A flange joint leaks after the plant reaches operating temperature, or the gasket crushes on assembly. The flange looked flat and the bolts were tightened, but the seal did not hold. Gasket selection is not just picking a rubber ring that fits the bore. The material, the pressure, the temperature, and the bolt load all have to work together.
What a gasket does
A gasket fills the microscopic gaps between two flange faces. Under bolt load, it deforms to conform to the surface irregularities, creating a seal. It must be soft enough to conform, strong enough not to extrude or crush, and stable enough to resist the fluid and temperature. Too hard and it cannot seal; too soft and it blows out or creeps. The selection is a compromise between conformability and resistance.
Common gasket types
Compressed non-asbestos gaskets are general purpose for water, oil, and moderate temperatures. Rubber elastomer gaskets suit low pressure and flange-facing seals. Spiral-wound gaskets, with metal and filler layers, handle higher pressure and temperature and are common in raised-face flanges. Ring-type joints use metal rings for extreme pressure and temperature. Non-asbestos is fine for light service; spiral-wound for process and steam; metal RTJ for high pressure and critical joints.
Temperature and fluid
The gasket must survive the fluid and the temperature. Oil, steam, acids, and solvents each rule out certain materials. A gasket rated for water will swell in oil, or harden in steam. Check the chemical compatibility and the temperature range, including transient upsets. A gasket that works at steady temperature may fail during a heat-up cycle. The operating envelope, not the nominal rating, decides.
Pressure and flange rating
Flanges have pressure-temperature ratings. The gasket must be rated at or above the flange rating at the service temperature. A spiral-wound gasket in a low-pressure flange may be too stiff and not conform; a soft gasket in a high-pressure joint may extrude. Match the gasket class to the flange class. Don’t use a high-pressure gasket on a low-pressure face expecting a better seal; it often leaks because it does not compress enough.
Bolt load and torque
The gasket seals because the bolts compress it. Under-tighten and the joint leaks; over-tighten and the gasket crushes or the flange distorts. Use the flange and gasket manufacturer’s torque values, tighten in a crisscross pattern, and repeat in passes. Don’t torque one bolt fully before the next. Lubricate the bolt nuts so the torque translates to load, not friction. Record the torque and the number of passes for repeatable assembly.
Surface finish
Flange face finish matters. A smooth face helps soft gaskets seal; a rough or damaged face leaks even with the best gasket. Standard raised-face finishes are about 125 to 250 microinches. A face scored, rusted, or warped must be repaired or replaced before assembly. The gasket cannot bridge a deep groove or a warped flange.
Compression and relaxation
Gaskets relax over time as the material creeps and the bolts stretch. Metal and spiral-wound gaskets relax less than soft ones. Retorque after initial heat-up where the design allows, and use washers or lubrication to maintain load. A joint that seals cold but leaks hot often relaxed, not misassembled. Account for this in the maintenance schedule.
A worked selection
Take a steam line at 10 bar and 180 C on raised-face flanges. A plain rubber gasket is unsuitable because steam hardens it. A compressed non-asbestos sheet gasket may work at lower pressure, but for process steam a spiral-wound gasket with inner and outer rings is the safe choice. The flange rating is matched, the spiral-wound filler handles the temperature, and the inner ring prevents extrusion. Torque to the manufacturer’s value in three passes, crisscross, and retorque after warm-up. The joint then holds through cycles. Choosing a cheap sheet gasket here leaks within weeks.
Inner and outer rings
Spiral-wound gaskets use an outer guide ring for centering and, in critical service, an inner ring to prevent the winding from buckling inward. Use the rings specified for the flange type. Without them, the winding can collapse or extrude. The outer ring also sets compression; don’t trim or modify it.
Bolt material and lubrication
Bolt load depends heavily on friction. Dry nuts give inconsistent load; lubricated nuts give a predictable relationship between torque and bolt tension. Use the same lubrication the torque values assume, usually specified as a factor. For high-temperature service, choose bolting that retains strength at temperature, and account for thermal expansion differences between bolts and flange. Stretch and relaxation matter as much as initial torque.
Retorque practice
After the joint reaches temperature and cools, some gaskets relax. Retorque in service, where the design allows, in the same crisscross pattern and to the same torque. Don’t retorque a hot, pressurized joint without procedures. Record that retorque was performed. A joint that leaks after heat-up often needs a controlled retorque, not a new gasket.
Flange alignment and gap
Misaligned flanges put uneven load on the gasket. Parallel misalignment, angular misalignment, or a gap larger than allowed distorts the gasket and causes a leak on the low side. Align the flanges with alignment devices before installing the gasket. Don’t draw flanges together with the bolts; that preloads them and distorts the gasket.
Why a new gasket still leaks
If a new gasket leaks after correct installation, check the flange face for rust, scoring, or warping, check bolt torque and pattern, and verify the gasket material matches the fluid. A recurring leak on one flange often means a damaged or warped face. Refinish or replace the flange rather than trying a different gasket each time.
Safety and pressure testing
Test the joint hydrostatically where required, before putting the line in service. Stand clear during testing. Re-torque and inspect after test. Gasket failures can expel hot or hazardous fluid, so follow pressure-test procedures and isolation rules. Never assume a new gasket is a leak-free guarantee.
Common mistakes
Mismatching material and fluid, over-tightening one bolt, ignoring flange finish, reusing gaskets, and skipping retorque are the recurring errors. A gasket is a consumable; replace it each time the flange is opened. Don’t reuse an old gasket expecting it to seal again. Treat the joint as a system and the seal will hold.
Soft gaskets vs metal joints
For low-pressure water or air, a compressed sheet or elastomer gasket is forgiving. It conforms easily and does not need extreme bolt load. For high pressure, steam, or toxic service, spiral-wound or ring joints carry the load but demand flatter faces and precise torque. Mixing them up causes the classic failure: a soft gasket crushed in a high-pressure flange, or a hard metal gasket that never conforms on a warped low-pressure face. Match the gasket technology to the service class, and don’t borrow a gasket from a different line.
Once installed, mark the bolt torque on the flange with a paint marker. This lets maintenance see at a glance whether a bolt has loosened or been disturbed. A visual check during inspections catches the obvious problems before a leak becomes a release.
On-site assembly checklist
Before assembling a flange, inspect both faces for rust, grooves, and old gasket material. Clean them with a solvent and check flatness with a straight edge. Verify the gasket part number matches the line and the flange size, and that it is not old or stored improperly. Install the gasket without sealants unless specified, because sealants can interfere with some gasket materials. Tighten in three passes at roughly 30, 60, and 100 percent of final torque, crisscross, so the load seats evenly. Record the torque and date. After start-up, inspect the joint during the first heat cycle and retorque if allowed. This discipline prevents most field leaks, which usually come from skipped steps rather than from the gasket itself.
Document which joints require retorque and at what interval, so the maintenance crew does not treat every leak as a failed gasket. Many weeping flanges are just a relaxation that a controlled retorque fixes. Treat the gasket as part of a maintained system, not a one-time install.
Document which joints require retorque and at what interval, so the maintenance crew does not treat every leak as a failed gasket. Many weeping flanges are just a normal relaxation that a controlled retorque fixes. Treat the gasket as part of a maintained system, not a one-time install.
Document which joints require retorque and at what interval, so the maintenance crew does not treat every weep as a failed gasket. Many flanges just relax. Treat the gasket as a maintained system, not a one-time install.
Bottom line
Choose the gasket material for the fluid and temperature, match its pressure class to the flange, control bolt torque with a crisscross pattern, and inspect the flange face. The gasket must be soft enough to conform and strong enough to hold. Leaks usually come from wrong material, wrong torque, or a damaged face, not from luck. Treat flange joints as a system: gasket, flange, bolts, and surface must all be right.
