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?
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 = ^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.)
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