1. Overview & Applications
Hot tap welding attaches fittings to pressurized pipelines containing flammable fluids. Specialized procedures prevent catastrophic burn-through failures.
Burn-through risk
Arc penetration
Welding arc can penetrate thin wall, causing pressurized release and ignition.
Hydrogen cracking
Absorbed hydrogen
Wet hydrogen from fluids can cause cold cracking in weld metal/HAZ.
Thermal stress
Restrained joints
Internal pressure restrains thermal contraction, increasing residual stress.
Quality assurance
Limited access
Cannot inspect internal surface or perform backgouging for root repair.
Key Concepts
- WPS: Welding Procedure Specification - detailed instructions for welding
- PQR: Procedure Qualification Record - test results proving WPS is acceptable
- WPQ: Welder Performance Qualification - test proving welder can follow WPS
- Heat input: Energy delivered to weld (kJ/inch), function of amperage, voltage, travel speed
- HAZ: Heat-Affected Zone - base metal adjacent to weld that experiences microstructure changes
Why hot tap welding is unique: Unlike standard pipeline welding, hot tap welds are made on pressurized pipe containing hydrocarbons. Internal cooling from flowing product, hydrogen absorption from wet gas, and inability to repair inside surface make qualification and execution critical.
Applicable Codes and Standards
| Code/Standard | Title | Application |
|---|---|---|
| ASME Section IX | Welding and Brazing Qualifications | WPS development, PQR testing, welder qualification |
| ASME B31.8 | Gas Transmission and Distribution | Design criteria, stress limits, hot tap restrictions |
| ASME B31.4 | Pipeline Transportation Systems for Liquids and Slurries | Liquid pipeline hot tap requirements |
| API 1104 | Welding of Pipelines and Related Facilities | Alternative to ASME Section IX for pipeline welding |
| AWS D1.1 | Structural Welding Code - Steel | Supplementary requirements for structural attachments |
| API 1160 | Managing System Integrity for Hazardous Liquid Pipelines | Integrity management considerations for modifications |
Weld Joint Configurations
Image: Hot Tap Fitting Assembly Cross-Section
Show split-tee fitting on pipe with labeled weld locations: longitudinal seam, circumferential fillet, and branch nozzle.
| Joint Type | Configuration | Application | Typical Groove |
|---|---|---|---|
| Longitudinal fillet | Split-tee half-shells to each other | Joining two halves of fitting | Full-penetration T-joint with backing |
| Circumferential fillet | Fitting to pipe OD | Primary seal weld | Fillet weld, 1/4" to 3/8" leg size typical |
| Nozzle attachment | Branch outlet to fitting shell | Branch connection | Full-penetration groove or fillet |
| Pad reinforcement | Reinforcing pad to pipe OD | Area reinforcement for large branches | Fillet weld, seal weld around perimeter |
Essential Variables (ASME Section IX)
Changes to essential variables require re-qualification of WPS:
- Base metal: P-Number grouping (carbon steel = P-1, stainless = P-8)
- Filler metal: F-Number grouping (E7018 = F-4)
- Welding process: SMAW, GMAW, FCAW, GTAW
- Position: 1G (flat), 2G (horizontal), 3G (vertical), 4G (overhead), 5G/6G (pipe)
- Preheat and interpass temperature: Minimum/maximum values
- Post-weld heat treatment: Required or not required
- Shielding gas: Type and flow rate (for GMAW/FCAW/GTAW)
- Electrical characteristics: AC or DC, polarity
2. Procedure Qualification (PQR Development)
Hot tap welding procedures must be qualified through destructive testing per ASME Section IX.
Procedure Qualification Record (PQR) Requirements
PQR Test Assembly:
Test configuration:
- Pipe section matching actual service (diameter, wall thickness, grade)
- Fitting identical to production design
- Pressurized during welding (simulates hot tap conditions)
- Typical: 50-80% of service pressure
- Contains water or inert gas (not flammable)
- Internal cooling (flowing water to simulate product cooling)
Test procedure:
1. Assemble fitting on test pipe
2. Pressurize to test pressure
3. Initiate flow (if applicable)
4. Weld per proposed WPS
5. Monitor for burn-through, leaks, weld defects
6. Cool naturally (no accelerated cooling)
7. Depressurize and remove test assembly
8. Perform destructive testing
Required tests per ASME Section IX:
- Tensile test (2 specimens): Ultimate strength ≥ base metal minimum
- Side bend tests (2-4 specimens): No cracks > 1/8" on convex surface
- Macro-etch (1 specimen): Verify complete fusion, no porosity
- Hardness survey (optional but recommended): HAZ hardness < 350 HV10
Additional tests for sour service:
- Impact testing (3 specimens): Charpy V-notch at service temp - 20°F
- HIC testing (per NACE TM0284): No cracking
- Hydrogen embrittlement testing: Per NACE TM0177 if required
WPS Development
Welding Procedure Specification Format:
Section 1: Joint Design
- Joint type: Fillet weld, fitting to pipe
- Groove preparation: Not applicable (fillet on OD)
- Backing: Split-tee provides backing
- Root opening: Fit-up gap ≤ 1/16"
- Included angle: Not applicable
Section 2: Base Metal
- Specification: API 5L Grade X52 (per pipe material)
- P-Number: P-1 (carbon steel)
- Thickness: 0.250" minimum to unlimited
Section 3: Filler Metal
- Specification: AWS A5.1 or A5.5
- Classification: E7018 or E71T-1 (for FCAW)
- F-Number: F-4 (low-hydrogen)
- A-Number: A-1 (carbon steel)
- Electrode diameter: 1/8" (root), 5/32" (fill/cap)
Section 4: Positions
- Position of groove: 5G (horizontal fixed pipe)
- Welding progression: Upward (for vertical sections)
Section 5: Preheat
- Preheat temperature: 50°F minimum, 100°F typical
- Interpass temperature: 50-500°F
- Preheat method: Electric blankets, propane torches
Section 6: Post-Weld Heat Treatment
- PWHT: None required (carbon steel, thickness < 1.25")
Section 7: Gas
- Shielding gas (if GMAW/FCAW): 75% Ar / 25% CO₂
- Flow rate: 30-40 cfh
Section 8: Electrical Characteristics
- Current type: DC (DCEP for electrode positive)
- Amperage range: 80-120A (1/8" electrode), 110-150A (5/32")
- Voltage range: 22-28V
- Travel speed: 6-10 in/min
Section 9: Technique
- String bead or weave: Stringer beads (no weave)
- Multi-pass: Yes
- Peening: Not permitted
- Back gouging: Not permitted (no access to interior)
Section 10: Heat Input Control
- Maximum heat input: 30 kJ/inch per pass
Calculation:
HI (kJ/in) = (Volts × Amps × 60) / (Travel speed [in/min] × 1,000)
Example:
V = 24 volts
I = 110 amps
TS = 8 in/min
HI = (24 × 110 × 60) / (8 × 1,000)
HI = 158,400 / 8,000
HI = 19.8 kJ/in ✓ OK (below 30 kJ/in)
Welder Qualification (WPQ)
Welder Performance Qualification Test:
Test requirements:
- Welder must weld test coupon per qualified WPS
- Joint configuration representative of production (fillet weld to pipe)
- Pressurized during welding (if WPS qualified with pressure)
- Visual inspection + NDE (radiography, bend tests, or macro)
Pass/fail criteria:
Visual:
- No cracks
- No incomplete fusion
- Undercut ≤ 1/32" deep
- Reinforcement not excessive (< 1/8" above surface)
Radiography (if used):
- No cracks
- Porosity: max 1/4t or 1/8", whichever is less
- Slag inclusions: max 3/4" long, 1/2t wide
Bend test (if used):
- No cracks > 1/8" after 180° bend
Qualification expiration:
- 6 months if not actively welding (requires re-test)
- Continuous if welding at least once every 6 months
- Revoked if weld fails inspection (requires re-test)
Qualified positions:
- Test in 5G position qualifies: 1G, 2G, 5G
- Does not qualify: 6G (45° incline)
- Separate test required for each process (SMAW vs. FCAW)
Typical test duration: 2-4 hours
Cost: $500-2,000 per welder (testing facility, materials, NDE)
Carbon Equivalent and Weldability
Carbon Equivalent Formula:
IIW (International Institute of Welding) formula:
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
Where elements are in weight %
ASTM formula (used in North America):
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
Acceptable CE for hot tap welding:
CE < 0.40: Excellent weldability, no preheat required
CE = 0.40-0.45: Good weldability, preheat recommended (50-100°F)
CE = 0.45-0.50: Fair weldability, preheat required (100-200°F)
CE > 0.50: Poor weldability, preheat > 200°F, PWHT may be required
Example - API 5L X52:
C = 0.20%
Mn = 1.35%
Si = 0.25%
Cr+Mo+V = negligible
Ni+Cu = negligible
CE = 0.20 + 1.35/6
CE = 0.20 + 0.225
CE = 0.425
CE = 0.43 → Good weldability, preheat 50-100°F recommended
Modern TMCP (thermo-mechanically controlled process) steels have lower CE:
API 5L X52 TMCP: CE ≈ 0.38 (excellent weldability)
API 5L X70 TMCP: CE ≈ 0.42 (good weldability with preheat)
Essential Variables Comparison
| Variable | ASME Section IX | API 1104 |
|---|---|---|
| Wall thickness change | Beyond qualified range requires re-test | ±50% or ±1/8" (whichever is less) requires re-test |
| Diameter change | Not essential variable for fillet welds | OD < 2.375" requires separate qualification |
| Process change | Each process requires separate PQR | Same (SMAW, GMAW, FCAW are separate) |
| Position change | Test in highest difficulty position | 5G qualifies 1G, 2G; 6G qualifies all |
| Filler metal change | F-number change requires re-test | Tensile strength change > 10 ksi requires re-test |
Qualification cost vs. risk: Developing and qualifying a WPS costs $10k-30k (test pipe, welding, NDE, lab testing). Welder qualification costs $500-2k per welder. Compare to consequence of burn-through: potential fire, explosion, fatalities, millions in damages, regulatory penalties, and criminal liability. Proper qualification is essential and cost-effective risk management.
3. Burn-Through Prevention Strategies
Burn-through occurs when welding arc penetrates pipe wall, causing pressurized fluid release through weld puddle.
Burn-Through Mechanism
Heat Balance During Welding:
Heat applied by arc:
Q_arc = V × I × t × η
Where:
Q_arc = Heat energy (Joules)
V = Arc voltage (volts)
I = Welding current (amps)
t = Time (seconds)
η = Arc efficiency (0.7-0.9 for SMAW)
Heat removed by:
1. Conduction into base metal: Q_conduction
2. Convection to internal fluid: Q_convection
3. Radiation to surroundings: Q_radiation
If Q_arc > (Q_conduction + Q_convection + Q_radiation):
→ Base metal melts through → Burn-through
Burn-through temperature:
For carbon steel: 2,800°F (melting point)
Critical factors increasing burn-through risk:
- Thin wall (< 0.250")
- High heat input (> 35 kJ/in)
- Slow travel speed (allows heat accumulation)
- Stagnant fluid (no internal cooling)
- Multiple passes on same location (cumulative heating)
- Gap/mismatch (thin section exposed to arc)
Critical factors reducing burn-through risk:
- Thick wall (> 0.375")
- Low heat input (< 25 kJ/in)
- Fast travel speed (6-10 in/min)
- Flowing fluid (convective cooling)
- Distributed passes (allow cooling between passes)
- Good fit-up (no gaps exposing thin metal)
Image: Burn-Through Heat Balance Diagram
Cross-section showing heat flow: arc heat input → conduction through wall → convection to flowing product. Show critical zone where melt-through occurs.
Heat Input Control
Heat Input Calculation and Limits:
Heat input formula:
HI = (V × I × 60) / (TS × 1,000)
Where:
HI = Heat input (kJ/inch)
V = Arc voltage (volts)
I = Welding current (amps)
TS = Travel speed (inches/minute)
60 = Conversion factor (seconds/minute)
1,000 = Conversion factor (J to kJ)
Industry guidelines:
Hot tap welding maximum HI:
- Normal: 30 kJ/in per pass
- Thin wall (< 0.300"): 25 kJ/in
- Extra-thin (< 0.250"): 20 kJ/in (if allowed at all)
Comparison to standard pipeline welding:
- Standard field girth welds: 40-60 kJ/in typical
- Hot tap welding: 20-30 kJ/in (30-50% reduction)
Example calculation:
Target: HI ≤ 30 kJ/in
Using E7018, 1/8" diameter:
Recommended range: 80-120 amps, 22-26 volts
At 100 amps, 24 volts:
HI = (24 × 100 × 60) / (TS × 1,000)
30 = 144,000 / (TS × 1,000)
TS = 144,000 / 30,000
TS = 4.8 in/min minimum
Travel speed must be ≥ 4.8 in/min to stay below 30 kJ/in
Faster travel (8 in/min):
HI = 144,000 / (8 × 1,000) = 18 kJ/in ✓ Better
Monitoring heat input:
- Weld bead width: Narrower = lower HI
- Interpass temperature: Stay below 500°F
- Temp indicating sticks on interior (if accessible)
Electrode Selection for Low Heat Input
| Electrode | Diameter | Current Range | Application |
|---|---|---|---|
| E7018 | 3/32" | 60-90 A | Root pass, thin wall, low HI |
| E7018 | 1/8" | 90-120 A | All passes, most common for hot taps |
| E7018 | 5/32" | 120-165 A | Fill/cap on thick wall (> 0.500") |
| E71T-1 | 0.045" | 150-200 A | FCAW for higher productivity (if HI can be controlled) |
Internal Cooling Effects
Convective Heat Transfer:
Heat removed by flowing product:
Q_convection = h × A × ΔT
Where:
h = Heat transfer coefficient (Btu/hr·ft²·°F)
A = Internal surface area exposed to arc heat
ΔT = Temperature difference (weld metal temp - fluid temp)
Heat transfer coefficients:
Stagnant gas: h = 1-5 Btu/hr·ft²·°F (very low cooling)
Flowing gas (10 ft/s): h = 10-30 Btu/hr·ft²·°F
Stagnant liquid: h = 50-100 Btu/hr·ft²·°F
Flowing liquid (5 ft/s): h = 200-500 Btu/hr·ft²·°F (excellent cooling)
Effect on burn-through risk:
Flowing liquid: Low risk (high convection, rapid cooling)
Flowing gas: Moderate risk (moderate convection)
Stagnant gas: High risk (minimal convection, heat accumulates)
Industry practice:
Gas lines: Reduce flow to < 60 ft/s but maintain some flow (cooling benefit)
Liquid lines: Maintain flow during welding (excellent cooling, low burn-through risk)
Empty lines: NOT suitable for hot tap welding (no cooling, very high burn-through risk)
Multi-Pass Welding Strategy
Interpass Temperature Control:
Interpass temperature = Base metal temperature between weld passes
Requirements:
- Minimum interpass: Same as preheat (50-150°F typical)
- Maximum interpass: 500°F (ASME Section IX default)
- Exceeding 500°F can cause excessive grain growth, reduced toughness
Cooling time between passes:
For thin wall (< 0.375"), allow 2-5 minutes between passes
For thick wall (> 0.500"), allow 1-3 minutes between passes
Temperature measurement:
- Temp indicating crayons (Tempilstiks)
- Infrared thermometer
- Thermocouple (if accessible)
Weld sequence for circumferential fillet (skip welding):
1. Divide circumference into quarters (4 zones)
2. Weld 2-3" in Zone 1 (stop)
3. Skip to Zone 3 (opposite side), weld 2-3"
4. Skip to Zone 2, weld 2-3"
5. Skip to Zone 4, weld 2-3"
6. Continue pattern, filling gaps between initial welds
7. Cap pass last, maintaining skip sequence
Image: Skip Welding Sequence Diagram
Top-down view of pipe showing 4-zone skip welding sequence. Numbers 1-4 indicate weld order. Show distributed heat pattern vs concentrated heat.
Why skip welding works: By distributing heat around the circumference, no single zone accumulates enough thermal energy to approach burn-through temperature. Each weld segment cools while welding proceeds on the opposite side. This technique is mandatory for thin wall hot tap welding.
Number of passes:
Fillet size: 1/4" leg → 2-3 passes
Fillet size: 3/8" leg → 4-6 passes
Fillet size: 1/2" leg → 6-9 passes
More passes = Lower heat input per pass = Reduced burn-through risk
Emergency Response to Burn-Through
If Burn-Through Occurs During Welding:
Immediate actions:
1. Stop welding immediately (do not add more heat)
2. Evacuate to safe distance (100+ ft upwind)
3. Alert control room and site supervisor
4. Call emergency services (911) if fire or large release
DO NOT attempt to "weld over" burn-through:
- Arc will blow out (pressure prevents puddle formation)
- Additional heat will enlarge hole
- Risk of ignition is extreme
Response options (in order of preference):
A. Small burn-through (< 1/8" hole), low pressure (< 100 psi):
- Apply leak sealant (epoxy putty, Belzona)
- Clamp small plate over leak
- Reduce pressure if possible
- Monitor and plan repair shutdown
B. Medium burn-through (1/8" to 1/2" hole), moderate pressure (100-500 psi):
- Do not attempt to stop leak
- Close block valves (isolate section)
- Allow depressurization
- Repair after pressure relief
C. Large burn-through (> 1/2" hole) or high pressure (> 500 psi):
- Evacuate all personnel
- Close remote block valves if safe to do so
- Protect exposures (cool adjacent equipment with water)
- Allow line to blow down
- If ignited, let burn under control (safer than gas cloud)
Prevention is critical:
- Burn-through is nearly impossible to repair while pressurized
- Results in extended outage (depressurize, drain, repair, test, re-commission)
- Cost: $100k-$1M+ (lost production, repair, regulatory response)
Temperature monitoring: If possible, install temp monitoring on internal surface (thermocouples through temporary hot tap upstream). Set alarm at 400-500°F. If alarm activates during welding, STOP immediately, allow cooling, and adjust parameters (lower current, faster travel, longer interpass time). This provides early warning before catastrophic burn-through.
4. Welding Execution and Quality Control
Disciplined execution of qualified procedures ensures successful hot tap welding without burn-through or defects.
Pre-Weld Surface Preparation
SSPC Surface Preparation Standards:
Coating removal:
- Remove coating 6" beyond fitting footprint (all sides)
- Blast clean to SSPC-SP-10 (near-white metal, 95% clean)
- Alternative: Grind to bare metal (SSPC-SP-11, power tool cleaning)
Surface cleanliness:
- No rust, scale, coating, or contamination
- Dry (no moisture)
- Profile: 1-3 mils (25-75 microns) for coating adhesion after welding
Fit-up requirements:
- Root gap: 0-1/16" (tight fit preferred)
- Hi-lo (mismatch): ≤ 1/8" or 10% of wall thickness, whichever is less
- Alignment: Fitting perpendicular to pipe (±2° tolerance)
Visual inspection before welding:
- Wall thickness verification (UT): ≥ 0.250" + CA
- No dents, buckles, or gouges in weld zone
- No laminations or defects (UT scan if suspected)
- No existing welds within 3× wall thickness of hot tap location
Temporary attachments:
- Alignment lugs: Tack weld to pipe, remove after fitting aligned
- Do not weld temporary attachments within 2" of final weld location
- Grind smooth after removal
Preheat and Interpass Temperature
Preheat Application:
Preheat purpose:
1. Reduce cooling rate (prevent hard HAZ microstructure)
2. Drive off moisture (reduce hydrogen)
3. Reduce thermal gradients (lower residual stress)
Preheat temperature determination:
Based on carbon equivalent (CE) and wall thickness:
CE < 0.40: No preheat required (50°F minimum)
CE = 0.40-0.45: Preheat 50-100°F
CE = 0.45-0.50: Preheat 100-200°F
CE > 0.50: Preheat > 200°F
Increase preheat for:
- Thick wall (> 0.750"): Add 50°F
- Cold ambient (< 32°F): Add 50-100°F
- Restrained joints: Add 50°F
- Sour service (H₂S): Add 50-100°F
Preheat methods:
1. Electric heating blankets (best control, uniform heating)
2. Propane/natural gas torches (localized, requires care)
3. Induction coils (fast, expensive equipment)
Preheat zone:
- 3× wall thickness on each side of weld (6t total width)
- Full circumference around pipe
Verification:
- Measure with temp indicating crayons or IR thermometer
- Record preheat temperature on WPS traveler
- Do not begin welding until preheat achieved
Interpass temperature maintenance:
- Monitor between each pass
- If temp exceeds 500°F, stop welding and allow cooling
- If temp drops below minimum preheat, re-apply heat before next pass
Welding Sequence and Technique
Multi-Pass Welding Procedure:
Pass 1 (Root pass):
- Electrode: E7018, 1/8" diameter
- Current: 90-110 A (DCEP)
- Travel speed: 6-8 in/min
- Bead size: 1/8" to 3/16" width
- Purpose: Establish fusion to base metal
Technique:
- Slight drag (10-15° push angle)
- Stringer bead (no weaving)
- Maintain consistent arc length (1/8")
- Steady travel speed (no stopping)
Pass 2-3 (Fill passes):
- Electrode: E7018, 1/8" or 5/32" diameter
- Current: 100-130 A
- Travel speed: 6-10 in/min
- Overlap: 50% of previous bead
Technique:
- Fill to 1/16" below final profile
- Stringer beads, no weaving
- Tie-in at toe of previous pass (avoid trapped slag)
Final pass (Cap pass):
- Electrode: E7018, 1/8" diameter
- Current: 90-110 A (lower to control profile)
- Travel speed: 6-8 in/min
- Profile: Smooth transition, no undercut
Technique:
- Slight weave (1-2× electrode diameter)
- Feather edges (prevent undercut at toes)
- Smooth, convex profile
Slag removal between passes:
- Wire brush and chipping hammer
- Remove all slag before next pass
- Inspect for defects (cracks, porosity, incomplete fusion)
Weld sequence for longitudinal seams:
- Start at bottom (6 o'clock)
- Weld upward to top (12 o'clock)
- Flip to opposite side, repeat
- This "uphand" technique improves quality
Weld sequence for circumferential fillet:
- Use skip sequence (described in Section 3)
- Prevents overheating and distortion
Common Weld Defects and Causes
| Defect | Cause | Prevention |
|---|---|---|
| Undercut | Excessive current, fast travel, improper angle | Lower current, slower travel, pause at toes |
| Overlap (cold lap) | Low current, slow travel, insufficient heat | Increase current, improve technique |
| Porosity | Moisture, contamination, long arc, wet electrodes | Clean surface, dry electrodes, short arc |
| Slag inclusions | Incomplete slag removal, improper technique | Thorough cleaning between passes, proper tie-in |
| Incomplete fusion | Low heat input, fast travel, incorrect angle | Increase current, slower travel, aim at base metal |
| Cracks (hot/cold) | High restraint, hydrogen, rapid cooling, high carbon | Preheat, low-hydrogen electrodes, controlled cooling |
| Excessive spatter | High current, long arc, dirty base metal | Lower current, short arc, clean surface |
Welding Consumable Storage
Low-Hydrogen Electrode Handling (E7018):
Storage requirements (AWS A5.1):
- Unopened containers: Store in dry location (< 50% RH)
- Opened containers: Store in electrode oven at 250-300°F
- Maximum exposure time (after removal from oven): 4 hours
- Re-drying: If exposed > 4 hours, re-dry at 650-700°F for 1 hour
Moisture effects:
- Hydrogen source: H₂O → H₂ (absorbed in weld metal)
- Hydrogen-induced cracking (cold cracking)
- Porosity (hydrogen gas trapped in solidifying metal)
Field practice:
- Portable electrode ovens (120V or propane heated)
- Issue electrodes in small quantities (1-2 hour supply)
- Discard exposed electrodes at end of shift (do not re-use)
- Do not use electrodes that have been dropped in mud/water
Electrode conditioning:
Visual inspection before use:
- Flux coating intact (no cracks, spalling)
- No rust or contamination
- Not bent or damaged
FCAW wire:
- Less sensitive to moisture (flux-cored)
- Store in original packaging
- Protect from weather (rain, snow)
Weld bead appearance: Experienced welders can judge weld quality by visual appearance. Good weld: Uniform ripples (8-12 per inch), smooth profile, clean tie-in at edges, no undercut/overlap, minimal spatter. Poor weld: Erratic ripples, excessive reinforcement, rough surface, undercut, heavy spatter. Visual appearance correlates well with internal soundness.
5. Inspection, Testing, and Acceptance
Comprehensive inspection verifies weld integrity and absence of defects before pipeline returns to service.
Visual Inspection (VT)
Visual Weld Inspection Criteria (API 1104):
Inspection timing:
- During welding: Monitor each pass for defects
- After welding: 100% visual inspection of completed welds
- After cooling: Final inspection when weld reaches ambient temperature
Inspection procedure:
1. Remove all slag, spatter, scale
2. Wire brush to bright metal
3. Inspect under good lighting (natural light or 100+ foot-candles)
4. Magnifying glass (2-3×) for close examination
5. Measure dimensions (weld size, profile)
Acceptance criteria (API 1104):
Cracks: None permitted (any size)
Incomplete fusion: None permitted
Undercut depth: ≤ 1/32" (0.031")
Undercut length: ≤ 2" continuous, ≤ 10% of weld length total
Overlap: None permitted
Porosity: Surface porosity prohibited in structural welds
Weld profile: Smooth, gradual transition to base metal
Reinforcement: ≤ 1/8" above surface
Rejection criteria:
- Any crack (even pinhole size)
- Incomplete fusion visible on surface
- Undercut deeper than 1/32"
- Surface porosity (interconnected voids)
- Overlap (unfused metal lapping on base metal)
Documentation:
- Weld map (identify each weld by number)
- Visual inspection report (accept/reject, defect location)
- Photographs (before, during, after welding)
Magnetic Particle Testing (MT)
MT Inspection Procedure (ASTM E709):
When required:
- 100% of production hot tap welds (standard practice)
- Per operator procedures or specifications
- After repair welding
MT principle:
- Magnetic field applied to weld area
- Magnetic particles (iron powder) applied to surface
- Defects (cracks, lack of fusion) disrupt magnetic field
- Particles concentrate at discontinuities (visible indication)
Procedure:
1. Clean surface (remove slag, oil, loose scale)
2. Apply magnetizing current (yoke, prods, or coil)
3. Apply magnetic particles (wet or dry method)
- Wet method: Fluorescent particles in suspension (viewed under UV light)
- Dry method: Iron powder (viewed under white light)
4. Inspect for indications immediately
5. Evaluate indications (relevant vs. non-relevant)
6. Demagnetize after inspection
7. Document results
Acceptance criteria (ASME Section V):
Linear indications (cracks, lack of fusion):
- > 1/16" long: Unacceptable, requires repair
- Aligned (within 1/16"): Count as single indication
Rounded indications (porosity):
- > 3/16" diameter: Unacceptable
- Cluster: 4+ indications in 1/2" length, unacceptable
Re-inspection:
After repair, 100% MT re-inspection required
Typical cost: $150-300 per weld (vendor mobilization, inspection)
Liquid Penetrant Testing (PT)
Alternative to MT for non-ferromagnetic materials (stainless steel, aluminum):
PT Procedure (ASTM E165):
Process:
1. Clean surface (remove all contamination)
2. Apply penetrant (dye or fluorescent)
3. Dwell time (5-30 minutes, penetrant seeps into defects)
4. Remove excess penetrant (wipe or rinse)
5. Apply developer (white powder, draws penetrant out of defects)
6. Inspect for indications (colored dye or UV fluorescent)
Advantages:
- Detects surface-breaking defects only (like MT)
- Works on any material (metals, ceramics, plastics)
- Portable (aerosol cans)
- Lower cost than MT
Disadvantages:
- Less sensitive than MT for ferromagnetic materials
- Longer process time (dwell + development)
- Messy (chemicals, residue)
Acceptance criteria: Same as MT
Radiographic Testing (RT)
RT Inspection (ASME Section V):
When required:
- If specified in contract or operator procedures
- Typically 10-25% of hot tap welds (statistical sample)
- 100% if critical service (sour gas, high pressure)
RT principle:
- X-ray or gamma ray passes through weld
- Film or digital detector records image
- Defects (voids, slag, cracks) appear as dark areas on film
Procedure:
1. Set up radiation source and film on opposite sides of weld
2. Expose film (1-10 minutes depending on thickness, source strength)
3. Develop film (chemical process)
4. Interpret film (certified radiographer)
5. Evaluate defects per acceptance criteria
Acceptance criteria (ASME B31.8, Appendix I):
Porosity: Scattered porosity acceptable if ≤ 1/8" diameter
Slag inclusions: Max length = 3/4", width ≤ 1/2× wall thickness
Lack of fusion: Not acceptable (any size)
Cracks: Not acceptable (any size)
Undercut: Measurable on film if > 1/32" deep
Limitations for hot tap welds:
- Single-wall exposure only (cannot access inside for double-wall technique)
- Reduced sensitivity (overlap of fitting and pipe)
- Fillet welds difficult to interpret (complex geometry)
Typical cost: $300-800 per weld (RT vendor, film, interpretation)
Ultrasonic Testing (UT)
UT Inspection (ASTM E164):
UT advantages over RT:
- No radiation safety concerns (no exclusion zone)
- Faster (immediate results)
- Better crack detection (perpendicular to beam)
- Volumetric examination (not just 2D image)
UT limitations:
- Requires skilled operator (interpretation)
- Surface preparation critical (smooth, clean)
- Access required (may not be possible on hot tap fitting)
Typical application for hot taps:
- Wall thickness measurement (before welding): 100%
- Weld inspection: Rarely used (geometry complex)
- Lamination check (if suspected): As needed
UT thickness measurement:
Equipment: Ultrasonic thickness gauge (Olympus, GE)
Procedure:
1. Clean surface (remove paint, scale)
2. Apply couplant (gel)
3. Place probe on surface
4. Read digital display (thickness in inches or mm)
Acceptance for hot tap welding:
Wall thickness ≥ 0.250" + Corrosion allowance (CA)
If measured thickness < minimum:
- Relocate hot tap to thicker wall section
- Upgrade fitting design (reinforcing pad)
- Re-evaluate design pressure
Pressure Testing
Pneumatic Pressure Test:
Purpose: Verify leak-tight integrity before service
Test procedure:
1. Install completion plug or test blind flange
2. Isolate from pipeline (close valve)
3. Pressurize with air or nitrogen to test pressure
- Test pressure = 1.5× design pressure or MAOP
- Alternative: 110% MAOP (per some operator specs)
4. Hold test pressure for 10 minutes minimum
5. Monitor pressure gauge (no pressure drop)
6. Apply soap bubble solution to all welds
7. Observe for bubbles (indicates leak)
Acceptance:
- No pressure drop
- No visible leaks (soap bubbles)
- No audible leaks
If leak detected:
- Depressurize
- Mark leak location
- Repair weld (add additional passes or grind out and re-weld)
- Re-test after repair
Safety precautions:
- Pneumatic test is hazardous (stored energy)
- Evacuate area during pressurization (100+ ft)
- Use remote pressure gauge or transducer (monitor from distance)
- Wear face shield and safety glasses
- Do not approach pressurized assembly until test complete
Hydrostatic test:
- Preferred if fitting can be filled with water
- Less hazardous than pneumatic (water not compressible)
- Test pressure: 1.5× MAOP for 4 hours
- Acceptance: No leaks, no visible weeping
Repair Procedures
Weld Repair (If Defects Found):
Minor defects (undercut, surface porosity):
1. Mark defect location with soapstone
2. Grind out defect (remove completely)
3. Check grind depth (do not reduce wall thickness excessively)
4. Clean ground area (wire brush, solvent wipe)
5. Re-weld using original WPS
6. Visual inspect repaired area
7. MT or PT inspect repaired area
8. Accept or reject
Major defects (cracks, lack of fusion):
1. Grind out defect plus 1" beyond each end (ensure complete removal)
2. Grind to sound metal (check with MT/PT)
3. Create groove (U-shape) for good access
4. Preheat if required
5. Fill groove with multiple weld passes (per WPS)
6. Final pass flush with original surface
7. Full NDE of repair (MT/PT, possibly RT)
Acceptance of repair:
- Same criteria as original weld
- No defects in repaired area
- If defect reappears after repair: Cut out and replace fitting
Repair limitations:
- Maximum 3 repair attempts on same location
- If still failing after 3 repairs: Replace fitting
- Document all repairs (location, method, NDE results)
Complete rejection:
If weld cannot be repaired (extensive cracking, burn-through):
- Cut out fitting (after line depressurization)
- Install new fitting
- Re-weld per original WPS
- Full inspection and testing
Inspection sequence: Always perform visual inspection first (100%), then MT/PT (100%), then RT/UT if required (10-25% sample). Visual catches most defects (80%), MT/PT catches surface defects missed by visual, RT/UT catches internal defects. This layered approach provides high confidence in weld integrity while controlling inspection costs.
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