Designing Commercial Sewage Lift Stations Part 1: How To Determine Flow Rate

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By Chris Edmondson

Sometimes codes just aren’t very helpful.  Take Section 712.4.2 of the International Plumbing Code (IPC) and its insight into the selecting flow capacity for a sewage and ejector pumps:

712.4.2 Capacity. A sewage pump or sewage ejector shall have the capacity and head for the application requirements.

(Thanks, IPC….)

Clearly, having the proper flow is necessary to efficiently discharge wastewater from a commercial plumbing system – but how do you determine the flow rate for the sewage system so you can properly size the pump? 

It’s really not so hard – common sense actually.  Most engineers rely on the “What goes in must come out” method.  In other words, they use a fixture count to determine the total flow rate, much like you would to determine the supply flow rate.

The Flow Calculation Sheet (Figure 1) shows the list of values assigned to the various types of plumbing fixtures that might exist in a project.  Using this calculation sheet as an example, these are the steps for determining the flow rate for a sewage system:

  • ·  Identify all the fixtures and the associated unit value (column A) that contribute to the installation
  • ·  List the total number of each type of fixture (column B)
  • ·  Multiply the fixture unit value (column A) with the total number of each type of individual fixture (column B) and list that value in the "Total Fixture Unit" column 
  • ·  Add all the values in the Total fixture Unit column together to obtain the Total Building Unit Count, which, in this case, is 800      
  • ·  Find the Toatl Building Fixture Count (800) on the Hunter's Curve to convert the total fixture units to pump capacity in GPM and size your pump accordingly          

 Keep in mind that the IPC also requires a minimum flow rate of 2 feet per second to keep any entrained solids moving down the line.

Pipe Size and Pump Discharge Size

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The IPC specifies that pumps or ejectors that receive the discharge from water closets be capable of handling spherical solid of up to 2-inch.  Other pumps or ejectors shall be capable of handling spherical solids with a diameter of up to and including 1 inch.  The IPC provides the following chart for reference on the diameter of the discharge pipe.

The above values assure that the velocity stays above 2 feet per second to prevent clogging of pipes.  If a grinder pump is used, a smaller pipe size may also be used – as small as 1.25” to maintain adequate flow.

A Few Other Things To Keep In Mind…

Along with your standard fixture counts, don’t forget to include higher volume components such as drainage from body showers, cooling towers, etc.

Also, don’t be confused by the following Drainage Fixture values shown in the IPC Table 709.1 below.  These values (as the Code states) should only be used for calculating drainage pipe size, not the flow capacity needed for the sewage or ejector pump.  It is best to use supply side fixture values for calculating flow.

Finally, check with local codes, which always supersede the IPC.  Chances are adhering to the IPC will be okay….but you never know until you check!

For more information, check out JMP’s webinar on designing sump and sewage systems.

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