The manufacture, inspection and acceptance of pressure vessels constitute a systematic engineering process, which must be carried out in strict accordance with national standards (such as GB 150) and special equipment safety technical specifications (such as TSG 21). The following is an analysis of the whole process of pressure vessels from raw material incoming inspection to final delivery:
Phase 1: Manufacturing Process
The manufacture of pressure vessels generally follows the process route below:
1. Raw Material Preparation and Acceptance
- Material selection: Select standard-compliant steel plates, forgings, steel pipes, etc., according to design drawing requirements.
- Re-inspection: After raw materials enter the factory, chemical composition analysis and mechanical property re-inspection must be performed to ensure material qualification.
- Mark transfer: Before material cutting, information such as heat batch number and specification shall be transferred to parts to ensure traceability.
2. Cutting and Forming
- Cutting: Cut steel plates to required dimensions by flame cutting, plasma cutting or shearing machine.
- Groove machining: Chamfer (machine grooves) on welding areas to ensure welding quality.
- Forming:
- Cylinder: Roll steel plates into cylindrical shape by plate rolling machine.
- Head: Stamp or spin steel plates into elliptical, dished or spherical heads.
3. Assembly and Welding (Core Process)
- Assembly: Fit cylinder sections to each other and cylinders to heads, adjust roundness and misalignment.
- Welding:
- Methods: Commonly used submerged arc welding (cylinder longitudinal/circumferential seams), TIG welding (root pass), manual arc welding.
- Welders: All welders must hold valid pressure vessel welder certificates for corresponding positions.
- Welding Procedure Specification / Procedure Qualification Record (WPS/PQR): Qualified and approved welding procedure documents must be available before welding.
4. Heat Treatment
- Purpose: Eliminate welding residual stress, improve material properties and prevent cracking.
- Methods: Overall heat treatment or local heat treatment.
5. Non-Destructive Testing (NDT)
Welded joints must undergo non-destructive testing during manufacturing.
- Methods:
- RT (Radiographic Testing): Most commonly used to detect internal defects (porosity, slag inclusion, incomplete penetration, etc.).
- UT (Ultrasonic Testing): Detect internal defects, more effective for thick plates.
- MT (Magnetic Particle Testing) / PT (Penetrant Testing): Detect surface and near-surface defects (cracks, etc.).
6. Opening and Nozzle Installation
Open holes on cylinders or heads and weld nozzles (manhole, process nozzle, instrument nozzle).Reinforcement rings or thick-walled pipes are usually welded at openings to compensate for strength reduction caused by perforation.
7. Final Assembly
Install flanges, supports (saddle supports, skirt supports), internal components, etc.
Phase 2: Inspection Process
Inspection runs through the entire manufacturing process, including self-inspection, mutual inspection and special inspection.
1. In-Process Quality Control (IPQC)
- Dimensional inspection: Check cylinder diameter, straightness, wall thickness, head shape deviation, etc.
- Welding inspection: Inspect weld appearance; no undercut, porosity, cracks or other surface defects allowed.
2. Pressure Test (Hydrostatic Test) — Most Critical Step
- Purpose: Verify overall strength and tightness of the vessel.
- Medium: Normally clean water.
- Pressure: Test pressure is usually 1.25 or 1.5 times the design pressure (determined by material and temperature).
- Pressure holding: After reaching test pressure, hold for a specified time (e.g. 30 minutes), then reduce to design pressure for inspection.
- Acceptance criteria: No leakage, no visible deformation, no abnormal noise.
3. Pneumatic Tightness Test
- Application: Vessels containing flammable, explosive, extremely hazardous or highly hazardous media, or vessels that cannot be filled with water due to structural reasons.
- Medium: Air or nitrogen.
- Pressure: Normally 1.05 times the design pressure.
- Method: Apply soapy water to check for leaks.
4. Leak Test
For vessels containing extremely or highly hazardous media, in addition to hydrostatic test, ammonia leak test, halogen leak detection or helium leak detection shall also be performed.
Phase 3: Acceptance and Delivery
Acceptance is the final step for the user (buyer) to confirm whether the equipment meets contract and drawing requirements.
1. Document Review (Technical Dossier)
During acceptance, the manufacturer shall be required to provide complete as-built documents, including:
- As-built drawings: Final drawings stamped with “As-Built Drawing” seal (reflecting actual manufacturing conditions).
- Quality certificates: Raw material certificates, re-inspection reports, WPS/PQR, welder registration forms.
- NDT reports: All RT/UT/MT/PT films or electronic data.
- Test reports: Hydrostatic test records, pneumatic tightness test records.
- Nameplate rubbing/photocopy: Including vessel name, serial number, design pressure, design temperature, volume, manufacturing date, license number, etc.
2. Appearance and Geometric Dimension Acceptance
- Inspect equipment surface for dents, scratches and corrosion.
- Check integrity of flange sealing surfaces.
- Verify nozzle orientation consistency with drawings (critical for on-site piping).
- Check anchor bolt hole positions on support base plates.
3. Accessory Acceptance
- Check completeness and validity period of safety accessories (safety valve, pressure gauge, level gauge).
- Verify availability of spare parts (gaskets, bolts, etc.).
4. Supervision Inspection Certificate (Critical Document)
During pressure vessel manufacture, on-site supervision and inspection must be conducted by inspectors from special equipment inspection institutions.Upon acceptance, confirm that the manufacturer has obtained the Pressure Vessel Supervision Inspection Certificate issued by the inspection institute. Without this certificate, the equipment cannot be registered with the local market supervision bureau for legal use.
5. Packaging and Shipment
- Nozzles are usually sealed with blind flanges or caps to prevent ingress of debris or rainwater during transportation.
- Equipment exterior is painted (primer and topcoat), with equipment tag number and flow direction arrows marked.
- Over-limit transportation formalities are required for large-scale equipment.
