Velocity Head

What is velocity head?

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.

Velocity head is the head developed by a liquid due to its velocity. If the liquid
velocity is reduced, part of the internal energy of the liquid is released and may be
harnessed as additional head. If the liquid velocity is increased the velocity head
is reduced as part of the internal energy of the liquid is used to accelerate the
liquid.


The formula for calculating  velocity head is:

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

One practical application of this formula is in estimating the head that an impeller
will develop.

Example: How much head will a 15" diameter impeller develop in a pump
operating at 1780 RPM?

Solution: The peripheral velocity of a 15" diameter impeller is:

V = [D x RPM] /229

where v -s the peripheral velocity









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.)


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:
Related topics:

Pump basics
Types of pumps
Horizontal vs. Vertical pump
Axial split vs. Radial split case pump
Parallel-series operation
Self-priming pump
Q  -  What is velocity head?

A  -  Velocity head is that portion of a pump's developed head caused by the
increase in the liquid discharge velocity due to the difference in the pump's
suction and discharge nozzle sizes.



Q -  When do you add, or subtract,  velocity head in calculating the total head of
a pump?

A  -  The velocity head is added in the calculation only when the inside diameter
(ID) of the discharge nozzle is smaller than the ID of the suction nozzle. (The
velocity head is zero when the ID of nozzles are of the same.) Since the suction
nozzle is never smaller than the discharge nozzle, then the velocity head is never
subtracted in calculating the total head of a pump.

The velocity head (Hv), in FT, is calculated from:

Hv  =  [V^2]  / [ 2g]

where g  =  32.2 (gravitational acceleration), and

V  =  [Q x  0.321]  /  [A]

where Q is the capacity in GPM, and A is the difference in flow area, in square
inches, between  the  suction and discharge nozzles.
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