Head is pressure, flow is volume, and one engine has to supply both. A high-pressure pump pushes water higher or further, but moves less of it per hour. If you ask a supplier for “high pressure and high flow”, you either get a much larger engine than you need, or a machine that falls short on both.
Here is that trade-off in our own range. Compare the first two rows in particular:
| Model | Port | Rated flow | Rated head | Max head | Engine |
|---|---|---|---|---|---|
| HTX-WP-HP20 | 2” | 12 m³/h | 60 m | 80 m | 3.7 kW |
| HTX-WP20 | 2” | 20 m³/h | 20 m | 35 m | 3.4 kW |
| HTX-WP30 | 3” | 40 m³/h | 18 m | 30 m | 3.7 kW |
| HTX-WP40 | 4” | 50 m³/h | 10 m | 18 m | 5.7 kW |
| HTX-WP60 | 6” | 130 m³/h | 13 m | 15 m | 7.5 kW |
The first two have the same port size and almost the same engine power. One delivers three times the head; the other delivers nearly twice the flow. Neither is better built. The same power is simply used in a different way.
Head, bar and psi
Pump head is given in metres because it is a physical height, which makes it easy to check against your site. To convert:
10 m of head ≈ 1 bar ≈ 14.5 psi.
The HTX-WP-HP20 at 60 m rated head delivers about 6 bar. That is enough for sprinkler lines or for lifting water several floors. A drainage pump rated at 10 m delivers about 1 bar. It moves water across a yard well, but it cannot push water uphill.
How to calculate the head you need
Add three figures. Leaving one out is the usual reason a pump seems to “underperform” when it is working exactly as specified:
- Static lift: the vertical distance from the water surface to the discharge point, in metres. Measure from the lowest water level, not today’s level.
- Friction loss: pressure lost to the pipe walls. It rises sharply with pipe length and as the pipe diameter gets smaller. A long run of undersized hose can cost more head than the lift itself.
- Required outlet pressure: sprinklers, nozzles and spray systems need pressure at the outlet. A sprinkler rated at 3 bar needs 30 m of head still available after the first two items.
Example: water is lifted 18 m, through 120 m of pipe with about 12 m of friction loss, to a sprinkler that needs 3 bar (30 m). Total = 18 + 12 + 30 = 60 m. This job needs a high-pressure pump, not a 20 m general-purpose pump, even though the actual lift is only 18 m.
Size the pump by its rated head, not its maximum head. Maximum head is the point where the pump pushes so hard that the flow falls almost to zero. It is a limit on the datasheet, not an operating point.
The suction limit no pump can exceed
Everything above is about pushing water. Suction has a physical limit. A pump does not pull water up. Atmospheric pressure pushes the water into the low-pressure space the pump creates. At sea level the theoretical limit is about 10.3 m. In practice it is 7–9 m, after pipe friction and the vapour pressure of the water.
Our datasheets state this clearly: 6 m rated suction head, 9 m maximum. A bigger engine does not change it. If the water source is more than about 7 m below the pump, the solution is not a bigger pump. Move the pump closer to the water, or use a submersible pump.
At altitude the limit is lower, because there is less atmospheric pressure to push the water. At 3,000 m, expect to lose about 3 m of usable suction lift. The same thin air also reduces engine output; see high-altitude derating.
When you really need high pressure
- Sprinkler and centre-pivot irrigation: the nozzles themselves need 2–4 bar.
- Firefighting and washdown: hose reels and lances are rated by pressure, not by flow.
- Long pipe runs: friction loss dominates even where the lift is small.
- Delivery to upper floors or hillsides: roof tanks, terraced fields, mine benches.
When you do not: flood drainage, emptying trenches or ponds, and moving water across flat ground. Those jobs need flow. A high-pressure pump there moves less water per hour for the same fuel.
What to tell a supplier
With these four figures, a competent supplier can size the pump in one step:
- Vertical lift in metres, measured from the lowest water level
- Pipe length and internal diameter, plus an approximate count of bends and valves
- Required pressure at the outlet, or the sprinkler or nozzle model, whose datasheet states it
- Flow you need, in m³/h or litres per minute
If a quotation gives only a “maximum head” figure and no rated duty point, ask again. A proper pump datasheet gives the flow at a stated head, because only that pair describes what the pump will do.
Where HTX fits
Our diesel pump range covers 12–130 m³/h and 10–60 m rated head, with 2” to 6” ports, including the dedicated high-pressure HTX-WP-HP20. Every model publishes rated and maximum figures for head and flow, plus the suction limit, so you can check the figures above against the datasheet yourself. For sizing by application, see how to choose a diesel water pump and what size pump for irrigation.
Not sure which side of the trade-off your site is on? Send us your lift, pipe length and required outlet pressure, or message us on WhatsApp. Our engineers will reply with a duty point and the matching model within 24 business hours.