# The C Factor of Pipe, What Exactly is It??

Often times in designing our sprinkler systems, we need to use formulas to arrive at a value that is useful to us and our work.  In these formulas there are variables and constants that we may or may not understand but we mindlessly plug them into equations anyway because we know them by heart.  The Hazen-Williams formula is the formula we will most often use when calculating our sprinkler system.  In Hazen-Williams we need to input the inside diameter of the pipe be used, the flow in gallons per minute, and the C Factor of the pipe to derive the friction loss for 1′-0″ of pipe.  From memory we can probably recite the C values, 150 for CPVC, 120 for black pipe used in a wet system, and 100 for black pipe used in a dry system, etc, etc.  Perhaps in the course of our experience we learned that the C Factor represents the roughness on inside of the pipe, which is true.  But what does the number actually represent and how does it actually affect our calculations?

As stated before, the C Factor is a representation of the roughness of pipe.  To find out the reason why we need to measure or represent the roughness of the pipe, we must first learn a little about how water likes to flow.  Water likes a nice, smooth, easy, and straight path to flow in, this is called a laminar flow.  In laminar flow conditions, there are few obstructions disturbing the flow path of water which results in very little friction loss. It should be fairly obvious that obstructions to the flow path of water causes friction loss through the pipe meaning it takes more pressure for a given amount of water to flow through a section of pipe with many obstructions then it would have it there were fewer obstructions.