What does a system head curve show?
A system head curve is the performance curve of the piping, not the pump. It plots the energy needed to push a given flow through the system, with head on the vertical axis and capacity (m³/h, L/s or gpm) on the horizontal axis. It is the sum of static suction head, static discharge head and friction head losses.
Static head stays roughly constant as flow changes. Friction losses rise approximately with the square of liquid velocity. That is why every system curve starts at the static head value at zero flow and bends upward as capacity increases.
Hsystem = Hstatic + Hfriction ≈ Hstatic + k × Q²
Here H is in feet or meters of liquid, Q is flow, and k is a constant for the piping (length, diameter, fittings and roughness) found from a friction loss calculation at one or more flows.
How do pipe size and static head change the system curve?
Pipe size controls how steep the curve is, and static head sets where it starts. A larger pipe lowers velocity, which lowers friction, so the curve rises more gradually. Comparing two layouts on one graph lets a designer see the head each needs at the design flow and pick the most economical option.
A classic comparison plots two systems on the same graph:
| System | Static head | Piping | Head needed at 80 L/s |
|---|---|---|---|
| A | 10 m | 800 m of 200 mm pipe | 40 m |
| B | 5 m | 800 m of 250 mm pipe | 15 m |
Friction losses for each system are calculated at several capacities and joined into a smooth curve. The lower velocity in the 250 mm pipe means less friction, so curve B rises more gradually than curve A.
System B allows a slower pump speed, lower power and lower operating cost. The tradeoffs: the larger pipe costs more to buy and install, and that capital cost may outweigh the energy savings. In slurry service, the lower velocity may also be too low to keep solids in suspension.
Where does a pump actually operate on its curve?
A pump always operates at the point where the system curve intersects the pump performance curve. At that point the energy the piping requires equals the energy the pump adds to the liquid. If either curve moves, the operating point moves with it, so both curves need to be accurate to predict real flow.
Why can one system have more than one system curve?
Most systems have a range of curves because static head changes during normal operation. As a pump empties a sump or tank, the liquid level falls, static head rises and the whole curve shifts upward by that amount. At constant speed, the pump then delivers less flow against the higher head.
How much the flow changes depends on the shape of both curves and on the size of the static head increase. Slurry pumps have fairly flat performance curves, so a small change in static head can cause a large change in flow. Plot the system curve at both high and low sump levels. The pump will deliver a higher capacity at high level and a lower capacity at low level, and both points need to fall in an acceptable range.
Who calculates the system curve, and what can go wrong?
The system designer or pump purchaser usually calculates the system curve using methods from pump industry handbooks. Many inputs, especially the friction factor, are chosen from a range of values, so judgment is involved. If the friction factor chosen is higher than actual, the calculated curve is steeper than the real one.
When that happens, the pump is selected to run at one capacity on the calculated curve but actually runs further out on its curve, at a higher capacity, where it meets the flatter actual curve.
- Clean, nonabrasive liquids: a discharge control valve can add the missing resistance and correct the flow.
- Abrasive slurries: control valves are seldom used because they wear quickly.
- No speed control: the pump runs at the higher capacity. If that exceeds the rate the sump fills, the pump periodically empties the sump and surges.
What does pump surging look like?
Surging is a repeating cycle of high flow, collapse and recovery that happens when a pump empties its sump faster than the sump refills. Test data from a surging pump shows the pattern clearly: flow starts high, drops sharply once the suction inlet is uncovered, then recovers once the level comes back up.
- The pump starts at about 90 L/s.
- The sump level drops until the suction inlet is exposed, and flow falls rapidly.
- Flow stays low until the sump refills enough to submerge the inlet again.
- Flow rises back to nearly the original value, and the cycle repeats.
Surging sharply shortens wear life, can damage other system components and can upset the process.
What other problems come from running above design capacity?
Running further out on the curve than planned causes more than surging. Power, NPSH required and internal velocities all climb with flow, and each brings its own risk. Good practice is to select a pump that can run safely at a flow slightly above the design point, so an optimistic system curve does not cause damage.
- Higher power draw than expected, which may overload the motor.
- Higher NPSH required. If the system cannot supply it, the pump cavitates (see NPSH and cavitation).
- Higher internal velocities, which can accelerate wear.
If you are sizing a pump against a calculated system curve, PCT can review the conditions with you and help select equipment. Request a quote or browse our process pumps.