Pump curves refer to plots of total head $H^*$, of the power tranferred to the fluid $N^*$ and the efficiency of the pump $\eta$ versus the flow rate $Q^*$.
For reference on definitions of $H^*$, $N^*$, $\eta$ and $Q^*$ please check on the post: Theoretical foundations for pump mesurements and estimations.
You need to measure the pump impeller diameter or at least have a reasonable estimation of it. Another way of doing it is by checking on the pump manufacturer datasheet. The fluid properties will also be required.
Calculation of $H^*$
For this you may refer to the following equation,
$H=\dfrac{p_D-p_A}{\gamma}+\left(z_D-z_A\right)+\dfrac{Q^2}{2g}\left( \dfrac{1}{A_D^2} -\dfrac{1}{A_A^2}\right)$ Eq. (01)
where subscripts $A$ and $D$ stand for admision (or suction) line and discharge line, respectively. $z$ can be measured as the vertical distance between the two points of pressure measurements.
Once, $H$ has been calculated its dimensionless form can be also found from,
$H^*=\dfrac{Hg}{n^2D^2}$ Eq. (02)
Calulation of the flow rate $Q^*$
The dimensionless flow rate can be estimated from,
$Q^*=\dfrac{Q}{n^2D^3}$ Eq. (03)
Calculation of the power transferred to the fluid $N^*$
This parameter can be estimated from,
$N_H=\gamma Q H_P$ Eq. (04)
where $\gamma$ is a property of the fluid. The dimensionless form of $N_H$ is,
$N_H^*=\dfrac{N}{n^3D^5}$ Eq. (05)
Calculation of the motor real power $N$
The power consumed by the electric motor is estimated from,
$N_M=\dfrac{\pi}{30}FRn$ Eq. (06)
where $r$ is the shaft length.
Calculation of the pump efficiency $\eta$
In this case, the formula is simply,
$\eta=\dfrac{N_H}{N_M}$ Eq. (07)
Curves construction
For simplicity, the data is better collected in a sheet so that all calculations can be automated. For data measurement proceed as follows,
- Start the pump
- Set the pump to the lowest possible speed so that flow rate can be measured
- Increase slowly the pump velocity so that several sets of flow rate and pressures can be recorded
- Register the data in a sheet so that calculations can be easily done and plot
- Once, your data are calculated, make plots of it
- Check for smooth of your plots.
Other stuff of interest
- Theoretical foundations for pump mesurements and estimations
- The pros and cons of bulb thermometers
- Some examples of temperature instruments
- Minor losses - Formulas
- What is a process variable?
- What are the most important process variables?
- Time dependence of process variables
- A list of process variables
Ildebrando.
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