1. Flange Joint Overview
Bolted flange joints are the primary mechanical connection in midstream piping systems, allowing disassembly for maintenance, inspection, and equipment access. A flange joint consists of three interacting components: the flanges, the gasket, and the bolting. Failure of any one component can result in a leak.
Flanges
Structural members
Provide rigidity and distribute bolt load to the gasket. Must resist bending from internal pressure and external loads.
Gasket
Sealing element
Deforms under bolt load to fill surface irregularities. Must maintain seal under operating pressure and temperature cycles.
Bolting
Clamping force
Provides and maintains compressive load on the gasket. Must account for relaxation, thermal expansion, and pressure loads.
Leakage Mechanisms
Flange leaks occur when the gasket stress drops below the minimum required to maintain a seal. Common causes include:
- Insufficient bolt load: Inadequate torque during assembly, uneven bolt loading, or failure to follow multi-pass tightening sequence.
- Gasket relaxation: Creep and stress relaxation of gasket material over time, especially at elevated temperatures.
- Thermal cycling: Differential thermal expansion between bolts and flanges reduces bolt load during temperature changes.
- Pressure and bending: Internal pressure creates a hydrostatic end force that tends to separate the flanges. External piping loads add bending moments.
- Vibration: Mechanical vibration from rotating equipment or flow-induced vibration can loosen bolts over time.
- Gasket damage: Incorrect installation, reuse of gaskets, chemical attack, or exceeding temperature limits.
Industry data: Studies indicate that over 50% of flange leaks are caused by improper assembly procedures rather than design deficiencies. Following ASME PCC-1 assembly guidelines significantly reduces leak frequency.
2. Flange Types & Facings
Common Flange Types
| Flange Type | Description | Typical Application |
|---|---|---|
| Weld Neck (WN) | Long tapered hub butt-welded to pipe. Best stress distribution. | High-pressure, high-temperature, critical service |
| Slip-On (SO) | Slides over pipe, fillet welded inside and outside. Lower cost. | Low to moderate pressure utility service |
| Socket Weld (SW) | Pipe inserts into socket, fillet welded outside. Small bore. | NPS 2 and smaller, high pressure |
| Threaded (TH) | Threaded bore screwed onto pipe. No welding required. | Low-pressure utility, instrument connections |
| Blind (BL) | No bore. Used to blank off piping or vessel nozzles. | Pipe ends, vessel nozzles, test blinds |
| Lap Joint (LJ) | Used with stub end. Allows rotation for bolt hole alignment. | Alloy piping with carbon steel flanges |
Flange Facing Types
| Facing | Abbreviation | Gasket Type | Pressure Class |
|---|---|---|---|
| Raised Face | RF | Spiral wound, sheet, PTFE | Class 150–2500 |
| Flat Face | FF | Full-face sheet gaskets | Class 150 (cast iron mating) |
| Ring Type Joint | RTJ | Metal ring gaskets (R, RX, BX) | Class 600–2500 |
| Tongue and Groove | T&G | Sheet or spiral wound | Special applications |
| Male and Female | M&F | Sheet or spiral wound | Special applications |
Raised Face Dimensions
Raised face height per ASME B16.5:
- Class 150 and 300: 1/16-inch raised face (0.0625 in.)
- Class 400 through 2500: 1/4-inch raised face (0.250 in.)
Surface Finish Requirements
| Gasket Type | Surface Finish (AARH μin) | Notes |
|---|---|---|
| Spiral wound | 125–250 | Concentric serrations preferred |
| Sheet (compressed fiber) | 125–250 | Stock finish acceptable |
| PTFE envelope | 63–125 | Smoother finish for soft gaskets |
| Metal ring joint (RTJ) | 63 max | Groove finish is critical |
| Kammprofile | 125–250 | Serrated metal core with soft facing |
Facing rule: Never bolt a raised face flange to a flat face flange. The raised face creates a bending moment on the flat face flange that can crack cast iron flanges. Use a flat face flange or machine the raised face down to flat when mating to cast iron or FRP flanges.
3. Gasket Selection
The gasket is the most critical component for sealing. Selection depends on fluid service, pressure, temperature, and flange type. ASME B16.20 covers metallic gaskets and ASME B16.21 covers non-metallic gaskets.
ASME Gasket Factors (m and y)
The ASME Boiler and Pressure Vessel Code defines two gasket factors used in flange design calculations:
| Gasket Type | m Factor | y (psi) | Max Temp (°F) |
|---|---|---|---|
| Rubber sheet (1/8 in.) | 1.00 | 200 | 200 |
| Compressed fiber (1/16 in.) | 2.00 | 1,600 | 750 |
| PTFE filled | 2.00 | 1,500 | 500 |
| Spiral wound (with filler) | 2.50 | 10,000 | 800 |
| Spiral wound (graphite fill) | 3.00 | 10,000 | 850 |
| Kammprofile (graphite faced) | 3.00 | 8,800 | 850 |
| Metal ring joint (soft iron) | 5.50 | 18,000 | 1,000 |
| Metal ring joint (SS 304) | 6.50 | 26,000 | 1,200 |
Gasket Factors:
m = Gasket maintenance factor (dimensionless)
Ratio of gasket stress to internal pressure required to maintain seal
Higher m = gasket needs more residual stress relative to pressure
y = Minimum gasket seating stress (psi)
Minimum compressive stress to initially seat the gasket
Must be achieved during bolt-up before pressurization
Gasket Selection Guide for Midstream Service
| Service | Recommended Gasket | Notes |
|---|---|---|
| Natural gas (sweet, <600 psig) | Spiral wound with graphite filler | Standard choice for most midstream applications |
| Natural gas (sour, H2S) | Spiral wound with graphite filler, SS 316 windings | Avoid rubber or PTFE in sour service |
| High-pressure gas (>900 psig) | Ring type joint (RTJ) or spiral wound | RTJ preferred for Class 900 and above |
| Amine service | Spiral wound with PTFE filler | Graphite can catalyze amine degradation |
| Glycol service | Spiral wound with graphite filler | Standard selection |
| Hot oil (>400°F) | Spiral wound with graphite filler | Verify gasket temperature rating |
| Instrument air / utility | Compressed fiber sheet | Low-cost, adequate for low pressure |
| Cryogenic (NGL, LNG) | Spiral wound with PTFE filler | Graphite may not seal at cryogenic temps |
Spiral Wound Gasket Components
- Inner ring: Prevents gasket blowout into the pipe bore. Required for Class 600 and above, and for all vacuum services.
- Winding: Alternating layers of metal strip and filler material. Metal provides resilience; filler provides sealing.
- Outer ring (centering ring): Centers the gasket on the flange and acts as a compression stop to prevent over-compression.
Critical rule: Never reuse gaskets. Even if a gasket appears undamaged, the seating surfaces have already conformed and compressed. A reused gasket cannot achieve proper seating stress and will leak.
4. Bolt Torque & ASME PCC-1
ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) provides comprehensive procedures for achieving proper bolt load on flanged joints. Following PCC-1 is the single most effective measure to prevent flange leaks.
Target Bolt Stress Method
Bolt Torque Equation:
T = K × D × Fb
Where:
T = Torque per bolt (ft-lbs)
K = Nut factor (dimensionless)
D = Nominal bolt diameter (inches)
Fb = Target bolt load (lbs)
Target bolt load:
Fb = Sb × Ab
Where:
Sb = Target bolt stress (psi) = typically 40,000–50,000 psi for B7 bolts
Ab = Bolt root area (in²)
Nut Factors (K Values)
| Condition | K Factor | Notes |
|---|---|---|
| As-received (lightly oiled) | 0.16–0.18 | Standard for new bolts with machine oil |
| Moly paste (MoS2) | 0.12–0.14 | Most common in midstream; recommended by PCC-1 |
| Copper-based anti-seize | 0.13–0.15 | Good for high-temperature applications |
| Nickel-based anti-seize | 0.14–0.16 | Stainless steel bolting |
| PTFE-based lubricant | 0.10–0.12 | Lowest friction; use with caution (easy to over-torque) |
| Dry / rusty / corroded | 0.20–0.35 | Unpredictable; never bolt up dry |
ASME PCC-1 Assembly Procedure
PCC-1 specifies a multi-pass cross-pattern tightening sequence to achieve uniform gasket loading:
Pre-Assembly Checks
- Inspect flange faces for damage, corrosion, or radial scratches across sealing surface
- Verify flange alignment: maximum 1/32 inch gap variation around the circumference
- Confirm correct gasket type and size; verify inner ring presence if required
- Clean bolt threads, nut faces, and flange bolt holes
- Apply lubricant to bolt threads and nut bearing faces (both sides)
Tightening Sequence
| Pass | % of Target Torque | Pattern | Purpose |
|---|---|---|---|
| 1 | 20–30% | Star (cross) pattern | Snug gasket, verify alignment |
| 2 | 50–70% | Star (cross) pattern | Begin seating gasket uniformly |
| 3 | 100% | Star (cross) pattern | Achieve target bolt load |
| 4 | 100% | Circular (clockwise) | Verify and equalize all bolts |
| 5 (optional) | 100% | Circular (clockwise) | Final verification pass |
Bolt Material Properties
| Bolt Material | ASTM Spec | Yield (psi) | Tensile (psi) | Max Temp (°F) |
|---|---|---|---|---|
| B7 (Cr-Mo steel) | A193 | 105,000 | 125,000 | 1,000 |
| B7M (tempered B7) | A193 | 80,000 | 100,000 | 1,000 |
| B16 (Cr-Mo-V) | A193 | 105,000 | 125,000 | 1,100 |
| B8 (304 SS) | A193 | 30,000 | 75,000 | 1,500 |
| B8M (316 SS) | A193 | 30,000 | 75,000 | 1,500 |
| L7 (low temp Cr-Mo) | A320 | 105,000 | 125,000 | -150 |
Nut material: ASTM A194 Grade 2H for B7 bolts. Grade 8 or 8M for stainless steel bolts.
Target stress rule: For ASTM A193 B7 bolts with spiral wound gaskets, target 50% of yield strength (approximately 50,000 psi bolt stress). This provides adequate gasket compression while maintaining bolt elasticity for service conditions. Never exceed 67% of yield in any bolt.
5. Pressure-Temperature Ratings
ASME B16.5 provides pressure-temperature ratings for flanged fittings in seven pressure classes. Ratings decrease as temperature increases due to reduced material strength at elevated temperatures.
Pressure Classes
| Class | Max Rating at −20 to 100°F (psig) | Rating at 400°F (psig) | Rating at 750°F (psig) |
|---|---|---|---|
| 150 | 285 | 230 | 170 |
| 300 | 740 | 600 | 440 |
| 600 | 1,480 | 1,200 | 880 |
| 900 | 2,220 | 1,795 | 1,320 |
| 1500 | 3,705 | 2,995 | 2,200 |
| 2500 | 6,170 | 4,990 | 3,670 |
Values shown for Material Group 1.1 (A105, A216 WCB). Other material groups have different ratings. Always verify with the full ASME B16.5 tables.
Flange Class Selection
Selection Procedure:
1. Determine maximum operating pressure (MAWP or design pressure)
2. Determine maximum operating temperature (design temperature)
3. Identify material group from ASME B16.5 Table 1A
4. Look up pressure rating at design temperature
5. Select lowest class where rated pressure >= design pressure
6. Apply corrosion allowance considerations
Common Material Groups
| Group | Materials | Typical Application |
|---|---|---|
| 1.1 | A105, A216 WCB, A516-70 | Carbon steel, general service |
| 1.2 | A350 LF2 | Low-temperature carbon steel |
| 1.5 | A350 LF6 | Low-temp carbon steel (lower toughness) |
| 2.1 | A182 F11, A217 WC6 | 1-1/4 Cr - 1/2 Mo alloy steel |
| 2.3 | A182 F22, A217 WC9 | 2-1/4 Cr - 1 Mo alloy steel |
| 2.6 | A182 F304, A351 CF8 | 304 stainless steel |
| 2.7 | A182 F316, A351 CF8M | 316 stainless steel |
Temperature de-rating: A Class 150 flange rated at 285 psig at 100°F drops to only 170 psig at 750°F. Always check the rating at the maximum design temperature, not ambient conditions. This is one of the most common specification errors in midstream piping design.
6. Worked Example
Calculate the bolt torque for a 6-inch Class 300 raised face weld neck flange joint in natural gas service using spiral wound gaskets with graphite filler.
Given:
Flange: 6-inch, Class 300, RF, WN per ASME B16.5
Gasket: Spiral wound with graphite filler (ASME B16.20)
Bolts: 8 x 7/8-inch diameter, ASTM A193 B7
Nuts: ASTM A194 Grade 2H
Lubricant: Moly paste (K = 0.13)
Service: Sweet natural gas, 600 psig at 200°F
Step 1: Determine Bolt Root Area
For 7/8-inch A193 B7 stud bolt:
Thread: 8 UNC
Root area (Ab) = 0.462 in² per bolt
Step 2: Calculate Target Bolt Load
Target bolt stress = 50% of yield = 0.50 x 105,000 = 52,500 psi
Target bolt load per bolt:
Fb = Sb x Ab = 52,500 x 0.462 = 24,255 lbs per bolt
Total bolt load = 8 x 24,255 = 194,040 lbs
Step 3: Verify Gasket Stress
Gasket seating area (from B16.20 for 6-inch spiral wound):
Gasket OD = 8.75 in., Gasket ID = 6.75 in.
Effective gasket width: N = (8.75 - 6.75) / 2 = 1.00 in.
Mean gasket diameter: G = (8.75 + 6.75) / 2 = 7.75 in.
Gasket contact area = pi x G x N = 3.14159 x 7.75 x 1.00 = 24.35 in²
Gasket seating stress = Total bolt load / Gasket area
= 194,040 / 24.35 = 7,968 psi
Required y = 10,000 psi (for spiral wound graphite)
7,968 psi < 10,000 psi --> Gasket may not fully seat
Increase target stress to 55% of yield:
Fb = 57,750 x 0.462 = 26,681 lbs per bolt
Total = 213,445 lbs
Gasket stress = 213,445 / 24.35 = 8,765 psi
This is still below y = 10,000, but within practical range.
Use maximum recommended: 60% of yield = 63,000 psi bolt stress
Fb = 63,000 x 0.462 = 29,106 lbs per bolt
Total = 232,848 lbs
Gasket stress = 232,848 / 24.35 = 9,562 psi
Step 4: Calculate Bolt Torque
T = K x D x Fb
T = 0.13 x (7/8 / 12) x 29,106
T = 0.13 x 0.0729 x 29,106
T = 276 ft-lbs per bolt
Tightening sequence (per PCC-1):
Pass 1: 30% = 83 ft-lbs (star pattern)
Pass 2: 70% = 193 ft-lbs (star pattern)
Pass 3: 100% = 276 ft-lbs (star pattern)
Pass 4: 100% = 276 ft-lbs (circular check)
Step 5: Verify Operating Condition
Hydrostatic end force from internal pressure:
H = (pi/4) x G² x P = 0.7854 x 7.75² x 600 = 28,302 lbs
Required gasket operating stress = m x P = 3.0 x 600 = 1,800 psi
Required gasket load = 1,800 x 24.35 = 43,830 lbs
Total bolt load required in service = H + gasket load
= 28,302 + 43,830 = 72,132 lbs
Available bolt load = 232,848 lbs >> 72,132 lbs [OK]
Residual gasket stress = (232,848 - 28,302) / 24.35 = 8,400 psi
This exceeds m x P = 1,800 psi [OK - adequate margin]
Design check: The 6-inch Class 300 joint with 8 x 7/8-inch B7 bolts at 276 ft-lbs provides adequate gasket seating and operating load. The residual gasket stress of 8,400 psi far exceeds the minimum required 1,800 psi, providing a comfortable margin for relaxation and thermal cycling.
7. Leak Prevention & Troubleshooting
Pre-Assembly Best Practices
- Flange face inspection: Check for radial scratches, pitting, corrosion, or warping. Any radial defect crossing the gasket seating surface can provide a leak path.
- Alignment: Flanges must be parallel within 1/32 inch per foot of flange OD. Use alignment pins if needed. Never use bolt force to pull flanges into alignment.
- Gasket centering: Center the gasket on the bolt circle. Off-center gaskets result in uneven compression and potential blowout.
- Bolt lubrication: Apply lubricant to both the bolt threads and nut bearing surfaces. Dry bolts can lose 40-90% of applied torque to friction, resulting in inadequate gasket load.
- Washer use: Use hardened washers (F436) under both the nut and the bolt head. This prevents galling and provides consistent K factor.
Common Leak Causes and Remedies
| Symptom | Likely Cause | Remedy |
|---|---|---|
| Leak at startup | Insufficient bolt torque or uneven loading | Re-torque following PCC-1 multi-pass procedure |
| Leak after thermal cycle | Bolt relaxation from differential thermal expansion | Hot re-torque at operating temperature; consider Belleville washers |
| Leak develops over time | Gasket creep/relaxation | Re-torque or replace gasket with lower-creep type |
| Leak at one location on circumference | Flange misalignment or uneven bolt load | Check alignment; re-torque with calibrated wrench |
| Gasket blowout | Gasket exceeded pressure rating or no inner ring | Replace with higher-rated gasket; add inner ring |
| Leak after hydrotest | Gasket over-compressed during test | Replace gasket; limit test pressure to 1.5x design |
Hot Bolting Procedures
Hot bolting (re-torquing bolts while the system is pressurized and at temperature) is sometimes necessary to address leaks in service:
- Follow site-specific hot bolting procedures and JSA/JHA requirements
- Never loosen more than one bolt at a time on a pressurized joint
- Re-torque in a circular pattern, one bolt at a time
- Use a calibrated hydraulic torque wrench for consistency
- Maximum additional torque per pass: 10-15% of target
- If leak persists after two re-torque attempts, plan for gasket replacement during shutdown
Live Leak Repair (Clamp Method)
When a flange leak cannot be stopped by re-torquing, an engineered leak repair clamp may be installed as a temporary measure. These must be designed per the applicable piping code (ASME B31.3 or B31.8) and require an engineering assessment. Live leak repair is a last resort before shutdown.
Record Keeping
ASME PCC-1 recommends maintaining records for critical joints:
- Joint identification number and location
- Gasket type, manufacturer, and lot number
- Bolt material, size, and lubrication used
- Target torque value and actual torque applied
- Torque wrench calibration date and certificate number
- Name of the assembler and date of assembly
- Any re-torque or leak repair actions taken
Quality tip: Implement a Qualified Joint Assembly Program per ASME PCC-1 Appendix A. Train and qualify all bolting technicians. Use calibrated torque equipment. These three steps alone can reduce flange leak rates by over 90% based on industry experience.
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