11.3 Pump and System Characteristics

Pump Performance and System Interaction

Pump performance curves are the primary tools for understanding how a pump behaves within a fluid system. They let you find the operating point, check efficiency, and determine power requirements, all of which are necessary for selecting the right pump and keeping it running well.

Interpretation of Pump Performance Curves

A pump's performance is described by several curves plotted against flow rate ($Q$):

• Head-capacity (H-Q) curve relates the pump's discharge head to its volumetric flow rate. As flow rate increases, the head a centrifugal pump can deliver generally decreases.
• Efficiency curve shows how efficiently the pump converts input power to hydraulic energy at each flow rate. Efficiency rises to a peak and then drops off on either side.
• Power curve shows the shaft power (brake horsepower) required at each flow rate. For many centrifugal pumps, power increases with flow rate.
• NPSHR curve gives the net positive suction head required to prevent cavitation at each flow rate. This value typically increases at higher flow rates because fluid velocities at the impeller eye are greater.

The operating point is where the pump's H-Q curve intersects the system resistance curve. That intersection tells you the actual flow rate and head the pump will deliver once installed. For example, a pump might settle at 200 gpm and 50 ft of head in a particular system.

Pump and System Interaction Analysis

The system resistance curve (also called the system curve) plots total head loss as a function of flow rate. It has two components:

• Static head: the elevation difference the fluid must overcome, plus any pressure difference between source and destination. This is constant regardless of flow rate.
• Friction head: losses from pipe friction, fittings (elbows, valves), and changes in pipe diameter. Friction losses increase roughly with $Q^2$, so the system curve is parabolic.

Factors that shape the system curve include pipe diameter, length, material roughness, and the number and type of fittings.

Pump selection means choosing a pump whose H-Q curve intersects the system curve near the pump's best efficiency point (BEP). A well-matched pump might operate at 80% efficiency delivering 500 gpm, for instance. Operating far from BEP wastes energy and accelerates wear.

To adjust the operating point after installation, you can:

• Throttle a valve to increase system resistance and shift the operating point left (lower flow, higher head).
• Use a variable frequency drive (VFD) to change pump speed. This shifts the entire H-Q curve up or down according to the affinity laws, and it's more energy-efficient than throttling.

Effects of System Resistance Changes

When system resistance increases (e.g., a partially closed valve or fouled pipe), the system curve steepens. The operating point slides left along the H-Q curve:

• Flow rate drops.
• Head developed by the pump increases (say, from 50 ft to 60 ft).
• Efficiency often falls (e.g., from 85% to 75%) because you've moved away from BEP.

When system resistance decreases (e.g., opening a bypass line), the system curve flattens. The operating point slides right:

• Flow rate rises.
• Head developed by the pump decreases (say, from 50 ft to 40 ft).
• Efficiency may improve if you move closer to BEP, but if flow increases too much, you risk overloading the motor (current exceeds the rated value) and pushing NPSHR above NPSHA, which invites cavitation.

Pump System Design and Troubleshooting

Causes of Pump Surging and Stalling

Surging is a cyclic oscillation between high and low flow rates. It happens when the pump's H-Q curve and the system curve intersect at an unstable point, typically on a portion of the H-Q curve that has a positive slope (head increases with flow). The pump hunts back and forth, producing excessive vibration, noise, and fatigue damage to seals and piping.

Stalling occurs when the pump cannot generate enough head to overcome the system's resistance. Common causes include:

• A sudden increase in system resistance (clogged filter, closed valve).
• A drop in pump speed (power supply issue, belt slip).

The result is a drastic loss of flow. With little or no fluid moving through the pump, internal temperatures rise quickly, potentially seizing bearings or damaging seals.

Design of Pump Piping Systems

Head loss calculation is central to piping design. Friction losses in straight pipe sections are found with the Darcy-Weisbach equation:

$h_f = f \frac{L}{D} \frac{v^2}{2g}$

where $f$ is the Darcy friction factor, $L$ is pipe length, $D$ is pipe diameter, $v$ is flow velocity, and $g$ is gravitational acceleration. Minor losses from fittings are added using loss coefficients ($K$-values) or equivalent lengths.

To minimize head loss, use larger-diameter pipes, smoother materials, and layouts with fewer bends and fittings.

Cavitation is the formation and violent collapse of vapor bubbles in low-pressure regions, typically at the impeller inlet. It causes pitting and erosion of impeller surfaces, reduced pump performance, and loud crackling noise. Preventing cavitation comes down to managing NPSH.

Net Positive Suction Head (NPSH) design check:

1. NPSHA (available) is the absolute pressure at the pump suction minus the fluid's vapor pressure, expressed as head. It depends on suction tank pressure, elevation, fluid temperature, and suction-side losses.
2. NPSHR (required) is the minimum NPSH the pump needs to avoid cavitation. The manufacturer provides this on the performance curve.
3. The design rule is straightforward: NPSHA must exceed NPSHR, with a safety margin (a common guideline is NPSHA ≥ NPSHR + 2–3 ft, though this varies by application).

To increase NPSHA, you can shorten and enlarge suction piping, raise the liquid level above the pump (flooded suction), reduce fluid temperature, or pressurize the suction vessel.