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A force main is a pressurized main pipe that can carry water, sewage, and other materials. Forced mains, as they are also known, are used in a variety of setting. Some systems combine this pipe with other types of mains, as terrain and circumstances change, to make sure that the needs of the system will be met. Such piping is typically installed by sanitation workers, who are also responsible for maintaining it in a fit condition so that public health risks are not created.
This name is often used in reference to discharges for water and sewer piping. This type of piping is often gravity fed. There are several reasons for this, including the fact that the system will function when there is no power, the fact that it is easier to maintain, and the fact that gravity feeding can make the system more efficient. Gravity feeding can be accomplished by laying the pipe in such a way that the contents tend to flow in a particular direction.
Such systems are designed so that the pressure of water entering inflow pipes, combined with the flow of the gravity-fed outflow pipes, prevents backflow. This is done to ensure that untreated sewage does not back up into the water supply, which would pose a serious public health risk in addition to being rather unpleasant.
There may be circumstances in which gravity feeding of outflow pipes is not sufficient, however. Low-lying areas, for example, can be difficult to fit with gravity-fed water and sewer, requiring a force main instead. In a forced system, the pipe is pressurized to force the water and sewage along in a particular direction. An entire length of pipe may be forced, or the pipeline may alternate forced and gravity fed areas, allowing nature to do some of the work of moving the water.
Engineers usually determine whether or not a force main is needed in a particular area, and perform calculations to figure out the best size for the pipe, the amount of pressure needed, and how to create that pressure and keep it consistent and stable to avoid problems. This is done during the planning phase, as public works departments prepare to renovate or expand a sewer system. The details of the project are often discussed at public meetings in which members of a community have an opportunity to offer input.
Breaking a force main to a sewer, makes a mess that you don't want to clean up, believe me. So always check with your local DPW before digging in your yard. I wish I had.