Head and Shut-off head of the centrifugal pump

What is "head"?

Head is a concept in fluid dynamics that relates the energy in an incompressible fluid to the height of an equivalent static column of that fluid. According to Bernoulli's principle, the total energy at a given point in a fluid is the energy associated with the fluid's movement plus energy from the fluid's static pressure plus energy from the fluid's height relative to an arbitrary datum. Head is expressed in distance units such as metres or feet. In a gravitational field, the force per unit volume on a fluid is equal to g, where is the density of the fluid and g is the gravitational acceleration. On Earth, each additional foot of fresh water column height adds a static pressure of about 9.8 kPa per metre (0.098 bar/m) or 0.433 psi per foot of water column height.

A pump's static head is the maximum height (pressure) that it can deliver. The Q-H curve of the pump can be used to determine its capability at a given RPM (flow vs. height).

A common misconception is that the head equals the fluid's energy per unit weight, when in fact, the term with pressure does not represent any type of energy (in the Bernoulli equation for an incompressible fluid this term represents work of pressure forces). Because centrifugal pumps' pumping characteristics are generally independent of fluid density, head is useful in specifying them.

There are generally four types of "head":

?1.Velocity head is due to the bulk motion of a fluid (kinetic energy):

hv = 1/2 v^2/g

2.Elevation head is due to the fluid's weight, the gravitational force acting on a column of fluid. The elevation head is simply the elevation (h) of the fluid above an arbitrarily designated zero point:

he = [rho x g x h] / [rho x g]

3.Pressure head is due to the static pressure, the internal molecular motion of a fluid that exerts a force on its container. It is equal to the pressure divided by the force/volume of the fluid in a gravitational field:

hp = p/rho x g

4. Resistance head (or friction head or Head Loss) is due to the frictional forces acting against a fluid's motion by the container. In a piped system head losses are described by the Hagen–Poiseuille equation and the Bernoulli Equation.

Pump head

The head of a pump is the maximum height that it can reach when pumping against gravity. Intuitively, a pump with higher pressure can pump water higher and produce a higher head. Also, because of the head exerted by the liquid in the suction tank, the higher the liquid in the tank, the higher the pump will be able to pump the water into the vertical discharge pipe.

Total head

The difference between the liquid level in the suction tank and the head in the vertical discharge pipe is a much more useful measure of the head. This is known as the "total head" that the pump can generate.

Increasing the level of liquid in the suction tank causes an increase in the head while decreasing the level causes a decrease in the head. Pump manufacturers and suppliers frequently refuse to tell you how much head a pump can generate because they cannot predict the height of the liquid in your suction tank. They will instead report the total head of the pump, which is the difference in height between the level of liquid in the suction tank and the height of a column of water that the pump can achieve. The total head is independent of the liquid level in the suction tank.

The total head Ht is calculated as Ht = Hd - Hs.

The discharge head is denoted by Hd, and the suction head is denoted by Hs. Also, keep in mind that this equation applies whether the suction head is positive (the level of liquid in the suction tank is higher than the pump) or negative (level of the liquid in the suction tank is below the pump). The pump will still produce the same total head if the level is negative, but because the suction head is negative, the discharge head will be reduced by this amount, according to our equation.

Shut- off "head"

In the image below, a pump moves liquid from the suction tank into a vertical pipe, where it rises until it can't overcome gravity and stops rising. The flow of the pump is zero in this case. The pump is running, but gravity causes the water in the vertical discharge pipe to rise and the net flow to stop. This is referred to as the "shut-off head," and it is the amount of head that a pump can produce at zero flow.

Why shut-off head is important?

To select the appropriate pump, you must first determine the total head and the required flow rate. These two quantities are, as one might expect, related. At a flow rate of zero, the maximum head (shut-off head) is achieved. As the liquid travels through the pipes from the suction tank to the pump and from the pump to the discharge pipe, the flow rate increases, introducing friction into the system. This friction reduces the total head that the pump can generate. In fact, as the flow increases, so does the friction, and the total head decreases. The amount of head lost due to friction is referred to as "friction head" or "friction loss."

Why is ''head', used to determine a pump's ability to pump liquids?

Many pumps were historically used to pump water uphill to a higher level, such as into a storage tank at the top of a hill. If you have to pump water to a height of 60 metres to get it up the hill, using head (measured in metres) makes sense. If a pump does not have 60 metres of head, you know it is not suitable for your application. Another reason for using that head is that as long as the liquid being pumped has a viscosity similar to water, the head will be identical for different liquids.

When using pressure to define a pump's characteristics, this may or may not be the case. Although some pump manufacturers use pressure to describe their products, the vast majority of pumps are still defined by the total head they produce.

References:

Wikipedia

Global pumps