5.1 Centrifugal & Positive Displacement Pumps
Key Takeaways
- Centrifugal pumps convert velocity head to pressure head using an impeller and volute casing.
- Positive displacement pumps deliver a constant flow and must never run against a closed discharge valve.
- Cavitation occurs when local pressure drops below vapor pressure, causing pitting and a gravel-like noise.
- Gland packing requires a controlled drip of 20 to 60 drops per minute for lubrication and cooling.
5.1 Centrifugal & Positive Displacement Pumps
Why Pump Operations Matter for the Exam
Pumps are the primary mechanical devices in water treatment to transport raw water, move process water, dose chemicals, and distribute finished water. Operators must understand the design, components, hydraulics, and failures of centrifugal and positive displacement pumps. Proper pump operation affects treatment efficiency, dosing accuracy, and equipment longevity.
Centrifugal Pumps: Theory and Components
A centrifugal pump is a dynamic machine converting electrical energy into kinetic energy using a rotating impeller. Water enters the impeller eye, is thrown outward by centrifugal force, and enters the volute, a spiral-shaped casing. The volute converts velocity head into pressure head before discharge.
Impeller design determines suitability for specific water quality conditions:
- An open impeller has vanes attached to a central hub without shrouds, used for solids or high turbidity to resist clogging.
- A semi-open impeller features a shroud on one side for moderate solids-handling.
- A closed impeller has shrouds on both sides enclosing the flow channels. This design is highly efficient and is used exclusively for clean, treated water because solids clog the narrow channels.
To prevent casing leaks around the rotating shaft, a stuffing box containing packing rings is used. Mechanical packing consists of braided fiber rings wrapped around the shaft, compressed by a gland follower. This follower must be adjusted to allow a cooling leak of 20 to 60 drops per minute. This leakage dissipates heat and lubricates the sleeve; dry packing destroys both. A lantern ring (a perforated spacer) is positioned between packing rings to allow seal water to enter. Seal water cools the shaft and creates a pressure barrier preventing air from entering under vacuum.
Positive Displacement Pumps: Principles and Safety
A positive displacement pump traps a fixed volume of water and forces it through the discharge line. Unlike centrifugal pumps, which experience a drop in flow as pressure increases, positive displacement pumps deliver a constant flow rate regardless of pressure. The two main types are reciprocating pumps (using a piston, plunger, or diaphragm pump) and rotary pumps (using gears, lobes, or vanes).
A critical safety rule for positive displacement pumps is that they must never be operated against a closed discharge valve. Because they displace a fixed volume, blocking the discharge causes pressure to rise rapidly until physical failure occurs, such as a ruptured pipe or motor burnout. Thus, they must have a calibrated pressure relief valve on the discharge piping.
Pump Hydraulics: Suction Lift and Priming
Pump hydraulics are categorized by the water level relative to the pump centerline:
- Suction lift exists when the source liquid level is below the pump centerline. The pump must draw a partial vacuum, allowing atmospheric pressure to push water up. The practical limit is 15 to 22 feet.
- Suction head exists when the source level is above the pump centerline (flooded suction), where gravity feeds water directly into the pump.
Centrifugal pumps require priming—filling the casing and suction pipe and venting trapped air before startup.
Cavitation: Causes, Consequences, and Prevention
Cavitation occurs when local pressure inside a pump drops below the liquid's vapor pressure, causing water to boil and form vapor bubbles. As these bubbles travel into higher pressure regions, they collapse violently, generating extreme shock waves. The classic symptom is a loud noise described as "pumping gravel". Over time, cavitation causes severe physical damage, including pitting of impeller vanes, bearing failure, and drops in flow and discharge head.
To prevent cavitation, Net Positive Suction Head Available (NPSHa) must exceed Net Positive Suction Head Required (NPSHr) specified by the manufacturer. Operators can increase NPSHa by cleaning suction strainers, lowering water temperature, decreasing suction lift, or throttling the discharge valve. Operators must never throttle the suction valve, which drops pressure further.
| Parameter | Centrifugal Pumps | Positive Displacement Pumps |
|---|---|---|
| Flow vs. Pressure | Flow decreases as pressure increases | Flow remains constant as pressure changes |
| Viscosity Effect | Efficiency drops with high viscosity | Handles high viscosity fluids easily |
| Discharge Valve | Can run briefly against closed valve | Never run against closed valve (requires relief valve) |
| Priming | Not self-priming (requires water fill) | Self-priming (can pump air/gases) |
Realistic Operational Scenario
In a realistic operational scenario, an operator hears a loud, rattling noise like gravel tumbling inside a centrifugal pump. The discharge pressure gauge fluctuates, and flow drops by 25%. The pump operates under a suction lift of 12 feet. Suspecting cavitation, the operator checks the suction screen and finds it clogged with debris. This blockage restricted flow, dropping suction pressure below vapor pressure. The operator shuts down the pump, clears the screen, and restarts it. The suction pressure returns to normal and the noise disappears, confirming that removing the suction restriction resolved the cavitation.
Cavitation is caused by what physical condition?
What is a critical operational safety rule for positive displacement pumps?