How to Select a Dewatering Pump

Choosing a dewatering pump is an engineering decision before it is a purchasing one. A pump that looks right in a brochure can underperform on site if the flow, total head, suction conditions or fluid were not defined first. This guide sets out the information a project team should gather, how that information relates to a pump's performance curve, and the practical points that shape the final configuration.

Start with the duty: flow and total head

Every pump selection begins with two questions: how much water must be moved, and how much energy is needed to move it? Together, the answers define the duty point the pump has to meet.

Flow

Flow is the volume of water to be pumped per unit of time, usually expressed in m³/h or L/s. On a dewatering job it depends on how much water enters the excavation or work area, whether from groundwater inflow, rainfall, process water or a combination, and on how quickly the water level must be lowered or held. Where inflow is uncertain, the site investigation and the temporary works designer's assessment are the starting point. It is good practice to agree a design flow and to understand whether peaks are expected, for example after heavy rain.

Total head

Total head is the energy the pump must add to the water, expressed as a height in metres of water column. It is made up of three parts:

• Static head: the vertical difference between the water level on the suction side and the discharge point.
• Friction losses: the head lost as water flows through hoses, pipes, fittings, valves and manifolds.
• Required discharge pressure: any pressure needed at the outlet, for example to feed a treatment unit or a settlement tank inlet, converted to metres of head (1 bar is roughly 10.2 m of water column).

Friction losses are often underestimated on long discharge runs. They grow quickly as velocity increases, so the diameter and length of the discharge line have a direct influence on the head the pump must deliver.

Understand the suction side

Surface-mounted dewatering pumps draw water upwards through a suction hose. The vertical distance between the water level and the pump inlet, together with the losses in the suction line, determines how hard the pump has to work to pull water in. Many on-site problems start here.

Suction lift is conditional. What a pump can achieve depends on fluid temperature, site altitude, suction hose diameter and length, the condition of seals and the pump's priming system. Suction lift is also not total head: a pump's ability to lift water on the suction side says nothing about how far it can push water on the discharge side, and the two figures should never be added together or compared directly.

Practical points to check:

• Position the pump as close to the water level as the site safely allows, to reduce suction lift.
• Keep suction hoses short, straight and airtight; air leaks weaken priming.
• Fit a suction strainer suited to the expected solids.
• Where the water level is deep, consider whether a submersible pump or a staged arrangement is more appropriate.

Know what you are pumping

Clean groundwater, silty runoff and water carrying stones or debris place very different demands on a pump. Describe the fluid as accurately as you can:

• Solids: type, approximate size and proportion. Trash pumps are designed for solids-laden water, but each model has a maximum allowable solids size, and selection should be checked against that limit rather than port size alone.
• Abrasives: sand and silt increase wear on impellers, wear plates and seals.
• Chemistry and salinity: saline or chemically aggressive water affects material selection. Compatibility should be reviewed against documented material data for the specific model.
• Temperature and density: both affect suction performance and power demand.

A trash pump is intended for solids-laden water; it should not be assumed suitable for sewage or other specialised fluids without a specific review.

Read the performance curve and find the duty point

A pump performance curve shows the relationship between flow and head for a specific pump at a stated speed and configuration. As head increases, flow decreases. The point where the pump curve crosses the system curve, which describes the head your pipework requires at each flow, is the operating point.

A common mistake is to compare pumps by their headline maximum figures. Maximum flow and maximum head do not necessarily occur together. Selection depends on the performance curve and operating duty. A pump listed with a high maximum flow may deliver considerably less at the head your site actually requires.

When reviewing a curve, check that:

• The duty point sits within the recommended operating range, not at the extreme ends of the curve.
• The speed, impeller diameter and fluid basis stated on the curve match the configuration being offered.
• Power, efficiency and NPSH data, where approved data is available, are consistent with the application.

Our guide to reading a pump performance curve explains these elements in more detail.

Choose the drive and the configuration

Once the duty is clear, the drive and configuration follow from site conditions and how long the pump will run:

• Diesel-driven pumps suit sites without a reliable power supply, remote work areas and jobs where equipment moves as the works progress. Fuel logistics, refuelling access, noise and exhaust need planning.
• Electric pumps, including submersible units, suit sites where a suitable power supply is available and the installation stays in one place for longer periods.
• Accessories such as suction and discharge hoses, couplings, manifolds, non-return valves and strainers must match the pump's connection sizes, the system's working pressure and the fluid.
• Standby capacity: where a loss of pumping would flood an excavation or delay the works, consider whether a standby pump should form part of the arrangement.

You can browse the equipment types available in our dewatering pumps category.

Information to prepare before requesting a selection

The more of the following you can provide, the more reliable the selection review will be:

1. Required flow, in m³/h or L/s, and whether pumping is continuous or intermittent.
2. Water level relative to the proposed pump position, and the discharge elevation.
3. Suction and discharge line lengths, diameters and the number and type of fittings.
4. Any pressure required at the discharge point.
5. Fluid type, solids, temperature and any known water chemistry.
6. Power availability on site and the expected duration of the works.
7. Project location and any access or space constraints.

The pump selection checklist brings this information together in one place.

Share your requirements

If you are planning a dewatering scope, send the flow, head, fluid type and project location you have so far. You do not need to know the model in advance. SmartCo reviews the duty and may ask for further details before proposing a pump and its accessories. Supply and service in Saudi Arabia and Bahrain, and supply to Kuwait by arrangement, are discussed according to the agreed scope for each project.