How To Calculate Pressure Head For Pump – I received an email the other day from two consulting engineers in North Carolina. The engineers, let’s call them R and G, wanted me to resolve their debate between “head” and “pressure” when interpreting pump performance curves.
R was convinced that the pump performance curves represented head and flow. G believed that all pump curves should represent pressure difference (psid) and flow. He argued that new pumps are bench tested using differential pressure, which is then converted to feet of water for the published pump curve. G thought “head to foot” was a misleading conversion since different liquids have different densities. He believed that the curve should show the pressure difference of the actual liquid with its unique specific gravity. Specific gravity is the density or weight of a liquid relative to an equal volume of water.
How To Calculate Pressure Head For Pump
This debate highlights the differences in the training and experience of the two engineers. Parts of G’s reasoning are correct.
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For simplicity and practical purposes, most pump companies design the performance curve for a new pump by installing the pump in a pipe loop and including a tank of ambient water. The test engineer records the pressure difference across the pump at flows controlled by different valves. Later, the pressure differences are converted to feet (or meters) on the published performance curve.
What is neither easy nor practical is to stretch a pipe 800 feet. up into the sky just to justify a pump that develops a head of 800 feet. Head can easily be converted to pressure difference and pressure difference can be easily converted to head. It is easy to measure the pressure difference in a loop at the ground surface and then convert it to head.
The pumps are also tested on ambient water. A test stand can hold 2,000 or 5,000 liters of water. The pump company does not dump the water or fill the tank with soy sauce, vodka, sulfuric acid or coffee grounds just to check the pressure difference in the pump with a certain liquid. If the customer specifies the fluid’s specific gravity, viscosity, temperature and pump head, the pump company will issue a certified curve that matches the current fluid’s properties.
There are applications in industry where the pump is defined by pressure. For example, if the pressure in a steam boiler is 20 psi, the boiler feedwater pump discharge pressure must be 20 psi or slightly higher to get more water into the boiler.
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There are several industrial applications where the pump is designed to lift liquid. For example, suppose an industrial tank rises 40 meters above the pump and you want to use the pump to fill the tank. The pump must reach a height of 40 feet to fill the tank.
At this point, G’s reasoning goes awry and R’s opinion is correct. The pump is designed to lift 40 feet and lifts cold water 40 feet. The same pump raises the gasoline 40 feet. The same pump raises the sulfuric acid 40 feet. We don’t care about the liquid’s name or specific gravity. What we need is 40 feet. These applications abound in the chemical processing industry.
Cold water, petrol and sulfuric acid have unique specific gravity. I don’t need the specific gravity if the pump is measured in feet (or meters). I need the specific gravity of the liquid if I want to convert the foot of the head to pressure.
By definition, “head” is a measure of energy. The unit of energy is the foot (or meter). “Pressure” is the force exerted per unit area, such as one pound of force per square inch area (psi). The density of the liquid determines the force.
What Is Npsh (net Positive Suction Head) And Why Does It Matter?
The classification of pumps by “head height” is standard in the pump industry for several reasons. One reason for this is that there are many applications where the pump needs to lift liquid. Another reason is that the density of the liquid is not part of the “head”. Density is a component of pressure.
There is another reason rooted in history. About 2,400 years ago, the Greek philosopher Aristotle proposed that gravitational attraction is a function of an object’s mass. Simply put, Aristotle said a £10. The rock falls to the ground twice as fast as a 5-lb. cut. A 10 lb. ball falls to the ground ten times as fast as a 1 lb. ball. ball. It seemed logical at the time. There were no helicopters, sky cranes or tall buildings back then. There was no practical way to prove or disprove Aristotle’s theory. Aristotle’s theory was not disputed for 2000 years.
So in 1589, Italian engineer/astronomer/physicist Galileo carried two objects of different weights to the top of the Leaning Tower of Pisa, Italy. The Leaning Tower is nearly 200 feet tall. Galileo released the objects together. The two objects collided with Earth and hit the ground at the same moment. Galileo proclaimed that the attraction of gravity is constant, independent of the object’s mass or weight.
Galileo continued his experiments and found that the acceleration of gravity was also constant. All objects in free fall accelerate towards the Earth with a speed of 9.8 m/sec2. These values often appear in formulas when engineering students study fluid mechanics at university.
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Therefore, if the acceleration due to gravity is constant, then the acceleration of an object against gravity is also constant. Launching a rocket into space corresponds to Galileo’s theory. And lifting a gallon of water, or a gallon of house paint, or a gallon of orange juice is also constant.
What does this mean for liquid lift pumps? This means that the pump performance curve, in feet or meters, applies to all fluids. When I use the term “feet” (or meter) when talking about pumps, the density of the fluid is irrelevant. If I am discussing pumps that use pressure (psi), the specific gravity of the liquid is a component of the pressure.
I know what you mean. Even if gravity is constant, there MUST be an observable difference between dropping two objects of different weights from a great height or lifting two different liquids in a pipe. Yes, there is a difference. Let’s go back to Galileo and the Leaning Tower of Pisa.
Equally important, but rarely mentioned in history books, is the fact that it took proportionally more effort to get a heavier object to the top of the Leaning Tower than a lighter object. And the heavier object left a proportionally larger/deeper crater in the ground at the point of impact.
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This means multiplying the horsepower or kilowatts by the specific gravity of the liquid. On most standard pump curves, the rated power (BHp or kW) is for water with a specific gravity of 1.0.
Galileo made great strides in understanding planetary gravity, the solar system, and modern engineering. But when asked in 1589, Galileo could not explain, “If gravity is constant, why does it take so long for a feather or a leaf falling from a tree to reach the earth?”
In 1589 he had no concept of atmospheric pressure, air pressure, air currents or air friction. Galileo could not explain that the air prevented the free fall of the feather to Earth by gravity. These concepts were explored and explained a century later by Isaac Newton, Blaise Pascal and Daniel Bernoulli.
In 1971, the Apollo 15 astronauts finally answered the question posed to Galileo back in 1589. Go to “You Tube” and type in “Apollo 15 Hammer and Feather Experiment”. You will see the video of what happens when a pen and a hammer are dropped together in the absence of air on the moon.
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I hope this answers the “head” and “pressure” question. If you have any questions about the pumps, ask them in the comments!
Many call Larry “The Pump Guy”. Larry writes the “Pump Guy” column in Flow Control Magazine, World Pumps Magazine and Mechanical Technology Magazine to approximately 40,000 monthly newsletter subscribers. Some of Pump Guy’s articles are available on our articles page. Larry writes like he talks. If you like the articles, you will love the books “Knowing and Understanding Centrifugal Pumps” and “Centrifugal Pumps and Everything You Need to Know About Pumps” written in both English and Spanish (“Bombas Centrífugas, y Todo lo que Necesita Saber Sobre “Ellas “). Larry continues to invent products when he is not lecturing or consulting. As a member of ASME (American Society of Mechanical Engineers), most of Larry’s products can be used in construction and mechanical engineering.
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How To Calculate Npsh Of Pump With Examples And Illustrations
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