One curve slopes and one is vertical. That single fact decides the relief valve, the flow control method, the viscosity limit, the shear behavior, and which pump belongs on your polymer feed.
A positive displacement pump MUST have a relief valve.
Deadhead a centrifugal and it heats up. Deadhead a PD and something ruptures.
| CENTRIFUGAL | POSITIVE DISPLACEMENT | |
|---|---|---|
| Mechanism | A rotating impeller adds velocity, which the casing converts to pressure | Traps a fixed volume in a chamber and physically displaces it |
| What's constant | Energy added per unit mass, at a given speed | Volume moved per cycle |
| Class name | Rotodynamic — works by imparting momentum | Displacement — works by mechanical exclusion |
| Result | Flow varies with system pressure | Flow stays nearly constant regardless of pressure |
| Behavior | Centrifugal | Positive displacement |
|---|---|---|
| Shape of the H-Q curve | Sloping — head falls as flow rises | Nearly vertical — flow essentially fixed |
| As discharge pressure rises | Flow decreases along the curve | Flow stays near constant |
| At shutoff (zero flow) | Reaches shutoff head and stops delivering | Keeps trying to deliver. Pressure climbs without limit. |
| Why PD isn't perfectly vertical | — | Slip — internal leakage from discharge back to suction through clearances |
If your process demands the same flow across varying backpressure, a PD pump is the safer starting point.
And the inverse: if you want flow to adjust itself as the system changes — a wet well level rising and falling, a filter loading up — the centrifugal's sloping curve is doing useful work for you.
Slip is worth understanding on its own. It's the small backflow through a PD pump's internal clearances, and it's the only reason that curve isn't perfectly vertical. Two properties matter: slip increases with discharge pressure, and it decreases with viscosity, because thicker fluid doesn't leak through tight clearances as readily. That second point drives everything in Part 3.
Never install a positive displacement pump without a fully rated safety relief valve on the discharge line, upstream of the first isolation valve.
Because a PD pump will attempt to deliver its fixed volume regardless of downstream resistance, closing a discharge valve causes pressure to rise instantly until the motor stalls, the piping ruptures, or the pump casing explodes.
A relief valve installed downstream of an isolation valve can itself be isolated out of service.
The moment somebody closes that valve for maintenance and then starts the pump, the protection is gone and the pump is deadheaded against a closed line with no relief path.
The relief must sit between the pump discharge and the first valve that can be closed. Verify this on every PD installation you inherit — it's a common as-built error.
| Discharge closed | Centrifugal | Positive displacement |
|---|---|---|
| Pressure | Rises to shutoff head and stops. Bounded by the impeller. | Rises without a natural limit until something yields |
| Flow | Zero | Zero out the discharge — but the pump keeps displacing |
| Immediate risk | Fluid recirculates and heats up. Can flash, damage seals and bearings. | Catastrophic mechanical failure — stalled motor, ruptured piping, cracked casing |
| Time to damage | Minutes, depending on the pump | Effectively immediate |
| Protection needed | Minimum-flow bypass on critical service | Relief valve or bypass — mandatory |
Throttling a discharge valve is the standard way to reduce flow on a centrifugal — the pump simply rides up its curve to a lower flow at higher head.
Do the same to a PD pump and you are deadheading it by degrees. The flow doesn't fall. The pressure rises.
To reduce PD flow you change speed, or recirculate through a controlled bypass. Never the discharge valve.
"Fluid viscosity is the ultimate decider; high-viscosity fluids degrade centrifugal efficiency but actually improve positive displacement volumetric efficiency." — EPCLand
| Centrifugal | Positive displacement | |
|---|---|---|
| Mechanism | Relies on high-speed fluid movement. Viscous drag inside the impeller and casing eats that energy. | Relies on trapping volume. Viscous fluid seals the clearances better. |
| Effect of rising viscosity | Head, flow and efficiency all drop. Power draw rises. | Volumetric efficiency improves slightly, because slip decreases |
| Practical ceiling | Best below roughly 100 cSt | Handles thick fluids readily |
| Sensitivity to change | Even 50 to 100 cP of change has a large impact | Stable delivery as viscosity rises |
What is the viscosity, and does it change?
Variable viscosity matters as much as absolute viscosity. A fluid that thickens when cold, or a product whose viscosity varies batch to batch, will move a centrifugal all over its curve. A PD pump barely notices.
| Factor | Detail |
|---|---|
| Speed | Centrifugals are higher-speed machines, and pumps shear liquids more as speed increases |
| Mechanism | The impeller accelerates fluid across a small clearance at high velocity. That velocity gradient is shear. |
| PD by contrast | Rotary PD pumps with larger chambers — progressive cavity, lobe, peristaltic — run at lower speed and produce low internal velocity, so little shear |
Polymer works because long chain molecules bridge particles together. Shear breaks those chains, and sheared polymer does not flocculate.
A plant feeding polymer through a centrifugal pump is paying for chemical it is simultaneously destroying.
The symptom is a dewatering or thickening process that underperforms no matter how much polymer is dosed — because the polymer arriving at the mixing point is not the polymer that left the day tank.
If polymer performance is unexplained, check what is pumping it before you change products.
Other shear-sensitive cases: flocculated sludge, where shearing formed floc defeats the thickening or dewatering step downstream; emulsions, which can break or over-emulsify; and biological cultures.
| Centrifugal | Positive displacement | |
|---|---|---|
| Efficiency shape | Peaks sharply at the best efficiency point and falls off either side | Relatively flat across the pressure range |
| Away from design point | Efficiency drops, allowable operating region is limited, cavitation and damage possible | Can run at any point on the curve without efficiency loss or damage |
| Typical efficiency | 50% to over 90% depending on type and size | Can exceed 90% |
| Low flow, high pressure | Relatively low efficiency | PD territory |
| Pressure capability | Head per stage limited by impeller diameter and speed; high pressure needs multistage | 20 bar not unusual; some models over 70 bar |
Operating well away from BEP produces recirculation, elevated vibration, and increased radial thrust on the shaft and bearings.
That's why the allowable operating region exists on the curve, and why "it still pumps" is not the same as "it's fine."
If a centrifugal is running at 40 percent or 130 percent of BEP flow, the fix is a different impeller, a trim, a VFD, or a different pump — not tolerance.
| Centrifugal | Positive displacement | |
|---|---|---|
| Priming | Standard designs must be primed. Self-priming designs exist. | Most designs are self-priming |
| Suction lift | Standard designs cannot produce suction lift | A vacuum is created on the inlet side — suction lift is possible |
| Air handling | An air pocket stops it — loses prime | Most can handle air pockets and run with the suction line not completely full |
| Flow control method | Centrifugal | Positive displacement |
|---|---|---|
| Discharge throttling | The standard method. The pump rides up its curve. | NEVER. You're deadheading it by degrees. |
| Speed control (VFD) | Works well. Affinity laws apply. | The standard method. Flow is proportional to speed. |
| Bypass / recirculation | Minimum flow protection | Flow control and overpressure protection |
| Impeller trim | Available | Not applicable |
| Stroke adjustment | Not applicable | Available on many reciprocating and metering designs |
Because flow is proportional to speed and nearly independent of pressure, a PD pump delivers a known volume per revolution or per stroke. That's what makes accurate dosing possible.
You cannot meter with a centrifugal, because you don't know the flow without measuring it — the discharge pressure moves it.
| Centrifugal type | Note |
|---|---|
| End suction | The general-purpose workhorse |
| Split case | Higher flow, easier maintenance access |
| Vertical turbine | Deep wet wells, high head |
| Submersible | Wastewater lift stations, dewatering |
| Chopper / solids-handling | Non-clog impellers and chopping designs |
| Multistage | High head where a single impeller isn't enough |
| Self-priming | Trash pumps, suction lift applications |
| PD type — rotary | Note |
|---|---|
| Progressive cavity | Rotor turning in a stator. Excellent for sludge and polymer — low shear, handles solids and high viscosity. |
| Lobe | Sanitary and viscous service. Low shear, non-contacting rotors. |
| Gear (internal / external) | Viscous clean fluids, oils. Internal gear used for very shear-sensitive liquids. |
| Vane | Thin fluids, self-priming, consistent flow |
| Peristaltic (hose) | The fluid touches nothing but the hose. Ideal for polymer, lime slurry and abrasives. |
| Screw | High flow at high pressure, low pulsation |
Reciprocating PD: piston for high pressure and chemical injection, plunger for very high pressure, and diaphragm for chemical metering and leak-free containment — including air-operated double diaphragm for portable and sump duty.
| Service | Pump type | Why |
|---|---|---|
| Raw wastewater lift station | Centrifugal — submersible or dry-pit non-clog | High flow, low viscosity, variable head |
| Plant water and effluent | Centrifugal | Clean, thin, high flow |
| RAS and WAS | Centrifugal, or PD where solids are heavy | Depends on concentration |
| Thickened and primary sludge | Progressive cavity | High viscosity, high solids, flow must stay constant against variable back pressure |
| Digested sludge to dewatering | Progressive cavity or lobe | Same reasoning, plus low shear preserves conditioning |
| Polymer feed | Progressive cavity or peristaltic | Shear destroys polymer chains |
| Chemical metering | Diaphragm metering pump | Accurate dosing against pressure. Leak-free. |
| Lime slurry | Peristaltic | Abrasive and scaling. The hose isolates the pump. |
| Filter press feed | PD — diaphragm or progressive cavity | Pressure rises steeply as cake forms — constant flow against rising back pressure |
| Scum and grease | Progressive cavity or chopper | Viscous and stringy |
| Dewatering and bypass | Self-priming centrifugal or diaphragm | Suction lift and air handling matter |
As a press cake builds, resistance rises dramatically.
A centrifugal responds by riding up its curve — flow collapses as pressure climbs, and the cycle takes forever or never completes.
A PD pump responds by holding flow and raising pressure, which is exactly what the process needs.
That's the sloping-curve-versus-vertical-curve difference showing up as a process outcome rather than a specification.
| If your application is… | Choose |
|---|---|
| High flow, low viscosity, variable head | CENTRIFUGAL |
| Thin clean liquid, water or similar | CENTRIFUGAL |
| Viscosity above ~100 cSt | PD |
| Variable viscosity | PD |
| Shear sensitive product | PD |
| Accurate metering or dosing | PD |
| Low flow, high pressure | PD |
| Constant flow against varying back pressure | PD |
| Suction lift required | PD or self-priming centrifugal |
| Very high flow at moderate head | CENTRIFUGAL |
Standards: API 610 covers centrifugal pumps, API 676 covers rotary positive displacement pumps, and Hydraulic Institute standards cover both including BEP and allowable operating region definitions.
| Error | Consequence |
|---|---|
| Installing a PD pump without a relief valve | Pressure rises until the motor stalls, piping ruptures, or the casing fails |
| Relief valve downstream of the first isolation valve | The protection can be isolated out of service. Common as-built error. |
| Throttling a PD pump to reduce flow | You're deadheading it by degrees. Change speed instead. |
| Using a centrifugal on a viscous fluid | Efficiency, head and flow drop; power rises |
| Using a centrifugal where viscosity varies | Even 50–100 cP of change moves it substantially on its curve |
| Pumping polymer with a centrifugal | Shear breaks the chains. You're destroying the chemical you're paying for. |
| Running a centrifugal far off BEP | Lower efficiency, recirculation, vibration, bearing and seal damage |
| Trying to meter with a centrifugal | Flow depends on discharge pressure. You don't know what you dosed. |
| Assuming a centrifugal can lift suction | Standard designs cannot |
| Deadheading a centrifugal and assuming it's harmless | Recirculating fluid heats up and can damage seals and bearings |
| Using a centrifugal on a filter press | Flow collapses as cake resistance rises |
| Ignoring slip when specifying a PD pump | Slip rises with pressure and falls with viscosity. Delivered flow isn't quite displacement. |
| Item | Value |
|---|---|
| Centrifugal principle | Adds velocity, converts to pressure. Rotodynamic. |
| PD principle | Traps and displaces a fixed volume per cycle |
| Centrifugal H-Q curve | Sloping — flow varies with pressure |
| PD H-Q curve | Nearly vertical — flow near constant |
| Slip | Rises with pressure, falls with viscosity |
| Centrifugal viscosity guideline | Best below ~100 cSt |
| Viscosity change sensitivity | 50–100 cP has a large impact on a centrifugal |
| Shear | Centrifugal high; PD low, especially at low speed |
| Centrifugal efficiency range | 50% to over 90% |
| PD efficiency | Can exceed 90% |
| PD pressure capability | 20 bar not unusual; some over 70 bar |
| Centrifugal deadhead | Recirculates and heats up |
| PD deadhead | Pressure rises until something fails |
| PD protection | Relief valve, upstream of the first isolation valve |
| Centrifugal flow control | Discharge throttling, VFD, impeller trim |
| PD flow control | Speed or bypass — never throttling |
| Suction lift | Standard centrifugal cannot; PD can |
| Metering duty | PD |
| Polymer feed | PD — progressive cavity or peristaltic |
| Standards | API 610 (centrifugal), API 676 (rotary PD) |
The ~100 cSt guideline is a commonly cited practical threshold, not a hard limit. Centrifugal viscosity correction methods exist for higher values, at a performance penalty.
Efficiency and pressure figures come from manufacturer and distributor sources. Actual efficiency depends heavily on size, type and duty point; the 20 and 70 bar figures describe general PD capability, not any specific pump.
All sources are pump manufacturers, distributors or trade publications with commercial interests. The physics — curve shape, slip, shear, deadhead behavior — is consistent across all of them and is not in dispute.
API 676 was confirmed via source; the API 610 and Hydraulic Institute references are general knowledge rather than verified against a source in this research.
The wastewater application table represents common practice. Specific selections depend on solids content, concentration and site conditions. Pump selection for a specific service should involve the pump vendor and, for anything critical, a qualified engineer.
The curve shape is the whole story. A sloping curve means flow adjusts itself as the system changes. A vertical curve means flow holds and pressure adjusts — without limit, unless you provide one.
That's why a PD pump needs a relief valve and a centrifugal doesn't, why you throttle one and not the other, and why a filter press feed pump is a PD pump.
And in a wastewater plant specifically: if polymer performance is unexplained, look at what is pumping it before you change chemical.
The Complete Exam Guide covers all 12 major exam topics with 200 conceptual questions and a detailed explanation behind every answer — including pumps, hydraulics and mechanical systems.
Get the Complete Exam Guide — $17Instant PDF download · 200 questions · No math · All topics