How to Read Pump Curves
A pump curve is the manufacturer's measured map of what a centrifugal pump will actually do. Horsepower on a nameplate does not size a pump. The curve does. If you can read flow versus head, find the duty point, stay near BEP, and check NPSHr, you can pick a Goulds 3656, a Berkeley B3ZRM, or a PACO LC without guessing.
This is the same process we use when a contractor emails a duty and a model number. The sheets in this guide are the ones in our Resource Center: Goulds Water Technology B363756S R7 for the 3656/3756 S-Group, and Berkeley Curve 5021 for the B3ZRM 3 x 4 x 9 M water-truck pump. If you only came here for B3ZRM pump curves, skip to the second worked example. If you came for a Goulds 3656 pump curve or a Goulds GT20 pump curve, both are walked below.
Read the Axes
Capacity (GPM) on X, total dynamic head (feet) on Y. Confirm RPM, frequency, and the liquid the sheet is based on before you pick a point.
Plot the Duty Point
Intersect required flow with required TDH. The impeller trim or speed curve that passes through that point is the candidate.
Check BEP, Power, NPSH
Confirm the point sits in a healthy efficiency island, under a non-overloading HP line, and that NPSHa exceeds the printed NPSHr.
What you need in hand before the curve matters: required GPM, TDH (or discharge PSI plus elevation and friction), liquid, temperature, and suction condition (flooded or lift). Without those, you are shopping horsepower.
Axes, Duty Point, and Best Efficiency Point
Every centrifugal pump performance curve is the same picture. The horizontal axis is capacity (U.S. GPM on Goulds and Berkeley sheets). The vertical axis is total dynamic head in feet of the pumped liquid. Downward-sloping solid lines are impeller diameters or, on a multi-speed sheet, RPM. The point where your system needs to operate is the duty point.
Flow (capacity)
Required delivery at the operating point, not the pump's published maximum. Oversizing "just in case" pushes you right on the curve, into runout, high NPSHr, and extra power.
Total dynamic head
Static lift plus friction plus any pressure you are pumping into. Curves are in feet. If you were given PSI, convert before you plot.
Head (ft) = PSI x 2.31 Â |Â PSI = Head (ft) / 2.31
Duty point
The intersection of required GPM and required TDH. The impeller trim or speed curve that passes through it is the hydraulic match. Goulds prints this warning on every 3656 sheet: not recommended for operation beyond the printed H-Q curve.
Best Efficiency Point (BEP)
The center of the highest efficiency island on that trim. U.S. DOE pumping-system guidance is to keep the system-curve intersection within about 20% of BEP flow. That is how you avoid chronic throttling, vibration, and wasted kWh.
TDH (ft) = static head (ft) + friction head (ft) + pressure head (ft) Â |Â Pressure head (ft) = (discharge PSI - suction PSI) x 2.31 / SG Â |Â On water (SG = 1.0): 10 PSI = 23.1 ft, 40 PSI = 92.4 ft
The 2.31 factor is the standard U.S. conversion for water at SG = 1.0. Berkeley Curve 5021 states it is based on fresh water at 68 F; centrifugal curve sheets are conventionally rated on clear water unless marked otherwise. If you are on glycol or a heavier fluid, divide by specific gravity before you plot, and scale brake horsepower by SG.
Header (model, RPM, Hz, size)
Meaning: Tells you which hydraulic you are on. A 3656 1-1/2 x 2-6 at 3500 RPM is a different map than the same size at 1750 RPM.
Check: RPM and suction x discharge x impeller match the nameplate.
H-Q curves (solid)
Meaning: Head the pump makes at each flow for one impeller diameter or one speed.
Check: Duty point sits on or just under a printed trim, not past the right-hand end.
Efficiency islands (ovals)
Meaning: Percent of hydraulic output versus input power. Peak island is BEP.
Check: Duty is inside a high island, not on the skinny left or far right.
HP / BHP lines (diagonals)
Meaning: Input power at that point, usually on water.
Check: Motor HP is at or above the line, preferably non-overloading across the usable curve.
NPSHr (dashed, often a second plot)
Meaning: Head the pump needs at the suction nozzle to avoid cavitation.
Check: NPSHa exceeds NPSHr. Margin gets thinner on hot water and on lift.
Anatomy taken from Goulds B363756S R7 (3656/3756 S-Group) and Berkeley Curve 5021 (B3ZRM). Other OEMs draw the same five layers.
Efficiency Islands, Impeller Trims, and VFD Curve Families
A pump curve chart is rarely one line. Goulds prints a family of impeller diameters on one speed. Berkeley prints either a family of diameters at one RPM (B3ZRM at 1750) or a family of speeds at one diameter (B3ZRM at 9.00 in, 1300 to 2500 RPM). A VFD is the second picture: you ride down a speed curve instead of throttling a valve.
Impeller trims
On the Goulds 3656 1-1/2 x 2-6 sheet (B363756S R7, 3500 RPM, Imp. Dwg. 114-04) the standard trims are A 5-15/16 in, B 5-5/8 in, C 5-1/8 in, and D 4-3/4 in. Each smaller diameter drops the whole H-Q curve down and to the left. Goulds notes that trims are cut in 1/16 in increments only, and that operation past the printed H-Q curve is not recommended. That is why we would rather start with a mid-range impeller we can trim than a max-diameter pump we have to throttle.
Speed families and VFDs
Berkeley Curve 5021 page 46 is the multi-speed view of the same 3 x 4 x 9 M hydraulic. The B3ZRM frame-mount coupling listing is rated 1400 to 2500 RPM (belt drive 1400 to 2300 RPM). The sheet marks a 2500 RPM limit for coupling, SAE, and hydraulic drives, and a 2300 RPM limit for belt. NPSH data on that page is at maximum speed. If you put a VFD on that pump, you are sliding down those RPM curves, not inventing a new hydraulic.
Flow: Q2 / Q1 = N2 / N1 = D2 / D1 Â |Â Head: H2 / H1 = (N2 / N1)^2 = (D2 / D1)^2 Â |Â Power: P2 / P1 = (N2 / N1)^3 = (D2 / D1)^3 Â |Â N = speed (RPM), D = impeller diameter. Water, similar operating region.
Those three relations are the standard centrifugal affinity laws used across Hydraulic Institute materials and OEM VFD notes. They are accurate near the original duty. They are not a license to run a 1750 RPM pump at 3500 RPM, or to treat a trimmed impeller as a new catalog model without reading the printed curve.
Power falls faster than flow. Head falls faster than flow.
Cut speed 20% and flow drops about 20%, but head drops about 36% and power drops about 49%. That is why a VFD that "almost" makes the duty on a hot afternoon often does not, and why we still size the pump on the 60 Hz curve first, then check the reduced-speed family.
NPSHr vs NPSHa
NPSHr is the head the pump requires at the suction nozzle to keep the eye of the impeller from cavitating. NPSHa is the head the system actually provides. The curve only prints NPSHr. You have to calculate NPSHa from tank elevation, atmospheric pressure, vapor pressure (temperature), and suction-line friction.
NPSHr (from the curve)
Printed as dashed verticals on Goulds 3656 sheets, or as a second plot under the B3ZRM H-Q chart. It rises as flow increases. On Curve 5021 the 1750 RPM NPSHr is about 10 ft near 400 GPM and climbs toward 25 ft near 750 GPM. The multi-speed page states NPSH data is at maximum speed.
NPSHa (from your system)
Atmospheric head plus static suction head (or minus lift) minus vapor pressure minus suction friction. Hot water raises vapor pressure and eats NPSHa. A lift application subtracts elevation. If NPSHa is not greater than NPSHr, the pump cavitates: noise, lost head, pitted impellers, repeated seal failures.
NPSHa must exceed NPSHr. The curve will not warn you twice.
We will not quote a one-size margin here. Hydraulic Institute practice is to keep a real surplus, and the surplus has to get larger as temperature rises or as you approach the right-hand side of the curve. If you are on condensate, hot water above about 180 F, or a suction lift, send us the tank elevation, liquid temperature, and suction piping and we will run the numbers against the sheet.
NPSH check before you order
Have these four numbers before we sign off a hot-water or lift selection.
Worked Example: Goulds 3656 Pump Curve
The Goulds 3656 is close-coupled; the 3756 is the same hydraulic on a bearing frame. S-Group sizes on B363756S R7 are 1 x 2-7, 1 x 2-8, 1-1/2 x 2-6, 1-1/2 x 2-6H, 1-1/2 x 2-8, 2-1/2 x 3-7, and 3 x 4-7, at 3500 or 1750 RPM. Construction is all iron, bronze fitted, or all bronze. Factory-assembled units are NSF/ANSI 61 certified (minimum usage 2 L/day for bronze-fitted models; Buna seal models are not certified).
S-Group published limits on that brochure: working pressure 175 PSIG, suction pressure 100 PSIG, 212 F with the standard Type 21 carbon/ceramic/BUNA seal or 250 F with the optional seal. Those same 175 PSIG and 212/250 F figures repeat on the S-Group product rows in our catalog. The page-3 coverage chart shows the 3500 RPM family envelope out to about 500 GPM on the 3 x 4-7 and about 300 ft of head on the 1 x 2-8. Catalog rows also print a series range of 550 GPM and 280 ft TDH at 3500 RPM (350 GPM and 67 ft at 1750 RPM). Treat 550/280 as the catalog series callout; use the coverage chart and the individual size curve for the duty.
M and L-Group (B3656M/L R6, confirmed on the 3656/3756 M&L product rows) step up to 1600 GPM and 490 ft TDH at 3500 RPM, or 4500 GPM and 300 ft at 1750 RPM, with working pressure to 200 PSIG. The 3656/3756 LH is the low-head sibling. Open the S-Group sheet for the example below.
Duty: 100 GPM at 120 ft, 3500 RPM, water
Open the right sheet. B363756S R7, 60 Hz, 3500 RPM, size 1-1/2 x 2-6 (Imp. Dwg. 114-04). Header must say 3500 RPM. The 1750 RPM page is a different map.
Plot 100 GPM on X and 120 ft on Y. On the full A trim (5-15/16 in) that size makes about 135 ft at 100 GPM, so the duty sits under the A curve with head to spare.
Read the island. 100 GPM at 120 to 135 ft sits in the low-70s efficiency band, right around the 73% peak island on this sheet. That is a BEP-side selection, not a runout selection.
Read power. The 3 HP and 5 HP diagonals cross this chart. 100 GPM on the A trim is a 5 HP motor if you want non-overloading coverage. 3AI1JBA3 is that exact hydraulic: 1-1/2 x 2-6, 5 HP, 3500 RPM, 5.94 in (5-15/16 in) impeller, all iron.
Read NPSHr. Dashed NPSHr lines on this page run from 5.5 ft on the left to 20 ft at the far right. At 100 GPM you are near the 7 and 8 ft lines. NPSHa has to beat that number. Do not skip this on a lift or on water above about 180 F.
Decide on a trim. If 135 ft is more head than the system wants, Goulds will trim in 1/16 in steps toward B (5-5/8 in). Do not pick a smaller catalog size and hope. Read the next size's curve.
Same process, different size. A 2 HP, 1750 RPM, 2-1/2 x 3-7 bronze-fitted unit such as 4BF2G7C0 is a different page of the same brochure (6-9/16 in impeller). Never reuse the 1-1/2 x 2-6 numbers on a 2-1/2 x 3-7. Open that size's curve.
S-Group
Range: Coverage to about 500 GPM / 300 ft at 3500 RPM (B363756S R7 p.3); 175 PSIG; 212 F std / 250 F optional
Link: 3656 / 3756 S
M & L-Group
Range: 1600 GPM and 490 ft at 3500 RPM; 4500 GPM and 300 ft at 1750 RPM; 200 PSIG (B3656M/L R6)
Link: 3656 / 3756 M&L
LH
Range: Low-head S-family sibling; use its own curve sheet, not the S-Group 3500 RPM pages above
Link: 3656 / 3756 LH
Worked Example: B3ZRM Pump Curves
The B3ZRM is a Berkeley Type B frame-mount centrifugal, not a Burks pump. Search traffic still lands on "b3zrm pump curves" because water-truck and irrigation buyers look up the hydraulic, then the BM number. The pump we stock is the 3 x 4 x 9 M (suction 4 in, discharge 3 in, nominal 9 in impeller) on Berkeley Curve 5021. Catalog model-number is B3ZRM. Common NPT builds are B66161 (CW) and B66168 (CCW). Victaulic twins are B66899 (CW) and B66895 (CCW). All four are packed stuffing box, cast iron impeller, standard frame.
Curve 5021 page 45 is the 1750 RPM family. Fresh water at 68 F. Maximum working pressure 266 PSI (18 bar). Impeller diameters on that page: 9.00, 8.438, 7.875, 7.00, and 6.063 in. Efficiency islands run 39, 55, 68, 76, 81, and 82%. Horsepower overlays include 3, 5, 8, and 10 HP. Flow axis goes to about 850 GPM; head axis to 100 ft.
Duty: 400 GPM at 65 ft, 1750 RPM, water
Open Curve 5021, 1750 RPM page. Confirm size 3 x 4 x 9 M and model family B3Z_M (B3ZRM is the frame-mount coupling and belt listing).
Plot 400 GPM and 65 ft. That point lands at the 8.438 in trim (the full 9.00 in impeller makes about 80 ft there), inside the 82% island, the best efficiency this sheet prints.
Read power. You are above the 8 HP overlay and onto the 10 HP band. Size the driver from the curve, not from a neighbor's 7.5 HP truck PTO.
Read NPSHr. The lower plot on page 45 shows about 10 ft NPSHr at 400 GPM, rising to about 25 ft near 750 GPM. A short flooded suction on a water truck usually clears 10 ft. A long lift or a hot fill will not.
If the truck runs a different RPM, turn the page. Page 46 holds the 9.00 in impeller at 1300, 1500, 1700, 1900, 2100, 2300, and 2500 RPM. B3ZRM coupling is listed 1400 to 2500 RPM. NPSH on that page is at maximum speed, so do not use the 1750 RPM NPSHr blob for a 2500 RPM duty.
Rotation and connection are not on the curve
CW versus CCW, and NPT versus Victaulic, do not change Curve 5021. They do change the part you receive. B66161 is CW NPT; B66168 is CCW NPT. Order the rotation that matches the PTO or engine. The hydraulic you just read is the same on all four BM numbers.
Where to Find Pump Curves on PumpCatalog
Curves live in two places on this site: the Resource Center (filter by manufacturer and series) and the Literature tab on the product or series page. We do not keep a separate "curve finder" per SKU. If the series page lists the technical brochure, that is the sheet to open.
Close-coupled (3656) and frame-mount (3756) end suction. Open B363756S R7 from the series documents for the 3500 and 1750 RPM families used in the first worked example.
Watch for: Each pump size is its own curve page. A 1-1/2 x 2-6 number is useless on a 2-1/2 x 3-7.
Frame-mount 3 x 4 x 9 M. Curve 5021 is the B3ZRM performance sheet (1750 RPM trims plus the 9.00 in multi-speed family). Search the BM number (B plus five digits) if you already have a truck pump.
Watch for: B3ZRM is Berkeley, not Burks. Rotation (CW/CCW) and NPT vs Victaulic are order details, not curve details.
The GT20 we stock is the 2 HP GT IRRI-GATOR self-priming centrifugal (GT20), not a different residential jet SKU. BGT R7 rates the family to 110 GPM at 5 ft suction lift and 128 ft of head, 3500 RPM, 140 F, 125 PSI, reprime to 25 ft.
Watch for: GT20 performance is a ratings table (GPM vs discharge PSI vs suction lift), not only an H-Q plot. Read the table on BGT R7 page 3.
PACO curve PDFs are not in the re-hosted library as standalone H-Q sheets. Series ranges below are from the 2022 PACO All Products brochure. For a specific duty, open the series page or contact us and we will pull the current selection sheet.
Watch for: Brochure maxima are family envelopes, not a single pump.
20 PSI (46 ft of water)
5 ft: 86
10 ft: 77
15 ft: 70
20 ft: 59
25 ft: 46
30 PSI (69 ft of water)
5 ft: 80
10 ft: 72
15 ft: 67
20 ft: 57
25 ft: 44
40 PSI (92 ft of water)
5 ft: 65
10 ft: 60
15 ft: 57
20 ft: 50
25 ft: 43
GPM from Goulds BGT R7 performance ratings table. PSI-to-feet column uses Head (ft) = PSI x 2.31 on water. Single- and three-phase ratings are the same on that sheet. GT20 is 2 HP (BGT R7 model table).
LC / LCV close-coupled end suction
Range: To 4,000 GPM, 400 ft, 175 psi, 275 F (brochure only)
Link: LC
Envelopes from PACO All Products Sales Brochure (2022). No standalone PACO H-Q PDF is in the document library as of this writing; ask us for the selection sheet on a real duty.
Common Curve-Reading Mistakes
Selecting on horsepower instead of the duty point
HP is an output of flow, head, and efficiency. Start with GPM and TDH, then let the HP overlay tell you the motor. A 5 HP 3656 and a 5 HP B3ZRM are not interchangeable.
Confusing the 1750 RPM page with the 3500 RPM page
Same pump size, different map. Goulds prints both in B363756S R7. Plotting a 3500 RPM duty on the 1750 RPM curve (or the reverse) is the fastest way to buy the wrong impeller.
Ignoring BEP and running off the right-hand end
Past the printed H-Q curve Goulds says do not operate. Runout raises NPSHr, radial load, and vibration. Stay inside a healthy island, ideally within about 20% of BEP flow.
Reading NPSHr and calling it NPSHa
The dashed line is what the pump needs. Your tank, temperature, and suction pipe decide what you have. Curve 5021 even warns that the multi-speed NPSH plot is at maximum speed.
Treating a VFD as a new pump
Affinity laws scale an existing curve. They do not add head you never had at 60 Hz. Size on the full-speed sheet, then check the reduced-speed family (B3ZRM page 46 is the picture).
Using a brochure envelope as a duty curve
PACO LC "to 4,000 GPM" is the family maximum, not a pump that makes 4,000 GPM at 400 ft. Open the size-specific selection sheet before you buy.
Frequently Asked Questions
How do you read a pump curve?
Confirm the header (model, RPM, Hz, size). Plot required GPM on the X axis and required TDH on the Y axis. The impeller trim or speed curve through that duty point is the hydraulic. Then read the efficiency island, the HP overlay, and NPSHr at that same GPM. NPSHa must exceed NPSHr.
What is a pump curve chart?
A pump curve chart is the manufacturer's plot of head versus flow for one hydraulic, usually with efficiency islands, horsepower lines, and NPSHr. Goulds 3656 sheets also show a family of impeller diameters. Berkeley B3ZRM Curve 5021 shows diameters at 1750 RPM and a speed family at 9.00 in.
How do I read a Goulds 3656 pump curve?
Open B363756S R7 (S-Group) or B3656M/L R6 (M&L) from the series page or Resource Center. Pick the page that matches your size and RPM. Example: 100 GPM at 120 ft on the 1-1/2 x 2-6, 3500 RPM sheet sits under the 5-15/16 in trim near the 73% island and wants a 5 HP non-overloading motor (see 3AI1JBA3).
Where do I find PACO pump curves?
We do not have standalone PACO H-Q PDFs in the document library. Family envelopes are in the 2022 PACO All Products brochure (LC to 4,000 GPM / 400 ft, LF to 6,500 GPM / 400 ft, VL/VLS to 3,900 GPM / 420 ft, KP to 20,000 GPM / 730 ft). For a real duty, start at PACO or contact us and we will pull the current selection sheet.
How do I read a Goulds GT20 pump curve?
The GT20 on this site is the 2 HP GT IRRI-GATOR. BGT R7 gives a ratings table, not only an H-Q plot: at 5 ft suction lift the GT20 makes 86 GPM at 20 PSI, 80 GPM at 30 PSI, and 65 GPM at 40 PSI. Family limits are 110 GPM at 5 ft lift and 128 ft of head, 3500 RPM, 140 F, 125 PSI.
Where are B3ZRM pump curves?
Berkeley Curve 5021, filed as B3ZRM Performance Curves in our library and on the B3ZRM series page. Page 45 is 1750 RPM impeller trims (3 x 4 x 9 M). Page 46 is the 9.00 in multi-speed family (B3ZRM coupling 1400 to 2500 RPM). Stock NPT examples: B66161 CW and B66168 CCW.
If the duty is clean water, flooded suction, and sits on a printed trim near BEP, you can order from the curve. Call or send the application when any of these are true:
Hot water, condensate, or anything where vapor pressure will eat NPSHa.
Suction lift, a long suction run, or a vacuum-side strainer you have not counted.
A VFD duty that has to make head at reduced speed, not only at 60 Hz.
A B3ZRM replacement where rotation, PTO speed, or packing versus seal is unclear.
A PACO or Goulds size that is on the edge of two trims and you want a factory selection.
Talk to a Pump Catalog application engineer
Send the duty (GPM, TDH or PSI, liquid, temperature, suction condition) and the curve you are looking at. We will confirm the trim, the motor, and the NPSH margin before you buy.