Introduction to the Basic Hydraulic Principles for Pumps

This is going to be a little bit more difficult for some and likely incredibly boring for a lot of you, but necessary if you want to really understand water flow as it relates to pumps and irrigation.  So hang in there!  These are somewhat difficult to understand hydraulic principles and we’ll simplify them to the extent possible.  Tip:  If you don’t fully understand a paragraph after reading it a couple of times, continue on with the next paragraph.  The concepts here are all very interrelated, making it hard to decide which to present to you first.  It’s the old “which comes first, the chicken or egg?” routine.  Often an explanation of something else later in the article will clarify a concept presented earlier.

Measuring Water Pressure:

Feet Head:

In the USA the pressure output of pumps is sometimes measured as “feet head” and abbreviated as ft.hd..  (Now and then you may see the term “feet of head” used.) If you need metric measurements you’ll want to make reference to the Conversion Formulas where you’ll find the necessary information for converting to your favorite measurement system!



Feet of head is really pretty easy, it is simply height or elevation.  Feet as in the measurement.  You know, one foot = 12 inches!  So no special definition, feet is simply feet.  Easy enough so far, right?

Head: if you pull out your dictionary you will find 70 or more definitions for the word “head.”  Look way down that list and you will find “In Physics: the vertical distance between two points in a fluid.”  That’s the definition we are using here.  So feet head is the number of vertical feet between two points.  Remember it isn’t the total distance between the points, it is the vertical distance.

Feet Head is the Vertical Distance Between Two Points

As everyone knows, water is pretty heavy. (Try carrying a 5 gallon jug of water up a flight or two of stairs!) “Water Pressure” is the weight of the water above the point where the pressure is measured.  Think of a tall column of water. The”water pressure” at the bottom of that column is simply the total weight of all the water in the column above the point where you are measuring it. In fact, at any point in the column the water pressure is equal to the weight of the water above that point. So as you move up toward the top of the column the water pressure decreases. Inversely, just like in the ocean or a swimming pool, the deeper you go, the greater the water pressure! That greater pressure is what makes your ears hurt if you dive down to the bottom of a deep swimming pool!  Still following?

So water has weight, but the water pressure at any point is only equal to the weight of the water directly above that point.  We can state this in an interesting way.  If we have a barrel of water that is 3 feet deep, then the weight of the water at the bottom of the barrel is equal to 3 feet.  We use the term “Head” to show we are talking about vertical distance.  So at the bottom of that 3 foot deep barrel of water the water pressure is 3 feet head.  Get it?  What if the barrel was 2 feet deep?  Then at the bottom it is 2 feet head.  Feet head is simply the depth in feet of the water above the point at which we are measuring the pressure.

OK, so let’s say we have a tank that is 10 feet deep.  If you are following along, you know that the water pressure at the bottom of the tank will be 10 feet head.  However, what would the water pressure be at 5 feet below the surface?  Did you answer 5 feet head?  If so you are right!  Since there is 5 feet of water above the point you are measuring the pressure at, then there will be 5 feet head at that point.  If you measure the pressure 2 feet from the top there will be 2 feet head.  8 feet below the water surface there will be 8 feet head.  Get it?  It doesn’t matter how deep the tank is, the pressure is only measured based on the water depth at the point you are measuring the pressure.

One last example; let’s go scuba diving with a friend.  Your friend is 80 feet below the surface.  What is the pressure there?  80 feet head.   How about you at 120 feet below the surface?  120 feet head!  You’re both wearing depth gauges to tell you how deep you are.  Want to guess how that depth gauge works?  Yep, it measures the water pressure in feet head to determine how deep you are.   The gauge face just says “feet depth” instead of “feet head.”  They are the same!

(Technical point:  air is a liquid and also has weight, so altitude does have an impact on the water pressure readings made with a gauge.  Why?  Because the weight of the air above the water is also pressing down on the water surface and adding to the weight of the water.  Therefore technically water pressure should be measured at sea level or adjusted for altitude.  In irrigation design we ignore this as it doesn’t create a significant impact for our purposes.   If you were doing very precise scientific work you would want to adjust the water pressure value based on the altitude.  Temperature also impacts pressure, as water expands and contracts with temperature changes.  For the most accuracy the water temperature should be 4ºc when the pressure or depth is measured.  Again, this is only applicable when doing very precise scientific work.  From here out we will ignore air pressure and temperature.)

OK, let’s move on and build on what we just learned…

Pounds per Square Inch (PSI):

In the USA water pressure is most often expressed as “pounds per square inch” usually abbreviated as “PSI”.  (If you maintain a car you are familiar with PSI, it is used to measure the air pressure in car tires.)   PSI and feet head are both values used to measure water pressure.  PSI measures the amount of pressure in pounds that the water would exert against a 1 square inch area.  Don’t worry if you don’t fully understand that weight per square inch idea, that is not something you will need to deal with much when working with pumps and irrigation usage.  However a lot of pumps and almost all irrigation equipment performance data is provided using PSI values, and most pressure measuring equipment (pressure gauges) also give you values in PSI,  so you will very likely be using PSI a lot.

One thing that is important is for you to know that you can easily convert PSI values to feet head and visa versa.  You can easily convert back and forth between Feet Head and PSI using two formulas:

Feet Head x 0.433 = PSI
PSI x 2.31 = Feet Head

You don’t need to memorize those formulas, just remember that you can switch back and forth between feet head and PSI by using them.  You can easily look up the formulas.

Example: Let’s say you have a swimming pool that is 8 feet deep. At the very bottom of the pool the water pressure will be equal to 8 feet of head. Pretty simple! If you want to know the pressure in PSI at the bottom of that 8 foot deep pool you can use the depth to get feet head (8 feet deep = 8 feet head), then convert it by multiplying feet head times 0.433 to get the PSI.  (8 ft. hd. x 0.433 = 3.46 PSI.)  If you swam under water at a depth of 5 feet below the surface then the water pressure on your body would be 5 feet head or 2.17 PSI. The Titanic rests on the sea floor at a depth of 12,600 feet below the surface. Therefore the pressure on the hull of the Titanic is 12,600 feet of head or a bone crushing 5,456 PSI! Consider that the plastic pipe in your sprinkler system will burst at somewhere around 300 PSI of pressure!

Measuring the Water Pressure:

Water pressure in feet head is measured using a ruler or other device that measures distance.  For a simple water tank this is easy to do, just measure the water depth and you have feet head.  Unfortunately with water in pipes it is often rather hard to determine the vertical distance between points to get the depth as the pipe may be hidden from view or you may not even know where the top of the water level is in the pipe.  Therefore water pressure is more often measured using a “Pressure Gauge”, a simple gauge that is attached to a pipe or tank and measures the pressure in PSI.  There are both analog and digital pressure gauges available.

Example of an inexpensive water pressure gauge:

Hydraulics in Pipes or Tubes:

Static vs. Dynamic Pressure:

When using a water pressure value it is often helpful to know if the water was in a static or dynamic state at the time the pressure was measured.  For that reason the terms “Static” or “Dynamic” are often indicated after pressure value that was measured using a gauge.  Example: “20 PSI static”, or “35 feet head dynamic”.

Static Pressure means the water was not in motion in the pipe when the measurement was taken.
Dynamic Pressure means the water was moving or flowing in the pipe when the measurement was taken.

When measuring water pressure in pipes using a pressure gauge it is important to know if the measurement was static water pressure or dynamic water pressure.   There can be a substantial difference between the two when measured at the same location, usually a dynamic pressure reading will give a much lower pressure value than a static pressure reading.  Most of the time static water pressure measurements are used.

Water Pressure in Pipes and Tubes:

Now the more difficult to understand part.  Since water is essentially a non-compressible liquid it exhibits the unique trait of transferring pressure horizontally when in a confined space like inside a pipe or tube.  What this means is the water pressure in a pipe is based on the vertical distance between points that we talked about earlier, not the actual distance.

Remember this diagram from above?

Feet Head is the Vertical Distance Between Two Points

The static water pressure at Point B is always based on the vertical distance between Point A (which would be the water surface) and Point B.  The total distance between the points  has no effect on the water pressure as long as the water is static, or not flowing.  (We’ll look at what happens when water in the pipe is dynamic, or flowing, later in the article.)



Look at this diagram showing a really simple water piping system:

Height Creates Water Pressure
Height Creates Water Pressure

In the picture above the water pressure in the water tank at the water surface is 0 feet, or 0 PSI.  (No water depth means no water pressure.)

The ground level is 40 feet below the water level in the tank. Therefore the water pressure at ground level is 40 feet of head, or about 17 PSI. So far, pretty straight forward.

Now the hard to understand part. The water enters the house at a level 100 feet below the water level in the tank. So the static water pressure at the house is 100 feet of head, or about 43.3 PSI.   Remember pressure is determined by vertical distance, not total distance. That total distance does not matter when the water is static (not moving) in the pipes. Because the water is a non-compressible liquid it transfers the pressure horizontally along the pipe route without any lose of pressure! If we measured the pressure with the water flowing the pressure would be termed “dynamic pressure”. With the water in a dynamic state (flowing in the pipe) the water would loose pressure due to friction on the sides of the pipe and we would get a lower pressure reading at the house. But static pressure means no flow, no friction, and no pressure loss! Read that last sentence again! Think about it for a second, go back, look at the picture again if you need to.  It makes sense if you think about it.  Don’t feel bad if you still don’t get it,  college students struggle with this after 2-3 lectures on it.   If you can’t wrap your head around it just accept it as true (it is!)   Don’t feel bad and don’t get discouraged! Just continue on with the next paragraph.

In most cases we measure water pressure in the static state when designing irrigation systems (or any other water piping system for that matter). Then we use calculations to figure out the friction loss that will occur in the pipes and subtract it from the static pressure to arrive at the dynamic pressure. Why not just turn the water on and measure the dynamic pressure with the water flowing? It would seem that then we would not have to prepare a separate calculation for friction loss, right? Well, that is correct, however dynamic pressure is extremely difficult to measure. You have to get the flow just right and then hold the flow at that level for a minute or two while the pressure stabilizes. This is a real pain in the rear to do and not nearly as easy as it sounds! If there is an air pocket in the pipe it can completely throw you an erroneous pressure reading. Plus, what if the pipe isn’t installed yet? Then you can’t measure the dynamic pressure at all. So, the result is that we almost always will work with static pressure. It’s just easier, and who wants to do it the hard way?

Now go back and look at that picture above again. As the water flows to the house the water level in the tank will go down. So the elevation of the top of the water in the tank will not be as high above the house. When the tank is almost empty the vertical distance between the water surface level and the house might be only 95 feet. So the water pressure would also be lower. This happens all the time and is normal! If the elevation varies, then so will the water pressure.

Still confused? Don’t worry about it, just follow through the procedures as outlined and you’ll be alright even if you don’t fully understand why you’re doing some of these things! Just remember that most of the time if you are taking a pressure reading with a gauge you probably should turn off the water flow in the pipes.  For example if you are taking a pressure reading at your house you would attach the pressure gauge to a faucet and turn on the faucet so water goes into the gauge.  Then make sure all the water in the house is off, no toilets filling , no ice makers making ice, nobody taking a shower, so that the water in the house’s pipes is not moving.  Then you read the static water pressure on the gauge.

Pumps and Hydraulics

I know it’s boring!!! Hang in there!

If you’re planning to use a booster pump jump down to the heading Booster Pumps. Everyone else just continue on…

The following is oriented toward wells. If you don’t pump out of a well don’t panic, just substitute river, lake, pond, spring, mud-puddle, or whatever for “well “in the following procedures. “Top of well “would be the high water level of the river, lake… etc.

First you will need to find out the “Dynamic Water Depth “of the water in your well. Dynamic Water Depth is the depth of the water below the top of the well, in feet, when the pump is running.  OK, so you’re thinking- “if I don’t have a pump yet, how am I supposed to know what the water level is when the pump (which I don’t have yet!!) is running? Grrrr!!!” Well, of course you’re right, but as you probably guessed by now, there is a solution. When a well company drills a new well they insert a temporary pump to “break in “and test the well. They refer to this as “developing “the well. As part of this process they also measure the Dynamic Water Depth of your new well at various pumping rates. Your pump company should have a record of this information which they can give you. One warning- you really should have the test repeated if the well is more than 5 years old. Water levels often drop over time. If you can’t find the dynamic water depth and are too cheap to have it tested, you can guesstimate using the well depth and assuming the Dynamic Water Depth is about 10′ above the bottom of the well. Your pump will likely waste some energy if you use this guesstimate depth, this is because the pump may be somewhat oversized (probably not by much however.)  You may also find you have problems with the pump cycling on and off if you use a pressure switch to control it (pressure switches will be described later. ) There are cures for the pump cycling problem. They include special cycle-stopping valves installed at the pump outlet or using the pump start circuit on the irrigation controller to override the pressure switch and lock the pump on.  The best cure however is to use a variable speed pump controller.  In fact, a variable speed pump is really the way to go, pressure switches and large pressure tanks are very much “old technology.”  More on that later.

If you’re not going to be pumping from a well (ie; you are using water from a pond or stream) just use the lowest “dry year “water level of your water supply in place of the Dynamic Water Depth.

Note that the term “draw-down “is often erroneously used in place of Dynamic Water Depth. I often do this myself. So be sure to clarify when talking to your pump company. When the pump is running, the water level in the well drops below the water table. It may drop a few inches or more than 100 feet depending on the type of soil (or rock) the well is drilled into. Often the water level in wells drilled into rock will drop well over 100 feet when the pump is running, as the water can’t easily move into the well from the surrounding rock. At any rate, the real definition of “draw-down “is the distance the water drops in the well when the pump is running. But keep in mind that many people interchange the terms draw-down and Dynamic Water Depth. See the diagram below for clarification.

Now you need to figure out the “Elevation Difference “between the top of your well and the highest point in the area to be irrigated. That is, how much higher (or lower) is the highest point in the irrigated area than the top of the well. This may be a negative number if the well is higher than the irrigated area. See the drawing below.

Elevation Related Impacts on Water Pressure
Height Related Impacts on Water Pressure

Irrigation System operating pressure.

There is one additional ingredient you need to add to the pressure needs, which is the pressure to operate the actual irrigation system.  In other words, in the diagram above we got the pressure from down in the well up to the irrigation system on hill.  Now we need enough additional pressure to get it through all the irrigation system’s pipes, valves and sprinklers.   This pressure will probably be stated in PSI rather than feet head.

Existing Irrigation System.  If you have an existing irrigation system that you want to add a pump to then you need to figure out how much pressure the existing system will need.  If the existing system has a working water supply then you can simply use a gauge to measure the pressure  in the existing supply (static).  If there isn’t a working existing water supply you really have two choices, take a wild guess or “reverse engineer” the existing system.

Wild guess:  In most cases 65 PSI is a good starting guess (this is one situation where it might pay to buy an off-the-shelf pump from someplace that will allow you to return it if it doesn’t work.  Remember this is just a wild guess, there is a really good chance it won’t work!)
Reverse Engineer:  To be real honest this may be a good place to hire someone who is an expert.   If you insist on doing it yourself the best way is to read through and learn the tutorial on how to design sprinkler systems.  Then using your new knowledge you should be able to reverse engineer the system.  It will require many hours  of work and study.

Irrigation System Already Designed and you have the plans.  The pressure required by the irrigation system will have been calculated as part of the irrigation design process and  it should be noted on the irrigation design. If not, ask the designer what it is, he/she should be able to give it to you.




Future Irrigation System.  Chances are you don’t have an irrigation system yet, or even a design. In this case you will need to make an “educated guess”. The following table will help you with your guess:

Typical Pressures for Irrigation Systems:

Drip Irrigation:  30 PSI (70 feet head)
Spray Type Sprinkler Heads and Rotary Nozzles:  45 PSI (104 feet head)
Rotor Type Sprinkler Heads:  60 PSI (139 feet head)

Remember, the values above are estimates. Dependent on your actual design you may need more or less pressure. You should design your irrigation system and adjust these values for the actual design before purchasing a pump! Spray sprinklers feature a steady fan shaped pattern of water a bit like a shower head. Rotor type sprinklers feature streams of water that rotate around the sprinkler. See the Selecting A Sprinkler Head for more information.

Add it all together:  To finish up your pressure requirement calculations you simply add the values of the Dynamic Water Depth, elevation head, and operating pressure head together to get the total head required. Remember that all the values should be in feet of head, not PSI!

Example: You measure a Dynamic Water Depth of 25 feet in your well. The irrigation system is 10 feet higher than the top of the well. You’re going to use rotor type sprinkler heads so you select an operating pressure of 104 feet head. Your total head required would be 25 + 10 + 104 = 139 feet of head!

Pressure vs. Flow: Inverse Relationship

When you are using a pump, flow and pressure have an inverse relationship.  For any given pump, as the output flow INCREASES, the output pressure DECREASES.  That’s all you really need to know at this point, but you do need to know it.

Example:  let’s say you have a 5 hp pump installed with a pressure controller.  You reset the pressure so that it will be higher.  As a result the flow from the pump will decrease.  Lower the pressure and the flow will increase.  Simple, but knowing how that relationship works will save a lot of grief if you start fooling around with the pressure settings!  If you want both more pressure and more flow you need to get a bigger pump!

Here’s another example:  You have a sprinkler zone with 8 sprinklers on it, but the pressure is not enough to make the sprinklers spray a nice pattern and the sprinklers are just piddling water.  You remove one of the sprinklers from the zone and cap off the pipe.  Now you have lots of pressure and the sprinklers spray great!  What happened?  By removing a sprinkler you reduced the flow, so when the flow went down the pressure went up!


Now, something just for the gear-heads…

Formulas:

Note that it is always preferable to get an actual performance table or graph for your specific pump from the manufacturer.  The formulas below only provide very rough estimates of performance!  Every pump is different depending on how it is made.

Estimated Pump Performance based on Horsepower:

Pressure for pumps is usually (in the U.S.A. that is!) measured in feet of head.

FT.HD. = HP x 2178 / GPM

GPM = HP x 2178 / FT.HD.

HP = GPM x FT.HD. / 2178

HP is brake horsepower
GPM is gallons per minute of flow
FT.HD. is pressure in feet of head
Note: the formulas above have been simplified to assume a pump efficiency of 55% which is a good average figure to work with if you don’t know the exact efficiency of your pump, which is likely the case.  (New pumps often perform at much better than 55% efficiency, however it is industry practice to use the 55% value so that the pump will still perform adequately as it ages and the efficiency drops due to wear. )

Pressure Conversion Formulas:

PSI x 2.31 = FT.HD.
FT.HD. x 0.433 = PSI