Installation of Dry Pipe and Preaction Sprinkler Systems

Introduction

Dry pipe sprinkler systems and preaction systems are essential components of fire protection in buildings where ambient temperatures can drop below 4 °C (40 °F) or where accidental water damage would be catastrophic. Unlike wet systems that maintain water under pressure in all piping, these systems use pressurized air or nitrogen to control an alarm valve. This chapter covers operating principles, design requirements, air volume calculations, bypass devices (accelerators and exhausters), as well as the rules of the National Fire Code of Canada (NFC) and NFPA 13, as adopted in Canada.

Definitions and Fundamental Principles

Dry Pipe System

A dry pipe system is an automatic sprinkler system in which the piping is filled with pressurized air or nitrogen (supervisory pressure) rather than water. Water is held back upstream of a dry pipe valve. When a sprinkler opens, the air pressure drops, the valve opens, and water enters the piping to reach the open sprinkler.

Main components:

Dry pipe valve (with differential clapper)

Air compressor or nitrogen cylinder with regulator

Alarm device (water motor gong or pressure switch)

Accelerator (optional, for large systems)

Exhauster (optional, for rapid air evacuation)

Special orifice sprinklers (dry sprinklers for pendants)

Test and drain valves

Preaction System

A preaction system is an automatic sprinkler system connected to a piping network containing air at atmospheric pressure or under low supervisory pressure, with a preaction valve controlled by a smoke, heat, or flame detector. Water is admitted into the piping only when a detector is activated (single interlock) or when a detector AND a sprinkler are both activated (double interlock).

Types of preaction systems:

19.Single interlock: water enters when the detector activates, before a sprinkler opens.

20.Double interlock: water enters only when both the detector AND the sprinkler are activated. This type is used where accidental water damage is unacceptable (e.g., computer rooms, museums).

Key Difference Between the Two Systems

Characteristic	Dry Pipe	Preaction
Air pressure in piping	Supervisory pressure (typ. 140–350 kPa)	Atmospheric or low pressure (20–35 kPa)
Activation	Opening of a sprinkler	Detector (single interlock) or detector + sprinkler (double interlock)
Response time	Slower (air evacuation required)	Variable depending on type
Risk of accidental water damage	Low (requires an open sprinkler)	Very low (double interlock)
Typical use	Parking garages, unheated warehouses, loading docks	Computer rooms, archives, museums

Dry Pipe Valve

Differential Clapper Principle

The dry pipe valve uses a differential clapper that maintains the balance between water pressure (upstream side) and air pressure (downstream side). The ratio of the clapper surface areas is typically 5:1 to 6:1. This means that for each unit of air pressure, the valve can withstand 5 to 6 units of water pressure.

Formula for calculating minimum air pressure:

P_air_min = (P_water_max) / (Differential ratio)

Where:

P_air_min = minimum required air pressure (kPa)

P_water_max = maximum water pressure at the valve (kPa)

Differential ratio = ratio of surface areas (e.g., 5.5)

Example: If the maximum water pressure is 1,200 kPa and the differential ratio is 5.5:

P_air_min = 1,200 / 5.5 = 218 kPa

The supervisory air pressure must be maintained at least 20 kPa above P_air_min to prevent unintentional valve opening.

Valve Components

Air chamber (downstream side): contains the pressurized air

Water chamber (upstream side): contains the pressurized water from the supply

Differential clapper: separates the two chambers

Intermediate chamber: space between the clapper and the seat, connected to the alarm and drain

Valve test device: test valve of 25 mm (1 in) minimum

Isolation valve: main OS&Y (outside stem and yoke) or indicator valve

Operation During a Fire

43.A sprinkler opens under the effect of heat.

44.The compressed air escapes through the sprinkler orifice.

45.The air pressure drops in the piping.

46.The water pressure on the upstream side exceeds the holding force of the clapper.

47.The clapper opens, and water enters the piping.

48.Water flows toward the open sprinkler.

49.Water enters the intermediate chamber and activates the water motor gong (bell) and/or the pressure switch.

Air Volume Calculation and Response Time

Response Time Requirement

The NFC (Article 3.2.5.14) and NFPA 13 (Section 7.2.3) require that water reach the most remote sprinkler within a maximum of 60 seconds for dry pipe systems. This time is measured from the opening of the sprinkler until water arrives at the sprinkler orifice.

Air Volume Calculation

The air volume in the piping determines the evacuation time. The total volume is calculated from the pipe diameters and lengths:

V_air = Σ (π × d² / 4) × L

Where:

V_air = total air volume (L)

d = inside diameter of the pipe (m)

L = length of the pipe (m)

π ≈ 3.14159

Table of air volumes per metre of pipe (Schedule 40 steel pipe):

Nominal Diameter (in)	Inside Diameter (mm)	Volume (L/m)
1	26.6	0.556
1 ¼	35.1	0.968
1 ½	40.9	1.314
2	52.5	2.165
2 ½	62.7	3.088
3	77.9	4.766
4	102.3	8.219
6	154.1	18.650

Volume Limits for Systems Without an Accelerator

NFPA 13 (Article 7.2.3.6.2) limits the air volume to 3,785 L (1,000 US gal) for dry pipe systems without an accelerator. Beyond this volume, an accelerator or exhauster is required to meet the 60-second time limit.

Rule of Thumb for Calculating Evacuation Time

The air evacuation time can be estimated using the following formula:

t = V_air / Q_air

Where:

t = evacuation time (s)

V_air = air volume (L)

Q_air = air flow through the open sprinkler (L/s)

The air flow through an open sprinkler depends on the air pressure and the discharge coefficient (K) of the sprinkler. For a standard sprinkler K = 5.6 (80.6 in metric units), the air flow can be approximated by:

Q_air ≈ K × √(P_air)

Where P_air is in kPa and K is in L/min/√kPa.

Accelerators and Exhausters

Accelerator

The accelerator is a device that detects a rapid drop in air pressure in the piping (due to a sprinkler opening) and allows air or water to enter the intermediate chamber of the valve to speed up its opening. It reduces the response time of the system.

Operation:

The accelerator is connected to the air piping on the downstream side of the valve.

When a sprinkler opens, the pressure drop is detected.

The accelerator opens and admits air (or water) into the intermediate chamber.

This creates an additional imbalance on the clapper, causing it to open more quickly.

Installation requirements:

The accelerator must be installed near the alarm valve.

An isolation valve must be installed upstream of the accelerator.

A restriction orifice (3.2 mm or 1/8 in orifice) must be installed in the bypass line to prevent false activations.

The accelerator must be tested at least once per year.

Exhauster

The exhauster is a device that evacuates air from the piping in a controlled manner when water is admitted. It is installed at the end of the piping, often at the highest or most remote point.

Operation:

When water enters the piping, the air must be evacuated to allow water to reach the sprinklers.

The exhauster opens automatically when the air pressure drops and water approaches.

It evacuates air at a controlled rate, preventing water hammer and reducing fill time.

Types of exhausters:

Mechanical exhauster (float type)

Automatic priming exhauster

Electrically controlled exhauster

Comparison: Accelerator vs. Exhauster

Characteristic	Accelerator	Exhauster
Primary function	Accelerate valve opening	Evacuate air from the piping
Location	Near the alarm valve	End of the piping
Activation	Air pressure drop	Water arrival
Effect	Reduces initial response time	Reduces final fill time
Maintenance	Annual test required	Periodic inspection

Sprinklers for Dry Pipe Systems

Dry Sprinklers

Dry sprinklers are used for pendants in dry pipe systems. They consist of an outer tube containing a fusible link element and an inner tube (or stem) that transmits the closing force. Water only enters the sprinkler when the fusible element melts.

Characteristics:

Available lengths: 76 mm (3 in) to 1,220 mm (48 in) and longer

The fusible element is located at the end of the tube, in the heated area

The inner tube is filled with air or a heat-transfer fluid

The temperature rating is indicated on the sprinkler nameplate

Installation requirements:

Dry sprinklers must be installed with the appropriate slope (see following section)

The maximum length of a dry sprinkler is limited by the manufacturer (typ. 1,220 mm)

Dry sprinklers must be supported to prevent stress on the fitting

Temperature Ratings and Colours

Temperature Rating (°C)	Classification	Liquid Colour (Bulb)
57	Ordinary	Orange
68	Ordinary	Red
79	Intermediate	Yellow
93	High	Green
141	High	Blue
182	Very High	Purple
227	Very High	Black

Pipe Slopes and Drainage

Slope Requirement

The NFC (Article 3.2.5.15) and NFPA 13 (Article 7.2.6) require that dry pipe system piping be installed with a minimum slope to allow complete drainage:

Minimum slope: 4 mm per metre (1/2 in per 10 ft) for mains and branch lines

The slope must be directed toward the alarm valve or toward the drain valves

Rule of thumb: The slope must allow water to drain completely by gravity to the drain points. Low points must be equipped with drain valves.

Drain Points

Each low point in the piping must be provided with a drain valve of at least 25 mm (1 in) diameter. Drain valves must be accessible and identified.

Consequences of Poor Drainage

Water accumulation in pipes → freezing and obstruction

Accelerated corrosion of the piping

Risk of ice blockage during activation

Non-compliance with the code → inspection failure

Alarm and Supervision Devices

Water Motor Gong (Alarm Bell)

The water motor gong is operated by the flow of water into the intermediate chamber of the valve. Water passes through a 3.2 mm (1/8 in) orifice and drives a turbine that sounds the bell.

Requirements:

The bell must be installed in an audible location

A bell test valve must be installed

The bell drain must be sized to evacuate water without backflow

Pressure Switch

The pressure switch detects the pressure change in the intermediate chamber and sends an electrical signal to the fire alarm control panel. It must be ULC (Underwriters Laboratories of Canada) or cUL certified.

Valve Supervision Switch

A tamper switch (limit switch) must be installed on the main isolation valve to signal any unauthorized closure of the valve.

Air Compressors and Air Supply Systems

Air Compressor

The air compressor maintains the supervisory pressure in the piping. It must be sized to compensate for normal small leaks in the system.

Sizing requirements:

The compressor must be capable of maintaining the required pressure

A pressure-reducing valve (pressure regulator) must be installed to limit the air pressure to the supervisory value

An air filter and dryer are recommended to prevent moisture in the piping

Nitrogen Supply

Nitrogen is increasingly used as a supervisory gas to reduce internal corrosion of the piping. Nitrogen is supplied by high-pressure cylinders with a pressure-reducing valve.

Advantages of nitrogen:

Reduces corrosion (no oxygen)

More effective drying of the piping

Extended service life of the piping

Typical Supervisory Pressure

System Type	Supervisory Pressure
Dry pipe (air)	140–350 kPa (depending on water pressure)
Dry pipe (nitrogen)	140–350 kPa
Preaction (air)	20–35 kPa (atmospheric +)
Preaction (nitrogen)	20–35 kPa

Preaction Systems: Design and Requirements

Preaction Valve

The preaction valve is an electrically or pneumatically actuated valve that controls the admission of water into the piping. It is typically a clapper valve or a motorized butterfly valve.

Components:

Solenoid release

Detectors (smoke, heat, flame)

Fire alarm control panel

Air compressor (for supervision)

Test device

Single Interlock vs. Double Interlock

Single interlock:

Water enters when the detector is activated

The piping fills with water before the sprinkler opens

Faster response time

Risk of water damage if the detector is activated without a fire

Double interlock:

Water enters only when both the detector AND the sprinkler are activated

The piping remains dry until the fire is confirmed

Slower response time (similar to dry pipe)

Maximum protection against accidental water damage

Special Requirements

Preaction systems must be designed to meet the NFC response time requirements

Detectors must be installed in all protected areas

A manual release device must be installed at each exit from the protected area

The system must be monitored by a ULC-certified fire alarm control panel

National Fire Code of Canada (NFC) Rules

Article 3.2.5.14 — Dry Pipe Systems

The NFC requires that:

Dry pipe systems be installed in areas where the temperature can drop below 4 °C

The maximum time for water to reach the most remote sprinkler be 60 seconds

The piping be sloped to allow complete drainage

Drain valves be installed at low points

Article 3.2.5.15 — Slopes and Drainage

Minimum slope of 4 mm/m for mains

Minimum slope of 4 mm/m for branch lines

Low points must be provided with drain valves

Article 3.2.5.16 — Preaction Systems

Preaction systems must be installed in accordance with NFPA 13

Detectors must be approved for the intended use

A manual release device is required

NFPA 13 — Key References

NFPA 13 Section	Subject
7.2.3	Response time (60 s)
7.2.3.6.2	Maximum air volume without accelerator (3,785 L)
7.2.6	Slopes and drainage
7.3	Dry sprinklers
8.2	Preaction systems
8.3	Valves and components

Practical Calculations for the Exam

Calculating Required Air Pressure

Formula: P_air = (P_water / R) + 20 kPa

Where:

P_air = supervisory air pressure (kPa)

P_water = maximum water pressure at the valve (kPa)

R = differential ratio of the valve (e.g., 5.5)

20 kPa = minimum safety margin

Example: P_water = 1,100 kPa, R = 5.5

P_air = (1,100 / 5.5) + 20 = 200 + 20 = 220 kPa

Calculating Total Air Volume

Formula: V_total = Σ (V_unit × L)

Example: 50 m of 2 in pipe (2.165 L/m) + 30 m of 1 ½ in pipe (1.314 L/m)

V_total = (50 × 2.165) + (30 × 1.314) = 108.25 + 39.42 = 147.67 L

Verifying the 60-Second Time Limit

Formula: t = V_total / Q_air

Example: V_total = 147.67 L, Q_air = 40 L/s (sprinkler K = 80.6, P_air = 220 kPa)

Q_air = 80.6 × √220 = 80.6 × 14.83 = 1,195 L/min = 19.9 L/s

t = 147.67 / 19.9 = 7.4 seconds

This system easily meets the 60-second time limit.

Pitfalls to Avoid

216.Forgetting the 20 kPa safety margin when calculating the supervisory air pressure. The complete formula always includes this margin.

217.Confusing units: volumes are often given in litres (L) but flow rates in L/min or L/s. Always convert before calculating.

218.Neglecting the slope: an insufficient slope or one in the wrong direction is a major non-compliance. The slope must always be directed toward the valve or drain points.

219.Ignoring the maximum air volume of 3,785 L: beyond this value, an accelerator is mandatory. Always check the total volume before deciding whether to add an accelerator.

220.Confusing accelerator and exhauster: the accelerator speeds up the opening of the valve; the exhauster evacuates air from the piping. They are not interchangeable.

221.Forgetting dry sprinklers for pendants: in a dry pipe system, pendant sprinklers must be special dry sprinklers, not standard sprinklers.

222.Not considering the ambient temperature: dry pipe systems are required when the temperature can drop below 4 °C. Always check the ambient conditions.

223.Mixing up preaction types: single interlock and double interlock have different requirements and response times. Read the question carefully.

224.Forgetting the bell test valve: every installation must include a test valve for the water motor gong.

225.Neglecting ULC certification: all electrical components and alarm devices must be ULC or cUL certified to be accepted in Canada.

Summary

Dry pipe systems use pressurized air or nitrogen to control a differential clapper alarm valve. Water only enters when a sprinkler opens.

Preaction systems use detectors to control the admission of water. Single interlock admits water upon detection; double interlock requires detection AND the opening of a sprinkler.

The maximum 60-second time limit for water to reach the most remote sprinkler is a fundamental requirement of the NFC and NFPA 13.

The maximum air volume without an accelerator is 3,785 L. Beyond this, an accelerator is required.

The supervisory air pressure is calculated using the formula: P_air = (P_water / R) + 20 kPa.

Minimum slopes of 4 mm/m toward drain points are mandatory to ensure complete drainage.

Dry sprinklers are required for pendants in dry pipe systems.

Accelerators reduce valve opening time; exhausters evacuate air from the piping.

All components must be ULC certified and installed in accordance with the NFC and NFPA 13.

Pitfalls to Avoid (Quick Recap)

Pitfall	Consequence
Forgetting the 20 kPa margin	Insufficient air pressure → unintentional opening
Confusing L/min and L/s	Calculation errors in response time
Insufficient slope	Incomplete drainage → freezing and corrosion
Volume > 3,785 L without accelerator	NFPA 13 non-compliance
Standard sprinklers for pendants	Sprinklers not suitable for dry system
Single vs. double interlock	Different times and requirements

Self-Assessment Questions

239.What is the required supervisory air pressure if the maximum water pressure is 1,300 kPa and the differential ratio is 5.5?

(Answer: (1,300 / 5.5) + 20 = 256 kPa)

241.A dry pipe system has a total air volume of 4,200 L. Is it compliant without an accelerator?

(Answer: No, the maximum volume is 3,785 L. An accelerator is required.)

243.What is the minimum required slope for a main line of a dry pipe system?

(Answer: 4 mm per metre, directed toward the valve or drain points.)

245.What is the fundamental difference between a single interlock and a double interlock preaction system?

(Answer: Single interlock admits water upon detector activation alone; double interlock requires both detector activation AND the opening of a sprinkler.)

247.Which device is used to accelerate the opening of the alarm valve in a large dry pipe system?

(Answer: The accelerator.)

Installation of Dry Pipe and Preaction Sprinkler Systems