or marine class rules, depending on project location and service.

A well-planned on-site audit helps you:

• Validate that the supplier can actually comply with the codes on their datasheet
• Identify capacity and bottlenecks vs your schedule
• Check the real maturity of QA/QC and documentation
• Align expectations early on welding, NDT, FAT, and packaging

Done right, the audit becomes a risk management tool, not just a procurement box-ticking exercise.

2. Why Supplier On-Site Audits Drive Project Success

A supplier audit is essentially a structured way to examine the quality and risk profile of a supplier before problems appear in your project. For EPC and Owners, this translates into four big levers:

1. Technical integrity
   • Are welders and procedures actually qualified to the relevant ASME/PED/EN codes?
   • Does the shop understand heat treatment, pressure testing, and material traceability?
2. Schedule reliability
   • Does the factory have realistic capacity, or is your order fighting space with several mega-projects?
   • Are there visual signs of chaos: unfinished items everywhere, poor planning boards, long queues for critical NDT?
3. Compliance & certification
   • Are they truly authorized to apply ASME "U" / "U2" stamps, DNV/CCS certificates, etc., via approved Authorized Inspection Agencies?
   • Is there an internal system to keep up with code updates and customer specs?
4. Documentation and traceability
   • Can they actually produce complete MDRs (Manufacturing Data Records)?
   • Are MTRs, WPS/PQR, NDT reports, and calibration certificates structured and retrievable?

If a vendor fails here, your risk is hidden until late in the project — exactly when it's most expensive to fix.

3. Key Engineering & Design Points to Check During the Visit

During an on-site audit for pressure equipment, skids, or refrigeration units, technical depth matters. Use engineers, not just buyers, for the visit.

3.1 Design and Engineering Capability

Ask to see:

• Sample design calculation files for ASME Section VIII / EN 13445 / PD 5500.
• Thermal design software used for heat exchangers (e.g. HTRI, HTFS, in-house tools).
• Typical design limits: max design pressure, temperature, materials (e.g. low-temp carbon steel, stainless, duplex, aluminum for refrigeration).
• How they manage design changes and client comments (revision control, transmittal logs).

Red flags: no formal design review records, "black box" vendor calcs, or over-reliance on a single senior engineer with no backup.

3.2 Materials and Traceability

Check:

• Material receiving, identification, and storage process
• Marking and traceability from plate/bar to finished vessel or skid
• Control of low-temperature impact-tested materials or special grades (e.g. for LNG and cryogenic refrigeration units)

Ask to follow a random heat number through their system — from MTR to shop traveler to current location in the shop.

3.3 Welding, NDT, and PWHT

For pressure vessels, skids, and piping spools, welding capability is a core differentiator:

• Review WPS/PQR bank and welder qualification records
• Inspect weld maps and NDT plans for sample jobs
• Check availability of in-house NDT (RT, UT, MT, PT) and third-party tie-ups
• Look at PWHT furnaces, temperature recording, and calibration certificates

Compare what you see to code requirements and class or directive expectations (e.g. ASME BPVC, PED, and relevant DNV rules for pressure equipment).

3.4 Skid Integration and Functional Testing

For Modular Skid Packages, Gas Compression Units, and Industrial Refrigeration Units:

• Confirm ability to handle 3D piping models and clash checks
• Inspect wiring, cable routing, and tagging practices
• Check test facilities for mechanical run tests, leak tests, control function tests, and interlock simulation
• Review how they integrate third-party items (compressors, valves, instruments) into their quality system

4. Risk, Compliance & Certification — What to Verify on Site

For high-risk process and pressure equipment, compliance is not just a logo on the website. It must show up consistently in the shop.

4.1 Quality Management System

Look beyond the ISO 9001 certificate:

• How is supplier control (ISO 9001:2015 clause 8.4) implemented for their own sub-vendors (forging shops, machining, galvanizing, etc.)?
• Are internal audit findings and corrective actions visible and recent?
• Do they have KPIs on on-time delivery, non-conformities, and rework?

4.2 Pressure Equipment Certifications

Verify:

• Valid ASME Certificates of Authorization with scope (Sections I, IV, VIII Div. 1/2/3 as relevant).
• PED / CE, and any marine class certifications (DNV, ABS, BV, CCS) relevant to your project.
• Relationship with Authorized Inspection Agencies (e.g. DNV as ASME AIA).

4.3 Documentation Packs

Ask to see a complete MDR from a similar past project, including:

• Datasheets, GA/ISO drawings and calculation reports
• MTRs, WPS/PQR, NDT and PWHT records
• FAT procedures and reports
• Nameplate samples and certificates of conformity

If pulling a complete set takes more than a few minutes, expect similar delays on your project.

5. Practical On-Site Audit Checklist Table

Use a simple matrix during your visit to keep observations structured:

Audit Area	What to Check on Site	Typical Red Flags
Design & Engineering	Calcs, design reviews, software, revision control	No formal reviews, outdated software, no backups
Materials & Traceability	MTRs, marking, storage, traveler system	Mixed materials, manual lists, missing heat numbers
Welding & NDT	WPS/PQR, welder quals, NDT plans, PWHT facilities	Expired qual records, external NDT only, no PWHT logs
QA/QC & Documentation	ISO 9001 system, NCRs, calibration, sample MDR	Paper-only system, scattered docs, weak NCR closure
Capacity & Planning	Load charts, planning boards, manning, shift patterns	Chronic backlog, no clear plan, overtime everywhere
HSE & Housekeeping	PPE use, housekeeping, lifting practices, confined space control	Poor PPE, cluttered shop, unsafe lifting
After-Sales & Service	Service team, spare parts control, long-term support capability	No dedicated service group, no parts management

6. Cost, Schedule & Total Cost of Ownership (TCO)

On-site audits are often seen as a cost, but for high-value equipment they are usually a TCO saving.

• Avoiding late surprises: catching under-sized lifting beams, missing NDT, or unrealistic schedules before PO saves weeks of RFIs and rework.
• Energy and lifecycle performance: for Pressure Vessels & Heat Exchangers and refrigeration units, design quality directly affects operating energy costs over 20+ years.
• Standardization across projects: using a consistent checklist allows you to benchmark suppliers globally, not just locally.

Standards such as the ASME BPVC, PED, and DNV rules exist partly to reduce lifecycle risk and ensure consistent safety and performance. A good supplier uses these as a baseline and builds repeatable internal processes around them.

7. EPC On-Site Audit Workflow

A typical on-site audit follows a structured seven-step process:

1. Plan Audit Scope & Criteria — Define which equipment categories, codes, and risk areas to cover.
2. Pre-Audit Document Review — Request and review certificates, QMS manual, sample MDRs, and capacity data before the visit.
3. On-Site Visit & Interviews — Meet engineering, QA/QC, production, and planning teams. Ask open-ended questions.
4. Shop Walkdown & Sampling — Walk the entire facility. Trace heat numbers, inspect weld quality, check housekeeping.
5. Review Findings & Evidence — Consolidate observations against the checklist. Document gaps with photos.
6. Score Supplier & Classify Risk — Apply your scoring matrix. Categorize as Approved / Conditionally Approved / Rejected.
7. Agree Actions & Follow Up — Share findings with the supplier. Set deadlines for corrective actions. Schedule re-evaluation.

You can adapt each step to your internal procedures and tie it to your ISO 9001 supplier evaluation process.

8. Mini Scenario: Skid Supplier Audit That Changed the Shortlist

An EPC team was bidding a mid-size gas processing project needing several compressor and process skids with integrated cooling and control systems. Two suppliers looked similar on paper — both ISO 9001 certified, both claiming ASME and DNV experience.

The EPC conducted short on-site audits at each:

• Supplier A
  • Strong welding and pressure vessel capability
  • Clean shop, clear production planning, visible KPIs
  • Well-structured MDR from a previous offshore project
• Supplier B
  • Heavy focus on structural fabrication, limited in-house NDT
  • Significant backlog on PWHT and hydro-test areas
  • Documentation scattered across local PCs, no central MDR structure

Technically, both could probably deliver. But the audit showed Supplier A had lower schedule and documentation risk, especially for third-party inspections and class approvals. That insight changed the internal risk scoring and helped the project select a more robust partner, reducing the likelihood of late-stage surprises.

9. How Lmart Adds Value as a Manufacturing Partner

At Suzhou Lmart Energy Equipment Co., Ltd., we work with EPC and Owner-Operator teams globally on:

• Modular skid packages and gas compression units
• Industrial refrigeration units for energy, chemical and marine service
• Certified pressure vessels and heat exchangers

From a supplier audit point of view, we focus on being transparent and audit-friendly:

• Structured documentation — Standard MDR templates for pressure vessels, skids, and refrigeration units, aligned with typical EPC requirements and international codes.
• Certified fabrication — Design and fabrication processes aligned with ASME BPVC, PED and relevant class rules, with third-party inspection access and clear weld/NDT traceability.
• Modular thinking — Skid layout, piping, cabling, and control integration optimized for transport, installation, and maintenance access.
• QA/QC culture — Formal internal audits, NCR tracking and closure, and a supplier management process aligned with ISO 9001 supplier control principles.
• Export and inspection experience — Familiarity with third-party agencies (e.g. DNV, BV, ABS, CCS), documentation expectations, and remote FAT or hybrid inspection setups.

Whether you're visiting us for a pre-qualification audit or a project-specific review, the goal is always the same: give your project team clear evidence that risk is understood, controlled, and transparent.

10. Key Takeaways & Next Steps

• On-site supplier audits are no longer optional for high-risk process and pressure equipment; they are part of your ISO 9001 and project risk management obligations.
• Look beyond certificates to actual practice in design, materials, welding, NDT, and documentation.
• Use a structured checklist and scoring to compare suppliers objectively across regions and projects.
• For skid packages and industrial refrigeration units, pay special attention to integration, FAT capability, and documentation.
• Choose suppliers who welcome audits and can show repeatable systems, not just individual heroics.

If you're preparing an RFQ or planning a supplier audit for skids, refrigeration units, or pressure equipment, you can adapt this article into your internal checklist. Feel free to share it with your project, QA/QC, and procurement colleagues.

For deeper discussions or to schedule a technical review of your upcoming project, you're welcome to reach out to Lmart's engineering team or explore our Quality & Certifications pages.