In the study of centrifugal pumps one will come across different head
parameters or terminology such as suction head, differential head, discharge
head, total dynamic head or TDH, velocity head, static or elevation head, shut-
off head, etc.

One will also come across the terms net positive suction head (NPSH), net
positive suction head available (NPSHA), net positive suction head required
(NPSHR and NPSHR3) - these terms are discussed in a separate article.


What is head?

Head is a unit of pressure converted to its equivalent column of liquid (in vertical
linear feet of that liquid) instead of gauge pressure in PSIG; that liquid column
exerts the same pressure in PSIG at the bottom of that liquid column. The height
of that liquid column, in feet, is referred to as its head.

[For simplicity, the U.S. customary unit of head, in feet, is used in this article.
The metric system uses meter as unit of head.]

The equation to convert gauge pressure to its equivalent head of liquid is:

H = [PSIG x 2.31] / SG

Where:

H = head, in feet
PSIG = gauge pressure, in PSIG
SG = specific gravity of liquid
2.31= unit conversion constant

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
head, is that a
pressure of 100 PSIG exerted at the bottom of a vertical pipe will push the
gasoline 289 feet above its starting level at the bottom of the pipe.


Why use head (in feet) instead of pressure (in PSIG)?

There are many reasons why it is preferable to express a pressure in a unit of
feet, 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 will always develop the same head regardless of the specific gravity of the
pumped liquid.

If the unit of gauge pressure (in PSIG) is used, instead of head (in Feet), then a
pump performance curve will change for every application where the liquid
specific gravity is different. It will be impractical and confusing to plot different
performance curves with varying pressures for that one pump.

In a system analysis where several factors have to be taken into consideration
such as the height of elevation, length of pipes, equivalent pipe lengths of
valves, fittings, friction loss, etc., the analysis is simplified because of the
consistent unit (Feet) being used. The equilibrium point of a pump performance
curve and a system-head curve when plotted against each other is easier to
visualize because both data are expressed in consistent unit of Feet.

Suction head - the suction pressure converted into head. If the suction
pressure is a negative number the term is sometimes referred to as suction lift,
instead of suction head.

Discharge head - the discharge pressure, measured by a pressure gauge,
converted into head.

Differential head - the differential pressure converted into head. It is equal to
the discharge head minus the suction head. Or, it is equal to the discharge head
plus the suction lift.

Shut-off head – the head measured at the discharge nozzle at zero flow or
when the discharge valve is fully close.

Velocity head - the head needed, or spent, to increase the velocity of the liquid
at the pump discharge nozzle over the velocity of the liquid at the suction nozzle.
Velocity head is present only if the discharge nozzle is smaller than the suction
nozzle; if the nozzles are of the same size the velocity head is zero.

Similar to differential head, if the pump changes speed, the velocity head varies
in direct proportion to the square of the ratio of speed change.

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 equation 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 is the difference in the flow area between the suction nozzle and discharge
nozzle, in square inches.
2.31 is a unit conversion factor

Example: What is the velocity head of a 4x6x11 single stage, horizontal, end
suction pump with a flow rate of 800 GPM?

Solution: 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 velocity head developed by the peripheral velocity of an impeller is
discussed in a separate article on this web site.)

Total dynamic head - the total dynamic pressure converted into head. It is the
sum of the differential head and velocity head.

Static head - head due to difference in vertical elevation between a reference
point (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 lift (instead of static
head.) The static head is numerically equal to the difference in elevation
expressed in Feet. Static head is also known as elevation head.

Elevation head - same as static head.

Friction head - the head loss due to friction, or to overcome friction, such as in
valves, pipes, fittings, and other surface areas in contact with the liquid (the
wetted areas.)


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Understanding the different head parameters
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