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~J L~ assumed that the total flow will be less than the rated flow of. the pump. At these lower flows, out- put pressures will be higher. Finally, it is assumed that if the fire grows, either straight stream nozzles will be utilized or the pumpers supplying the fire department connections will provide greater pres- sures. 'Designers must be aware of this information for a number of reasons. First, designers should only use 65 psi minimum design pressure when a partic- ular fire department so specifies, based on their equipment and tactics. An example would be if a department used only 2'/2 inch hose and straight stream nozzles for standpipe operations. Other design conditions such as additional fire hose con- nections to enable shorter hose lines may also fac- tor into the decision. In all cases where lower pressure is not specifi- cally approved by the fire department, 100 psi basic pressure should be considered the minimum. However, if any of the above assumptions about the fire pump or the fire department equipment and tactics are invalid in a particular building or jurisdiction, designers should consider providing pressures greater than the basic 100 psi at the hose connections to facilitate adequate fire streams. NFPA 14 imposes a maximum pressure limit of 175 psi on standpipe systems for fire department use. Pressures in excess of 175 psi will invoke requirements for pressure reducing devices, which are covered in the next section, Pressure Regulating Devices. Considerations - Design Pressure ■ Use minimum 100 psi outlet pressure unless specifically exempted by the fire depart- ment, with justification based on tactics. ■ If fog or combination nozzles are used for standpipe operations, investigate fire department procedures „and tactics to check the need for pressures over 100 psi (but not exceeding 175 psi). 38 PRESSURE REGULATING DEVICES Pressure regulating devices (PRDs) restrict system pressures, usually below 175 psi for Class I systems (Figure 4.10). This is considered the maximum safe operating pressure as well as the maximum working pressure limit of most fire protection com- ponents. Proper design of PRDs is imperative so that firefighters have adequate pressure for hose streams. As a stark example, failure to coordinate settings on these devices with fire department tac- tics emerged as a key issue in the 1991 Meridian Plaza high-rise fire in Philadelphia, which resulted in the death of three firefighters. PRDs fall into three categories: pressure reducing valves (PRVs), pressure control valves, and pressure restricting devices. Pressure restricting devices do not limit pressure during static (non-flowing) condi- tions, nor do they maintain a constant discharge pressure. These devices incorporate orifice plates, mechanical pressure restrictors, or valve limiting stops. Pressure restricting devices are not used for new Class I standpipe systems. However, designers may encounter these when redesigning existing systems, which would provide the opportunity to implement some or all of the considerations below. PRVs and pressure control valves limit both stat- ic and residual (flowing) pressures. However, many of these valves are factory preset to attain specific outlet pressures with specific inlet pressures. It is important for designers to specify the inlet pressure range for valves as well as the desired outlet pres- sure, so that they may be designed properly and then installed on the correct floors. Careful atten- tion during design, installation, acceptance testing, and maintenance ensures that systems with PRDs will function properly. 0JiZt'1• Occupational Safety and Health Administration 154 (Fig. 4.10) Hose connection equipped with a pressure regulating device.