“What’s the advantage of a wet-pipe fire sprinkler system over a dry-pipe fire sprinkler system?”
If you’ve been in the industry a long time you might scoff at the question, but I’ve been asked a couple times from different non-fire protection clients.
Grab a pen real quick. Identify all the reasons why we don’t do dry systems everywhere. Seriously – see how many you come up with.
If you only said cost – you hit the big one. Dry systems are more expensive than wet. But there’s more to it than that. A lot more.
Here’s my reasoning why dry-pipe systems are more challenging than wet systems. Compare it to your list and post your thoughts below in the comment section here.
The biggest driver (as is with much in construction) for wet over dry is the cost.
Cost is impacted by
- the inclusion of a dry valve,
- air compressor (or nitrogen generator)
- potentially different pipe types
- additional labor to design and install sloped pipe
- inclusion of a remote inspector’s test
- potentially additional low-point auxiliary drains with drum drips, and
- use of dry-pendent style sprinklers in unheated areas
2. System Configuration
With wet systems, we’re able to design tree, looped, or gridded sprinkler systems. Dry systems are limited to tree or looped systems (NFPA 13 2002 126.96.36.199, 2007-16 188.8.131.52, 2019 184.108.40.206).
Gridded systems specifically can be great for bringing down branch pipe sizes by distributing the flow across mains and gridded branches. With more pathways to flow, there’s less overall friction loss from supply to sprinkler.
Looped systems can benefit from a similar premise, but looped systems don’t benefit from flow down gridded branch lines. Looped systems with long branchlines can still have larger branch pipe diameters.
Dry systems must slope to a drainable location (NFPA 13 2002 220.127.116.11.1, 2007-16 18.104.22.168.1, 2019 22.214.171.124). All dry system pipe must be sloped. For large or complex areas, these slopes can add up over time and result in big differences in pipe elevation.
I worked on a pre-engineered metal building once which was several hundred feet long. We originally planned for a dry system due to a large exposed material storage overhang at the end of the building.
The three pipe slopes that appear in NFPA 13. Non-refrigerated mains require 1/4-inch per 10 feet slope, while branches and any refrigerated locations require 1/2-inch per 10 feet slope (NFPA 13 2002 126.96.36.199.1, 2007-16 188.8.131.52.1, 2019 184.108.40.206)
The slope on the main from one end of the building to the other resulted in a difference of about 8-inches in height. Even splitting the difference and sloping to a high-point in the middle of the building was too much height difference for the building. We were trying to stay tight to structure and above wide overhead doors. The pre-engineered building had such little elevation tolerance (it was intended to house commercial trucks) that the slope on the dry mains were causing issues.
Long story short – the slope of the pipe caused enough issues that the design of the building was shortened by six feet to accommodate dry sidewall sprinkler throws and not need a dry-pipe system. Keeping the entire system wet allowed level main runs and reduced overall cost to the project. It may be the only project I ever work on where the building size was adjusted to accommodate sprinklers, but it resulted in a much more cost-effective solution.
See more about pipe slope in a prior article here.
Dry systems suffer accelerated corrosion compared to wet-pipe systems. Those who inspect or replace dry systems know that their expected lifetime can be as short as a few years to as long as about a decade.
Why do dry systems corrode faster than wet? They have more oxygen molecules introduced to the interior pipe network than wet systems do. A combination of water vapor (from originally filling the system, trapping water, or introducing moisture through air compressors) and oxygen will corrode the system.
Wet systems suffer the same, but in much smaller quantities. In wet systems oxygen is only introduced from trapped water when the system is drained and refilled, or within the fresh water to the system.
5. Pipe Types
Some specifiers differ in pipe specifications between wet and dry systems. Many do not, but some do. While galvanized pipe is no longer a standard for dry systems in the industry (and for good reason), dry systems may necessitate schedule 40 pipe to slow the progression of corrosion in the system.
Pipe wall thickness not only affects cost and time to install, but it affects hydraulics too.
Speaking of hydraulics, dry systems require a 30% increase in the remote area (NFPA 13 2002-16 220.127.116.11.1, 2019 10.2.4.2.1). The system essentially must accommodate a larger fire because a fire has the ability to be larger in size before the sprinkler system can introduce water. This 30% increase in the remote area results in significantly more water and often larger main size than a similarly designed wet system.
Additionally, NFPA 13 requires that dry-pipe systems use a Hazen Williams C-Factor of 100 in lieu of 120. While this may change in future editions of NFPA 13 when paired with nitrogen inertion (as UFC criteria has), it’s still currently only 100 (NFPA 13 2013 Table 18.104.22.168.1, 2016 22.214.171.124.1, 2019 126.96.36.199.1) for black steel. This higher friction loss can also result in larger pipe sizes.
7. Dry Pendents
Not all sprinkler types are allowed to be used in dry systems. If a pendent sprinkler is located in an area where the return bend is not kept above 40-degrees, then it must be a dry pendent (NFPA 13 2002-16 7.2.2, 2019 8.2.2).
Dry pendent sprinklers are significantly more expensive than a traditional pendent sprinkler, and introduce other manufacturer requirements (minimum shaft length, insertion into tees and not elbows).
8. Remote Inspector’s Tests & Drum Drips
Wet systems can locate inspector’s tests (included to show water flow and test the waterflow switch) just past the flow switch as a riser.
Dry systems, however, require that an inspector’s test be located at the most remote point of the system (NFPA 13 2002 188.8.131.52, 2007-13 184.108.40.206, 2016 220.127.116.11, 2019 16.14.2). This accessible valve at the most remote portion requires more pipe & coordination than a test at the riser often does.
Remote Inspector's Test (and drain shown here) come with an assortment of requirements. See a full detailing and breakout of the Inspector's Test here.
We use dry systems when we need to accommodate temperatures less than 40-degrees (F). Much of the time there isn’t a choice between a wet and dry system.
Some applications, though, could go either way. Early in design is often a great time to discuss heating options for spaces throughout a building. While the difference between 30 and 50 degree setpoints may not have major ramifications mechanically, it can have a major impact on the design of the suppression system.
What impacts have affected your projects the most? Comment below here.
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Joseph Meyer, PE, owns/operates his own Fire Protection Engineering practice in St. Louis, Missouri. See bio on About page.