1. Design Basis
Facility piping at natural gas compressor stations and processing plants must comply with applicable codes and standards. The design basis establishes pressure, temperature, and material requirements for all piping components.
Typical High-Pressure Gas Service Design Basis
| Service | Natural Gas |
| Maximum Operating Pressure | 800 psig (typical) |
| Design Factor | 50% SMYS |
| Operating Temperature | -20°F minimum, 200°F maximum |
| Hydrostatic Test | 1,200 psig minimum, 1,606 psig maximum |
| Flange Rating | ANSI Class 600 |
Governing Codes and Standards
| Code/Standard | Scope | Application |
|---|---|---|
| ASME B31.8 | Gas Transmission & Distribution | Primary code for gas facility piping |
| ASME B31.3 | Process Piping | Process equipment connections |
| 49 CFR 192 | DOT Pipeline Safety | Federal regulatory requirements |
| API 5L | Line Pipe | Pipe material specification |
| API 6D | Pipeline Valves | Valve specification |
| ASME B16.5 | Pipe Flanges & Fittings | Flange dimensions and ratings |
| MSS SP-75 | High-Test Wrought Fittings | Large diameter fittings |
Design Factor Selection
The design factor (F) determines the allowable stress as a percentage of SMYS. For facility piping, lower design factors provide additional safety margin:
| Location Class | Design Factor | Application |
|---|---|---|
| Class 1 | 0.72 | Rural areas, low population |
| Class 2 | 0.60 | Semi-rural, scattered buildings |
| Class 3 | 0.50 | Suburban, commercial |
| Class 4 | 0.40 | Multi-story buildings |
| Compressor Stations | 0.50 | Typical facility design |
2. Pipe Specifications
Pipe specifications define material grade, wall thickness, and manufacturing method for each pipe size. Selection depends on operating pressure, design factor, and service conditions.
Small Diameter Pipe (≤16" NPS)
Small diameter pipe for facility use is typically seamless carbon steel per ASTM A-106:
| Pipe Size (OD) | Wall Thickness | Standard | SMYS (psi) | Type |
|---|---|---|---|---|
| 0.500" | 0.147" | ASTM A-106 | 35,000 | SMLS, Plain End |
| 0.750" | 0.154" | ASTM A-106 | 35,000 | SMLS, Plain End |
| 1.000" | 0.179" | ASTM A-106 | 35,000 | SMLS, Plain End |
| 1.500" | 0.200" | ASTM A-106 | 35,000 | SMLS, Plain End |
| 2.375" | 0.218" | ASTM A-106 | 35,000 | SMLS |
| 3.500" | 0.216" | ASTM A-106 | 35,000 | SMLS |
| 4.500" | 0.237" | ASTM A-106 | 35,000 | SMLS |
| 6.625" | 0.280" | ASTM A-106 | 35,000 | SMLS |
| 8.625" | 0.322" | ASTM A-106 | 35,000 | SMLS |
| 10.750" | 0.365" | ASTM A-106 | 35,000 | SMLS |
| 12.750" | 0.375" | ASTM A-106 | 35,000 | SMLS |
| 14.000" | 0.375" | ASTM A-106 | 35,000 | SMLS |
| 16.000" | 0.375" | ASTM A-106 | 35,000 | SMLS, DSAW |
Large Diameter Pipe (≥18" NPS)
Large diameter pipe transitions to API 5L specification with higher strength grades:
| Pipe Size (OD) | Wall Thickness | Standard | SMYS (psi) | Type |
|---|---|---|---|---|
| 18.000" | 0.375" | API 5L X-52 | 52,000 | SMLS, DSAW |
| 20.000" | 0.375" | API 5L X-52 | 52,000 | SMLS, DSAW |
| 24.000" | 0.375" | API 5L X-52 | 52,000 | SMLS, DSAW |
| 26.000" | 0.438" | API 5L X-52 | 52,000 | SMLS, DSAW |
| 30.000" | 0.500" | API 5L X-52 | 52,000 | DSAW |
| 36.000" | 0.562" | API 5L X-52 | 52,000 | DSAW |
| 42.000" | 0.562" | API 5L X-60 | 60,000 | DSAW |
| 48.000" | 0.688" | API 5L X-60 | 60,000 | DSAW |
Abbreviations
- SMLS: Seamless
- DSAW: Double Submerged Arc Welded
- SMYS: Specified Minimum Yield Strength
- ASTM: American Society for Testing and Materials
- API: American Petroleum Institute
Weld Preparation
Standard weld bevel is 30 degrees unless otherwise specified. Pipe ends must be prepared per welding procedure specification (WPS) requirements.
3. Weld Fittings
Butt-weld fittings (elbows, tees, reducers, caps) must match the pipe material and wall thickness. Fitting specifications transition at 18" diameter similar to pipe.
Standard Fittings (≤16" NPS)
| Fitting Size | Wall Thickness | Standard | SMYS (psi) |
|---|---|---|---|
| 1.000" - 16.000" | Match pipe | ASTM A-234 WPB | 35,000 |
High-Test Fittings (≥18" NPS)
| Fitting Size | Wall Thickness | Standard | SMYS (psi) |
|---|---|---|---|
| 18.000" - 36.000" | Match pipe | MSS SP-75 | 52,000 |
| 42.000" - 48.000" | Match pipe | MSS SP-75 | 60,000 |
Swage Nipples
Swage nipples for reducing pipe size shall be ASTM A-234 Grade WPB, Extra Heavy (X-HVY) wall. Concentric swages are used for vertical runs; eccentric swages with flat side down for horizontal runs to facilitate drainage.
4. Flanges
Flanges provide bolted connections for valves, equipment, and piping sections. Proper flange selection ensures leak-free connections at design pressure and temperature.
Welding Neck Flanges
| Flange Size | Class | Standard | Notes |
|---|---|---|---|
| Less than 42" | ANSI 600 | ANSI B16.5, ASTM A-105 | Bore to match pipe ID |
| 42" and larger | ANSI 600 | MSS SP-44 | Grade F-36 minimum |
Blind Flanges
| Flange Size | Class | Standard |
|---|---|---|
| Less than 42" | ANSI 600 | ANSI B16.5 |
| 42" and larger | ANSI 600 | MSS SP-44 |
Flange-to-Pipe Grade Matching
When flange yield strength is less than pipe yield strength, the welding end hub thickness may be increased per ANSI B31 Case 61:
| API 5L Pipe Grade | Minimum MSS SP-44 Flange Grade |
|---|---|
| B / X-42 | F-36 or equivalent B16.5 |
| X-46 | F-36 or equivalent B16.5 |
| X-52 | F-36 or equivalent B16.5 |
| X-56 | F-42 |
| X-60 | F-42 |
| X-65 | F-46 |
| X-70 and higher | F-56 |
Anchor Forgings
Anchor forgings (anchor flanges) are used at thrust block locations and pipe supports. They shall match pipe SMYS and bore, and be designed for minimum 90°F temperature differential. Class 600 rating required for high-pressure service.
5. Bolting Requirements
Proper bolting is critical for flange joint integrity. Bolt and nut materials must be compatible and rated for service conditions.
Standard Bolting
| Item | Specification | Class | Grade | Notes |
|---|---|---|---|---|
| Stud Bolts | ASTM A-193 | 2A | B-7 | Threads rolled after heat treatment |
| Hex Nuts | ASTM A-194 | 2B | 2-H | Two nuts per bolt |
| Anchor Bolts | Cold Rolled Steel | Class 2 | - | With hex nuts |
Gasket Requirements
| Parameter | Specification |
|---|---|
| Class | ANSI Class 600 |
| Type | Non-asbestos ring |
| Standard | Per ANSI B16.5 |
| Documentation | Specify OD and ID on all gaskets |
Insulating Gasket Sets
Insulating gasket sets are used for cathodic protection isolation:
- For ANSI Class 600 raised face flanges
- Complete with full-length Micarta sleeves
- Double washers (2 Micarta + 2 steel per stud)
6. Valve Requirements
Valve selection depends on size, service, and operational requirements. Full-bore valves are required for piggable lines.
Instrumentation Valves (1/4" - 3/4")
| Parameter | Specification |
|---|---|
| Connection | Screwed |
| Material | Carbon steel |
| Rating | 3,000 psig |
| Type | Needle or plug |
Small Valves (1/2" - 1-1/2")
| Parameter | Specification |
|---|---|
| Connection | Screwed |
| Material | Carbon steel |
| Rating | 3,000 psig W.O.G. |
| Type | Ball, plug, or check |
Large Valves (2" - 48")
| Parameter | Specification |
|---|---|
| Class | 600 |
| Standard | API 6D |
| Material | Carbon steel |
| End Connection | Bore weld end to match pipe |
| Types | Ball, plug, gate, check |
Closure Requirements
Quick-opening closures (2" through 16") for pig launchers and receivers shall be:
- Class 600, API 6D with monogram
- Hinged for horizontal operation
- Equipped with safety bleed
7. Branch Connections
Branch connections require reinforcement when the branch opening weakens the header pipe. Reinforcement is provided by saddles, pads, or integrally reinforced fittings.
O-Let Fittings
| Parameter | Specification |
|---|---|
| Maximum branch size | 1-1/2" |
| Material | ASTM A-105 |
| Thread-O-Let / Sock-O-Let | 3,000 psig only |
| Thermowell O-Lets | 3/4" NPT minimum |
| Weld-O-Let | Specify header and branch wall thickness |
Saddle and Pad Requirements
For branch connections larger than O-let capacity, saddles or reinforcing pads are required:
| Type | Material | Standard |
|---|---|---|
| Standard Saddle (SS) | Carbon Steel | ASTM A-234 WPB, Grade B |
| Extra Strong Saddle (XSS) | High Strength | MSS SP-75, Grade Y60 |
| Reinforcing Pad (R-PAD) | High Strength | MSS SP-75, Grade Y60 |
Vent Hole Required
All reinforcing pads must have a vent hole (typically 1/4" NPT) per ASME B31.8 Section 831.4. The vent hole allows escape of gases during welding and serves as a leak detection point during hydrostatic testing.
8. Small Bore Piping
Small bore piping (typically ≤1-1/2") for instruments, drains, vents, and sample connections requires special attention due to vibration and fatigue concerns.
Screwed and Socketweld Fittings
| Item | Material | Rating | Standard |
|---|---|---|---|
| Fittings | Forged Steel | 3,000 psig | ASTM A-105 |
| Bull Plugs | Carbon Steel | X-HVY | ASTM A-234, Gr. WPB |
| Swage Nipples | Carbon Steel | X-HVY | ASTM A-234, Gr. WPB |
| Unions | Forged Steel | 3,000 psig | ASTM A-105 |
Tubing Requirements
Instrument tubing specifications:
| Size | Copper (ASTM B280) | Stainless Steel (ASTM A269) |
|---|---|---|
| 1/8" - 1/2" | 0.035" wall | 0.035" wall |
Approved tubing fitting manufacturers: Parker and Swagelok
Thermowell Requirements
- Material: 304 stainless steel with cap and chain
- Connection: 3/4" NPT minimum outside threads
Orifice Plate Requirements
| Plate Size | Thickness |
|---|---|
| 1.5" to 10" | 1/8" |
| 12" to 18" | 1/4" |
| 20" to 24" | 3/8" |
| 26" to 36" | 1/2" |
Material: 304 stainless steel with paddle-type handle
9. Hydrostatic Testing
Hydrostatic testing verifies piping integrity before commissioning. Test pressure is based on pipe SMYS and design requirements.
Maximum Test Pressures by Pipe Size
| Pipe Size (OD) | Wall Thickness | SMYS (psi) | Max Test Pressure (psig) |
|---|---|---|---|
| 0.500" | 0.147" | 35,000 | 20,580 |
| 2.375" | 0.218" | 35,000 | 6,425 |
| 6.625" | 0.280" | 35,000 | 2,958 |
| 12.750" | 0.375" | 35,000 | 2,059 |
| 16.000" | 0.375" | 35,000 | 1,641 |
| 24.000" | 0.375" | 52,000 | 1,625 |
| 36.000" | 0.562" | 52,000 | 1,624 |
| 48.000" | 0.688" | 60,000 | 1,720 |
Test Pressure Determination
The test pressure for a piping section must be based on the component with the lowest allowable pressure rating. In many cases, FITTINGS OR VALVES (not pipe) may determine the maximum test pressure. Always verify the pressure rating of all components in the test section.
Test Requirements
- Minimum test pressure: 1,200 psig (for 800 psig MOP)
- Maximum test pressure: Per component ratings
- Test medium: Clean water
- Hold time: Per applicable code (typically 4-8 hours)
- Temperature: Test medium must be above freezing