Reference to the bubble point temperature of the fluid at the pump inlet
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Dear colleagues from Sichuan, when calculating the net positive suction head at the pump inlet, it is necessary to consider the saturated temperature of the fluid. In systems equipped with nitrogen sealing, the fluid is essentially in a state of nitrogen saturation; the temperature at this point (usually 40°C) is theoretically also the bubble point temperature. The solubility of nitrogen in such fluids is around 1–3‰. When determining this bubble point temperature, do you take into account the influence of nitrogen? Or consider part of it? If everything is taken into account, then the bubble point temperature is basically around 40°C or slightly higher; if nothing is taken into account, then the bubble point is essentially equal to the boiling point of that substance (in the case of a pure substance), and there can be a significant difference between these temperatures
When calculating the net positive suction head at the pump inlet, the saturated vapor pressure of the fluid is generally considered to determine the bubble point temperature. If nitrogen saturation is present in the system, it is indeed necessary to consider the effect of nitrogen on the saturated vapor pressure of the fluid. Although the solubility of nitrogen is low, it can reduce the local saturated vapor pressure of the liquid, thereby increasing the bubble point temperature. In practical engineering calculations, it is often not necessary to determine the effect of nitrogen with high precision; instead, a safety factor is used to ensure that cavitation does not occur at the pump inlet. If more accurate results are required, experimental data or more complex thermodynamic models can be used to calculate the bubble point temperature in the presence of nitrogen. In simple terms, when taking the effect of nitrogen into account, the bubble point temperature is usually higher than the boiling point of the pure substance; it is not equal to 40°C, but rather some value between the two. .
I am from the process engineering field; dissolved nitrogen is not taken into account when considering pumping conditions. Nor is it included as a component of nitrogen when using Aspen to simulate flow streams and determine their physical properties. The NPSH margin can be increased by 1 m accordingly.