College Of Product Knowledge: Private Water Systems
THE BASICSAll of our discussions about plumbing products and components up to this point have had one rather basic assumption in mind - that something is delivering water to them. In all cases, the “something” involved will be one of two possibilities: 1) service from a local utility; or 2) a private water system - usually a well.
In the case of buildings and houses served by wells, “private water system” is really more than just a fancy name for a pump, because there is more to the average installation than just that. There are actually two basic components in the typical installation - the pump and the pressure tank.
PUMPS: There are two primary criteria to look for here: 1) the location of the pump - above ground or submerged in water; and 2) the type of pump - the operating principle involved.
The first is rather self-explanatory, but let's expand on the second part. Probably the two most common identifications of water well pumps today are the jet type and the submersible type. Unfortunately, these terms give you an “apples to oranges” match-up, since the first one tells you something about the operating principle involved, whereas the second tells you where it is installed - down in the water. Actually, the two types are pretty closely related when you get down to the basics of how they move the water, the operating principle in both cases being a “centrifugal” action, using a high speed impeller.
But here is where the difference comes. Whereas the jet type typically uses a single impeller of a relatively large diameter, the submersible uses a “stack” of impellers of relatively small diameter.
In addition to these most common types of pumps, there are also a few other designs in what is called the “positive displacement” category. These include the reciprocating (or piston) variety, the helical rotor type, and regenerative turbine designs.
PRESSURE TANKS: The other basic component of a private water system is the storage tank. In its simplest definition, a storage tank stores not only the water itself, but also pressure to move that water through the supply system when called for. (The pressure aspect is made possible through the compressing of air in the tank that acts like a coiled spring to push the water back out again.) This results in three benefits: 1) It prevents excessive cycling of the pump, protecting the motor from overuse and abuse; 2) It permits delivery of water to outlets between cycles of the pump; and 3) It can provide pressure and capacity beyond what the pump itself could deliver on a strictly “demand basis.”
WELL SYSTEMSBoth “shallow” and “deep” well systems involve unique requirements in terms of their mechanical components.
SHALLOW WELL SYSTEMS: Shallow wells are those in which the water inlet is located no deeper than about 25 ft. below the level of the pump. The operating principle here is a simple one, involving the creation of a partial vacuum on the inlet side, leading down into the water. Atmospheric pressure on the water surface around the inlet pushes water up the suction pipe and on through the pump.
The limit of effectiveness of such a system is about 25 ft. at sea level, and somewhat less at higher elevations, due to lower atmospheric pressures. Pump manufacturers specify the capacity of their pumps at a given suction (number of feet), plus a friction factor. It is important to calculate the friction factor in addition to the suction lift in order to properly determine the resulting performance of a pump. Several different specific pump designs can be used for shallow well applications.
DEEP WELL SYSTEMS: When water in a well is located at levels too deep for suction limits, another type of pumping system must be used. While there is more than one pumping concept employed for such use, all of them place at least part of the pumping mechanism beneath the water level. This essentially reduces or eliminates the consideration of suction lift as a specification factor, since water is typically not required to rise a long distance in a suction pipe. Centrifugal jet pumps and submersibles are most often used for such wells.
PUMPING CONCEPTSThere are several operating principles currently employed in water pumps today.
CENTRIFUGAL PUMPS: By far the most commonly used pumping concept today, this type (as the name suggests) employs centrifugal force as a means of moving water. The basic operating theory is as follows:
A rapidly rotating component takes in water at its center and “throws” it out to the walls of the pump housing by centrifugal force. This outward spinning of the water creates a vacuum toward the inside, which in turn, draws in more water at the center. The illustration gives an elementary example of this pumping principle. As mentioned above, the two most common versions of centrifugal pumps are the jet and submersible types. They differ in this way: 1) The jet type places the primary pump component above water, pushing water down a pipe and through a venturi jet near the bottom of the well to pull “new” water up another pipe with it. 2) A submersible pump is assembled into an integral housing that is placed at or near the bottom of the well (motor and all).
PISTON (RECIPROCATING) PUMPS: This is one of several pump designs that are classified as positive displacement types. By this, we mean that water is forced (displaced) from an area (usually a cylinder) by some action of the mechanism. In the case of a piston pump, this is accomplished by a plunger-like component that pushes water out of a chamber.
Such pumps are classified as single-acting and double-acting designs. The simplest, the single-acting type, consists of an inlet valve, a discharge valve and a plunger. The working cycle is accomplished in two strokes - upward (water intake) and downward (discharge). During the upward stroke, a vacuum is created in the cylinder into which water is forced by atmospheric pressure. At this time, the inlet valve is open and the discharge valve closed (these are essentially check valves, which open and close in response to pressure). During the downward stroke, the inlet valve closes, the discharge valve opens, and the plunger forces the water out of the cylinder and through the discharge outlet of the pump.
HELICAL ROTOR PUMPS: Another example of a positive displacement concept is the helical rotor type. This basically consists of a molded rubber sleeve with a helical (spiral) configuration inside, and a spiral metal shaft that turns inside the sleeve. The relationship between these two components forces or “squeezes” water through the internal channel formed between them, forcing it out under pressure at the discharge end.
REGENERATIVE TURBINE PUMPS: This type of pump uses a rotating wheel or impeller that has a series of blades (like little “buckets”) near its perimeter. These fins mate with a groove in the housing called the raceway. Incoming water is picked up by the fins of the impeller and carried along in a revolving, spiraling path. Such pumps are able to develop pressures far greater than centrifugal types.
PRESSURE TANK OPTIONSSince one characteristic of water is its tendency to absorb surrounding air, the design of the tank must either make provision to replenish such air, or prevent the absorption in the first place. A tank in which a considerable portion of the air has been absorbed is often referred to as being “waterlogged.” The reason this condition is undesirable relates to the function of air in the tank. Without an adequate air supply, there will be less compression, and accordingly, less sustained flow of water without assistance from the pump. This, in turn, means that the pump must cycle that much more frequently, which wastes energy and accelerates the wear of that component.
Approaches to handling this problem range from tank designs, which are called “plain steel”' and are equipped with a device to automatically introduce more air with each operating cycle, to types that isolate most or all contact between water and air. Here's a rundown on the three basic types:
PLAIN STEEL TANK: This type can be installed in either a vertical or horizontal position, with separate connections provided for inlet and outlet. In addition, there is typically a connection provided for installing an air volume control device, located at the level the water would be at the time the pump activates.
PLAIN STEEL TANK WITH FLOATING SEPARATION: This style uses a floating wafer at the top of the water level, which reduces the absorption by eliminating most of the contact between the water and the air. Like the plain tank, both an inlet and outlet connection are provided. Upon installation and periodically in use, such tanks are supercharged with air externally through a tire-like valve. Because of the floating orientation of the wafer, these tanks are limited to vertical installations.
DIAPHRAGM AND BLADDER TYPES: Finally, we have a type of tank that provides a complete separation of water and air by means of a flexible membrane. This membrane typically takes the form of either a diaphragm across the cross-section of the tank, or a self-contained bladder within the tank. Tanks of this sort are pre-charged with air at the factory, and are fitted, as a rule, with just one connection for both inlet and outlet. <<