Pump curves are generally the graphical representation of their performance parameters. The pump manufacturers design various pump curves of different standards. These curves show the relation between the pump’s head, bhp, efficiency and NPSH against various flowrates. The pump curves also depend upon the size of the impeller and its operating speed.
Why to use a Pump Curve?
The pump curves inform us about the limitations of a particular pump. The manufacturer uses them to compare with the parameters given by their customers. This helps them to select a pump that satisfies their process requirement and its driver consumes minimum electric power.
An important recognition is that a pump always operates near its performance curve. But due to pump wearing, some of the fluid from discharge leaks back to the suction because of the increased clearance. Consequently, the operating curve moves downward to a line parallel to the original curve. This helps in troubleshooting the pumps.
We just measure its performance parameters and compare them with the original curve. If the current parameters are too below, we must change the pump.
How to read Pump’s Operating Curves?
Whenever we discuss the performance of any mechanical device, the first thing coming to our minds is its efficiency. We define the pump’s efficiency as the ratio of useful power (output) to supplied power (input).
ղpump = ρgV̇H/ꙌTshaft … (1)
The product “ρgV̇H” is called the Water Horsepower. It is the actual power delivered by the pump to the fluid. It is a conventional name, no matter even if the fluid is not water and the power is not measured in HP.
‘ρ’ is the density of the fluid
‘g’ is the gravitational acceleration
‘V̇’ is the volumetric flow rate and we also call it the capacity of the pump. The product of ‘ρV̇’ is actually the mass flow rate through the pump.
‘H’ is the net head i.e, the pressure rise across the pump expressed in the dimension of length
Brake Horsepower (BHP)
Brake Horsepower is the total amount of power required by a particular pump. It must always be greater than the Water horsepower since we have frictional and other losses in mechanical devices. The product “ꙌTshaft” is Brake Horsepower (bhp) where:
‘Ꙍ’ is the rotational speed of the impeller
‘Tshaft‘ is the torque supplied to the shaft
Pump Curves (considering above parameters)
The figure below shows the typical curves of the pump at one particular speed.
If we look at the figure below: The maximum flow rate occurs when the net head of the pump is zero. This is the condition when the pump has no restriction to its path. We can also conclude that the pump has no load on it. This is the free delivery state. But the equation (1) suggests; the pump’s efficiency at this point is zero. We can observe this condition when the pump discharges fluid at its outlet. In other words, the outlet has no elevation.
Similarly, the pump has a zero flow rate at the maximum head which is Shutoff Head. This is the condition when the pump has a blocked outlet port. Again equation (1) suggests that the pump has zero efficiency at this state. Because the volumetric flow rate is zero i.e, the pump isn’t doing any useful work. The pump’s head decreases by increasing the flow rate. And the value of its efficiency increases with the flow rate, reaches a maximum value somewhere between, and then drops to zero at free delivery condition.
The point of maximum efficiency is called the best efficiency point or BEP. And the Net Head, bhp, and Volumetric flow rate against BEP are the ideal states to operate the pump at.
In a practical application, the required net head of the pump increases with an increasing flow rate. This is because of the major and minor losses of the piping system. The major losses in piping systems are due to the friction between fluid and piping walls. The minor losses occur due to the flow restrictions. For example, pipe fittings etc.
Operating or Duty Point
While the available net head decreases with increasing flow rate as discussed above. The important recognition here is; the pump only operates along its performance curve. So, there is always a point where the curves of the pump available head and required head have a unique value, i.e, Hreq = Hav. Yunus A. Cengel (in his book) has called this point, the operating point or duty point of the pumping system. And shows it in a graph in the following way:
It is important to note that the pump’s operating/duty point may or may not be the best efficiency point as well. Therefore, while designing a pump, we must acknowledge what is required. If efficiency is our prime concern, we’ve to make changes so as to bring the duty point near the BEP, if it isn’t. In extreme cases when it is not possible to match the duty point with BEP or at least bring it near to it, we must design a new pump.
We can have a number of curves depending upon the speed and size of the impeller. The pump manufacturing industries provide more than one size of impellers for a single casing. This provides the following added benefits:
- To save manufacturing cost
- Capacity can be increased by only changing the impeller
Below is a figure taken from the same book of Yunus A. Cengel. This gives us an example of a family of centrifugal pumps. The manufacturer has plotted the pump curves so as to accumulate the same casing for different sizes of impellers.