Design issues for safety facilities in Class A large tanks
Thread Content
I am seeking help from the expert Hai Chuan. The specific issue is as follows: Our company has a large tank for storing Class A chemicals, with a capacity of 1,000 cubic meters. It is in the form of a dome tank. 1. The large tank is equipped with a nitrogen pressurization system, featuring a single-exhale valve, a breathing valve, and a pressure relief manhole at the top. The designer has considered using nitrogen to maintain pressure equilibrium in the large tank under normal conditions; when the pressure rises, a one-way vent is used to release excess pressure. If this vent is not sufficient, a breathing valve takes over to discharge pressure as well. In extreme situations, pressure is released through a pressure relief hatch, thereby preventing the large tank from experiencing overpressure ； At present, the breather valves and pressure relief manholes discharge directly into the atmosphere, as there is no recycling system designed for this purpose. I would like to know whether this complies with regulatory requirements; if not, which standard should be followed? ； 2. The breather valve installed in the large tank is used to supply air into the tank when the pressure is low. Considering the oxygen content in the air, a explosive substance may form within the large tank. May I ask whether the installation of this breather valve complies with the relevant standards?
1. What is the normal pressure control for storage tanks? Are there control valves with step control? When the pressure is low, nitrogen is added; when the pressure is high, pressure is released. What mechanism does your system use to automatically maintain the pressure in the storage tank? 2. It cannot be discharged directly into the atmosphere nor introduced directly into the air. Isn’t there a nitrogen seal in place to supply nitrogen?
It’s fine for the breather valve and pressure relief manhole to vent directly to the atmosphere； To understand the drawings, it is essential to comprehend the designer’s intent ； Under normal operating conditions: 1. The function of the nitrogen seal in the storage tank is primarily to prevent negative pressure from forming in the tank, which could otherwise cause air to be drawn in through the breather valve, thereby maintaining a slight positive pressure inside the tank ； When the gas pressure inside the tank is below the opening pressure of the nitrogen seal valve, the valve opens to supply nitrogen into the tank ； When the gas pressure inside the tank reaches the closing pressure of the nitrogen seal valve, the valve closes, stopping the supply of nitrogen to the tank. 2. When the gas pressure inside the tank exceeds the set pressure of the one-way valve, the gas is discharged outside the tank through the one-way valve and into the waste gas collection system. In abnormal situations (such as failures in the nitrogen sealing system that prevent proper gas injection or exhaust), 1) when nitrogen cannot be injected properly, discharging material may cause negative pressure to develop in the storage tank; if this pressure exceeds the designed range, an accident can occur. Therefore, a vent valve is necessary to prevent such accidents. Although the intake of air may create a risk of explosion, this risk is considered acceptable compared to the risk of the tank leaking due to excessive negative pressure. The same goes for the other way around! 2. Similarly, the pressure relief manhole serves as the third safety barrier in cases of positive pressure.
I remember there seems to be a requirement to design dual breathing valves
Whether to install a dual breathing valve or not depends on the tank volume and the calculation results
1. The normal control pressure is 2 KpaG; the exhalation pressure at the single-exhalation valve is 3 KpaG. There is only a pressure-regulating valve for filling, no pressure-regulating valve for release, and the pressure release is achieved by using the single-exhalation valve to discharge the material into the exhaust gas treatment system； 2. There is a nitrogen seal in place, but under extreme conditions, a failure of this nitrogen seal can prevent air from entering, causing the pressure in the large tank to continue dropping; the breather valve currently installed is only capable of allowing air to enter.
Then it’s normal, just as what was said on the 3rd floor. Normally, controlling the supply of nitrogen at 2 KPa is sufficient to maintain the pressure in the storage tank; the breathing valve, which prevents negative pressure in the tank and the absorption of air, comes into use only when there is insufficient nitrogen supply or when the nitrogen control valve fails and cannot supply nitrogen. Just confirm the control pressure of the breather valve again.
The methanol tank here is also equipped with a nitrogen seal and a vent valve; it seems to be standard equipment: lol