Pressure Vessels
Compressed air storage tanks are pressure vessels that must comply with strict codes for design, manufacturing, and inspection.
ASME Standards
Section VIII - Pressure Vessels
┌─────────────────────────────────────────────────────────┐
│ ASME BPVC SECTION VIII │
├─────────────────────────────────────────────────────────┤
│ │
│ Division 1: General rules │
│ • Pressures up to 3,000 psi │
│ • Safety factor 3.5 │
│ • Most common in industry │
│ │
│ Division 2: Alternative rules │
│ • Detailed stress analysis │
│ • Safety factor 2.4 │
│ • Specialized applications │
│ │
│ Division 3: High pressure │
│ • Pressures greater than 10,000 psi │
│ • Special requirements │
│ │
└─────────────────────────────────────────────────────────┘
U Stamp
Typical nameplate (U Stamp):
┌─────────────────────────────────────────┐
│ [U] │
│ │
│ MFG: ABC Vessel Co. │
│ S/N: 12345 │
│ YEAR: 2024 │
│ │
│ MAWP: 200 PSIG @ 650°F │
│ MDMT: -20°F @ 200 PSIG │
│ │
│ SA-516-70 / NB-12345 │
└─────────────────────────────────────────┘
MAWP = Maximum Allowable Working Pressure
MDMT = Minimum Design Metal Temperature
NB = National Board Registration Number
Manufacturing Requirements
| Element | Requirement |
|---|---|
| Materials | ASME specification |
| Welding | Qualified procedures |
| Welders | Certified |
| Heat treatment | Per code |
| Inspection | By Authorized Inspector |
| Testing | Hydrostatic 1.3× MAWP |
Vessel Components
Tank Anatomy
Relief connection
│
┌─────┴─────┐
Air ════════►│ │
inlet │ │
│ │◄─── Level sight glass
│ TANK │
Air ◄════════│ │
outlet │ │
│ │◄─── Pressure gauge
│ │
└─────┬─────┘
│
Drain
(manual or auto)
│
Supports
Required Accessories
| Accessory | Function | Code |
|---|---|---|
| Relief valve | Overpressure protection | ASME Section VIII |
| Pressure gauge | Pressure indication | Recommended |
| Drain | Condensate removal | Recommended |
| Manway | Inspection access | >18" diameter |
| Nameplate | Identification | Required |
Relief Valves
Types
Conventional relief valve:
Outlet
↑
┌──────┴──────┐
│ Spring │
│ ◯◯◯ │
│ ◯◯◯ │
│ ┌─────┐ │
│ │Disc │ │
│ └──┬──┘ │
│ │ │
└──────┼──────┘
│
Inlet
(pressure)
| Type | Application | Characteristics |
|---|---|---|
| Conventional | Standard | Atmospheric discharge |
| Balanced | Variable backpressure | Internal bellows |
| Pilot operated | High capacity | Pilot-operated |
Sizing
Required capacity:
W = Air inlet to system (SCFM)
The valve must be able to discharge all capacity
from compressors feeding the tank.
Example:
2 compressors at 500 CFM each
Required PSV capacity = 1,000 SCFM minimum
Pressure Settings
| Parameter | Typical Value |
|---|---|
| Set pressure | Vessel MAWP |
| Overpressure | 3% (fire) or 10% (process) |
| Reseat pressure | 93-96% of set |
| Maximum accumulation | 10% above MAWP |
Vessel Inspection
Inspection Types
┌─────────────────────────────────────────────────────────┐
│ INSPECTION PROGRAM │
│ │
│ INTERNAL (requires entry): │
│ ├─ Visual surface inspection │
│ ├─ Thickness measurement │
│ ├─ Corrosion detection │
│ └─ Weld inspection │
│ │
│ EXTERNAL (in-service): │
│ ├─ Paint condition │
│ ├─ Supports and anchors │
│ ├─ Visible leaks │
│ └─ Insulation condition │
│ │
│ ACCESSORIES: │
│ ├─ Relief valves (annual) │
│ ├─ Gauges (calibration) │
│ └─ Drains (operation) │
│ │
└─────────────────────────────────────────────────────────┘
Inspection Frequency
| Type | Frequency | Responsible |
|---|---|---|
| External visual | Monthly | Operator |
| Internal visual | 5 years | Certified inspector |
| Thickness measurement | 5-10 years | Certified inspector |
| Hydrostatic test | If concerns exist | Per code |
| PSV | Annual | Certified shop |
Rejection Criteria
| Condition | Action |
|---|---|
| Thickness < minimum | Repair or replace |
| Active corrosion | Clean, evaluate, repair |
| Deep pitting | Evaluate with engineer |
| Cracks | Stop use, evaluate |
| Deformation | Stop use, evaluate |
Repairs and Alterations
Repair Code
ASME/NB repair process:
1. Detect problem
│
▼
2. Evaluate extent
│
▼
3. Develop procedure ◄── Qualified engineer
│
▼
4. Approve by inspector ◄── Authorized Inspector
│
▼
5. Execute repair ◄────── Certified welders
│
▼
6. Inspect result
│
▼
7. Document (R-Form) ◄── National Board
Permitted Repairs
| Type | Requirements |
|---|---|
| Welding | Qualified procedure, certified welder |
| Patch | Engineering calculations |
| Nozzle replacement | Per original design |
| Adding connections | Calculations, AI approval |
Prohibited Repairs
- Hammering to stop leaks
- Welding without qualification
- Material changes without approval
- Exceeding original MAWP
- Permanent "temporary" patches
Retirement from Service
Criteria
| Condition | Action |
|---|---|
| Excessive age | Evaluate, possible retirement |
| Insufficient thickness | Derate pressure or retire |
| Repair history | Evaluate viability |
| Missing documentation | Special inspection |
| Obsolescence | Replace |
Retirement Process
1. Fully depressurize
2. Purge with clean air
3. Disconnect from system
4. Remove relief valve
5. Mark "OUT OF SERVICE"
6. Render unusable (drill or cut)
7. Document retirement
8. Dispose per regulations
Documentation
Required File
| Document | Contents |
|---|---|
| Manufacturer's report | U-1, calculations, MTR |
| National Board certificate | NB registration |
| Inspection history | All inspections |
| Repair reports | R-Forms if applicable |
| PSV certificates | Annual tests |
Legal Responsibility
The owner/operator is legally responsible for vessel integrity. Inspections must be performed by qualified personnel and properly documented.