| characteristic | flange | blind plate |
| Core functions | Connect components such as pipelines, valves, and equipment. | Isolate/block fluid flow in pipelines or equipment. |
| structure | A circular disc-shaped body with a hole in the middle (allowing the medium to pass through) | Solid circular plate (without holes, blocking medium) |
Function in the flow channel | A part of the channel through which the medium must flow | The blocker of the flow channel is installed between the flanges to prevent the medium from passing through that point |
| main purpose | Establish a detachable pressure sealed connection | Provide absolutely reliable physical isolation |
| Installation position | Pipeline ends, valve interfaces, equipment interfaces (need to be used in pairs) | Installed between two flanges (replacing the position of the gasket) |
| Is it under pressure | Yes (the connection point needs to withstand system pressure) | Yes (needs to withstand system pressure and maintain sealing) |
| Material requirements | Compatible with pipeline/equipment materials, media, pressure and temperature | It needs to be compatible with flange materials, media, pressure and temperature, and have stronger sealing surface performance |
| Analogy | Connectors or sockets in pipeline systems | A"plug"or"gate"in a pipeline system |
Simply put, flanges are used for connecting, while blind flanges are used for blocking. Flanges are the interfaces that allow fluid to"flow through", while blind plates are the barriers that allow fluid to"stop here". Blind plates require flanges (between two flanges) to achieve their isolation function.
The types of blind plates can be classified from many perspectives, such as structural form, material, connection method, sealing surface form, etc. In today's article, we will first classify them from the perspective of structural form. According to the different structural forms, they can be divided into four categories: flat plate blind plates, 8-shaped blind plates, insert plates, and gasket rings (insert plates and gasket rings are blind to each other), which will be introduced one by one below.
1、 Plate type blind plate
(1) Definition:
Plate blind plate is a circular plate-shaped component that is enclosed as a whole, with evenly distributed bolt holes on the edges. It is installed at the flange end of the pipeline to permanently or temporarily block the pipeline and prevent fluid flow.
(2) Characteristics:
The sealing surface has no holes overall, ensuring complete sealing. Easy to install, usually fixed to the flange end with bolts. Suitable for long-term sealing and can be used in situations such as shutdown and maintenance. Can withstand certain pressure, and different materials and thicknesses can be used for pipeline systems with different pressure levels.
(3) Common materials:
Carbon steel (such as Q235, A105)
——Economically applicable, suitable for general working conditions;
Stainless steel (such as 304, 316L)
——Corrosion resistant, suitable for industries such as chemical and food;
Alloy steel (such as 12Cr1MoV, F11, F22)
——Used in high temperature and high pressure environments;
Non metallic materials (such as PVC, PP, PE, fiberglass)
——Used for low-pressure, corrosion-resistant pipelines.
(4) Application scenarios
Chemical, petroleum, and natural gas pipelines - used for temporary sealing before pipeline maintenance or system expansion;
Water treatment system - installed at the end of unused pipelines to prevent water from flowing in;
Boiler and pressure vessel systems - as safety sealing devices to prevent leaks.
2、 8-shaped blind board
(1) Definition:
The 8-shaped blind plate is a type of blind plate composed of two connected disks, with one end being solid (blind plate) and the other end being an opening (through-hole part). By rotating the installation position, the on/off state of the pipeline can be switched.
(2) Characteristics:
Unique structure, shaped like the number"8"or glasses, hence also known as"glasses blind plate";
● It can quickly switch the pipeline status by simply loosening the bolt and flipping it over, without the need to disassemble the entire flange;
Suitable for situations where pipeline status needs to be changed frequently, such as equipment maintenance, medium conversion, etc;
Can effectively isolate the medium and prevent accidents caused by misoperation.
(3) Common materials:
Carbon steel (Q235, A105) - generally used in low-pressure, moderately corrosive environments;
Stainless steel (304, 316L) - corrosion-resistant, suitable for industries such as chemical and food;
Alloy steel (F11, F22, 12Cr1MoV) - suitable for high temperature and high pressure working conditions.
(4) Application scenarios:
Oil and gas pipelines - used for equipment maintenance or emergency shutdown to ensure system safety;
Chemical and refining facilities - providing reliable medium isolation in high-risk environments;
Power plant pipelines - used for isolation of steam systems to meet high temperature and pressure requirements.
3、 Insert blind board
(1) Definition:
Insertion blind plate is a type of single piece blind plate, usually equipped with a handle, which facilitates temporary sealing between pipeline flanges and is suitable for working conditions that require frequent changes in pipeline status.
(2) Characteristics:
Single piece structure, easy to operate, thin thickness, light weight, and convenient to carry.;
Equipped with a handle or logo, easy to plug and unplug without the need for additional fasteners, making it convenient to confirm the closed state of the pipeline;
Suitable for occasions that require temporary sealing, such as pipeline testing and maintenance.
(3) Common materials:
Carbon steel (A105, Q235) - suitable for general pressure and environment;
Stainless steel (304, 316L) - suitable for corrosion-resistant environments;
Alloy steel (F11, F22) - suitable for high temperature and high pressure environments.
(4) Application scenarios:
Petrochemical pipelines - used for sealing equipment replacement, pipeline pressure testing, or temporary isolation and maintenance sections. ；
Liquefied natural gas (LNG) system - isolate gas flow during maintenance;
Power plant boiler system - used for rapid isolation of steam pipelines, facilitating the restoration of circulation.
4、 Pad ring
(1) Definition:
A gasket ring is a ring-shaped component with an opening, usually used in conjunction with a plug plate as a placeholder to restore pipeline flow when removing the plug plate, while maintaining the same flange spacing.
(2) Characteristics:
● Simple structure, shaped as a ring with a hole in the middle, usually with a handle for easy insertion and removal;
● Used in conjunction with the plug-in board, it is easy to switch from a closed to a smooth pipeline state;
● It can avoid changes in flange spacing and reduce the workload of disassembling and assembling flanges.
(3) Common materials:
Carbon steel (A105, Q235) - commonly used in general pipeline systems;
Stainless steel (304, 316L) - suitable for corrosive media environments;
Alloy steel (F11, F22) - occasionally used for high temperature and high pressure conditions.
(4) Application scenarios:
Petrochemical industry - restoring the flow of high-temperature and high-pressure medium pipelines after removing the plug;
Natural gas transportation - replace the gasket ring after inserting the isolation board to restore pipeline smoothness;
Power plant steam system - After steam pipeline maintenance, replace the plug plate to restore high-temperature steam circulation.
As a practitioner in the pipeline industry, the pipeline standards that I usually come into contact with are mainly"GB/T"and"CJ/T". When I first started working on them, I often didn't understand what they meant. After careful research, I realized that there are still many ways to implement these standards. Here, I will share them with you.
1、 What is the difference between GB and GB/T?
There are two types of national standards that are encountered in daily work. One type starts with"GB", such as GB 50268-2008; Another one starts with"GB/T", such as GB/T 13663.2-2018. What is the difference between the two?
GB"is the abbreviation for"National Standard", which is a mandatory national standard that must be strictly followed by products and services. Failure to strictly implement it may violate the law and may result in administrative penalties, product bans, or legal proceedings.
The GB standard is mainly applicable to important fields such as safety, health, and environmental protection. For example, the"GB 50268-2008"mentioned earlier refers to the"Code for Construction and Acceptance of Water Supply and Drainage Pipeline Engineering", which specifies the construction and acceptance standards for various types of water supply and drainage pipelines and must be strictly implemented.
The"T"in"GB/T"represents"recommended"and refers to national recommended standards. This standard has no legal enforceability and can be voluntarily adopted by enterprises and organizations. The"GB/T"standard usually involves product quality, technical specifications, testing methods, and other aspects. For example, the HDPE water supply pipe standard GB/T 13663.2-2018 provides comprehensive regulations on pipe specifications, dimensions, parameters, performance, etc. Although the standard is not mandatory, products that adopt recommended standards are usually more likely to gain market and consumer recognition.
In addition to"GB"and"GB/T", there is also a guiding technical document starting with"GB/Z", which mainly provides technical guidance and suggestions, and is not legally mandatory or recommended. Enterprises or organizations can selectively refer to and adopt it. These standards are mostly used for emerging technologies or experimental specifications, while the pipeline industry relies more on mature mandatory or recommended standards, which will not be further elaborated here.
2、 What other standard classifications are there besides national standards?
In addition to national standards starting with"GB", there are also industry standards, local standards (DB), group standards, and enterprise standards. Here are examples.
(1) Industry standards:
Technical and management standards applicable to specific industries, formulated by supervisory departments at all levels. Example: CJ/T 125-2014"Steel skeleton polyethylene plastic composite pipes and fittings for gas use"
● Establishment institution: formulated by the competent department of urban construction (CJ) industry, specifically for the gas industry.
Scope of application: It covers the technical requirements of steel skeleton polyethylene plastic composite pipes and their fittings for gas use, ensuring the safety and reliability of gas transmission pipelines.
Compulsory: Although it is a recommended standard, it may have high authority in practice due to public safety concerns, relevant regulations or local policies that require mandatory implementation in the gas industry.
(2) Local standards:
Standards formulated by the standardization administrative departments of each province, city, and autonomous region, applicable to specific regions. Example: DB11/T 1835-2021"Technical Code for Construction of Water Supply and Drainage Pipeline Engineering"
● Institution: Developed by the Beijing Municipal Commission of Housing and Urban Rural Development, targeting specific water supply and drainage construction requirements in Beijing.
Scope of application: Applicable to the design of urban water supply and drainage engineering within the scope of Beijing, which specifies design principles, methods, and technical requirements.
Regional: It has regional applicability and specifically considers the geographical, climatic, and environmental characteristics of Beijing to ensure that the design scheme is in line with the local actual situation.
(3) Group standard:
Developed by social organizations, associations, or alliances, applicable to technical and management standards within the organization or industry. Example: T/ZZB 1607.1-2020"Chlorinated polyvinyl chloride (PVC-C) piping systems for industrial use - Part 1: Pipes"
● Institution: Developed by the Zhejiang Brand Building Federation.
Scope of application: Suitable for chlorinated polyvinyl chloride (PVC-C) pipes used in industrial fields, including chemical plants, pharmaceutical factories, power plants, and other industrial sites.
Voluntary: Enterprises and organizations can voluntarily adopt it, which plays an important guiding and regulatory role in the design, selection, and use of pipeline systems in the industrial field.
(4) Enterprise Standard (Starting with Q)
Developed and published independently by the enterprise, applicable to internal standards of the enterprise. Usually starts with the enterprise code, such as Q/XYZ 001-2019, where Q represents the enterprise standard.
3、 What are the common pipeline standards internationally?
The above mentioned are all domestic standard systems. For international standard systems, friends who often engage in foreign trade will also frequently encounter them. Here is a brief list:
● ISO (International Organization for Standardization): International Organization for Standardization ● ASTM (American Society of Testing Materials): American Society for Testing and Materials ● ANSI (American National Standards Institute): American National Standards Institute ● EN (European Norm): European Standards ● DIN (Deutsche Industrie Norm): German Industrial Standards ● JIS (Japanese Industrial Standards): Japanese Industrial Organization
There are different opinions on the introduction of DN, dn, and de, but there is a problem that the specific standard basis has not been clearly explained. This article intends to start from the standard basis and clarify these concepts.
1、 Capitalized DN: Nominal Diameter
Firstly, let's look at the standards,
Standard 1:
GB/T 1047-2019
Definition and selection of DN (nominal size) for pipeline components
Standard 2:
GB/T19278-2018
General terms and definitions for thermoplastic pipes, fittings, and valves
Standard Three:
International ISO 6708-1996"Definition and selection of nominal dimensions for pipeline components"
● Technical definition:
DN belongs to the category of nominal size, with its full English name being Nominal Diameter, also known as"nominal diameter". It is a standardized diameter specification that does not directly represent the outer diameter of a pipeline, nor is it equivalent to the inner diameter. Instead, it is a nominal size that is conventionally agreed upon in the design and manufacture of pipelines and their accessories (such as valves and fittings).
DN originated from early pipeline engineering, especially the galvanized steel pipe standard related to imperial units such as inches. For example, DN15 corresponds to a 4-inch British pipe (1/2 inch), and DN25 corresponds to a 1-inch pipe.
● Scope of application:
DN is mainly used in metal pipeline systems, such as galvanized steel pipes, cast iron pipes, steel plastic composite pipes, and some plastic pipes. In water supply and drainage, gas transmission, and HVAC engineering, DN is the preferred way to label pipeline and accessory specifications.
Industry characteristics:
The standardization of DN is supported by international standards (such as ISO 6708) and national standards (such as GB/T 1047). For example, DN50 represents a pipeline with a nominal diameter of 50mm, but its actual outer diameter may be 60.3mm (galvanized steel pipe standard), and the inner diameter may vary due to changes in wall thickness.
● Precautions:
In the design drawings, if DN is used to represent the pipe diameter, a comparison table between DN and actual product specifications should be attached to ensure accurate material selection during construction.
2、 Lowercase dn: nominal outside diameter
Standard 1:
GB/T19278-2018
General terms and definitions for thermoplastic pipes, fittings, and valves
Standard 2:
GB/T 42952.1-2023"Thermoplastic pipes for fluid transport - Dimensions and tolerances - Part 1: Metric series"(equivalent to ISO international standard: ISO 11922-1:2018)
● Technical definition:
The full name of DN in lowercase English is nominaloutside diameter, usually referring to the"nominal outer diameter", which represents the standardized outer diameter of the pipeline and is used in conjunction with wall thickness parameters (such as the standard size ratio SDR). For example, dn200 represents a PE pipe with a nominal outer diameter of 200mm, and its wall thickness is determined by the SDR value (such as SDR17 corresponding to a wall thickness of 11.8mm).
● Scope of application:
DN is commonly found in the labeling of plastic water supply and drainage pipes (such as PE, PVC, PPR, etc.), especially in scenarios where the outer diameter size needs to be clearly defined. Compared to DN, DN tends to directly reflect the physical outer diameter of the pipeline.
Industry characteristics:
In practical applications, the difference between DN and DN is not always strictly distinguished, and many practitioners tend to consider the two as equivalent. However, in the design of plastic pipes, the labeling of dn is more common and attention should be paid to matching it with SDR values.
● Precautions:
Due to the frequent use of DN in plastic pipelines, it is necessary to avoid confusion with DN, especially in mixed systems involving the connection of metal and plastic pipelines.
3、 De: Outer diameter (at any point)
Standard 1:
GB/T19278-2018
General terms and definitions for thermoplastic pipes, fittings, and valves
Standard 2:
GB/T 42952.1-2023"Thermoplastic pipes for fluid transport - Dimensions and tolerances - Part 1: Metric series"(equivalent to ISO international standard: ISO 11922-1:2018)
Technical definition: 'de' is the abbreviation for the outer diameter of a pipeline, which directly represents the actual diameter outside the pipeline, usually measured in millimeters. Unlike DN and dn, de is a clear physical dimension commonly used in scenarios where precise annotation of the outer diameter and wall thickness of the pipeline is required. The format is"de x wall thickness", such as de63 x 4.7 representing a pipeline with an outer diameter of 63mm and a wall thickness of 4.7mm. Its value is equal to the actual measured value at the outer edge of the pipeline. Scope of application: mainly used for specification description of plastic pipes (such as PPR, PE, PVC pipes) and some seamless steel pipes. In water supply, heating, and chemical pipeline systems, de is a common labeling method, especially suitable for situations where wall thickness needs to be specified to calculate pressure capacity. Industry characteristics: In actual construction, de is commonly used to describe the specifications of plastic pipes. For example, the"25 pipes"and"32 pipes"referred to by construction personnel usually refer to PPR pipes of de25 or de32, rather than DN specifications. Attention to use: The difference in values between de and DN may lead to procurement errors. For example, the PE pipe of de63 may correspond to a nominal diameter of DN50, and specific matching needs to refer to product standards or design specifications.
Finally, to summarize, DN, dn, and de each have their own strengths in the pipeline industry: DN is a standardized nominal diameter suitable for a wide range of pipeline systems; DN emphasizes the nominal outer diameter, which is commonly found in plastic pipes; De is the actual outer diameter, suitable for precise design and construction.
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