By Chris Edmondson
In a previous blog we discussed how to determine the flow for a commercial sewage lift station so you determine the necessary pump capacity. Today we are going to discuss Total Dynamic Head, which is also critical to sizing a pump for a sewage lift station. Not only is it absolutely necessary to select a pump to overcome the static head required to pump the highest point in the system but friction head must also be calculated. Thus, a total head loss calculation is required.
What is Total Dynamic Head (TDH)?
Total Dynamic Head (TDH) is the total static head plus the friction head imposed on the sewage lift station – a value that is typically express in feet of head.
Static Head + Friction Head = Total Dynamic Head
Friction head includes all piping head losses through the pipe length, elbows, and other fittings. Static head refers to the vertical distance from the minimum water level in the basin to the highest point of discharge. Just remember that the discharge point may not be the highest point in the sewage lift system. Thus, static head will also includes any “back pressure” that might exist beyond the point of discharge, i.e. in cases where you are pumping up hill. That vertical height applies as well. In the system example shown in Figure 1, note there is a back pressure of 19.7 feet.
Table 1 shows a complete TDH calculation for a system. The static head (vertical lift) in this example is 28 ft., which gets added to all of the friction losses in our example, including threaded fittings, tee branches, etc. In this case our total equivalent length of pipe is 126, which we convert over to feet of head, giving us a total friction loss of 2.3.
Obviously, not many engineers today are going to spend time looking up friction loss values when there are excellent online tools to do it for the. Bell & Gossett’s System Syzer has an excellent tool, which can be downloaded that will calculate all the values for you, once you all of the pertinent piping information. It will also complete the TDH calculation for you and give you the proper pump head so that you have a complete record of the design and selection of the system.
A word of caution – A sewage pump cannot be balanced. A detailed head calculation is required to make sure we do not overhead the pump.
For more information on this topic, see our video, “Sewage Lift Stations – Determining TDH”.