PLUMBING e Continued from p 26
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. l
Deron Oberkorn
is the product development manager at Zoeller Pump Company.
e Circle 43 on reader reply form on page 80
phc july 2011
www.phcnews.com
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