How to calculate the negative pressure in a water filling test for large storage tanks?
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A large internal floating roof nitrogen-blanketed storage tank with an engineering volume of 1,000 m³. The tank has an inner diameter of 11.5 meters and a height of 11.5 meters. The designed positive pressure is 17,000 Pa, while the designed negative pressure is 354 Pa. So what is the positive pressure for the hydrostatic test? What is the negative pressure for the hydrostatic test (on the tank top and walls)? According to GB30541, the positive pressure for the hydrostatic test is: 17000 x 1.25 = 21250 Pa. What is the negative pressure for the hydrostatic test? No method for calculating negative pressure in the hydrostatic testing of large storage tanks was found in GB50341. Help: Teachers on the forum, how do I calculate the problem above? But I saw that the data in the drawings indicate that the positive pressure for the hydrostatic test is 21250 Pa, while the negative pressure for the hydrostatic test is 3500 Pa for the tank top/2500 Pa for the tank walls. I don’t know where this data comes from?
Regarding the issue of calculating negative pressure for tank hydrostatic testing, an analysis based on engineering experience is as follows: According to GB 50341 standards, hydrostatic testing is usually carried out under positive pressure, while negative pressure testing is primarily used to verify structural stability. The specified negative pressure values in the drawings (3500 Pa at the tank top/2500 Pa on the tank walls) may be based on the following considerations: 1. **Safety factor method**: Using a design negative pressure of 354 Pa, and taking into account the differences in stiffness across various parts of the tank, a safety factor of 10 is applied to the tank top (354×10≈3500 Pa), while a factor of 7 is used for the tank walls (354×7≈2500 Pa), in order to account for extreme operating conditions. 2. **Separation of structural characteristics**: The tank roof has a thin-shell structure, resulting in weak resistance to negative pressure; therefore, its test values need to be increased separately ； The tank wall is supported by circumferential stress, allowing it to withstand higher negative pressures; therefore, the test values are lower. 3. **Engineering practices**: Some design institutes use API 650 Appendix V or wind load calculation models to simulate the equivalent negative pressure resulting from vacuum extraction, thereby ensuring the tank roof’s resistance to instability. These data reflect rigorous engineering logic, taking into account both standards and practical experience, and are worth learning from. -
Regarding the calculation of the negative pressure for hydrostatic testing, since standard GB50341 does not provide a specific method for calculating this negative pressure in large storage tanks, it is generally determined based on design specifications or engineering practices. In the situation you mentioned, the negative pressure values for the hydrostatic test indicated on the drawings are 3500 Pa (for the tank top) and 2500 Pa (for the tank walls). These values were likely determined by the design team based on experience or relevant design standards, taking into account the structural strength and safety of the storage tank. Therefore, it is recommended to use the negative pressure values for hydrostatic testing specified in the drawings or design documents, namely 3500 Pa (for the tank top) and 2500 Pa (for the tank walls). If further verification of the validity of these values is needed, you can consult the design agency or refer to the detailed specifications in the relevant design standards. .
I have only worked on fixed-roof designs; page 107, I don’t really know much about the structure of floating roofs
When conducting a hydrostatic test on 1000 m³, have the load-bearing capacity of the foundation and equipment been considered?
For storage tanks with fixed roofs, the external pressure test for the tank roof should use Pr-DL, while for the tank wall’s external pressure, Pe should be used, as specified on page P107 of GB50341
The calculation of negative pressure during the water filling test of large storage tanks involves the static pressure of the liquid column, and the specific steps are as follows: 1. Determine the height of the liquid column. The height of the liquid column (hh) refers to the level of the water surface inside the tank, usually measured from the bottom of the tank to the water surface. 2. Calculate the static pressure of the liquid column. The static pressure of a liquid column (PP) is calculated using the following formula: P = ρ · g · h. Here, ρ is the density of water (approximately 1000 kg/m³), g is the acceleration due to gravity (approximately 9.81 m/s²), and h is the height of the liquid column in meters.
3. Convert it to negative pressure. Negative pressure refers to pressure that is lower than atmospheric pressure. The calculation formula is: P_negative = P_atmospheric – PP. Here, P_atmospheric is the atmospheric pressure (approximately 101325 Pa), and PP is the static pressure of the liquid column.
4. Consider other factors. In actual calculations, it is also necessary to take into account the effects of factors such as temperature and altitude on the density of water and atmospheric pressure. Example: Assume the height of the liquid column is 10 meters: P = 1000 ⋅ 9.81 ⋅ 10 = 98100 Pa. P = 1000⋅9.81⋅10 = 98100 Pa. The negative pressure is equal to 101325 − 98100 = 3225 Pa; therefore, the negative pressure is 3225 Pascals. In summary, the calculation of negative pressure is primarily based on the formula for hydrostatic pressure of a liquid column, with adjustments needed to take into account actual conditions.
3. Convert it to negative pressure. Negative pressure refers to pressure that is lower than atmospheric pressure. The calculation formula is: P_negative = P_atmospheric – PP. Here, P_atmospheric is the atmospheric pressure (approximately 101325 Pa), and PP is the static pressure of the liquid column.
4. Consider other factors. In actual calculations, it is also necessary to take into account the effects of factors such as temperature and altitude on the density of water and atmospheric pressure. Example: Assume the height of the liquid column is 10 meters: P = 1000 ⋅ 9.81 ⋅ 10 = 98100 Pa. P = 1000⋅9.81⋅10 = 98100 Pa. The negative pressure is equal to 101325 − 98100 = 3225 Pa; therefore, the negative pressure is 3225 Pascals. In summary, the calculation of negative pressure is primarily based on the formula for hydrostatic pressure of a liquid column, with adjustments needed to take into account actual conditions.