7.1 Pumps and Total Dynamic Head
Key Takeaways
- Total dynamic head (TDH) is static lift plus friction losses through pipe, fittings, the hair-and-lint strainer, filter, heater, flow meter, and valves — the resistance the recirculation pump must work against.
- A centrifugal pump curve plots flow versus head; the operating point is the intersection with today's system curve. A dirty filter raises the system curve, so gallons per minute fall even though the motor nameplate did not change.
- Flooded suction keeps the case full from a water source above the impeller; self-priming pumps can re-prime only if the case still holds a water seal and the strainer lid is airtight.
- Cavitation is vapor bubbles forming and collapsing at the impeller when suction-side pressure is too low — gravel noise, vibration, and pitted vanes. The 2025 public AFO handbook does not print NPSH tables; treat NPSH as a suction-condition story, not a memorized unpublished number.
- Never dead-head a pool pump against a closed discharge valve as a daily habit. Isolation valves belong in the designed open running position before you press START.
The recirculation pump is the heart of Domain 2C mechanical devices. Chapter 6 already taught that turnover is a time that comes from gallons per minute, and that a dirty filter lengthens actual turnover even when the design plaque still says six hours. This section owns the hardware that creates that flow: the centrifugal pump, the resistance it must overcome, and the operator habits that keep the impeller in liquid water instead of vapor.
The 2025 National Recreation and Park Association (NRPA) Aquatic Facility Operator (AFO) Exam Candidate Handbook lists mechanical devices as Domain 2 topic 2C. Independent teaching here uses that public outline, ordinary pump-curve physics, and the 2024 Model Aquatic Health Code (MAHC) as a voluntary model. Your authority having jurisdiction (AHJ) wins when it is more specific. This is not a quotation of the copyrighted AFO Manual. Extra hydraulic word problems and preventive-maintenance programs belong in later chapters; here you learn the machine.
What the pump is for
A pool pump does not "make turnover" by magic. It moves water against resistance. That resistance has a name: total dynamic head (TDH).
If the pump cannot produce enough head at the gallons per minute the design needs, the flow meter reads low, actual turnover stretches, inlets go lazy, the heater may trip on low flow, and chemical feed may interlock off. The basin still looks full. The loop is starving.
Total dynamic head
TDH is the sum of two families of terms:
- Static head (static lift) — the elevation work. How far must the pump lift water from the free surface on the suction side (surge tank, gutter trough, or pool waterline) to the free surface or residual pressure on the discharge side? A pump in a pit below the waterline often enjoys flooded suction: water wants to fill the case. A pump on a pad above the waterline must lift on the suction side.
- Friction losses — every obstruction in the loop: pipe length and diameter, elbows and tees, valves (especially a partly closed valve), the hair-and-lint strainer, the filter (clean versus dirty), the heater heat exchanger, the flow meter, and the returns.
TDH is expressed in feet of head. A public conversion operators use constantly is about 2.31 feet of water column per 1 psi. A discharge gauge that climbs from 18 psi to 26 psi while suction stays the same is telling you the loop got "taller" in friction terms. Chapter 8 will make you do more arithmetic with those numbers. Here you need the parts list of TDH so you can hunt the part that changed.
Trap: reading only the filter influent gauge and calling that "the pump." Influent pressure is one term. Suction gauge, strainer-lid leaks, heater bypass, and a throttled return valve are other terms. TDH is the whole path.
Pump curve, system curve, operating point
A centrifugal recirculation pump does not deliver one GPM. The manufacturer publishes a pump curve: flow versus head. As required head rises, flow falls. As required head falls, flow rises — until the pump runs out on the right side of the curve (too little resistance, possible motor overload).
The piping loop has a system curve: the head the system demands at each flow. More gallons per minute means more friction (friction rises roughly with the square of flow). A dirty filter, a clogged strainer, or a heater full of scale raises the system curve — more head for the same GPM.
The operating point is the intersection. The pump will run where its curve crosses today's system curve, not where last year's design drawing hoped.
Worked qualitative: design needs 333 GPM, then the filter clogs
Chapter 6's handbook-scale example needed 333 GPM to turn 120,000 gallons in 6 hours. Suppose the engineer selected a pump that, on a clean filter and open valves, intersects the system curve at 333 GPM at 50 feet of TDH. That is the design operating point. The flow meter should read about 333 GPM. The plaque still says 6-hour turnover.
Monday, the filter is dirty. Cake or sand loading adds friction. The system curve shifts up. The same pump curve now intersects at, say, 260 GPM at 58 feet. Nobody changed the motor. Flow fell because head rose. Actual turnover is now longer than the 6-hour design. Inlets throw less. Corners cloud. A heater flow switch may chatter. This is the same systems story as Chapter 6, told from the pump curve instead of from the turnover formula.
If someone "fixes" low flow by closing a discharge valve to raise pressure, they have moved the operating point the wrong way on purpose. Pressure went up because flow went down. That is not filtration.
Diagram reading on exam items
A 2C-style item that shows a pump delivering less GPM after a storm weekend is often a dirty strainer or dirty filter item (system curve up). An item about a pump that is loud and the impeller looks sandblasted is cavitation. An item about a melted impeller and a closed valve is dead-heading.
Self-priming versus flooded suction
Flooded suction means the water source sits above the impeller. Gravity keeps the case full. Indoor rooms fed from a surge tank, or pumps in a pit, often run this way. Lose the waterline or run the surge tank dry and you still lose prime — flooded is a layout, not a guarantee.
Self-priming pumps can re-establish a water seal in the case after a brief loss of water if the case still holds enough liquid and the suction lid is airtight. Outdoor pads above the waterline use them. They are not air compressors. A dry case, a cracked lid, or a missing strainer gasket will not prime no matter what the nameplate says.
Hair-and-lint strainer (pump basket) sits on the suction side, before the impeller. Chapter 6 listed it as a collection-side screen. As hardware: the basket must be in place, the lid gasketed, and the pot full of water before a start. A missing basket sends hair into the impeller. A leaking lid sucks air — the pump sounds like gravel, flow collapses, and the mechanical seal can run dry. 2024 MAHC §5.7.1.7 expects strainers in place and cleaned so the pump can do its job.
Open the lid only with the pump off and suction isolated as the plant's procedure requires. Start-up order: basket in, lid sealed, valves in the running position, then motor.
NPSH and cavitation — conceptually
Net positive suction head (NPSH) is the suction-side pressure budget at the impeller eye. The pump requires a certain NPSH so water stays liquid. The installation must supply more than that. The public 2025 AFO handbook does not print an NPSH table. Do not memorize a fake required-feet number as if NRPA published it. Memorize the failure.
When suction pressure is too low, water flashes to vapor at the impeller. Bubbles collapse on the vane faces. That is cavitation. Operators hear gravel in the pump, feel vibration, and later see pitting on the impeller. Flow is erratic. Seals fail.
Typical operator causes — all suction-side:
- Clogged hair-and-lint basket, or skimmer/gutter starvation
- Waterline below the weir; surge tank too low
- Suction valve throttled "to quiet the pump"
- Air leak at the strainer lid or union
- Pump sitting too high above the water with undersized suction pipe
Fix the suction condition. Do not "turn up the motor" into a cavitating pump.
Impeller, volute, seal, motor
| Part | Job | Operator tell |
|---|---|---|
| Impeller | Spinning vanes add energy to the water | Worn, clogged, or pitted → low flow at a given head |
| Volute | Spiral casing converts velocity into pressure | A crack or gasket leak at the volute is a flood, not a chemistry problem |
| Mechanical seal (or packing) | Seals the spinning shaft | A drip is a warning; a spray is a shutdown; running dry destroys it |
| Motor | Turns the shaft (often C-face coupled) | Heat, a tripped overload, and wrong rotation on three-phase power are electrical/mechanical, not "low chlorine" |
| Strainer pot | Last debris trap before the eye | Basket missing or lid leaking → cavitation and impeller damage |
Never dead-head a pump against a closed discharge valve as a habit. Dead-heading means the impeller stirs a slug of water with nowhere to go. Energy becomes heat. Plastic impellers can melt. Seals cook. Some service procedures mention a brief shutoff-head reading; many manufacturers forbid even that. Daily operator practice is: valves in the designed running position before you press START. Isolation valves exist so you can service the strainer and the pump, not so you can see how high the gauge will go.
Start sequence (hardware)
- Confirm suction and discharge isolation valves are open to the designed path (including heater bypass as labeled).
- Confirm the strainer basket is in, the lid gasket is seated, and the pot is full if the plant is flooded or primed.
- Prime a self-priming pump per the nameplate if the case was opened.
- Start the motor.
- Read suction and discharge gauges, the flow meter, and listen. Gravel sound is cavitation until proven otherwise.
Variable-speed drives shift the entire pump curve down as speed drops. They are a tool for matching flow to a night setback only when the AHJ allows reduced flow. They are not a license to starve the filter or the heater. Speed control is still the same machine: less speed, less head capability, less GPM at a given system curve.
Hold TDH as static lift plus friction. Read the operating point as pump curve versus today's system curve. Treat a dirty filter as extra head and less GPM. Keep the suction side flooded, gasketed, and unthrottled. Open the valves before you start. That is pump hardware for an aquatic facility operator.
Total dynamic head (TDH) on a recirculation loop is best described as which of the following?
A plant is designed for 333 GPM at a stated head. After a busy weekend the filter is dirty. What happens at the pump's operating point?
An operator hears gravel in the pump, feels vibration, and later finds pitting on the impeller vanes. What failure is that pattern describing?
What is the correct daily habit for isolation valves when starting a recirculation pump?