Conversion of atmospheric-pressure rainwater tanks into low-pressure storage tanks
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Dear sea friends: The rainwater tanks in our company are designed for a pressure of ATM, which means atmospheric pressure； For fixed-roof storage tanks ； There are emission ports for the atmosphere at the top (DN250). Since this tank collects initial rainwater, it contains a certain amount of oil. The safety department suggested that a nitrogen seal and a vent valve should be added to this tank. However, with the nitrogen seal in place, the pressure of the nitrogen gas supplied to it is 25 mmH2O, while the set pressure for the vent valve is 100 mmH2O ； The set pressure for the emergency relief manhole is 140 mmH2O ； However, the design pressure of this tank is ATM. Clearly, this pressure exceeds the tank’s designed value. May I ask: can this storage tank be used under conditions with a pressure higher than its designed pressure? Are there any relevant standards?
It is recommended that the original poster consult GB50341-2014. The design pressure for oil tanks is usually set at 2 KPa; obviously, the pressure used for nitrogen sealing in your case exceeds this value. It is advised to reduce the pressure of the nitrogen gas used for sealing, as nitrogen sealing does not require a very high pressure – a slight positive pressure is sufficient.
This post was last edited by altpage on 2017-12-26 at 16:54. The absolute pressure at standard atmospheric conditions is approximately 103.3 kPa, which is what is referred to as normal pressure. The maximum operating pressure for the modified tank is 140 mm of water column, equivalent to 1.4 kPa (gauge pressure); when converted to absolute pressure, this amounts to 104.7 kPa. I believe that the modifications were carried out in accordance with the GB50341-2014 standards regarding micro-pressure storage tanks.
What are the requirements for this? All the atmospheric storage tanks that emit odors in our factory have been sealed with nitrogen
We specialize in the production of nitrogen sealing devices; a slight positive pressure of 1.2 kPa is generally sufficient, with the minimum value capable of being adjusted down to 500 Pa
The design pressure for atmospheric storage tanks is also 2 kPa; your nitrogen seal, on the other hand, is only 1.4 kPa. As long as the tank has not been in use for a long time, there should be no problem
Why were tanks built instead of rainwater collection ponds?
140 mmH2O is equivalent to filling the tank and then having an additional 14 cm of pressure; this is a minor pressure level, unless the tank is of such poor quality that it deforms even when filled with water
The design pressure for atmospheric storage tanks can also be 1960 PaG
The pressure setting for the nitrogen seal valve is 0.25 KPa; for the breathing valve, 0.4 KPa (or 0.5 KPa) is sufficient. The pressure setting for the emergency pressure relief valve is 0.6 KPa (or 0.8 KPa), which is also quite safe. tang.caiyi@kaneko.com.cn – Tang, you can also contact me by email. We have done something similar at CNOOC
Changing a normal-pressure tank into a pressure-resistant tank involves a range of issues related to container design, so it shouldn’t be a good solution. Why not treat the rainwater slightly before it enters the tank, in order to remove floating oil and similar substances? Can’t the nitrogen seal pressure be maintained at a slight positive pressure?