Build a system curve and read the operating point

Static head, with the pump centerline as datum

CurvePoint measures elevations from the pump suction centerline. Liquid above that centerline is flooded suction head. Liquid below it is suction lift, and it adds to the head the pump must make. Discharge static head is the discharge surface, or the pressure equivalent of a closed tank, above the same datum. Total static head is discharge static minus the suction surface. A 6 ft lift and 24 ft of discharge head is 30 ft of static head before a single foot of pipe is counted. Flip the suction selector to flooded head and that static term falls, because the wet well is already helping.

Get the datum wrong and every later number moves with it. Measuring from grade, from the floor, or from the waterline at one tank and the nozzle at the other will invent or hide static head. Sketch the surfaces, mark the centerline, and write the two elevations in the same units. Pressure on a closed suction tank is static head too, once it is converted to feet of the pumped fluid. This page keeps the fluid as water so the conversion stays out of the way. The full calculator accepts other fluids, including fuels and dilute wastewater.

Friction head from Darcy-Weisbach

Pipe friction head is f (L/D) V²/2g. The length L is the pipe you type. The diameter D is the inside diameter of Schedule 40 steel at the nominal size you pick, not the nominal size itself. A 4 in Schedule 40 pipe is not 4.00 inches inside, and that difference shows up in velocity. Velocity is flow divided by the internal area, so dropping from 4 in to 3 in does more than a proportional increase in loss. The system curve steepens, and the intersection with the same pump curve moves left, toward less flow and more head.

The friction factor f is not a constant you pick from a table in this tool. Churchill’s 1977 equation covers laminar, transitional, and turbulent flow from the Reynolds number and the relative roughness of commercial steel. Cold water in a few-inch steel line is usually turbulent at the flows on this chart, so f sits in a narrow band, but a long suction line at low flow, or a viscous fluid in the full app, can leave that band. Changing length scales friction almost directly. Changing diameter changes velocity, Reynolds number, and f together. That is why “add 50 ft” and “go down one pipe size” are not interchangeable edits.

Fittings are part of the system curve

A fitting loss is K V²/2g, with K from the same catalog the app uses. On this page the suction line always includes a sharp-edged inlet plus the number of standard 90° elbows you set. The discharge line includes the long-radius elbows you set, one full-open gate valve, one swing check, and an outlet into a tank or basin. The outlet is easy to forget. It is a full velocity head, and on a short discharge line it can be a large share of the friction. A swing check is larger again. If the real station has a different check, a strainer, or a partly closed valve, this estimate is low until you account for them in the app.

Count elbows in the plane you will actually build, including the ones hidden in a valve vault. Equivalent length is a shortcut this page does not need, because the loss is evaluated as K at the velocity of that segment. Suction fittings hurt twice: they raise the system curve, and they consume NPSH. Discharge fittings raise only the system curve. If you are sizing a lift, keep the suction line short and large, and spend the length on the discharge side where the pump has already done its work. The suction lift sizing guide walks through that layout, and the NPSH calculator isolates the suction terms.

The operating point is an intersection

The red curve on the chart is total head versus flow: static head plus suction friction plus discharge friction. It starts at the static head when flow is zero and rises as friction grows. The blue curve is the illustrative pump, falling from shutoff head to the runout head at maximum catalog flow, then stopping. Head at that end stays near half of shutoff. Where the curves cross inside that flow range, the readout lists flow, head, the static split, both friction terms, and NPSH available. Those are estimates. NPSH available here is the app’s result for water at the default site elevation, with the suction loss of this simplified line. It is not a substitute for reading a published pump curve.

If the curves do not cross inside the pump’s flow range, there is no operating point on this curve. The chart says so. It does not extend the pump line to zero head to force a crossing. Raising shutoff head can create an intersection. So can shortening the discharge or opening the pipe size, which drops the system curve. Neither edit tells you the pump will be happy there. The intersection can sit far left of a pump’s preferred range, where radial load and temperature rise, or far right, where NPSH margin disappears. The pump curve versus system curve page shows what a throttle and a speed change do to that same intersection without resizing the pipe.

Three static levels, one piping layout

A wet well does not sit at one elevation. Houston’s lift-station manual asks for system curves at the low water level, the high water level, and a flooded suction. The figure above keeps the pipe and the illustrative pump fixed and moves only the suction surface. Low water adds lift, so the red curve starts higher and the intersection moves left. High water and a flooded suction spend less static head, and the same pump delivers more flow. Design the duty at the level that is hardest for the pump you are checking. For NPSH that is low water. For force-main velocity it is often the flooded case, when friction has the most flow to carry.

Aging belongs in that picture as a change in absolute roughness, ε, in millimetres. CurvePoint does not use a C factor. Pipe friction is Darcy-Weisbach with Churchill’s friction factor, from Reynolds number and ε/D. Fitting losses are the Darby 3-K method already in the app: K depends on Reynolds number, the fitting, and the nominal size. An older force main is a larger ε, which steepens the system curve and moves the intersection. The wastewater example runs that comparison on one pump, with new steel and an illustrative aged roughness side by side.

A branch is not the force main

A duplex station gives each pump its own suction and discharge branch, then a shared header and a force main. A swing check and a plug valve usually sit on each branch so one pump can be isolated and so a stopped pump does not spin backward. CurvePoint can add those pipes in series for the pump that is running: branch, header, and force main as one path. Two pumps on one force main are a different system. Each branch then sees its own flow, and the shared main sees the sum, so the second pump adds less than a second copy of the first duty. This page does not print that combined curve. The wastewater example describes the station and calculates only the one-pump path.

What this page leaves for the app

One steel segment per side is enough to see the shape of the problem. A real station often has a hose, a buried run, a different material, and a second diameter. CurvePoint accepts multiple segments, HDPE with a dimension ratio, PVC, and a custom roughness. It also accepts the fluid you are actually pumping, including a viscosity correction when the liquid is no longer water-like, and it will overlay a curve you trace or import instead of this illustration. Pro adds the PDF: duty, friction on each side, the NPSH check, the methods, a sketch, and the curve.

Use the numbers on this page to test your instincts. Then rebuild the line in the calculator with the fittings you will buy. The button below opens this page’s mid-level example already filled in. A worked file with 4 in steel, a 12 ft lift, and a reported intersection of 301 gpm at 49.6 ft is on the suction-lift wastewater example, and the fictional Lakeshore PDF for that study is the sample report. Pricing for the report export is on the pricing page, and the product overview is on the CurvePoint homepage.