For rural drinking water, residential secondary supply, industrial park make-up water, and overseas village water supply, the booster pump station is the key asset that delivers water far enough and steady enough. Many projects later suffer from low pressure, high energy use, or frequent failures, and the root cause is usually in the early selection: too small a flow leaves users short, too little head fails to lift, and a wrong pipe size either chokes the line or wastes energy on friction. This article lays out a four-step selection method you can apply directly, with a full worked example at 100 m3/h and 35 m head.
1. How to calculate the design flow Q
The design flow is the peak hourly flow, not the average daily flow. For a community water supply, follow two steps.
Step 1. Daily water demand
Qd = q x N x (1 + a)
- q is the per-capita daily water quota: 130 to 300 L per person per day for residential use (GB 50015-2019, Table 3.2.1).
- N is the served population.
- a is the leakage and unforeseen loss coefficient: 8 percent to 12 percent (GB 50015-2019, clause 3.2.9).
Step 2. Peak hourly flow
Qh = Qd x Kh / T
- Kh is the hourly variation coefficient: about 2.0 to 2.5 for residential areas.
- T is the daily supply hours; for continuous 24 h supply, T = 24.
Worked: a community of 2000 people, q = 200 L/(person-day), a = 10 percent, Kh = 2.2, T = 24 h. Then Qd = 200 x 2000 x 1.10 = 440,000 L/d = 440 m3/d, and Qh = 440 x 2.2 / 24 = 40.3 m3/h. For fire protection or industrial make-up, add the corresponding simultaneous demand required by the local code.
2. How to calculate the design head H
Use the component summation method:
H = H_st + h_f + h_j + h_v + H_free
- H_st: static lift, the vertical distance from the source water surface to the highest or farthest demand point.
-
h_f: friction loss along the pipe, by the Hazen-Williams formula:
h_f = 10.67 x L x Q^1.852 / (C^1.852 x d^4.87)
L is length in m, Q is flow in m3/s, C is the pipe roughness coefficient (about 120 for steel, 130 to 150 for well-operated ductile iron, PE or PVC per AWWA M23), and d is the internal diameter in m. - h_j: minor loss from elbows, valves, and fittings, usually 10 percent to 20 percent of h_f.
- h_v: outlet velocity head, v^2 / (2g), often negligible.
- H_free: required free head at the outlet. For a building connection, the minimum service head per GB 50013-2018, clause 3.0.10 is 10 m for the first floor, plus 4 m for each additional floor. For filling an elevated tank, a free head of about 2 m is sufficient.
3. How to size the suction and discharge pipes
Sizing follows the economic velocity method. The velocity v = Q / A, where A = pi x d^2 / 4. Recommended ranges per GB 50013-2018, clause 6.3.1:
| Pipe | Economic velocity | Maximum | Note |
|---|---|---|---|
| Suction pipe | 1.0 to 1.6 m/s | 1.6 m/s | Lower velocity reduces cavitation risk |
| Discharge pipe | 1.5 to 2.5 m/s | 3.0 m/s | Balance of energy and capital cost |
Pick the diameter so the velocity lands in the recommended band, then check that the friction loss is acceptable. A larger diameter lowers friction loss and energy use but raises capital cost; a smaller diameter does the opposite. The economic band above balances both.
4. Pump units and standby configuration
Select the number of pump sets from the duty flow. Per GB 50013-2018, clause 6.1.6:
- When there are 4 or fewer working pumps, provide 1 standby.
- When more than 4 working pumps, provide 2 standby.
For critical supplies such as hospitals or continuous industrial processes, use 2 duty + 2 standby or N+1 redundancy. Variable-frequency drives let one standby cover several duty pumps and improve part-load efficiency.
5. Worked example: 100 m3/h residential secondary supply
| Item | Value | Note |
|---|---|---|
| Design flow Q | 100 m3/h (27.8 L/s) | Residential peak |
| Static lift H_st | 30 m | Source to roof tank |
| Discharge pipe | DN150, L = 120 m, C = 120 | v = 0.0278 / (pi x 0.15^2 / 4) = 1.57 m/s, within 1.5 to 2.5, compliant |
| Friction h_f | about 2.9 m | By Hazen-Williams |
| Suction pipe | DN200, L = 8 m | v about 0.885 m/s, compliant; low velocity reduces cavitation risk |
| Minor loss h_j | about 0.5 m | About 18 percent of h_f |
| Outlet velocity head h_v | about 0.13 m | 1.57^2 / 19.6 |
| Free head H_free | 2 m | Residual into tank |
| Design head H | about 35 m | 30 + 2.9 + 0.5 + 0.13 + 2 |
| Pump set | 2 duty + 1 standby | Each 50 m3/h at 35 m |
| Shaft power per pump | about 6.4 kW | P = rho g Q H / eta = 9810 x 0.0139 x 35 / 0.75; fit a 7.5 kW motor |
6. Common mistakes and a verification checklist
- Mistake 1: using average daily flow instead of peak hourly flow. Result: chronic low pressure at peak hours.
- Mistake 2: ignoring friction loss and counting only static lift. Result: pumps cannot reach the far end.
- Mistake 3: discharge pipe too small, velocity over 3.0 m/s. Result: high friction loss and wasted energy.
- Mistake 4: no standby pump. Result: one failure stops the whole supply.
- Mistake 5: suction velocity too high, over 1.6 m/s. Result: cavitation and shorter pump life.
Checklist: confirm peak flow, static lift, pipe length and material, free head requirement, standby rule, and motor power margin before ordering.
7. Summary
Good selection starts with the right peak flow, adds every head component, sizes pipes by economic velocity, and keeps a standby. Use the four steps and the 100 m3/h example as a template, then adjust the numbers to your site.