The purpose of installing dual valves

The dual-valve feed inlet and the dual-valve reserve ports are designed to facilitate isolation: when it is necessary to add new pipelines, perform equipment maintenance, enter confined spaces, or carry out welding work, this device can be easily isolated from other systems (by using blind flanges) and have its contents replaced.

Valves are prone to internal leakage and failure; using two valves is safer than using just one. Especially in areas where isolation is required, removing the pipes to install blind flanges is not necessarily easy to do

It’s better to include a simple diagram for reference, to facilitate discussion. In typical setups, the dual valves are usually such that the first valve is kept fully open, while the second valve serves as the control valve for normal operation. The purpose is for replacement or repair in the event that the second valve is damaged. There is also a two-valve arrangement designed to prevent leaks; typically, a branch line is installed between the two valves, and an exhaust valve is placed on this branch line to allow any leaked fluid to be removed, thereby preventing pressure from building up and reaching the second valve. For reference.

For details, refer to HG/T 20570.18—95 \"Valve Installation\" (which differs slightly from the text below). 6 Regulations on dual-valve installation 6.1 Function of dual valves In petroleum and chemical production, the process media handled include flammable, explosive, and toxic substances as well as those in high-pressure systems; in some batch-type production facilities, dual valves are installed under certain operating conditions to ensure safe, continuous, and stable production. 6.2 Installation of double valves 6.2.1 For equipment used to handle hydrocarbons, toxic gases, and hazardous chemicals, where the operating pressure or temperature exceeds the values listed in Table 1–1, double valves shall be used on the pipelines connected to such equipment. Table 1-1 Temperature and pressure conditions for using double valves (omitted) 6.2.2 Double 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 needed 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 double 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 need for double 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. 6.2.6 All isolations on the hydrogen pipeline, including pressure gauge valves and sampling valves, shall use 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 pipe connecting the two valves, as shown in Figure 1-1. In 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. (Less) 6.2.10 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), 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, 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 the pipelines of the process system are connected to the utility pipes, double valves must be installed, with a drain valve placed between the two valves. 6.2.13 Sampling valves are switched on and off 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 pipes (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 completely, double valves shall be installed.