6-Inch vs 8-Inch Borehole Pumps: How to Choose？

Choosing between a 6-inch and an 8-inch borehole pump is not simply a matter of buying the largest unit that fits the well. The correct pump diameter depends on three essential checks:

• The complete pump assembly must pass through the narrowest part of the casing.
• The pump performance curve must cover the required flow rate and total dynamic head.
• The submersible motor must receive adequate cooling under the actual installation conditions.

In many projects, the well diameter is known, but the pump’s actual outside diameter is not. In other cases, a supplier receives only a request such as, “We need a 6-inch pump for a 6-inch well."

That information is not sufficient for accurate pump selection. Nominal inch sizes are product series classifications. They do not replace the dimensional drawing, the actual casing internal diameter, or an inspection of couplings, cable guards and other accessories.

The safest selection rule is simple: first define the hydraulic duty, then identify the smallest suitable pump series, and finally verify the maximum installed outside diameter against the minimum clear bore of the well.

The inch designation normally identifies the approximate diameter class of a submersible borehole pump. It does not guarantee that every part of the assembly is exactly 6 or 8 inches in diameter.

The pump bowl, motor, cable guard, coupling, check valve and discharge connection may all have different dimensions. Different manufacturers may also use different dimensional conventions.

The SLAPK SP catalogue includes 6SP and 8SP stainless-steel borehole pump series. Catalogue data shows representative maximum body diameters of approximately 144 mm for listed 6SP models and 184 mm for listed 8SP models.

These values demonstrate why a nominal series name must not be treated as the final installation diameter. Always confirm the exact dimensional drawing for the selected pump and motor combination.

For field verification, the most important well measurement is the minimum internal diameter along the complete installation path. A casing may be described by its nominal size, while its actual internal diameter varies because of:

• Casing wall thickness
• Joints and couplings
• Internal liners
• Mineral deposits
• Corrosion or deformation
• Changes in casing section
• Previous downhole equipment

Choose a 6-inch pump series when its performance curve covers the required duty efficiently and the casing provides verified installation clearance.

Choose an 8-inch pump series for higher-flow duties when the borehole yield, casing size, power supply and rising main are designed for the larger hydraulic capacity.

Do not select a borehole pump by diameter alone. Pumps of different diameters may both produce the required head, but they may operate at very different flow ranges and efficiencies.

A 6-inch borehole pump provides a practical combination of flow capacity, head range and installation size.

It is commonly considered for:

• Agricultural irrigation
• Commercial water supply
• Industrial process water
• Municipal water supply
• Deep wells requiring both flow and high head

The SLAPK 6SP catalogue includes nominal flow families such as 17, 20, 30, 46 and 60 m³/h. Different numbers of stages provide different head options.

However, the nominal model designation should not be treated as a guaranteed operating point. The buyer should compare the required duty with the exact performance curve of the selected model.

A 6-inch pump series is often appropriate when:

• The required duty falls within an efficient region of an available 6SP performance curve.
• The casing and all internal restrictions provide sufficient clearance for the complete assembly.
• The well can sustainably supply the selected flow rate.
• The rising main, valves and electrical system are sized for the required capacity.
• Future removal and maintenance will not be restricted by an excessively tight installation.

An 8-inch borehole pump is generally considered for large irrigation projects, industrial water transfer, municipal water supply and other high-flow deep-well applications.

The larger hydraulic section can deliver more water efficiently, but the entire system must be designed for the increased flow. Installing a large pump in a low-yield well will lower the pumping water level more quickly and increase the risk of dry running.

The SLAPK 8SP catalogue includes nominal flow families such as 77 and 95 m³/h, with multiple stage options for different heads.

An 8SP assembly is materially larger than a 6SP unit. Therefore, the casing diameter, casing straightness and any internal restrictions become critical installation considerations.

An 8-inch pump series is normally justified when:

• The required flow exceeds the efficient capability of the smaller pump series.
• A pumping test confirms that the borehole can sustain the required extraction rate.
• The casing provides sufficient internal diameter and straightness.
• The power supply, cable, starter or VFD can support the larger motor.
• The discharge pipe, wellhead, lifting equipment and control system are designed for the larger unit.

Measure or obtain the actual internal diameter of the casing. Do not rely only on its nominal size.

Check every casing section, liner, reducer and joint. In an older well, consider scale, corrosion, deformation and previous equipment that could restrict the clear bore.

If the condition of the well is uncertain, a downhole inspection or gauge check may be necessary before ordering the pump.

Request a certified dimensional drawing for the exact pump and motor combination.

The dimensional check should include:

• Pump body
• Submersible motor
• Cable guard
• Cable lead connection
• Splice protection
• Cable straps
• Centralizers
• External check valves
• Other projecting accessories

The largest component or accessory determines whether the complete assembly can pass through the casing.

A theoretical metal-to-metal fit is not an acceptable installation plan.

Clearance is required for safe lowering, retrieval, cable protection and minor well deviation. The necessary allowance depends on:

• Pump length
• Casing condition
• Casing straightness
• Accessory arrangement
• Installation method
• Manufacturer requirements

Do not apply one universal clearance value to every well. The pump manufacturer’s drawing and the actual condition of the borehole must be reviewed together.

Pump diameter narrows the possible product family, but it does not complete the hydraulic selection.

Determine the required flow rate and total dynamic head, then locate the operating point on the exact pump performance curve.

Choose a model that operates within the manufacturer’s recommended range and reasonably close to its best-efficiency region. Avoid selecting excessive capacity and controlling it through continuous throttling.

For the full calculation method, read the Deep Well Pump Sizing Guide: Flow Rate and Total Dynamic Head.

A larger pump may provide more flow, but the well must be able to recharge at the same rate.

Use a stabilized pumping water level measured at or near the proposed operating flow. Before increasing the pump size, confirm:

• Sustainable well yield
• Stabilized pumping water level
• Seasonal water-level decline
• Required pump submergence
• Dry-running protection

For more information, see Static Water Level vs Dynamic Water Level for Deep Well Pump Selection.

A submersible motor is cooled by water moving past its outer surface.

In some wells, water naturally flows upward around the motor toward the pump intake. In a large casing, open reservoir or installation where water enters above the motor, the flow velocity around the motor may be insufficient.

A flow sleeve or cooling shroud may therefore be required.

Confirm the minimum cooling velocity, installation orientation and flow-sleeve requirements with the motor manufacturer.

Changing from a 6-inch to an 8-inch borehole pump can affect much more than the pump itself.

The complete system review should include:

• Rising-main diameter
• Check valves
• Wellhead loading
• Power cable size
• Starting current
• Starter or VFD capacity
• Generator capacity
• Lifting equipment
• Pressure and level controls
• Electrical protection

The correct borehole pump diameter is a system decision, not an isolated product dimension.

Assume an irrigation project requires a flow rate of 42 m³/h at 115 m total dynamic head.

The well has a verified minimum casing internal diameter of 200 mm. A pumping test confirms that the well can sustain 45 m³/h with acceptable drawdown.

A 6SP pump family includes performance curves around this flow range, while an 8SP family may offer more capacity than the irrigation system requires.

The engineer should therefore begin by evaluating suitable 6SP curves. The following items must then be verified:

• The exact operating point
• Pump efficiency at the duty point
• Required number of stages
• Motor power
• Maximum pump and motor outside diameter
• Cable guard and accessory dimensions
• Clearance within the 200 mm casing
• Electrical supply and starting method

The correct conclusion is not, “A 6-inch well needs a 6-inch pump."

The correct conclusion is, “The 6SP hydraulic range appears suitable, but the exact pump assembly must be dimensionally, hydraulically and electrically verified for this well."

If an available 6SP model cannot produce 42 m³/h at 115 m efficiently, the engineer should reconsider the pump series, hydraulic duty or well construction instead of choosing by nominal diameter alone.

Choose a borehole pump diameter by combining the required hydraulic duty with verified installation dimensions.

A 6-inch pump series covers a broad range of irrigation and industrial water-supply duties. An 8-inch series generally serves higher-flow systems. However, the final selection must be based on the exact performance curve, the well’s sustainable yield and a dimensional check of the complete assembly.

Do not order a pump based only on its nominal inch designation.

Before production, confirm:

• Minimum casing internal diameter
• Pump and motor outside diameter
• Maximum accessory envelope
• Installation clearance
• Pump performance at the required duty
• Well yield and pumping water level
• Motor cooling conditions
• Electrical and piping requirements

Send SLAPK your required flow rate, total dynamic head, well depth, minimum casing internal diameter, static water level, pumping water level, water quality, voltage, frequency and cable length.

Our engineers can compare suitable QJ and SP borehole pump options and provide a performance curve, dimensional drawing and recommended motor configuration for your project.