8.1 Raw Water Intake, Screening & Pumping Equipment
Key Takeaways
- Net positive suction head available must always exceed net positive suction head required, or the pump will cavitate and destroy its impeller.
- Cavitation sounds like gravel passing through the pump and is corrected by raising suction pressure, lowering suction lift, or reducing the flow rate, never by throttling the suction valve.
- Throttle a centrifugal pump on the discharge side only; throttling the suction starves the impeller and induces cavitation.
- Rising differential head loss across an intake screen means the screen is blinding and needs cleaning before the low-lift pumps lose suction.
- Vertical turbine pumps are the standard for wells and wet pits because the bowl assembly stays submerged and self-primed.
8.1 Raw Water Intake, Screening & Pumping Equipment
The Need-to-Know Criteria require Water Treatment operators to inspect, maintain, and operate "raw water intake, screening, and pumping," to "operate and maintain pumps, drivers, and auxiliary equipment," and to "perform efficiency tests on pumps and related equipment (e.g., pump curves)." Pumping questions appear on all four discipline exams, so this material pays off no matter which test you sit.
Intake Structures and Screening
| Component | Purpose | Operator attention |
|---|---|---|
| Trash rack | Excludes logs, branches, large debris | Rake or clear; watch differential |
| Bar screen | Excludes intermediate debris | Manual or mechanically raked |
| Traveling water screen | Continuous fine screening on a moving belt | Spray wash pressure, belt tracking, drive chain |
| Intake tower ports and gates | Depth selection | Gate operator lubrication, position indication, stem alignment |
| Low-lift (raw water) pumps | Move raw water to the plant | Suction conditions, bearing and seal condition |
Differential head loss across a screen is the primary diagnostic. As debris, algae, or mussels blind the screen, the water level upstream rises relative to downstream. Left alone, the differential eventually starves the low-lift pumps of suction and causes cavitation or loss of prime — and in extreme cases can structurally damage the screen panel.
Arizona-specific concerns: quagga mussel colonization on screen faces, guides, and in raw water piping from Colorado River water; algal mats during summer; and debris surges during monsoon runoff.
Pump Types in Raw Water Service
| Type | Description | Typical use |
|---|---|---|
| Horizontal centrifugal (end suction) | Impeller in a volute casing, horizontal shaft | General plant service |
| Split case centrifugal | Casing splits for access; often double suction | High-flow raw and finished water pumping |
| Vertical turbine | Multiple bowl-and-impeller stages on a vertical shaft, bowls submerged | Wells and wet pits — self-primed because the bowls stay under water |
| Submersible | Motor and pump both submerged | Wells, wet wells, sumps |
| Axial flow (propeller) | Moves large volumes at low head | Raw water lifts, flood pumping |
| Positive displacement | Fixed volume per stroke or revolution | Chemical metering and sludge |
Vertical turbines dominate well and wet-pit service for one practical reason: the bowl assembly is always submerged, so the pump never needs priming and never loses suction as long as the water level stays above the bowls.
Pump Theory Operators Must Apply
Head Terms
- Static suction lift: the pump is above the water source; the pump must lift water to itself.
- Static suction head: the source is above the pump centerline; water flows in under pressure.
- Static discharge head: elevation the water must be raised on the discharge side.
- Total static head: total elevation difference from source surface to discharge surface.
- Friction head: pressure lost to pipe wall friction, fittings, and valves.
- Total dynamic head (TDH): total static head plus friction head plus velocity head — the actual work the pump does.
Net Positive Suction Head and Cavitation
Net positive suction head available (NPSHA) is the absolute pressure at the pump suction above the vapor pressure of the liquid. Net positive suction head required (NPSHR) is what the pump needs to avoid vaporizing the liquid at the impeller eye, and it comes from the manufacturer's curve.
When it does not, water flashes to vapor at the impeller eye and the bubbles collapse violently as pressure recovers across the impeller. That is cavitation.
| Cavitation sign | Detail |
|---|---|
| Sound | Like gravel or marbles passing through the pump — the classic description |
| Vibration | Rough, erratic running |
| Gauges | Fluctuating discharge pressure and flow |
| Damage | Pitting and erosion of the impeller vanes and casing |
Corrective actions: raise the suction water level, lower the pump elevation, shorten and enlarge the suction piping, clean a blinded screen or clogged suction strainer, open a partially closed suction valve, or reduce the flow rate.
[!WARNING] Never throttle a centrifugal pump on the suction side. Throttling suction reduces NPSHA and directly causes the cavitation you are trying to avoid. All throttling is done on the discharge valve. This is one of the most frequently tested operating rules in the entire pump section.
Priming
A centrifugal pump moves water, not air. If the casing contains air, the pump spins without developing head — it is air bound. Suction-lift installations must be primed by a foot valve with fill connection, a vacuum priming system, or a self-priming pump design. Submerged installations such as vertical turbines and submersibles are self-primed.
Pump Curves and Efficiency
The manufacturer's curve plots head against flow. As flow increases, head decreases. The operating point is where the pump curve intersects the system head curve.
- Best efficiency point (BEP) is where the pump runs most efficiently and with the least vibration and bearing load. Running far off BEP shortens life.
- Shutoff head is head at zero flow; running against a closed discharge valve for more than a moment overheats the pump.
- Pumps in series add head. Pumps in parallel add flow.
Affinity Laws
For a change in speed (N):
Flow varies directly with speed, head with the square, and power with the cube. This cubic relationship is why variable frequency drives save so much energy: dropping to 80% speed cuts power to about 51%.
Routine Maintenance
| Item | Task |
|---|---|
| Packing (stuffing box) | Should drip slowly, roughly 30 to 60 drops per minute, to lubricate and cool. A packed gland that is bone dry is over-tightened and will score the shaft sleeve |
| Mechanical seals | Should not leak at all; any visible leakage means seal failure |
| Bearings | Lubricate on schedule; monitor temperature and vibration; over-greasing is as damaging as under-greasing |
| Coupling alignment | Check with dial indicator or laser; misalignment destroys bearings and seals |
| Vibration analysis | Trending identifies imbalance, misalignment, looseness, and bearing wear before failure |
| Suction strainer | Clean; monitor differential |
| Motor | Amperage draw trending, insulation resistance testing, ventilation, cleanliness |
Rising amperage at constant flow and head signals mechanical binding or a failing bearing. Falling flow at constant speed signals impeller wear, a partially blocked suction, or an increase in system head.
A low-lift raw water pump begins making a sound like gravel passing through the casing, discharge pressure fluctuates, and the operator later finds pitting on the impeller vanes. Which action correctly addresses the cause?
A variable frequency drive reduces a pump from 1,800 rpm to 1,440 rpm. Using the affinity laws, what happens to the power the pump draws?
An operator observes that the stuffing box on a raw water pump is completely dry with no visible drip, and the shaft sleeve shows scoring. What is the most likely cause?