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In psychology, especially when it comes to matters of introspection, there is a saying: \"What you resist will persist.\" ” I’ve heard it before, and I’ll hear it again. It is said that in the world of pumps, the most misunderstood concept is Net Positive Suction Head (NPSH). Many pump technicians, engineers, and so-called experts have written many such articles. The acronym NPSH is itself confusing to most beginners in pumps. The topics it deals with and the calculations required confuse many newcomers to the industry, outsiders (operators or managers), as well as professionals; even after working in this field for 25 years, some still mistakenly believe they fully understand what NPSH means. It is recommended that everyone pay attention to this issue, as errors in calculating the effective net positive suction head (NPSHA) occur very frequently, and correcting them is costly. The definitions of NPSH, NPSHa, and NPSHr: Net positive suction head is the total suction head in feet (or meters), minus the vapor pressure of the liquid being pumped, also expressed in feet or meters. Think of the indenter as an energy level, rather than a force like pressure. All values are absolute values. NPSHa is measured at the pump centerline or impeller bore. The two can be measured at different locations or heights. Consider NPSHa as the available energy level of the liquid at the pump inlet or at the impeller opening. If there is not enough NPSHa, the liquid will flash into vapor. Do not confuse NPSHa with suction pressure. Although inhalation pressure is to some extent a component of the mixture, it is more complex. NPSHa is the NPSH level of the system at the pump impeller opening. This NPSHa value is entirely a function of the liquid and its properties, environmental conditions, as well as the design and geometry of the suction system. Essentially, the calculation is related to the inhalation system itself and should be carried out by the system owner, end user, and/or their engineers or consultants, and is unrelated to the pump. For reasons of responsibility, manufacturers usually do not participate in the customer’s calculations. But over time, manufacturers will become more involved to provide maintenance support. The required net positive suction head (NPSHr) is usually determined by pump manufacturers using empirical methods, as well as the standards and specifications of the Hydraulic Institute (HI). The NPSHr value is usually indicated on the pump’s performance curve. Note that NPSHr and NPSH3 are essentially the same. At the given head and flow rate operating point, the pump has experienced slight cavitation due to insufficient NPSH; as a result, the head decreased by 3%, while the flow rate remained constant. The NPSH margin refers to the level at which the NPSHa value exceeds NPSHr. Some suggestions or guidelines regarding appropriate margins suggest that the higher the margin, the better. For more information, please refer to the section of ANSI/HI 9.6.1-2012 related to this topic. NPSHa: Formula or number? Formulas can be regarded as our “friends,” because once we know the correct formula, we can simply fill in the appropriate values and conditions as required by the formula, carry out the mathematical steps, and arrive at the correct answer. If you don’t like formulas, you might want to examine the diagram below to see what happens before using the formulas. In the figure above, there is a tank of clean water at ambient temperature (68 degrees Fahrenheit, specific gravity of 1.0), under atmospheric pressure. The height of the tank and pump system here is close to sea level. The water level in the tank is 10 feet above the centerline of the pump; this is referred to as \"priming suction,\" as the liquid comes from above the pump impeller. There is a pipe of appropriate size running from the tank to the pump’s suction inlet, equipped with an elbow and a fully open isolation valve. Assume that the water level remains constant at 10 feet, but in reality it is necessary to calculate NPSHA for worse-case scenarios, as the water level is likely to be lower. At this point, based on the information provided in the figure, all the data required to calculate NPSHa can be obtained except for the friction head. In the first example, the friction head will be calculated to simplify the problem. Ignore the velocity head, as its value is usually very small. To calculate the NPSHa value, it is necessary to know: • the absolute pressure above the liquid surface or at the liquid surface • the static head, which is the vertical distance from the top of the liquid surface to the pump impeller opening (or, if they are at the same level, to the centerline of the pump). •The vapor pressure of a pumped liquid can be easily calculated based on temperature, and it is relatively simple to determine this value. •The friction ram has been calculated; in this case, it is recommended to set it at 3.2 feet. Calculate the NPSHa1 for perfusion inhalation. For readers using the metric system, these values can also be expressed in meters, but the units must be consistent. 2. Vapor pressure and friction are always negative, hence they are always unfavorable. 3. Absolute pressure can be zero, but by definition it cannot be negative. 4. If the liquid source is located below the pump rather than above it, in a \"lift\" situation, when the liquid level is below the pump’s suction inlet (as opposed to a \"filling\" situation), the static height is negative (this condition is referred to as \"lift\"); this value in the equation works against you. 5. There is no need to concern oneself with the discharge side of the pump or system in order to calculate NPSHa. 6. In summary, most of the components in the NPSHa formula work against you. Now let’s understand why the inhalation source needs to be elevated, submerged, exposed to the atmosphere, and/or pressurized with a liquid at lower temperatures. Calculation formula: NPSHa = absolute pressure – absolute steam pressure + hydrostatic head – friction head; or NPSHa = A – V + S – F, or NPSHa = ha – hvpa + hst – hf. NPSHa = 34 – 0.783 + 10 – 3.2 = 40.017 feet, which rounds to 40 feet. In the formula, absolute pressure = ha, absolute vapor pressure = hvpa, hydrostatic head = hst, and friction head = hf, all in feet. As mentioned earlier, we have the information for the four components in the actual data filling formula to carry out the calculations and determine NPSHa. The first variable in the equation (ha) represents the absolute pressure value above the open tank. As mentioned above, the system is at sea level. The liquid in an open tank is affected by atmospheric pressure. At sea level, it can be assumed that the atmospheric pressure is approximately 14.7 pounds per square inch of absolute pressure (psia), or 0 pounds per square inch (psig). Note that changes in atmospheric pressure affect the NPSHa value. Now, simply convert the atmospheric pressure from psia to feet of head. Multiplying 14.7 by 2.31 gives 33.957 feet, which rounds to 34 feet. The value of the first variable in the equation is 34 feet. The second variable in the equation is hvpa, which is the vapor pressure of the liquid at a given temperature of 68 F. To obtain the steam pressure values, simply look them up in the reference book “Cameron Hydraulic Data Book”. This value (absolute pressure of saturated steam) is usually expressed in psia, varies directly with temperature, and varies depending on the type of liquid. The value found should be 0.33889 psia. Multiply this number by 2.31 to convert the indenter to feet. The resulting value is 0.7828 feet. Rounded to 0.783 feet. The second value in the equation now is: 0.783 feet. The third variable in the equation is the static head (hst). This is the vertical distance from the liquid surface to the pump’s centerline (impeller hole). Remember to measure the worst-case scenario (the lowest expected level). The static height indicated in this example is 10 feet, and no conversion is needed as the correct unit has already been used. Now, the value of the third variable in the equation has been calculated to be 10 feet. The fourth variable in the equation is (hf), which represents the friction loss in the pipe; the value provided earlier was 3.2 inches. There are now four calculated answers. Please note that the given friction coefficient of 3.2 feet is a function of the liquid properties, flow velocity, pipe (suction system) material, and geometry. In simple terms, for a given liquid flow rate, friction losses occur due to the pipe length, bends, valves, outlet losses at the water tank (a transition zone from large to small diameters), and inlet losses of the pump (due to the change in diameter from the pipe to the pump nozzle). Finally, it should be remembered that the fifth variable in the formula, namely the velocity head (hvel), was not discussed. In properly designed systems (Newtonian fluids under non-slurry conditions), the value of the velocity head is usually less than 1 foot. The value of the velocity head is positive. Now there are four required values to fill into the formula in order to calculate the NPSHA value. All units are in feet. The pressure is an absolute value. As a high-quality manufacturer and seller of chemical pumps, our product range includes: stainless steel magnetic pumps (insulated magnetic chain pumps, high-temperature magnetic pumps, magnetic pumps that can operate without load), fluorinated lining magnetic pumps, self-priming magnetic pumps, vortex magnetic pumps, pipeline magnetic pumps, chemical shielded pumps, pneumatic diaphragm pumps, electric diaphragm pumps, and chemical centrifugal pumps – all of which are corrosion-resistant, high-performing, and leak-free (RMD/RMI) pumps