Wood’s Data

Tools of the past are still tools of the trade today.

While perusing the office library I came across a blast from the past, or at least I thought so. It was a black, plastic-covered, three-ring binder entitled “Hydraulic Data for Fire Protection Systems.” The author, Clyde Matheson Woods, can be considered the father of sprinkler hydraulics. Graduated as a civil engineer from Georgia Tech in 1915, his first and last job was with the Automatic Sprinkler Corp. (ASCOA) of Cleveland, Ohio. 

This binder or book was first published in 1944, with later editions published in 1961 and 1972. Our library contains a 1982 reprint of the 1972 edition. The book was intended to teach young engineers and technicians in the sprinkler business how to do hydraulic calculations and to serve as a reference.

My first encounter with Wood’s data, was while learning hydraulic calculations in ENFP 312 from Dr. Hickey at the University of Maryland in the fall of 1977. Our class was learning the basics of hydraulic calculations for sprinkler systems.  

Keep in mind that provisions for the hydraulic calculation of fire sprinkler systems did not appear in NFPA 13 until the 1970s. Though NFPA 15 water spray systems had been hydraulically calculated for years by then, NFPA 13 sprinkler system hydraulic calculation was a relatively new technology. Of course, the calculations were primarily done by hand, with the help of a four-function calculator (after we had walked five miles through knee-deep snow to get to class).

As a young engineer reviewing sprinkler calculations, Wood’s data was always near at hand. Though I no longer have occasion to refer to it, I still have my Xerox copy of selected tables for pipe friction losses and fitting equivalent lengths still near at hand.

The book is comprised of 10 sections. Section 1 is an introduction, Section 2 is the table of contents, and Section 10 is the index. 

Section 3, “Elementary Principles of Hydraulics,” introduces the reader to the basic concepts with which one must be familiar to perform sprinkler system hydraulic calculations. Some sample excerpts include this definition of a pitot tube:

 Pitot tube (named for its inventor Pitot) is used to measure the pressure of water discharging from a nozzle or flowing in a pipe by having its open end in the water and the other end connected to a gauge or manometer. The tube is generally about 1/16 inch in diameter, bent at right angles, and mounted with a gauge or manometer connected to the long end. The short end is held by hand in a hose stream. 

I also found the following brief discussion interesting:

Friction loss is not affected by pressure. Consider two pieces of pipe equal in size, length and roughness with outlets of different sizes but with the same gpm flowing through each, the pressure in one piece being 200 psi and in the other piece 50 psi. Since the friction loss is caused only by roughness of the pipe and velocity of the water, as explained above, and since in both pieces of pipe the roughness and velocity are the same, it is easily seen that the friction loss will be the same in both pipes, and that the difference in pressure in the two pipes has no bearing on the friction loss.

Section 4, “Fittings, Valves, Strainers — Pressure Loss,” discusses fittings and friction loss, noting that fitting losses are determined using equivalent pipe lengths (for commonly used fittings) or direct losses determined by a lab test for such special fittings such as strainers. One thing missing was information about pressure-regulating devices. 

Section 5, “Friction Formula,” contains a very detailed analysis justifying the proper friction formula that should be used for calculations of fire sprinkler systems. 

Having shown that the Williams-Hazen solution requires substantially less time, requires fewer calculations, and therefore is less productive of errors than the Darcy-Weisbach formula, it now remains to find which formula produces the most accurate results. (Woods obviously seems to think Williams deserves top billing over Hazens). And later in this section …

On the basis of the evidence just offered, under the conditions of fresh water weight at temperatures from 45 F to 75 gpm range of flows shown on 5-13, it is the author’s opinion that there is no essential, practical difference in friction loss calculated by the Darcy-Weisbach and Williams-Hazen formulas as given herein.

Section 6, “Calculation Methods,” details the procedures for calculating standard tree sprinkler. This is the real meat of the book. The following paragraph tells of the author intent with this section while giving us some of the history.

In an effort to arrive at a uniform practice in hydraulic calculations for fire protection systems, a recommended standard method and standard forms were introduced in the 1957 revision of NFPA-15, Standards for Water Spray Systems for Fire Protection. This standard method embraced the principles and practice set up and used by ASCOA since 1930 and published in “Automatic Hydraulic Data” in November 1944.

 For the reader’s information, those standard forms, consisting of Title Sheet, Hydraulic Calculation Sheet, N1.85 Graph Sheet and Summary Sheet are the same forms found in Figures A.23.3.2(a) from the annex of the 2016 edition of NFPA 13.

The calculation method and rules are described in detail. The concepts of supply and demand curves are introduced. The chapter also shows one how to calculate systems using one or more of the typical water supplies, i.e., municipal connection, gravity tank, or fire pump with tank. 

Section 7, “Velocity Pressure in Hydraulic Calculations,” goes through the painstaking process of how to perform a velocity pressure calculation. Clearly, the information is provided for completeness as the author has strong opinions about the value of this method. 

He writes: The data in this section is given in support of the continuance of our policy of omitting velocity pressure from normal hydraulic calculations in our field. In the 1944 Edition of  “Automatic” Sprinkler Hydraulic Data,” after a brief explanation of the minor influence of velocity head in our work, the following statement was made:

“The extra work of calculating the velocity head for every piece of pipe, in addition to calculating the friction and obstruction heads, would not be justified. The velocity head is therefore neglected entirely in the calculation method described in this book.”  

Velocity pressure calculation is made optional in A-23.20-D of NFPA-NBFU No. 15 — Standards for Water Spray Systems for Fire Protection. (Similarly, NFPA 13 permits the use of velocity pressure.) As will be seen by reading the velocity pressure calculations noted in this section, the amount of extra work required to calculate velocity pressure is difficult, tedious and time-consuming. 

Section 8, “Loop Calculations,” focuses on the calculation method for a simple loop and a compound loop consisting of two loops. The author recognizes the limitations involved with hand calculation of more complex loops and grids suggesting that these problems may be better solved with the use of “electronic computers.”

In the age before computers or even programmable calculators, engineers relied heavily on data taken from tables. Section 9 provides the sprinkler engineer with a treasure of cool tables to assist in calculations. There are the expected pipe friction loss and equivalent length tables. There is a table for friction loss for fire hose. There is a table calculating the sprinkler pressure given the flow and orifice size (but only up to 17/32-inch sprinklers.) One table which saves several calculation steps allows one to determine the minimum flow and pressure required for a given sprinkler orifice size, at a given design density and area of sprinkler coverage.

After the section on tables, there are three full system calculation examples. These are a calculation for an aircraft hangar deluge system, a rubber tire storage warehouse and a complex fixed water spray system. These calculations are quite detailed and include looped exterior mains and various types of water supplies. 

I had recently brought the Wood’s data with me on a trip so I could begin writing this column and soon got an email from my office asking where the binder was. What a pleasant surprise. This old fuddy-duddy thought no one else in the office has looked at this stuff for 20 years. Come to find out that it is being used by our senior engineers to teach our young engineers how to hand calculate sprinkler systems. This is done before we allow them to use the computer hydraulics software.

In 1964, Wood published a companion Study Guide for “‘Automatic’ Sprinkler Hydraulic Data,” containing problems and solutions for commonly encountered calculations. Wood hits the nail on the head regarding what we are trying to accomplish with hydraulic calculations.

It has been stated by more than one observer that the greatest weakness among hydraulic calculators is inability or inattention in following through with a comparison of demand and supply. This is of utmost importance, as finding just what the supply will actually deliver is of the essence of the calculation.

This statement echoes what I often say, “If there is one thing FPEs need to get right when designing sprinkler systems, it is the water supply.” 

Woods, who by the way, is a SFPE Fellow, goes on to say: “One of the greatest hazards in the mechanical equipment industry is to get an incorrect specification changed once it has been set up. The prime designer and specification writer should therefore avail himself of the services of a fire protection engineer who is competent to calculate and compare both demand and supply.”

Though ASCOA of Cleveland, Ohio is no longer around, parts of its legacy lives on, as young FPEs take their first baby steps in practice guided by Wood’s data.    

Samuel S. Dannaway, P.E., FSFPE, is a licensed fire protection engineer and mechanical engineer with bachelor’s and master’s degrees from the University of Maryland Department of Fire Protection Engineering. He is a past president and Fellow of the Society of Fire Protection Engineers. He is vice president of Fire Protection Technology at Coffman Engineers Inc., a multi-discipline engineering firm with over 360 employees across eight offices. Dannaway can be reached at dannaway@coffman.com.

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