Hydraulic Analysis of Fire Protection Water Supplies

Vol 21 No 6

In Part 1 of this series, we introduced the value of hydraulic analysis of fire protection water supplies. We also recommended that to make the most of future articles, the reader should have the slide rule that we will be discussing available. Please request a slide rule through www.fireworld.com using the IFW Store header link. The slide rule is available to the first 100 readers to request one for a two-dollar shipping fee. A few are still available.

Introducing the slide rule:

The front of the slide rule (Photo 1) is used to solve for any of the variables in fire main friction loss if the other variables are known.

The variables (from the top) as seen in photo 1 are:

- Friction loss in pounds per square inch (psi)
- Pipe length in feet
- Friction loss in feet of head (this will rarely be used and will be ignored for the remainder of this series of articles.) Do not confuse this with pipe length in feet.
- Pipe diameter from 6" - 24."
- Pipe C factors from 70-150. This is a measure of pipe roughness with 70 being the roughest and 150 being the smoothest. The figure to use is given on the back of the calculator and will be discussed later.
- The flow in gallons per minute (gpm) from 100 - 15,000 gpm.

The back of the slide rule (Photo 2) is used to calculate water flows from Underwriters playpipe nozzles and common hydrant outlets. Once the pitot pressure is known, the flow in gpm is read in the window under the appropriate outlet size.

A hydrant coefficient of 0.9 is already included for smooth round hydrant outlets. For other hydrant outlets, a coefficient is applied. Please note that the coefficients shown differ from those in National Fire Protection Association and International Fire Service Training Association publications. This is because the slide rule values already include the 0.9 coefficient. The final answers will be the same. Coefficients have already been applied for the Underwriters playpipes as well.

The back side also has rulers so that hydrant or nozzle openings can be measured. Common conversion factors are also provided.

Finally, a table of pipe "C" factors is included. The "C" factor is needed when solving problems involving pipe friction loss. The multiplier (Multiply by) is not used with the front of the calculator since it is already accounted for. The multiplier is only needed if the problem under consideration is outside the range of the front of the calculator.

Lets look at some basic problems. We'll start with the back of the slide rule because that information is needed to solve friction loss problems on the front.

Problem 1:

What is the flow from a 1?" Underwriters playpipe nozzle with a pitot pressure of 30 psi?

Solution: Adjust the back of the slide rule so that 30 psi appears in the pitot pressure window. Read across to the window corresponding to a 1?" nozzle. The window shows 498 gpm.

Problem 2: What is the C factor for an eight-inch diameter plastic pipe?

Solution: By inspection of the Friction Loss Constants table at the bottom of the back of the slide rule, the C factor is 150.

Problem 3: 600 gpm is flowing through 700 feet of six-inch unlined cast iron pipe. The pipe is 30 years old. What is the expected friction loss?

Solution: From the Friction Loss Constants table on the back of the slide rule, the C factor is 80.

Now, line the calculator up as shown in Photo 3. You know the pipe diameter (6"), the C factor (80), the flow 600 gpm), and the length (700 feet.) The remaining variable is the friction loss in psi. Look just above the 700 feet mark and note that the friction loss is approximately 22 psi.

Congratulations! You understood everything so far; you can answer many industrial fire protection water supply questions. In Part 3, we'll show you how.

Questions can be directed to the author at John_Frank@swissre.com or +1 770-569-7082.

*Swiss Re is the world's leading and most diversified global reinsurer. The company operates through offices in over 30 countries. Founded in Zurich, Switzerland, in 1863, Swiss Re offers financial services products that enable risk-taking essential to enterprise and progress. The company's traditional reinsurance products and related services for property and casualty, as well as the life and health business are complemented by insurance-based corporate financial solutions and supplementary services for comprehensive risk management. *