We usually select a centrifugal pump for our project based on the following main parameters:
- Required Head (H)
- Required Flow Rate (Q)
- Fluid characteristics
The pump manufacturers mostly illustrate their pump characteristics by using a performance curve (H-Q curve). A centrifugal pump performance or characteristic curve shows the correlation between media Flow Rate (Q) and the Total Developed Head (TDH) created by the pump. A performance curve tells us how a pump imparts the kinetic energy to the pumping fluid. Since the definition of "Head" can sometimes be confusing, it's important to understand the meaning of the head in pumping systems when we come to the pump selection and work with performance curves.
What is Head?
"Head" in the unit of length (m, ft, etc.) refers to the energy per unit weight of the fluid. The distinction between weight and mass should not be forgotten.
Elevation Head
The Elevation Head shows the fluid potential energy per unit of weight, and it's the result of the gravity force acting on a column of fluid.
Static Head
The Static Head, with the physical dimension of length, is a term to denote the pressure in a pipe. Static head is equivalent to the vertical distance between the surface of the fluid and a given geodetic datum. For example, at point A in a tank, the static head is 10.2 meters and the pressure gauge shows 1 bar pressure (P) when the fluid is water (Static Head = P/ρg). But if we change the fluid to gasoline (with a specific gravity of 0.72), then the pressure gauge will show 0.72 bar.
Suction Static Head
The Suction Static Head is a function of the fluid level in the pump suction tank. If we install a pump above the fluid level, the suction static head is negative and we usually call it "Suction Lift". Since centrifugal pumps are not capable of pumping air, in a suction lift condition the pump should be primed before starting.
Discharge Static Head
Discharge Static Head is the vertical distance between the pump discharge point and the maximum level to which the pump should raise the fluid in the pumping system. We should take siphon effects into consideration when measuring or calculating discharge static head.
System Static Head or Total Static Head
System Static Head, or Total Static Head, or just simply Static Head, is a system characteristic and is equal to the difference between the fluid level at the suction and discharge points.
System Static Head = Discharge Static Head − Suction Static Head
The static head can be negative when the fluid level at the final discharge point is lower than the fluid level in the pump suction. In fact, when the static head is negative, we don't need a pump to transfer the fluid, since gravity energy moves the fluid through the pipes. However, to increase velocity and flow rate, we need a pump to overcome piping system resistance.
We can calculate the static head by reading the pressure at the pump's suction and discharge points when the pump is turned off. (Siphon effects should be taken into consideration to avoid overestimating the static head.)
Pressure Head or Pump Head
Pressure Head is a kind of fluid internal energy attributed to its pressure. Simply, we can read the pressure at any point of a pumping system by using a pressure gauge in bar, atm or psi. In fact, the gauge pressure shows the amount of force on the surface of the fluid container (P = F/A).
We can convert pressure to pressure head by using the following equation:
H = P / ρg (g is the acceleration of gravity and ρ is the density)
For example, for converting psi to ft, we may use the following equation:
P (psi) = 0.433 × H (ft) × SG
where SG is the dimensionless specific gravity of the pumping fluid.
Velocity Head
Velocity Head indicates the kinetic energy of the fluid and it is equal to v² / 2g, where v is the fluid velocity. We can calculate the velocity head at the suction and discharge of a pump by using a flow metering system.
Friction Head (Head-loss)
Friction Head shows the amount of energy lost to the environment as a result of the pipes and fittings interacting with the moving fluid. Head-loss or friction head is similar to the voltage drop in an electrical cable and shows the amount of energy dissipated in the piping system through friction, turbulence and heat. Friction Head is a function of fluid velocity in the piping system and it gets bigger with increasing flow rate.
System Head
System Head is the total of the system friction head (at a specific flow rate) and the system static head. The curve that shows the relation between system head and flow rate is the System Curve, and usually it has a parabolic shape.
Pump Total Developed Head (TDH)
The pump Total Developed Head is the total amount of energy given to the fluid by a pump, including kinetic and static energies. The pump performance curve shows the relation between TDH and flow rate of a pump (with a certain hydraulic characteristic). The point where the system curve meets the pump performance curve is the actual pump working point.
We can use the energy equation in a pumping system to calculate the TDH of a pump. The Bernoulli equation (head based) is:
Suction Pressure Head + Suction Velocity Head + Pump TDH = Elevation Head + Discharge Pressure Head + Discharge Velocity Head + Total Friction Head
which is:
P(in)/ρg + v(in)²/2g + TDH = Z + P(out)/ρg + v(out)²/2g + Hf
Shut-off Head
The TDH at zero flow rate is the Shut-off Head. We can run a centrifugal pump with a closed discharge valve (for a short period of time) and read the maximum created pressure at the pump discharge to calculate the pump shut-off head. Running centrifugal pumps near the shut-off head is not recommended and it can damage the pump over a short period of time.
When we talk about "Head", in fact we are talking about the energy transformation, transfer and waste in a pumping system.