I. STANDARDS FOR MEASURING HEADS AND CAPACITY.
Head is measured in feet, pounds per square inch (PSI), or in inches of mercury. However, so that a common means of head measurement is understood, it is recommended that all heads be expressed in feet of water. Measurement of liquid should be expressed in U.S. gallons.

II. ATMOSPHERIC PRESSURE.
At sea level it is 14.7 PSI. This will maintain a column of mercury 29.9 inches or a column of water 33.9 ft. high. This is the theoretical height of which water may be lifted by suction. The practical limit for cold water (60 F) is 25 feet.

III. SUCTION AND DISCHARGE HEAD.
Static Suction Lift – Is the vertical distance from the center line of the pump’s suction inlet to the constant level of the water. This is added to discharge head to obtain total dynamic head.
Positive Suction Head – Is the vertical distance above the center line of the pump’s suction to the constant level of the water . This is subtracted from the discharge head to obtain total dynamic head.
Dynamic Suction Head – Is the suction lift (or head) plus suction line friction loss. May be positive or negative.
Static Discharge Elevation – Is the vertical distance from the pump’s discharge to the highest point in the discharge line.
TDH (Total Dynamic Head) – Is the total head and is the total of static suction lift (head), friction loss in suction line, static discharge elevation, friction loss in discharge line and fittings, plus discharge pressure, if any. To be hydraulically correct, we should not include “Static Head” in total dynamic head. Dynamic means “moving” and “Dynamic Head” only includes velocity head and friction loss. However, most pump people use TDH interchangeably with TH (Total Head).
Friction Head – Is the heat loss experienced by the movement of the liquid through the suction and discharge lines. Charts are available showing loss in feet of head at various flows through various pipe or hose sizes. Charts also show velocity in feet/sec, which is particularly important when pumping liquids with solids in suspension. Fittings, valves, etc. must be considered.

IV. NPSH.
Net Positive Suction Head is defined as head that causes liquid to flow through the suction line and enter the impeller eye. This head comes from either atmospheric pressure or from a static suction head plus atmospheric pressure. Two types of NPSH will be considered.
Required NPSH – Is a function of pump design. It varies between different makes, between different models, and with capacity of any one pump. This value is supplied by the manufacturer, if available. Refer to pump curves or contact the factory.
Available NPSH – Is a function of the system in which pumps operate. Can be calculated for any installation. For a pump to operate properly, available NPSH should be greater than the required NPSH, plus 2 feet for safety factor, at a desired head and capacity. In simple terms, available NPSH is calculated by deducting from barometric pressure, in feet, the static suction head ( + or-), friction loss, and the vapor pressure (ft.) of liquid being pumped. Velocity heads should also be deducted.
NPSH does not indicate the priming capabilities of self­priming centrifugal pumps. This capability is shown, generally on engine driven pumps, by respective “break­off” lines representing 10, 15, 20, 25′ static suction lifts.

V. USEFUL FACTORS OR FORMULAS.
a) Feet head x .433 = PSI (pounds per square inch).
b) PSI (water) x 2.31 = Ft. Head
c) Specific gravity of water (sp.gr.) = 1.0.
d) PSI (water) x 2.31/sp.gr. = Ft. Head
e) Weight of one U.S. gallon of water= 8.33 pounds
f) One cubit foot (cu.ft.) of water contains 7.48 gallons.
g) GPM = Gallons Per Minute.
h) Imperial gallon x 1.2 = U.S. gallon; U.S. GPM x .833 = Imp. GPM.
i) TDH = Total Head or total dynamic head.
j) WHP = Water Horsepower.
k) BHP = Brake Horsepower.
I) EFF = Pump Efficiency.
m) WHP = Ft .Head x GPM/3960
n) BHP = WHP/EFF or BHP = Ft. Head x GPM/3960 x EFF (Pump)
o) EFF = WHP/BHP x 100
p) For liquids having different specific gravity other than 1.0.
WHP = Ft. Head x GPM x sp.gr./3960
BPH = Ft. Head x GPM x sp.gr./3960 x EFF
BHP (for liquids other than water)
= BHP (for water) x sp.gr.

VI. EFFECT ON CENTRIFUGAL PUMPS ON CHANGE OF SPEED OR CHANGE OF IMPELLER DIAMETER.
Three rules govern the operation of centrifugal pumps: a) Capacity varies directly with changes of speed or of the impeller diameter.
GPM1/GPM2 = RPM1/RPM2
or GPM1/GPM2 = Dia.1/Dia.2
GPM2 = GPM1/RPMlxRPM2
and GPM2 = GPMl/Dia.lxDia.2
b) Head varies as the square of the speed or the impeller diameter.
Headl/Head2= RPM12/RPM22
or Headl/Head2 = Dia.12/Dia.22
Hd2 = Hdl/RPM12/RPM22
and Hd2 = Hdl/Dia.12 /Dia.2

c) Power (BHP) varies as the cube of the speed or impeller diameter
BHP1/BHP2 = RPM13/RPM13
or BHPl = Dia13/Dia23
BHP2 = BHP13/RPM13xRPM23
and BPH2= BHP13/Dia.13xDia23

VII. EFFECT OF ALTITUDE ON PUMPS
At elevations above sea level, suction lift should be reduced accordingly to insure that the same amount of water can get into the pump as would occur at an equivalent sea level lift. Lower atmospheric pressure reduces horsepower output of gas engines, thus
causing a drop in speed which reduces pump performance. Enginepower will decrease 3.5% for each 1000 ft. above sea level and 1 % for each 10° F above standard temperature at 60° F.

VIII. GUIDELINES FOR PUMPING WARM WATER
MAXIMUM PRACTICAL DYNAMIC SUCTION LIFTAND VAPOR PRESSURE

IX. EFFECT OF SPECIFIC GRAVITY
The specific gravity of a substance is the ratio of the weight of a given volume to the weight of an equal volume of water at standard conditions.

1. A centrifugal pump will alway s develop the same head in feet no matter what the specific gravity of the liquid pumped; however, the pressure (in pounds per square inch) will be increased or decreased in direct proportion to the specific gravity.
2. The brake horsepower (BHP) of a pump varies directly with specific gravity. If the liquid has a specific gravity other than water (1.0), multiply the BHP for water by the sp.gr. of liquid handled.

X. VISCOSITY
The viscosity of a fluid is the internal friction or resistance to motion of its particles. The coefficient of viscosity of a fluid is the measure of its resistance to flow. Fluids having a high viscosity are sluggish in flow, for example: heavy oil or molasses. Liquids such as water or gasoline have relatively low viscosity and flow readily. Viscosity is a fluid property independent of specific gravity. Viscosities vary with temperature; as temperature increases, viscosity decreases. Pressure changes have negligible influence on viscosity. There are many types of viscometers and expressed in many terms. Commonly used is SSU (Seconds Saybolt Universal). This is actually the time in seconds required for a given quantity of fluid to pass through a standard orifice under standard conditions. Viscous liquids tend to reduce the capacity, head, and efficiency, and increase the BHP.
Kinematic Viscosity (in Centistokes)
= Absolute Viscosity (in centipoise)/Specific Gravity
Centistrokes = SSU/4.64
This is an approximation for Viscosities greater than 250 S.S.U.The approximated performance for pumping fluids more viscous than water is determined from the following formula:
BHPvis = Qvis X Hvis X S.G./3960/Evis

HOW CENTRIFUGAL PUMPS WORK
Liquid is supplied to the inlet at the center of the pump head. Since centrifugal pumps are not self-priming, liquid must be supplied by gravity feed or the pump must be primed. The spinning impeller propels the liquid outward by centrifugal force, providing the motive force required to move the liquid. The specially shaped volute receives the liquid and converts the radial motion to a smooth pulseless flow. Easily adjust the flow rate by restricting the flow at the outlet.

CENTRIFUGAL PUMP TERMS
IMPELLER – A rotating vaned disck that provides the pumping force.
VOLUTE – The body of the pump that is shaped to receive liquid from the inlet and direct it through the outlet.