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APPLICATION NOTE



      Fixed fire
      protection and booster/spray sprinkler design

      
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INTRODUCTION

            
NFPA 15 "Standard
            for Water Spray Fixed Fire Systems for Fire Protection"
            is the industry-accepted standard for the design and testing
            of Firewater Protection systems. However, in the particular area
            of hydraulic design it makes no allowance for modern computer
            simulations in terms of both the possible benefits and/or limitations
            that such a method of analysis might provide.

            
Fixed fire protection
            systems involve large numbers of pipes, valves and fittings.
            It is unlikely that any current hydraulic design would be carried
            out using the manual calculation method described in Appendix
            A of NFPA 15.

            
Section 1.2 of
            the standard states …

            
"Nothing
            in this standard is intended to restrict new technologies or
            alternate arrangements providing the level of safety prescribed
            by the standard is not lowered."

            
Given the accuracy
            of a computer simulation over a hand-calculated design, and even
            taking into account the differences between a computer simulation
            and a calculation carried out in accordance with Appendix A of
            NFPA 15 described below, it is argued that the computer simulation
            increases the probability of achieving adequate firewater distribution
            during system operation and does not lower the level of integrity
            or safety of the system.

            
DESIGN CRITERIA

            
There are a number
            of design criteria described in NFPA 15, which need to be compared
            with the methods utilised by the various software programs avaialble
            from Accutech. These are:

            

              
Pressure loss
              formula for pipes.
              
Pressure loss
              formula for valves and junctions.
              
Velocity head.
            

            
Pressure Loss Formula for
            Pipes

            
NFPA 15 utilises
            the Hazen-Williams formula for calculating the pressured drop
            due to flow in a straight pipe. The Hazen-Williams formula utilises
            a coefficient C that varies depending on the roughness and size
            of the pipe. The value of C can vary for new steel pipes between120
            and 140 depending on size.

            
Figure A-7-2
            (e) in NFPA 15 provides a head drop nomograph for varying flows
            through steel pipes of different diameters. The Figure states
            that the value of C is constant for all diameters at C=120.

            
By contrast,
            the Pipe Friction Handbook by the Australian Pump Manufactures
            Association Ltd provides values of C, which vary with pipe diameter.
            Values for new steel pipe closer to 140 are given. Since the
            range in value of C between 120 and 140 can make a significant
            difference in pressure loss, this inconsistency is significant.

            
An alternative
            method of calculating the pressure loss due to flow in a pipe
            is the Darcy-Weisbach equation (sometimes referred to as the
            Moody diagram method). This approach relies on knowledge of the
            absolute roughness of a pipe to determine a friction coefficient.
            Outside the water and fire protection industries, the Darcy equation
            is universally used and the method highly accurate.

            
A comparison
            between test values from Figure A-7-2 (e) in Appendix 'A' and
            the same calculation in Piping Systems FluidFlow and FluidFlow3
            using either method gave almost identical results for C values
            of 120.

            
Pressure Loss Formula for
            Valves and Junctions

            
NFPA Table A-7-2
            (g) gives equivalent lengths for various valves and pipe fittings.
            The equivalent length method utilises the concept of a length
            of straight pipe which produces the same pressure loss as the
            valve or fitting it is simulating. A drawback with this method
            is that a different equivalent length is required for each "size"
            of fitting, i.e. there is one equivalent length for a bend in
            a 100mm pipe and a different equivalent length for a the same
            type of bend in a 150mm pipe. Other drawbacks with the equivalent
            length method are:

            

              
Table A-7-2
              (g) requires that the equivalent lengths be corrected for values
              of C different from 120.
              
Elbows are referred
              to as "standard" and no account is given for longer
              radius bends if used.
              
The method does
              not take into account fittings with changing diameters, for instance
              a tee junction with a branch diameter different from the barrel
              diameter.
              
Values are generic,
              it they do not necessarily apply to a particular manufacturer's
              valve or junction.
              
The list of
              valves and fittings is extremely limited, i.e. only a generic
              "swing check" valve is given and the table actually
              notes that the given value is to be considered "average".
            

            
Modern computer
            simulation techniques utilising a schematic flow sheet representation
            of a pipe network can provide a more accurate and sophisticated
            modelling of valves and junctions, for instance the Ft method
            described in Crane or the methods developed by Idelchik which
            take into account unequal tees, wyes and bends of different r/D
            values.

            
Velocity Head

            
Water flowing
            in a pipe has an energy component due to its velocity - a kinetic
            energy. If this velocity is reduced, the kinetic energy is converted
            to pressure energy. Theoretically this additional pressure energy
            would be available to discharge slightly more flow through a
            nozzle than would be the case if the velocity head was ignored.
            NFPA 15 states that velocity head can be either included in a
            calculation or ignored.

            
Velocity head
            becomes significant only if the velocity of flow is quite high,
            say well above 3.5 m/s. High velocities are to be avoided because
            of increased frictional resistance. In a Piping Systems FluidFlow
            or FluidFlow3 simulation, a maximum pipe velocity can be set
            such that a warning is enunciated for every pipe where the velocity
            exceeds the set maximum. This feature allows the design engineer
            to conveniently monitor the whole system for excessive velocities.

            
SUMMARY

            
FluidFlow3 and Piping Systems
            FluidFlow are both suitable for the design and simulation of
            fixed fire protection and spray/sprinkler systems and can produce
            designs that satisfy the requirements of NFPA. FluidFlow3's advanced
            user interface and flowsheet display make this the ideal software
            both for the design and reporting of a fire protection system.

            
For a summary of Piping Systems
            FluidFlow and FluidFlow3 capabilities in the area of fixed fire
            protection design, click here.
          
        
      

      

        

          
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