Questions: Centrifugal Pump Performance Curves and System Selection

Question 1

An existing piping system requires 400 GPM and is served by a single pump at its design point. To increase flow to 600 GPM without changing the piping, you consider adding a second identical pump. Should you add it in series or in parallel?

• A. In series — series pumps add their flow rates, so two pumps give 800 GPM
• B. In parallel — parallel pumps add their heads, doubling the pressure and forcing more flow through the system
• C. In parallel — parallel pumps add their flow rates at the same head, shifting the combined H-Q curve to higher flows
• D. In series — series pumps add their heads, and higher head is always needed for higher flow

Answer: (C)

Pumps in parallel add flow rates at the same head — the combined H-Q curve is the horizontal sum of the individual curves. When the combined curve intersects the unchanged system curve, the new operating point is at a higher flow rate. Pumps in series add heads at the same flow rate, appropriate when you need more head (e.g., to overcome additional elevation or friction) rather than more flow. Since the piping hasn't changed, parallel is the correct approach.

Question 2

A pump H-Q curve shows 100 ft of head at zero flow and 60 ft at 500 GPM. The system curve requires 60 ft at 500 GPM. Where will the pump actually operate?

• A. At shutoff head (100 ft, 0 GPM) because that is where the pump delivers maximum head
• B. At 500 GPM and 60 ft — the intersection of the pump curve and the system curve
• C. At the best efficiency point, regardless of the system curve
• D. At maximum flow, where the pump curve crosses zero head

Answer: (B)

The operating point is always the intersection of the pump H-Q curve and the system curve — the unique combination where both are simultaneously satisfied. Here, the pump delivers 60 ft at 500 GPM and the system requires exactly 60 ft at 500 GPM, so that is the operating point. The pump does not 'choose' the BEP — the BEP is a property of the pump design, not a constraint that overrides the system curve.

Question 3

The best efficiency point (BEP) is the flow rate at which a centrifugal pump converts shaft power to fluid head with maximum efficiency.

• T. True
• F. False

Answer: True

The BEP is the design point where hydraulic, volumetric, and mechanical losses are minimized together, yielding the highest ratio of hydraulic power output to shaft power input. Operating away from BEP causes internal recirculation, increased turbulence, and additional mechanical stress. Selecting a pump so the expected operating point falls near the BEP is essential for both reliability and energy cost.

Question 4

Adding two identical pumps in parallel doubles the head available to the system at any given flow rate.

• T. True
• F. False

Answer: False

Pumps in parallel add their flow rates at the same head — the combined H-Q curve is the horizontal sum of the individual curves. At any given head, the combined pair can deliver twice the flow of one pump. Doubling the head at the same flow requires pumps in series (vertical sum of curves). Confusing series and parallel is one of the most common errors in pump system design.

Question 5

What is the system curve, and how does it interact with the pump H-Q curve to determine the actual operating flow rate and head?

Model answer: The system curve describes how much head is required to push flow through the piping system at every possible flow rate: H_system = H_static + R·Q², where H_static is the fixed head from elevation and pressure differences, and R·Q² captures friction losses that scale with the square of flow. When plotted on the same axes as the pump H-Q curve, the operating point is their intersection — the only flow rate and head at which the pump's output exactly matches what the system demands.

The intersection is a self-regulating equilibrium. If the pump produced more head than the system needed at some flow, the excess would accelerate fluid to higher flow rates until balance is reached. If it produced less, flow would decelerate to lower rates. The intersection is stable — the system naturally finds it. This is why you cannot simply 'select' an arbitrary operating point; it is determined by the physics of both the pump and the piping system together.