Understanding Pump Head



Head-related terms

In dealing with pumps, someone is likely to come across different head-related
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
Net Positive Suction Head
Available
(NPSHA), and Net Positive Suction Head Required (NPSHR.) These
terms are discussed in a separate article on this web site.

What does head mean?

Head is a unit of pressure expressed as a column of liquid, in Feet, instead of the
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
head.

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
head ,is that a
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 head means, understanding other head-related terms
becomes easier.

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.

Velocity head - the head needed 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 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

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 -  the head due to difference in 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 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
elevation head.

Elevation head - same as static head.

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

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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