10 Common Gasket & Bolt Mistakes

Gasket and bolt errors are the leading cause of flanged joint leaks. These 10 mistakes cover gasket selection, bolt torquing, and material mismatches that cause leakage, blowout, and rework.

Gasket Mistakes

1. Reusing a Gasket After Disassembly

Error: Reinstalling a used gasket after opening a flanged joint for inspection or maintenance. Risk: Gaskets take a permanent compression set after initial tightening. The deformed gasket cannot re-conform to the flange face irregularities, resulting in leakage. Fix: Always install a new gasket when reassembling any flanged joint. Used gaskets should be discarded.

2. Wrong Gasket Type for the Flange Face

Error: Using a full-face gasket on a raised face (RF) flange, or a ring gasket on a flat face (FF) flange without checking compatibility. Risk: A full-face gasket on an RF flange wastes bolt load on the area outside the raised face, reducing sealing pressure. A ring gasket on an FF flange without centering leaves the gasket unsupported. Fix: Match gasket type to the flange face: ring-type spiral wound for RF flanges, full-face for FF flanges, and ring gaskets (R, RX, BX) for RTJ flanges.

3. Using PTFE Envelope Gaskets Above Their Temperature Limit

Error: Specifying PTFE or PTFE-envelope gaskets for services above 260°C (500°F). Risk: PTFE creeps (cold-flows) under bolt load at elevated temperatures, extruding from between the flanges. Above 327°C, PTFE decomposes. The joint loses seal integrity. Fix: Use spiral wound gaskets with graphite filler or solid metal gaskets for services above 260°C. See gasket selection guide.

4. Wrong Spiral Wound Gasket Inner Ring Material

Error: Using a carbon steel inner ring in a stainless steel piping system, or vice versa. Risk: Galvanic corrosion between the inner ring and the pipe bore. The corroded ring contaminates the process fluid and weakens the gasket. Fix: Match the inner ring material to the piping material (e.g., 304SS inner ring for 304SS pipe). The outer centering ring is typically carbon steel.

5. Incorrect Gasket ID/OD for the Flange Class

Error: Ordering gaskets by NPS only, without specifying the pressure class (which affects the gasket dimensions). Risk: Different flange classes have different raised face diameters and bolt circle dimensions. A Class 150 gasket is physically different from a Class 600 gasket for the same NPS. Fix: Specify gaskets by NPS, class, and gasket type per ASME B16.20 or B16.21.

Bolt Mistakes

6. Wrong Torque Sequence (Sequential Instead of Star Pattern)

Error: Tightening flange bolts sequentially around the circle instead of using a star (cross) pattern. Risk: Sequential tightening causes uneven gasket compression; one side is fully loaded while the opposite side is loose. The gasket blows out on the loose side. Fix: Use a star (cross) pattern in minimum 3 passes (30%, 60%, 100% of target torque) per ASME PCC-1. Add a final 360° check pass.

7. Using Machine Bolts Instead of Stud Bolts

Error: Using hex-head machine bolts for ASME B16.5 flanged joints instead of stud bolts. Risk: Machine bolts have a shorter thread length and a head that may not distribute load evenly. ASME B16.5 specifies stud bolts with a nut on each end. Machine bolts are a code violation for pressure flanged joints. Fix: Use ASTM A193 stud bolts with ASTM A194 hex nuts for all ASME B16.5 flanged joints.

8. Wrong Bolt Material for the Service Temperature

Error: Using A193 B7 stud bolts for service above 450°C or below -40°C. Risk: B7 (4140 alloy steel) loses yield strength above 450°C and becomes brittle below -40°C. The bolt fails to maintain gasket load, causing leakage. Fix: Use B16 bolts for high-temperature service (up to 540°C) and B7M or L7 bolts for low-temperature service. Check bolt material selection.

9. Not Lubricating Bolt Threads Before Torquing

Error: Torquing dry bolts without thread and nut-face lubrication. Risk: Without lubrication, 80-90% of applied torque is lost to friction. The actual bolt stretch (and gasket load) is only 10-20% of what the torque chart assumes. The joint is under-loaded and leaks. Fix: Apply thread lubricant (anti-seize compound) per ASME PCC-1 before torquing. Re-calibrate torque values if using a different lubricant than the torque chart assumes.

10. Bolt Length Too Short for Proper Thread Engagement

Error: Using bolts that are flush with or recessed inside the nut, with no thread protrusion. Risk: Insufficient thread engagement reduces the clamping force and joint integrity. Under vibration or thermal cycling, the nut can work loose. Fix: Bolts should protrude at least 1-3 full threads past the nut face per ASME PCC-1. Calculate bolt length based on flange thickness + gasket + nut height + protrusion.

Frequently Asked Questions

Can I reuse a gasket after opening a flanged joint?

Never. Gaskets undergo permanent deformation (compression set) during initial bolt-up. A reused gasket cannot re-conform to the microscopic flange face irregularities, and the seal will leak. Always install a new gasket when reassembling any flanged connection.

What is the correct bolt tightening sequence for flanges?

Use a star (cross) tightening pattern in at least 3 passes: 30%, 60%, and 100% of the target torque. After reaching full torque, make a final 360° verification pass. This ensures even gasket compression across the entire flange face. See torque charts for target values.

What is the difference between a stud bolt and a machine bolt?

A stud bolt is a fully or partially threaded rod with a nut on each end. A machine bolt has an integral hex head on one end. ASME B16.5 flanged joints require stud bolts (ASTM A193 + A194 nuts), not machine bolts. Machine bolts are only acceptable for structural or non-pressure applications.

Why can't I use PTFE gaskets above 260°C?

PTFE begins to creep (cold-flow) under compressive load above 260°C, gradually extruding from between the flanges. Above 327°C, PTFE decomposes and releases toxic fumes. For high-temperature services, use spiral wound gaskets with flexible graphite filler or solid metal gaskets.

What happens if I use the wrong gasket type for a raised face flange?

Using a full-face gasket on an RF flange extends the gasket beyond the raised face area. The bolt load is distributed over the full flange face instead of concentrated on the raised face, reducing the gasket seating stress below the minimum required for sealing. Use ring-type gaskets dimensioned to the RF diameter.