Installation of storage tank breather valves/pressure relief manholes
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Here, I would like to discuss with all of you professionals the issue of installing vent valves/pressure relief manholes on storage tanks. According to clause 5.1.5 of SH/T3007-2014, storage tanks that use nitrogen or other inert gases for sealing and protection systems shall be equipped with emergency pressure relief devices, which must meet the following requirements: a) The operating pressure of these emergency pressure relief devices should be higher than the exhaust pressure of the vent valves, and it should be less than or equal to the design positive pressure of the storage tank; b) These emergency pressure relief devices must ensure proper ventilation of the storage tank in case of failures in the ammonia seal system, other inert gas sealing systems, or vent valves; c) The emergency pressure relief devices should be able to discharge directly into the atmosphere; d) It is advisable to use emergency vent covers or vent manholes with a diameter of at least DN500 for such devices. GB50160-2008 (2018 edition) 6.2.19 Fixed-roof tanks for Class A B and Class C liquids shall be equipped with flame arresters and vent valves ； Storage tanks for Class A and Class B substances that use nitrogen or other gases as a gas seal should also be equipped with emergency pressure relief devices. Regarding the above specification requirements, my views are as follows: 1. For storage tanks equipped with a nitrogen seal, normal pressure (inlet and outlet) control is carried out through the nitrogen seal valve and an automatically regulating pressure relief valve; in the event of a fault-induced overpressure, a 24/7 flame arrestor breather valve is used (which does not operate under normal conditions). If the amount of gas that can be released through the breather valve is insufficient in case of a fault, then a breather manhole is utilized. In this case, if the breather valve can meet the pressure relief requirements, there is no need to use a breather manhole for pressure relief. 2. For storage tanks equipped with a nitrogen seal, normal inlet pressure control is carried out through the nitrogen seal valve, while outlet pressure control is achieved using a flame arrestor breather valve (in normal operation); in the event of a fault-related overpressure, a breathing manhole should be used. The above safety design concepts are proposed because some small storage tanks (under 200 m3) have very low leakage rates in the event of a failure; installing both a vent valve and a pressure relief manhole would be excessive. However, designers hold different opinions on this matter, which can easily lead to over-design or inadequate consideration. The above are my own design perspectives; I hope fellow experts in this field will join the discussion and offer their valuable opinions.
Your concern stems from the small size of the storage tank; even in the event of a failure, the exhaust capacity of the breather valve is sufficient to handle such emergencies, eliminating the need for a pressure relief manhole, which would otherwise be redundant. In fact, pressure relief manholes are installed primarily to prevent fires in storage tanks. Imagine if a fire breaks out while the tank is filled with oil – in such a situation, even the maximum exhaust capacity of the breather valve will not be sufficient; hence, large-diameter pressure relief manholes are necessary.
First, thank you for your reply. The explanation is as follows: According to API2000, the fire venting capacity can be calculated; for small storage tanks, the venting capacity of the available breather valves is more than sufficient to meet the requirements.
If the discharge capacity of the breather valve calculated is sufficient to meet the requirements, then installing a breather valve will suffice. Vent valves and emergency relief valves serve essentially the same purpose: to release excess pressure from storage tanks.
The interpretations of the two provisions listed by the original poster are as follows: SHT 3007-2014 5.1.5 This provision specifies the measures to ensure the safety of storage tanks in the event of a failure in the nitrogen or other inert gas sealing system or vent valves. GB 50160-2008 6.2.19 For fixed-roof tanks storing Class A and B liquids that are purged with nitrogen or other gases, emergency pressure relief devices such as pressure relief manholes and vent manholes shall be installed to ensure the safety of the tanks. It can be seen from the interpretation of these two provisions that the purpose of installing accident relief devices such as pressure relief manholes is to ensure safety in emergency situations, and it has nothing to do with whether the volume of gas released through breathing methods is sufficient for such situations. Referring to the typical protective layers specified in AQT 3054-2015 \"Guidelines for the Application of LOPA Methods\", the breathing valve constitutes an intrinsically safe design layer, while the pressure relief manhole represents a physical protection layer. One is intended to meet the requirements of process control, and the other to fulfill safety protection needs; the necessity of safety measures cannot be dismissed based on considerations related to process control. I suspect that in most cases, it is still necessary to install accident pressure relief equipment, so there is nothing wrong with the standards imposing strict requirements for safety’s sake.
I think what you say is not correct; just as what the SIS interlock can do, the DCS interlock can also do, then why is it still necessary to use SIS? The fundamental functions are of course the same, but safety is the main consideration.
In my opinion, the normal exhaust design for the nitrogen seal of storage tanks should involve control valves or self-acting valves; the vent valve is used for pressure relief in case of abnormal situations and is not utilized under normal conditions. If the breather valve is used during normal venting, then an alternative pressure relief device (such as a breather manhole) must be employed for emergency pressure relief measures. Do you agree with this design philosophy?
Should an emergency vent manhole also be considered for horizontal low-pressure nitrogen-sealed tanks?
However, the standard mentions that in the event of a malfunction of the breather valve, pressure should be released during a fire. How should this situation be considered?
I’ve never seen horizontal tanks equipped with emergency relief manholes; the standards refer to fixed-roof storage tanks