Why Can Two QJ Submersible Well Pumps With the Same Motor Power Deliver Different Flow and Head?

When customers ask us to replace a deep well pump, they often begin with motor power: “I need a 30 kW pump.”

Motor power is important, but it does not tell us how much water the pump will deliver or how much pressure it will produce. Two QJ submersible well pumps can use motors of the same rated power while having very different flow rates, heads and numbers of stages.

At SLAPK, we do not select a QJ pump from kilowatts alone. We confirm the required flow, total dynamic head and well conditions, then match the hydraulic model with a suitable motor.

Motor power tells us how much mechanical power is available to drive the pump. The duty point tells us what the pump must actually do.

A duty point consists of two main values:

• Required flow rate, usually expressed in m³/h

• Required total dynamic head, usually expressed in metres

Flow describes how much water must be delivered. Head includes vertical lift, outlet pressure and pipeline losses. A large-flow, moderate-head project is different from a smaller-flow, high-head project, even when both use the same motor power.

This is why an enquiry that only states “30 kW QJ pump” is incomplete. We still need to know the required flow and head before we can identify the correct model.

The difference can be seen clearly in two models from our QJ catalogue:

• 200QJ50-130/10: rated flow of 50 m³/h, rated head of 130 m, 10 stages and a 30 kW motor

• 200QJ32-176/11: rated flow of 32 m³/h, rated head of 176 m, 11 stages and a 30 kW motor

Both pumps use the same motor power, but their hydraulic duties are different. The first model is designed for a higher flow and lower head. The second delivers a lower flow against a higher head.

Neither model is automatically better. The right choice depends on the well, pipeline and required discharge conditions. We still check the performance curve, motor configuration and installation dimensions before confirming an order.

The pump section of a QJ deep well pump contains impellers and guide casings. Their design determines how water moves through the pump.

Different QJ models can use different impeller passages, dimensions, guide casings and numbers of stages. A larger-flow design must allow more water to pass, while a higher-pressure design may use different impeller geometry and more stages.

The motor provides the power, but the hydraulic section determines how that power is converted into flow and head.

QJ pumps are multistage centrifugal pumps. Water passes through several impellers arranged in series. Each stage adds head before the water moves to the next stage.

Increasing the stages generally raises the head within the same hydraulic family. It also increases the pump length and can change motor power, shaft load and installation requirements.

However, more stages do not automatically make a pump more suitable. If the selected head is much higher than the system requires, the pump may operate away from its intended duty point. This can cause excessive pressure, unsuitable flow, higher power demand or unstable operation.

We select the number of stages from the calculated total dynamic head. We do not simply choose the model with the highest published head.

Power demand depends on flow, head and hydraulic efficiency. Two hydraulic designs can therefore require similar motor power at different duty points. Efficiency also changes across the curve, so we check whether the required point falls in a suitable operating region.

For this reason, we review the complete curve rather than comparing only the maximum flow or maximum head. We want the required duty point to fall within a suitable operating range while leaving a reasonable motor power margin according to the confirmed pump and motor data.

Choosing a larger motor does not correct an unsuitable hydraulic model. It may provide more available power, but it does not make the pump curve match the system.

The pump does not operate independently from the well and discharge system. Its actual operating point is affected by:

• Dynamic water level

• Vertical lift to the discharge point

• Required outlet pressure

• Rising-main diameter and length

• Friction losses

• Check valves, elbows and other fittings

• Water level changes during operation

The operating point is where pump performance and system resistance meet. A smaller or longer pipe increases friction, while a lower dynamic water level increases the required head. Therefore, two systems using the same QJ model may not produce exactly the same field flow.

Selecting a replacement only because the motor power matches can create several problems.

The replacement pump may deliver too little water if its hydraulic design is intended for a smaller flow. It may not produce enough pressure if its rated head is too low. A pump designed for much higher head may operate at an unsuitable point in a low-resistance system.

There is also a risk of overload. A correctly rated motor can still draw excessive current if the pump operates outside its permitted range or if the electrical supply is unsuitable.

When replacing an existing pump, we ask for more than the old motor power. Useful information includes:

• Complete pump model and nameplate photo

• Existing pump curve, if available

• Measured flow, pressure and running current

• Static and dynamic water levels

• Pump installation depth

• Rising-main diameter and length

• Reason for replacing the original pump

These details show whether the old duty point was correct or whether the replacement must solve an existing selection problem.

Our selection normally follows a clear sequence.

First, we confirm the required flow from irrigation demand, tank-filling time or industrial consumption. Second, we calculate the total dynamic head from the pumping water level, outlet pressure and pipeline losses.

Third, we check the well conditions. The pump must fit inside the minimum casing internal diameter, remain adequately submerged during operation and stay clear of sediment at the well bottom.

Fourth, we compare suitable QJ performance curves. We select the hydraulic model and number of stages that place the required duty point in an appropriate operating region.

Finally, we match the motor, voltage, frequency, cable and starting method before confirming the complete model.

To select a QJ submersible well pump accurately, please provide:

• Required flow rate

• Required total dynamic head, or data for us to calculate it

• Well depth

• Static water level

• Dynamic water level

• Planned pump setting depth

• Minimum casing internal diameter

• Rising-main diameter, length and required outlet pressure

• Available voltage, frequency and phase

• Daily operating hours

• Water temperature and water analysis

If some information is unavailable, tell us how the pump will be used. We can identify what must be measured before confirming the model.

No. The replacement must match the required flow, total dynamic head, well diameter, voltage, frequency and installation conditions. Equal motor power does not mean equal pump performance.

No. Additional stages mainly increase head within the same hydraulic family. Flow is determined by the hydraulic design and the actual system operating point.

No. Maximum values are not necessarily available at the same operating point. Use the performance curve and select the pump at the required flow and total dynamic head.

Not by itself. Flow is governed by the pump curve and system resistance. A larger motor does not redesign the impellers or change the hydraulic curve.

Two QJ submersible well pumps can use the same motor power and still deliver different flow and head because their hydraulic designs and numbers of stages are different.

Motor power tells us how much power is available. The curve shows how the pump converts it into flow and pressure. We begin with the required duty and well conditions, then select the hydraulic model, stages and motor.

Send SLAPK your flow, head, well depth, dynamic water level, casing diameter, voltage and frequency. We will recommend a suitable QJ pump and provide the corresponding performance curve and dimensional information for review.