Need to dewater a jobsite with a dewatering pump? Don’t simply grab a pump and hose from the back of the yard and hope for the best. Failure of a dewatering system will quickly flood a site and erode profitability. Instead, choose the right type of water pump for the task. There are many pumps available capable of handling the large-scale movement of liquids.

Sizing a pump correctly is also essential to creating an effective dewatering system. Pump manufacturers will tell you that the vast majority of pump failures result from improper pump sizing or failure to consider all the parameters of the pumping application.

Wondering how to size a pump? Ask yourself these nine questions when choosing a water pump and other dewatering system components.

What is the scope of the dewatering task?

If you need to remove deep groundwater in order to place a new cement foundation, for example, you may need a different type of pump and/or more powerful pump than you’d need to remove water from a trench after a few days of heavy rain.

What will you be pumping?

Will your pump primarily be moving water, or does the fluid contain sludge or other materials? You may need a more powerful pump to move thicker fluids.

What is the NPSH?

Another critical factor when determining pump size is the net positive suction head (NPSH). NPSH is the difference between the pressure available at the pump’s suction port (the NPHS available, or NPHSa) and the minimum pressure required to keep the pump operating properly (the NPSH required, or NPSHr).

The NPSH calculation involves several factors, including the atmospheric pressure at the location of pumping, the static suction lift, friction losses through the suction pipe, hose and fittings, and the vapor pressure of the fluid. The warmer the fluid, the higher its vapor pressure. If you’re using a pump to dewater an area that’s close to a geothermal well, for example, the water may be warmer and the vapor pressure higher. Your local pump expert can help you determine the NPSHa/NPSHr.

Using a pump with the right NPSH is important in avoiding cavitation, which can cause severe damage to pumps.

What’s the fluid’s pH?

When the pH readings are above or below the normal (neutral) range, you may need to use a pump made of a corrosion-resistant material, with seals that can withstand more acidic or more basic pH ranges.

What flow rate do you need?

To keep your excavation dry, you need to determine the inflow rate of water. More inflow will require more pump capacity.

For example, an excavation with a high groundwater table and sandy soil near the Pacific Ocean will need a higher flow capacity pump than a project far from any water source with a clay soil content.

A pump vendor with expertise in dewatering systems can help you do the water pump design calculations necessary to determine the required flow rate and the pump capacity you need.

What is the required suction lift?

What is the vertical lift from the lowest point pumped to the center or eye of the impeller? Site elevation affects the amount of water that can be pushed into the pump by the atmospheric pressure during priming.

A good rule of thumb is that at sea level, a properly operating vacuum assisted priming pump can perform a static lift of 28 feet to the eye of impeller. For every 1,000 feet of elevation, deduct 1 foot of priming capability. At 5,000 feet of elevation, the pump priming capability would be 23 feet. The greater the lift, the lower the pump flow capacity.

United Rentals’ most frequently rented pump, a 6-inch, diesel-powered vacuum-assisted pump, is designed to lift fluid 28 vertical feet to the impeller. If the required suction lift is greater than 25 feet, the dewatering system designer may suggest a different type of pump, such as a submersible pump.

How close will the pump be to the water?

The closer the pump to the water source, the more likely you will be able to pump out the water at the required flow rate. If the pump must be 100 feet away from the water source instead of 20 feet, you may have to increase the size of the piping or conduit to increase capacity and reduce friction losses.

How far away is the discharge point?

Consider discharge requirements when determining the best pump system design for your application.

If you’re discharging 400 gpm in close proximity to the pump, a 4-inch hose will be adequate. A longer discharge distance, such as 2,000 feet, may require a larger size discharge pipe or hose and potentially, additional pump horsepower to discharge the same 400 gpm. Helpful hint: For every 1,000 feet from the pump discharge, increase the discharge pipe or hose by one size.

Remember to check local, state, and federal regulations before discharging any liquid on or off site.

Are there any elevation changes along the discharge route?

Elevation changes along the discharge route will impact pump selection. Whether there’s a dirt mound in the path of discharge or you’re pumping into a temporary sediment tank, you’ll need to factor in the static discharge head, or the vertical distance between the centerline of the pump and the point of free discharge.

Where will you be discharging the water?

Discharging into a pressurized point requires a different pump system design than discharging into a field or other open area. Take any additional pressure into account when calculating pump size.

Many factors affect the choice of a pump and the design of a pump system. Working with a pump expert benefits even the simplest pump applications. A person who really knows and understands pumps can engineer a fluid movement system that provides the best results for minimal cost.

Ensuring in advance that your pumping solution is up to the job is the best way to avoid pump failure, keep the dewatering process running smoothly and stay on schedule.

United Rentals offers dewatering equipment and expertise. Contact the United Rentals Fluid Solutions group or browse our selection of pumps.