How to Size the Rising Main for a Borehole Pump？

The rising main is the vertical pipe that carries water from a submerged borehole pump to the wellhead. Selecting it may appear straightforward: connect a pipe to the pump outlet and extend it to the surface.

In practice, the rising main affects pump performance, energy consumption, installation load, water hammer, maintenance and long-term reliability.

A pipe that is too small produces high water velocity and excessive friction loss. The pump must then operate against additional head, which can reduce the delivered flow and increase energy consumption.

A pipe that is unnecessarily large costs more, adds weight and may be difficult to install inside the available well casing.

The pump discharge connection is also not automatically the correct diameter for the entire rising main. The outlet size identifies the mechanical connection on the selected pump. The final pipe diameter must be checked using the design flow, internal pipe diameter, length, material, pressure rating and installation conditions.

A rising main, also called a column pipe, drop pipe or discharge pipe, connects the submersible pump outlet to the wellhead.

Depending on the installation, it may be manufactured from:

• Carbon steel

• Galvanized steel

• Stainless steel

• HDPE

• uPVC

• Reinforced flexible pipe

• Another approved pressure-pipe material

The pipe must carry the pumped water while supporting or accommodating the loads created by the pump assembly, cable, check valves and the water column.

In some installations, the rising main supports the pump mechanically. In others, a separate safety cable or support system is provided. The complete arrangement must comply with the pipe manufacturer’s instructions and local installation requirements.

Pipe diameter affects water velocity and friction loss.

For a given flow rate, reducing the internal diameter increases velocity. Higher velocity generally increases friction loss and makes the system more sensitive to sudden valve movement and pump starting or stopping.

Increasing the pipe diameter reduces velocity and friction loss, but it also increases material cost, pipe weight and the space required inside the well.

The correct diameter is therefore a balance between:

• Required pump flow

• Acceptable water velocity

• Allowable friction loss

• Total installation cost

• Pipe pressure rating

• Available casing clearance

• Mechanical strength

• Installation and retrieval requirements

Pipe size should be selected as part of the complete pumping system rather than after the pump has already been ordered.

A borehole pump catalogue normally provides a discharge outlet size for each pump family.

For example, different SLAPK 6SP model families can use different discharge connections because their nominal flow ranges are different. The 8SP series generally serves higher-flow duties and uses larger outlet connections.

However, the outlet dimension does not independently determine the best rising-main diameter.

A short engineered transition may be used when the calculated pipe diameter differs from the pump outlet, provided that:

• The transition is mechanically suitable.

• The pressure rating is adequate.

• The connection does not create excessive local loss.

• The transition fits inside the casing.

• The assembly can be lowered and retrieved safely.

• The manufacturer approves the configuration.

Do not select a pipe only because its nominal size matches the outlet thread or flange.

The rising main must be sized at the actual design flow, not the pump’s maximum published flow.

The design flow should come from the application, such as:

• Irrigation demand

• Municipal water consumption

• Storage-tank filling time

• Industrial process demand

• Livestock water requirements

• Fire-water requirements

• Mine or construction water transfer

The selected pump must deliver this flow at the calculated total dynamic head.

If the system will operate at several flow rates, check the pipe at:

• Minimum continuous flow

• Normal design flow

• Maximum expected flow

• Variable-speed operating limits

• Future expansion flow, if confirmed

Friction loss does not increase linearly with flow. A relatively small increase in flow can cause a much larger increase in pipe loss.

Hydraulic calculations must use the pipe’s internal diameter.

Two pipes with the same nominal size can have different internal diameters because of:

• Wall thickness

• Pipe schedule

• Pressure class

• Material

• Manufacturing standard

• Internal lining

A high-pressure pipe normally has a thicker wall than a lower-pressure pipe of the same outside diameter. The thicker wall reduces the internal flow area.

Using the nominal diameter instead of the true bore can underestimate water velocity and friction loss.

Obtain the pipe manufacturer’s dimensional data before completing the calculation.

The average water velocity can be calculated from the flow rate and the pipe’s internal cross-sectional area:

Velocity = Flow rate ÷ Internal pipe area

For a circular pipe:

v = 4Q ÷ (πD²)

Where:

• v is average water velocity

• Q is volumetric flow rate

• D is the actual internal pipe diameter

• π is approximately 3.1416

All units must be consistent. If flow is provided in cubic metres per hour, convert it to cubic metres per second before calculating velocity in metres per second.

Velocity is a screening parameter, not the only selection criterion. The acceptable range depends on the pipe material, system duty, starting method, water-hammer analysis and applicable engineering standard.

Avoid applying a single universal velocity limit to every project.

Friction loss is the head consumed as water moves through the rising main.

It depends on:

• Flow rate

• Actual internal diameter

• Pipe length

• Pipe roughness

• Pipe material

• Pipe age and internal condition

• Water properties

Darcy-Weisbach and Hazen-Williams are commonly used methods for water-pipeline calculations.

Darcy-Weisbach can be applied to a broad range of fluids and pipe materials when the correct friction factor is used. Hazen-Williams is frequently used for water-system estimates, but its coefficient must be selected appropriately.

For a defensible calculation:

1. Identify the complete vertical pipe length.

2. Confirm the internal diameter and material.

3. Calculate the loss per unit length at the design flow.

4. Multiply by the actual pipe length.

5. Repeat the calculation at other expected operating flows.

6. Include the result in the system’s total dynamic head.

Do not use a general percentage of well depth as a substitute for a pipe-loss calculation.

For the complete head method, see Deep Well Pump Sizing Guide: How to Calculate Flow Rate and Total Dynamic Head.

The rising main is not the only source of hydraulic resistance.

Additional losses can occur through:

• Pump discharge transitions

• Check valves

• Gate valves

• Butterfly valves

• Elbows

• Tees

• Reducers

• Flowmeters

• Pressure gauges and gauge fittings

• Filters

• Wellhead connections

• Control valves

• Tank inlet assemblies

These losses may be calculated using loss coefficients, equivalent pipe lengths or manufacturer data.

Check valves deserve particular attention. A valve selected only by nominal size may create significant loss at high flow. Confirm its full-open flow area, pressure rating, orientation and suitability for vertical service.

The pipe must withstand more than the normal outlet pressure observed at the surface.

Depending on the operating condition, the highest pressure may occur near the pump or during a transient event.

The pressure review should include:

• Pump shutoff head

• Static water-column pressure

• Normal operating pressure

• Required wellhead pressure

• Valve closure

• Pump starting and stopping

• Water hammer

• Elevation changes

• Pressure-control settings

• Test pressure

• Safety margin required by the pipe standard

The lowest sections of a deep rising main can be exposed to high internal pressure.

Do not select the pressure class from the normal wellhead gauge reading alone. The designer should check the most severe credible operating and transient condition.

Water hammer is a temporary pressure change caused by a rapid change in water velocity.

It may occur when:

• The pump starts suddenly.

• The pump stops after a power failure.

• A check valve closes rapidly.

• A surface valve is closed too quickly.

• A VFD changes speed too aggressively.

• Flow reverses before the check valve closes.

The severity depends on pipe length, water velocity, pipe material, valve behavior and the rate of velocity change.

A long vertical rising main contains a substantial moving water column. An incorrectly selected check valve or rapid shutdown can produce damaging pressure surges.

Possible controls include:

• Non-slam check valves

• Controlled valve operation

• Soft starting

• Controlled VFD acceleration and deceleration

• Surge vessels

• Pressure-relief devices

• Other engineered surge-control equipment

A detailed transient analysis may be necessary for deep wells, long pipelines and high-flow systems.

The rising main must safely carry its own weight, the weight of the contained water and any equipment it supports.

Relevant loads include:

• Dry pipe weight

• Water-column weight

• Pump and motor weight

• Check-valve weight

• Cable and cable-guard weight

• Coupling weight

• Dynamic loads during starting and stopping

• Loads during installation and removal

• Water-hammer forces

• Wellhead support loads

For threaded steel pipe, verify the thread and coupling strength. For flanged pipe, verify the flange, bolts and gasket arrangement.

For plastic pipe, check tensile load, creep, temperature effects, joint rating and manufacturer limits for suspended vertical service.

A pipe that meets the internal pressure requirement may still be unsuitable for the suspended mechanical load.

The outside diameter of the rising main, couplings and accessories must fit through the minimum casing internal diameter.

Check the complete installation envelope, including:

• Pipe couplings

• Flanges

• Pump outlet transition

• Check valves

• Power cable

• Cable guards

• Cable clamps

• Splice protection

• Centralizers

• Safety cable

• Lifting attachments

The largest component may be a coupling or valve rather than the pipe itself.

Also consider:

• Well deviation

• Casing joints

• Internal welds

• Liners

• Scale deposits

• Deformation

• Changes in casing diameter

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

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

Carbon-Steel Rising Main

Carbon steel provides high mechanical strength and is widely available. It can be suitable for deep and high-pressure installations.

However, corrosion must be considered. Internal corrosion increases roughness and friction loss, while external corrosion reduces wall thickness.

Coatings, linings and corrosion allowance may be required.

Galvanized-Steel Rising Main

Galvanized steel is commonly used in water-well installations, particularly at moderate sizes.

Its suitability depends on water chemistry, installation life, threaded connections and local standards. Damaged galvanizing at threads or joints can become a corrosion point.

Stainless-Steel Rising Main

Stainless steel provides improved corrosion resistance, but the correct grade must be selected from a water analysis.

SS304 is not automatically suitable for high-chloride water. SS316L, duplex stainless steel or another alloy may be required in more aggressive conditions.

Galvanic interaction between dissimilar metals should also be reviewed.

HDPE Rising Main

HDPE can offer corrosion resistance, smooth internal surfaces and fewer rigid joints.

Its design requires careful review of:

• Pressure class

• Tensile load

• Temperature

• Creep

• Jointing method

• Elongation

• Compatibility with the pump connection

• Vertical support requirements

The pipe manufacturer should confirm suitability for a suspended borehole installation.

uPVC Rising Main

Specialized uPVC column pipes are used in some borehole systems because they are lightweight and corrosion resistant.

The pipe and threaded joints must be specifically rated for the installation depth, pressure and suspended load. Ordinary pressure pipe should not automatically be used as a deep-well column pipe.

Assume a borehole pump must deliver 50 m³/h through a 140 m vertical rising main.

Two candidate pipes meet the basic connection and pressure requirements. One has a smaller internal diameter, while the other has a larger internal diameter.

The smaller pipe will produce:

• Higher water velocity

• Greater friction loss

• Higher total dynamic head

• Greater sensitivity to water hammer

• Lower initial pipe cost

The larger pipe will produce:

• Lower water velocity

• Lower friction loss

• A lower required pump head

• Potentially lower operating energy

• Higher initial cost and weight

The correct decision cannot be made from the purchase price alone.

The engineer should calculate the operating point for both alternatives, add each friction loss to TDH and compare:

• Pump model and motor power

• Pump efficiency

• Annual energy consumption

• Pipe and installation cost

• Pressure class

• Well clearance

• Expected service life

A larger pipe can reduce lifetime energy cost, but increasing diameter beyond the practical requirement may provide little additional benefit.

Many borehole pumps include a check valve at or near the discharge. Additional check valves may be considered in long rising mains, but they should not be added using a universal spacing rule.

Incorrectly positioned or unsuitable check valves can contribute to:

• Trapped pressure

• Hydraulic shock

• Valve chatter

• Delayed closure

• Reverse rotation

• Increased friction loss

• Difficult maintenance

The valve arrangement should be selected according to pump depth, pipe length, pressure, flow, starting method and manufacturer instructions.

Confirm whether the selected pump already includes an integral check valve before specifying additional valves.

Should the rising main be the same size as the pump outlet?

Not necessarily. The outlet determines the immediate connection. The rising-main diameter should be checked using flow, velocity, friction loss, pressure, mechanical load and casing clearance.

Can I reduce the rising-main diameter above the pump?

A reducer may be possible, but it increases velocity and can add hydraulic loss. The complete system must be recalculated, and the transition must be mechanically suitable.

Does a larger pipe always improve pump performance?

A larger internal diameter reduces friction loss, but excessive size can increase cost, weight and installation difficulty. Choose the diameter through hydraulic and lifecycle evaluation.

Should friction loss be calculated from well depth?

No. Use the actual length, internal diameter, material and design flow of each pipe section. Well depth alone does not determine friction loss.

Can HDPE be used as a borehole pump rising main?

Yes, in suitable installations, but the specific pipe, joints and support system must be rated for internal pressure, suspended load, temperature, creep and installation depth.

How many check valves should a deep-well pump use?

There is no universal quantity or spacing suitable for every well. Follow the pump, valve and system designer’s recommendations after reviewing the depth, pressure and transient conditions.

Does the rising main affect pump motor power?

Yes. Pipe friction adds to TDH. Higher TDH can change the selected pump model, number of stages, operating point and required motor power.

A reliable borehole-pump rising main must satisfy hydraulic, pressure, mechanical and installation requirements at the same time.

The selection process should:

• Confirm the design flow.

• Use the actual pipe internal diameter.

• Calculate water velocity and friction loss.

• Include valves and fittings.

• Check operating, shutoff and transient pressure.

• Verify pipe and joint strength.

• Confirm casing clearance.

• Select a material compatible with the water.

• Evaluate lifecycle energy cost.

Do not select the complete rising main only from the pump outlet size. The final pipe diameter and pressure class must be verified against the actual well and operating conditions.

Send SLAPK your required flow and head, well depth, pump installation depth, minimum casing internal diameter, proposed pipe material and diameter, required outlet pressure, voltage, frequency and water analysis.

Our engineers can recommend a suitable QJ or SP borehole pump and provide the pump curve, outlet dimensions and installation data required for your rising-main design.