Subscribe to our newsletters & stay updated
Many of you have had the opportunity to work with “FM Global Property Loss Prevention Data Sheets” during your career — an excellent, free resource at https://bit.ly/2Tu9SCm. However, have you ever heard of the forerunner of the FM data sheets, the “FM Handbook of Industrial Loss Prevention?”
First published in 1959 (brown binder), 2019 year marks the 60th anniversary of the handbook. A second edition was published in 1967 (green binder). There are similarities to the NFPA Fire Protection Handbook, but also important differences. The FM handbook focuses on property protection and takes more of an engineering approach to loss prevention.
The 1959 and 1967 editions contain 76 and 80 chapters, respectively. Topics run the gamut and include building fire protection features, as well as a wide range of industrial processes and industrial hazards.
In this column, I would like to focus on the handbook’s approach to sprinkler system hydraulic calculations. Note that hydraulically calculated sprinkler systems did not appear in NFPA 13 until the 1972 edition.
The 1959 FM handbook “Chapter 18 Hydraulics of Sprinkler Systems” presents the same method for hand calculating tree systems as is used today.
Given the limitations of hydraulic calculations in the pre-programmable calculator world, the handbook also provides a series of flow curves as a simplified substitute for detailed calculation of the hydraulic characteristics of sprinkler-system piping.
Sets of curves are provided for tree-systems, given the following system features:
• Center-central fed or side-central fed;
• 1895 pipe schedule or 1940 pipe schedule (for old-style sprinklers) or 1953 pipe schedule for standard sprinklers;
• Number of sprinklers on a branch line;
• Spacing between branch lines;
• Spacing between sprinklers on a branch line; and
• Protection area per sprinkler.
Once the total sprinkler system demand flow has been estimated (see Figure 18-12), one can go to the appropriate tree system curve and, based on the size of the system (regarding number of total sprinklers), one can determine the riser pressure required. Then one accounts for the friction loss and elevation between the feed main connection to the cross main and the water supply source, plus this riser pressure — resulting in sprinkler flow and pressure demand.
Add in the number of 250 gpm hose streams needed and you can determine what is needed to provide an adequate water supply.
Though it appears these curves could be used for establishing requirements for new tree systems, I imagine the curves were more often used by FM field engineers to evaluate existing systems.
Primary vs. Total
So where does one get the sprinkler system demand flow? This is addressed in “Chapter 12 Water Supplies for Private Fire Service.” Here is where engineering judgment kicks in and it gets tricky. This chapter indicates there should be a primary sprinkler demand and a total sprinkler demand. Depending on the severity of the hazard, the primary sprinkler demand will be in the range of 50 to 100 percent of the total sprinkler demand (based on the total number of sprinklers expected to operate).
Where there is a difference between the primary and total sprinkler demands, a secondary water supply is required to make up the difference.
The secondary water supply may be: (1) a gravity tank; (2) one or more fire pumps (taking suction from a ground level tank or body of water); (3) a booster pump taking suction from a low pressure public main; or (4) occasionally, fire department pumper connections.
To establish the estimated total sprinkler demand, one determines if the occupancy is in the low, moderate or high demand class. Then estimate the total sprinkler demand from the ranges listed in Table 12-2. According to the handbook:
“The lower demands … are applicable to small areas of 2,500 to 5,000 [sq. ft.] These demands are based on the operation of all sprinklers in the area, with an average discharge of 20 [gal./minute (gpm)] per sprinkler.”
“The upper demands are applicable to large fire areas (75,000 to 100,000 [sq. ft.]) and represent only 5 to 10 [percent] of all sprinklers in a particular area with an average discharge of 20 [gpm/sprinkler].”
For the upper end of the range, it would appear this is similar to a sprinkler design area in the range of 3,750 to 10,000 sq. ft. Hose allowances are based on the number of 250 gpm hose streams needed: 0 to 2 for low hazard, 1 to 3 for moderate and 2 to 4 for high hazard.The required duration of the water supply also is judged by the degree of hazard and is in the range of one to four hours.
For example, let us choose a tree system design which conforms to the curves for Figure 18-12, a center-central feed with 6 sprinklers on a branch line, spacing of sprinklers on the branch line at 8.67 ft. and branch line spacing of 12.1 ft. If the system size is 250 sprinklers and one has determined the estimated sprinkler demand is 1,000 gpm, then the riser pressure will be 35 psi. Note that the figure indicates this flow is equivalent to approximately 53 sprinklers in operation.
All these curves are based on providing a minimum end head pressure of 5 psi. If one wishes to use a higher-end head pressure, then an adjustment must be made using the system equivalent k-factor.
Now, it appears to me that Chapter 18 bases the curves on ordinary hazard pipe schedules, so I wonder if a different set of curves were available for extra hazard pipe schedules.
The handbook notes: “Detailed calculations of the performance of sprinkler systems are ordinarily limited to infrequent instances where the occupancy may require a greater than normal water density or where the adequacy of the available water supply is in question.”
The handbook references specific sprinkler densities in numerous instances. For drum storage of flammable (Class I) and combustible (Class 2) liquids, densities in the range of 0.30 to 0.66 gpm/sq. ft. for old-type (old-style) sprinklers and 0.22 to 0.50 gpm/sq. ft. for standard sprinklers. The section states:
“These discharge densities (0.66 and 0.50), at minimum pressures indicated (35 and 20 psi, respectively) require 50 [sq. ft.] per head spacing and usually the use of deluge sprinkler schedule (extra hazard pipe schedule) of pipe sizes to prevent excessive friction loss.”
Note that per the FM Handbook, the deluge pipe schedule is the same as the extra hazard pipe schedule. The handbook goes on to recommend that the water supply be based on the operation of all sprinklers in the area.
Sprinkler densities also are listed for protection of rubber tire storage. Table 66-1 lists densities for various storage arrangements of rubber tires, ranging from 0.27 to 0.60 gpm/sq. ft. for old-type sprinklers and 0.22 to 0.48 gpm/sq. ft. for standard sprinklers.
What is intersting about these design densities is that they are intended for establishing the initial (primary) supply for sprinklers. This initial supply is based on either 15 or 20 operating sprinklers and must be provided for at least one hour. For the total sprinkler demand, one is referred to Chapter 12. I suppose where rubber tires are concerned, if the initial demand is inadequate that we cross our fingers.
The FM Handbook has much more to offer than information on early hydraulic calculation methods. In browsing through the pages of the handbook, it is possible to get a glimpse of the basis for many of the things we still do today.
© 2023 All Rights Reserved