One of my wanted items on my personal to-do list was a full lineup of flow charts that incorporate discharge coefficients coming off a fire hydrant. These (in my line of work) are most commonly used in fire hydrant flow tests, where flow comes out of the side (commonly 2-1/2") outlets or the main pumper outlet (single 4" or larger).
So what is a discharge coefficient?
When we flow out of an orifice, water is restricted and forced into a smaller cylindrical shape. The discharge coefficient is what helps us adjust the measured pitot pressure, which is taken along the centerline of the stream, and adjust for about how much flow is coming through the opening.
It's simply comparing the size of the actual stream against the size of the opening.
When a long straight stream-straightener is used, this coefficient is high. The stream is less tightly condensed as compared to the opening dimension.
When water takes a more difficult approach and is restricted in its flow, this coefficient is lower.
Discharge Coefficients as Identified in NFPA 291
We'll discuss this in more detail later. For today, we've created flow charts for these three different discharge coefficients. See the PDF link below to get access to all three flow charts, broken out across a number of different orifice sizes.
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Joe Meyer, PE, is a Fire Protection Engineer out of St. Louis, Missouri who writes & develops resources for Fire Protection Professionals. See bio here: About