Allegedly, the most misunderstood concept in the pump world is net positive suction head NPSH. I have written several articles on the subject and so have many other pump technicians or engineers and so-called experts. The NPSH name itself, an acronym, confuses most pump neophytes. The subject and required calculations confounds people who are new to the industry, those on the periphery operators or administrators and professionals who incorrectly believe they fully understand the subject even after 25 years in the business. I suggest we need to be concerned about this issue, because mistakes with respect to NPSH available NPSHa calculations are all too frequent and expensive to correct. One of the fun parts of my position is teaching at several pump schools per year and devoting a major portion of the course to the subject of understanding the concept of NPSH and how to complete the calculations.
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Allegedly, the most misunderstood concept in the pump world is net positive suction head NPSH. I have written several articles on the subject and so have many other pump technicians or engineers and so-called experts.
The NPSH name itself, an acronym, confuses most pump neophytes. The subject and required calculations confounds people who are new to the industry, those on the periphery operators or administrators and professionals who incorrectly believe they fully understand the subject even after 25 years in the business.
I suggest we need to be concerned about this issue, because mistakes with respect to NPSH available NPSHa calculations are all too frequent and expensive to correct. One of the fun parts of my position is teaching at several pump schools per year and devoting a major portion of the course to the subject of understanding the concept of NPSH and how to complete the calculations.
In the teaching process, I cover the five main examples that you will likely encounter in normal industry applications. I will explain these five examples first in this column and over the next four months with the basic optimistic intent that once you learn these five examples and a few variations of each you will be able to handle the applications encountered in the real world.
Between now and Thanksgiving, you may wish to read and file all five columns as a handy reference. The net positive suction head is the total suction head in feet of liquid or meters , less the vapor pressure in feet or meters of the liquid being pumped.
NPSHa is measured at the pump centerline or the impeller eye. These two things can be at different places or elevations. The liquid will flash to vapor if there is not enough NPSHa.
Do not confuse NPSHa with suction pressure. While suction pressure is in some ways a component in the mix, there is something more complex to the story. This NPSHa value is entirely a function of the liquid, its properties, ambient conditions and the suction system design and geometry.
Essentially, the calculation is about the suction system itself and has nothing to do with the pump. NPSHr values are normally reported on the performance curves for the pump. At the given operating point of head and flow, the pump is already slightly cavitating due to insufficient NPSH and the developed head has dropped by 3 percent while the flow rate is fixed at some value. There are guidelines for recommended or proper margins, and I say the higher the margin, the better.
In Image 1, we have a tank of clear water at ambient temperature 68 F, which also means the specific gravity will be 1.
Here, the tank and pump system are at an elevation near sea level. The top surface of the water level in the tank is 10 feet above the pump center line. There is adequately sized piping from the tank to the pump suction with an elbow and a fully open isolation valve. We will assume that the water level remains constant at 10 feet for the example, but in the real world you will want to calculate the NPSHa for the worse condition, which will likely be at a lower level.
At this point, with just the information I have given you from the figure, you have all the data you need to calculate the NPSHa, except the friction head. For this first example I will calculate the friction head to keep the problem simple. We will ignore velocity head since the value is normally small.
In future examples, we will discuss the methods of calculation for the friction head, and I will also calculate and show how the velocity head can affect the outcome or not.
As mentioned before, you have the information to fill these four components in with real numbers and complete the calculation to determine NPSHa. It was given earlier that the system is at sea level. The liquid in the open tank is subject to atmospheric pressure. At sea level, the atmospheric pressure can be assumed to be near Now you just need to convert the atmospheric pressure from psia to feet of head. Multiply The value for the first component in the equation is 34 feet.
The value you should obtain from your search is 0. Multiply that number by 2. You will obtain the value of 0. We will round off to 0.
You now have the second value in the equation: 0. This is a vertical measurement from the surface of the liquid to the center line of the pump impeller eye. Remember to figure for the worst case lowest level expected. In our example, it was stated the static height is 10 feet. No need to convert as it is already in the correct units. You now have the value for the third component in the equation: 10 feet.
You now have all four values for figuring the answer. Note that the given friction factor of 3. You should understand in simple terms that for a given liquid flow rate, there will be friction losses for the pipe length, the elbow, the valve, the exit loss from the tank large to small transition and the entrance loss into the pump change in diameters from the pipe to the pump nozzle.
We will address some pipe friction fundamentals in the later examples in the coming months. Lastly, remember we are not addressing the fifth factor in the formula, which is velocity head h vel. In a properly designed system with Newtonian liquids in a nonslurry application , the value of velocity head will typically be under 1 foot in value.
For now, we have the four values required to fill in for the components and calculate the answer for NPSHa. Next month, we will calculate NPSHa for a lift condition. Read it here. In later editions, we will look at the effects of high elevations, hot liquids, hydrocarbons and pressurized suction tanks.
We will also address suction conditions under vacuum. Jim Elsey is a mechanical engineer who has focused on rotating equipment design and applications for the military and several large original equipment manufacturers for 47 years in most industrial markets around the world. He is the general manager for Summit Pump Inc. Elsey may be reached at jim summitpump. View our digital edition here.
Subscribe Now and Get: In-depth articles on pump industry issues Expert insights into important topics in the field Keep up with trends, basics, and more. Breadcrumbs You are here: Home. The first of a five-part NPSH primer series. Image 1. Flooded open system with ambient temperature water Image courtesy of the author. Issue July Related Articles Pumps.
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In a hydraulic circuit, net positive suction head NPSH may refer to one of two quantities in the analysis of cavitation :. NPSH is particularly relevant inside centrifugal pumps and turbines , which are parts of a hydraulic system that are most vulnerable to cavitation. If cavitation occurs, the drag coefficient of the impeller vanes will increase drastically—possibly stopping flow altogether—and prolonged exposure will damage the impeller. In a pump, cavitation will first occur at the inlet of the impeller. Note that it is equivalent to the sum of both the static and dynamic heads — that is, the stagnation head — from which one deducts the head corresponding to the equilibrium vapor pressure, hence "net positive suction head". Applying the first law of thermodynamics for control volumes enclosing the suction free surface 0 and the pump inlet i , under the assumption that the kinetic energy at 0 is negligible, that the fluid is inviscid , and that the fluid density is constant:. Using the above application of Bernoulli to eliminate the velocity term and local pressure terms in the definition of NPSH A :.
Net positive suction head
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