I. Introduction
In pressure piping engineering such as chemical and oil & gas, power and thermal power, new energy storage, water treatment, and municipal heating, flange connections are the most commonly used detachable connection form between pipelines, equipment, and valves.A complete flange connection unit is not just the flange body itself; it also includes the mating flange, gasket, bolts and nuts, matching short pipe, as well as connected straight pipes, elbows, reducers, and other pressure-bearing pipe fittings.Flange connection is a combined sealing pressure-bearing structure that requires coordinated matching of the entire set of parameters; it is not enough for the flange specification alone to meet the standard to satisfy service requirements.
A phenomenon often occurs on engineering sites: the flange parameters are verified to be correct individually, but when assembled with pipes, valves, and mating flanges, they cannot be connected, or leakage occurs during the pressure test stage.The root cause is mostly that the concept of "complete set matching" was not established during the procurement stage. Procurement personnel only focus on the flange DN nominal diameter and ignore supporting parameters such as pressure rating, sealing face type, flange outer diameter, bolt hole center circle, bolt quantity and specification, material, and governing standard, randomly combining components of different standards, different ratings, and different sealing forms.This is especially true for overseas EPC projects, where multiple standards such as GB, ASME, EN, DIN, GOST, and JIS coexist. Flange dimension bases, pressure logic, and sealing systems differ greatly, and cross-standard mixing can easily cause batch material scrapping, schedule delays, and cost increases, and in severe cases can lead to medium leakage after commissioning and cause safety accidents.
The core of complete-set matching procurement for flanges is to coordinate the flange as a connection unit as a whole, using the design working conditions and drawing pipeline parameters as the basis, to ensure that all parameters of the flange, mating parts, pipes and fittings, sealing elements, and fasteners form a closed and unified loop.Establishing standardized complete-set procurement rules, verification points, and acceptance mechanisms is the key to avoiding the risk of flange connection failure from the source and ensuring the long-term stable operation of pipelines.
II. Core Technical Parameter Matching Requirements for Complete-Set Matching Procurement of Flanges
(I) Unified Matching of Applicable Standards
Standard unification is the first prerequisite for complete-set matching. The same flange connection unit must be locked to the same standard system, and cross-standard mixing is strictly prohibited.If national standard GB/T 9113 to GB/T 9124 series flanges are selected, the mating flanges, matching pipe fittings, and pipes must all comply with the national standard;If American standard ASME B16.5 flanges are selected, the entire unit must follow the ASME system;European standard EN 1092-1, German standard DIN, Russian standard GOST, and Japanese standard JIS flanges likewise follow the principle of adhering to one standard throughout.
Flanges under different standards are completely independent in bolt hole quantity, hole spacing, flange thickness, sealing face height, and flange outer diameter, and there is no universal interchangeability relationship.For example, American standard Class150 flanges and national standard PN10 flanges have the same nominal diameter, but their bolt hole layouts differ, so the bolts cannot be aligned for installation;The sealing face reference for EN European standard flanges and ASME American standard flanges is different; direct pairing will result in poor sealing face contact and leakage during pressure testing.The procurement list must clearly indicate the governing standard for each flange item, and the matching short sections and connecting pipes must use the same standard to eliminate mixing of standards.
(II) Principles for Matching Pressure Ratings
The flange pressure rating must be greater than or equal to the maximum design pressure of the pipeline, and the pressure rating of the entire unit must not be downgraded.The rated pressure ratings of flanges, mating flanges, connected pipe fittings, and piping materials should remain consistent; high-pressure pipelines must not be paired with low-pressure flanges, and high-grade flanges must not be paired with low-pressure gaskets or low-strength bolts.
Particular attention should be paid to the fact that the American standard Class pound rating is a dynamic temperature-pressure system, and its allowable pressure decreases as the medium temperature rises; it cannot simply be converted to the PN kilogram rating at equivalent value;National standard PN ratings mark the rated pressure based on normal temperature.During procurement, one must not equate Class150 with PN16 or Class300 with PN40 based on experience alone; it is necessary to verify against the temperature-pressure rating tables of the corresponding standards in combination with the project design temperature and medium, so as to ensure that the pressure-bearing capacity of the flange meets the requirements under actual working conditions.For the same set of flange connections, the pressure ratings of the flanges on both sides must be consistent; pairing high and low ratings is prohibited.
(III) Complete matching of sealing face types
The sealing face is the core of flange sealing; the sealing face types of mating flanges must be compatible with each other, and the gasket model must correspond one-to-one with the sealing face and must not be replaced arbitrarily.Common sealing faces for industrial flanges include RF raised face, MFM male-and-female face, RJ ring joint face, TG tongue-and-groove face, etc.
RF raised-face flanges can only be paired with RF raised-face flanges, together with non-metallic flat gaskets or flexible graphite composite gaskets;MFM male-and-female face flanges must be used in pairs with the female face and male face together;The RJ ring joint face is a dedicated structure for high-pressure flanges and can only be paired with metal ring gaskets; it imposes extremely high requirements on the machining accuracy of the flange sealing groove.Many procurement mismatch problems are concentrated here: on one side is an RF raised-face flange, while the other side is procured as an MFM male-and-female flange, or an RJ high-pressure flange is paired with an ordinary non-metallic gasket, so after assembly it cannot be compressed tightly for sealing, resulting directly in leakage.When preparing the list, the sealing face form of the flanges at both ends and the matching gasket type must be marked simultaneously.
(IV) Complete material matching
The material grades of the flange, mating flange, pipe stub, and connected pipes and fittings shall remain from the same source and match; the materials of bolts and gaskets shall be selected to match according to the medium, temperature, and corrosive environment.20# carbon steel flanges are matched with 20# carbon steel pipes and carbon steel butt-welded fittings;304 and 316L stainless steel flanges are matched with stainless steel pipes of the same grade;For low-temperature working conditions, LF2 and A350 LF Series low-temperature flanges are selected and matched with low-temperature fittings;Alloy steel pipelines are matched with alloy flanges of the same grade to avoid direct connection of dissimilar steels.
Directly combining dissimilar metals, on the one hand, results in poor weldability and a tendency to produce welding cracks;on the other hand, different materials have different coefficients of thermal expansion, so under alternating temperature conditions gaps are prone to appear at the sealing surface, and electrochemical corrosion occurs at the same time, accelerating the failure of flanges and seals.The material of bolts and nuts must likewise not be neglected; for high-temperature and corrosive conditions, high-strength alloy steel bolts should be selected, and ordinary carbon steel bolts must not be used for high-temperature and high-pressure flange connections.
(5) Matching of external dimensions and connection structure
Flange connection types are divided into welded neck flange WNF, slip-on flange with neck SO, plate-type slip-on flange PL, socket weld SW, threaded flange, etc., and the connection type of the flange must match the nozzle structure.Welded neck flanges are used for high-pressure and high-temperature pipelines, butting against butt-welded pipe fittings and straight pipes;Plate-type slip-on flanges are mostly used for low-pressure and normal-temperature conditions, with the nozzle inserted into the flange for welding.
The flange inner diameter and nozzle bevel dimensions need to match the outer diameter and wall thickness of the matching pipe material, ensuring that the root gap and misalignment meet welding specifications.Especially for thick-walled high-pressure flanges and non-standard custom flanges, the inner diameter of the flange neck end and the groove angle must match the pipe Sch wall thickness; there must not be an excessive difference between the flange neck wall thickness and the pipe wall thickness, which would cause poor weld transition and stress concentration.
(6) Fasteners and gaskets must be supplied as a complete set
Bolts, nuts, and gaskets are inseparable components of a flanged connection; procurement must not involve purchasing only the flanges while omitting the sealing and fastening components.The specification, quantity, and length of bolts are jointly determined by the flange standard, sealing face, and gasket thickness. For flanges of the same standard and the same pressure rating, the number and diameter of bolts are fixed. Bolt length must be determined after calculating the flange thickness, gasket thickness, and nut and washer thickness; insufficient length prevents proper compression, while excessive length causes assembly interference.
Gasket selection must comprehensively consider the corrosiveness of the medium, temperature, and pressure: for room-temperature water media, rubber gaskets or non-metallic composite gaskets may be selected;for high-temperature oil and gas and steam pipelines, flexible graphite gaskets are selected;for high-risk working conditions such as high-pressure hydrogen and oil and gas, spiral-wound metal gaskets or metal ring gaskets are selected.The outer diameter and inner diameter of the gasket must match the flange sealing face, and the gasket width must not exceed the effective compression area of the sealing face.
III. Common Misconceptions in Complete Flange Set Procurement
First, looking only at the DN nominal diameter while ignoring the standard and pressure rating, mistakenly assuming that matching diameters mean the flanges can be paired—this is the most frequent error in flange procurement.
Second, statically converting pressure ratings by directly equating Class pound ratings with PN values, ignoring temperature derating, which results in insufficient pressure-bearing capacity under high-temperature working conditions.
Third, arbitrarily substituting sealing faces, mixing RF, MFM, and RJ, with mismatched gaskets, planting hidden leakage risks.
Fourth, the flange material and the pipe material are inconsistent, considering only the pressure-bearing capacity of the flange while ignoring the corrosion and welding risks of dissimilar materials.
Fifth, treating bolts and gaskets as minor accessory parts and not purchasing them together with the flange set, then purchasing them separately later with non-matching specifications, delaying assembly.
Sixth, mixing old and new standard versions; under the same standard, flange bolt hole spacing differs between old and new versions, making assembly impossible.
Seventh, purchasing equipment flanges and pipeline flanges separately without checking the equipment interface flange parameters, so they cannot be connected after arrival on site.
IV. Implementation Control Process for Complete Flange Set Matching Procurement
Step 1: Lock down drawing parameters. Based on the P&ID diagram, pipeline single-line diagram, and equipment interface diagram, extract item by item for each flange group the pipeline number, medium, design temperature and pressure, applicable standard, flange type, sealing face form, and material grade, and simultaneously record the component to which the mating flange belongs, forming a flange-specific data table. Equipment nozzle flanges shall serve as the priority benchmark, and the parameters of pipeline-side flanges must remain consistent with those of equipment flanges.
Step 2: Compile the complete set list.The list shall use "one flange connection unit" as one record, and simultaneously enter the parameters of the flange, mating flange, gasket, bolts and nuts, matching short pipe section, and connected pipe fittings, with unified standards, ratings, and materials; parameter fields shall be complete without abbreviations, and marking only DN is prohibited.
Step 3: Technical disclosure and supplier confirmation.Before placing an order, send the complete set of parameters to the supplying manufacturer; the manufacturer shall review the matching relationships among flanges, fasteners, and gaskets, with emphasis on verifying the matching schemes for high-pressure RJ ring joint flanges, low-temperature flanges, and alloy flanges; production scheduling may proceed only after the supplier issues a matching confirmation sheet.
Step 4: Factory documentation package.Each batch of flanges is accompanied by material certificates, spectral reports, non-destructive testing reports, and dimensional inspection records. For high-pressure flanges, heat treatment and hardness testing documentation is provided in accordance with specifications. The entire documentation set corresponds one-to-one with the flange heat batch numbers.
Step 5: Complete set verification upon arrival.After arrival, acceptance is carried out by set. Verify the standard, pressure rating, and sealing face type; measure the flange outer diameter, bolt hole spacing, and thickness; recheck the material by spectral analysis; trial-fit bolts to check alignment; compare and verify gaskets and sealing faces. Reject the entire set if any item fails inspection; do not replace just one component to make do.
Step 6: Closed-loop change management.When a design change occurs at a pipeline or equipment interface, update all parameters of the entire flange unit simultaneously, and change gaskets and bolts at the same time, avoiding the situation where only the flange body is changed while the parameters of matching seals and fasteners lag behind.
V. Conclusion
Flanged pipeline connections are the weak link in pressure-bearing systems. Most sealing failures are not due to quality defects in the flange body itself, but rather systematic problems caused by mismatched parameters across the entire unit.Procurement of complete flange sets must be matched as a whole; individual flange items cannot be viewed in isolation. Flanges, mating flanges, pipes and fittings, gaskets, and bolts and nuts must be treated as a single integrated connection unit, strictly following the six major procurement principles of unified standards, pressure compatibility, matching sealing faces, consistent material origin, dimensional matching, and complete fastener sets, with closed-loop control throughout the entire process from drawing material requisition, list preparation, and supplier confirmation to incoming acceptance inspection.
Beijing Guojie Pipeline Co., Ltd. has long been deeply engaged in the complete-set supply of multi-standard flanges, fittings, and pipes. It is familiar with the flange parameter systems of national standards, American standards, European standards, Russian standards, and Japanese standards. According to the working conditions of chemical, oil and gas, energy storage, thermal power, and foreign trade EPC projects, it can provide one-stop services including complete flange matching solutions, parameter verification, list preparation, and material supporting, avoiding the risks of flange mismatch and leakage, and ensuring the smooth pressure testing and safe commissioning of pipeline projects.