Understand chemical process pipelines, double valves, and blind flanges in one article! Comprehensive analysis of regulatory basis, applicable scenarios, and selection criteria

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Hello, colleagues in the chemical industry. I’m Brother Donkey! In hazardous chemical production facilities, pipeline isolation serves as the first line of defense for safety during startup and shutdown, routine inspections, and special operations. As the two most commonly used isolation methods on site, double valves and blind flanges can be found at almost every location, including vent ports, drain ports, sampling points, the boundaries of equipment units, and equipment connections. But many workers on the front line still have doubts: does closing two valves completely mean isolation? When is it compliant to use only dual valves? In what scenarios is it necessary to install blind flanges? What are the mandatory requirements for the double-break double-drain (DBB) structure? In recent years, there have been multiple incidents of fires, poisoning in confined spaces, and material leakage; in most cases, the root cause was the incorrect selection of isolation measures or inadequate implementation of such measures. Today, by drawing on **current regulations, industry standards, and practical field experience, we will thoroughly explain the legal basis, key differences, usage restrictions, and important precautions related to double valves and blind flanges. It is recommended to save and share this content for all team members to study. I. First, draw a red line: The supporting regulations and standards dictate that the choice of any pipeline isolation method is not based on experience, but rather on strict adherence to relevant industry mandates. The following are the key guidelines for on-site implementation, as well as the core criteria used for regulatory inspections and the identification of potential hazards: 1. The “Code for Selection of Materials for Petrochemical Pipelines” SH/T 3059 specifies that for pipelines carrying highly hazardous toxins, Class A flammable gases, liquefied hydrocarbons, high-pressure/cryogenic fluids, as well as for venting, drain, sampling, and instrument pressure tapping branches, the use of dual isolation valves along with an intermediate release mechanism is mandatory ； The use of a single valve as a long-term isolation measure for high-hazard media is prohibited. 2. The \"Specifications for the Design of Chemical Process Systems\" HG/T 20546 defines the double-break double-relief (DBB) standard structure, and specifies requirements for valve selection, pipeline layout, and dead zone control; it serves as the core guideline for the design of dual-valve systems in chemical pipelines. 3. According to the \"Code for Fire Protection Design of Petrochemical Enterprises\" GB 50160, for pipelines carrying flammable and explosive media of Class A and B, as well as at the boundaries of installations and for branch pipelines spanning different units, reliable isolation facilities must be installed; separate requirements are specified for temporary isolation and permanent isolation. 4. The \"Safety Specifications for Special Operations in Hazardous Chemical Enterprises\" AQ 3022 (hot work) and AQ 3027 (confined spaces) stipulate explicitly that for tasks involving high-risk hot work or level 1 hot work, entry into confined spaces, or pipeline disassembly, it is strictly prohibited to rely solely on valves for isolation; blind plates must be used to create a physical barrier. This is an absolute requirement that must be adhered to in such special operations. 5. The \"Criteria for Identifying Major Hidden Dangers in Production Safety Incidents in Chemical and Hazardous Chemicals Production and Operation Units\" specify one major hidden danger: when performing maintenance on hazardous chemical equipment and pipelines, if blind plates are not installed as required and valves are used solely as isolation measures, it is directly classified as a major hidden danger for production safety incidents. 6. The “Code for Construction and Acceptance of Piping in Petrochemical Projects” SH/T 3501 specifies the requirements for blind plate installation and removal, the use of cross-shaped blind plates, isolation labeling, and record-keeping. Additional note: Large-scale refining and joint venture projects follow the international standard ASME B31.3 for process piping specifications; for pipes handling highly toxic substances, ultra-high pressures, or cryogenic conditions, a DBB double-break double-relief design is mandatory. II. Dual-valve piping (DBB: dual shutdown and dual relief): structure, principle, and application scenarios 1. What is a compliant dual valve? What is commonly referred to as a “double valve” on-site has the full professional name of double isolation valve system; a compliant design must employ a DBB (Double Block and Bleed) structure: two valves are connected in series to serve as the main isolation devices, with a small vent/condensate drain valve installed between them. The fact that only two valves are installed without a relief valve in between constitutes a design flaw, preventing the achievement of a safe isolation function. 2. Working principle: After closing the two main shut-off valves, open the intermediate relief valve. If there is an internal leakage at the first valve upstream, the leaking fluid will be discharged and depressurized through the intermediate drain pipe, preventing it from flowing backward past the second valve. ⚠️Key feature: The dual valves achieve isolation through sealing at the valve seats, which is a form of sealed isolation. Due to exposure to the medium, temperature changes, and valve body aging, there is a very low probability of long-term leakage; a 100% absolute physical separation cannot be ensured. 3. Valve selection and layout requirements: For root valves (located near the equipment/main pipeline), those that need to remain open or closed for extended periods with low operation frequency should preferably be gate valves or ball valves, as they offer low flow resistance and stable static sealing performance ； Valves for operational use: When they need to be opened and closed frequently on a daily basis, globe valves, gate valves, and ball valves can be used, as they can withstand erosion caused by the medium ； Installation requirement: The valve should be as close as possible to the equipment outlet and the main pipe wall, in order to minimize the \"dead zone\" in the pipe section. For media that are prone to crystallization, polymerization, and have high viscosity, an excessively long dead zone can cause the media to remain and solidify, leading to valve blockage and corrosion, which in turn results in the failure of isolation. 4. Scenarios where the dual-valve (DBB) system must/should be used preferentially: routine operation interfaces such as venting at the high points of the pipeline, draining condensate at the low points, and on-site sampling ports ； Instrument connections: pressure gauge, level gauge, transmitter, and other instrument-related pressure introduction pipelines ； Device auxiliary interfaces: device spare ports, temporary connection ports, small branch pipelines ； Routine isolation for high-risk media: daily branch pipelines for highly toxic media, Class A flammable gases, liquefied hydrocarbons, and high-pressure/cryogenic media ； Online maintenance points: No need to shut down the main system; only the pipeline interfaces that require partial isolation for online maintenance. In short: for applications involving routine operations, frequent switching, and online monitoring, the dual-valve DBB structure should be preferred. III. Blind plate isolation: A physical barrier, the safety baseline for special operations. 1. What is a blind plate? A blind plate is a solid metal plug installed between two flanges; it represents a purely physical barrier that completely blocks the flow cross-section of the pipeline, thereby eliminating the risk of fluid leakage and internal valve leaks. It represents the highest level of protection for isolation in the chemical industry. The main types on site fall into two categories: dead blind flanges (flange-type blind flanges): these are solid steel plates that provide the highest level of sealing; they are used for permanently isolating pipelines and equipment that will not be in use for an extended period, though their installation and removal require significant effort ； Eight-character blind flange: One end is a blind flange and the other end is an open flange; they are manufactured as one unit, allowing for quick switching between open and closed states without the need to remove or install flanges. This makes it convenient to use, and it is the preferred choice for frequent switching of isolation points in industrial facilities. https://mmbiz.qpic.cn/mmbiz_png/8UuC2kmmvWdqgBJnWdcMdFtKt0niavic9pB7icefBFh2eMceojW95V43WcicrDfjUUNpK6lFjgdjh5syibypicL7ePt8wLecXYfPokAjAWl1Sf5eg/0?wx_fmt=png&from=appmsg2. Key features: no seals, no risk of internal leakage, 100% reliability in isolation ； The downside is that switching and disassembly require the system to be depressurized, purged, and shut down for material flow, resulting in a complex operational process that makes it unsuitable for frequent use on a daily basis. 3. Specify the scenarios where the installation of blind plates is mandatory (red-line requirements). In line with special operation protocols and criteria for identifying major hazards, isolation using valves alone is strictly prohibited in the following scenarios; blind plates must be installed: Special operation scenarios: Class A/Class B hot work, entry into confined spaces, pipeline cutting and disassembly, and equipment opening for maintenance ； Comprehensive system maintenance: shutdown of the entire installation or individual equipment for extended periods, coordinated maintenance across the entire plant, as well as maintenance of the main pipelines within the installation area and the branch pipelines between different units ； Pipelines out of use for extended periods: Those that have been unused for more than 7 days, abandoned pipelines, and standby pipelines that have been idle for a long time – it is prohibited to isolate them by simply closing the valves ； In-depth maintenance of high-hazard media: In-depth maintenance of pipelines/equipment handling highly toxic, highly corrosive, or high-pressure flammable media, at locations requiring dual isolation ； Error prevention isolation: At the junctions of public pipeline networks and interconnected pipelines with a risk of cross-contamination, this mechanism prevents accidents caused by accidental opening of valves leading to cross-contamination of fluids. 4. Strict requirements for the on-site management of blind plates: Material compatibility – The material, pressure rating, and thickness of the blind plate must match those of the pipeline itself; it is strictly prohibited to use iron sheets, plastic sheets, or non-metallic sheets as substitutes ； Identification ledger: All blind flanges are uniformly labeled and numbered, and a dedicated ledger is maintained indicating the installation location, medium used, installation date, and responsible person ； Job control: Removing and installing blind flanges is considered a special type of task; it is necessary to obtain a work permit, conduct gas testing, and assign a dedicated person to supervise the process ； Reproduction verification: Before resuming production after maintenance is completed, check the records item by item to ensure that the same number of components are installed and removed, thereby preventing blockages in the pipelines or pipe explosions due to leftover blind flanges. IV. Dual valves vs. blind flanges: Key differences + selection guide
Comparison criteria:
Dual valves (DBB – double isolation and double relief)
Blind flanges (physical barriers)

Isolation type:
Sealed isolation (reliant on valve seat sealing)
Physical barrier (blocked by a solid plate)

Reliability of isolation:
Higher reliability, but there is a 100% chance of internal leakage.
Reliable, with no risk of internal leakage.

Frequency of operation:
Suitable for frequent opening and closing, as well as daily operations.
Suitable for long-term isolation, with very few adjustments needed.

Applications:
Inspections, venting, condensate removal, sampling, on-line maintenance, hot work, confined space operations, pipeline disassembly, long-term shutdowns.

Main advantages:
Simple to operate, no need to shut down the system, high efficiency.
Complete isolation, high safety levels, prevents accidental operations.

Main disadvantages:
Prone to sticking when left in use for long periods, potential for internal leakage.
Complex installation and removal procedures, requires pressure release and gas replacement, heavy workload.

Regulatory requirements:
Standard isolation method for regular use.
Mandatory for special operations and long-term isolation.

Additional note: Dual isolation (highest safety level)
For extremely high-risk situations such as highly toxic substances, high-pressure liquefied hydrocarbons, and equipment in high-hazard areas, a combination of dual valves (DBB) and blind flanges is used as a form of dual-layer isolation, representing the highest standard of isolation in chemical plants. V. Analysis of Common Mistakes on Site (Key Pitfalls to Avoid for Teams) 1. Mistake No. 1: Thinking that closing two valves is sufficient to carry out welding work or enter confined spaces ❌ This is a serious violation and constitutes a major hazard! Valves are inevitably subject to aging and internal leakage risks; for special operations, physical isolation using blind flanges is mandatory, as it is a strict requirement imposed by regulations. 2. Misconception 2: Installing only dual valves while omitting the vent valve in between ❌ This completely negates the purpose of the DBB design; when the first valve leaks, pressure cannot be released, and the risk of material leakage persists, which constitutes an installation defect. 3. Misconception 3: When pipelines are to be out of use for an extended period, only the valves are closed without installing blind flanges. ❌ Valves that remain unused for long periods tend to stick and lose their sealing capacity; fluctuations in pressure upstream and downstream can easily cause the flow of fluid to bypass the valves, which represents one of the most common sources of leakage in industrial facilities. 4. Misconception 4: Blind plates are not labeled or recorded, and non-standard plates are used indiscriminately ❌ This can lead to the omission of blind plates during restart, resulting in pipe blockages and overpressure ； Non-standard blind flanges do not have sufficient pressure resistance and are prone to cracking, leading to leakage accidents. 5. Misconception 5: Excessively long dual-valve section in crystallizable/polymerizing media ❌ Media retention and solidification can block the valves, thereby rendering the isolation function ineffective; it is necessary to shorten the dead zone length. VI. Mnemonics for practical use on the front line (easy to remember, useful, and easy to apply): For daily operations, pay attention to the two valves; use DBB for venting and sampling ； For special operations, blind plates are necessary; only physical barriers ensure safety ； For short-term use, switch to dual-valve protection; for long-term shutdown, use blind flanges for blocking ； Mark the red line for restricted hot work areas; neither single valves nor double valves are allowed ； Double protection for high-risk areas: both valves and blind flanges are installed together. There is no superiority or inferiority between double valves and blind flanges; only their applicable scenarios differ. The dual-valve (DBB) system is designed for daily production and frequent operations, balancing efficiency with basic safety ； Blind plates ensure the safety baseline for special operations and prolonged shutdowns, using physical barriers to eliminate all risks. As professionals in the chemical industry, one must not seek shortcuts by simplifying isolation measures. It is essential to fully understand the guidelines and clearly distinguish between different application scenarios, so as to strengthen this first line of defense through pipeline isolation – only in this way can various accidents such as cross-contamination, poisoning, fires, and explosions be prevented at the source.