How to Select Gaskets for Flanges
Created on 09.26
In pipeline systems and equipment connections, flangegaskets are core components ensuring sealing performance. The rationality oftheir selection directly affects the safety, stability, and economy of systemoperation. Improper gasket selection may lead to medium leakage, equipmentcorrosion, energy waste, and even serious safety accidents such as fires andexplosions. Starting from the core considerations for flange gasket selection,this article will elaborate on the scientific selection method by breaking downfour key dimensions—operating pressure and temperature, chemical compatibility,flange surface condition, and bolt load—providing practical guidance forengineering and technical personnel.
I. Core Considerations for Flange GasketSelection
The selection of flange gaskets must adhere to thethree principles of "adaptability, stability, and safety". Beforeformal selection, the following 4 basic prerequisites should be focused on toavoid 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 passsealing tests and safety certifications to avoid hidden risks due to non-compliance with standards.
II. Scientific Selection Method for FlangeGaskets: Breakdown of Four Key Dimensions
The selection of flange gaskets should center on"working condition adaptation". Through the analysis of operatingpressure and temperature, chemical compatibility, flange surface condition, andbolt load one by one, accurate matching can be achieved.
(1) Step 1: Determine the general category ofgasket material based on operating pressure and temperature
Operating pressure and temperature are core factorsdetermining gasket material, and together they form the "basicboundary" for gasket selection. Gaskets of different materials havesignificant 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 rubbergaskets (better oil resistance than natural rubber), and asbestos rubbergaskets (gradually restricted, with asbestos-free fiber gaskets asalternatives). These gaskets are easy to install, but attention should be paidto avoiding exceeding the temperature upper limit, which may cause aging andhardening.
2、Medium-pressure and medium-temperature conditions
(pressure: 1.6-10MPa, temperature: 100-350℃):
Applicable to process pipelines in the petrochemicaland chemical industries. Semi-metallic gaskets are recommended, such asmetal-clad gaskets (metal shell + non-metallic core material, with both sealingperformance and temperature resistance) and spiral wound gaskets (wound withmetal 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., 304stainless steel, 316L stainless steel) should be further confirmed based on thecorrosion 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-metallicgaskets are required, such as metal serrated gaskets (achieving sealingthrough serrated compression, suitable for male-female face flanges) and metalring gaskets (e.g., octagonal, oval gaskets, suitable for tongue-and-grooveface flanges, with extremely strong sealing performance but high requirementsfor the precision of the flange sealing surface). Full-metallic gaskets shouldmatch the flange material (e.g., carbon steel flanges with carbon steelgaskets, stainless steel flanges with stainless steel gaskets) to avoidelectrochemical 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 materialsthrough chemical compatibility verification
Chemical compatibility is crucial for ensuring thelong-term stable operation of gaskets. If a gasket reacts chemically with themedium (e.g., corrosion, swelling, degradation), it will not only reduce thegasket's sealing performance but also may contaminate the medium or produceharmful 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 acidicmedia, and corrosion-resistant materials such as 316L stainless steel andHastelloy should be selected; natural rubber gaskets (prone to swelling) shouldbe avoided for oily media, and nitrile rubber or fluororubber gaskets should bechosen.
2、Refer to "compatibility charts" and experimental data:
Gasket manufacturers usually provide"material-medium compatibility charts", marking the tolerance levelsof 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 conductsmall-scale simulated working condition tests (e.g., immersion test: immersethe 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 "hiddenreactions" with the gasket material under high temperature and pressure.For example: high-temperature water vapor may react with additives in somenon-metallic gaskets to generate impurities. Therefore, even if compatible atroom temperature, the stability under high temperature and pressure must stillbe confirmed.
(3) Step 3: Evaluate the flange surfacecondition and match the gasket structure and compressibility
The flatness, roughness, and damage status of theflange sealing surface directly affect the compression and fitting effect ofthe gasket. Even if the gasket material is suitable, leakage will occur if theflange surface has scratches, dents, or inconsistent roughness. The following 3points should be focused on:
1、Flange surface roughness (Ra):
Different types of gaskets have different requirementsfor 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 groundwith fine sandpaper, while flanges adapted to full-metallic gaskets need to beprocessed with precision grinders).
1、Inspection of flange surface damage:
Before installation, visually inspect or use feelergauges to check whether the flange sealing surface has scratches (repairrequired if depth > 0.2mm), dents (filling required if area > 5mm²), ordeformation (correction required if flatness deviation > 0.1mm/m). Minorscratches can be repaired through lapping; if the damage is severe, the flangemust be replaced before installing the gasket.
2、Matching of flange sealing surface forms:
The gasket structure must strictly correspond to theflange 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 ensurethe gasket reaches the optimal compression amount
Bolt load is the "power source" forachieving gasket sealing. Insufficient bolt pre-tightening force will preventthe gasket from being fully compressed, resulting in poor fitting of thesealing surface; excessive pre-tightening force will cause over-compression ofthe gasket (non-metallic gaskets may be crushed, while metallic gaskets mayundergo plastic deformation), losing resilience and easily leading to leakagewhen the system pressure fluctuates subsequently. The specific operation stepsare as follows:
1、Determine the "minimum sealing load"
required for the gasket:Gasket manufacturers usually provide two keyparameters: "gasket factor (m)" and "minimum pre-tighteningspecific 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 boltmaterial (e.g., Q235, 304 stainless steel), specification (e.g., M16, M20), andlubrication condition (whether thread grease is applied), calculate thepre-tightening torque using the formula: T = K × F × d (K is the torquecoefficient, 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 totighten the bolts evenly according to the calculated torque (following the"diagonal step-by-step tightening method" to avoid flangedeformation), 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 thetotal 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 orfracture). If the bolt load is insufficient, it can be solved by increasing thenumber of bolts or replacing them with high-strength bolts; if the bolt load isexcessive, select gaskets with larger compression amounts (e.g., flexible graphitespiral wound gaskets) or adjust the gasket size (increase the effective sealingarea A to reduce the required load).
III. Post-Selection Verification andMaintenance: Ensuring Long-Term Sealing Performance
After selecting the gasket, installation verificationand 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 atimely manner to avoid expanding the fault.
Conclusion
The selection of flange gaskets is a "systematicproject" that requires comprehensive consideration of working conditionparameters, medium characteristics, and flange and bolt conditions. Accurateselection can be achieved through the four-step process of "determiningthe general material category → verifying chemical compatibility → matching theflange 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 sealingsystem be ensured, providing guarantee for the safety and efficiency ofindustrial production.