HVAC Psychrometric Chart Practice Questions Engineering Analysis: Fire Sprinkler System Design

Engineering Analysis: Fire Sprinkler System Design

Comprehensive technical review based on BS EN 12845 and associated standards

Technical Document

August 2023

This analysis provides a comprehensive review of the fire sprinkler system design, highlighting critical engineering considerations and compliance with relevant standards.

1Hazard Classification & System Type

Hazard Classification

Classification: The building is classified as Ordinary Hazard (OH) due to combustible materials with a medium fire load and medium combustibility.

Standard: BS EN 12845, Clause 6.2.2

Elevation Consideration: With a height difference of 44.45m between the highest and lowest sprinklers, the system is designed to Ordinary Hazard Group III (OH3) requirements.

Standard: BS EN 12845, Clause E.2.1

Installation Type: A wet pipe system was selected as ambient conditions rule out frost damage and temperatures will not exceed 95°C.

Standard: BS EN 12845, Clause 11.1.1

Protection of Concealed Spaces

Ceiling voids with a height exceeding 0.8m are identified as concealed spaces requiring sprinkler protection.

Standard: BS EN 12845, Clause 5.4

Hazard Class: These spaces are classified as Ordinary Hazard 1 (OH1).

2Zoning Strategy & Compliance

Zoning Strategy & Compliance with Annex D

The total protected area (41,598 m²) far exceeds the standard limit for a single control valve set (12,000 m²). The design successfully utilizes zoning per Annex D of the standard.

Key Compliance Points for Zoning:

  • Wet pipe OH installation: Clause D.1
  • Max protected area on any one floor (6,411 m²) < 12,000 m²: Clause D.2(a)
  • No hazard greater than OH3 present: Clause D.2(c)
  • Building is sprinkler protected throughout: Clause D.2(e)
  • Total area (41,598 m²) < 120,000 m²: Clause D.2(f)

Implementation: Each floor is designated as a separate zone, with the largest zone area being 6,411 m².

3Component Specifications & Monitoring

Critical Component Specifications & Monitoring

The design meticulously specifies components and integrates comprehensive monitoring to ensure system integrity and provide immediate alarms.

Main Control Valve Set Includes: Wet alarm valve, test/drain valves, pressure gauges, alarm line strainer, pressure switch, retard chamber, water motor gong, and supervised stop valves (upstream, downstream, bypass).

Per-Zone Valve Set Includes: Supervised zone stop valve, flow switch, pressure gauge, test valve with orifice plate, drain valve, and flushing connections.

Monitoring: All supervisory switches (valve tamper alarms) and flow switches are interfaced with the building's Fire Alarm Control Panel (FACP) as detailed in Tables 1-3 and 1-4 of the source document.

Sprinkler Selection & Characteristics

The specification of sprinklers is a critical design choice impacting performance.

Parameter Specification Standard Reference
Type & Pattern Concealed Pendent (Spray) for ceilings; Upright (Conventional) for no ceilings/voids; Flat Spray above open-cell ceilings. BS EN 12845, Table 37 & Clause 14.2.4
K-Factor K=80 (Nominal), with tolerance between 76 and 84. BS EN 12845, Table 37; BS 5306-2:1990, Table 69
Orifice & Thread Size 15mm BS 5306-2:1990, Table 66 & 69
Temperature Rating 68°C (Red bulb) for all areas, based on ambient +30°C rule. BS EN 12845, Clause 14.4
Thermal Sensitivity Quick Response BS EN 12845, Table 38

4Hydraulic Design Parameters

Hydraulic Design Parameters

The core parameters governing the hydraulic calculation ensure adequate water delivery to suppress a fire.

Parameter Value Standard Reference
Design Density 5.0 mm/min (5.0 l/min/m²) BS EN 12845, Table 3 & 37
Assumed Max Area of Operation (AMAO) 216 m² BS EN 12845, Clause 13.4.3
Min. Discharge Pressure 0.35 bar BS EN 12845, Clause 13.4.4
Max. Flow Velocity 6 m/s (at valves); 10 m/s (in pipes) BS EN 12845, Clause 13.2.3
Calculation Method Hazen-Williams formula (C=120 for steel pipe) BS EN 12845, Clause 13.2.1

5Spacing and Location Criteria

Spacing and Location Criteria

Sprinkler placement is crucial for effective coverage and avoiding obstructions.

Criteria Requirement Standard Reference
Max. Spacing between Sprinklers 4.0 m (between & along range pipes) BS EN 12845, Clause 12.2 & Table 19
Max. Coverage Area per Sprinkler 12.0 m²
Distance from Boundaries ≤ 2.0 m (½ of max spacing) BS EN 12845, Clause 12.4.1
Min. Distance from Columns 0.6 m BS EN 12845, Clause 12.4.9
Clearance below Deflector ≥ 0.5 m BS EN 12845, Clause 12.4.1

6Design Summary

Key Design Parameters
  • Hazard Classification OH3
  • System Type Wet Pipe
  • Design Density 5.0 mm/min
  • Total Protected Area 41,598 m²
  • Number of Zones 10 Floors
Compliance Status
  • BS EN 12845 Zoning Requirements
  • Component Specifications
  • Hydraulic Design Parameters
  • Spacing & Location Criteria
  • Concealed Space Protection