Under what conditions do storage device media inlets and outlets require dual valves?

We use double valves for pipelines carrying high-pressure, highly toxic substances

Reply 1# Papaya Melon: For equipment used to handle hydrocarbons, toxic gases, and harmful chemical agents, when the operating pressure or temperature exceeds certain values, double 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 shall be equipped with double valves. 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. 3. In the factory, the entire installation or unit can be shut down through a bypass pipe; except for equipment that requires frequent cleaning or maintenance, no dual valves are necessary for other equipment. 4. 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). 5. 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 intermediate cooling pump, dual valves are not installed regardless of temperature or pressure levels. 6. Experience has shown that double valves can be used for heat exchangers where failures may occur due to salt deposition, severe scaling, corrosion, and other factors. 7. 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. 8. All shut-offs on the hydrogen pipeline, including pressure gauge valves and sampling valves, shall employ double valves. 9. For liquid butane and lighter hydrocarbons, dual valves must be used for sampling and draining. 10. All pipelines on the liquid chlorine storage tank shall be equipped with double valves. 11. 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. 12. 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). When the boiler uses intermittent blowdown, 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. 13. 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. It should be noted, however, that the bypass valve inside the high-pressure system is a single-valve type. 14. When the process system pipelines are connected to the utility pipelines, double valves must be installed, with a drain valve placed between the two valves. 15. 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. 16. 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. 17. The drain pipes of towers and vessels should be equipped with double valves for normal operation. 18. When draining hazardous materials, double valves should be installed. The content is taken from a post by a netizen on Haichuan’s forum: http://bbs.hcbbs.com/viewthread.php?tid=810290