Dry pipe sprinkler systems protect areas where water-filled piping could be exposed to freezing. This guide explains the dry pipe valve, air or nitrogen supervision, waterflow alarm, pressure switches and fire alarm interface from a technician's perspective.
What Is a Dry Pipe Fire Sprinkler System?
A dry pipe fire sprinkler system is an automatic sprinkler system in which the system piping normally contains pressurized air or nitrogen instead of water.
Water is held back by a dry pipe valve until operation of one or more automatic sprinklers causes the system pressure to fall sufficiently for the dry pipe valve to operate.
Once the valve operates, water enters the sprinkler piping and travels to the sprinkler or sprinklers that have opened.
Why Are Dry Pipe Sprinkler Systems Used?
Dry pipe systems are commonly used where portions of the sprinkler piping can be exposed to temperatures that make a conventional water-filled wet pipe system unsuitable.
Typical applications can include unheated areas such as:
- Parking structures
- Exterior loading areas
- Unheated warehouses
- Freezers and cold-storage applications
- Canopies and other spaces exposed to freezing conditions
Where piping can remain protected from freezing, a wet pipe system is generally much simpler because water is already present throughout the sprinkler piping.
For the complete wet-system sequence, see: Wet Pipe Fire Sprinkler Systems | How They Work →
How Does a Dry Pipe Sprinkler System Work?
During normal conditions, the sprinkler piping above the dry pipe valve contains pressurized air or nitrogen.
The dry pipe valve separates that pressurized piping from the sprinkler water supply.
When sufficient heat causes an automatic sprinkler to operate, air or nitrogen begins escaping through the open sprinkler.
As system pressure falls to the operating point of the dry pipe valve, the valve trips and allows water to enter the sprinkler piping.
The water then travels through the piping and discharges through the sprinkler or sprinklers that have operated.
Heat → Sprinkler Operates → Air/Nitrogen Escapes → System Pressure Falls → Dry Pipe Valve Trips → Water Enters Piping → Water Discharges
Does a Smoke Detector Trip a Dry Pipe System?
Not in a conventional dry pipe sprinkler system.
The basic dry pipe sequence begins when an automatic sprinkler operates from heat and releases the air or nitrogen contained in the sprinkler piping.
This is an important distinction between a dry pipe system and a preaction system.
A preaction system incorporates a supplemental detection/releasing arrangement as part of its operating sequence. A conventional dry pipe system does not require a smoke detector or heat detector to electrically release the dry pipe valve.
What Is a Dry Pipe Valve?
The dry pipe valve separates the pressurized air- or nitrogen-filled sprinkler piping from the water supply.
During normal operation, the valve remains closed while the system piping is maintained at the pressure specified for that particular valve and system.
When an automatic sprinkler opens and sufficient pneumatic pressure is released, the pressure relationship across the valve changes until the dry pipe valve operates.
Water can then pass through the valve and into the sprinkler piping.
Dry pipe valve designs vary considerably by manufacturer. Some use differential principles, latches, diaphragms or other listed operating arrangements.
Why Can Lower Air Pressure Hold Back Higher Water Pressure?
A common question is how a dry pipe valve can remain closed when the water-supply pressure below the valve is considerably greater than the air pressure in the sprinkler piping.
Traditional differential dry pipe valves use differences in effective surface area and mechanical design so that a lower pneumatic pressure can maintain the valve in its normal closed position against a higher water-supply pressure.
Modern valve designs vary, so the important field concept is not to memorize one universal pressure ratio. The listed valve determines the required normal system pressure and operating characteristics.
Where Does the Air or Nitrogen Come From?
Dry pipe systems require a source capable of maintaining the required pneumatic pressure.
Depending on the system design, this can involve equipment such as:
- Air compressor
- Plant air supply
- Nitrogen supply or nitrogen-generation equipment
- Air maintenance device
- Associated regulators, valves and trim
The air-maintenance arrangement maintains the required system pressure according to the sprinkler-system design.
Does the Fire Alarm Panel Control the Air Compressor?
Normally, no.
This is an important distinction for fire alarm technicians.
A low- or high-air supervisory signal tells the fire alarm system that sprinkler-system pressure has moved outside its intended range.
The fire alarm control unit does not normally respond by simply turning the sprinkler air compressor on or off to regulate system pressure.
Pressure maintenance is handled by the sprinkler system's air-supply and air-maintenance equipment according to its listed design.
What Is a Low-Air Supervisory Switch?
A supervisory pressure switch monitors the pneumatic pressure in the dry sprinkler piping.
If the system pressure falls outside the acceptable range, the switch can initiate a supervisory signal through the fire alarm system.
Low air can indicate conditions such as:
- Leakage from sprinkler piping
- Air-supply failure
- Air-maintenance equipment problems
- Improper valve or system condition
- Pressure approaching the operating range of the dry pipe valve
A low-air supervisory signal provides warning of an abnormal condition before it develops into a more serious sprinkler-system impairment or unintended valve operation.
What Is a High-Air Supervisory Switch?
High pneumatic pressure can also create problems in a dry pipe system.
Excessive system pressure can delay operation of the dry pipe valve because more air or nitrogen must be released before the valve reaches its operating condition.
Where required by the system design and applicable standards, high pressure is also supervised so abnormal pneumatic pressure can be identified.
How Much Air Pressure Should a Dry Pipe System Have?
There is no single pressure setting that applies to every dry pipe sprinkler system.
Different dry pipe valves operate at different normal pneumatic pressures.
Some traditional systems operate around substantially higher air pressures, while newer low-differential valve designs can operate at much lower pressures.
The correct normal pressure and supervisory set points must be established using the requirements for the specific dry pipe valve, air-maintenance arrangement and system design.
Low Air vs High Air vs Waterflow
These signals are easy to confuse because they can originate from pressure-operated devices mounted close together around the same dry pipe valve.
| Condition | What It Means | Typical Fire Alarm Signal |
|---|---|---|
| Low Air / Nitrogen | System pneumatic pressure has fallen outside its normal range | Supervisory |
| High Air / Nitrogen | System pneumatic pressure has risen outside its normal range | Supervisory |
| Sprinkler Waterflow | Dry pipe valve has operated and the sprinkler system has entered an alarm condition | Alarm |
How Is Waterflow Detected on a Dry Pipe System?
This is another major difference between typical wet and dry systems.
A wet pipe system commonly uses a vane-type waterflow switch mounted directly on water-filled piping.
A dry pipe valve arrangement commonly uses a pressure-type waterflow alarm switch associated with the valve's alarm trim.
When the dry pipe valve operates, water pressure becomes available to the alarm line. A pressure-operated waterflow device can detect that pressure and initiate the sprinkler waterflow alarm.
Dry Valve Trips → Alarm Line Pressurizes → Waterflow Pressure Switch Operates → FACU Receives Alarm
For a deeper explanation of vane-type devices used on wet systems, see: Fire Sprinkler Waterflow Switch | How It Works & Fire Alarm Monitoring →
Alarm Pressure Switch vs Supervisory Pressure Switch
Although both devices can respond to pressure, they do not necessarily monitor the same pressure or perform the same fire alarm function.
A supervisory pressure switch monitors the air or nitrogen pressure maintained in the sprinkler piping.
A pressure-type waterflow alarm switch can monitor pressure associated with operation of the dry pipe valve and its alarm trim.
What Does the Fire Alarm System Monitor?
Depending on the system and applicable requirements, the fire alarm system can monitor several conditions associated with a dry pipe sprinkler system.
Common examples include:
- Sprinkler waterflow alarm
- Low pneumatic pressure supervisory
- High pneumatic pressure supervisory where applicable
- Sprinkler control-valve supervisory conditions
- Other required sprinkler-system supervisory conditions
Each point should be programmed and identified according to the physical condition it actually represents.
How Does a Dry Pipe System Differ From a Wet Pipe System?
| Feature | Wet Pipe | Dry Pipe |
|---|---|---|
| Normal sprinkler piping | Filled with water | Contains pressurized air or nitrogen |
| Typical environment | Protected from freezing | Areas where water-filled piping could freeze |
| Automatic sprinklers | Yes | Yes |
| Additional valve operation before water enters system piping | No dry pipe valve | Dry pipe valve must operate |
| Pneumatic pressure supervision | Not inherent to a conventional wet system | Important part of system supervision |
Dry Pipe vs Preaction Sprinkler Systems
Dry pipe and preaction systems both can have sprinkler piping that is not normally filled with water, but their operating sequences are not the same.
A conventional dry pipe system operates when an automatic sprinkler opens and the resulting pneumatic pressure loss causes the dry pipe valve to trip.
A preaction system incorporates an additional detection and releasing arrangement. Depending on the type of preaction system, one or more events must occur before water is admitted to the sprinkler piping.
This distinction is particularly important for fire alarm technicians because the detection and releasing equipment associated with a preaction system can become a much larger part of the fire alarm/releasing interface.
Dry Pipe vs Deluge Sprinkler Systems
A dry pipe system normally uses closed automatic sprinklers.
A deluge system uses open sprinklers or nozzles. When the deluge valve opens, water can discharge through all open outlets connected to that system.
That difference completely changes the operating sequence.
See: Deluge Fire Sprinkler Systems | How They Work →
Why Can Dry Pipe Systems Take Longer to Deliver Water?
A wet pipe system already contains water throughout its sprinkler piping.
A dry pipe system has an additional sequence that must occur:
Sprinkler Opens → Air/Nitrogen Escapes → Valve Trips → Piping Fills With Water → Water Reaches Open Sprinkler
The size and arrangement of the system, pneumatic pressure, dry pipe valve characteristics and other design factors influence water-delivery performance.
This is another reason excessive pneumatic pressure can be important: more pressure can mean additional time must pass before the dry pipe valve reaches its operating condition.
What Is an Accelerator or Quick-Opening Device?
Some dry pipe systems use a listed quick-opening device, such as an accelerator, to help reduce the time required for the dry pipe valve to operate.
These devices respond to pneumatic pressure changes and assist valve operation according to the particular listed system design.
Not every dry pipe system uses one, and their operation varies by manufacturer.
Why Must the Dry Pipe Valve Be Protected From Freezing?
The irony of a dry pipe system is that although much of the distribution piping is normally free of water, the dry pipe valve and portions of its trim are associated with the water supply.
The valve equipment therefore must be installed and protected according to the applicable sprinkler requirements and manufacturer's instructions.
A dry system does not mean that every component can simply be installed in an unheated environment without considering freezing.
Common Dry Pipe Fire Alarm Troubleshooting Problems
From the fire alarm side, common problems can include:
- Low-air supervisory that will not restore
- High-air supervisory condition
- Pressure switch adjusted incorrectly
- Alarm and supervisory pressure switches confused during wiring
- Incorrect monitor-module programming
- Waterflow programmed as supervisory
- Low-air pressure programmed as alarm
- Incorrect point descriptions
- Damaged or loose field wiring
- Sprinkler-system pressure problem incorrectly blamed on the FACU
Testing Dry Pipe Sprinkler Fire Alarm Signals
Testing should verify the actual function being monitored rather than merely shorting wires or manually operating a switch.
From the fire alarm technician's perspective, the goal is to verify that the appropriate sprinkler-system condition produces the correct signal at the FACU.
That means confirming distinctions such as:
- Low air → supervisory
- High air → supervisory where applicable
- Waterflow → alarm
- Control valve moved from normal position → supervisory
Full sprinkler-system trip testing, valve operation, drainage and restoration should be coordinated with qualified sprinkler personnel and performed in accordance with the applicable inspection, testing and maintenance requirements.
Common Dry Pipe Sprinkler System Mistakes
- Calling every air-filled sprinkler arrangement a dry pipe system.
- Describing preaction and deluge as simply types of dry pipe systems.
- Assuming the FACU maintains sprinkler air pressure.
- Using one universal normal-air-pressure setting for every dry valve.
- Using a universal "10 psi below normal" rule for low-air supervision.
- Assuming alarm and supervisory pressure switches monitor the same condition.
- Programming low air as a fire alarm instead of the required supervisory condition.
- Programming actual sprinkler waterflow as supervisory.
- Assuming a dry pipe system has no water anywhere in the system.
- Ignoring the need to protect the dry pipe valve and associated equipment from freezing.
Frequently Asked Questions
Why is it called a dry pipe sprinkler system?
Because the sprinkler distribution piping is normally maintained with pressurized air or nitrogen rather than being continuously filled with water.
What causes a dry pipe valve to trip?
Operation of an automatic sprinkler releases pneumatic pressure from the system. When system conditions reach the operating point of the dry pipe valve, the valve opens and allows water into the sprinkler piping.
Does a smoke detector activate a dry pipe system?
Not in a conventional dry pipe sprinkler system. The basic operating sequence begins with heat operating an automatic sprinkler and the resulting loss of pneumatic pressure. Detection/releasing-system operation is characteristic of preaction and deluge arrangements rather than a conventional dry pipe system.
Is low air an alarm or supervisory signal?
Low pneumatic pressure on a dry pipe sprinkler system is a supervisory condition when monitored by the fire alarm system.
Is high air an alarm?
No. Where high pneumatic pressure is monitored, it is a supervisory condition rather than a sprinkler waterflow alarm.
Is sprinkler waterflow an alarm?
Yes. Operation of the sprinkler system resulting in the required waterflow condition is an alarm function.
Does the fire alarm panel turn the sprinkler air compressor on?
Not as the normal method of maintaining dry-system pressure. The sprinkler system's air-supply and air-maintenance equipment controls the pneumatic supply according to its design. The FACU monitors and annunciates the required supervisory conditions.
What pressure should a dry pipe system operate at?
There is no universal pressure. The correct pneumatic pressure depends on the specific dry pipe valve, system design and manufacturer's requirements.
Can a vane-type waterflow switch be used the same way as on a wet system?
Dry pipe valve arrangements commonly use pressure-operated alarm devices associated with the valve's alarm trim rather than relying on the same vane-type arrangement commonly encountered on continuously water-filled wet piping.
Related Fire Sprinkler & Fire Alarm Guides
- Wet Pipe Fire Sprinkler Systems | How They Work
- Fire Sprinkler Waterflow Switch | How It Works & Fire Alarm Monitoring
- Deluge Fire Sprinkler Systems | How They Work
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Bottom Line
A dry pipe fire sprinkler system keeps water out of sprinkler distribution piping that could be exposed to freezing by maintaining pressurized air or nitrogen downstream of a dry pipe valve.
When heat operates an automatic sprinkler, pneumatic pressure begins to fall. The dry pipe valve eventually operates, water enters the sprinkler piping and the system produces the required sprinkler alarm condition.
For fire alarm technicians, the most important distinction is understanding the three different jobs being performed: pneumatic pressure supervision tells you whether the sprinkler system is in a normal condition, the dry pipe valve controls admission of water, and the waterflow alarm tells the fire alarm system that the sprinkler system has operated.
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