How should the saturated vapor pressure be determined for the pumps located behind the flash tank and large storage tanks in the calculations?
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This post was last edited by longkui1990 on 2015-6-18 at 17:04. For example, if I feed acidic water with parameters of 50ºC and 0.5 MPaG (2% H2S, 2% NH3, 96% H2O, along with small amounts of light hydrocarbons and light naphtha, all expressed in moles) into a flash tank operating at a pressure of 0.15 MPaG, then the small amount of H2S and vaporized oil and gas that emerge as a result of flashing go into the flare system. The liquid phase is then pumped via pump P1 to an acidic water storage tank located 1 km away, where the operating pressure is 2 kPaG. After staying in this storage tank, some more H2S and oil and gas vaporize; the liquid is then pumped again via pump P2 to the acidic water stripping tower. When calculating the NPSH and installation height for pump P1, should the saturated vapor pressure of the liquid phase be taken as 0.15 MPaG, that is, 250 kPaA? When calculating the NPSH and installation height for the P2 pump, should the saturated vapor pressure of the liquid phase be taken as 0.2 kPaG, that is, 103 kPaA? O(∩_∩)O Thank you. If calculated in this way, it’s easy to reach a negative NPSH value. At present, the only ways to increase the NPSH to 2–3 meters are by raising the height of the supports of the flash tank and increasing the liquid level at which the pump stops due to the low-level interlock in the acidic water storage tank. Is this appropriate? Is there any other way?
In my personal understanding, the saturated vapor pressure of the liquid phase is 0.2 MPaG, as there is a gas-liquid equilibrium process taking place inside the flash tank. Just like the bottom of a distillation column
The same is true for large storage tanks: the components remain unchanged, and the saturated vapor pressure also stays the same. This situation can only lead to higher costs for the equipment
Maybe it wasn’t made clear before; I’ve just added that the composition will change slightly after pressure reduction, but the change isn’t significant. It’s easier to raise a flash tank, but raising a large storage tank with a capacity of around a thousand cubic meters is quite challenging
This post was last edited by arpcd on 2015-6-17 21:28. How do you calculate NPSH? ? ? Write down the process. Note that when performing the calculations, the suction inlet of your pump is below the liquid level. Logically, there’s no need for the levels in these two tanks to be too high. Handling acidic water is something that happens frequently; for that flash tank, it’s sufficient if the liquid level is about 1 meter higher than the pump’s suction inlet. I’m not sure how you did your calculations – why such hesitation? Besides, there are also many options for pumps with a low NPSH available these days.
I haven’t calculated pumps by hand in a long time; I just use the company’s spreadsheets for those calculations. . . For example, in the case where fluid is drawn from a flash tank at 0.15 MPaG and sent to a larger tank, the saturated vapor pressure of the fluid being transported is also 250 kPaA. This requires a relatively high liquid level in the flash tank in order to prevent cavitation inside the impeller
Adding some nitrogen can also meet the requirements; don’t be so fixated. . When it comes to lifting large tanks weighing several thousand cubic meters, you need to think of different ways to solve the problem. The amount of nitrogen required is not large.
Both the flash tank and the large tank are equipped with nitrogen seals. It doesn’t really matter if the skirt of the flash tank is raised a bit, so we can ignore that. The large tank is a tank at atmospheric pressure, so it’s unlikely that nitrogen can be used to increase its pressure