Sizing Control Valve for a Fixed-Speed Centrifugal Pump

A fixed-speed pump reaches all three modes (10 m3/h at 3.0 bar, 15 m3/h at 3.2 bar, 20 m3/h at 3.4 bar) only if its curve at 10 and 15 m3/h sits above 3.0 and 3.2 bar with enough margin to leave the control valve a workable pressure drop. The valve removes head. It never adds any. The source text lists the third duty as "Q=3.4 bars"; read here as H = 3.4 bar. The bar values are treated as the pressure the process needs at the valve outlet for each flow, and that reference point must be confirmed before anyone sizes anything.

Which fixes fail for a three-mode fixed-speed pump?

Each of these is a common first move, and each breaks on the same physics: the pump always operates on its own H-Q curve.

• Size the pump for the middle mode (15 m3/h at 3.2 bar) and throttle for the others. Throttling only moves the operating point up-curve toward lower flow. It cannot deliver 20 m3/h at 3.4 bar from a pump that peaks out at 15 m3/h at 3.2 bar.
• Size the pump for 20 m3/h at exactly 3.4 bar with a fully open valve. The valve then has zero drop at maximum flow, so it has no control authority at the mode where it is most open. Line and valve losses at 20 m3/h also push the real requirement above 3.4 bar.
• Ask the pump supplier for three separate duty points. A single-speed pump has one curve. Three points that do not lie on that curve cannot all be satisfied by the pump alone. The gap between the curve and the required head at each flow is the valve drop.
• Assume "15 m3/h can never match 3.2 bar" from someone else's curve. That statement depends entirely on the specific curve. This pump has not been purchased, so no curve exists yet. On a normal falling H-Q curve, the head at 15 m3/h is higher than at 20 m3/h. The valve burns the excess.
• Give the valve vendor one dP for all three flows. The valve drop differs per mode because the pump head differs per flow. One dP produces a valve that is oversized at low flow or undersized at high flow.
• Jump to a variable-speed drive because the duty points differ. A VSD is a valid alternative, but the decision comes from an energy and curve-fit comparison (below), not from the fact that three points exist.

Where does the operating point sit on the H-Q curve with a valve in series?

The pump delivers head H_pump(Q) at each flow. The system plus valve consumes H_required(Q) + Δp_valve(Q). The two must be equal at steady state, so:

Δp_valve(Q) = H_pump(Q) - H_required(Q)

Closing the valve raises its resistance until this balance is met at the target flow. There is always a valve position that hits 15 m3/h, and at that position the pump head equals 3.2 bar plus the valve drop. The requirement for feasibility is H_pump(Q) ≥ H_required(Q) at every mode, with a minimum controllable drop on top.

The three required points do not fit a clean H0 + kQ² system curve. Between 10 and 15 m3/h the implied k is 0.0016 bar/(m3/h)². Between 15 and 20 m3/h it is about 0.0011. Either the reference point contains process-side pressure control, or the numbers are rounded design points. Treat them as three independent targets, not a curve. The required head rises only 0.4 bar across a doubling of flow, while a fixed-speed curve typically falls with flow. That divergence is what the valve absorbs.

Unit conversion for water: 1 bar ≈ 10.2 m of head (100,000 Pa / (1000 kg/m³ × 9.81 m/s²)). Supplier curves quote head in metres; convert before subtracting.

How do I pick the pump so all three modes are reachable?

Select on the worst case, which is the highest flow at the highest required head, plus the valve allowance. Using a 0.5 bar allowance for the valve at 20 m3/h:

H_pump(20 m3/h) ≥ 3.4 + 0.5 = 3.9 bar (≈ 39.8 m)

If the pump curve falls monotonically with flow (a stable curve; confirm on the supplier's curve), head at 15 and 10 m3/h is at least 3.9 bar. That gives lower bounds on the valve drop:

The real drops are larger, because a normal curve climbs toward shutoff. Read the actual head at 10 and 15 m3/h from the supplier curve at the actual impeller diameter and speed. When choosing among candidate pumps, check three items on the curve:

• Mode 1 (10 m3/h) sits within the manufacturer's minimum continuous stable flow and preferred operating region. Throttling pushes the pump to the left of its best efficiency point.
• Motor rating covers the power at the end of the curve, since a valve that fails open or a low-resistance line drives flow beyond 20 m3/h.
• NPSH available exceeds NPSH required at 20 m3/h with margin. Read both values from the datasheet and the suction system calculation.

What goes on the control valve datasheet?

Send the valve vendor three complete cases, one per mode: flow, inlet pressure, outlet pressure, fluid, temperature. The vendor computes Kv (or Cv) per case. For water, with Q in m3/h and Δp in bar:

Kv = Q * sqrt(SG / Δp)     (SG = 1 for water)

Applying the lower-bound drops from the table gives the upper bound on required Kv in each mode:

These are preliminary numbers. The final datasheet uses the real pump head at each flow, so the required Kv at modes 1 and 2 comes out lower and the spread between modes widens. The valve must cover that spread inside its controllable travel, with the smallest Kv well above the valve's minimum controllable opening. Ask the vendor to state the installed characteristic and the travel percentage at each of the three cases, and to check cavitation and noise at the highest Δp, which occurs at the lowest flow.

1. Confirm the pressure reference point for the 3.0, 3.2, and 3.4 bar values (valve outlet, or system inlet including downstream line losses).
2. Choose the pump on 20 m3/h at 3.4 bar plus the valve allowance.
3. Read pump head at 10, 15, and 20 m3/h from the supplier curve.
4. Compute Δp_valve per mode as pump head minus required head.
5. Issue the three-case datasheet to the valve vendor with the pump curve attached.
6. Review the vendor's travel percentages, characteristic, and cavitation check before purchase.

What does each signal tell you when the valve runs the modes?

Three discrete valve positions replace a flow loop, so there is no feedback on the result unless you add it. Measure before adjusting: a wrong flow is usually a wrong pressure reference or a wrong curve, not a controller gain.

If the modes must be held within a flow tolerance, close a flow loop on the valve with the flowmeter as the process variable and use the three modes as setpoints. Discrete position presets alone drift with pump wear and downstream pressure changes.

When does a variable-speed drive beat the throttling valve?

Compare the hydraulic power wasted in the valve, P = Q × Δp, against the cost and complexity of a drive. Hydraulic power at each duty point:

Mode 1: (10/3600) m3/s * 300 kPa = 0.83 kW
Mode 2: (15/3600) m3/s * 320 kPa = 1.33 kW
Mode 3: (20/3600) m3/s * 340 kPa = 1.89 kW

At the lower-bound valve drops in the table, the valve dissipates about 0.25 kW at mode 1 (10/3600 × 90 kPa) and about 0.29 kW at mode 2 (15/3600 × 70 kPa) in hydraulic terms. Shaft power lost is higher by the reciprocal of pump efficiency. Multiply by the annual hours in each mode to get the energy case. On this duty the throttling loss is small compared with the useful hydraulic power, so a VSD is justified by curve fit or operating hours, not by the difference in duty points.

A VSD follows the affinity laws: Q ∝ N, H ∝ N², P ∝ N³. Because the required head stays near 3 bar across the whole flow range, the head has a large static component, and the speed cannot drop far before the pump falls below the static head and delivers zero flow. Have the pump supplier plot the reduced-speed curves against the three required points and confirm each is reachable. A VSD also removes the need for a valve at the price of a drive, filtering, and a minimum-speed limit. Throttling on a fixed-speed pump stays the simpler design if the supplier curve clears all three points with margin.

How do I confirm each mode on the installed system?

1. Record pump discharge pressure, valve outlet pressure, and flow at each of the three valve settings with the process stable.
2. Compare the pump discharge pressure at each flow to the supplier curve. A shortfall of the head at the measured flow points to the pump, not the valve.
3. Compute the measured valve drop (upstream minus downstream) and back-calculate Kv with the formula above. Compare to the vendor's Kv at that travel.
4. Check the outlet pressure against 3.0, 3.2, and 3.4 bar at the target flows. A consistent offset points to the pressure reference point, not the valve.
5. Cycle from mode 1 to mode 3 and back several times. Flow at each mode must repeat within the tolerance the process needs; a spread indicates positioner deadband or a flow loop that is needed.
6. At mode 3, read motor current against the nameplate to confirm the pump is not running out on its curve. At mode 1, listen and watch vibration for recirculation or cavitation at the highest valve drop.

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