Submersible Motor Cooling: When Does a Borehole Pump Need a Flow Sleeve?

A submersible motor operates underwater, but being submerged does not automatically guarantee adequate cooling.

The motor generates heat whenever the pump is running. This heat must be transferred to water flowing along the motor’s outer surface. If water does not move past the motor at the required velocity, the motor can overheat even though the complete pump assembly remains underwater.

A flow sleeve, also called a flow inducer sleeve, cooling shroud or cooling jacket, controls the path of the water around the motor. It forces the pumped water to travel along the motor surface before entering the pump intake.

A flow sleeve is commonly required in large-diameter wells, open tanks, reservoirs, top-feeding wells and other installations where natural water movement may not provide sufficient motor cooling.

However, it should not be added according to a general rule alone. The engineer must evaluate the motor diameter, casing diameter, pumping rate, water temperature, inflow position and installation orientation.

A borehole pump normally consists of two main sections:

• The hydraulic pump section

• The submersible electric motor

In a conventional vertical installation, the pump section is positioned above the motor. Water enters the pump through an intake located between the motor and the hydraulic section.

When groundwater enters the well below the motor, it flows upward along the motor surface before reaching the pump intake. This movement carries heat away from the motor.

Adequate cooling therefore depends on moving water, not simply on the presence of water.

If the casing is excessively large relative to the motor, the same pumping flow is distributed across a larger annular area. Water velocity around the motor decreases, and the cooling effect may become insufficient.

Cooling can also be inadequate when water reaches the pump intake without first passing along the motor.

A flow sleeve is a cylindrical tube fitted around the submersible motor and part of the pump assembly.

The upper end is normally sealed near or above the pump intake. The lower end extends below the bottom of the motor and remains open to the surrounding water.

This arrangement changes the water path.

Instead of entering the pump directly from the surrounding well or tank, water must enter through the lower opening of the sleeve. It then flows upward through the narrow space between the sleeve and the motor before reaching the pump intake.

The sleeve performs three important functions:

• It directs water along the motor surface.

• It increases cooling-water velocity around the motor.

• It reduces the chance that water will bypass the motor and enter the pump directly.

A flow sleeve does not increase the hydraulic performance of the pump. Its purpose is to provide the motor with an appropriate cooling environment.

A large casing provides more space around the pump, but more space does not necessarily improve motor cooling.

When the casing is much larger than the motor, the annular flow area increases. If the pumping rate remains unchanged, the average water velocity past the motor decreases.

For example, the same pump flow will produce a lower cooling velocity in a wide casing than in a casing that fits the pump more closely.

This does not mean that every large well automatically requires a flow sleeve. The actual cooling condition must be checked using:

• Pumping flow rate

• Motor outside diameter

• Minimum casing internal diameter

• Required motor cooling velocity

• Location of the well inflow

If the calculated or confirmed velocity is below the motor manufacturer’s requirement, a correctly sized sleeve should be considered.

Submersible borehole pumps are sometimes installed in water tanks, reservoirs, ponds or other open bodies of water.

In these applications, water can approach the pump intake from every direction. It may enter the hydraulic section without moving along the full motor surface.

The surrounding water can therefore remain relatively stagnant around the motor, particularly during continuous high-load operation.

A cooling sleeve provides a controlled water path and is commonly necessary for this type of installation.

The SLAPK QJ installation information specifically advises adding a cooling jacket around the pump motor when the unit is used in a pool. The final sleeve design should still be confirmed for the selected motor and installation.

A top-feeding well is one in which a significant portion of the groundwater enters the casing above the motor or pump intake.

Water flowing down from an upper formation can reach the pump intake without passing along the lower motor body. As a result, the motor may be submerged but receive little effective cooling flow.

A sleeve with its inlet below the motor forces the water to travel downward outside the sleeve and then upward along the motor before entering the pump.

The inflow position should therefore be considered when reviewing the well-completion drawing and pumping-test information.

If the pump is positioned below the main screened or perforated section of the well, water may enter above the pump.

This creates a cooling risk similar to a top-feeding installation. The water can move directly toward the intake without passing across the complete motor surface.

The installation depth must be reviewed in relation to:

• Screened intervals

• Perforated casing sections

• Water-producing formations

• Pump intake elevation

• Motor position

The dynamic water level alone does not reveal the direction of water movement inside the well.

A borehole pump may be installed horizontally in a tank or reservoir when vertical mounting is not practical.

Horizontal operation requires specific confirmation from the pump and motor manufacturer. Bearings, thrust conditions, intake clearance and motor cooling must all be considered.

Natural convection around a horizontal motor is not a substitute for the required cooling flow. A properly supported cooling sleeve may be necessary to direct water over the motor.

The sleeve must not rest directly against the tank floor or become obstructed by sediment. Adequate inlet clearance and structural support are required.

A pump may be physically large but operate at a relatively low flow because of throttling, system resistance or variable-speed control.

Lower flow can reduce the amount of water moving past the motor. A cooling arrangement that is adequate at full design flow may become inadequate at minimum operating speed or minimum continuous flow.

For a variable-frequency drive application, check motor cooling across the complete intended speed range rather than only at the rated operating point.

Continuous throttling should also be reviewed carefully. It may move the pump away from its preferred operating range while simultaneously reducing cooling flow.

Warm water removes motor heat less effectively than cooler water.

A cooling arrangement suitable for normal groundwater temperatures may not be sufficient in a geothermal well, hot-water tank or industrial process application.

High-temperature service can affect:

• Motor winding insulation

• Bearings

• Power cables

• Sealing components

• Lubricants

• Required cooling flow

• Allowable motor load

A flow sleeve can improve water movement, but it does not automatically make a standard motor suitable for hot water. A temperature-rated QJR pump, motor and cable configuration may be required.

Cooling-water velocity depends mainly on three variables:

• Actual pump flow rate

• Motor outside diameter

• Casing or sleeve internal diameter

The available flow passes through the annular space between the motor and the casing or sleeve. A smaller annular area produces a higher average velocity for the same flow.

The basic relationship is:

The annular flow area is determined from the internal diameter of the casing or sleeve and the outside diameter of the motor.

Before performing the calculation, make sure all units are consistent. Do not mix cubic metres per hour, litres per second, millimetres and metres without the correct conversions.

The calculated velocity must then be compared with the requirement for the exact motor model.

Do not apply one universal minimum velocity to all submersible motors. The required value may vary with:

• Motor diameter

• Motor power

• Manufacturer

• Water temperature

• Motor loading

• Vertical or horizontal orientation

• Continuous or intermittent service

Request the motor installation manual or written confirmation from the supplier.

The nominal pump series does not determine the cooling condition.

A 6SP or 8SP designation identifies the approximate pump diameter class, but cooling calculations depend on the actual motor outside diameter and the true internal diameter of the casing or sleeve.

The complete model drawing should be checked because the following dimensions may differ:

• Hydraulic pump diameter

• Motor diameter

• Cable-guard projection

• Coupling diameter

• Sleeve internal diameter

• Sleeve wall thickness

A sleeve must provide enough space for the motor, cable and assembly tolerances while still producing the required cooling velocity.

For more dimensional guidance, see 6-Inch vs 8-Inch Borehole Pumps: How to Choose.

The lower end of the sleeve should extend below the motor so that water must enter from beneath it.

If the lower opening ends above the bottom of the motor, part of the water may bypass the motor surface.

The required extension and inlet clearance should be confirmed for the specific assembly.

The upper area of the sleeve is normally sealed around the pump near the intake.

A poor seal allows water to enter directly at the top, reducing the amount of flow passing along the motor.

The sealing arrangement must accommodate:

• Pump geometry

• Power cable

• Cable guard

• Fasteners

• Assembly tolerances

• Sleeve material

The seal should guide the flow without damaging the power cable or restricting maintenance access.

The lower sleeve opening must have enough free area to admit the full pump flow without excessive entrance loss.

The inlet should not be blocked by:

• Well sediment

• Tank floors

• Structural supports

• Debris

• Nearby walls

• Improperly positioned strainers

In a horizontal installation, the sleeve should be supported so that its inlet remains clear.

The sleeve design must not pinch, rub or sharply bend the motor cable.

Cable routing should be planned before fabrication. Any cable opening should be smooth and protected against sharp metal edges.

The cable must remain secure during installation, operation and pump removal.

The sleeve material must be compatible with the pumped liquid and installation environment.

Possible materials include stainless steel, coated steel or suitable engineering plastics. The choice depends on:

• Water chemistry

• Temperature

• Corrosion risk

• Mechanical loads

• Sleeve diameter

• Installation depth

• Required service life

A carbon-steel sleeve should not be placed in corrosive water without reviewing coating life and galvanic interaction with stainless-steel components.

A motor can remain completely underwater and still overheat if there is insufficient water movement along its surface.

Submergence protects the pump intake from drawing air, but it does not prove that the required cooling velocity exists.

The lowest expected continuous operating flow may govern the cooling review.

Consider throttled operation, variable-speed operation, declining well yield and changes in system resistance.

Water level measurements show how much water is above the pump, but they do not show whether the water flows past the motor.

Review the well screen, perforations and producing formations.

An unnecessarily large sleeve leaves a wide annular space around the motor and may not generate sufficient cooling velocity.

The sleeve diameter should be selected through calculation and manufacturer review.

A sleeve that is too small can interfere with the motor cable, assembly tolerances and installation. It may also increase hydraulic loss or make removal difficult.

If the sleeve does not extend below the motor, water may bypass part of the surface that needs cooling.

A cooling sleeve cannot correct:

• Excessive pump flow

• Insufficient well yield

• Incorrect total dynamic head

• Inadequate pump submergence

• An unsuitable motor voltage

• Excessive water temperature

• Operation outside the pump curve

Each issue must be evaluated separately.

In borehole pump applications, the following terms are often used for similar devices:

• Flow sleeve

• Flow inducer sleeve

• Cooling sleeve

• Cooling shroud

• Cooling jacket

Terminology varies between manufacturers and markets.

The important question is not the name but the function: does the device force the pumped water to travel along the motor surface at the required velocity?

A jacket designed only as a protective cover may not provide the same hydraulic function as a properly sealed flow inducer sleeve.

Provide the following information when asking a pump supplier to check whether a flow sleeve is required:

• Complete pump model

• Motor model and rated power

• Actual operating flow

• Minimum continuous flow

• Motor outside diameter

• Well casing internal diameter

• Open tank or borehole installation

• Vertical or horizontal orientation

• Static water level

• Stabilized pumping water level

• Pump installation depth

• Screened or perforated casing intervals

• Location of water inflow

• Water temperature

• Continuous or intermittent operating schedule

• Variable-speed operating range

• Available sleeve dimensions and material

• Distance from the motor to the well bottom or tank floor

A dimensional drawing and well-completion diagram are particularly useful because they show whether water is likely to bypass the motor.

No. A conventional well may already direct sufficient water upward along the motor. A sleeve is required when the natural installation cannot provide the cooling conditions specified for the motor.

It may be required. A larger casing reduces water velocity around the motor for a given flow. Calculate the velocity and compare it with the exact motor requirement.

Frequently, yes. In an open tank, water can approach the pump intake without passing along the motor. A sleeve creates a controlled cooling path. Confirm the arrangement with the manufacturer.

No. A sleeve improves motor cooling but does not create water or maintain the dynamic water level. Dry-running protection should be provided through suitable level sensors, controls or motor-protection equipment.

No. It is not a hydraulic performance upgrade. Its purpose is to guide cooling water past the motor. An incorrectly designed sleeve can add intake loss instead of improving pump performance.

Possibly. The motor must receive sufficient cooling at the lowest continuous operating flow and speed. The complete VFD operating range should be reviewed.

No. The water inlet must remain open. Provide suitable supports and sufficient clearance from the tank floor, sediment and nearby walls.

A submersible motor is cooled by water moving along its outer surface. Being underwater is not enough if the surrounding water is stagnant or bypasses the motor.

A flow sleeve should be considered when:

• The well casing is much larger than the motor.

• Water enters the well above the pump or motor.

• The pump is installed below the screened or perforated section.

• The unit operates in a tank, reservoir or other open body of water.

• The pump is installed horizontally.

• The operating flow is too low to maintain the required cooling velocity.

• High water temperature reduces the available cooling margin.

Final selection must be based on the actual motor model, operating flow, casing or sleeve diameter, inflow direction and water temperature.

Send SLAPK your required flow and head, pump and motor model, casing internal diameter, installation drawing, static and pumping water levels, screen position, water temperature, installation orientation, voltage, frequency and operating schedule.

Our engineers can review the QJ or SP pump configuration, motor cooling conditions and whether a properly sized flow sleeve is required.

Contact SLAPK for a borehole pump selection and cooling review