Direct Answer

Open and closed water mist nozzles use different discharge philosophies. Open nozzles discharge when the associated zone control or deluge valve is activated, while closed/automatic nozzles incorporate an approved thermal operating mechanism and discharge in response to local heat conditions. The appropriate arrangement must be selected for the application and confirmed against the approved manufacturer design, hydraulic calculation and applicable fire testing.

1. Engineering Overview

Open and closed water mist nozzles represent two different discharge philosophies within engineered water mist systems. Open nozzles discharge when the associated zone control or deluge valve is activated, while closed / automatic nozzles incorporate an approved thermal operating mechanism and discharge in response to local heat conditions.

2. Nozzle Architecture

Open Nozzle System: Water Source → Pump → Zone / Deluge Valve → Open Nozzle Network → Simultaneous Zone Discharge

Closed Nozzle System: Water Source → Pump / Pressurised Network → Closed / Automatic Nozzles → Individual Thermal Operation

3. Open Water Mist Nozzles

Open nozzles do not contain an individual thermal operating element. When the zone release sequence operates the associated control valve, water is supplied to the complete group of open nozzles within the protected zone.

4. Closed / Automatic Water Mist Nozzles

Closed / automatic water mist nozzles contain an approved operating element that responds to the local thermal condition. When the operating condition is reached, the individual nozzle opens and discharges water mist.

5. Release Philosophy

Open nozzle systems generally depend on an external detection and releasing sequence to initiate water discharge. Closed nozzle systems can respond locally through the individual nozzle operating mechanism, subject to the approved system architecture.

6. Detection Requirement

Open nozzle arrangements normally require a coordinated detection and release system because the nozzles themselves do not individually initiate discharge. Closed nozzle arrangements may use thermal operation at the nozzle, while additional detection and alarm systems may still be provided as part of the overall fire protection strategy.

7. Deluge Arrangement

Open nozzles are commonly associated with deluge or zone control arrangements. Detection initiates the release sequence, the zone valve opens and water is distributed to the designated open nozzles.

8. Closed Nozzle Arrangement

Closed nozzle systems maintain the piping in the approved standby condition until individual nozzles operate. The final system architecture should be based on the selected manufacturer and approved application.

9. Zone Coverage

For open nozzle systems, the complete design zone may discharge simultaneously when the zone valve operates. For closed nozzle systems, discharge may be limited to the individual nozzles that operate in response to local thermal conditions, subject to the approved design.

10. Hydraulic Design

Hydraulic calculations differ according to the selected nozzle philosophy. Open systems should account for the approved number of simultaneously operating nozzles in the design zone. Closed systems should use the manufacturer’s approved operating-nozzle assumptions and hydraulic methodology.

11. Nozzle Selection

Both open and closed nozzles must be selected from the approved manufacturer range. Nozzle type, operating pressure, flow, spray pattern, orifice, spacing and application suitability should correspond with the approved hydraulic calculation and fire testing.

12. Coverage & Spacing

Coverage and spacing should be established from the approved manufacturer’s design criteria and application-specific testing. Generic spacing values should not be applied without confirming compatibility with the selected nozzle technology.

13. Pressure & Flow Considerations

Open and closed nozzle systems require adequate pressure and flow at the operating nozzles. The pump, piping and control valve arrangement should be coordinated with the applicable design condition.

14. Control Valve Arrangement

Open nozzle systems generally require a zone control or deluge valve to control water release to the open nozzles. Closed nozzle systems use a valve arrangement appropriate to the approved system and may require additional supervision and pressure maintenance.

15. Alarm & Detection Integration

Both arrangements may interface with fire alarm, BMS / IBMS, HVAC, equipment shutdown and remote monitoring systems. The exact interface sequence should be defined in the approved Cause and Effect Matrix.

16. Installation Requirements

Installation should verify:

• Correct nozzle type
• Correct operating element, where applicable
• Correct location
• Correct orientation
• Correct spacing
• Correct pipe connection
• Clear discharge path
• Secure mounting
• Correct identification

17. Testing & Commissioning

Testing should verify the selected nozzle philosophy and associated release sequence.

Checks may include:

• Detection testing
• Zone valve operation
• Nozzle arrangement verification
• Pump operation
• Pressure / flow verification
• Alarm indication
• Cause and Effect testing
• Interface testing

18. Engineering Comparison

Key comparison factors include:

• Discharge initiation
• Detection dependency
• Zone discharge philosophy
• Hydraulic design condition
• Control valve arrangement
• Nozzle operating mechanism
• Water release pattern
• Application suitability
• Maintenance requirements

19. Documentation

Documentation should include, as applicable:

• Nozzle datasheets
• Nozzle schedule
• Hydraulic calculation
• Zone control schematic
• Detection layout
• Cause and Effect Matrix
• Installation drawings
• Testing and commissioning records
• As-built drawings
• O&M manual

20. Engineering Objective

The engineering objective is:

Correct Application Assessment → Appropriate Open / Closed Nozzle Selection → Verified Release Philosophy → Correct Hydraulic Design → Coordinated Detection & Control → Optimised Coverage → Complete Testing → Reliable Water Mist Fire Protection

Frequently Asked Questions

What is the main difference between open and closed water mist nozzles?

Open nozzles discharge when their associated zone/deluge valve is activated. Closed/automatic nozzles open individually in response to local thermal conditions through an approved operating mechanism.

Do open water mist nozzles need a detection and release system?

Open nozzle arrangements normally require coordinated detection and release logic because the nozzles do not individually initiate discharge.

Do all closed nozzle systems discharge through every nozzle?

No. Discharge may be limited to the individual nozzles that operate in response to local thermal conditions, subject to the approved design.

How should nozzle spacing and coverage be selected?

Use the selected manufacturer’s approved design criteria and application-specific testing. Do not apply generic spacing without confirming compatibility with the nozzle technology.

What should be verified during testing and commissioning?

As applicable, verify detection, zone valve operation, nozzle arrangement, pump operation, pressure/flow, alarms, Cause and Effect logic and interfaces.

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Suggested internal links	Water Mist Deluge Arrangement; Water Mist Zone Control Arrangement; Water Mist Piping Network Design; Water Mist Pressure and Flow Calculation; Water Mist Nozzle Types and Applications.