Pump Curves and Speed Rules Made Simple

1. First, the 5 Words You Must Know

Don't memorise — just picture a pump on ship

Imagine a ballast pump moving water from one tank to another. These 5 words describe everything it does.

That's it. The whole chapter is just: how Q, H, N and power are connected.

2. The Main Curve: More Flow = Less Pressure

Every centrifugal pump comes with a graph from the maker called the H–Q curve. Don't fear the graph. It says only one thing:

The one line to remember

When flow (Q) increases, head / pressure (H) falls. When you block the flow, pressure rises. Same as putting your thumb on a garden hose.

How to read this sketch in 10 seconds

Left end = valve closed (no flow, full pressure). Middle dot = sweet spot, run here. Right end = valve full open (full flow, low pressure, motor overloaded). Follow the line from left to right and you are opening the valve slowly.

Why does it slope down? Inside the pump there is friction, swirling and shock as water hits the blades. At high flow these losses increase, so less pressure is left. In theory it would be a straight line, in real life it bends down. You don't need the maths — just remember the shape.

How to read Figure 1 — line by line

Bottom line (Q): flow, zero on left, more flow on right. Side line (H): pressure / head, low at bottom, high at top.

• Top straight line: theory — if there was no friction, head would fall in a straight line. This never happens in real life, just for understanding.
• Shaded gap (shock + eddy + friction losses): energy wasted as water hits blades and rubs the casing. Gap gets wider on the right because at high flow there is more rubbing and swirling.
• Lower curved line (Actual H/Q): what you really get. This is the curve printed in the pump manual.
• Bottom hump (η/Q): efficiency — starts low, rises to a top, then falls. Top = sweet spot (BEP).
• Bottom rising line (HP/Q): power — keeps rising to the right. Means: more flow = more current. So full-open valve = heaviest load on motor.

Two useful points on this curve:

• Shut-off head — discharge valve fully closed. Flow = zero, but pressure is maximum. Pump just churns the same water.
• Run-out end — valve fully open. Lots of flow, very little pressure, motor works hard.

How makers draw this curve? Simple: they run the pump at full speed, keep suction open, start with discharge valve closed, then open it step by step and note pressure and flow each time. Join the dots = curve.

How to read Figure 2 — 3 pumps compared

Bottom line: flow in % (100% = full flow). Side line: total head / pressure.

• Flat curving line (centrifugal): gentle slope down. If pressure rises, flow drops a little. This is our ship's normal water pump — safe and forgiving.
• Steep dashed line (axial): falls sharply. Small pressure rise kills the flow. Like a fan — moves huge air freely, but cannot push against resistance.
• Near-vertical line (reciprocating / piston): almost straight up. Means flow stays ~100% no matter how high the pressure goes — till something bursts. That is why piston pumps MUST have a relief valve, centrifugal need not.
• All 3 lines cross at one dot = the 100% design point where they are compared.

Why no relief valve on a centrifugal pump?

This is a favourite oral question. Answer in simple words:

• With discharge valve closed, the centrifugal pump uses minimum power. It just stirs water.
• Pressure also rises only a little above normal — it cannot keep building pressure like a piston pump.
• So even if the line is blocked, nothing bursts. No relief valve needed.

Contrast: a piston / gear pump (positive displacement) will keep building pressure till the pipe bursts — so it MUST have a relief valve.

Efficiency and the sweet spot (BEP)

Like a bike or car, the pump has a speed where mileage is best. That point is called BEP — Best Efficiency Point. Middle of the curve, smooth and quiet.

How to read Figure 3 — don't panic, it's simpler than it looks

Bottom (GPM): flow. Side (H FEET): pressure in feet. This one picture shows many situations together.

• Big curve on top (#4 Pump): the pump's own curve — what it CAN give.
• Curves starting from middle (Hf+Hv high / low): different pipe systems — "high" = long, rusty, half-closed valve system; "low" = short, clean, open valve system.
• #1, #2, #3 BEP dots: sweet spot shifts when the system changes. There is no single fixed BEP for all pipes.
• "BEST EFF. ZONE" arrow: the maker is saying — try to keep the crossing point inside this zone. That is the 70–120% band we talked about.
• Hp / Hs marks on side: tank pressure and height lift. Higher tank pressure or lift pushes the starting point up.
• For your exam: just point at the top curve and say "pump curve", point at rising curve and say "system curve", point at BEP and say "sweet spot". Enough.

Warning: What if you run far from BEP?

Too far left (valve almost closed): water recirculates inside, vibration, seal and bearing damage, water gets hot. Too far right (valve full open): motor overload, noise, cavitation damage. Stay in 70% to 120% of BEP flow.

3. What Changes the Curve? Bigger Impeller or Faster Speed

A pump's curve is not fixed. Two things shift it — like fan blades:

• 1. Bigger impeller diameter = bigger fan = more flow and more pressure. Whole curve moves UP.
• 2. Wider impeller = can pass more water, but pressure does not rise much. Curve moves RIGHT.
• 3. Faster speed = same effect as bigger impeller. Curve moves UP.

How to read Figure 4 — different diameters

Bottom: flow (Quantity). Side: head / pressure.

• 3 curves, one above the other: top = biggest impeller, bottom = smallest (trimmed) impeller. Same speed, only wheel size changed.
• See the left end: bigger wheel starts HIGHER (more shut-off pressure). Smaller wheel starts lower.
• Diagonal "Best efficiency" line: joins the sweet spot of each wheel. If you trim the wheel, your new sweet spot slides down this line.
• Boxes on the right: pick a pressure (horizontal line), go up to the curve, then down — that is the flow you will get with that wheel size.
• Simple takeaway: oversized pump giving too much pressure? Cut the wheel → whole curve comes down.

How to read Figure 5 — different widths (compare with Fig. 4)

Bottom: flow. Side: head / pressure. Same axes as Fig. 4 — spot the difference:

• All 3 curves start at the SAME point on the left: width does NOT change shut-off pressure much — unlike diameter which did.
• Curves spread out to the right: wider wheel = stretches further right (more flow at same pressure).
• So: need less pressure? trim diameter (Fig. 4). Need same pressure but less/more flow? change width (Fig. 5).
• Oral one-liner: "Diameter controls head, width controls flow."

Why should you care?

On ship we cannot buy a new pump every time. We trim (cut down) the impeller on a lathe to reduce an oversized pump, or we slow it with a VFD. Next section gives the exact rule.

4. Affinity Laws: Just Remember 2 - 4 - 8

Affinity laws sound scary. They are just 3 simple relations. Forget the formulas first, remember this story:

The 2-4-8 rule — if you DOUBLE the speed:

Flow becomes 2 times. Pressure becomes 4 times. Power becomes 8 times.

And reverse: HALF speed = half flow, quarter pressure, one-eighth power.

Why? Think of a ceiling fan: speed 2 gives a bit more air, but electricity and noise jump much faster. Same with pumps.

The 3 rules in one line each

• Flow follows speed directly: 10% faster = 10% more flow. Q ∝ N
• Pressure follows speed × speed: 10% faster = 21% more pressure. H ∝ N²
• Power follows speed × speed × speed: 10% faster = 33% more power. Power ∝ N³

Same rules apply if you trim the impeller (make diameter D smaller). Replace N with D: Q ∝ D, H ∝ D², Power ∝ D³.

Double speed → Flow × 2, Head × 4, Power × 8

Half speed → Flow ÷ 2, Head ÷ 4, Power ÷ 8

Example you will never forget

Pump takes 10 kW with a 10-inch impeller. Trim it to 9 inch (10% smaller):

New power = 10 × (9/10)³ = 10 × 0.729 = ~7.3 kW. Almost 30% saving for a 10% cut!

Same with VFD: run at 50% speed all night to empty a tank when there is no hurry — you use only 1/8th power. Big fuel saving.

Caution: VFD is not for everything

Some pumps MUST give full pressure always — e.g. boiler feed pump, or pump pushing through fine filters. At 50% speed you get only 25% pressure, so water will not reach the boiler drum. For such duties, use VFD only between 85–100% speed. For exam, give this example.

5. The Pipe System Decides the Final Flow

Pump proposes, system disposes

The pump can push, but the pipes, height, tank pressure and valves decide how much water will ACTUALLY flow. That total demand is called TDH.

TDH in plain words — 4 parts

1. Height (Hs): How high must water go? From lower tank level to upper tank level. Higher lift = more head needed.
2. Tank pressure (Hp): Is the discharge tank pressurised? Pushing into a 3 bar tank is harder than into open air. 10 psi ≈ 23 ft ≈ 7 m head.
3. Speed energy (Hv): Tiny part — energy to keep water moving. Usually so small we ignore it. Formula v²/2g only for exam.
4. Friction (Hf): Rubbing loss in pipes, bends, valves, filters, fouled / rusty pipe. Longer pipe and more valves = more loss.

TDH = Height + Tank pressure + Friction (+ tiny velocity part)

Example: ballast pump lifting 10 m + pushing into tank + long rusty pipe with half-closed valve = high TDH = less flow.

The operating point — where two curves meet

How to read Figure 6 — the most important diagram for exams

Bottom (Q/GPM): flow. Side (H FEET): head. There are 3 lines here:

• Downward curve (PUMP CURVE): what the pump CAN do — high pressure at low flow, low pressure at high flow.
• Upward curve (SYSTEM CURVE): what the pipes ASK for — at higher flow they need much more head because friction jumps up. Starts above zero because even at zero flow you must overcome height + tank pressure.
• Bottom hump (EFFICIENCY CURVE): rises to a top called BEP, then would fall. Dotted vertical line shows: ideally your crossing point sits exactly above BEP.
• Big black dot (POINT OF OPERATION): where pump and system curves cross. Pump will run ONLY here. Not where you wish, not where the maker tested — only at the crossing.
• Close the valve → system curve gets steeper → dot moves LEFT (less flow). Open valve → dot moves RIGHT (more flow, motor takes more amps).

• Close the valve / pipe gets fouled? System curve goes steeper → crossing moves LEFT → less flow, more pressure.
• Open the valve / clean pipe? Crossing moves RIGHT → more flow, less pressure, motor takes more current.

This is why a pump correct at commissioning may be wrong after 5 years — pipes foul, filters choke, process changes.

Two pumps together — parallel and series

• Parallel (side by side): Like two taps into one bucket. Flows ADD. Used when you need sometimes one pump, sometimes two, plus standby. Note: two pumps do NOT give exactly double flow because pipes resist more at high flow.
• Series (one after other): Like two pumps lifting water floor by floor. Pressures ADD. Used for very high head, e.g. boiler feed. Multi-stage pump = many small stages in series inside one casing.

6. Specific Speed: Just a Shape Number (Keep It Light)

Examiners love this word. But idea is simple: specific speed (Ns) is just a number that tells you the SHAPE of the impeller.

• Low Ns (500–1,500): Narrow radial wheel — like a pressure cooker pump. High pressure, small flow. E.g. boiler feed, fire pump.
• Medium Ns (1,500–7,000): Mixed / Francis wheel — compromise. Medium pressure, medium flow. Common pumps.
• High Ns (7,000–20,000): Axial / propeller type — like a ship's propeller or table fan. Huge flow, very low pressure. E.g. dock dewatering, cooling water.

Ns = N × √Q ÷ H^3/4   (at BEP, Q in gpm, H in feet)

For exam just say: "Ns is the rpm of a similar imaginary pump that gives 1 gpm at 1 ft head. It classifies impellers and helps select volute casing." No need to calculate unless asked.

Where does pump power get wasted?

Efficiency = useful water power ÷ shaft power. The rest is lost as:

• Rubbing — bearings, seals, disc friction on impeller sides (some impellers are polished to reduce this).
• Shock and swirling at blade entry / exit.
• Leakage back through wear rings, clearances, balance holes.

Pump types by construction (one-liners for oral)

• Overhung: impeller hanging at shaft end — simple, common.
• Between bearings: impeller supported both sides — stronger, for big loads.
• Vertical turbine: long shaft, impellers dipped in sump — no priming needed.
• ANSI / API: standard chemical / oil pumps; API is for hot high-pressure oil duties above ~350 °F.

7. Oral Questions — Short Simple Answers

Draw the pump curve and explain it.

It slopes down: more flow, less pressure. Zero flow gives max pressure (shut-off head). Real curve bends below ideal due to friction and shock losses in the impeller.

Why no relief valve on a centrifugal pump?

At closed discharge it takes least power and pressure rises only slightly. It just churns water. A piston pump keeps building pressure, so it needs a relief valve.

State affinity laws in one line.

Flow ∝ speed, head ∝ speed², power ∝ speed³. Same with impeller diameter.

Double the speed — what happens?

Flow × 2, head × 4, power × 8. Wear and maintenance also rise sharply.

What is BEP?

Best Efficiency Point — the sweet spot in the middle of the curve where pump runs smooth and efficient. Stay within 70–120% of BEP flow.

What is TDH?

Total Dynamic Head — total push the system needs: height lift + tank pressure + pipe friction (+ small velocity part).

What is specific speed?

A number that tells impeller shape: low = high pressure low flow (radial), high = high flow low pressure (axial / propeller).

Two pumps in parallel — double flow?

No, less than double, because pipe friction rises with flow. Each pump moves back left on its curve.

How to reduce flow from 100% to 60% to save power?

Use VFD to slow to 60% speed — power falls to about 0.6³ = 22%. Throttling the valve gives same flow but wastes power.