What size FDC to require?

Questions about Georgia Codes and Standards

What size FDC to require?

Postby Blackstone on Thu Nov 20, 2008 10:08 pm

Fire Department Connection sizing

The Fire Department Connection (FDC) must be sized with one 2½-inch inlet per 250-gpm required to supply the hydraulically most demanding system’s requirements. An alternate number of FDC inlets is acceptable provided hydraulic calculations are submitted indicating that with the friction losses (hose, fittings, valves, etc.) through this alternate number of FDC inlets, for the required pressure and quantity of water of the hydraulically most demanding system’s demand, does not exceed the pressure typically provided by a Fire Department Pumper Truck (approximately 155psi). Provided calculations are submitted which indicate the alternate number of FDC inlets will provide the system requirements based upon the pressure typically provided by a Fire Department Pumper Truck, the proposed arrangement would be acceptable.

FL=0.80(q^2)L [that's 0.80 times q squared times L]
Where FL=friction loss (psi), q= flow divided by 100 (gpm), L=hose length divided by 100-ft.
For a required flow of 1850gpm, assuming a hose length of 100-ft (typical), the loss in each hose to a four way FDC is calculated to be 17.11psi. The friction loss from the FDC to the base of the riser is calculated as follows:

2½-inch Piping:
18-inches of piping length
1 check valve
1 tee
Total Eq. Length – 19.55-ft

6-inch piping:
36-inches of piping length
2 elbows
1 tee
1 check valve
Total Eq. Length – 83.76-ft

A C-Factor of 100 is utilized based upon the piping being dry pipe up to the check valve. Resulting friction loss 18.41psi thru the 2½-inch FDC inlets plus 10.25psi friction loss thru the 6-inch FDC manifold and feed to the base of the riser. Combining all calculated friction loss results in 45.77psi leaving a balance of 109.23psi available for operation of the sprinkler system which exceed the base of riser sprinkler/hose demand [i.e. the 155psi pumper truck availble pressure minus the 45.77psi losses = 109.23 psi available pressure that can satisfy the sprinkler system pressure demand as long as that sprinkler demand is less than the 109.23psi].

NFPA 13, 14, and 24 all indicate the purpose of the FDC is to provide a connection whereby the Fire Department pumper truck(s) can provide an auxiliary/supplemental water supply for fire protection. The degree of supplementation is not specifically indicated. Supplementation is a requirement that can vary – 25%, 50%, 90%, etc., with 100% being the ‘worst case scenario’. The requirements give a minimum size for the FDC, but do not give any info on when a 2-way, 3-way, 4-way, or 6-way connection should be selected. An article written by AFSA noted a light and ordinary hazard building could use the minimum size, but a storage building would justify using a larger FDC. Manufacturer’s listing provide info that the FDC should be sized based on 250gpm per inlet. NFPA 14 (2003), Section indicates the maximum demand for any standpipe system to be 1250gpm, and since a 6-way FDC is designed for a flow of 1500gpm, this would not explain away the reason for such a device as being for standpipes.

Back to supplementing the flow and pressure, if a water supply had a static pressure of 90psi and residual of 80 flowing say 750gpm, when a pumper truck connects-up downstream of the alarm check, backflow, etc., and boosts the pressure to say 155psi, something interesting happens. Since the pressure downstream of the alarm or backflow check valve (FDC side) exceeds the pressure on the upstream side (city water supply), this forces the check valve to close. The pumper is now ‘supplementing’ most, if not all, of the pressure/flow to the system. There has been instances where a small FDC (i.e. Yard Siamese with 4-inch supply main running hundreds of feet) was used and proved to inadequately supplement high challenge fires resulting in extensive loss. NFPA 1142 (2001), Section F.4 indicates one of the uses of an FDC is in case the valve controlling the water supply to the sprinkler riser were shut off, the demand of the sprinklers can still be provided thru the pumper. Frequently, the valve controlling the sprinkler system is closed by the Fire Department once the pumper is connected to ensure a malfunctioning check valve between the system and the water supply (i.e. alarm valve) does not rob the system of the pumper’s pressure.

Closed valves (inaccessible or hidden); seized-up, improperly maintained, or worn-out Fire Pumps; poorly maintained private service mains (debris/corrosion in underground pipe, etc.) are but a few of the reasons the FDC’s supplementary role can change from augmenting a little pressure to providing almost the total fire protection demand.

This essay is to explain the concerns for facilities using the minimum sized FDC ~ the minimum size indicated in the codes may not be sufficient for all applications (especially where high-challenge fire may occur such as high-piled storage and other extra hazard occupancies where the sprinkler demand is relatively large). The 2003 International Fire Code (IFC), Section 104.1, authorizes the Authority Having Jurisdiction (AHJ) to render interpretations to the Fire Codes, such as when a minimum requirement is insufficient for a particular liability.
We find consulations, we learn tricks with which we deceive ourselves, but the essential thing - the way - we do not find. Listen to the river.

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