How to Size a Vertical Multistage Pump
Two numbers pick the pump: peak flow in GPM and the pressure the pump must add. Get these right and the curve does the rest.
1. Find peak flow
Add up the fixtures the pump serves in water supply fixture units (WSFU) and convert them to GPM with the Hunter method in the International Plumbing Code (Table 604.3 and Appendix E). For single- and multi-family dwellings, the IAPMO Water Demand Calculator gives a lower, more current peak. For process, irrigation or RO duty, use the equipment’s own flow requirement.
2. Work out the required boost
Required boost (psi) = target pressure + 0.433 psi/ft × elevation + friction − minimum inlet pressure. Target pressure is what you need at the highest outlet; elevation is the rise from the pump to that outlet; friction covers pipe, fittings and valves at peak flow. Multiply psi by 2.31 to get feet of head for the curve.
Worked example. A building needs 60 GPM with 45 psi at a top-floor fixture 45 ft above the pump. Friction through the riser and fittings is 18 ft (7.8 psi) at 60 GPM.
Elevation: 0.433 × 45 ft = 19.5 psi.
From a tank at atmospheric pressure: 45 + 19.5 + 7.8 = 72.3 psi, which is 167 ft of head.
From a city main holding 35 psi minimum: 72.3 − 35 = 37.3 psi of boost, which is 86.1 ft.
60 GPM is the nominal BEP of the 10VR, so that is the family to start with; the stage count comes from the 10VR curve at 60 GPM. For reference, Franklin’s own catalog example selects a 10VR05 for 60 GPM at 70 psi. We confirm the final model against the manufacturer curve.
3. Check suction (NPSH)
Available NPSH must exceed the pump’s required NPSH at the maximum flow it will see. Franklin’s catalog gives maximum lift as H = Ho − NPSHr − Hf − Hv − Hs (atmospheric head, NPSH required, suction friction, vapor pressure, safety margin of at least 0.5 ft, 3.95 ft suggested). Their worked example: sea level (33.95 ft), 10VR05 with 10 ft NPSHr, 4.78 ft suction losses, 65 °F water (0.72 ft) and a 3.95 ft margin gives 14.5 ft maximum lift. A negative result means the pump must sit below the water level.
4. Apply the 70% step-up rule
If your required head is more than about 70% of a model’s maximum (shut-off) head, step up one stage count or one size. That keeps the duty point near the middle of the curve and leaves room for aging pipe and future demand. Then confirm inlet pressure plus shut-off head stays within the flange rating. Run the numbers in our booster pump sizing calculator.
5. Read the model code
The part number is the full configuration. Example from the VR catalog, available in this collection: 30VR032AGF1A-K6E3.
| Code |
Meaning |
Options |
| 30VR |
Size and speed |
1, 3, 6, 10, 15, 20, 30, 45, 65, 95; VR = 2-pole, VRL = 4-pole |
| 03 |
Number of stages |
Always two digits |
| 2A |
Impeller trim |
00 = no trim, 1A = one stage trimmed, 2A = two stages trimmed |
| G |
Casing / hydraulics |
G = cast iron / 316 SS, N = 316 SS / 316 SS |
| F |
Connection |
F = round flange (T = oval, V = grooved in the catalog) |
| 1 |
Seal |
1 = graphite/SiC EPDM (standard); SiC, WC and FKM options in the catalog |
| A |
Motor type |
A = NEMA |
| K |
Motor frame and HP |
K = 250TC 15 HP (A through T letters in the catalog table) |
| 6 |
Speed |
6 = 3600 RPM, 8 = 1800 RPM (60 Hz) |
| E |
Enclosure |
D = premium ODP, E = premium TEFC |
| 3 |
Voltage |
1 = 115/230 1Ph, 2 = 230 1Ph, 3 = 230/460 3Ph, 5 = 575 3Ph |
Install it right
Mount vertically with the motor above the pump on a level concrete base, with at least 4 in of clearance, a gate valve and check valve in the discharge line and an airtight suction line rising at 2° or more toward the pump. Keep supply voltage within 6% of the motor rating and ambient at or below 104 °F. Prime before start, confirm rotation, and never run the pump dry. New to curves? See our Pump Curve 101 guide.
