Combining HVAC, fire suppression piping
- Understand the definition and NFPA 13 requirements for a circulating closed-loop system.
- Learn how to pipe the circulating closed-loop system to assure dependable performance of both fire suppression and HVAC systems.
- Grasp the basics of corrosion concerns and prevention.
With building operating and energy costs becoming more scrutinized, building owners are demanding more efficient yet simpler systems be installed in their buildings. This, coupled with the demand for green building systems and U.S. Green Building Council LEED Certification, requires engineers to provide high-performance heating, ventilation and air conditioning solutions, which just a few years ago were not seen in the industry.
Modern high-performance HVAC systems decouple the ventilation function of the HVAC system from the heating and cooling function and then treat each in the most efficient manner possible. In these systems the ventilation function will typically be served by a dedicated outdoor air system. This system would have an energy recovery device appropriate for the climate and would efficiently pre-treat the incoming outdoor ventilation air that is required by ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality to a neutral temperature.
The heating and cooling function then would be served by terminal equipment such as chilled beams where the heating and cooling would be handled on a zone–by–zone basis and reheat would be avoided. Such systems when designed properly can easily achieve a 30 to 40 percent energy savings over an ASHRAE Standard 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings compliant baseline system.
Although these systems are relatively simple in operation, lack of familiarity and first–cost concerns can sometimes cause these systems to be rejected in the early phases of a project, especially when a thorough life cycle cost analysis is not used. Although savings are realized in smaller ductwork, equipment, equipment space, shaft space, ceiling space and other mechanical and electrical infrastructure, chilled beam systems require a building wide network of above–ceiling piping to distribute chilled water to each of the chilled beams in each control zone.
This piping system is above and beyond what a typical HVAC system requires and can be perceived to make this system too first cost intensive, especially if decisions are made on first cost rather than life cycle.
Network of pipes
For most buildings, there is a simple solution to eliminate the majority of this additional piping system, thereby reducing the first cost of these high-performance HVAC systems. Most buildings have a vast network of piping, which typically goes unused for the entire life of the building. A wet pipe sprinkler system is installed above the ceiling and then sits idle and is never used except the event of a fire.
If this network of piping is used for distribution of chilled water to chilled beams as well as for sprinkler system use, overall first cost and ceiling space can be reduced.
An automatic sprinkler system with nonfire protection connections is defined in NFPA 13: Standard for the Installation of Sprinkler Systems as a circulating closed–loop system. Per NFPA 13: “A circulating closed-loop system is primarily a sprinkler system and shall comply with all provisions of this standard such as those for control valves, area limitations of a system, alarms, fire department connections, sprinkler spacing, etc.”
The circulating closed-loop system description and requirements were eliminated in the 2016 edition of NFPA 13. Specifically, it was removed in the first draft of the 2016 edition in first revision 64. The committee statement was: “Sprinkler systems have become more and more specialized with multiple components being used. With the issues of compatibility rampant in our industry, connection to systems that could cause failure of individual components should be avoided. Users of the standard can refer back to previous editions for this design concept.”
The substantiation notes that you can use previous editions of the standard to continue to make use of the circulating closed-loop system. With this language being in the committee statement there are grounds for allowing continued the use of circulating closed-loop systems.
Wet pipe sprinkler systems serving most buildings are still relatively simple. Careful thought and consideration as well as a thorough review of the applicability and of all aspects of a circulating closed-loop system design will keep it that way. When properly applied, this system will provide owners and operators with the desired results – reduced overall first cost, long life and simple operation and maintenance of this portion of the high-performance HVAC system.
Pipes and valves
For circulating closed-loop systems, piping materials and valving are required to comply with the standard. Auxiliary devices such as heat exchangers, pumps and terminal equipment like chilled beams must have a pressure rating to match the required rating of the sprinkler system components – either 175 or 300 psig. Specifications for all auxiliary equipment must indicate this requirement. These components also shall be constructed so that they will “maintain their physical integrity under fire conditions to avoid impairment of the fire protection system.”
This would be interpreted by the authority having jurisdiction, but most HVAC equipment used would likely comply with the exception of say plastic tubing or components, which would want to be avoided. These auxiliary devices also are required to be supported independent of the sprinkler system. The allowable water temperatures of the circulating closed-loop system must be controlled to limit the circulating fluid to between 40 F and 120 F.
A fire suppression system is engineered and the pipe size is calculated to handle the sprinkler system water flow. Allowable velocity in a calculated fire suppression system typically is limited by the amount of pressure available, and can be quite high. In a hydronic system it is necessary to use piping system velocities that limit pump head pressure to a reasonable level; because it is a high-performance HVAC system, energy consumption and the necessary pump horsepower are a concern. The flow requirements of the hydronic system function of a circulating closed-loop system typically will require slightly larger piping than that required for the sprinkler system function of the system and so the worst case (largest) sizing should be noted on the contract documents.
In a fire suppression system, the water flow is in one direction – toward the open sprinkler head. A hydronic system requires both a supply and a return side of the system. By definition, a circulating closed-loop system will require subtle changes in the piping layout. Rather than a single riser or branch main it will require two – one for the supply side of the system and one for the return side.
While operating as a fire suppression system, the flow will be out and toward the open sprinkler heads in both risers. While operating as a hydronic system, the flow will be reversed in one-half of the piping system and will return back to the pumps and heat exchanger. Sprinkler water flow must trip a flow switch indicating water flow to an open sprinkler head.
Careful consideration must be made with respect to flow switch locations to avoid false trips as this would be highly undesirable. Also, a direct path through the piping system from the water supply to each sprinkler head must be provided so that the fire suppression water is not required to flow through pumps, heat exchangers or other auxiliary devices, although it may.
Isolation valves located in a fire suppression system are required to have tamper switches to signal through the fire alarm system that portions of the system are shut down or impaired. Auxiliary equipment such as pumps, heat exchangers or chilled beams may require occasional maintenance and, per the NFPA standard, require isolation valves to allow repair or removal of the equipment without impairing the functioning of the sprinkler system.
As the fire suppression water does not flow through this auxiliary equipment and as closing of these auxiliary equipment isolation valves does not impair flow to sprinkler heads, tamper switches are not required on these isolation valves. At the same time, work on the fire suppression system that would require draining of the fire suppression system — although very rare — would impair operation of the hydronic system.
Many locations require trained union labor to perform work. Sprinkler system installers (sprinkler fitters) as well as hydronic system piping installers (pipe fitters) are highly protective of their work and for good reason – they are each specialists and it’s their livelihood. The drawings must clearly indicate which portions of the system are to be provided and installed by the sprinkler fitters and which portions of the system are to be installed by the pipe fitters.
This line can be blurry when you look at the bigger picture, but if you look at what specifically is isolated and identified as auxiliary equipment, the line becomes much clearer. Defining these individual scopes of work on the drawings is critical.
Corrosion in closed-loop hydronic piping systems typically is mitigated by modern treatment chemicals and is monitored by checking corrosion coupons, chemical levels and adjusting dosages as deemed necessary by the chemical treatment professional. Numerous very old hydronic piping systems exist that have never received chemical treatment and have crystal–clear water still circulating through them.
Wet pipe sprinkler systems, on the other hand, do not receive chemical treatment and can experience corrosion problems. Corrosion in wet pipe fire suppression systems can be from minimal to severe, resulting in everything from leaks and associated damage to property to potentially critical obstruction and reduced flow to the sprinkler heads.
Microbiologically influenced corrosion can occur in any piping system and seems to be problematic in some fire suppression systems – especially dry-pipe systems. MIC is described as a chemical corrosion process that is concentrated and accelerated by the activity of specific bacteria within the fire-sprinkler system resulting in the premature failure the system.
Volumes are written on causes and potential treatments of this affliction. The 2014 Corrosion and Corrosion Mitigation in Fire Protection Systems report written by FM Global goes into quite some detail and provides recommendations for extending the life of fire-sprinkler systems. In a circulating closed-loop system, the majority of the water is flowing throughout the system, to and from terminal equipment – with very little stagnant water.
Recommendations to reduce corrosion include:
- Minimize dead ends in the piping system where water does not flow.
- Use a high–quality combination air and dirt separator to keep the water clean and air–free as you would in any hydronic system.
- Use piping with internal antimicrobial coating as is standard for fire suppression piping.
- Employ a nitrogen purge system and air venting to reduce trapped air and oxygen.
- Use a side–stream ultraviolet filter to kill circulating bacteria.
- Follow other recommendations of NFPA and FM Global for corrosion prevention.
Although the nitrogen generator and the UV filter are additional cost over a typical fire suppression system, the hydronic system piping distribution savings will far outweigh this expense.
This method of hydronic water distribution has been specified on several projects, providing a simple solution to the perceived added cost of a chilled beam system as a result of the additional chilled water piping system required. Figure 1 illustrates a partial plan view of one such project with separate fire supply and fire return branches. Auxiliary valves are provided at chilled beam locations and chilled water piping is then extended to the chilled beams.
Figure 2 is a riser diagram indicating the complete pumping loop along with the heat exchanger and terminal equipment. It also indicates flow directions for both normal fire water flow and for normal chilled water flow. It is clear from this diagram that flow switches will not be tripped until a sprinkler head discharges and that fire sprinkler flow has a clear path from the source to the heads without requiring travel through the auxiliary equipment.