In need of a dewatering pump? 10 Factors to consider.

Need to dewater a jobsite with a dewatering pump? Avoid simply grabbing a pump and hose from the back of the yard and hoping 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.

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

1. What kind of dewatering challenge?

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 would need to remove water from a trench after a few days of heavy rain.

2. What kind of liquid 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.

3. Size of the dewatering pump and Net Positive Suction Head

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 vapour pressure of the fluid. The warmer the fluid, the higher its vapour pressure. If you are using a pump to dewater an area that is close to a geothermal well, for example, the water may be warmer and the vapour 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.

4. The role of the fluids pH

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

5. Determine the inflow ratio 

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 lagoon or lake 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.

6. Suction lift requirements?

What is the vertical lift from the lowest point pumped to the centre 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 8.5 metre to the eye of impeller. For every 300 metre of elevation, deduct 1 foot of priming capability. At 1500 metre of elevation, the pump priming capability would be 7 metres. The greater the lift, the lower the pump flow capacity.

7. Why the location of the pump is important

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 30 metre away from the water source instead of 6 metre, you may have to increase the size of the piping or conduit to increase capacity and reduce friction losses.

8. What’s the distance to the discharge point?

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

If you are discharging 90 m³/h in close proximity to the pump, a 4-inch hose will be adequate. A longer discharge distance, such as 600 metre, may require a larger size discharge pipe or hose and potentially, additional pump horsepower to discharge the same 90 m³/h. Helpful hint: For every 300 metres from the pump discharge, increase the discharge pipe or hose by one size.

Remember to check local regulations before discharging any liquid on or off site.

9. Are there elevation challenges?

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 centreline of the pump and the point of free discharge.

10. 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.