Explanation

Standpipe
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175 psi (1207 kPa).
For standpipe systems containing 11/2 - 21/2 in. (40 - 65 mm) hose connection outlets without preconnected hose, a pressure regulating device is required when the pressure exceeds 175 psi (1207 kPa), according to NFPA 14, Standard for the Installation of Standpipe and Hose Systems, 2019 edition. This requirement applies to Class I and Class III standpipe systems, which are intended for use by fire department personnel or trained occupants. A pressure regulating device is a device that automatically reduces and controls the pressure of the water flowing through the hose connection outlet to a predetermined value.This device helps to prevent excessive pressure that could damage the hose, nozzle, or fittings, or cause difficulty in handling the hose stream123 References:
NFPA 14: Standard for the Installation of Standpipe and Hose Systems1
Standpipe System Design and Calculations | NFPA | NFPA2
ENGINE COMPANY STANDPIPE OPERATIONS:PRESSURE-REGULATING DEVICES3