What is the difference between open flow and flow under pressure?
When you look at a pump specification sheet, you often see two very different flow numbers: one labeled “open flow” or “maximum flow”, and another that only appears on a performance curve. The difference between these two numbers can be huge – sometimes a pump rated for 5 L/min open flow may only deliver 1 L/min in your actual system.
Understanding the difference between open flow (flow at zero pressure) and flow under pressure is essential to selecting the right pump for your application. This guide explains both concepts, why they differ, and how to use this knowledge to avoid buying a pump that is too weak for your needs.
1. Definitions
Open flow (also called free flow, maximum flow, or flow at zero head) is the flow rate a pump delivers when the outlet is completely open and there is no resistance to flow. In this state, the pump does not need to build any pressure; the liquid simply flows out freely.
Flow under pressure (also called operating flow or actual flow) is the flow rate the pump delivers while overcoming resistance such as pipe friction, vertical lift, filters, valves, or nozzles. In this state, the pump builds pressure, and the flow rate is always lower than the open flow.
Simple analogy : Think of a water tap fully open – that is open flow. Now put your thumb partially over the outlet – the flow decreases, but you feel pressure against your thumb. That reduced flow is flow under pressure.
2. Why are these two numbers different?
The reason is the pump performance curve (Q‑H curve) . Every pump has a natural relationship between flow and pressure:
When pressure is zero (no resistance), flow is at its maximum – this is open flow.
As you increase resistance (higher pressure), the pump’s flow decreases.
When flow reaches zero (outlet blocked), pressure is at its maximum – this is called shut‑off head.
This inverse relationship is a physical law for all pumps. No pump can deliver its maximum flow while also producing significant pressure.
3. A practical example
Consider a small diaphragm pump with these specifications:
Open flow: 4 L/min
Maximum pressure (shut‑off head): 0.6 MPa (6 bar)
If you connect this pump to a short, wide hose with no nozzle (almost zero resistance), you will get approximately 4 L/min.
But if you connect it to a reverse osmosis membrane that requires 0.5 MPa pressure, the flow will drop dramatically – perhaps to 0.8 L/min. The pump is still working hard, but most of its energy is used to create pressure, not to move a large volume.
The manufacturer’s performance curve shows exactly how much flow remains at each pressure level. Without that curve, you cannot know the real flow under your operating pressure.
4. Why open flow alone is misleading
Many buyers look only at the open flow number and think that is what the pump will deliver in their system. This is a common and costly mistake.
Example : You need 2 L/min at 0.4 MPa. You see a pump advertised with “open flow 5 L/min”. Without checking the performance curve, you buy it. When installed, the pump may only deliver 0.5 L/min at 0.4 MPa – far below your requirement.
Always ask for the flow at your required pressure, not just the maximum (open) flow.
5. Factors that convert open flow to pressure‑dependent flow
In a real system, several factors create resistance and reduce flow below the open flow value:
Pipe or hose length and diameter : Longer and narrower pipes increase friction.
Vertical lift : Pumping water upward adds static head resistance.
Filters : Clean filters add some resistance; dirty filters add much more.
Valves : solenoid valves, check valves, and manual valves all create pressure drops.
Nozzles or spray tips : Small orifices create significant back pressure.
Restrictive tubing fittings : Elbows, reducers, and quick‑connects add local losses.
The sum of all these resistances is the total pressure (head) that the pump must overcome. At that total head, the pump’s flow is determined by its performance curve.
6. How to find the real flow you need
To select a pump that will deliver enough flow in your actual system, follow these steps:
Step 1 – Determine your required flow rate at the point of use For example, you need 2 L/min to fill a cup in 10 seconds.
Step 2 – Calculate the total resistance (head) in your system
Add vertical lift (meters from water surface to outlet)
Add friction loss from pipes and fittings (use online calculators or estimates)
Add back pressure from filters, valves, and nozzles
Step 3 – Look at pump performance curves Find a pump whose curve shows that at your calculated total head, the flow is equal to or greater than your required flow. Ignore the open flow number; focus on the curve.
Step 4 – Add a safety margin Choose a pump that delivers 10–20% more flow than needed at your operating pressure. This accounts for filter clogging and pump wear over time.
7. Common misconceptions
Misconception 1: “Open flow is what the pump will deliver in my system” False. Open flow is only achieved with zero resistance. Most real systems have significant resistance.
Misconception 2: “A higher open flow always means more flow under pressure” Not necessarily. Two pumps with the same open flow can have very different performance curves. One may hold flow well under pressure; the other may drop steeply. Always check the curve.
Misconception 3: “I can just use a pump with much higher open flow to compensate for pressure drop” Sometimes, but not always. A pump designed for high flow at low pressure may still perform poorly at high pressure. For example, a large centrifugal pump may have high open flow but cannot build high pressure. A small piston pump may have modest open flow but excellent pressure‑holding ability.
Misconception 4: “The pressure rating tells me the flow at that pressure” No. The maximum pressure rating (shut‑off head) is the pressure at zero flow. It does not tell you the flow at any intermediate pressure. Only the performance curve can provide that information.
8. When is open flow a useful number?
Open flow is useful only as a relative comparison between pumps of the same type and series. For example, within the same pump family, a pump with 4 L/min open flow will generally deliver more flow under pressure than a pump with 2 L/min open flow. But you still need the curve to know the exact performance at your operating point.
Open flow is also useful for applications with almost no resistance – for example, simply transferring water from one tank to another through a short, wide hose with no height difference.
9. How manufacturers test open flow
Manufacturers measure open flow by running the pump with the outlet completely open (no hose, no nozzle, no valve) and collecting water in a measuring container. The pump runs at its rated voltage, and the water temperature is usually room temperature. This is a repeatable laboratory test, but it does not represent real‑world installation conditions.
10. Conclusion
Open flow is the maximum flow a pump can deliver with no resistance – a laboratory number. Flow under pressure is the flow you will actually get in your real system, where pipes, filters, valves, and height differences create resistance.
Never select a pump based only on open flow.
Always ask for the performance curve.
Calculate your system’s total head, then read the pump’s flow at that head.
A pump that looks powerful on its open flow spec may fail in your application. A pump with a modest open flow but a flat curve may be perfect. Understand the difference, and you will choose the right pump every time.
Sammy