Pump NPSHa calculation

1. The saturated vapor pressure of the liquefied hydrocarbon should be the same as the operating pressure at the liquid level. Therefore, to prevent gasification, it can only be controlled by maintaining the liquid level. It is sufficient for (NPSHa-0.6) to be greater than NPSHr. 2. The higher the NPSH, the easier it is to select a pump. The wider the selection range. The original poster has the wrong concept; it is necessary to select an appropriate pump based on the calculation of NPSHa

NPSHA = P + H – Pv – Hs – H’. P: Pressure in the storage tank; H: Net liquid column pressure from the liquid surface to the center of the pump; Pv: Saturated vapor pressure of the liquid; Hs: Pressure loss in the suction pipeline; H’: Margin, typically taken as 1 meter of net liquid column pressure (it can also be 0.6 meters, depending on the company’s regulations). NPSHA >= NPSHR. If it is a saturated liquid, then P = Pv, and NPSHA = H – Hs – H’, meaning that the net liquid column pressure must be greater than or equal to the pressure loss in the suction pipeline minus the margin. Liquefied hydrocarbons should be stored under pressure; whether they are saturated depends on the pressure and temperature at which they are stored. If the control pressure is higher than the saturated vapor pressure corresponding to that storage temperature, that is, P > Pv, then the pressure can provide some margin against cavitation (liquefied hydrocarbons are usually pressurized using nitrogen). Based on the data you provided, the pressure in the storage tank is not mentioned; if it is a saturated liquid, it’s obvious that there must be a problem without having to do any calculations. I think you might not have considered storage pressure. Also, pay attention to whether the NPSHR provided by the supplier refers to a liquid column or a water column.

Answering the original poster’s question step by step: 1) The pressure at the liquid level of the liquefied hydrocarbon tank in this calculation certainly is not equal to the saturated vapor pressure of the liquefied hydrocarbon. The conditions provided by the original poster do not include the pressure of the liquefied hydrocarbon tank; here it is assumed to be P (barg). Regarding the first question, in addition to calculating NPSHa, there is one more factor that cannot be ignored: NPSHr needs to be corrected. NPSHa = (P – Pv) / (ρg – ΔH – Hf). Here, Pv is taken as its maximum value under operating conditions, namely 4 bar; this results in the lowest possible value for NPSHa ； △H is the height to which the material is lifted, which is -0.8m in this case. After taking into account the unit conversions, the above formula becomes NPSHa = (P-4)*10e5/(0.578*9.8*10e3)+0.8-0.5; by substituting the operating pressure of the storage tank into this formula, you can obtain NPSHa. Since the medium you are pumping is not clean water at 20°C, NPSHr also needs to be corrected. The correction formula is NPSHr = φ•NPSHrw, where NPSHrw is the NPSHr value provided by the pump manufacturer, which is 3 m in this case. φ —— correction factor; you can check the accompanying materials provided by the pump supplier, where information on how to determine this factor should be available. (I took a quick look; under the conditions described by the original poster, φ is roughly 1.1, for reference only.) Then compare these two values: NPSHa should be more than NPSHr by at least 0.5 m; of course, the greater the difference, the better. 2) If NPSHa is much larger than NPSHr, does such a pump exist? The answer is of course yes. NPSHa is actually not related to the pump; it is determined by the material being transported and the characteristics of the piping. Only NPSHr is related to the pump; it is determined by the pump’s type and structure, provided by the supplier, and has nothing to do with the piping. So, if you want NPSHa to be much larger than NPSHr, then just increase NPSHa significantly. For example, you can place the storage tank at a very high location and the pump on the ground, which results in NPSHa being much larger than NPSHr.

There should be an operating pressure above the storage tank, at least not lower than its saturated vapor pressure

First, it is necessary to determine the distance between the base elevation of the tank and the centerline of the pump; adding the liquid level height in the tank, this distance should be more than just 600 mm. Secondly, whether the system is equipped with compressors and vaporizers, as well as mechanisms to increase the pressure in the liquefied hydrocarbon storage tanks; if so, the operating pressure of these tanks can be higher than the saturated vapor pressure, thereby avoiding cavitation issues. The specific methods for calculating cavitation are described in many books; I believe your calculations should be correct. 1# Lake Orchid

P: Tank pressure; H: Net liquid column pressure from the liquid level to the center of the pump; Pv: Saturated vapor pressure of the liquid; Hs: Pressure loss in the suction pipeline; H’: Margin, typically taken as 1 meter of net liquid column (it can also be set at 0.6 meters for the second floor; regulations vary among different companies). NPSHA >= NPSHR. If it is a saturated liquid, then P = Pv, and NPSHA = H – Hs – H’, meaning that the net liquid column pressure should be greater than or equal to the pressure loss in the suction pipeline minus the margin. Liquefied hydrocarbons should be stored under pressure; whether they are saturated depends on the pressure and temperature at which they are stored. If the control pressure is higher than the saturated vapor pressure corresponding to that storage temperature, that is, P > Pv, then the pressure can provide some margin against cavitation (liquefied hydrocarbons are usually pressurized using nitrogen). NPSHA=P+H-Pv-Hs-H'

The vertical distance from the liquid level in the tank to the pump inlet is 800 mm – this is a very low height… Is it necessary to use a liquid bag in the pipeline? The operating pressure inside the tank does not necessarily equal the saturated vapor pressure of the liquid hydrocarbon; this value needs to be determined first before any calculations can be made

The calculation shows it’s over 7 meters, so of course it’s sufficient.

The saturated vapor pressure of the liquefied hydrocarbon is 0.2–0.4 MPa. With a vertical distance of 800 mm between the liquid level in the tank and the pump’s suction inlet, and a relative density of 0.578 for the liquefied hydrocarbon, the hydrostatic pressure is equal to 0.8 * 0.578 = 0.046 MPa. As a result, the liquefied hydrocarbon vaporizes at the pump’s inlet. The original poster is right.