terminology such as suction head, differential head, discharge head, total

dynamic head, velocity head, static head, etc.

Someone is also likely to come across the terms

Available

terms are discussed in a separate article on this web site.

usual unit of pounds per square inch gauge(PSIG). A pressure in PSIG is simply

converted to its equivalent column of liquid. That column of liquid develops the

same pressure in PSIG at the bottom of that liquid column. The length of that

column, in Feet, is referred to as the

The equation for converting a pressure to an equivalent head of liquid:

H = [PSIG x 2.31] / SG

where H = head, in feet

PSI = pressure, in PSIG

SG = specific gravity of liquid

2.31 = conversion factor

Example: How much head, or column of gasoline, will a pressure of 100 PSIG

produce if the gasoline has a specific gravity of 0.80?

H = [100 x 2.31] / 0.80 = 289 Feet

Another way of looking at this example, in understanding the term

pressure of 100 PSI acting at the bottom of a vertical pipe containing gasoline, will

push the gasoline 289 Feet above its original level inside that pipe.

There are many reasons why it is preferable to express a pressure in terms of its

equivalent head in Feet, or column of liquid, rather than in PSIG:

- A pump performance curve is usually plotted based on its flow rate in

gallons per minute (GPM), and its head in Feet. The unit of head is used

because a pump always develop the same head regardless of the specific

gravity of the liquid.

If the unit of pressure in PSIG is used, instead of the head (in Feet), then

the pump performance curve will change every time the liquid specific

gravity changes. It is impractical and confusing to plot different

performance curves with varying pressures for exactly the same pump.

- In a system analysis where several factors have to be taken into

consideration such as height of elevation, length of pipes, equivalent pipe

lengths of fittings, friction loss,etc., the analysis is simplified because of

the consistent unit (Feet) being used.

Now that we know what

becomes easier.

is a negative number the term is sometimes referred to as

suction head.

converted into head.

discharge head minus the suction head. Or, it is equal to the discharge head plus

the

discharge nozzle over the velocity of the liquid at the suction nozzle. Velocity head

is present only is the discharge nozzle is smaller than the suction nozzle. If the

nozzles are of the same size then the velocity head is zero. The velocity head is

calculated from the equation:

Hv = V^2 / 2g

where Hv = velocity head, in Feet

V = liquid velocity, in Feet per second

g = acceleration due to gravity, in Feet/sec^2

The formula for calculating the increase in liquid velocity is:

V = [Q x 0.321] / [A2 - A1]

where Hv = velocity head in Feet

Q = flow rate in gallons per minute

A = the difference in the flow area between the suction nozzle and

discharge nozzle, in square inches.

2.31 is a unit conversion factor

suction pump with a flow rate of 800 GPM?

In the universal pump designation consisting of three groups of figures, the first

figure refers to the discharge nozzle size, the second figure refers to the suction

nozzle size, and the third figure refers to the nominal impeller diameter size.

First, calculate the change in liquid velocity:

V = [800 x 0.321] / [A6 - A4]

V = [800 x0.321] / [28.274 - 12.566] = 4 Feet

Next, calculate the velocity head:

H = V^2 / 2g

H = [4]^2 / [2x32.2] = 0.248 feet

(More on

discussed in a separate article on this web site.)

sum of the differential head and velocity head.

(such as the top of a tank) and a datum point (such as the centerline of an

impeller.) If the reference point is above the datum point the static head is a

positive number. If the reference point is below the datum point the static head is

a negative number, and is sometimes referred to as suction life (instead of static

head.) The static head is numerically equal to the difference in elevation

expressed in Feet. Static head is also known as

pipes, fittings, and other surface areas in contact with the liquid (sometimes

referred to as the wetted areas.)

Pump basics

Types of pumps

Horizontal vs. Vertical pump

Axial split vs. Radial split case pump

Parallel-series operation

Self-priming pump

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