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D frame adapter: Used to connect and align the pump and motor (manufactured in metric dimensions). The Imperial version is called a "C" frame adapter.



D.E. - Diatomaceous Earth: A type of filtration media. Can also stand for Dual Element when talking about electrical fusing.



D.I.N. standard: The German standard for industrial products.



Darcy, Weisbach Head Loss Tables: Based on the head loss in clean, new pipe, these values are used by fluid engineers when designing pumping systems.



Darcy-Weisbach equation: An equation used for calculating the friction head loss for fluids in pipes, the friction factor f must be known and can be calculated by the Colebrook, the Swamee-Jain equations or the Moody diagram.



Datum plane: A reference plane. A conveniently accessible known surface from which all vertical measurements are taken or referred to.



Datum Plane-1-: The centerline of the impeller eye in an above-ground horizontal pump; the eye of the first stage or bottom impeller in a submersible pump. This can be closely estimated as being at the same height as the top of the suction screen.



DC - Direct Current - a one way push of electrons, electric charge.



DC Motor: A motor using either generated or rectified DC power (See "Motor"). A DC motor is often used when variable-speed operation is required.



Dead head: A situation that occurs when the pump's discharge is closed either due to a blockage in the line or an inadvertently closed valve. At this point, the pump will go to it's maximum shut-off head, the fluid will be recirculated within the pump resulting in overheating and possible damage.



Dead Head-1-: The point on a pump performance curve where the unit is pumping zero GPM - also known as Shut-off Head.



Dead Head-2-:  The ability of a pump to continue running, for a short period of time, without damage when discharge is closed off. Only recommended for centrifugal pumps.



Decks: Those areas surrounding a pool, spa or hot tub that are specifically constructed or installed for use by bathers.



Deep Areas: Portions of a pool, spa or hot tub having water depths in excess of five feet (5’).



Deep Cycling: Repeated subjection of a battery to deep discharging and recharging; i.e., frequent withdrawal of most of the battery’s total storage capacity.



Deep Well Jet Pump: A type of jet pump able to lift water less than 70'. It has two pipes leaving it - the suction pipe and the drive pipe.



Deflection: Movement or displacement of the shaft in a radial direction.



Deionization: Removal of ions from water by exchange with other ions associated with fixed charges on a resin. First, positively charged ions are removed, by a cation exchange resin, in exchange for a chemically equivalent amount of hydrogen ions. Second, negatively charged ions are removed, by an anion exchange resin for a chemically equivalent amount of hydroxide ions. The hydrogen and hydroxide ions introduced in this process unite to form water molecules. This process is also called demineralization by ion exchange.



Demineralization - The process of removing minerals from water e.g. deionization, reverse osmosis and distillation.



Density (specific weight of a fluid): Weight per unit volume, often expressed as pounds per cubic foot or grams per cubic centimeter.



Density: Measured in gm/cm2 or lb./in 2 A measure of the weight of the fluid. A better term than specific gravity.



Depth of Discharge: The relative amount of energy withdrawn from a battery relative to how much could be withdrawn if the battery were discharged until exhausted.



Desalination: The removal of salts from saline water to provide freshwater. This method is becoming a more popular way of providing freshwater to populations.



Desalination-1-: The removal of dissolved inorganic solids (salts) from a solution such as water to produce a liquid which is free of dissolved salts. Desalination is typically accomplished by distillation, reverse osmosis or electrodialysis. A common source water may be seawater.



Design A, B, C, D: For AC Motors NEMA has standard motor designs with various torque characteristics to meet specific requirements posed by different application loads. The design "B" is the most common design.



Design or Design Letter: Letter assigned by NEMA to denote standard performance characteristics relating to torque, starting current and slip.



Design Rate of Flow (Design Filter Rate) - The average rate of flow in a system which is used for design calculation (usually the flow in gallons per minute divided by the effective filter area in square feet).



Dewatering pump: Designed for clear water applications (agricultural, industrial, and residential). As a general rule, dewatering pumps are limited to a 10% solids concentration and a solids size of one-fourth the diameter of the suction inlet. For applications requiring greater solids handling capability, trash pumps should be used.



Dewatering: The removal of unwanted water (clear or dirty) but free from hazardous materials.



Dialysis Dementia: A severe, often fatal encephalopathy which has been attributed to accumulation in the brain of aluminum from dialysate prepared with inadequately purified water. May include consumption of tap water with high levels of alum used in most municipal water treatment processes.



Diaphragm pump: A positive displacement pump. Double Diaphragm pumps offer smooth flow, reliable operation, and the ability to pump a wide variety of viscous, chemically aggressive, abrasive and impure liquids. They are used in many industries such as mining, petro-chemical, pulp and paper and others. An air valve directs pressurized air to one of the chambers, this pushes the diaphragm across the chamber and fluid on the other side of the diaphragm is forced out. The diaphragm in the opposite chamber is pulled towards the centre by the connecting rod. This creates suction of liquid in chamber, when the diaphragm plate reaches the centre of the pump it pushes across the Pilot Valve rod diverting a pulse of air to the Air Valve. This moves across and diverts air to the opposite side of the pump reversing the operation. It also opens the air chamber to the exhaust. This type of diaphragm pump is driven by pneumatic air so these can be used where electric drives are not preferred, is self priming and can run dry for brief periods, an handle hazardous liquids with almost any viscosity, can pump solids up to certain sizes.



Diaphragm pump-1-:  Uses a positive displacement design rather than centrifugal force to move water through the casing, delivering a specific amount of flow per stroke, revolution, or cycle. Due to their great air handling capabilities, diaphragm pumps are ideal for applications involving slow seepage at the point of suction.



Diatomaceous Earth (D.E.): Porous silica from skeletal remains of one-celled plants, which when properly graded, acts as a precoat filter media for water filtration.



Diatomaceous Earth Filter: A filter that utilizes a thin layer of diatomaceous earth as its filter medium that periodically must be replaced.



Diatomaceous Earth: The filtering medium of the DE filter, this dry powder is the fossilized remains of the ancient plankton; diatom. Diatomaceous Earth Filter: A filter tank containing fabric covered grids which hold the DE powder up against the flow of the water. Dichloro -S-Triazinetrione: a chemical compound. It is an oxidizer, bacteriocide, algicide, and cleaning agent that reacts with water to form hypochlorous acid, which is related to bleach.



Differential Pressure: The difference between the outlet pressure and the inlet pressure.  Differential Pressure is sometimes Called the Pump Total Differential Pressure.



Diffuser: A staionary housing similar to a volute in which the impeller rotates. Compact in design, it enables the pump to produce higher heads/pressures.



Diffuser: Located in the discharge area of the pump, the diffuser is a set of fixed vanes often an integral part of the casing that reduces turbulence by promoting a more gradual reduction in velocity.



Diffuser-1-: The portion of a volute case from the cutwater to the pump flange. In submersibles, a piece, adjacent to the impeller exit, which has multiple passages of increasing area for converting velocity to pressure.



Digital Meter: Has an LCD display, a range select dial, and is used to measure electrical properties of circuits and components.



Dilatant: A liquid that thickens (increases its viscosity) with agitation.



Dilatant-1-:  The property of a fluid whose viscosity increases with strain or displacement.



Dimensions: NEMA has standard frame sizes and dimensions designating the height of the shaft, the distance between mounting bolt holes and various other measurements. Integral AC motor NEMA sizes run from 143T-445T, and the center of the shaft height in inches can be figured by taking the first two digits of the frame number and dividing it by 4. Fractional horsepower motors, for which NEMA spells out dimensions, utilize 42, 48 and 56 frames. The shaft height in inches can be established by dividing the frame number by 16.



Direct Connection or Dead Short: Electricity takes the shortest direct route through the motor, coil or other component; nothing is restricting the current flow.



Direct current:  A current formed when electrons flow in one continuous direction.



Directional Inlet Fitting: An inlet fitting which provides adjustment in direction and flow rate to produce proper distribution of incoming water.



Directions: What you should read before using any chemicals.



Dirt Demand: The demand that your pool has for dirt. This level is inversely proportional to available time for cleaning. If you remove the dirt from the pool, you have created a dirt deficit, and the pool will actually suck dirt out of the air to maintain its dirt demand.



Discharge Head: The difference in elevation between the liquid level of the discharge tank and the centerline of the pump. This head also includes any additional pressure head that may be present at the discharge tank fluid surface.



Discharge Head-1-: The total head, including static head and friction head, on the discharge side of the pump.



Discharge Head-2-: The outlet pressure of a pump operation.



Discharge Hose: A collapsible hose used to move the water discharged from the pump.



Discharge Port: Same as the outlet. The point where the discharge hose or pipe is connected to the pump.



Discharge recirculation: Connecting a line from the discharge side of the pump to the stuffing box. Should be used with a close fitting bushing in the end of the stuffing box to increase the stuffing box pressure. A common application when pumping a fluid close to its vapor point.



Discharge Static Head: The difference in elevation between the liquid level of the discharge tank if the pipe end is submerged and the centerline of the pump. If the discharge pipe end is open to atmosphere than it is the difference between the pipe end elevation and the suction tank fluid surface elevation. This head also includes any additional pressure head that may be present at the discharge tank fluid surface, for example as in a pressurized tank.



Discharge, deep: Withdrawal of all electrical energy to the end-point voltage before the cell or battery is recharged.



Discharge, high-rate: Withdrawal of large currents for short intervals of time, usually at a rate that would completely discharge a cell or battery in less than one hour.



Discharge, low-rate: Withdrawal of small currents for long periods of time, usually longer than one hour. Discharge Rate: The rate at which charge is withdrawn from a cell or battery during discharge; i.e., the discharge current, expressed in terms of the battery’s capacity. In other words the C rate, as applied to discharging.



Discharge (battery): The conversion of the chemical energy of the battery into electric energy.



Discharge: The pump orifice where water exits the pump.



Discharge-1-: The volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second.



Discharging: Withdrawing the stored energy from a cell or battery.



Disconnect Box: An on/off switch placed between the control box and power distribution (breaker) panel and permits power to be disconnected from a device for service purposes.



Disinfectant: A chemical (commonly chlorine, chloramine, or ozone) or physical process (e.g., ultraviolet light) that kills microorganisms such as bacteria, viruses, and protozoa.



Disinfectant-1-: Chemicals or processes which work to destroy vegetative forms of microorganisms and other contaminants. Examples are chlorine, bromine, Soft-Swim, and ionizers; also included are copper and silver algaecides.



Disinfection: A process for the destruction of bacteria. The process may be physical, as with heat or ultraviolet irradiation, or chemical, as with chlorination.



Dissolved Solids (Total Dissolved Solids) - TDS: Includes colloidal and suspended particles at sizes far less than one micron in diameter. Associated with evaporation processes such as distillation.



Distance: The straight line separation between an object and a reference point.



Distillation: Vaporous state liquid is condensed on a cool surface, collected and stored. Most contaminants do not vaporize and therefore do not pass to the condensate. Removes nearly 100 percent of salts and those organics that do not have a vaporizing temperature near or below that of the distilled liquid. Usually combined with carbon filtration to remove balance of remaining organics with vaporization temperatures below that of water.



Distilled Water: Water which has been purified by passing through an evaporation-condensation cycle; it generally contains less than 5 ppm of dissolved solids.



Distribution System: A network of pipes leading from a treatment plant to customers' plumbing systems.



Distributor (Top or Bottom): The device in a filter designed to divert the incoming water to prevent erosion of the filter media.



Diverter Valve: Used in a twin port skimmer, a diverter allows the operator to manipulate the amount of flow from the main drain and skimmer to the pump.



Diving Area: That area of a pool designed for diving. (NOTE: Diving Areas are defined in detail in various standards and regulations such as NSPI, Public and Residential Pool Standards, AAU, FINA, etc.).



Diving Board: A board especially designed to produce diver spring action when properly installed on an anchor (base) and fulcrum. (The term diving board includes non-spring types).



Diving Platform: Usually used for the standard 5-meter and 10-meter official diving platform.



Diving Tower: This term is usually used for the 3-meter (10-ft.) springboard support.



Dole Flow Valve: This is an excellent valve to use as there is nothing to turn or adjust. This means that when you put it in no one else can play with it. It helps to keep the pump set right where the professional puts it. Because of the nature of this valve it only allows a flow at a certain GPM.



Domestic water use: Water used for household purposes, such as drinking, food preparation, bathing, washing clothes, dishes, and dogs, flushing toilets, and watering lawns and gardens. About 85% of domestic water is delivered to homes by a public-supply facility, such as a county water department. About 15% of the Nation's population supply their own water, mainly from wells.



Double balanced seal: Hydraulically balanced in both directions. A desirable feature, but seldom provided by seal manufacturers.



Double Pipe: In a jet pump application means that there are two pipes going all the way down to the jet pump. This is a closed system. The industry standard has the suction pipe over the drive pipe.



Double Pipe or Single Pipe System - refers to the well size and how the well can be entered.



Double Pole Single Throw Circuit Breaker – the only safe method of connecting a 230VAC water pump. It consists of two separate circuit breakers within a housing and both are controlled by one single lever.



Double seal: An out dated term describing two seals in a pump. The latest terminology is "dual seals". In the past the term was used to describe a higher-pressure barrier fluid between dual seals.



Double suction pump: The liquid is channeled inside the pump casing to both sides of the impeller. This provides a very stable hydraulic performance because the hydraulic forces are balanced. The impeller sits in the middle of the shaft which is supported on each end by a bearing. Also the N.P.S.H.R. of this type of pump will be less than an equivalent end-suction pump. They are used in a wide variety of industries because of their reliabilty. Another important feature is that access to the impeller shaft and bearings is available by removing the top cover while all the piping can remain in place. This type of pump typically has a double volute.



Double suction pump-1-:  The rotor is suspended between two bearings with the fluid entering on either side of the impeller. Used at higher capacities.



Double volute pump: A pump where the immediate volute of the impeller is separated by a partition from the main body of the casing. This design reduces the radial load on the impeller making the pump run smoother and vibration free.



Double volute: A centrifugal pump design that incorporates two cut waters to prevent shaft deflection when the pump is operating off of the B.E.P. Lowers the efficiency of the pump and therefore seldom used on smaller size impellers.



Downthrust: The general direction of the thrust is toward the impeller eye. In sub turbines installed vertically this direction is down. Is the normal operating mode of the pump.



DPD: A method of testing for chlorine levels in the pool water. Unlike OTO, DPD testing allows determination of total and free available chlorine levels which through subtraction gives us combined levels.



Drain Plugs: Removable plugs used to drain water from the pump during periods of inactivity.



Drain: Also called the "main drain," this plumbing fitting is the start of one suction line to the pump and is usually situated at or near the center bottom of the pool.



Drain-1-: An outlet at the deep point of a vessel or trough through which waste water passes.



Drain-1-: Withdrawal of current from a cell.



Drainage basin: Land area where precipitation runs off into streams, rivers, lakes, and reservoirs. It is a land feature that can be identified by tracing a line along the highest elevations between two areas on a map, often a ridge. Large drainage basins, like the area that drains into the Mississippi River contain thousands of smaller drainage basins. Also called a "watershed."



Drawdown (tank): The amount of water that can be removed from a tank, when it is full, before the tank pressure drops and the pump is turned on to refill the tank. A larger tank gives more drawdown which means that more water is available before the pump is needed.



Drawdown (well): The distance the water level drops below the standing water level while the pump is operating.



Drawdown Level: The water level in a well measured while the pump is pumping at full capacity. Less than normal water level in the tank causes the pump to run more frequently and for shorter durations. More than normal water level in the tank causes the pump to run less frequently and for longer durations to replace the high quantity of water in the tank.



Draw-down: A lowering of the ground-water surface caused by pumping.



Drawdown: The difference between the static water level and the pumping water level in a well. Also can refer to the per cycle capacity rating of a pressure tank at a given pressure range. Occurs when the pump starts running.



Drip Cover: Umbrella type cover used to keep water out of motor



Drip irrigation: A common irrigation method where pipes or tubes filled with water slowly drip onto crops. Drip irrigation is a low-pressure method of irrigation and less water is lost to evaporation than high-pressure spray irrigation.



Drip-proof: Same as Open Drip-proof (ODP. Ventilation openings in bearing housings and some yokes placed so drops of liquid falling within an angle of 15° from vertical will not affect performance. Normally used indoors in fairly clean, dry locations.



Drive or Pressure Pipe: Usually the smaller of two pipes connecting a jet pump to the down-well venturi, it carries the water from the pump to the venturi.



Drive water: In a jet pump, the water which is supplied under pressure to drive the jet.



Drooping curve: Similar to the normal profile except at the low flow end where the head rises then drops as it gets to the shut-off head point.



Dry Acid: Sodium bisulfate, a granular form of acid used to lower pH and alkalinity in the water. It is safer and less caustic than muriatic acid. Usually available as a "pH decreaser."



Dry Cell: A primary cell in which the electrolyte is absorbed in a porous medium, or is otherwise restrained from flowing. Common practice limits the term "dry cell" to the Leclanché cell, which is the common commercial type. Dry Cell Battery: The most common type of cell or battery nowadays, where the electrolyte is either in non-liquid form (i.e., a paste or gel) or absorbed in a porous separator material. This allows the cell or battery to be sealed.



Dry Niche: A normal weatherproof fixture placed in an opening behind the pool wall which illuminates the pool through a watertight window in the pool wall.



Dry running: Running without fluid at the seal face.



Dual Purpose Pump: This versatile pond pump category is designed to operate submersed or in-line. (out of the water).



Dual Purpose: A pond pump designed to operate submersed or in-line.



Dual Purpose-1-: This versatile pump category is designed to operate either submersed or in-line.



Dual Seal: Two seals running in various configurations:  back to back, tandem, face to face, or concentric.



Dual Voltage Motors: A type of single or three phase motor that can be configured to operate on two or more voltage levels. Dual voltage motors allow the same motor to be used in many applications.



Ductile iron: Ductile iron has a composition similar to gray iron. Special treatment modifies metallurgical structure, which yields higher mechanical properties; some grades are heat-treated to improve ductility. Ductile iron has the strength properties of steel using similar casting techniques to that of gray iron.



Ductile Iron: Has composition similar to gray iron. Special treatment modifies metallurgical structure, which yields higher mechanical properties; some grades are heat-treated to improve ductility. Ductile iron has the strength properties of steel using similar casting techniques to that of gray iron.



Ductility: The property of a metal that allows a great deal of mechanical deformation without cracking.



Duplex System: A sewage or effluent system that uses two pumps to control the water level in the storage chamber via float switches and a control box.



Duty Cycle: Standard is continuous duty, suitable for 24 hour per day operation. Some special motors may be rated for intermittent use (15 min., 30 min., etc.).



Duty Point: The point on a performance curve that plots flow (GPM) and head (feet).



Dynamic Discharge Head: The static discharge head plus the additional discharge head created by friction or resistance (usually referred to as losses) from the liquid flowing through the hoses, fittings, sprinklers, nozzle, etc.



Dynamic discharge head-1-: The sum of the static discharge head and the discharge friction loss in the discharge line. Also referred to as Total Discharge Head.



Dynamic elastomer: The rubber part that has to move to move or flex to compensate for seal face wear or shaft movement.



Dynamic head (system head): The pump head created by friction in the piping system.



Dynamic Suction Head: The static suction head plus the additional suction head created by friction from the liquid flowing through the hoses, fittings, etc. Atmospheric pressure enables pumps to lift water. As a result, an atmospheric pressure of 14.7 psi at sea level limits practical dynamic suction head lift to less than approximately 26 feet for any pump.



Dynamic suction head: The sum of the static suction lift and the suction friction loss in the suction line. Also referred to as Total Suction Head.



Dynamometer: A device which places a load on the motor to accurately measure its output torque and speed by providing a calibrated dynamic load. Helpful in testing motors for nameplate information and an effective device in measuring efficiency.