Installation Overview
Proper flange gasket installation is critical for achieving leak-free bolted joints in pressure systems. The majority of gasket failures are traced not to material defects but to improper installation — particularly incorrect bolt tightening sequences and torque application.
A correctly installed gasket creates uniform compression across the entire sealing surface. This prevents flange warping, gasket extrusion, and the formation of leak paths. Conversely, poor technique can damage even the highest-quality gaskets.
Important
This guide is an introductory overview only. It is not a substitute for formal training, certification, manufacturer's guidelines, site-specific procedures, or applicable codes and standards. Improperly installed gaskets can cause dangerous leaks, equipment damage, and serious injury. Qualified personnel only, following approved procedures.
Standards & Guidelines
Several international standards provide detailed guidance on flange joint assembly, covering surface preparation, bolt tightening methods, torque calculation, tooling, and inspection. Always consult the applicable standards for your application and jurisdiction.
ASME PCC-1
Pressure Boundary Bolted Flange Joint Assembly
The primary reference for bolted flange joint assembly. Covers tightening sequences, torque methods, gasket seating stress, and quality control.
API 601 (Withdrawn)
Metallic Gaskets for Raised-Face Pipe Flanges
A gasket product standard for metallic gaskets on refinery pipe flanges, not an installation code. Its last edition was 1988; the content moved into ASME B16.20 and API 601 has since been withdrawn. Referenced here for legacy documentation only.
EN 1591-4
Assembler Competence
Part 4 of the European EN 1591 series on bolted circular flange connections. It sets out how to qualify the competence of personnel who assemble bolted joints on critical-service pressure systems. It covers who assembles the joint, not a tightening method.
This guide demonstrates the sequence pattern based on ASME PCC-1. Detailed requirements (torque tables, lubrication specifications, surface finish tolerances) must be obtained from the applicable standards. Modern methods such as controlled-yield and ultrasonic tensioning may differ.
Surface Preparation
Both the flange faces and the gasket must be clean, undamaged, and suitable for service. Inspect and prepare all components before assembly.
Flange Face Inspection
- Check for pitting, scoring, warping, or corrosion on sealing surfaces
- Verify surface finish meets gasket manufacturer requirements (typically 3.2–6.3 µm Ra for soft gaskets; solid metal gaskets need a smoother face, typically 1.6 µm Ra max)
- Check flange flatness using a straight edge and feeler gauge
- Circumferential scratches that follow the serration direction are generally tolerable; radial scratches or gouges that cross the gasket seating face create leak paths. Assess defects against ASME PCC-1 Appendix D.
Cleaning
- 1. Remove all old gasket material using brass scrapers, wire brushes, or approved solvents — never use steel tools on soft flanges. Always brush in the direction of the flange face grooves and use non-ferrous tools (e.g. brass drift) to avoid damaging the sealing surface
- 2. Clean all rust, scale, paint, and corrosion from sealing surfaces
- 3. Degrease with approved solvents (ensure compatibility with gasket material)
- 4. Do not touch cleaned surfaces with bare hands — skin oils can cause leaks
Install the gasket dry. Moisture, lubricant, or solvent residue on the sealing face can reduce sealing performance and promote gasket degradation in service.
Gasket & Bolt Inspection
- • Verify gasket is the correct size, style, and material. Inspect for damage or foreign material
- • Never reuse gaskets. Once a joint is broken, install a new gasket even if the old one appears undamaged; a replacement costs far less than unplanned downtime, leakage, or an environmental incident
- • Check all bolts for damaged, corroded, or stretched threads. Verify same length, grade, and material
- • Clean and lubricate threads per procedure — lubrication dramatically affects achieved bolt stress
Before installing the gasket, verify flange alignment. Check for pipe strain, parallelism, and bolt hole alignment, and use feeler gauges to measure flange gap uniformity around the circumference. Misaligned flanges will prevent uniform compression regardless of torque. For alignment acceptance limits, refer to ASME PCC-1 Appendix E or your site procedure.
Torque Values
Bolt torque values are not generic. The correct torque depends on bolt size, material grade, thread condition, lubrication, gasket type, flange class, and service conditions. Do not use "rule of thumb" torque values — requirements must be calculated based on the specific bolt and gasket combination.
Key Factors
- • Bolt properties: Diameter, material, proof strength (ASME PCC-1 Appendix O). Nut proof load, washer selection, and bolt grade must all be considered per the applicable standard
- • Gasket seating stress: From manufacturer data sheets
- • Lubrication: Dry vs. lubricated bolts can produce 30–50% difference in achieved stress at the same torque. Apply lubricant consistently and do not substitute without recalculating
- • Tooling: Use calibrated torque wrenches within their 20–80% accuracy range. For large or critical bolts, consider hydraulic tensioners
Apply lubricant to bolt threads and nut faces only, and keep it off the gasket and flange sealing surfaces. The lubricant’s rated service temperature must be within the process temperature limits.
Common Mistakes
- × Using generic torque charts without accounting for actual assembly conditions
- × Applying torque in a single pass to 100% — use a multi-pass tightening sequence
- × Using impact wrenches on critical joints (torque cannot be accurately controlled)
- × Pushing bolt stress past about 70% of yield without an engineering assessment (little safety margin, risks bolt failure)
In our experience supporting plant maintenance teams, the most underestimated variable in bolt torque is lubrication. We've seen the same torque value produce very different bolt loads because one crew used moly paste and another used copper anti-seize. The nut factor (K-factor, the coefficient relating applied torque to actual bolt load) can vary by over 50% between lubricant types. Our engineers always ask: "What lubricant are you using, and does your torque table account for it?" If the answer is uncertain, the torque value is meaningless.
Engineering calculation required: This guide does not provide specific torque values. Torque must be calculated by a qualified engineer based on the specific application, following ASME PCC-1 Appendix O or equivalent. Contact Universal Gaskets if you need assistance.
Bolt Tightening Sequence
The order in which bolts are tightened has a profound impact on gasket compression and flange alignment. Tightening in a simple circular pattern causes uneven compression and flange warping. The star pattern (cross-pattern) tightens bolts on opposite sides alternately, distributing compressive load evenly.
Bolts are tightened in multiple passes at increasing percentages of target torque (typically 20–30%, 50–70%, then 100%). This staged approach ensures gradual, uniform compression. A final verification pass confirms no bolts have relaxed.
One of the simplest ways to prevent tightening errors is to number the bolts with a paint marker before starting the sequence. On a 24-bolt flange during a turnaround, it's easy to lose track of which bolt is which in a cross-pattern. Our team always recommends this step.
This tool demonstrates the Legacy and Modified Legacy star-pattern sequences from ASME PCC-1. Modified Legacy loads the first group at 30%, the second at 70% and the rest at 100%, then runs a full star pass at 100% before the circular check pass. Bolts shown with a pale fill have been tightened in the current pass but are not yet at full torque. Other recognised methods (quadrant sequential, circular sequential, multi-bolt simultaneous tightening) may be faster or better suited to your application. Always follow site-specific procedures and consult a qualified bolting specialist.
Verification & Testing
A correctly tightened joint holds torque on the final clockwise check, shows a uniform flange gap, and has no visible gasket extrusion. The checks below cover what to look for before releasing the joint for service.
Visual Inspection
- • Gasket seated evenly with no visible gaps or extrusion
- • Flange gap is uniform around circumference (use feeler gauges)
- • For spiral-wound gaskets: outer ring should not be fully collapsed
Leak Testing
- • Hydrostatic test: Pressurise with water (safest method, typically 1.5× design pressure)
- • Pneumatic test: Low-pressure air (stored energy hazard — follow safety protocols)
- • Bubble test: Apply soap solution and look for bubbles under pressure
Verification Pass
After the final 100% torque pass, perform a full verification pass:
- Re-apply 100% torque to every bolt in sequential (clockwise) order.
- If any bolt moves, repeat the full verification pass.
- Continue until no bolt tightens further at 100% torque.
Only then is the joint properly seated.
For critical applications, allow the joint to rest (e.g. 4 hours) and then perform another verification pass. Gasket creep and bolt relaxation can cause load loss during this period, and re-verification ensures the joint is still at full torque before being put into service.
We recommend a re-torque pass 4 to 24 hours after initial bolt-up, especially on soft gaskets like PTFE or compressed fibre, because gasket creep and embedment relaxation reduce bolt load in the first few hours. For critical flanges, this single re-torque pass can be the difference between a leak-free start-up and an emergency shutdown in the first week.
Post-Installation
- • Some gaskets require re-torquing after initial heat-up (creep relaxation). Follow the gasket manufacturer’s advice for the gasket type
- • Hot bolting (removing and replacing bolts one at a time while the line is in service) is a separate, higher-risk task. Do it only under a specific approved procedure, with trained personnel
- • Record torque values, gasket details, assembly date, personnel, and procedure revision for audit trail
Important
If re-tightening is necessary, do so at ambient temperature before or during the first start-up. Never retighten compressed fibre gaskets at elevated operating temperatures or after extended service — the material may have taken a permanent set and further compression can cause blowout.
Related Resources
Proper installation is only one part of a reliable flange joint. These resources cover the remaining factors — gasket selection, failure diagnosis, and material compatibility.
Gasket Failure Modes
Visual diagnostic guide to nine common failure modes: identify symptoms, root causes, and corrective actions.
Diagnose Failures
Gasket Selection Guide
Understand the key factors — service conditions, gasket types, flange design — to choose the right gasket for the job.
Select a Gasket
Spiral Wound Gasket Guide
Anatomy, ASME B16.20 colour coding, gasket styles, and installation considerations specific to SWGs.
Read the Guide
Pressure-Temperature Ratings
Interactive ASME B16.5 PT tables — verify your flange class against the full pressure-temperature envelope.
Explore Ratings
Engineering Conversion Tables
Convert torque, pressure, temperature, and pipe-size units between metric and imperial — useful when bolt torque tables are quoted in foot-pounds and your wrench reads N·m.
Open Conversion Tool
Need Technical Assistance?
If you need help with gasket selection, torque calculation, or troubleshooting a persistent leak, our technical team can assist. We manufacture gaskets locally and can provide application-specific guidance.
Disclaimer
This guide is for general reference only and provides a high-level introduction to flange gasket installation principles. It does not constitute formal training and must not be used as a substitute for certified instruction, manufacturer's guidelines, site-specific procedures, or applicable codes and standards (including ASME PCC-1 and EN 1591-4). Gasket selection, torque calculation, and joint assembly for specific applications must be performed by qualified personnel following approved procedures. Universal Gaskets Pty Ltd accepts no liability for errors or omissions, as outlined in our Terms & Conditions.