NPSHNPSH

NPSH DISCUSSIONNPSH DISCUSSIONName the four letter word that pump salesmen least like to hear. NPSH Those four letters strike fear in the hearts of pump salesmen more than any others. Let’s take a quick look at why. The Hydraulic Institute defines Net Positive Suction Head as follows: “The Net Positive Suction head is the total suction head in feet of liquid absolute determined at the suction nozzle and the referred datum less the vapor pressure of the liquid in feet absolute”. After reading this definition, it is no wonder why many people become frightened and confused. What this study will attempt to do is break through the technical definitions, and put this NPSH study into layman’s terms, something everyone can understand. Our goal is for you to have a working understanding of how NPSH affects a pumping application.

You Are Not AloneYou Are Not AloneDon’t know much about NPSH? You first need to realize that you are not alone, and there is no reason to be embarrassed. I have met seasoned salesmen, brand new salesmen and engineers alike who have little to no working knowledge of NPSH. And you can continue to get by this way if you wish, but you will be more valuable to your customers, and more successful if you have a complete understanding of the products you sell. You will never know everything there is to know about pumps, but the real embarrassment should be having the opportunity to learn, and not taking it.

Making Sense Out of NPSHMaking Sense Out of NPSHSimply stated, most liquids can take on more than one form. Most commonly, liquid or gas form. Because a centrifugal pump cannot pump gas, we need to insure the liquids we are pumping remain in liquid form. NPSH or Net Positive Suction Head, is the term we use to calculate and insure the liquids we are pumping remain in liquid form. Pressure is what keeps the liquid in liquid form, so begin to think of NSPH as being the way to measure the pressure necessary to keep the liquids from turning to gas. We have all experienced water in three forms in our very homes, and only one form can be pumped with a centrifugal pump. The next three slides show three forms of water found in every home.

Liquid Water

Water Vapor

Ice

Terms For DiscussionTerms For Discussion

We will be covering the following terms in this study:NPSHNPSHa (available)NPSHr (required)Atmospheric PressureAbsolute PressureVapor PressureFriction & Elevation LossesCavitation

NPSHNPSH

As we have stated, NPSH is the abbreviation of Net Positive Suction Head. If you get beyond the term NPSH and begin to think of it somewhat the same as PSI or feet of head, you will take the mystery away from the term. The only aspect of NPSH that is really confusing is the part of the pressure being measured that you will not see on a standard pressure gauge. It is however very real, and it can be measured. NPSH is measured in feet of head. In fact, since we are at sea level in this meeting, the very glass of water in front of you contains about 34’ of NPSH. We will discuss that more in depth in a moment. The important point to remember is NPSH = Pressure in feet of head.

NPSHaNPSHaNPSHa is the abbreviation of Net Positive Suction Head available. Available for what you might ask? In the life of a pump salesman, it means available to keep the pump running properly. Think of it this way. NPSH can be used up by other factors in a pumping application, so we need to determine what is being used up, and what is left over. The amount left over is the NPSHa. Imagine this: You need to make three trips in your car, and you want to find out if you will have enough gas left over for the third trip. You start with 10 gallons of gas in the tank. The first trip will use 2 gallons, and the second trip will use 5 gallons, so you have 3 gallons left for the third trip. The NPSHa is equivalent to the 3 gallons of gas left over for the third trip. If your third trip is 15 miles, you have plenty of gas left for the third trip. However, if your third trip is 200 miles, you will definitely run out of gas. In much the same way, we can determine if we have enough NPSHa to properly operate a pump. Keep this example in mind, because we will refer to it again during this discussion.

NPSHrNPSHrNPSHr is the abbreviation for Net Positive Suction Head required. Required for what? Required to keep the pump operating properly. In our previous example, the NPSHr is the equivalent of the amount of gas required for the third trip. If the third trip was 15 miles, the NPSHr would have been about 1 gallon of gas assuming the car gets about 15 miles per gallon. Just as every car is different, each pump is different as well. Different types of cars get different gas mileage, and different pumps require different amounts of NPSHr. The pump manufacturer has already tested the pumps, and they will show what the pumps requirements are on the pump curve. The following slide shows a pump design point with an NPSHr of about 12 feet.

NPSH Curves

Design Point

12’ NPSH Curve

Atmospheric PressureAtmospheric PressureIn simple terms, atmospheric pressure is the amount of pressure the atmosphere is putting on the earths surface. In our example of the three car trips, the ten gallons of gas in the tank at the beginning is equivalent to the atmopsheric pressure, or the pressure inside a pressure tank. Think of the atmosphere as weighing 1 PSI per every 2000 feet of height. This is actually about right for the weight of the lower atmosphere, but we do not feel this weight because we are accustomed to it. If you flew in an airplane to get here, during your flight, yourears popped. This is because even though the airplane tries to keep the interior cabin pressure constant, high attitudes have alower pressure, and the higher pressure inside your ear tries toescape into the lower pressure surrounding your head in a high altitude airplane. When you land, the opposite is true, and thehigher pressure is pushing against the lower pressure inside your ear drum.

Atmospheric Pressure Atmospheric Pressure -- ContinuedContinued

Imagine blowing up a large balloon. I think everyone understands there is pressure trapped inside. But if you were inside the balloon, do you think you would feel the pressure? Probably not, unless it contained a high pressure. However, if you untie the balloon, the pressure contained inside immediately comes out. Think of the earth’s atmosphere as the inside of the balloon. Pressure is present, we just can’t usually feel it. If every 2000’ of atmosphere weighs about 1 PSI or 2.31 feet of head as we have stated, would you expect the atmospheric pressure in Jacksonville, Florida and Denver, Colorado to be the same? Certainly not! Jacksonville is about at sea level, while Denver is the mile high city. So Denver hasabout 5280’ (one mile) less atmosphere pushing down on the surface of the earth, which is equivalent to about 2.6 PSI or about 6’ of head.

Atmospheric Pressure Atmospheric Pressure -- ContinuedContinued

Now, can anyone guess why the glass of water sitting in front of you has 34’ of NPSH contained inside? And if we were sitting in Denver, Colorado for this meeting, can you guess how much NPSH would be in the glass? (The correct answers are found two slides down) Atmospheric pressure only becomes a factor in NPSH calculations when the system is open to the atmosphere. Examples would be taking suction from a cooling tower, a swimming pool, a pond, or a ground storage tank with a vent. Any totally enclosed system does not involve atmospheric pressure in the NPSH calculations. The following slide shows atmospheric pressures at different altitudes.

Atmospheric Pressure at Sea Level

Atmospheric Pressure About One Mile High

Absolute PressureAbsolute Pressure(The atmospheric pressure in Jacksonville {sea level} is about 34’, which is why the glass has 34’ of NPSH. In Denver, the glass would have about 28’ of NPSH which is equivalent to the atmospheric pressure in Denver)

Absolute pressure refers to the “total” pressure you will find in any system. In a system which is open to atmospheric pressure, absolute pressure is the pressure you would read on a pump suction gauge, plus the atmospheric pressure at that location. The absolute pressure in a pressurized tank is the same as the pressure you would read on a pressure gauge . Absolute pressure is expressed as psia and can be converted into feet of head using the standard formula to convert psi to feet of head.

Vapor PressureVapor PressureVapor pressure is the amount of pressure required to keep any given liquid in liquid form. Most liquids vapor pressures vary from liquid to liquid. Temperature is a key factor that will affect the vapor pressure needed to keep liquids in liquid form. The higher the temperature, the higher the vapor pressure. In our example of the three car trips, vapor pressure is equivalent to the gas used during the first car trip, and it is the first factor that uses up someof the NPSH. So, when doing NPSHa calculations, the vapor pressure is the first thing subtracted from the NPSH. Think of it this way, by subtracting the vapor pressure, we are actually reserving the vapor pressure needed to prevent the liquid from turning into gas. Any time the vapor pressure exceeds the atmospheric pressure, the liquid must be contained in a pressurized tank to prevent it from turning to vapor. The following slide shows the vapor pressure of water heated to various temperatures.

Friction & Elevation LossesFriction & Elevation LossesThe next step in calculating NPSHa, is to determine what friction and elevation losses are present in the suction line. These losses along with the vapor pressure must also be subtracted from the NPSH at the jobsite to determine the NPSHa. In our example of the three car trips, the gas used in the second trip is equivalent to the friction and elevation losses. Friction losses are calculated just as they are in the pump discharge line using charts or programs. Elevation calculations however, often take on a different form from discharge piping elevations. Most often, discharge line elevation calculations are actual losses since the pump is usually below the final destination of the water flow. In the case of suction elevation, elevation is most often calculated as a gain, and not a loss. For instance, a pump taking suction from a cooling tower is always below the surface water level in the cooling tower. If the pump is 6 feet below the water level in the cooling tower, this 6’ of elevation is now added to the NPSH calculations, while friction losses are always going to be subtracted in our formula. Whether a system is open or closed, elevationgains or losses still need to be calculated.

Tying It All TogetherTying It All TogetherLet’s begin by reviewing our 3 car trips.

We are starting out with 10 gallons of gas. This is the beginning atmospheric pressure or pressure contained in the pressure tank.Trip one will require 2 gallons of gas. This is the vapor pressure.Trip two will require 5 gallons of gas. This is the friction and elevation losses. Trip three will require 1 gallon of gas. This is the NPSHr.Subtracting the vapor pressure (2), and friction & elevation losses (5) from the atmospheric pressure (10) will result in the NPSHa. (3) Since the gas left over for the third trip is 3 gallons (NPSHa), and the gas required for the third trip is one gallon (NPSHr), we can safely make the third trip. In our example, trip one and trip two must always take place before trip three.

What You Have Just LearnedWhat You Have Just LearnedNPSHa = ha – hvpa + hst – hfsWhere:ha = atmospheric pressure in feet on the surface of the liquid if the suction is from an open tank or sump, or the absolute pressure existing in a closed tank.

hvpa = vapor pressure of the liquid at the temperature being pumped.

hst = static height in feet that the liquid supply level is above the eye of the impeller. (If the pump is above the water supply, this amount must be subtracted and not added in the formula)

hfs = all suction line losses in feet including friction losses through the pipe, valves, fittings, etc.

CavitationCavitationCavitation is the term we use to describe a pump that is pumping a combination of liquid and gas. Generally speaking, classic cavitationis caused by a pump operating with insufficient NPSHa, and the pump sounds as if small rocks are passing through it. Two other causes of cavitation are entrained air, and vortexes. Entrained air is tiny air bubbles that are still suspended in the water when it enters the pump suction. Entrained air is usually caused by some type of turbulence such as water falling into a sump. If the bubbles do not have enough time to rise to the surface and pop before being drawn into the pump, this is a type of cavitation. We have all seen an example of a vortex while letting the water out of a bath tub. The small tornado looking whirlpool that appears as the water gets low is actually a vortex, and air is being sucked into the drain. This same scenario can happen with a pump, most commonly, a vertical turbine. Minimum submergence values are given for vertical turbines to avoid vortexing. Poor sump design can also lead to vortexing, even when sufficient submergence is available. In our car example, cavitation will occur if the third trip requires more than three gallons of gas.

TemperatureTemperature

Temperature also plays an important role in keeping liquids in liquid form. As the temperature of a liquid rises, so does the vapor pressure. (the pressure required to keep the liquid in liquid form) Liquids with naturally high vapor pressures, and those that have been heated to the point of having a high vapor pressure are usually contained in enclosed pressurized tanks. For instance, water heated to 300 degrees F. must be kept in a pressurized tank with more than 67.005 psia to prevent it from flashing. (turning to vapor) Look at the following chart to understand why.

Margin of SafetyMargin of SafetyUp until this point, we have been explaining how to calculate and understand NPSHa. NPSHr also needs to be understood. Just as we have to calculate Max. BHP at runout on a pump curve, we also have to add in some margin of safety between the NPSHa and the NPSHr. As a pump flows more water, the NPSHr goes up. Unless the design point of the pump is constant and very accurate, the pump will more than likely operate at a number of points along the curve. There are many technical papers written about the proper margin of safety to use in an application, and most differ in opinion. When quoting jobs with borderline NPSHa, it is always a good idea to send a pump curve along with your quotation. This way, the customer or engineer has a chance to raise questions. The Hydraulic Institutes latest guidelines show a recommended safety factor for cooling tower applications at 30% or 5’, whichever is greater. Most other applications show a safety factor of 10% or 2’, whichever is greater. As you can see by the pump curve on the following slide, if the design point A of 360 GPM was used, and the pump actually runs at point B, 500 GPM, the NPSHr goes from 10’ to 15’.

NPSHr Curves

Design Point

Point B

POP QUIZPOP QUIZUsing the following two charts, calculate the NPSHa in the four piping diagrams that follow. Assume that the friction loss in the suction line for all four diagrams is 3’, and that the first and third applications are at sea level, and the second and fourth are at an elevation of 2500 feet. Because the first diagram includes a suction lift, it will be done for you on the slide. Whenever a pump is located above the supply water, it must have sufficient NPSHa to operate, and it must also be primed (the suction line and pump must be filled with water) before it will work. A self priming pump that has the priming chamber filled can complete the priming action. Answers will be found on the following slide for each example.

134.63

Using this formula, NPSHa = ha – hvpa + hst – hfs , we have the following answer. 34 - .78 – 10 – 3 = 20.22 NPSHaWhere 34 is the atmospheric pressure, .78 is the vapor pressure, 10 is the elevation loss, and 3 is the friction loss. You are on your own for the next three.

Elevation: Sea Level

Friction losses in suction piping are 3 feet.

Elevation: 2500 feet

Friction losses in suction piping are 3 feet.

Previous answer Previous answer NPSHa NPSHa = 31’ = 31’ -- .78’ + 10’ .78’ + 10’ -- 3’ = 37.22’3’ = 37.22’

Where 31’ is the atmospheric pressure at 2500’ elevation, .78 isWhere 31’ is the atmospheric pressure at 2500’ elevation, .78 is the the vapor pressure of 68 degree F water, 10’ is the plus elevation, vapor pressure of 68 degree F water, 10’ is the plus elevation, and 3’ and 3’ is the friction losses.is the friction losses.

Elevation: Sea Level

Friction losses in suction piping are 3 feet.

Previous answerPrevious answer NPSHaNPSHa = 34’ = 34’ -- 34’ + 10’ 34’ + 10’ -- 3’ = 7’.03’ = 7’.0

Where 34’ is the atmospheric pressure at sea level, 34’ is the vWhere 34’ is the atmospheric pressure at sea level, 34’ is the vapor apor pressure of water at 212 degrees F. 10’ is the plus elevation, apressure of water at 212 degrees F. 10’ is the plus elevation, and 3’ is nd 3’ is the friction losses.the friction losses.

Elevation: 2500’

Friction losses in suction piping are 3 feet.

Previous answerPrevious answer NPSHaNPSHa = 349’ = 349’ -- 349’ + 10’ 349’ + 10’ -- 3’ = 7’.03’ = 7’.0

Where 349’ is the absolute pressure in the tank, 349’ is the vapWhere 349’ is the absolute pressure in the tank, 349’ is the vapor or pressure of 350 degree F. water, 10 is the plus elevation, and 3pressure of 350 degree F. water, 10 is the plus elevation, and 3’ is the ’ is the friction losses.friction losses.

For those who got the correct answer of 7’ but used the wrong first two numbers (311’), you forgot to correct for a S.G. of .8904 @ 350 degrees F. Since you made the same mistake for the vapor pressure and the tank pressure, your answer was still correct. If you tried to use atmospheric pressure at 2500’ elevation in your equation, this is incorrect. This is a closed system and is not subject to atmospheric pressure.

Congratulate YourselfCongratulate YourselfIf you didn’t understand NPSH when you began this study today, you should be ending with a basic understanding of Net Positive Suction Head. This is not something you will use everyday unless you work consistently with boiler feed water pumps, or in industrial applications pumping volatile liquids. So to retain this knowledge, you should refer back to this study from time to time and study both the Cameron Hydraulic Data book, and the Hydraulic Institute Standards book as well. The more you know about the products you sell, the more valuable you will become to your customers, and you will also be more successful in this business.