Pressure Regulating Devices: Understanding the Nuances
There is a lot of confusion about pressure regulating devices, especially when referring to fire department hose connections on standpipes. The confusion lies somewhere between the standards and how we refer to the specific devices. The 2019 edition of NFPA 14: Standard for the Installation of Standpipes and Hose Connections has clearly defined not only what a pressure regulating device is but went a step further in defining both pressure regulating valves and pressure restricting devices. This is a key to understanding the operational differences between the design applications.
This article explores the importance of static and residual pressure, pressure regulating devices, and the design application of various devices.
Static versus residual pressure
To understand pressure regulating devices, we first need to understand the difference between the static and residual pressure of water. Static water pressure is simply the pressure of water when not flowing (motionless). Residual water pressure, on the other hand, is the pressure of water when flowing or, as used in early code terminology, under working pressure. This basic concept will allow us to better understand what we are regulating, and which devices are required.
What is a pressure regulating device?
Pressure regulating devices (NFPA 14, Section 3.3.16) are designed for the purpose of reducing, regulating, controlling, or restricting water pressure.
When a pressure regulating device is installed as part of a hose valve, it is either a pressure reducing valve (NFPA 14, Section 126.96.36.199) or a pressure restricting device (NFPA 14, section 188.8.131.52). The pressure reducing valve (PRV) is designed to reduce both residual (flowing) and static (non-flowing) water pressure. In contrast, pressure restricting devices (PRDs) are designed to only control residual (flowing) water pressure.
This differentiation is vital to understanding the design application and expectations of the system. The current edition of NFPA 14 requires the following:
- All class I & III standpipe systems to flow a minimum of 500 gpm though the two most remote 2½” hose connections (Section 184.108.40.206.1).
- You must provide a minimum residual design pressure of 100 psi at that most remote hose connection (Section 7.8.1).
- Where pressures exceed 175psi, a listed pressure regulating device shall be provided to limit both static and residual pressures (Section 220.127.116.11.3).
By definition, the only pressure regulating device capable of regulating both static and residual pressure is a pressure reducing valve (PRV).
It must be noted than when hose connections are above the maximum allowed, they may be restricted or reduced to any pressure within the acceptable range within the two design limitation criteria of pre-1993 (100 psi) vs post-1993 (175 psi). This pressure can also be factory-set or field-adjustable at the hose connection. This is a significant consideration, as field-adjustable set pressures may be able to be increased with specific tools and knowledge, while factory-set cannot.
Design application for pressure restricting devices
This leads us to a discussion of the design application for pressure restricting devices. PRDs were typically used in designs based on earlier editions of NFPA 14, prior to the 1996 standard that increased the 65psi minimum/100psi maximum to the current standard of 100 psi minimum/175 psi maximum. The pre-1996 standard required outlet pressures that exceeded 100 psi to be reduced with an approved device.
The device was only required to restrict residual (flowing) water pressure, making PRDs a common choice in the design of the day. This design justification was based on fire department hose/nozzle packages dating back to the 1950s and had not yet been adjusted until the 1996 edition of NFPA 14 was released.
In the 2017 edition of NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-based Fire Protection Systems—Chapter 13, Common Components and Valves—the requirements for Pressure reducing valves (PRV), call for them to be fully flow tested every five years (Section 13.5).
The confusion falls in Section 13.5.2 that governs testing requirements for Hose Connection Pressure Regulating Devices. The section is trying to reference pressure restricting devices but uses the umbrella term of regulating. This continues in 13.5.3, with requirements for hose rack assembly pressure regulating devices—once again, using “regulating” instead of “restricting.” Although the misuse of “regulating” devices unintentionally still covers the required maintenance for all devices designed for the purpose of reducing, regulating, controlling, or restricting water pressure, it should be addressed by the NFPA 25 committee.
The 2018 NFPA 1: Fire Code (Section 18.104.22.168.2) states that hose connections shall be in accordance with NFPA 13, unless class II or Class III standpipes in accordance with NFPA 14 are used. This is an extract from the 2015 NFPA 101: Life Safety Code (Sections 22.214.171.124.2 and 126.96.36.199.2)—specifically extracted from the assembly chapters of that document. This section applies to stages but was extracted to NFPA 1’s standpipe system chapter, leading to confusion about the standard governing requirements for hose connections. This section has been removed in the 2018 edition of NFPA 101.
The 2016 edition of NFPA 13: Standard for the Installation of Sprinkler Systems does address maximum pressure for 1½” hose connections (188.8.131.52.4 (6)). NFPA 13 also allows 2 ½” hose connections for “Fire Department Use” to be added to sprinkler-only designed systems (184.108.40.206) but does not address any pressure provisions for 2 ½” hose connections. The disconnect is “fire department use,” implying that the hose connection meets the requirements for structural firefighting. Section 220.127.116.11.2 indicates that an annex note in reference to this issue exists and that hose connections of 2 ½” are permitted to be attached to wet pipe sprinkler systems only.
These connections can be used for final extinguishment of the fire or to cool residual heat. They are also not to be treated as standpipe hose connections. If standpipes are installed, and the standpipe risers also supply the sprinklers, all provisions of NFPA 14, including water supply and pressure provisions, need to be followed. When either one of the hose connections described in 8.17.5 is provided, the flow rates need only be added to the sprinkler system at the design pressure available at the point of the connection to the sprinkler system pipe.
NFPA 13 clearly allows the use of a 2 ½” hose connection on a sprinkler system-only design, but that hose connection is not required to meet the fire service pressure provision and should not be consider for structural firefighting—only mop-up.
Knowing the differences between pressure regulating devices
It’s vital to understand the design difference between the umbrella term of pressure regulating—which covers all things that reduce, regulate, controll, or restrict water pressure—and the more specifically defined terms of restricting and reducing that delineate residual (flowing) and static (non-flowing) reductions. This is not only key to proper design and installation of hose connections but imperative to fire service operations. The fire service must understand the implications of the design criteria used in the installation of fire protection systems in buildings, and the limitations of the system design.
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