Heat tracing for freeze protection of fire sprinkler systems
BY BRIAN LARKIN
For years, plumbing engineers faced challenges when designing fire sprinkler systems in areas subject to freezing. Wet sprinklers are ideal for indoor areas or warm climates; however, when sprinkler systems were installed in areas subject to freezing, design options were limited to dry, glycol and dry pendant systems.
As the trend of integrating nature into our living spaces becomes more popular, complications arise when the installations include both indoor and freezing locations; such as walkways, enclosed and open patios, glassed entries and balconies. Parking garages are another area of fire sprinkler design that present challenges to engineers. Tight spaces and potential exposed areas to the elements require complex sprinkler systems, thus more and more, plumbing engineers have to design sprinklers systems that are suited both for indoor and freezing locations.
In addition to new construction, building modifications such as add-ons and retrofits, may require extensive reengineering of the sprinkler systems. Accounting for changes in water supply volume and dry system water delivery time can become an engineering nightmare for conventional sprinkler systems.
The issue of sprinkler system design is further complicated with Building Management System (BMS) integration. As the ability to monitor the performance of all aspects of building systems is increasing in popularity, BMS integration is becoming a requirement. A design engineer must ask, “How does the sprinkler system integrate into my BMS?” “Are the pipes frozen?” “How do I know the sprinkler system is working?”
Wet fire suppression systems are significantly simpler to design and test than dry or glycol systems. Buildings with combined wet and dry systems are vastly more complex. A single wet system for the entire installation provides a less complex engineering challenge and more reliable end result. In order to have one single wet system in a dual environment, with both indoor and freezing locations, electric self-regulating heat-tracing is the key.
The 2007 edition of the National Fire Protection Association (NFPA) standard NFPA-13
Standard for the Installation of Sprinkler Systems recognizes heating cables as a solution for freeze protection of sprinkler systems. Using heating cables enables designers to utilize a single wet fire suppression system for the entire installation.
These systems must be specifically listed for sprinkler applications which include sprinkler heads. Also these systems are required to be ”supervised.”
In the United States, the heating cable system is listed according to the requirements of ieee 515.1-2005 (ieee Standard for the Testing, Design, Installation, and Maintenance of Electrical Resistance Heat Tracing for Commercial Applications). This standard includes requirements for heating cables on fire sprinkler branch lines, including sprinkler heads. This standard defines tests to confirm that the heat-tracing system will provide protection in freezing conditions, while not over-heating the sprinklers in the highest expected ambient temperature. In Canada the standard used for certifying these systems is csa 22.2 130-03.
Self-regulating heating cables can be the ideal solution since each point in the cable automatically adjusts its power output in response to its local environmental temperature. Thus, each point of the cable increases or decreases its temperature as needed to keep the pipe within the designed temperature range. Additionally, this temperature-limiting feature makes self-regulating heating cables compatible with both metal and plastic sprinkler piping.
NFPA 13 also requires that the heating cable system is “supervised.” The recommended solution is to use electronic controllers that measure the ambient air temperature surrounding the pipes subject to freezing. When the air temperature drops below 40°F (4°C) the heat-tracing system is powered, providing the heat needed to keep the sprinklers from freezing. Additional sensors maybe applied to the sprinkler piping to monitor its temperature.
To satisfy the NFPA 13 supervision requirements the controllers should include alarm contacts that are connected to the fire control panel. This ensures that if an alarm condition is detected, the controllers will send a signal to the fire control panel. The events that trigger the alarm contacts include:
• Low pipe temperature
• High pipe temperature
• Temperature sensor failure
• Loss of power
• Ground-fault condition
• Ground-fault trip
• Power relay failure
• Communication failure
The controllers may also be connected to the BMS through rs-485 connections via bacnet ms/tp, bacnet-ip, Metasys N2 or LonWorks host protocols.
Beyond typical building sprinkler system freeze protection, self-regulating heat-tracing is also an ideal solution for preventing frozen condensate in dry pendant sprinklers. Dry pendant sprinklers, that are part of an overall wet system, are located in areas where the piping is in a temperature controlled area but the sprinklers extend into areas subject to freezing, such as inside freezers.
The base of the pendant attached to the branch sprinkler piping contains the sprinkler seal with the actuator in the sprinkler head. Due to the temperature differential, condensation may form in the “dry” section of the pendant. This condensate can freeze, causing an ice plug at the sprinkler head. Heat-tracing the pendant sprinkler prevents this ice plug from forming.
The changes to NFPA 13 provide an exciting new opportunity in sprinkler system design with performance enhancements over the previous design options.
Heat-tracing systems with control supervision provide:
• Simpler design without the need for dry actuator control or glycol filled systems for freezing areas;
• Simpler retrofit or building expansion designs without worries about water delivery times;
• Faster water delivery time compared to dry systems. Water is present at each sprinkler head so no time is spent waiting for the air to be purged from the air-filled piping;
• Longer sprinkler pipe life compared to dry systems. Dry systems have moisture trapped in the pipes after testing, leading to increased internal pipe corrosion, shortening the life of the pipe;
• Lower cost piping compared to galvanized pipe recommended for dry systems;
• Better compatibility with metal and plastic sprinkler piping;
• No ground water pollution or disposal issues from testing or operation of glycol systems;
• Prevention of frozen condensate in pendant sprinklers;
• Complete sprinkler pipe monitoring in case of system power loss or malfunction; and
• BMS integration allowing the system to be monitored remotely or as part of the BMS.
Brian Larkin is product marketing manager of Tyco Thermal Controls.