Calculating Worthington 6LR-13A Pump Curve at 1475 RPM

The plotted curve on the operator’s screen may look authoritative while still representing the wrong shaft speed. For the Worthington 6LR-13A, the installation data identifies 1475 RPM and 30 kW, while the located reference curve is described once as 1175 RPM and elsewhere as 1170 RPM. Resolve that five-revolution discrepancy from the curve title block, scale every data point with the same verified ratio, and treat the 30 kW value separately until its meaning is known.

What is the displayed pump curve telling you?

Start at the displayed axes and metadata. A centrifugal-pump curve binds flow, head, absorbed power, efficiency, impeller diameter, and speed. If the speed attached to the dataset is wrong, a correctly entered flow or head point still plots in the wrong operating envelope.

Record the curve revision, printed speed, impeller diameter, flow units, head units, and power basis alongside the digitized points. The check passes when the plotted dataset and its speed label refer to the same source curve.

Which reference speed should drive the calculation?

The reference is described as 1175 RPM, but a later instruction calls it a 1170 RPM curve. Do not average those values or choose one from memory. Read the curve’s title block or speed annotation and bind that value to the dataset.

The original request specifies 1475 RPM; a later commissioning note proposes 1480 RPM. These are separate targets. Moving a finished 1475 RPM curve to 1480 RPM multiplies flow by about 1.00339, head by about 1.00679, and power by about 1.01020. The check passes when the base speed comes from the curve sheet and the target speed matches the physical machine or measured operating speed.

How do you scale every point with the affinity laws?

Use the affinity laws when the pump, impeller diameter, liquid, and hydraulic configuration remain unchanged and the two operating states are sufficiently similar. Define N1 as the printed reference speed and N2 as the target speed:

r = N2 / N1
Q2 = Q1 × r
H2 = H1 × r²
P2 = P1 × r³

Flow Q changes directly with speed, head H changes with speed squared, and absorbed power P changes with speed cubed. Preserve the original units; the factors are dimensionless.

1. Digitize each reference-curve flow and head coordinate. Include shutoff, several intermediate points, the best-efficiency region, and the high-flow end.
2. Enter the verified reference speed as N1 and 1475 RPM as N2.
3. Calculate one speed ratio and apply it to every row. Do not round the ratio before calculating head and power.
4. Multiply each flow value by r and each head value by r².
5. If the sheet contains absorbed-power points, multiply them by r³. Keep motor nameplate power out of this column.
6. Plot the transformed points and label the result with the target speed and unchanged impeller diameter.

Do not move an efficiency curve merely by changing its axis label. Affinity scaling commonly treats efficiency as approximately unchanged for preliminary work, but speed-dependent hydraulic and mechanical losses can shift actual performance. Likewise, obtain net-positive-suction-head requirements from applicable pump data or a test instead of presenting an affinity-scaled estimate as a verified limit. The check passes when a back-calculation of any transformed row returns the original point.

What does the 30 kW value constrain?

30 kW is not enough by itself to establish a flow-head curve. First identify the field that supplied it.

If 30 kW were absorbed power on a 1175 RPM base curve, the affinity estimate at 1475 RPM would be about 59.3 kW. If it were absorbed power at 1170 RPM, the corresponding estimate would be about 60.1 kW. Those calculations do not mean the installed 30 kW motor can deliver either value. Conversely, if 30 kW is the installed motor rating at 1475 RPM, compare it against the maximum absorbed power over the intended operating region, not only the selected duty point.

The check passes when the 30 kW field has a documented meaning and the scaled absorbed-power curve stays distinct from the motor-rating line.

How should the scaled curve be connected to the duty point?

A scaled pump curve predicts pump capability; it does not identify the operating point on its own. The actual duty occurs where the 1475 RPM pump curve intersects the system curve. Static head sets the flow-independent portion, while piping, valves, fittings, and equipment losses contribute a flow-dependent component commonly represented as proportional to flow squared.

1. Plot the scaled Q-H coordinates at 1475 RPM.
2. Build the system curve from measured or calculated static and friction head using the same flow and head units.
3. Locate the intersection and read its predicted flow and head.
4. Interpolate the scaled absorbed-power curve at that flow.
5. Compare the point with the acceptable operating region, driver capability, suction conditions, and process requirement.

Throttling a discharge valve changes the system curve; changing rotational speed changes the pump curve. Keep those actions separate in the commissioning record. The check passes when the plotted intersection reproduces both the expected process flow and total developed head without exceeding the applicable equipment limits.

When is a reconstructed curve acceptable?

The model code was interpreted as indicating a 13-inch impeller and a 6-inch discharge, but confirm both dimensions from the pump, records, or a dimensional inspection before using that interpretation. Model-code conventions can vary with configuration and production history.

A rough centrifugal-pump curve can be constructed when no reference curve exists by combining confirmed geometry, speed, measured duty points, and a suitable curve shape. One proposed shortcut uses a quadratic head curve and estimates shutoff head as 1.28 × BEP head. Another estimates flow from pipe area using assumed velocities of 10 ft/s at suction and 22 ft/s at discharge. These are screening assumptions, not replacements for the located reference curve or a performance test.

Because a reference 6LR-13A curve is available, scaling that curve is preferable to reconstructing one from nominal geometry. The check passes when the reconstruction is either removed from the final dataset or clearly marked as an estimate and compared against the scaled reference.

How do you verify the 1475 RPM curve end to end?

Commission the data path from the physical pump back to the displayed curve. A correct formula cannot repair the wrong pump identity, impeller, speed, units, or power definition.

1. Match the equipment identification to Worthington 6LR-13A.
2. Read actual rotational speed with a suitable instrument or validated controller feedback; distinguish 1475 RPM from the proposed 1480 RPM.
3. Confirm the installed impeller diameter rather than relying only on the model-code interpretation.
4. Verify the base curve’s printed speed as either 1170 or 1175 RPM.
5. Check several spreadsheet rows manually using Q ∝ N, H ∝ N², and P ∝ N³.
6. Measure a stable operating point: flow, suction pressure, discharge pressure, and speed. Convert the pressure readings to developed head using the actual measurement elevations and liquid properties.
7. Plot the measured point against both the scaled pump curve and the system curve. Investigate material deviation through speed, impeller diameter, valve position, blockage, recirculation, wear, air entry, instrument calibration, and unit conversion.
8. Repeat at additional safe operating points where the process permits, then retain the final curve with its base-speed source, scaling ratio, target speed, units, and test date.

The check passes when measured flow and developed head plot at the expected intersection while measured speed, driver loading, and suction conditions remain within their documented limits.

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