The dehydrating valve for storage tanks should be a dual-valve setup; can a single valve combined with a blind flange be used as a substitute?

The last edit to this post was made by ylb913 on 2018-8-19 at 07:58. There are several possible purposes for using two valves: The first is less common; it is used to prevent leakage of the medium in the pipeline (in such cases, there are no pipes after the valve, so a single valve along with a blind flange will suffice), or to prevent the mixing of different media (in this case, it is necessary to close both valves, add an inspection valve in between, or use blind flanges)； The second scenario involves valves that are opened and closed frequently. As asked by the original poster, in such cases, dual valves are used as an additional safety measure. For example, a sampling valve is equipped with two valves – the sampling valve itself is usually connected to the atmosphere, whereas cut-off valves remain sealed. Valves that are opened and closed often experience seal failures; therefore, the first valve should not be closed easily, so that if there is internal leakage in the second valve, it is possible to replace it by closing the first valve. ——Furthermore, it is not very convenient to install blind flanges on valves that are opened and closed frequently; moreover, it is impossible to install blind flanges when there is no way to remove the material from the valve used for sealed discharge. The third type is designed to prevent internal leakage in the valves (with both valves closed under normal conditions, such as the valves used for venting acidic gases from sulfur recovery units, as well as those on some off-take lines where nitrogen is filled between the two valves). The fourth scenario is when there is a too large pressure difference before and after the valve (as mentioned, this is also to prevent leakage inside the valve; usually both valves are closed, as in the case of the auxiliary valves of some control valves) ；

The dehydration valve is a valve that is operated frequently, and installing a blind plate would certainly make operation inconvenient； A valve + blind flange provides complete isolation/separation, while a valve + valve offers double protection – facilitating maintenance and preventing internal leaks

It is mainly to prevent the failure of the first valve; that’s why a dual-valve system is used. Water cut-off valves are generally used quite frequently, so it’s not feasible to keep removing and installing plugs using blind flanges

Hello, thank you for your answer. Also, is there any standard support for the measure of using a single valve along with a blind flange?

This post was last edited by ylb913 on 2018-8-19 at 10:07. It can be found in SIEP D 0401 of Sinopec Engineering Construction Company, titled \"Design Specifications for Venting and Drainage Piping\". The capabilities of Sinopec Engineering Construction Company (SEI) can be trusted. Also: GB 50160-2008 \"Code for Fire Protection Design of Petrochemical Enterprises\" and SH 3012-2011 \"Code for Design of Layout of Metal Piping in Petrochemical Industries\"

According to 5.3.7 of the SHT 3007-2014 Standard for the Design of Tank Areas in Petrochemical Storage and Transportation Systems, the drain pipes of storage tanks shall be equipped with dual valves, as shown in the figure below.