Flange Bolting: ASME B16.5 Patterns, Gaskets & Torque Sequences

ASME B16.5 Bolt Patterns: What Goes Where

Every ASME B16.5 flange has a defined number of bolt holes, bolt hole diameter, bolt circle diameter, and bolt size. These are not engineering judgment calls; they are fixed by the standard. Using the wrong number of bolts or the wrong bolt size is a code violation and a leak waiting to happen.

Common bolt patterns for Class 150 flanges (the most common in plant utilities):

• 2" pipe: 4 bolts, 5/8" diameter, 4.75" bolt circle
• 4" pipe: 8 bolts, 5/8" diameter, 7.88" bolt circle
• 6" pipe: 8 bolts, 3/4" diameter, 9.50" bolt circle
• 8" pipe: 8 bolts, 3/4" diameter, 11.75" bolt circle
• 10" pipe: 12 bolts, 7/8" diameter, 14.25" bolt circle
• 12" pipe: 12 bolts, 7/8" diameter, 17.00" bolt circle
• 16" pipe: 16 bolts, 1" diameter, 21.25" bolt circle
• 24" pipe: 20 bolts, 1-1/8" diameter, 29.50" bolt circle

Higher pressure classes use more bolts and/or larger bolt sizes for the same pipe size. A 6" Class 300 flange uses 12 bolts at 3/4" diameter versus 8 bolts at 3/4" for Class 150. A 6" Class 600 uses 12 bolts at 7/8" diameter. The bolt circle diameter also changes with class because the flange OD is larger.

Bolt material is specified by the service conditions. ASTM A193 Grade B7 (chrome-moly alloy steel) is the standard for elevated temperature service. ASTM A193 Grade B8M (316 stainless) is used for corrosive environments. Nuts are typically ASTM A194 Grade 2H (heavy hex) for B7 bolts. Always match the bolt and nut grades; mixing grades can result in galling or incorrect preload behavior.

Gasket Selection: Raised Face, RTJ, and What Actually Seals

The gasket is the component that actually creates the seal. Flange faces compress the gasket, and the gasket material deforms to fill surface irregularities and create a leak-tight barrier. Different gasket types require different levels of bolt load (gasket seating stress) and have different abilities to recover from relaxation and thermal cycling.

Common gasket types for raised face (RF) flanges:

• Compressed fiber (non-asbestos): The simplest and cheapest option. Suitable for low-pressure, low-temperature, non-critical services (water, air, low-pressure steam). Requires moderate seating stress (3,500 to 6,000 psi on the gasket face). Poor recovery from relaxation.
• Spiral-wound (with inner and outer ring): The workhorse gasket for process piping. Alternating layers of metal (usually 304 or 316 stainless) and filler (flexible graphite or PTFE) wound into a spiral. The metal provides resilience (spring-back after compression), and the filler provides the seal. Requires seating stress of 10,000 to 25,000 psi. Good recovery from thermal cycling. The inner ring prevents inward buckling, and the outer centering ring positions the gasket on the bolt circle.
• Flexible graphite (Grafoil / Thermiculite): Sheet graphite cut to gasket dimensions or laminated to a metal core. Excellent chemical resistance and temperature range (-200°F to 850°F for graphite). Requires careful bolt loading because the material creeps under load. Commonly used in heat exchanger joints and high-temperature services.
• PTFE envelope: A filler gasket wrapped in PTFE for chemical resistance. Used in chemical plants where the process fluid attacks other gasket materials. PTFE cold-flows under load, which makes retorquing essential.

Ring-type joint (RTJ) gaskets are solid metal rings (soft iron, 304 SS, 316 SS, or Inconel) that seat in grooves machined into the flange faces. RTJ flanges are used in high-pressure services (Class 600 and above) and provide a metal-to-metal seal. The gasket material must be softer than the flange face material so the gasket deforms, not the flange. RTJ gaskets are single-use: once compressed, the ring takes a permanent set and cannot re-seal if removed.

Torque Sequence: The Star Pattern and Multi-Pass Method

The single most important assembly step for a flanged joint is the torque sequence. ASME PCC-1 (Guidelines for Pressure Boundary Bolted Flange Joint Assembly) defines the procedure that prevents the elastic interaction and uneven gasket loading that cause most flange leaks.

The multi-pass star pattern:

• Pass 1 (snug): Hand-tighten all nuts finger-tight, ensuring the gasket is centered and the flange faces are parallel. If the flanges are not parallel within 1/16 inch across the face, correct the pipe alignment before proceeding.
• Pass 2 (30% target torque): Tighten in star (cross) pattern. For an 8-bolt flange: 1-5-3-7-2-6-4-8. This brings the gasket into initial contact uniformly.
• Pass 3 (60% target torque): Same star pattern. The gasket is now loaded enough to begin sealing.
• Pass 4 (100% target torque): Same star pattern. Full design load on the gasket.
• Pass 5 (100% check, circular): Go around the flange sequentially (1-2-3-4-5-6-7-8) and re-torque any bolt that turns more than 5 degrees. This catches residual elastic interaction from the cross pattern.

For large flanges (12 bolts or more), ASME PCC-1 recommends additional intermediate passes. Some specifications call for passes at 20%, 40%, 60%, 80%, and 100% for critical services. The additional passes take time but produce more uniform gasket stress. On heat exchanger shell flanges with 40 or more bolts, the elastic interaction between bolts is severe, and skipping passes leads to predictable leaks at startup.

Never use an impact wrench for the final torque pass. Impact wrenches deliver torque in short pulses with poor accuracy and inconsistent friction behavior. Use them for running nuts down to snug, then switch to a calibrated torque wrench for the controlled passes. Hydraulic torque wrenches are preferred for large bolts (1-1/4 inch and above) where manual torque wrenches become impractical.

Why Flanges Leak: The Five Most Common Causes

Flange leaks have predictable causes. Understanding them prevents repeating the same mistakes:

• Uneven bolt loading: The most common cause. If one bolt is at 80% of target and the adjacent bolt is at 120%, the gasket is overloaded on one side and underloaded on the other. The underloaded zone leaks. This results from skipping the multi-pass star pattern or using uncalibrated tools.
• Gasket creep and relaxation: Spiral-wound and graphite gaskets creep (compress permanently) under sustained load, especially during the first thermal cycle. The bolt load drops as the gasket thins. If the residual bolt load falls below the gasket's minimum seating stress, the joint leaks. The fix is hot retorque: re-torque the joint after the first heat-up cycle while the system is at operating temperature.
• Flange face damage: Scratches, corrosion pitting, or weld spatter on the flange face create channels that the gasket cannot seal. Inspect flange faces before assembly. The ASME allowable surface finish for spiral-wound gaskets is 125 to 250 microinch Ra (roughness average). Smoother is not better; the gasket needs some surface roughness to grip.
• Wrong gasket type or material: A compressed fiber gasket in a steam service that sees 500°F will degrade and blow out. A PTFE gasket in a service above 400°F will cold-flow and extrude. Match the gasket material to the temperature, pressure, and chemical exposure.
• Pipe strain: If the piping exerts axial or bending loads on the flange, the bolt loading becomes non-uniform and the gasket is partially unloaded on one side. Pipe strain leaks often appear at startup when the system heats up and thermal expansion changes the pipe forces. The fix is proper pipe support and spring hangers, not additional bolt torque.

Most plants that adopt ASME PCC-1 procedures and train their mechanics on proper bolting technique see a 50% to 80% reduction in flange leaks within the first year. The procedures are not complicated. They are just different from the "run the bolts down and hope for the best" approach that persists in too many maintenance shops.

Hot Retorque: The Step Most Plants Skip

Gasket relaxation during the first thermal cycle typically reduces bolt preload by 5% to 20%. On spiral-wound gaskets, the relaxation is 5% to 10%. On flexible graphite, it can be 10% to 20%. On PTFE-based gaskets, it can exceed 20%. If the initial bolt torque barely achieved the minimum gasket seating stress, a 15% relaxation drops the joint below the seal threshold, and it leaks at startup.

Hot retorque means re-torquing the joint at operating temperature after the first heat-up. The procedure is: bring the system to operating temperature, hold for at least 4 hours to reach thermal equilibrium, then retorque all bolts in a circular pattern (not star pattern; the gasket is already seated) to the original target torque value. Any bolt that turns more than 5 degrees was under-loaded from relaxation. Most bolts will turn 10 to 30 degrees during hot retorque, indicating significant gasket creep that would have caused a leak on the next pressure cycle.

Hot retorque requires safety precautions: the system may be under pressure and at elevated temperature. Some facilities perform hot retorque with the system depressurized but at temperature. Others retorque under pressure using specific safety protocols (face shields, body positioning away from the flange face, approved hot-bolting procedures). Follow your facility's hot-work and bolted joint safety procedures.

For joints that cannot be retorqued in service (buried flanges, insulated joints, confined space locations), use live-loading: Belleville (disc spring) washers stacked under the nut maintain bolt load as the gasket relaxes. The spring washers provide a compliance buffer that keeps the gasket stress above the sealing threshold despite creep. Live-loading adds $5 to $15 per bolt but eliminates the need for hot retorque on critical or inaccessible joints.