Types of Water Pumps
Pumps commonly used for irrigation fall into the following categories based on the design of the pump. This tutorial addresses electric powered pumps only. While most of the information here also applies to fuel powered pumps the formulas don’t! You must use different formulas for calculating the size and flow information for fuel powered pumps. If you have an engine powered pump (gas, diesel, propane, corn liquor, etc.) you should contact the pump manufacturer and request a copy of the pump performance curve. As a general rule, fuel powered pumps require more horsepower than electric pumps. Let’s look at the major types of pumps, with a focus on those commonly used for irrigation systems.
(If you just jumped to this page without reading the first page of the tutorial you may be making a big mistake. Please take a look at the consumer warnings on the first page of the tutorial so you don’t get ripped off!)
Displacement pumps force the water to move by displacement (bet you couldn’t have guessed!) This means pumps such as piston pumps, diaphragm pumps, roller-tubes, and rotary pumps. The old fashioned hand-pumps, the ones you operate by moving a long lever handle up and down, are piston displacement pumps. So are those grasshopper-like oil well pumps. Displacement pumps are used for moving very thick liquids, creating very precise flow volumes, or creating very high pressures. In addition to oil wells they are also used for fertilizer injectors, spray pumps, air compressors, and hydraulic systems for machinery. With the exception of fertilizer injectors (used for mixing fertilizer into irrigation water) you will not see them typically used for irrigation systems, so that is all I’m going to say about them.
Almost all irrigation pumps fall into this category. A centrifugal pump uses an “impeller” (sort of like a propeller, but a little different) to spin the water rapidly inside a “casing”, “chamber”, or “housing” (any of those terms may be used). This spinning action moves the water through the pump by means of centrifugal force. Centrifugal pumps may be “multi-stage”, which means they have more than one impeller and casing, and the water is passed from one impeller to another with an increase in pressure occurring each time. Each impeller/casing combination is referred to as a “stage”. All centrifugal pumps must have a “wet inlet”, that is, there must be water in both the intake (inlet) pipe and the casing when the pump is started. They can’t suck water up into the intake pipe. They must be “primed” by filling the intake pipe and case with water. To prime a pump you simply fill the intake pipe with water and then quickly turn on the pump. Most centrifugal pumps are designed to trap water in the intake once they have been primed the first time, thus they “maintain their prime” between uses.
There are several types of centrifugal pumps. Here are the types you are most likely to encounter:
End-Suction Centrifugal Pumps
The most common type of centrifugal pump. Typically the pump is “close-coupled” to an electric motor, that is, the pump is connected directly onto motor’s drive shaft and the pump case is bolted to the motor so that it looks like a single unit. The water typically enters the pump through a “suction inlet “centered on one side of the pump, and exits at the top. Almost all portable pumps are end-suction centrifugal type pumps. End-suction centrifugal pumps generally need to be primed the first time they are used (including many so-called “self-priming” models,) after that most will not require priming unless a leak develops in the intake pipe. If the pump needs to be primed each time it is turned on this almost always means there is a tiny leak in the intake pipe.
End-Suction Centrifugal pumps are designed to push water, not pull it. They are great for use as irrigation booster pumps. They are very good for pumping water from any source where the pump is level with, or below, the water level. But any time they need to actually suck the water up into the pump they perform much less efficiently. Therefore end-suction centrifugal pumps are not the best choice for drawing water from a water source that is more than a few feet lower than the pump. When sucking water up into the pump they must be installed as close to the water surface level as possible, which is often inconvenient, especially for locations where the water level may go up and down, like some lakes, rivers, creeks and ponds. Each pump is different, so check with the manufacturer to determine the maximum height the pump can be above the water surface. As a general rule guideline, they perform very poorly if they are more than 5 feet above the water surface. Just remember, end-suction centrifugal pumps are great for pushing water, but they suck at sucking it!
Submersible centrifugal pumps are pumps that are installed completely underwater, including the motor. (Note that not all submersible pumps are centrifugal, displacement pumps are often used for hand-operated pumps and pumps for heavy fluids like oil.) A centrifugal submersible pump consists of an electric motor and pump combined in a single unit. Typically the pump will be shaped like a long narrow cylinder so that it can fit down inside of a well casing. Although most submersible pumps are designed to be installed in a well, many can also be lowered into a lake or stream provided the water is deep enough. Some (not all!) may be installed sideways in shallower water. Another common installation method for lakes and rivers is to mount the submersible pump underwater to the side of a pier piling or a post. Some are attached to the bottom of a float. Submersible pumps don’t need to be primed since they are already under water. They also tend to be more energy efficient because they only push the water, they don’t need to suck water into them. Most submersible pumps must be installed in a special sleeve if they are not installed in a well, and sometimes they need a sleeve even when installed in a well. The sleeve forces water coming into the pump to flow over the surface of the pump motor to keep the motor cool. Without the sleeve the pump may over-heat. Because the power cord runs down to the pump through the water it is very important that it be protected from accidental damage. You wouldn’t want a boat tangled up in the cord or a snapping turtle or alligator to bite through it!
Many submersible pumps are “multi-stage” pumps. This means they are actually several smaller centrifugal pumps stacked on top of each other to create higher flow, more water pressure, or a combination of both.
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A turbine pump is a centrifugal type pump mounted underwater and attached by a drive-shaft to a motor mounted above the water. The drive-shaft usually extends down the center of a large pipe. The water is pumped up this pipe and exits directly under the motor. Turbine pumps are comparable to submersible pumps in energy efficiency. They are used primarily for larger pump applications where the size of the motor would be difficult to fit in a submersible structure (ie; it wouldn’t fit in a well!) Often turbine pumps consist of multiple stages, each stage is essentially another pump stacked on top of the one below. It works like a train with multiple engines hitched together pulling it, each stage would be a engine. Turbine pumps are typically the type of pumps you see on farms or municipal water district wells. When you see a huge motor mounted on its end and a pipe coming out sideways below the motor, that is most likely the motor for a turbine pump down below it in the well or tank. A typical landscape use for a turbine pump would be in a large park or golf course where water is coming from lakes. The turbine pump is mounted in a large concrete vault with a pipe connecting the vault to the lake. The water flows by gravity from the lake through the pipe and into the vault. From there a turbine pump sends the water under pressure through pipes tot he irrigation system. Two or three different sized pumps are often placed side-by-side to handle different flow combinations.
Like all pumps the jet needs an energy source. The jet is hydraulically powered by a stream of water diverted to it from the centrifugal pump. When the centrifugal pump starts running most of the water would go to the irrigation system, however some of the water output is diverted through a small pipe (called the “drive line”) to the jet. This is the “drive water” that powers the jet. The jet is very simple and yet highly engineered for the exact shape and size of the water channels through it. The jet consists of two non-moving parts; a nozzle and a venturi, and it works using the venturi effect. At the jet the drive water is forced through a nozzle which massively increases the velocity of the drive water. This increase in velocity results in a water pressure decrease (Bernoulli’s principle.) As this high velocity water flows through the venturi a vacuum or suction results, which draws water from the well/river/pond into the venturi where it mixes with the drive water. At the exit of the venturi the pipe diameter increases greatly, which slows the water velocity back down. The decrease in velocity results in a rise in water pressure (Bernoulli’s principle again,) pushing the water back up toward the centrifugal pump. Back at the centrifugal pump the process begins again with most of the water sent off to run the irrigation system, but some is again diverted back down the drive line to power the jet.
Note that the exact sizes of the centrifugal pump, nozzle in the jet, and the venturi are very critical for efficient operation of a jet pump. It is important that you have exactly the right size combination to match the performance requirements of your irrigation system so that the irrigation will work properly and the pump will be energy efficient.
You may run into the term booster pump now and then as booster pumps are common in irrigation, so let’s start by defining it. Most pumps are used to take water from a standing (or non-pressurized) source and move it to another location. For example, a pump might take water from a lake and move it to a sprinkler system. A booster pump, on the other hand, is used to increase the water pressure of water that is already in a pipe and pressurized. Example: say you have a sprinkler system that needs 80 PSI of pressure to operate (you apparently have very big sprinklers if you need 80 PSI, but that’s a topic for the Sprinkler System Design Tutorial!) But the water line coming onto your property from the water district only has 50 PSI of pressure. In this case you could install a booster pump to raise the pressure of the water from 50 PSI up to the 80 PSI needed for your sprinkler system. So to put it another way, a booster pump is used to “boost” the water pressure. So what is special about a booster pump? Nothing! The term booster pump simply defines an ordinary pump by the job it does (boosts pressure,) there is nothing particularly special about it, you don’t need to buy a pump labeled by the manufacturer as a “booster pump.” That said, however, almost all booster pumps are the “end-suction centrifugal” type because they are simple, work excellent as booster pumps, and generally are less expensive.
What is a Floating Pump?
A floating pump is simply a pump that is attached to the bottom of a float. Most use a submersible pump that is suspended below the float, although some use a turbine, end-suction centrifugal, or jet pump mounted on top of the float. The float is anchored in a lake, pond, or river. A flexible tube or pipe is used to take the water from the pump to the irrigation system.
A floating pump is a good option to look into for installing a pump in a pond, lake, or slow moving river. It is often much easier to install a floating pump than trying to anchor a fixed pipe intake and intake filter screens on the bottom of a lake or river. Most floating pumps also come with a screening device built into the pump or float to keep garbage out of the pump. The screen is normally below the water surface but well above the bottom of the lake or river, thus it doesn’t suck floating debris onto the pump, or suction muck off the bottom. This helps reduce how often the screen needs cleaning. Floating fountains and pond aerators are another utilization of floating pump technology.
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End-suction centrifugal and jet pumps should have a foot valve installed on the intake pipe. (Submersible and turbine pumps don’t need a separate foot valve, there generally is not an intake pipe to install it on even if you wanted to.) A foot valve is a simple check valve that holds water in the intake pipe when the pump is turned off, so the pump maintains it’s prime. Generally the foot valve is installed on the beginning of the intake pipe, where the water is sucked into the pipe. Most of the pros suggest that you install a foot valve on your intake pipe, even for self-priming pumps that have built-in check valves in the pump. (Obviously if the pump manufacturer specifically states that you should not use a foot valve, don’t install one!) Most foot valves have a built in screen as part of the foot valve. While sufficient for most well water, the built in screen is not a big enough screen for protecting your pump if you are pumping from rivers, pond, or lakes where the water has any amount of debris. See the section “Pump Intake Screens” below.
Pump Intake Screens:
If you are pumping from a lake, pond, or river you need an intake screen of some type to keep sticks, moss, algae blooms, fish, amphibians, crustaceans, and especially rocks (yes, rocks!) out of your pump. Not just the little screen on the foot valve, consider it a back-up screen, what you need is a big, ugly, get-the-job-done intake screen! Even if you think you have the cleanest water in the world you still need something. There are numerous types and styles of intake screens available, some are self-cleaning. None are totally free from the need for periodic maintenance. Servicing the screens is an unavoidable part of having a pump in a river, pond or lake. Ponds with lots of algae or moss can be really tough on intake screens. The fibers of the algae and moss tend to wrap themselves around the wire screen material which makes removing them from the screen very difficult. You need a much larger screen in those situations and most likely one that is self-cleaning as well. Ask your local pump dealer for recommendations for your area, or search online for “intake screen.”
An intake screen is NOT a substitute for a filter on your irrigation system! A second filter should be installed after the pump. A filter is recommended for irrigation water regardless of the source; well, pond, stream, even city water should be passed through a filter. A good filter will save you a lot of money in repairs, way more than the cost of the filter. See the Irrigation Filter Tutorial for a discussion of the filter types and options.
Sand Separators & Sand Filters:
Some wells “pump sand”, ie; the well water has high levels of sand in it that can damage a pump, and clog up your water filter quickly resulting in the need for frequent cleaning. A well that pumps sand may indicate a problem with the well, so if you haven’t had your well checked it might be a good idea to start by having a pro look at it. The sand is coming from someplace and you don’t want to look out the back door one morning and find a huge sinkhole!
Sometimes other water sources like lakes, ponds and streams contain high levels of sand as well. Most often this is because the intake pipe is too close to the bottom. If the intake can’t be lifted out of the sand you may need a sand filter, sand separator or sand trap.
Special sand separators, sand filters, and sand traps are made that mount on the pump intake pipe to remove this sand from the water. Talk to your pump supplier or do Internet searches for “well sand separator” and “well sand filter.”
Pressure vs. Flow
Flow and pressure have an inverse relationship. For any given pump, as the flow INCREASES, the pressure DECREASES. That’s all you really need to know at this point, but you do need to know it.
Example: So let’s say you have a 5 hp pump installed with a pressure switch to turn it on and off (more on controls later.) 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 and the sprinklers are just piddling water. You remove one of the sprinklers 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 and Annoying Technical Stuff!
Here are the standard formulas used to estimate flow, pressure, and horsepower for all electric pumps. Note: these formulas 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.
Note that it is always preferable to get an actual performance table or graph for your specific pump from the manufacturer. These formulas only provide very rough estimates of performance!
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
To convert FT.HD. to PSI and visa versa.
PSI x 2.31 = FT.HD.
FT.HD. x 0.433 = PSI