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Halogen: Elements that attack carbon. Fluorine, bromine, chlorine, astatine and iodine.

Handhold/Handrail: A permanently installed device that can be gripped by a bather for the purpose of resting and/or steadying him/herself. Is not limited to but may be located within or without the pool, spa or hot tub or as part of a set of steps or deck-installed equipment.

Hard face: A seal face either rotating or stationary. The most common materials are silicone carbide, ceramic, tungsten carbide, Stellite, Ni-resist. The hard face must be the wider seal face.

Hard Water: That water which is high in calcium hardness and other salts which, as such, resists soap being lathered.

Hardness: A water-quality indication of the concentration of alkaline salts in water, mainly calcium and magnesium. If the water you use is "hard" then more soap, detergent or shampoo is necessary to raise a lather.

Hardness-1-: The amount of calcium and magnesium in the water in grains per gallon, (expressed as calcium carbonate).

Harmonic vibration: Vibrating in harmony with something near by. This can be a big problem for bearings in stationary or non- running equipment.

Hastelloy "C": A nickel rich, corrosion resistant metal used for mechanical seal springs and metal bellows because it is not sensitive to chloride stress corrosion.

HASTELLOY C®-1-:  A high nickel-chromium molybdenum alloy, which has outstanding resistance to a wide variety of chemical process environments, including strong oxidizers such as wet chlorine, chlorine gas, and ferric chloride. Hastelloy C is also resistant to nitric, hydrochloric, and sulfuric acids at moderate temperatures. HASTELLOY C is a registered trademark of Haynes International.

Hazardous Material: Any volatile, explosive or flammable liquid that requires special handling and should not be used with a dewatering pump.

Hazen-Williams equation: This equation is now rarely used but has been much used in the past and does yield good results although it has many limitations, one being that it does not consider viscosity. It therefore can only be applied to fluids with a similar viscosity to water at 60F. It has been replaced by the Darcy-Weisbach and the Colebrook equation. Interestingly the NFPA (National Fire Protection Association) mandates that the Hazen-Williams equation be used to do the friction calculations on sprinkler systems for example.

Hazmat: A hazardous material, a term used almost exclusively in the United States, is any solid, liquid, or gas that can harm people, other living organisms, property, or the environment. Hazmats may be radioactive, flammable, explosive, toxic, corrosive, biohazardous, an oxidizer, a pathogen, an allergen, or may have other characteristics that render it hazardous in specific circumstances.

Head water: The source and upper reaches of a stream; also the upper reaches of a reservoir. (2) the water upstream from a structure or point on a stream. (3) the small streams that come together to form a river. Also may be thought of as any and all parts of a river basin except the mainstream river and main tributaries.

Head (Marine Terms): A marine toilet. Also the upper corner of a triangular sail.

Head: Another measure of pressure, expressed in feet.  Indicates the height of a column of water being lifted by the pump, neglecting friction losses in piping. If pump is submerged, the head can be measured from the surface of the water. Head ( Ft. of Water) = 2.31 PSI. Example: 10 feet equals 4.32 PSI.

Head-1-: Refers to the height of a column of water that can be supported by the pressure or vacuum exerted at the pump.

Head-2-: short for "Feet Of Head", a term used to define water pressure in vertical feet. Or a term representing the energy content of a liquid, expressed as the height of an equivalent vertical column of water. a. This is expressed in feet of head. b. This can apply to both sides of a pump; the suction side and discharge side. c. It is also used to express the total losses in a pumping system (Total Dynamic Head). Used interchangeably with PSI (pounds per square inch). Head is a result of what happens when we start the pump and create a flow.

Head-3-:  Refers to the pressure produced by a vertical column of fluid expressed in feet.  A measure of the energy possessed by water at a given location in the water system expressed in feet; or a measure of the pressure or force exerted by water expressed in feet. Indicates the height of a column of water being moved by the pump (without friction losses). It can be measured in lbs. per square inch (PSI) or feet of water.

Head-4-: The equivalent height of the liquid. 20° C. Water is used, as the standard where 10 meters (33.9 ft.) of water equals one atmosphere (14.7 psi. or 1 bar). The term head is used instead of pressure in the centrifugal pump business.

Head-to-head: When sprinklers are spaced so that the spray from one sprinkler reaches the adjacent sprinkler head, this assures complete coverage.

Health Advisory: An EPA document that provides guidance and information on contaminants that can affect human health and that may occur in drinking water, but which EPA does not currently regulate in drinking water.

Heat Exchanger: A set of 8 or 10 ribbed copper tubes that absorb the heat produced below it and transfer it to the water cycling through its tubes.

Heat Loss: The heat lost by a system (i.e. the heat lost due to friction).

Heat Pump: The antithesis of the air conditioner, the heat pump's cooling coil removes heat from the air while the condenser coil transfers it to water cycling through it.

Heat transfer: the heat lost or gained by a system. This book has not considered the application of equipment that produce a significant change in the fluid temperature.

Heater: A device to automatically heat pool or spa water to the desired temperature and maintain it there. A heater typically has automatic thermostat controls, and may be controlled by a time clock.

Heavy Metals: Metallic elements with high atomic weights, such as mercury, chromium, cadmium, arsenic, and lead. Even at low levels these metals can damage living things. They do not break down or decompose and tend to build up in plants, animals, and people causing health concerns.

Hermetic: Completely sealed, especially against the escape or entry of air.

Hertz: One cycle of AC electric flow.

Hertz-1-: A unit of measurement indicating a single cycle in an alternating current. Same as Cycle.

High Head (high-pressure) Pump: Capable of handling flows at significantly higher total dynamic head ratings (TDH). They utilize a closed design impeller and a compact volute called a diffuser to generate the high discharge pressure needed and cannot handle large solids.

High Static Water Level: The ground water elevation without any influence from pumping.

Hi-Rate Permanent Media Filter: A swimming pool water filter capable of water flows of up to 20 GPM for 10 hours. Offer the best combination of economy, performance, durability, and ease of maintenance.

H-O-A (Hand-Off-Auto): A toggle switch which allows you to choose how you want the pump to run. Hand means you turn it on and off. Off means nothing will happen because the pump(s) are turned off. Auto means that the float switches are being used to turn the pump(s) on and off.

Homogeneous Membranes: See membranes.

Horizontal Run: The horizontal distance between the point where fluid enters a pipe and the point at which it leaves.

Horse power: 33,000 foot pounds per minute. Common methods of measuring work.

Horsepower -1-:  A unit of measurement that indicates the amount of electrical power in larger devices. Horsepower is used instead of watts to describe many electric motors.

Horsepower-2-: 'One horsepower' is defined as the ability to move 33,000 pounds one foot in one minute (or to move 550 pounds one foot in one second). It is a measure of work performed.

Horsepower-3-: The output power rating of the motor shown on the nameplate. One horsepower is equivalent to lifting 33,000 pounds to a height of one foot in one minute. Exactly 746 watts of electrical power will produce 1 HP if a motor could operate at 100% efficiency, but of course no motor is 100% efficient. A 1 HP motor operating at 84% efficiency will have a total watt consumption of 888 watts. This amounts to 746 watts of usable power and 142 watts loss due to heat, friction, etc. (888 x .84 = 746 = 1 HP).

Hose Connector: The fitting used to connect the hose to the wall or pump fitting (usually a combination hose sleeve and nut).

Hose length (or pipe): The suction and discharge hose or pipe lengths required for a given application. Longer hoses increase friction loss, thereby reducing pump performance. Therefore, hose lengths should be kept as short as possible.

Hot Tub: A spa constructed of wood with sides and bottom formed separately; and the whole shapes to join together by pressure from the surrounding hoops, bands or rods; as distinct from spa units formed from plastic, concrete, metal or other materials.

Hot Tub-1-: Usually considered a circular, wooden vessel filled with heated and circulated water.

HP: Horsepower (hp) is the name of several non-metric units of power. The most occurring conversion of horsepower to watt goes 1 horsepower = 745.7 watts.

Hydraulic balance: A method of reducing mechanical seal face loading by reducing the seal face closing area.

Hydraulic force: Occurs any time pressure acts on a seal face area. Force times distance divided by time is a measurement of work done.

Hydraulic gradient: All the energy terms of the system (for example velocity head and piping and fitting friction loss) are converted to head and graphed above an elevation drawing of the installation. It helps to visualize where all the energy terms are located and ensure that nothing is missed.

Hydraulic Shock: A damaging condition that occurs when a column of liquid changes direction quickly and increases in velocity. Also called Water Hammer. Weakest point in system will break. Causes can be sudden loss of power to the driver, valve closing too quickly, valve closing too slowly allowing backflow.

Hydrocarbon: A petroleum product consisting of hydrogen and carbon.

Hydrodynamic seal: Special geometric features on the seal face that provide lift by taking advantage of the rotation of one seal face upon the other.

Hydroelectric power water use: The use of water in the generation of electricity at plants where the turbine generators are driven by falling water.

Hydrogen Sulfide: A toxic gas that is detectable by a strong "rotten egg" odor. Associated with high levels of bacterial decay. Commonly found together with iron and manganese contaminants.

Hydrojets: A fitting that bleeds air and water creating a high velocity, turbulent stream of air enriched water.

Hydrologic cycle: The cyclic transfer of water vapor from the Earth's surface via evapotranspiration into the atmosphere, from the atmosphere via precipitation back to earth, and through runoff into streams, rivers, and lakes, and ultimately into the oceans.

Hydrologic Cycle-1-: The term used to describe how water travels through the environment by evaporation, condensation and precipitation. Identical process is observed in steam distillation systems.

Hydrolysis: A chemical process resulting from reactions with water; frequently used in reference to the breakdown of polymers.

Hydronic Heating System: Heating systems that use hot water from a boiler or hot water heater as the main source of heat. Radiant loops are gaining popularity as the medium for heat transfer with hydronic heating systems.

Hydrophilic: Pertaining to a substance, which readily absorbs water ("water-loving").

Hydrophobic: Pertaining to a substance, which does not readily absorb water ("water-hating").

Hydrostatic Pressure Valve: Fitting(s) installed in the floor of the pool designed to manually or automatically release hydrostatic pressure beneath the pool by allowing ground water into the pool.

Hydrostatic Pressure: A force involving built up ground water which creates upward pressure beneath the pool shell.

Hydrostatic seal-1-:  Maintains a controlled gap between the seal faces by balancing the open and closing forces. There is a small amount of leakage across the faces when the shaft is rotating. Used in some compressor applications, but not very practical for the chemicals found in the process industry.

Hydrotherapy Inlet Fitting: A special high velocity air entraining inlet fitting to produce a massage effect.

Hydrotherapy Spa or Hot Tub: A unit that may have a therapeutic use which is not drained, cleaned or refilled for each individual. It may include, but not be limited to, hydrojet circulation, hot water and cold water mineral baths, air induction bubbles or any combination thereof. Industry terminology for a spa includes, but is not limited to, “therapeutic pool,” “hydrotherapy pool,” “whirlpool,” “hot spa,” etc.

Hydroxides: The most common name for the diatomic anion OH−, consisting of oxygen and hydrogen atoms, usually derived from the dissociation of a base. It is one of the simplest diatomic ions known.

HYPALON® (CSM):  Hypalon has very good resistance to oxidation, ozone, and good flame resistance. It is similar to neoprene except with improved acid resistance where it will resist such oxidizing acids as nitric, hydrofluoric, and sulfuric acid. Abrasion resistance of Hypalon is excellent, about the equivalent of nitriles. Oil and solvent resistance is somewhat between that of neoprene and nitrile. Salts have little if any effect on Hypalon. Hypalon is not recommended for exposure to concentrated oxidizing acids, esters, ketones, chlorinated, aromatic and nitro hydrocarbons. Hypalon has a normal temperature range of -20°F to 200°F. HYPALON is a registered trademark of the DuPont Company.

Hypochlorinator: A device used to feed, control and measure a solution of sodium or calcium hypochlorite into a water being treated. There are three general types: The positive displacement type which is usually a motor driven unit, the aspirator type actuated by a pressure differential created within the hydraulic system, and the metering type connected to the pump suction using an orifice which is opened and closed by a timing mechanism.

Hypochlorite, Calcium: A compound of chlorine and calcium used in powder or granulated form usually containing 70% to 80% available chlorine by weight which is released in water solution to act as a germicide or algaecide.

Hypochlorite, Sodium: A compound usually containing 5% to 16%, or more, available chlorine by weight, in a caustic soda solution, which releases chlorine when added to pool water.

Hypochlorite: A family of chlorine compounds such as Calcium Hypochlorite and Lithium Hypochlorite, both granular, and the liquid Sodium Hypochlorite. When these compounds contact water, they release Hypochlorous Acid, the active sanitizing agent.

Hysteresis: The delay or lag that causes seal faces to open.