🚿 Wet vs. Dry Pipe Sprinkler Systems: What’s the Difference? (NFPA 13 Explained)

💧 What is a Wet Pipe Sprinkler System?

A Wet Pipe System is the most common and the simplest type of fire sprinkler system.
In this system:

• Pipes are always filled with water 💦
• When a sprinkler head activates, water flows immediately to extinguish the fire.
• Works best in areas where the temperature stays above freezing (above 40°F / 4°C).

📚 As per NFPA 13 (Section 8.1):

“A wet pipe system shall have water present at all times and must have proper pressure gauges, relief valves, and air vents installed.”

🛠️ Components of Wet Pipe Systems:

• Sprinklers (pendent, upright, or sidewall)
• Piping filled with pressurized water
• Alarm check valve
• Pressure gauges above and below valves
• Relief valve to release excess pressure

📈 How It Works:

1. Fire heats the air near a sprinkler.
2. The heat-sensitive element (like a glass bulb) bursts.
3. Water discharges immediately onto the fire! 🚒

🎯 Best Places for Wet Pipe Systems:

• Office Buildings 🏢
• Hotels 🏨
• Shopping Malls 🛍️
• Schools 🏫
• Residential Apartments 🏠

❄️ What is a Dry Pipe Sprinkler System?

A Dry Pipe System is specially designed for cold areas where water-filled pipes might freeze and burst! ❄️💥
In this system:

• Pipes are filled with pressurized air or nitrogen instead of water.
• Water is held back by a dry pipe valve.
• When a sprinkler activates, air escapes first, then water rushes into the pipes.

📚 As per NFPA 13 (Section 8.2):

“Dry pipe systems must have pressure gauges on both the air and water sides, and must deliver water within specific times to the most remote sprinklers.”

🛠️ Components of Dry Pipe Systems:

• Dry pipe valve assembly
• Pressurized air or nitrogen supply
• Quick-opening devices (sometimes used)
• Pressure gauges for both air and water
• Auxiliary drains

📈 How It Works:

1. Fire activates a sprinkler.
2. Pressurized air escapes from the system.
3. Dry pipe valve opens.
4. Water flows into the pipes and discharges from the open sprinkler! 🚿

🎯 Best Places for Dry Pipe Systems:

• Parking Garages 🅿️ (unheated areas)
• Warehouses in cold climates ❄️🏭
• Outdoor Loading Docks 📦
• Unheated Storage Buildings 🏚️

⚡ Key Differences Between Wet and Dry Pipe Systems

🔥 Important Requirements from NFPA 13 (2022 Edition)

✅ For Wet Pipe Systems:

• Relief valve minimum ½ inch size, set at 175 psi or 10 psi above system pressure​.
• Air vent required for metallic pipes to remove trapped air and reduce corrosion risks.

✅ For Dry Pipe Systems:

• Maximum water delivery time:
  • 15 seconds for dwelling units 🏠
  • 60 seconds for light hazard 🚿
  • 50 seconds for ordinary hazard 🛠️
  • 45 seconds for extra hazard ⚡
• System size restrictions without quick-opening device:
  • 500 gallons (1900 liters) maximum​.

💦 For Wet Pipe Systems (Section 8.1, NFPA 13)

✅ Water is Always Inside the Pipes:
The pipes stay filled with water, ready to jump into action the moment a fire breaks out! 🔥🚿

✅ Pressure Gauges Are Mandatory:

• You must install an approved pressure gauge on every wet pipe riser.
• If you have check valves (alarm check valves or riser check valves), you need pressure gauges both above and below these valves.

✅ Relief Valve – The Safety Net:

• A listed relief valve (minimum ½ inch size) is compulsory.
• Set it to operate at either:
  • 175 psi (12 bar) OR
  • 10 psi (0.7 bar) higher than your maximum system pressure — whichever is greater!

🎯 Purpose: To safely release pressure if it builds up too high (because trapped heat, mechanical issues, or water supply fluctuations can cause dangerous pressure spikes! 🛡️)

✅ Air Vent Requirement:

• Every wet pipe system using metallic piping must have at least one air vent installed.
• (Don’t worry, it can be manual or automatic!)
• The reason? To remove trapped air that causes internal corrosion over time — keeping your pipes healthy and happy! 🛠️

✅ No Heat Tracing Alone:

• Heat tracing (like wrapping pipes with electric heating cables) is NOT a substitute for proper heated valve enclosures.
• Pipes and valves must be protected inside heated spaces when exposed to freezing temperatures. ❄️🔥

❄️ For Dry Pipe Systems (Section 8.2, NFPA 13)

✅ Pipes Are Filled with Air/Nitrogen:
Dry pipe systems keep air (or nitrogen) under pressure inside the pipes until fire strikes. Then water rushes in!

✅ Pressure Gauges Everywhere!
You need pressure gauges:

• On the air side of the dry pipe valve
• On the water side of the dry pipe valve
• On air pumps and air receivers (if used)
• On any independent air supply lines

(Yes, lots of gauges — but it’s for your system’s safety! 📈)

✅ System Size Limits:
Depending on how quickly water must reach the fire after sprinkler activation:

• Without quick-opening device: Max system size = 500 gallons (1900 liters)
• With quick-opening device: Max system size = 750 gallons (2850 liters)

✅ Water Delivery Time:
Water must reach the fire FAST:

(Time starts from when the sprinkler opens, until water arrives at that spot.)