This part of the Pump Tutorial Series. The 1st page is at Pumps: Selecting-a pump step-by-step.
There are numerous types of pumps designed for various purposes. Pumps commonly used for irrigation fall into two broad categories: Displacement Pumps and Centrifugal Pumps. Within those categories there are sub-categories that further define the type of pump. This tutorial will focus on those types of pumps most often used for irrigation.
Displacement Pumps:
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 we’ll move along to centrifugal pumps.
Centrifugal Pumps:
Almost all irrigation pumps fall into this category. A centrifugal pump uses an “impeller” 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. Remember those fair rides (“the Twister”) where you had your girlfriend sit next to you at the start and then as it spun you both around she slowly wound up in your lap? Fun right? That was centrifugal force that moved her to where you wanted her! Centrifugal pumps move water in the same way, by spinning it very fast, which causes it to slide out to and off of the end of the impeller where the shape of the casing directs it to the pump outlet.
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”.
Most 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 easily suck water up into the intake pipe if it is full of air. Most centrifugal pumps must be “primed” before the first use. 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. Some centrifugal pumps are “self-priming” which means they are designed to get started without needing to be primed. Portable pumps for temporary use tend to be self-priming.
There are several types of centrifugal pumps. Here are the types you are most likely to encounter:
End-Suction Centrifugal Pumps
“End-Suction Centrifugal pumps” are 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 is a single unit. The water typically enters the pump casing through a “suction inlet “centered on one side of the pump, and exits at the top of the pump casing. 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. 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 installed level with, or below, the water level. But any time they need to draw the water up from a water source below the pump they perform much less efficiently. Therefore end-suction centrifugal pumps are not the best choice for pumping 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 installed above the water surface. If you can’t ask the manufacturer you can use a maximum of 4 feet above the water surface as a general guideline. The higher the pump is above the water surface, the less efficient that pump is going to be. Here’s a stick in your head little saying to help you remember: end-suctions suck at sucking!
Submersible Pumps
Submersible pumps are typically centrifugal type pumps that are installed completely underwater, often including the motor. (Not all submerged pumps are centrifugal types, but the term “submersible pump” is almost always a reference to a submersible centrifugal pump.) A centrifugal submersible pump consists of a water-proofed electric motor and a pump combined in a single unit. Typically a larger size submersible pump and motor will be shaped like a long narrow cylinder so that it can fit down inside of a water well. Smaller size submersibles are often designed to sit on the bottom of a sump, pond, or tank and are often used as fountain pumps or sump pumps. Although most larger submersible pumps are designed to be installed in a well, many can also be placed in a lake or stream provided the water is deep enough. Some (not most, so check the instructions!) may be installed sideways in shallower water. A common submersible pump installation method for lakes and rivers is to mount the submersible pump underwater in a “sleeve” made of well casing pipe that is attached underwater to the side of a pier piling or a post. Some are attached to the bottom of a float or floating dock.
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 (with large diameter wells they sometimes 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 a 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.
Turbine Pumps
A turbine pump is a centrifugal type pump mounted underwater and attached by a drive-shaft to a motor mounted above the water. 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 an 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 located down below it in the well or a underground 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 to the irrigation system. Two or three different sized turbine pumps are often placed side-by-side to handle different flow combinations.
Jet Pumps
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 some of the water output is diverted into a small pipe (called the “drive line”) that goes down 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 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.
Booster Pumps
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 “boost” the water pressure up to a higher pressure value. Example: say you have a sprinkler system that needs 80 PSI of pressure to operate (if you need 80 PSI you apparently have very big sprinklers, but that’s a topic for the Sprinkler System Design Tutorial!) But let’s say that 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 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 use a pump labeled as a “booster pump.” That said, almost all booster pumps are the “end-suction centrifugal” type because they are simple, work excellent as booster pumps, and generally are less expensive.
Floating Pump
A floating pump is simply any type of pump that is attached to a float. Most floating pumps use a submersible pump that is suspended in the water below the float, although there is no reason you couldn’t 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 into the pump, or suck up 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.
The following paragraphs briefly go through some of the pump related equipment you may need for your pump.
Foot Valves:
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 as it is usually built in to the pump.) 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. The little screen found on many foot valves is not good enough, consider that 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 even self-cleaning. Even “self-cleaning” units periodically need a manual 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.
Media Intake Beds:
For really nasty water “media beds” are used to filter the intake water. A media bed is a structure on the bottom of the pond where a slotted intake pipe is buried under a layer of sand and gravel. Water is drawn down through the sand and gravel layers into the holes in the intake pipe. The sharp edges of the sand snag algae, moss, etc. as the water passes through. The snagged moss is eaten by fish or slowly broken down by bacteria. Note that a special “sharp sand” is used, not normal sand. These media beds are large in order to provide enough sand surface area for the water to flow easily with minimal resistance through the sand down to the intake pipe. It is a large undertaking to install one thus they are seldom used for residences. Generally you need an expert to design one of these media beds for you. Try a member of the American Society of Irrigation Consultants.
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 where your well was!
Sometimes other water from 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.”
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