PUMPS
water systems: MATERIALS ENCYLOPEDIA
Pumps INTRO:
The heart of all water systems is the pump that makes the water flow. In most cases, the type of water system will dictate the style and size of pump required. Head (the height water must be moved), flow (volume of water moved) and pressure (force of the water) are the variables, and must be matched to meet the water needs of the users. Most pumps have electronic or mechanical pressure switches that turn the motor on or off as required.
Pump types
These pumps can range from tiny recirculation pumps to very large pressure pumps. As the name implies, the pump is submerged directly into the water source, whether it be a natural water body, a tank or a shallow or deep well. These pumps use a positive displacement pumping mechanism to move water, typically a rotary vane or screw pump. Lobe and gear pumps are also available, though not as common.
The advantage of submerging the pump is the elimination of any inlet piping and the need for the pump to have a “sucking” action to pull water in and a “blowing” action to push the water through the supply pipe. Because it sits in the water, there is no danger of the pump losing its “prime” and not being able to draw water.
On the down side, submersibles can be difficult to inspect and maintain as they are often in locations that require some effort to physically remove the pump. The majority of submersibles have the electric motor in the water as well, and rely on excellent seals and a housing (usually oil-filled) to keep this from getting wet. Should these seals fail, the motor will short-circuit and cease working, and may suffer permanent damage.
A subcategory of submersible pumps is the solar slow pump, which has its electric motor directly wired to a photovoltaic panel and moves water any time the sun is shining, at a speed proportional to the amount of sunlight available. These pumps do not produce the kind of pressure and volume required to directly supply fixtures in the home, but are used to slowly and steadily move water to a storage container, from where it can be used when needed. They do not require battery storage or any other components used in a household solar electric system, making them simple to install and use.
Solar slow pumps are often used to lift water to a storage tank at a high point in the home, from which the water can use gravity to produce usable pressure. They can also be used to pump water from deep wells to a storage tank in or near the house, from which a small electric or hand pump can provide pressure.
These pumps are positioned close to the water source. The electric motor powers a centrifugal pump that provides flow and pressure to the output line supplying the home. A small amount of the water exiting the pump is siphoned away and sent down to the well, where it is pushed through a venturi. The pressure difference at the venturi draws more water up to the centrifugal pump. For shallow (0–12 meters /0–40 feet) wells, the venturi may be located at the pump, while for deep wells (12–30 meters /40–100 feet) it will be at the bottom of the well. Jet pumps are limited to about 30 meters (100 feet) of depth.
The chief advantage of the jet pump is that it does not rely on a tight seal to move water, and so requires less precise machining and is less prone to loss of performance due to wear than other pumps. In an era when metallurgy and seal technology was not as refined, this was the preferred way to pump water.
Jet pumps are less common now because of their main disadvantage, the need to be primed and maintain a prime. This means the reservoir at the pump and the jet and intake lines must be filled with water before the pump will work. A spring-loaded foot valve keeps water in the system when the pump is not running, but any leak in the system will mean manually priming again.
A diaphragm pump uses a flexible membrane to exert positive and negative pressure on a chamber. In the state of negative pressure, water is drawn into the chamber through a one-way check valve. In the state of positive pressure, water is pushed out of the chamber through another one-way valve. Rapidly pushing and pulling on the diaphragm draws water into the pump and through the outlet pipe to the home.
Advances in rubber and plastic membrane quality now make these pumps very reliable. As there are no moving parts in direct contact with the water, diaphragm pumps are not prone to corrosion or seal and bearing failure. They can also move water with particulate without causing damage to the pump.
Diaphragm pumps with good-quality valves can be self-priming, as the pump will displace air and create a low-pressure area that draws water into the pump body. Pumps will specify the amount of head that can be overcome by the self-priming action, with ranges from 0–150 meters (0–500 feet).
These pumps tend to cost less, but will not provide flow and pressure rates as high as other options. However, many models on the market will easily provide standard household water pressure. The diaphragm is prone to wear and fails more frequently than other pumps, but can also be replaced quickly and affordably on some models. Ensure that the diaphragm can be replaced before purchasing a unit.
One of the important unrecognized technical revolutions came with the invention of the piston or siphon pump in ancient Greece. The ability to move water mechanically while remaining stationary eliminated the labor-intensive need to carry water in vessels from a source to a point of use, especially where there are large changes in elevation. This freed a huge amount of labor to be put to other uses. Until the invention of the electric motor, hand (or foot) pumping of water was common and effective.
Human-powered pumps can use almost any type of positive displacement pump action. They type of action is chosen based on anticipated speed of the pump, the length of the displacement stroke and the head to be overcome.
Regardless of the type of pump, the propelling force comes from human action. A pump exists to match any type of human movement, from the raising and lowering of the arm on an “old-fashioned” bucket pump to high-efficiency “bicycle” units to those driven by rowing machines, oscillating exercise equipment or a foot-powered diaphragm.
In most cases, human-powered pumps are used to move water to a storage vessel from which fixtures are supplied with gravity pressure. Hand or foot pumps can provide on-demand flow, but with at least one limb busy pumping, it can be difficult to simultaneously use the water!
Human-powered pumps have benefitted from the same advances in seal technology and metallurgy as electric pumps. It is possible to move 5–10 liters (1.25–2.5 gallons) per minute (lpm) at heads of 6–12 meters (20–40 feet), and as much as 40 lpm (10 gallons per minute) at low heads. Even at the low end of that range, 10–20 minutes of cycling could provide enough water for a full day’s use.
Typically, human-powered pumps are used as backups so that water can still be supplied when there is no electrical power.
The ram pump is a positive displacement pump that uses the kinetic energy of moving water to provide the power to lift water. Water entering the pump from the flow of the stream pushes against a one-way valve, closing it. The resulting shock wave provides a moment of flow in the opposite direction. This back flow is directed to the pump’s outlet hose, which has another one-way valve that allows the water to push into the outlet but not return. This process is repeated in rapid cycles, depending on the speed of flow and the volume. The output side of the pump is able to lift water approximately six times higher than the head of the river or stream providing the flow. Water is ejected from the pump between shock cycles, which is fine when using the run of a stream or river, where the ejected water rejoins the flow.
Ram pumps for a household would be coupled to a storage tank, as the pump produces a continuous low flow at relatively low pressure. Water would accumulate in the tank and a small electric pump or gravity would provide the delivery pressure.
If all the conditions are right, a ram pump is an excellent, no-energy means of moving water. In areas where the driving water would freeze in the winter, this type of pump provides only a seasonal option.
Commercial ram pumps may be purchased, or the parts to make one are available from any plumbing supply store. The costs are very low and the number of moving parts very limited, making ram pumps very reliable.