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1. Vent ports, sampling ports, etc. that must not allow leakage 2. Connections for two materials that cannot mix with each other

(1) For storage tanks holding liquefied petroleum gas, other flammable, toxic, valuable liquids, highly corrosive substances such as concentrated acids and caustic soda, as well as substances with special requirements (such as those with foul odors that cause severe environmental pollution), two valves in series (a dual valve arrangement) should be installed on the pipes leading from the bottom of the tank to other equipment, regardless of whether there are valves near those other devices; one of these valves should be located right next to the tank’s connection port. When the storage tank has a large capacity or is located at a great distance, it is advisable to use a remotely controlled valve for this purpose. The drain valve of the container containing the aforementioned medium should also be a double-valve. The sampling valves and drain valves on the aforementioned medium pipelines should have dual valves installed or not, depending on the frequency of operation and other factors ； (2) For equipment that needs to be shut off during operation of the unit for maintenance, cleaning, or regeneration, a double valve arrangement should be used, with an inspection valve installed between the two valves. When the device is disconnected from the system, both valves close and the check valve opens. (3) When designing high-pressure waste heat boilers and steam systems, it is advisable to follow the provisions in the \"Technical Specifications for the Design of Steam and Water Pipelines in Thermal Power Plants\" (DL/T5054-1996), which state that for pipelines with PN≥4.0MPa, two stop valves should be installed in series for drainage and water release purposes ； For the venting devices of pipes with PN≥4.0MPa, two stop valves should be installed in series. (4) Double valves shall be installed on the upstream side of the connections between hydrocarbons and toxic, hazardous chemical substances and other process materials, as well as on the vent and drain pipes

The double-valve technology is generally used in applications where leaks are not allowed and damage must be avoided； For example, in areas that are toxic, polluted, highly valuable, or require processes such as pressure maintenance or heat retention, or where the location is of great importance due to process requirements and it is not convenient to replace valves, it is possible to have one valve open and another closed for easier maintenance.

I. When to install dual valves: 1. In equipment used for handling hydrocarbons, toxic gases, and harmful chemicals, when the operating pressure or temperature exceeds certain values, dual valves should be installed on the pipelines connected to such equipment. 2. Devices that need to be shut off for cleaning or maintenance during operation must be equipped with double valves. 3. Double valves can be used for heat exchangers that may fail due to salt deposition, severe scaling, corrosion, and other reasons. 4. Use a bypass for the control valve; no need for two valves. The dual valves (installed in series) can only be used when severe corrosion or scaling is suspected, or when failure may occur for other reasons, or when manual adjustment using a bypass valve is required for an extended period of time. 5. All shut-offs on the hydrogen pipeline, including pressure gauge valves and sampling valves, shall employ double valves. 6. For liquid butane and lighter hydrocarbons, double valves must be used for sampling and draining. 7. All pipelines on the liquid chlorine storage tank shall be equipped with double valves. 8. In certain intermittent chemical production processes, if another medium leaks in while the reaction is ongoing, it could lead to explosions, fires, or serious quality issues; therefore, double valves should be installed on the pipelines for that medium, along with a vent valve on the pipe connecting the two valves. 9. Double valves should generally be installed on the blowdown pipes of industrial boilers (with a evaporation rate of ≥1 t/h, or an operating pressure of ≥0.7 MPa). 10. In the design of high-pressure pipelines, where valves must be installed, regardless of the pipe diameter, dual valves are generally required to prevent leakage of the medium ； The installation of the two valves should be closely connected to reduce the number of fittings and facilitate operation. 11. When the process system pipelines are connected to the utility pipelines, dual valves must be installed, with a drain valve placed between the two valves. 12. Sampling valve switches are used frequently and are prone to damage; therefore, two valves are generally installed on the sampling tube. One serves as a shut-off valve, and the other is used for sampling. 13. Large-diameter pipes (such as crude oil pipelines) are difficult to purge completely; a drain pipe at the lowest point should be installed, along with a double valve or a single valve followed by a blind flange. 14. The drain pipes of towers and vessels should be equipped with double valves for normal operation. 15. When draining hazardous materials, double valves should be installed. II. How to set up dual valves 1. When installing dual valves, the short circuit between them should be as short as possible; if necessary, a drain valve should be installed. 2. The dual valves should have a certain slope when installed. 3. A proper support should be installed after the dual valves are installed.

1. When the operating pressure is higher than 4.0 MPa, the isolation valve at the instrument base and the vent valve are both equipped with dual-valve isolation. 2. The connection points between different medium systems are isolated using double valves and 8-shaped blind plates. 3. Large-diameter pipelines (such as crude oil pipelines) are difficult to purge completely; a low-point drain pipe should be installed, and this drain pipe must be equipped with a double valve or a single valve followed by an 8-shaped blind plate. 4. If, at 40°C, the saturated vapor pressure of the medium in the system is greater than 0.5 MPa, double valves shall be used on the exhaust and drain pipes of such a system.

Sampling and venting of volatile substances should be done using double valves. The double valves intended for maintenance purposes are not likely to refer to these same valves. Last edited by tt80111 on 2009-4-14 12:44]

1. For storage tanks holding liquefied petroleum gas, other flammable, toxic, or valuable liquids, highly corrosive substances such as concentrated sulfuric acid and caustic soda, as well as substances with special requirements (such as those with unpleasant odors that cause severe environmental pollution), two valves in series should be installed on the pipes leading from the bottom of the tank to other equipment. Regardless of whether there are valves near those other devices, one of these valves should be placed right next to the tank’s connection port; when the tank has a large capacity or is located at a considerable distance, it is preferable that this valve be a remotely controlled one. To reduce the number of valves, where operation permits, several pipes are combined and connected to a single pipe outlet as shown in Figure 2.0.3—1. The drain valve of the container containing the aforementioned medium should also be a double-valve, as shown in Figure 2.0.3–1. The sampling valve and drain valve on the aforementioned medium pipeline should have dual valves installed or not, depending on the frequency of operation and other conditions. 2. For equipment that needs to be shut off during operation of the unit for maintenance, cleaning, or regeneration, a double valve arrangement should be used, with an inspection valve installed between the two valves. When the device is disconnected from the system, both valves close and the check valve opens. Other measures can be taken to replace the dual valves. For the backup reboiler, due to its larger valve diameter and strict requirements regarding pressure drop, a single valve (usually a stem-mounted gate valve) can be installed along with an 8-shaped blind plate; each side of the reboiler should be equipped with its own drain valve. For equipment that requires switching to regeneration mode, since the regeneration temperature is often much higher than the operating temperature, installing a reversible elbow allows for a safe switch-over while also preventing significant thermal stress. 3. Utility material pipelines should, as much as possible, not be fixedly connected to process material pipelines; instead, they should be connected using flexible hoses and quick connectors at a hose station. When direct connection is required for operation, a double-valve connection should be used, with a check valve in between; this check valve opens when feeding is stopped, or can be sealed with a lead seal (C.S.O.). Add a check valve in situations where pressure may fluctuate. If the pressure gauge for the common fluid is located far from this valve assembly, a pressure gauge can be installed between these two valves to allow for on-site monitoring of the pressure of that common fluid during use. This type of connection is also suitable for situations where auxiliary materials such as oxygen and hydrogen are frequently supplied to the process system. To prevent liquid materials from contaminating the water system, when water needs to be added frequently, the water pipe should be connected to the gas space of the equipment; in this case, it is also not necessary to use a double valve. 4. When designing high-pressure waste heat boilers and steam systems, professionals in the field of chemical process engineering may refer to the relevant regulations set forth by the General Administration for Electric Power Construction under the Ministry of Electric Power: The \"Technical Specifications for the Design of Steam and Water Pipelines in Thermal Power Plants\" (DLGJ 233—81) (provisional version), Articles 7–7.1: For pipelines where Pg > 40*, two stop valves should be installed in series for drainage and water release purposes. Articles 7–8: For pipes with Pg＞40*, two stop valves shall be installed in series as the venting device. The pressure unit is kg/cm2 (table). Please be sure to follow the latest regulations when using it. 5. For materials such as hydrocarbons and toxic or harmful chemical agents, at the connections to other process materials, as well as on the vent and drain pipes, dual valves should be installed; refer to Table 2.0.3 for details. Temperature and pressure conditions for the reaction dual valves Medium name Operating temperature (°C) Operating pressure Heavy hydrocarbons (kerosene, lubricating oils, asphalt, etc.) ≥200 ≥20 Raught Hydrocarbons with a vapor pressure of less than 1.05×105 Pa (at atmospheric pressure) and a flash point of less than 37.8°C (e.g., crude gas oil) ≥180 ≥20 Raught Hydrocarbons with a vapor pressure higher than 1.05×105 Pa (at atmospheric pressure) but lower than 4.57×105 Pa (at atmospheric pressure) (e.g., butane, light crude gas oil) ≥150 ≥18 Raught Hydrocarbons with a vapor pressure higher than 4.57×105 Pa (at atmospheric pressure) (e.g., propane) ≥120 ≥18 Raught H2, liquefied petroleum gas Any Any Any flammable gas ≥120 ≥25 Toxic gases and harmful chemicals Any ≥3.5

Texaco gasification: 1. The oxygen cut-off valve must be equipped with a dual valve； Reason: Leakage can cause a major accident ； (Two valves in series) 2. The slurry cut-off valve shall be equipped with two valves ； Reason: Fluid leakage into the high-temperature furnace can cause serious accidents ； (Two valves in series) 3. A double valve should be installed at the connection between the lock hopper and the quench chamber ； Reason: One valve serves a safety function, while the second valve is used during normal production ； (Two valves in series)

6.2 Installation of double valves 6.2.1 For equipment used to handle hydrocarbons, toxic gases, and harmful chemicals, where the operating pressure or temperature exceeds the values listed in Table 1–1, double valves should be used on the pipelines connected to such equipment. Table 1–1 Temperature and pressure conditions for using dual valves
Medium name, Temperature (°C), Pressure (MPa(G))
Heavy hydrocarbons (kerosene, lubricating oil, asphalt): 204, 2.11
Hydrocarbons with a vapor pressure below 1.05×105 PaG and a flash point below 37.8°C (crude gasoline): 177, 1.93
Hydrocarbons with a vapor pressure higher than 1.05×105 PaG but lower than 4.57×106 PaG (butane, light crude gasoline, etc.): 149, 1.76
Hydrocarbons with a vapor pressure higher than 4.57×105 PaG (propane, etc.): 121, 1.76
Hydrogen, liquefied petroleum gas: –
Any flammable gas: 121, 2.46
Toxic gases and harmful chemicals (acids, bases, etc.): –, 0.352
6.2.2 Dual valves should be installed on equipment that needs to be shut down for cleaning or maintenance during operation of the plant. Steel valves with a diameter greater than 250 mm and alloy steel valves with a diameter greater than 150 mm require special consideration due to their high cost. In factories, the entire installation or unit can be shut down through bypass pipes, and except for equipment that requires frequent cleaning or maintenance, no dual valves are necessary for other equipment. When using a single valve to isolate the equipment from operation, a drain valve should be installed between the valve and the equipment. When using a dual valve, a check valve is installed between the two valves. (DBD, Double block Plus Drain). 6.2.3 All major pumps, such as feed pumps, return pumps, feed pumps for the heating furnace, and bottom extraction pumps, shall be equipped with double valves when the above operating conditions apply. For non-primary pumps such as the counterflow pump and the intercooler pump, dual valves are not provided regardless of temperature or pressure levels. 6.2.4 Experience has shown that for heat exchangers prone to failure due to salt deposition, severe scaling, corrosion, and other factors, dual valves can be used when the above conditions are met. 6.2.5 Bypass for control valves – no dual valves required. The dual valves (installed in series) can only be used when severe corrosion or scaling is suspected, or when failure may occur for other reasons, or when manual adjustment using a bypass valve is required for an extended period of time. 6.2.6 All isolations on hydrogen pipelines, including pressure gauge valves and sampling valves, shall employ double valves. 6.2.7 For liquid butane and lighter hydrocarbons, double valves shall be used for sampling and draining. 6.2.8 All pipelines on the liquid chlorine storage tank shall be equipped with double valves. 6.2.9 In certain intermittent chemical production processes, if another medium leaks in while the reaction is ongoing, it may cause explosions, fires, or serious quality issues; therefore, double valves should be installed on the pipelines for that medium, and a vent valve should be placed on the connecting pipe between the two valves. During production, valve 1 is closed and valve 2 is open. When the previous production is completed and preparation is made for the next production run, valve 2 is closed and valve 1 is opened. 6.2.10 Double valves should generally be installed on the blowdown pipes of industrial boilers (with a evaporation capacity of ≥1 t/h, or an operating pressure of ≥0.7 MPa), as shown in Figure 1–2. When intermittent blowdown is used in the boiler, the valve is opened and closed 3 to 4 times per hour; under the influence of pressure and temperature, it operates frequently and is prone to leakage. Severe leakage from this valve can cause the boiler to stop operating, so a dual-valve system is installed to ensure the boiler can function properly over the long term. 6.2.11 In the design of high-pressure pipelines, where valves must be installed, dual valves are generally required regardless of the pipe diameter, in order to prevent leakage of the medium ； The installation of the two valves should be closely connected to reduce the number of fittings and facilitate operation. In the case of angle valves, the direction and elevation of the handwheel shall meet the requirements for operation and maintenance. For corner valves with bottom inlet and side outlet, or side inlet and bottom outlet, attention should be paid to the installation direction. Its installation method is shown in Figure 1-3. It should be noted, however, that the bypass valve inside the high-pressure system is a single-valve type. 6.2.12 When process system pipelines are connected to utility pipes, dual valves shall be installed, with a drain valve placed between the two valves. 6.2.13 Sampling valves are operated frequently and are prone to damage; therefore, two valves are generally installed on the sampling tube. One serves as a shut-off valve, and the other is used for sampling. 6.2.14 Large-diameter pipelines (such as crude oil pipelines) are difficult to purge completely; a low-point drain pipe should be installed, along with a double valve or a single valve followed by a blind flange. 6.2.15 The drain pipes of towers and vessels shall be equipped with double valves for normal operation. 6.2.16 When draining hazardous materials, double valves should be installed.

Dual valves are required under the following operating conditions: first, in high-pressure pipeline systems； The second is the highly toxic pipeline system ； Third are pipes with special requirements. For the dual valves installed on the high-pressure pipeline at the pump outlet, when the pipe diameter is DN50, a connection pipe should be provided at both ends of the dual valves; the specification for this connection pipe is generally DN20. .

As far as I know, there are two situations in which dual valves need to be installed: 1) the outlet valve at the bottom of the large acid tank; 2) the blowdown valve of the boiler (high pressure). Setting up dual valves is mainly for safety and ease of maintenance, especially in cases where the valves are used frequently and thus prone to damage. The dual-valve setup involves installing two valves in sequence. There are two points to keep in mind when installing double valves: first, the type of valve must be chosen appropriately (stainless steel ball valves are a good option) ； Secondly, it is necessary to strengthen the maintenance of valves; for specific details, one can refer to the information on the installation, maintenance, and operation of valves at http://bbs.hcbbs.com/viewthread.php?tid=127977.

I know there are three situations in which dual valves need to be installed: 1. High-pressure pipelines； 2. External pipelines of the highly toxic system and pipelines leading to the catalytic system ； 3. External pipelines of the fermentation system. During installation, the two valves can be installed consecutively, with the short circuit being as short as possible. But try to use one of them when in use, to avoid both valves breaking at the same time. Because in a dual-valve system, one valve is used for normal production operations, while the other is used in case the valve used for those operations gets damaged. This post was last edited by ntdbm on 2009-4-14 22:18]