In industrial plants, piping systems are like the factory’s blood vessels, and the “flanges” that connect these pipes—along with the “gaskets” in between—are the critical factors that determine whether the system will suffer a “major leak.” This guide provides engineers and procurement professionals with a thorough analysis of the material differences among flange gaskets and the logic behind matching pressure ratings and thicknesses, while also revealing the standard SOP for leak-proof installation. Finally, we’ll debunk critical myths—such as the idea that “gaskets can be reused”—to help you safeguard the last line of defense for your plant’s safety.
Chapter 1: What Is a Flange Gasket? Why Is It the Last Line of Defense for System Safety?
Understand Gasket Seals in 60 Seconds
In industrial piping systems, it is not feasible to weld all pipes together seamlessly, as this would make future maintenance or cleaning impossible. Therefore, engineers weld disc-shaped components—known as “flanges”—to both ends of the pipes and then secure the two flanges together with bolts.
However, no matter how smooth a metal surface may be, there will still be microscopic irregularities and gaps when viewed under a microscope. If two metal flanges are bolted together directly, the fluid or gas inside will inevitably leak. **A flange gasket is a cushioning sealing element sandwiched between these two flange surfaces.** When the bolts are tightened, the gasket undergoes plastic deformation, perfectly filling the microscopic gaps in the metal surfaces to achieve a 100% level of containment and sealing.
The Evolution from Asbestos to Teflon
Early industrial gaskets were often made of oiled paper or asbestos. Although asbestos is heat-resistant and inexpensive, its fibers have been proven to be highly carcinogenic, and it has now been completely banned in most countries around the world.
As industrial environments have evolved from standard temperature and pressure conditions to the highly corrosive environments of the semiconductor and petrochemical industries—where temperatures frequently reach hundreds of degrees—gasket materials have also undergone a generational shift. Modern plants have now fully transitioned to using non-toxic synthetic rubber, PTFE (Teflon) with extreme corrosion resistance, and semi-metallic spiral-wound gaskets capable of withstanding high-pressure, extreme environments. It can be said that the history of gasket material upgrades is, in essence, a history of the evolution of industrial safety.
Chapter 2: Comparison of the Three Major Types of Flange Gasket Materials—Soft, Semi-metallic, and Metallic
When selecting flange gaskets, it is not necessarily true that “the more expensive, the better”; rather, they must be precisely matched based on the medium flowing through the pipe (water, acids, alkalis, or gases), temperature, and system pressure. The industry typically classifies gaskets into three major categories:
(H3) Comprehensive Table of Gasket Material Classifications
| Classification of Gaskets | Examples of Common Materials | Operating Temperature Range | Stress Tolerance | Main Use Cases | Price Range |
| Non-metallic, flexible | Rubber (NBR/EPDM), PTFE, non-asbestos fibers | -50°C to 250°C | Low to Medium Pressure (Class 150) | General water supply lines, low-pressure chemical pipelines, and high-purity fluids in cleanrooms | Economy to Mid-Range |
| Semimetal Composite | Spiral-Wound Gasket | -200°C ~ 800°C | Medium- to High-Pressure (Class 300–900) | High-temperature, high-pressure pipelines in petrochemical plants; steam systems in power plants | Upper Elementary and Middle School |
| Pure Metal | Stainless Steel Ring Gaskets (RTJ), Copper/Aluminum Gaskets | Up to 1000°C | Ultra-High Voltage (Class 1500+) | Deep-sea oil well extraction, high-temperature heat exchangers, extreme-condition reactors | Extremely high |
Material Selection Dilemmas in Practice: PTFE vs. Rubber vs. Spiral-Wound
In plant procurement practice, decision-makers are most often faced with choosing among these three mainstream types of gaskets. Below is the decision-making logic of a senior engineer:
- General Cold/Warm Water Piping: Select directly EPDM Rubber Gasket. It has the lowest cost, excellent elasticity, and is not prone to breaking when tightened.
- Highly Corrosive Acid and Alkali Pipelines: Must be selected PTFE (Polytetrafluoroethylene) Gasket. PTFE is virtually immune to all chemical solvents. However, pure PTFE has a fatal flaw—“creep”—which means that prolonged exposure to pressure causes it to slowly deform and thin, leading to loosening of bolts. Therefore, it is strongly recommended to choose products reinforced with glass fiber or carbon fiberModified PTFE Gaskets, to enhance its compressive strength and creep resistance.
- High-Temperature Steam or Gas Systems: When temperatures exceed 200°C and are accompanied by high pressure, soft gaskets will melt or be blown out. In this case, the only option is to Spiral Wrap Gasket, which is made by alternately winding V-shaped metal strips and graphite, uses the tension of the metal to press firmly against the flange surface like a spring, achieving a perfect high-temperature seal.
Chapter 3: Quick Reference for Flange Pressure Ratings and Gasket Thickness—Stop Relying on Gut Feel
Thicker isn't always better when it comes to gaskets.
Many novice engineers have a dangerous misconception when it comes to preventing leaks: “Since it’s leaking, I’ll just use a thicker gasket and tighten it even more—that should do the trick, right?”
That is definitely wrong! In high-pressure systems, the thicker the gasket, the larger the “load-bearing area” on its sides that is exposed to fluid pressure. When the pressure inside the pipe surges, an excessively thick soft gasket is highly susceptible to being “blown out” from the flange face by the intense fluid pressure, resulting in an instantaneous, catastrophic leak. The correct physical principle is:Provided that the flange surface is flat, the gasket should be as thin as possible., to reduce the surface area exposed to lateral forces.
Weight × Thickness Matching Matrix
So, what thickness of gasket should you choose? It depends on the flange size and pressure rating (Class). The following is a quick reference guide to the most commonly used non-metallic gasket thicknesses in engineering practice:
| Flange Diameter (DN) / Size | Class 150 (Low Voltage) | Class 300 (Medium Pressure) | Class 600 (High Voltage) |
| Small Diameter (DN15–DN50) | 1.5 mm | 1.5 mm | We recommend switching to semi-metallic gaskets. |
| Medium Diameter (DN65–DN150) | 3.0 mm | 1.5 mm to 3.0 mm | We recommend switching to semi-metallic gaskets. |
| Large Diameter (DN200 and above) | 3.0 mm | 3.0 mm | We recommend switching to semi-metallic gaskets. |
Note: Only if the flange face exhibits obvious pitting corrosion or poor flatness is it permissible to moderately increase the gasket thickness (e.g., using a 3 mm gasket instead of a 1.5 mm one) in low-pressure systems to compensate for defects on the metal surface. For high-pressure environments rated Class 600 or higher, spiral-wound gaskets must always be used.
Conceptual Diagram: Flange Pressure Rating vs. Gasket Thickness
Chapter 4: The Key to Leak Prevention—Proper SOP Implementation and Common Failure Cases
Choosing the right gasket material is only half the battle; as many as 70% of flange leakage incidents are caused by “improper installation.” Tightening a flange is by no means simply a matter of “just tightening the bolts”; rather, it requires adherence to a strict stress distribution process.
"Five-Step Installation SOP" to Prevent Leaks
Please use the following steps as the Standard Operating Procedure (SOP) for plant maintenance:
- Step 1: Thoroughly Clean and Inspect the Flange Surface
After removing the old gasket, you must use a brass brush or a special scraper to thoroughly remove any debris from the old gasket, adhesive residue, and rust spots from the flange surface. If there are radial scratches deeper than 0.5 mm on the flange face (which can create pathways for fluid leakage), the flange face must be re-machined or the flange replaced. - Step 2: Alignment and Lubrication
Place the new gasket exactly in the center of the two flanges, ensuring it is not off-center. Apply anti-seize lubricant to the threads of the bolts and the contact surfaces of the nuts (Note:Never apply butter or any lubricant to the surface of the gasket., which can cause the gasket to slip off the flange surface when under pressure). - Step 3: Manual Pre-locking
Gently tighten all bolts by hand until they are snug, ensuring that the flange faces remain parallel and aligned with the gasket. - Step 4: Use a torque wrench to “tighten in a crisscross pattern in three stages”
This is the most critical step for preventing leaks! Never “tighten the left side first, then the right side”—this will cause uneven stress on the gasket, leading to it being compressed unevenly and rupturing. You must use a torque wrench and tighten the bolts in a “diagonal crisscross” pattern (e.g., 12 o’clock → 6 o’clock → 9 o’clock → 3 o’clock). Additionally, you must increase the torque in three stages:- First Round: Tighten to the target torque 30%
- Round 2: Tighten to the target torque 60%
- Round 3: Tighten to the target torque 100%
- Step 5: Pressure Hold Test
After installation is complete, introduce the fluid and increase the pressure to 1.5 times the operating pressure. Allow the system to stand under pressure for at least 30 minutes, and confirm that there are no leaks before putting it into service.
Warning on the Xingda Power Plant Gas Explosion: The Missing Gasket and the Tragedy of Workplace Safety
How serious are the consequences of neglecting gasket installation? In September 2025, a gas explosion at Taiwan’s Xingda Power Plant shocked the entire island. According to the investigation report, the cause was directly attributed to a “missing flange gasket.”
It appears that after conducting a hydrostatic test on the piping, the contractor failed to replace the temporary gasket used for the test with a high-temperature-resistant metal gasket. When high-temperature natural gas was introduced, the non-heat-resistant gasket instantly melted and vaporized, causing a massive natural gas leak and triggering a gas explosion. This tragic incident serves as a stark warning to all plant operations personnel:Although small, gaskets serve as the final line of defense for workplace safety throughout the entire plant and the stability of Taiwan’s power grid.。
Chapter 5: Stress Relaxation and Thermal Cycling—Why Do Installed Gaskets “Loosen on Their Own”?
Many engineers have encountered this situation: “I tightened it perfectly with a torque wrench on the day of installation, so why did the flange start leaking after three months of operation?” This isn’t a ghost; rather, it’s the result of the physical phenomena known as “stress relaxation” and “creep.”
Gaskets "breathe," and torque is lost
All soft gaskets undergo gradual plastic deformation in the material itself after being subjected to high compressive forces from bolts over an extended period, causing them to gradually become thinner. As the gasket thins, the originally taut bolt becomes “relatively longer,” resulting in a decrease in the bolt’s preload.
In addition, if high-temperature steam or hot water is flowing through the piping, the flanges and bolts will undergo thermal cycling due to thermal expansion and contraction. Each time the system cools, the metal contracts at a different rate than the gasket, further accelerating torque loss. When the preload falls below the fluid thrust within the system, leakage occurs.
Preventive Measures: Hot Retorque
To counteract stress relaxation, the industry-standard preventive measure is to perform “hot tightening.” After a high-temperature system is started up, reaches operating temperature, and has been running for 24 to 48 hours, maintenance personnel must, while the system is still at temperature (or after cooling it down as recommended by the manufacturer),Use the torque wrench again to retighten all bolts in a circular pattern to the target torque of 100%.. This effectively compensates for initial stress loss and ensures long-term seal stability.
Chapter 6: Debunking Myths About Flange Gaskets—FAQ
In the field of flange sealing, there are many dangerous "quick fixes" and misconceptions. Below, we’ve compiled eight of the most frequently asked questions from engineers:
Q1: Can flange gaskets be reused?
Absolutely not! When the bolt is first tightened, the gasket undergoes “irreversible plastic deformation” to fill in microscopic gaps. Once the bolt is removed, the gasket loses its resilience. Whenever the flange is disassembled, a brand-new gasket must be installed.
Q2: What is the difference between a flange gasket and an O-ring? Are they interchangeable?
They are completely different and not interchangeable. A flange gasket is flat and relies on compression across a large flat surface to achieve a static seal; an O-ring, on the other hand, is a rubber ring with a circular cross-section that must be installed in a precisely machined “groove.” If an O-ring is placed between two flat flanges, it will be immediately forced out (blow-out) as soon as pressure is applied.
Q3: Does a thicker gasket provide a better seal?
Incorrect. The thicker the gasket, the larger the “contact area” on its sides that bears the thrust of the internal fluid, making it extremely prone to rupture under high pressure. Provided the flange faces are flat, the gasket should be as thin as possible (the standard thickness for low-pressure applications is 1.5 mm).
Q4: If you discover a leak (water or gas) in a flange, what should you do immediately?
Never attempt to tighten the screws with a wrench while the system is under pressure, as this may cause the gasket to burst and spray out suddenly. The correct procedure is: first shut down the system and relieve pressure → remove the flange for inspection → install a brand-new gasket → retighten according to the diagonal crisscross SOP.
Q5: Why do pure PTFE (Teflon) gaskets tend to leak?
Because pure PTFE exhibits extremely high “creep,” it will slowly flow outward and deform like clay when subjected to continuous pressure at room temperature, causing bolts to loosen. You should instead use “modified PTFE gaskets” reinforced with glass or carbon fiber.
Q6: When installing a gasket, should you apply grease or lubricating oil to its surface?
Never apply it! Applying lubricant will significantly reduce the friction between the gasket and the flange surface. When the pressure inside the pipe rises, the gasket will slip out of the flange surface as easily as stepping on a banana peel.
Q7: Can traditional asbestos gaskets still be used?
We strongly recommend replacing them immediately. Asbestos fibers have been proven to cause serious lung cancer, and most countries around the world have enacted laws banning their use. Currently, there are “non-asbestos” gaskets or composite materials with superior performance available on the market that serve as perfect replacements.
Q8: How tight should the bolts be tightened? Is it okay to rely on feel?
Relying on feel (tightening as hard as possible) is the main cause of uneven stress distribution and gasket failure. The tightening torque for bolts must be determined by consulting the corresponding “recommended torque values” based on “flange rating, bolt material, and gasket material,” and a torque wrench must be used to ensure precise tightening.
Conclusion and Technical Support
From an unassuming rubber O-ring to a spiral-wound metal gasket designed to withstand high temperatures and pressures, the selection and installation of flange gaskets are critical to the production capacity and workplace safety of an entire plant. Don’t let incorrect installation methods or the misconception of “saving a few pennies by buying the wrong gasket” become a ticking time bomb that could lead to system explosions or plant shutdowns.
Still choosing shim thickness based on a hunch? We’ve prepared the most comprehensive “Gauge × Material Thickness Matching Matrix” and “Torque Cross-Locking Reference Table” for engineers.
