It’s Not All About the Pump

Other Factors Play a Role in Sizing a Sump System

By Deron Oberkorn

The fluid mechanics of sump pump operation is based upon the principles of Daniel Bernoulli’s (Hydrodynamica in 1738) which define the effects of balanced energy on fluid systems. This equation employs kinetic, pressure and potential energies to move a matrix of solids and water through a specified conduit of specific properties. What does that mean? To further explain, movement is maintained at a minimum velocity of 2 feet (61 cm) per second to maintain solids and particulate suspended in the flow and to prevent the build up of any scaling on the walls of the piping conduit.

Contrary to popular belief, pumps do not create pressure; pumps create flow. When this flow is restricted, either within the confines of a pump or the boundaries created with pipe, pressure is the result. As the areas that maintain the flow are changed (through orifice sizes, clearances between parts, diameters, pipe lengths, pipe diameters, etc.), the pressure and flow rate reflects those changes.

A common practice after a severe rain event is to increase the size of the sump pump. Say the homeowner has a ? HP, 115V, submersible sump pump in an adequately sized (18" 5 22") sump basin and their community is hit by a 25-year storm. The homeowner calls and explains to the contractor that their existing pump could not keep up with the incoming flow and a small flooding event was experienced within the home. This was not due to a power outage, just a deluge of water in a short amount of time. The first response from the homeowner is, “How much will it cost to install the next largest size pump?” As you set up the visit to see what is installed and assess the proper application of their request, inspection of the piping configurations should be considered as well. When attempting to move more water, “bigger pump means bigger pipe.” As the information in the first paragraph defined, flow and pressure are all relative to the conduit of piping used to carry the water from point A to point B.

The system described above has a ? HP submersible pump with 1½" discharge. Water is lifted from the bottom of the sump to the floor joist of the first floor, 10 feet total in height. Then, the system runs 60 feet of 1½" PVC Sch40 pipe through three 90 degree elbows before exiting the foundation wall and feeding a 3" gravity corrugated pipe to the point of discharge. Using the industry coefficients for friction losses and velocities in 1½" PVC pipe at 35 gpm, a design point of 35 gpm at 15' TDH (total dynamic head) is calculated. Total Dynamic Head (TDH) refers to all the friction losses, head pressures and minor losses combined at a specific flow rate through a specific pipe size and construction. This calculated design point is very typical for any residential sump pump.

Let’s look at the same system with a ½ HP unit installed. The assumed flow rate of this size unit is 60 gpm, or at least that is what your homeowner wants to get out of the unit for the expense of the installation. The chart included shows how the friction losses in 1½" PVC pipe become a detriment to the efficiency of the existing piping system. At 60 gpm through the existing piping system, the new pump will feel like it is pushing against 23.5' of head as compared to the 15' of the 35 gpm unit. The ½ HP pump will reach equilibrium and fall somewhere in the 40 to 42 gpm range unless you install a high head ½ HP unit. These are not typical for residential applications and usually carry a slightly higher cost. Instead of your customer getting 60 gpm as they requested, this installation yields 40-42 gpm—or only a 17% increase in flow rate.

If your customer truly needs to move 60 gpm, consider increasing the pipe diameter to 2" for as much, if not all, of the system as possible. Just because the pump has a 1½" diameter discharge doesn’t mean that this is a required pipe size throughout the system. The pump discharge is an industry standard that is representative of the solid size the unit can pass. A 1½" discharge will typically pass a ¾" solids size or smaller, 2" discharge is typically a sewage ejector (unless it is labeled a sump/effluent pump) that will pass up to a 2" spherical solid. That is another article all together. The point is, “bigger pump means bigger pipe,” for the best system efficiency and pump life to meet your customer’s expectations.

Deron Oberkorn is the product development manager at Zoeller Pump Company.