How to Size a Submersible Dewatering Pump

A pump's maximum flow number doesn't tell you how much water it will move on your site. Once the pump has to push water uphill and through a long hose, the actual flow drops.

The simple way to size a submersible dewatering pump is to find two numbers: how much water you need to move each minute, and how much resistance the pump must overcome. That resistance is called total dynamic head, or TDH. Then you check both numbers together on the pump curve.

We'll use one example from start to finish. The job needs 100 gallons per minute (gpm), and the installed system creates about 21 feet of TDH. So the correct requirement isn't simply a “100 gpm pump.” It is a pump whose curve shows at least 100 gpm at about 21 feet of head.

The Simple Answer

Here is the complete example before we explain each step.

What we measured	Result
Water that must be removed	100 gpm
Vertical lift	14 ft
Hose, fitting and exit loss	7.1 ft
Required pump duty	100 gpm at about 21.1 ft TDH

Step 1: Work Out How Much Water Must Move

This step gives you the required flow. Include both the water already in the sump and the water that keeps entering while the pump runs.

In our example, the sump contains 3,000 US gallons. It must be emptied in 60 minutes. Dividing 3,000 gallons by 60 minutes gives 50 gpm.

Stored water: 3,000 gal ÷ 60 min = 50 gpm

But another 50 gpm is flowing into the sump. The pump has to handle that water too.

Required flow: 50 gpm stored water + 50 gpm incoming water = 100 gpm

If inflow changes with rain, excavation depth, or groundwater conditions, measure it more than once. A pump sized from a dry-day reading may fall behind during the condition that matters most.

Step 2: Measure How High the Water Must Go

Next, measure the vertical distance from the lowest water level you plan to maintain to the point where the discharge water comes out. This is a static head.

Suppose the discharge point is 14 feet above the lowest sump level. The static head is 14 feet. Measure vertical rise only. A 200-foot hose running across level ground doesn't create 200 feet of static head, though its length does create friction.

Static head: 14 ft

Use the lowest planned water level because the lift increases as the sump is drawn down. If the water starts higher, the pump may face less head at the beginning than it does near shutoff.

Step 3: Add Hose and Fitting Losses

Water rubbing against the inside of a hose loses energy. Bends, valves, fittings and the discharge exit add more loss. Together, these losses act like extra lift.

Our example uses 200 feet of 3-inch inside-diameter hose at 100 gpm. With water and a Darcy friction factor of 0.020, the straight hose loses about 5.1 feet of head. We then add an illustrative 2 feet for fittings and the exit.

Discharge loss: 5.1 ft hose loss + 2 ft fittings and exit = 7.1 ft

Use the actual hose inside diameter and the hose supplier's loss data for final sizing. Diameter matters a lot. Under the same example assumptions, 200 feet of 2-inch hose loses about 38.9 feet—not 5.1 feet. A smaller hose can turn an easy pumping job into a high-head job.

Step 4: Calculate Total Dynamic Head

Now add the vertical lift and the discharge losses. If the hose discharges openly to the atmosphere, no extra pressure head is required.

TDH = static head + hose and fitting losses

TDH = 14 ft + 7.1 ft = 21.1 ft

That gives us the two numbers needed to select the pump: 100 gpm at about 21 feet of TDH. Keep them together. A pump advertised at 100 gpm may deliver much less than 100 gpm when it has to work against 21 feet of head.

Step 5 Check the Pump Curve

A pump curve is a chart showing how much flow a pump can deliver at different amounts of head. Flow is usually along the bottom. Head is usually up the side.

Find 100 gpm along the bottom of the chart. Move upward until you reach 21 feet of head. The selected pump's curve must reach that point. If the curve sits below or to the left of it, the pump won't deliver the required flow in this setup.

And don't combine the maximum flow and maximum head printed on a product page. They are opposite ends of the curve. Maximum flow is normally measured at a very low head. Maximum head occurs where flow is close to zero. Neither number describes the middle of the curve where the pump will probably run.

Use the curve for the exact pump version, voltage, phase, frequency and impeller. Then check the permitted operating range and motor current in the installation manual.

What Could Change the Answer

A useful selection covers the real operating range, not only one perfect-day calculation.

• A storm can increase inflow and raise the required gpm.
• A lower sump level increases the vertical lift.
• More hose, a smaller hose or extra fittings increase TDH.
• A filter, settlement tank or treatment system can add resistance.
• Kinks, wear and sediment buildup can reduce delivered flow.

Check the normal condition, the lowest water level and a realistic high-inflow condition. If one pump can't cover that range without poor operation or constant cycling, staged pumps may be the better setup.

Match the Pump to the Water

The curve tells you whether the pump can meet the flow and head. The water tells you what kind of pump can survive the job. Clear rainwater, silty groundwater, abrasive sand and debris-filled pits aren't the same service.

Before choosing a model, record:

• The largest solids that can reach the pump
• Whether the water contains abrasive sand or fine silt
• Whether rags, leaves or stringy debris are possible
• Water temperature and any chemicals present

For water with larger debris, compare electric submersible trash pumps. A solids-passage number tells you what can pass through the pump. It doesn't tell you how quickly abrasive grit will wear the impeller and casing.

Examples to Compare After You Know the Duty Point

These models show why the curve and the application both matter. The stated maximum flow and maximum head remain separate endpoints.

Site need	Example to compare
Portable 2-inch dewatering	HCP GD-400 — 1/2 HP; up to 64 gpm; 39 ft maximum head; 1/4-inch solids passage.
Water left on a flat surface	HCP GDR-400 — removes water to about 1 mm; up to 64 gpm; 39 ft maximum head.
Muddy or silty water	HCP A-31V — vortex impeller; 3/8-inch solids passage; 140 gpm at 5 ft of head and 4 gpm at 39 ft.
High flow at low head	HCP L-41A-6117EP — 4-inch discharge; up to 255 gpm; 16 ft maximum head; 3/8-inch solids passage.

Controls Power and Discharge

A correctly sized pump still needs the right controls and power. Confirm voltage, phase, running current, starting current, and voltage drop over the full cable length. If a generator is used, it must handle motor starting as well as the other loads running at the same time.

Choose float switches and accessories that provide enough distance between the start and stop levels. Too little usable sump volume can make the pump start and stop repeatedly, which adds heat and wear. Follow the motor's stated limit for starts per hour.

Also confirm where the water can legally go. Water-taking and sewer-discharge requirements depend on the location, daily volume, and water quality. Sediment may need to be settled or filtered before discharge.

Quick Site Check

Before ordering, make sure you can answer each question:

• What flow is required during normal and high-inflow conditions?
• What is the TDH at that flow?
• Does the exact pump curve cover the required duty point?
• Can the pump handle the solids and abrasiveness of the water?
• Are the hose, fittings, controls, cable and power supply suitable?
• Is the discharge route approved and prepared?

Choose the Pump From the Duty Point

Bring the required flow, calculated TDH, and water conditions together before comparing models. Then review the exact curves in the submersible dewatering pump collection and choose a pump that covers the site's real operating range.

Frequently Asked Questions