June 1, 2026
A flange joint may begin to leak after the system is returned to operation, even when it has been tightened to specification and the correct gasket has been installed for the service. When that happens, the leak usually points to a change in the joint after installation: loss of bolt load, gasket relaxation, flange face damage, or external piping stress.
This scenario is common across refineries, chemical plants, and power generation facilities. Flange leaks are usually predictable failures with identifiable causes. When facilities respond only by replacing the gasket or retightening bolts, the original cause often remains in place, and the same joint may leak again during a future operating cycle. Understanding how to stop a leaking flange requires working through that chain of causes systematically, not just getting the line back online.
To stop a leaking flange, maintenance teams should:
• Isolate and depressurize the system
• Inspect the failed gasket and flange faces
• Confirm that the gasket material matches the service conditions
• Reinstall using proper flange preparation and controlled bolt loading
• Verify the joint after startup
A lasting repair depends on identifying why the seal failed, not simply replacing the gasket.
Below, we explain where flange leaks originate, how gasket selection affects sealing performance, how to complete a repair that holds, and what high-pressure connections require beyond standard field practice.
Why Flange Leaks Occur in Industrial Piping Systems
Bolted flange joints depend on maintaining sufficient compressive force across the gasket face to prevent process fluid from leaking. When that compressive force drops below the threshold required for the gasket to maintain an effective seal, leakage can occur.
Several mechanisms consistently drive the loss of compressive force in real operating conditions:
• Thermal cycling imposes repeated mechanical stress on the bolted joint. Each startup and shutdown causes differential expansion between the flange body, bolts, and gasket material. Over multiple cycles, this can reduce residual bolt load and lower gasket contact stress, especially in services with wide temperature swings or frequent cycling.
• Gasket creep reduces contact stress under sustained load. As the gasket compresses and loses thickness, the bolt load that was adequate at installation can become insufficient weeks or months later. The rate depends on gasket material, operating temperature, compressive load, and time in service.
• Flange face condition determines the quality of the sealing surface that the gasket must work with. Corrosion pitting, scratches from improper gasket removal, and residue from previous installations all create irregular surfaces that allow fluid to bypass the gasket regardless of bolt load.
• Piping stress from thermal expansion, inadequate support, or vibration introduces bending moments at the flange connection. Those forces can load one side of the gasket more than the other, reducing contact stress in one area while potentially overloading another.
Identifying which of these is driving a specific leak matter. A joint that leaks repeatedly in the same location after multiple gasket changes usually indicates that the gasket is not the root cause.
Common Causes of Gasket Leakage in Process Piping
Most flange leaks trace back to a short list of recurring failure modes. Working through them systematically helps prevent a temporary repair that fails again within the next maintenance cycle.
| Common Issue | What It May Indicate | What to Check |
|---|---|---|
| Leaks return at the same flange location | The root cause was not corrected | Flange face condition, alignment, piping stress |
| Compression marks near bolt holes only | Uneven bolt loading | Torque sequence, lubrication, bolt condition |
| Chemical degradation near the bore | Gasket compatibility issue | Media, concentration, temperature, exposure time |
| Leakage after startup or heat-up | Loss of bolt load or gasket relaxation | Retorquing guidance, gasket creep, thermal cycling |
| Leak at a high-pressure connection | The gasket type may not match the pressure class | Pressure rating, temperature, gasket design |
Bolt Load Problems
Insufficient bolt load is one of the most common causes of gasket leakage in process piping. When bolts are not tightened to the specified torque, the gasket never reaches the seating stress required to seal the joint. This is more common than it should be, particularly where installers estimate torque by feel or use impact wrenches without torque verification.
Friction makes this worse than it appears. In dry conditions, much of the torque applied to a fastener is consumed by thread and bearing surface friction. Only a smaller portion of the applied torque generates clamping force. Without lubricated threads and bearing surfaces, even a calibrated torque wrench consistently underloads the joint.
Creep Relaxation and the Wrong Material for the Service
Gasket creep is inevitable, but the rate varies significantly by material and service conditions. Some compressed fiber gaskets can lose a substantial portion of initial seating stress shortly after installation, especially under sustained load or elevated temperature. Materials such as expanded PTFE and flexible graphite generally offer stronger stress retention under suitable service conditions, helping the joint maintain contact stress for longer.
This often happens when a gasket specification is reused in a new service without being checked against the actual operating conditions. A material that performed adequately in one application may be unsuitable for a hotter, higher-pressure, or more frequently cycled connection.
Flange Surface Defects
Even the right gasket material cannot reliably compensate for a damaged flange face. Surface defects can create leak paths along the gasket-to-flange interface, even when bolt load is correct. Common issues include:
• Corrosion pitting
• Scratches or gouges from improper gasket removal
• Residual gasket material from a previous installation
• Waviness or uneven flange surfaces
Even shallow pitting can allow process fluid to migrate across the seating surface. For many gasketed connections, industry practice targets flange facing finishes in the 125 to 250 microinch Ra range, though requirements vary by gasket type, flange specification, and service conditions. Surfaces outside that window, too smooth or too rough, can compromise gasket seating. Flange faces should be inspected at every gasket change, not assumed to be acceptable.
Chemical Incompatibility
A gasket that is chemically incompatible with the process fluid degrades from the contact surface inward. Strong oxidizing acids attack carbon-based materials; organic solvents swell elastomers; alkaline environments break down certain synthetic fibers. The failure is not always immediate; the gasket can appear intact for weeks before degradation has progressed far enough to open a leak path.
Compatibility must be checked against the actual service conditions, including fluid concentration, operating temperature, pressure, and exposure time, not just a general compatibility chart. Many fluid-material combinations that are acceptable at ambient conditions become incompatible at operating temperatures.
Selecting the Right Gasket Material for Industrial Flange Sealing
Material selection is where many leaking flange repair strategies fall short. Reusing the previous gasket specification, or choosing whatever is in stock, can lead to repeat failures if the service conditions have changed. The right gasket material is defined by the specific service conditions of that joint: temperature range, pressure class, process chemistry, and how frequently the system cycles.
High-Temperature Services
Steam lines applications, and process services in the oil, gas, petrochemical, refining, and chemical industries require gasket materials that maintain sealing integrity through repeated thermal excursions. Flexible graphite with tanged metal inserts is well established in these services. It provides strong recovery under thermal cycling, helping maintain contact stress as the joint expands and contracts. This helps reduce the progressive loss of sealing stress that can eventually lead to leakage.
Flexible graphite also has low gas permeability, which matters in facilities subject to EPA leak detection and repair (LDAR) monitoring. A joint that holds against liquid process pressure may still emit measurable vapor concentrations through a gasket with high permeability.
Corrosive Chemical Services
When process chemistry would degrade many other materials, expanded PTFE gaskets are often a strong choice. PTFE is chemically inert across a wide pH range: resistant to most acids, alkalis, and solvents. Its low creep under sustained compressive load also works in its favor in services where bolt relaxation is a chronic issue.
Its limits are worth knowing, though. PTFE softens at elevated temperatures and can extrude from a joint under high bolt loads if the flange design does not constrain it properly. Temperature and pressure limits should always be verified against the manufacturer’s data for the specific product grade, flange configuration, and service conditions.
High-Pressure Connections
Soft sheet gaskets are often not the best choice for high-pressure flanged connections. In many Class 600-and-above services, the bolt loads required to achieve adequate seating stress can over-compress or extrude some sheet materials before the seal reaches the required contact stress. Semi-metallic designs handle these conditions.
When specified correctly, spiral wound gaskets pair a metallic structural winding with a soft filler, typically flexible graphite or PTFE, to provide the rigidity needed to withstand higher bolt loads and the conformability needed to fill minor surface irregularities. They are widely used in process piping, heat exchangers, and pressure vessels in demanding services.
How to Repair a Leaking Flange in an Industrial Environment
A leaking flange repair that fails within one maintenance cycle usually points to: a flange face not inspected, bolts not lubricated, or a torquing sequence that cut corners. Before any repair begins, the line must be isolated, depressurized, drained, or purged as required, and verified safe according to site procedures. The repair process is straightforward, but each step must be controlled and documented.
Read the Failed Gasket Before You Replace It
The removed gasket can show why the joint failed and what needs to be changed before the replacement gasket is installed.
Look for patterns such as:
• Uniform compression marks with chemical degradation at the bore, which may indicate a compatibility issue.
• Compression marks concentrated near bolt holes but absent between them, which may indicate uneven bolt loading.
• A clean zone on one side of the gasket, which may indicate flange misalignment or external piping stress.
Skipping this inspection can carry the same root cause into the next service interval.
Flange Face Preparation
Flange preparation is the step most often cut short when the maintenance window is tight. Both mating faces should be cleaned thoroughly using approved methods. Remove old gasket material with non-metallic scrapers, clean off corrosion products, and wipe both surfaces with an approved solvent to remove oils and residues that could interfere with gasket seating.
After cleaning, inspect both faces for pitting, scratches, or waviness. Defects that exceed the acceptable finish range for the gasket type need to be addressed before the new gasket goes in. Installing a new gasket against a damaged flange face recreates the original leak path.
Bolt Lubrication, Sequencing, and Torquing
Lubricate all threads and bearing surfaces using the lubricant specified by the installation procedure. Proper lubrication reduces friction and helps the applied torque generate the intended clamping force instead of mostly overcoming thread resistance.
Then follow a controlled tightening procedure:
• Use a cross-pattern or star-pattern tightening sequence
• Tighten in multiple passes according to the approved procedure
• Use 30 percent of the target torque on the first pass, 60 percent on the second, and 100 percent torque on the final pass, where applicable
• Avoid circumferential tightening or single-pass torquing, which can displace gasket material and create uneven contact stress
Durlon’s bolt tightening worksheet provides a practical field reference for torque targets.
Hot Retorquing
For high-temperature services, evaluate whether hot retorquing is recommended by the gasket manufacturer and permitted under site safety procedures. Some gasket materials compress further after heat-up, which can reduce bolt load and lower seating stress. Where hot retorquing is approved, it can help recover initial relaxation and improve sealing reliability, especially on joints with a history of recurring leaks.
Best Practices for Sealing High-Pressure Industrial Flange Connections
High-pressure flanged connections require more rigorous attention at every stage. The bolt loads are higher, the consequences of a leak are more severe, and the tolerance for installation error is smaller. Practices that may be adequate for lower-pressure services may not provide enough control for Class 600 connections and above.
Match Gasket Type to Pressure Class
The gasket type must be matched to the pressure class of the connection, not just the process chemistry. In many Class 600-and-above services, some soft sheet gaskets may over-compress or extrude under the bolt loads required to achieve adequate seating stress. Semi-metallic or metallic gaskets are often better suited to these pressure classes, depending on the full-service conditions.
Before specifying a gasket, verify that it meets the combined requirements of:
• Pressure rating
• Operating temperature
• Process chemistry
• Thermal cycling
• Flange configuration
A material may be rated for the pressure alone but not for the operating temperature at that pressure. Evaluating pressure, temperature, chemistry, cycling, and flange configuration together helps prevent selecting a gasket that looks acceptable on paper but fails in service.
Use Precision Bolt Load Control
Torque wrenches that are calibrated regularly and verified before use give consistent results. Impact wrenches without torque verification are not appropriate for controlled bolt loading on critical joints. For critical high-pressure connections, hydraulic bolt tensioners apply load directly to the fastener, bypassing the friction variables that make torque an imprecise proxy for bolt tension. They are widely used in high-pressure applications precisely because they produce more repeatable and verifiable clamping force.
Document target torque values, actual final values, lubricant used, gasket type, and installation date for each critical connection. This supports future maintenance decisions, provides a baseline for retorquing, and creates an audit trail for facilities operating under regulatory oversight.
Build in Proactive Inspection
High-pressure connections, especially insulated, high-temperature, critical, or historically problematic joints, should be included in the routine inspection program. Thermal imaging detects temperature anomalies at insulated flanges, localized hot spots, or cool zones from evaporating fluid before the leak reaches a visible or reportable threshold. Ultrasonic detection identifies the high-frequency signatures of fluid escaping through small leak paths that cannot be seen or smelled at the flange face.
A flanged connection that requires more than one gasket replacement within a normal maintenance interval likely has an unresolved root cause. Repeating the same repair without investigating the underlying condition postpones it at increasing cost in labor, parts, and production time.
Fix the Cause, Not Just the Leak
Many leaking flange repairs fail because they address only the symptom, the leaking joint, without identifying why the gasket failed in the first place. Installing a new gasket without addressing the root cause is a temporary fix, not a true solution.
The combination of accurate gasket material selection matched to actual service conditions, proper flange preparation, and controlled bolt loading addresses many of the conditions that drive flange leaks in process plants. For high-pressure and high-temperature connections, adding structured inspection to that foundation reduces leak rates further and allows corrective action before a monitoring item becomes a forced shutdown.
Need Help Stopping a Recurring Flange Leak?
For technical support on gasket material selection, leaking flange repair, or installation practices at your facility, contact Durlon’s team. We support engineers and maintenance professionals in oil and gas, chemical processing, power generation, and many other demanding industrial applications.