HOME > Mechanicals

"Head" is not the same as pressure. Head is measure in units of feet while pressure is measured in pounds per square inch, and is independent of pressure or liquid density.

The pressure at any point in a liquid can be thought of as being a vertical column of the liquid. The height of this column is called the static head and is expressed in terms of feet of liquid.

In a vertical column of water, the weight of the water at any given height determines the amount of pressure it exerts in any direction. Static head is defined as the height of this column expressed in terms of feet of liquid.

 
The principle of pressure head can be demonstrated with a drinking straw inserted into a glass of water.

The water level in the straw will be the same as the water the level in the glass. If air is blown into the straw, pressure builds in the straw and pushes the water out.

The pressure created in the straw is equal to the head pressure of the water in the glass. It is a simple matter to convert the created pressure to a desired unit of measure from wherever the water level may be in the glass.

Hydrostatic head
The fluid height in columns A and B is identical, but the pressure reading on the gauges differs because of the different fluid densities. Hydrostatic head refers to the vertical column height; hydrostatic pressure refers to the force exerted by the fluid.

"Head" is also used to measure the kinetic energy created by a pump. If a pump were to force a stream of water straight up into the air, the height the pump was able to achieve would be the head. A given pump with a given impeller diameter and speed will raise a liquid to a specific height regardless of the weight of the liquid.

Head, rather than pressure, is used to measure a pump's power because the pressure a pump creates will change if the specific gravity (weight) of the liquid changes, but the head will not change. Since a pump can move a lot of different fluids having different specific gravities, it is easier to refer to the pump's head and disregard pressure.

Liquids have specific gravities typically ranging from 0.5 to 1.8. Water is the benchmark with a specific gravity of 1.0. Motors are rated assuming this benchmark and the resultant calculations are considered to be in "feet absolute."

Head (ft) = Pressure (PSI) X 2.31 Specific Gravity

The various categories of head are listed below. (Note: The Subscripts 's' refers to suction conditions and 'd' refers to discharge conditions.)

Static Suction Head (hS): This refers to the level of the liquid relative to the level of the pump. When the liquid level is above pump centerline this number will be positive, and when it is below pump centerline this will be negative. A negative hS condition is commonly called a "suction lift" condition.

Static Discharge Head (hd): This refers to the vertical distance in feet between the level of the pump and the level of the liquid in the discharge tank.

Friction Head (hfs) or (hfd): This refers to the resistance in the pipe and fittings. It varies with the size, condition and type of pipe, the number and type of fittings, the flow rate, and type of liquid.

Vapor Pressure Head (hvp): This is the pressure level at which a liquid will vaporize at a given temperature. This pressure rises as the temperature of the liquid ruses, which also effectively reduces suction pressure head.

Pressure Head (hps) or (hpd): This refers to the pressure on the liquid in the reservoir feeding the pump. It occurs when a pumping system operates in a pressurized tank. In an open system, this is atmospheric pressure.

Velocity Head (hvs) or (hvd): This measures pressure developed by liquid moving at a selected velocity 'v'. It represents the height from which the liquid would have to fall to achieve the same velocity. It can be a significant factor in low head systems but may be ignored in most high head systems.

Total Suction Head (HS): This reflects the reading a pressure gauge would show if it were mounted at the pump inlet. It is based on the following formula: Suction Pressure head (hpS) plus Static Suction head (hS) plus the Velocity head at the inlet (hVS) minus the Friction head in the suction line (hfS).

HS = hpS + hS + hvS - hfS

Total Discharge Head (Hd): This reflects the reading a pressure gauge would show if it were mounted at the pump outlet. It is based on the following formula: Discharge Pressure head (hpd) plus Static Discharge head (hd) plus the Velocity head at the outlet (hvd) plus the Friction head in the discharge line (hfd).

Hd = hpd + hd + hvd + hfd

Total Differential Head (HT): This is the difference between the total discharge head and the total suction head:

HT = Hd + HS (with a suction lift)
HT = Hd - HS (with a suction head)

Net Positive Suction Head (NPSH) is the total head at the pump inlet less the vapor pressure. Pumps cannot pump vapors, only liquids. If the external pressure drops or if the operating temperature rises, vaporization may occur and the pump will stop pumping. Therefore, the two most critical measurements of head are NPSHr and NPSHa.

NPSH required is the pressure required at the inlet to overcome internal pressure drops and keep the liquid above its vapor pressure. NPSHr is always positive since it is expressed in terms of absolute fluid column height. It varies with speed and capacity of the pump. It increases as the velocity of the liquid increases, but it is independent of the fluid density. NPSHr is based on actual testing by the manufacturer.

NPSH available is the excess pressure of the liquid at the inlet over its vapor pressure. NPSHa is defined in the formula below:

NPSHas = hps + hs - hvps - hfs

(hps) = Pressure Head, or Barometric Pressure of the suction vessel.
(hs) = Static suction Head, or the distance between the first stage impeller and the liquid level.
(hvps) = Vapor Pressure of the liquid at maximum pumping temperature.
(hfs) = Friction Head, or the friction losses on the suction side.

NPSHa can also be approximated by a gauge at the inlet using the formula:

NPSHa = hpS - hvpS ± hgS + hvS

Where:
(hpS) = Pressure head or Barometric pressure of the suction vessel.
(hvpS) = Vapor pressure of the liquid at maximum pumping temperature.
(hgS) = Gauge reading at the pump corrected to the pump centerline.
(hvS) = Velocity head in the suction pipe at the gauge.

NPSH available must always be greater than NPSH required for the pump to operate properly. It is normal practice to have at least 2 to 3 feet of extra NPSH available at the suction flange to avoid any problems at the duty point.

See Also:
All About Pumps
All About Water


 BACK

Was this article helpful?   Yes    No