In pipeline systems and equipment connections, flange gaskets are core components ensuring sealing performance. The rationality of their selection directly affects the safety, stability, and economy of system operation. Improper gasket selection may lead to medium leakage, equipment corrosion, energy waste, and even serious safety accidents such as fires and explosions. Starting from the core considerations for flange gasket selection, this article will elaborate on the scientific selection method by breaking down four key dimensions—operating pressure and temperature, chemical compatibility, flange surface condition, and bolt load—providing practical guidance for engineering and technical personnel.
I. Core Considerations for Flange Gasket Selection
The selection of flange gaskets must adhere to the three principles of "adaptability, stability, and safety". Before formal selection, the following 4 basic prerequisites should be focused on to avoid selection deviations caused by preliminary omissions:
1、Prioritize clarifying medium characteristics: It is necessary to first confirm the type of medium (e.g., gas, liquid, steam, corrosive fluid), concentration, purity, and whether it contains particulate impurities that the flange conveys or contacts. For instance, flanges conveying strong acids have drastically different requirements for the corrosion resistance of gaskets compared to those conveying ordinary water. Neglecting medium characteristics will quickly lead to gasket failure even if other parameters match.
2、Anchor the system's operating boundary conditions: Obtain the rated operating pressure, instantaneous maximum pressure, and long-term operating temperature, extreme temperature fluctuation range of the system where the flange is located, rather than merely referring to the designed nominal values. For example, high-temperature steam pipelines may experience "sudden temperature rise and drop" conditions, requiring gaskets to meet both high-temperature stability and thermal shock resistance.
3、Match basic parameters of flanges and bolts: Confirm the flange type (e.g., slip-on flange, weld neck flange, socket weld flange), sealing surface form (e.g., flat face, raised face, male-female face, tongue-and-groove face), as well as the bolt material, specification, and pre-tightening torque requirements. Different sealing surface forms correspond to gaskets of different structures (e.g., tongue-and-groove faces require gaskets without locating rings), and bolt load directly affects the compression amount and sealing effect of gaskets.
4、Comply with industry and safety standards: Ensure the selected gaskets meet relevant industry specifications (e.g., GB/T 9126 for the petrochemical industry, HG/T 20606 for the chemical industry, and ASME B16.20 for international standards). Especially in scenarios involving flammable, explosive, toxic, or harmful media, gaskets must pass sealing tests and safety certifications to avoid hidden risks due to non-compliance with standards.
II. Scientific Selection Method for Flange Gaskets: Breakdown of Four Key Dimensions
The selection of flange gaskets should center on "working condition adaptation". Through the analysis of operating pressure and temperature, chemical compatibility, flange surface condition, and bolt load one by one, accurate matching can be achieved.
(1) Step 1: Determine the general category of gasket material based on operating pressure and temperature
Operating pressure and temperature are core factors determining gasket material, and together they form the "basic boundary" for gasket selection. Gaskets of different materials have significant differences in their tolerance within the pressure-temperature (P-T) range, requiring strict matching:
1、Low-pressure and low-temperature conditions (pressure ≤ 1.6MPa, temperature ≤ 100℃):
Suitable for scenarios conveying media such as water, air, and low-pressure steam. Non-metallic gaskets are recommended, including natural rubber gaskets (good elasticity, low cost), nitrile rubber gaskets (better oil resistance than natural rubber), and asbestos rubber gaskets (gradually restricted, with asbestos-free fiber gaskets as alternatives). These gaskets are easy to install, but attention should be paid to avoiding exceeding the temperature upper limit, which may cause aging and hardening.
2、Medium-pressure and medium-temperature conditions (pressure: 1.6-10MPa, temperature: 100-350℃):
Applicable to process pipelines in the petrochemical and chemical industries. Semi-metallic gaskets are recommended, such as metal-clad gaskets (metal shell + non-metallic core material, with both sealing performance and temperature resistance) and spiral wound gaskets (wound with metal strips and non-metallic strips, with excellent compression and resilience, making them the "universal choice" for medium-pressure conditions). Note: The material of the metal strip in spiral wound gaskets (e.g., 304 stainless steel, 316L stainless steel) should be further confirmed based on the corrosion of the medium.
3、High-pressure and high-temperature conditions (pressure ≥ 10MPa, temperature ≥ 350℃):
Suitable for high-pressure steam pipelines, hydrogenation reactors, high-temperature heat exchangers, etc. Full-metallic gaskets are required, such as metal serrated gaskets (achieving sealing through serrated compression, suitable for male-female face flanges) and metal ring gaskets (e.g., octagonal, oval gaskets, suitable for tongue-and-groove face flanges, with extremely strong sealing performance but high requirements for the precision of the flange sealing surface). Full-metallic gaskets should match the flange material (e.g., carbon steel flanges with carbon steel gaskets, stainless steel flanges with stainless steel gaskets) to avoid electrochemical corrosion.
Key Notes:
The "synergistic effect of pressure and temperature" must be considered simultaneously. For example, a certain gasket can withstand 300℃ under a pressure of 2MPa, but may only withstand 250℃ under 5MPa. The "P-T curve" provided by the gasket manufacturer should be referred to instead of judging based on a single parameter.
For scenarios with extreme temperature fluctuations (e.g., sudden temperature drop ≥ 100℃), materials with good thermal shock resistance should be selected, such as flexible graphite spiral wound gaskets (graphite has high-temperature resistance and low thermal expansion coefficient), to prevent gasket sealing failure due to thermal expansion and contraction.
(2) Step 2: Eliminate unsuitable materials through chemical compatibility verification
Chemical compatibility is crucial for ensuring the long-term stable operation of gaskets. If a gasket reacts chemically with the medium (e.g., corrosion, swelling, degradation), it will not only reduce the gasket's sealing performance but also may contaminate the medium or produce harmful substances. The verification methods are as follows:
1、Clarify the "corrosion factors" of the medium:
First, analyze the chemical properties of the medium: whether it is acidic (e.g., hydrochloric acid, sulfuric acid), alkaline (e.g., sodium hydroxide, ammonia water), oxidizing (e.g., nitric acid, chlorine gas), reducing (e.g., hydrogen sulfide, hydrogen), or contains solvents (e.g., methanol, ethanol) or oils (e.g., crude oil, lubricating oil). For example: ordinary carbon steel gaskets (prone to corrosion) should be avoided for acidic media, and corrosion-resistant materials such as 316L stainless steel and Hastelloy should be selected; natural rubber gaskets (prone to swelling) should be avoided for oily media, and nitrile rubber or fluororubber gaskets should be chosen.
2、Refer to "compatibility charts" and experimental data:
Gasket manufacturers usually provide "material-medium compatibility charts", marking the tolerance levels of different materials in specific media (e.g., "excellent, good, poor"). For special media (e.g., highly corrosive, high-purity media), request third-party compatibility test reports from the manufacturer or conduct small-scale simulated working condition tests (e.g., immersion test: immerse the gasket sample in the medium and observe whether there are weight changes, deformation, dissolution, etc. after 72 hours).
3、Beware of "hidden chemical reactions":
Some media may decompose or undergo "hidden reactions" with the gasket material under high temperature and pressure. For example: high-temperature water vapor may react with additives in some non-metallic gaskets to generate impurities. Therefore, even if compatible at room temperature, the stability under high temperature and pressure must still be confirmed.
(3) Step 3: Evaluate the flange surface condition and match the gasket structure and compressibility
The flatness, roughness, and damage status of the flange sealing surface directly affect the compression and fitting effect of the gasket. Even if the gasket material is suitable, leakage will occur if the flange surface has scratches, dents, or inconsistent roughness. The following 3 points should be focused on:
1、Flange surface roughness (Ra):
Different types of gaskets have different requirements for roughness:
Non-metallic gaskets (e.g., rubber, asbestos-free gaskets): The flange surface needs to be relatively smooth, with a roughness of Ra ≤ 3.2μm (to prevent the rough surface from scratching the gasket or causing leakage due to excessive gaps).
Semi-metallic gaskets (e.g., spiral wound gaskets): Moderate roughness is required, with Ra = 1.6-6.3μm (an overly smooth surface may cause the gasket to slip, while an overly rough surface may damage the gasket's surface sealing layer).
Full-metallic gaskets (e.g., serrated, ring gaskets): The flange surface requires high-precision processing, with a roughness of Ra ≤ 1.6μm (to ensure tight metal-to-metal fitting and avoid sealing failure due to uneven surfaces).
If the flange surface roughness is inconsistent, grinding is required (flanges adapted to non-metallic gaskets should be ground with fine sandpaper, while flanges adapted to full-metallic gaskets need to be processed with precision grinders).
2、Inspection of flange surface damage:
Before installation, visually inspect or use feeler gauges to check whether the flange sealing surface has scratches (repair required if depth > 0.2mm), dents (filling required if area > 5mm²), or deformation (correction required if flatness deviation > 0.1mm/m). Minor scratches can be repaired through lapping; if the damage is severe, the flange must be replaced before installing the gasket.
3、Matching of flange sealing surface forms:
The gasket structure must strictly correspond to the flange sealing surface form:
Flat Face (FF) flanges: Adapt to non-metallic gaskets (e.g., flat gaskets). Note that the outer diameter of the gasket should cover the flange bolt holes to prevent medium leakage from the bolt holes.
Raised Face (RF) flanges: Adapt to semi-metallic gaskets (e.g., spiral wound gaskets, metal-clad gaskets) or thick non-metallic gaskets. The inner diameter of the gasket should be consistent with the inner diameter of the flange to avoid "necking" which causes medium retention.
Male-Female (MFM) and Tongue-and-Groove (TG) flanges: Adapt to semi-metallic gaskets or full-metallic gaskets (e.g., spiral wound gaskets with locating rings, metal ring gaskets). The male-female/tongue-and-groove structure is used for positioning to prevent gasket displacement, which is especially suitable for high-pressure conditions.
(4) Step 4: Calculate the bolt load to ensure the gasket reaches the optimal compression amount
Bolt load is the "power source" for achieving gasket sealing. Insufficient bolt pre-tightening force will prevent the gasket from being fully compressed, resulting in poor fitting of the sealing surface; excessive pre-tightening force will cause over-compression of the gasket (non-metallic gaskets may be crushed, while metallic gaskets may undergo plastic deformation), losing resilience and easily leading to leakage when the system pressure fluctuates subsequently. The specific operation steps are as follows:
1、Determine the "minimum sealing load" required for the gasket:
Gasket manufacturers usually provide two key parameters: "gasket factor (m)" and "minimum pre-tightening specific pressure (y)" (refer to ASME B16.5 or GB/T 9126):
Minimum pre-tightening load (Fp): The load ensuring the gasket fits tightly without medium pressure, calculated by the formula: Fp = y × A (A is the effective sealing area of the gasket, which needs to be calculated based on the gasket size).
Operating load (Fm): The load preventing the gasket from being pushed open by the medium pressure during system operation, calculated by the formula: Fm = m × P × A (P is the system operating pressure).
The total bolt load must meet both Fp and Fm, i.e., total bolt load F ≥ max (Fp, Fm).
2、Calculate the bolt pre-tightening torque:
Based on the total bolt load F, combined with the bolt material (e.g., Q235, 304 stainless steel), specification (e.g., M16, M20), and lubrication condition (whether thread grease is applied), calculate the pre-tightening torque using the formula: T = K × F × d (K is the torque coefficient, usually 0.12-0.2, determined by the bolt surface treatment method; d is the nominal diameter of the bolt).
In actual operation, a torque wrench should be used to tighten the bolts evenly according to the calculated torque (following the "diagonal step-by-step tightening method" to avoid flange deformation), and tightening based on experience is strictly prohibited.
3、Avoid bolt load overload:
Confirm the maximum allowable load of the bolt (calculated based on the yield strength of the bolt material) and ensure the total bolt load F does not exceed 80% of the maximum allowable load of the bolt (a safety margin is reserved to prevent bolt stretching deformation or fracture). If the bolt load is insufficient, it can be solved by increasing the number of bolts or replacing them with high-strength bolts; if the bolt load is excessive, select gaskets with larger compression amounts (e.g., flexible graphite spiral wound gaskets) or adjust the gasket size (increase the effective sealing area A to reduce the required load).
III. Post-Selection Verification and Maintenance: Ensuring Long-Term Sealing Performance
After selecting the gasket, installation verification and post-maintenance are required to further ensure the sealing effect:
1、Installation verification: After installing the gasket, conduct a pressure test (e.g., water pressure test, air tightness test), maintain the pressure at the rated operating pressure for 30 minutes, and observe for leakage (soapy water can be applied to the sealing surface; no bubbles indicate qualification). If leakage occurs, check whether the bolt torque is sufficient, whether the gasket is misaligned, and whether the flange surface is damaged, and re-test after troubleshooting one by one.
2、Regular maintenance: Develop a gasket maintenance cycle based on the severity of the working conditions (e.g., inspection every 1-2 years for low-pressure and normal-temperature conditions, and every 3-6 months for high-pressure and high-temperature conditions). Focus on the following:
Whether the gasket has aging, hardening, or cracking (for non-metallic gaskets) or corrosion, deformation (for metallic gaskets).
Whether the bolts are loose (re-tightening can be done with a torque wrench).
Whether there are signs of medium leakage (e.g., medium residue, corrosion spots near the sealing surface).
If signs of gasket failure are found, replace it in a timely manner to avoid expanding the fault.
Conclusion
The selection of flange gaskets is a "systematic project" that requires comprehensive consideration of working condition parameters, medium characteristics, and flange and bolt conditions. Accurate selection can be achieved through the four-step process of "determining the general material category → verifying chemical compatibility → matching the flange surface → calculating the bolt load". At the same time, installation verification and regular maintenance after selection are also indispensable. Only by forming a closed loop of "selection - installation - maintenance" can the long-term stable operation of the flange sealing system be ensured, providing guarantee for the safety and efficiency of industrial production.