COMPUTERIZING FIRE SPRINKLER HYDRAULIC CALCULATIONS

COMPUTERIZING FIRE SPRINKLER HYDRAULIC CALCULATIONS©
by Bill Smith, president of Elite Software

New legislation coupled with the lure of dramatically reduced insurance costs have contributed to an unprecedented increase in the demand for automatic fire sprinkler systems. For those in fire sprinkler related businesses this new legislation has been a welcome shot in the arm. However, along with increased demand has also come increased competition and scrutiny of designs. The need to perform more work and price it competitively has added to the burden of fire protection engineers and contractors. Most designers and contractors will agree that the design and review stages in the engineering of fire sprinkler systems are among the most time consuming and tedious aspects of the design and installation process.

In complying with the "authority having jurisdiction", designers usually have the choice of using pipe schedules listed in NFPA 13 [1] or hydraulically designing the sprinkler system. Since pipe schedules are simply tables showing the maximum number of sprinklers that a pipe can handle for a given hazard condition, this procedure is very fast and easy to use. With such advantages one might think that pipe schedules are the way to go for time conscious designers. However, the use of pipe schedules has a host of disadvantages that outweigh their speed and simplicity.

A major shortcoming of pipe schedules is that they are based on the assumption that a certain minimum "historical" water supply pressure is available. Since no specific minimum water pressure requirement is given, other than a 15 psi residual pressure at the base of the riser, it is possible that a pipe schedule designed system will not have enough pressure to function properly. A designer attempting to safely use pipe schedules must review hydrant test results and use judgement as to whether sufficient pressure is available.

Another shortcoming of designing by pipe schedules is that they implicitly require a tree type of pipe layout. Although tree systems use a minimum amount of pipe, they tend to cause much more pressure loss than an equivalent grid system. The use of pipe schedules discourages evaluating grid, loop, and hybrid pipe layouts in the search of an optimal design.

From the perspective of minimum system cost, the major drawback of pipe schedule designed systems is that in most cases they tend to use larger, more expensive pipe sizes than actually needed. This bias towards conservative pipe sizes is necessary because designing with pipe schedules is so imprecise.

Perhaps the greatest drawback to using pipe schedules is that the designer has no way of knowing exactly how the fire protection system will perform in the event of fire.

He simply must hope that if he uses the pipe sizes specified by the schedules and locates sprinkler heads according to NFPA 13, that the system will perform as required when called upon. Ideally, a designer likes to know the exact flow rates and coverage he can expect for any section of the covered area of operation. It is little solace to the designer that he can defend his design as being in strict accordance with NFPA 13 schedules in the event of failure or inadequate operation.

Hydraulic calculations are the alternative to the use of pipe schedules. The purpose of these calculations is to determine flow rates, water velocity, and pressure losses through each pipe section. In addition, the flow and residual pressure available at each sprinkler are calculated. Accurate hydraulic calculations show exactly how a sprinkler system will perform if called upon. The primary advantages of using hydraulic calculations are first, that there is much more certainty that the system will operate as designed, and secondly, that the hydraulically designed system will also tend to use smaller, more economical pipe sizes.

The only problem with hydraulic calculations is that they are very tedious to perform, and for some systems virtually impossible to do by hand. Some designers actually perform manual hydraulic calculations for simple tree type systems, but very few will attempt manual calculations for grid type systems. The complexity and number of calculations required to analyze a grid system is simply impractical by hand.

For anyone who routinely wants to perform hydraulic calculations, a computer program is a virtual necessity. These programs make possible the impossible. In addition, all standard calculations are done much faster and more accurately with the help of a computer program.

Most designers recognize the value of hydraulic calculations over pipe schedules, yet many engineers and contractors are intimidated by the idea of performing hydraulic calculations. This is understandable to some degree, because even with a computer program to automate the calculation drudgery, a designer must understand a few more hydraulic concepts than is necessary for use with pipe schedules. The most important points to understand for performing hydraulic calculations involve pressure. A designer most know very well the differences between total pressure, static pressure, residual pressure, and elevation pressure. In addition, it is also useful to understand the terms HGL (hydraulic grade line) pressure and EGL (energy grade line). Unfortunately, NFPA 13 does not go into great detail concerning pressure definitions. However, there are supplementary books on the subject [2,3], and most hydraulic calculation program user manuals cover the subject as well.

Once a designer understands the basics of hydraulic calculations, he definitely appreciates what a computer program can do, but he still may be squeamish about using one. After all, the computer may be an unfamiliar tool, and it certainly has its own nuances that must be learned.

However, most program vendors would say that computer knowledge is much less important than fire sprinkler system design knowledge. Most hydraulic calculation programs are geared towards novice computer users so that learning how to use the computer is not a barrier to learning how to handle the program.

Before embarking on the search for a hydraulic calculations program, it is important to know in general how they work. Many designers mistakenly believe that hydraulic programs can suggest a pipe layout or even compute optimal pipe sizes. This is not really possible because there are an infinite number of pipe layout and pipe size combinations that will work satisfactorily for a given project. What constitutes an optimal pipe layout and size configuration is something decided by the designer.

The main purpose of a hydraulic calculations program is to tell the designer how a proposed system will perform. Although the hydraulic calculations generated by a program often show that a proposed sprinkler system will perform adequately, the designer may still not be entirely satisfied with the results. For example, the designer might note that water velocities and pressure losses are minimal in his initial design. He might wonder whether smaller and less expensive pipe sizes could be used without incurring excessive velocities and pressure losses. The beauty of using a program is that alternative designs can be quickly reviewed and evaluated. Thus, a designer is encouraged to seek an optimal design, and not just the first one that happens to barely work.

Most hydraulic calculation programs are easy to use. The basic premise is that the designer lays out the sprinkler pipe network and numbers it in a logical fashion. Sprinkler heads are given a node number while each pipe section is given both a beginning and ending node number. Nodes in the pipe network occur at every sprinkler, and at the junction of two or more pipes. Therefore, the first step in using a hydraulics program is to layout the pipe network and assign a number to each node.

After node numbers have been assigned, the designer is ready to enter pipe and sprinkler data. This is usually accomplished through the use of "menus" and "fill in the blank" type input screens. Pipe data typically consists of the node numbers, pipe material (copper, steel, etc.), pipe length, and the quantities of any fittings on the pipe. The sprinkler data usually entails only a sprinkler node number and the sprinkler K-Factor. Fortunately, most programs allow the user to specify "default" data so that much of the redundant data for pipes and sprinklers need not be re-entered. Some programs even have automatic grid and tree builders that can create a detailed pipe network based on the designer's description. In addition, virtually all programs check the data as it is entered so that mistakes are minimized.

Other key data required by hydraulic programs include the minimum allowable sprinkler residual pressure, minimum desired sprinkler flow per square foot (density), system type (wet, dry, deluge, pre-action), and the maximum allowable nodal flow and pressure imbalance. Most hydraulic programs will also request water supply information including maximum gpm flow, static pressure, and residual pressure available. In addition, some advanced hydraulic programs include provisions for fire pumps, multiple inflow points, supply/demand graphs, bill of materials, cost estimating, and integration with CAD.

The output reports from a hydraulics calculation program show the flow rates and pressure loss data for each pipe section, and the discharge flow and residual pressure for each sprinkler. The format of this output data varies from program to program, but many programs follow the NFPA format. Key output includes the actual density obtained and the lowest residual pressure occurring for any sprinkler. For most programs, the output information can be based upon a known water supply pressure or it can be based on the minimum supply pressure needed to meet the system demand pressure. These two output modes are typically referred to as supply and demand based output. Supply based output reflects the system operation given a certain water supply pressure. Demand based output shows how the system will work when supplied with just enough water pressure to drive all sprinklers at the minimum allowed residual pressure.

One other report often produced by hydraulic calculation programs is a water supply and demand graph. This graph is sometimes called a "Q" to the 1.85 graph because pipe friction losses are proportional to the water flow rate raised to the 1.85 power. If two flow and pressure conditions are known for the water supply, then a straight line can be plotted on a semi-logarithmic graph that relates available water supply with the system demand requirements of the sprinkler system. If the system demand point is below the water supply line, then the proposed sprinkler system will work with the available water pressure. This graph is an effective way to quickly assess the margin of safety in terms of gpm and pressure there is available to the sprinkler system.

If reports show that all sprinklers have adequate pressure with available water pressure and flow greater than the system demand pressure and flow, then the proposed sprinkler system is considered workable, but not necessarily optimal. An optimal sprinkler system is a workable system that blends economics and conservative operation together. Designing an optimal system takes creative design talent, and usually multiple runs of a hydraulic calculations program.

Just now on the market are new computer aided (CAD) systems that nicely interface with modern hydraulic calculations programs. This capability is extremely beneficial since the sprinkler system can be drawn on the computer screen and then automatically analyzed.

After reviewing the hydraulic calculations, the designer can then make changes to the drawing and specify recalculation until an optimal design is reached. CAD systems completely eliminate the need for designers to manually type in pipe sections, fittings, node numbers, and sprinkler heads into a hydraulic program. All that information automatically goes from the drawing directly into the hydraulic calculation program. Such systems deserve close examination and are being employed by more and more designers.

With the advent of low cost, powerful hydraulic calculation programs, designers have little reason not to use them. The advantages of hydraulically designed systems are simply too great to ignore. All fire protection designers wanting to design faster and optimally, should certainly consider the use of a hydraulic calculations program.

1. Standard for the Installation of Sprinkler Systems, NFPA 13, 1987 Edition, National Fire Protection Association, Inc., Quincy, Massachusetts, 1987

2. John K. Bouchard, Automatic Sprinkler Systems Handbook, Elizabeth M. Leahy, National Fire Protection Association, Inc., Quincy, Massachusetts, 1987

3. Warren Ng, Fire Sprinkler System Hydraulic Calculations and Computer Aided Design Methods, Warren Ng, Oakland, California, 1982

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