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Title39930201 SPD E04 Zoning of Hazardous Areas
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Table of Contents
                            CONFIDENTIALITY
CONTENTS LIST
	List of Tables
	List of Figures
NOTES
REGISTERED HOLDERS
REVIEW AND AMENDMENT PROCEDURE
INTRODUCTION
	ENQUIRIES AND AUTHORS
	GENERAL
	SPECIFIC MEANINGS
	GLOSSARY AND ABBREVIATIONS
		Definition of Area Classification Terms
		Abbreviations (used within this SPD)
SCOPE
REFERENCED MATERIAL
	LEGISLATION
	STANDARDS
	OTHER DOCUMENTS
DSEAR COMPLIANCE
	DESIGN OF PLANT AND EQUIPMENT
	STORAGE OF DANGEROUS SUBSTANCES
	CONTROL AND MITIGATION MEASURES
	MAINTENANCE AND REPAIR PROCEDURES
	MARKING OF HAZARDOUS AREAS
PRINCIPLES OF AREA CLASSIFICATION
	INTRODUCTION
	KEY TERMS FOR AREA CLASSIFICATION
	HAZARDOUS AREA ZONING
	EXTENT OF ZONE
	NON- HAZARDOUS AREA
	GRADE OF RELEASE
	RELATIONSHIP BETWEEN GRADE OF RELEASE AND CLASS OF ZONE
PROCEDURE FOR AREA CLASSIFICATION
	ASSESSING THE RISK
		Risk Assessment Process
		Flammable Liquids in Drainage Systems (FLIDS)
	DETERMINING HAZARDOUS AREAS
		Personnel
		Timing
		Information Required
		Equipment Details
		Recording Area Classifications
	CLASSIFICATION OF AREAS WHERE FLAMMABLE GASES OR VAPOURS MAY
	CLASSIFICATION OF AREAS WHERE COMBUSTIBLE DUSTS MAY BE PRESE
		Classification Procedure for Explosive Dust Clouds
		Dust Layer Hazard
MODEL SOLUTIONS FOR SEWERAGE AND SEWAGE TREATMENT FACILITIES
	INTRODUCTION
	VENTILATION AND ODOUR CONTROL SYSTEMS
	SEWERS
		Domestic Foul Sewer
		Combined Sewer
	SEWAGE PUMPING STATIONS
		Domestic Effluent Wet Well
		Combined Effluent Wet Well
		Combined Effluent Dry Well
		Equipment in Wet Wells
		Example Zoning for Sewage Pumping Stations
	STORAGE CHAMBERS AND CSO’S
	PRELIMINARY AND PRIMARY TREATMENT (INCLUDING STORM WATER STO
		General
		Classification of Inlet Channels and Chambers Open to Atmosp
		Classification of Enclosed or Sheltered Inlet Channels and C
		Enclosed, Open and Ventilated Channels in Buildings
		Primary Settlement (including Storm Water Storage)
	INTERSTAGE PUMPING STATIONS
	SLUDGE STORAGE, THICKENING/DEWATERING
	DIGESTERS
	DIGESTED SLUDGE VESSELS, SUMPS AND LAGOONS
	PIPEWORK CONTAINING DIGESTED SLUDGE
	DIGESTED SLUDGE CENTRIFUGES
	DIGESTED SLUDGE PRESSES AND PRESS HOUSES
		Press Liquors
	GAS MAINS
		Definitions
		Gas Pipework and Fittings
		Condensate Drains
		Low Pressure Gas Mains Installed in the Open Air
		Medium Pressure Gas Mains Installed in the Open Air
		Medium and Low Pressure Gas Mains Installed in Covered Ducts
		Gas Mains Installed in Plant Rooms
		Gas Receivers
	GASHOLDERS
	GAS COMPRESSORS AND BLOWERS
		Gas Compressors and Blowers Installed in the Open Air
		Enclosed Gas Compressors and Blowers
	FLARES / WASTE GAS BURNERS
	PLANT ROOMS
		Plant Rooms for Engines, Boilers or Other Fired Equipment
		Plant Rooms for Pumps
		Other Enclosed Spaces
	LPG FACILITIES
	DISTILLATE FUEL
	ROADWAYS AND VEHICLES FOR USE IN ZONED AREAS
	ODOUR CONTROL PLANT
CLASSIFICATION OF FACILITIES (WATER TREATMENT)
	INTRODUCTION
	ELECTROCHLORINATION PLANT
	AMMONIATION FACILITIES
	UNDERGROUND STRUCTURES AND TUNNELS
	PUMPING STATIONS
	PLANT ROOMS FOR ENGINES, BOILERS AND OTHER FIRED EQUIPMENT
	LPG FACILITIES
	OTHER PLANT
CLASSIFICATION OF FACILITIES (WORKSHOPS AND LABORATORIES)
	INTRODUCTION
	WORKSHOPS
	LABORATORIES
		Fire Control Measures
		Fume Cupboards
		Releases into enclosed spaces
SELECTION OF EQUIPMENT FOR USE IN POTENTIALLY EXPLOSIVE ATMO
	SOURCES OF IGNITION
		Unprotected Flames
		Mechanically Generated Sparks
		Static Electricity
		Lightning
		Electrical Equipment
		Hot Surfaces (non-electrical equipment)
		Other Ignition Sources
	GAS DETECTORS
		Introduction
		Use of Gas Detectors
		Use of Gas Detectors in a Safety Instrumented System
		Methods of Flammable Gas Detection
		Response characteristics
		Location of Sensors
		References
	SELECTION OF MECHANICAL AND ELECTRICAL EQUIPMENT
	EQUIPMENT CATEGORY
	SELECTION OF TYPES OF PROTECTION ACCORDING TO HAZARD ZONE
	SELECTION ACCORDING TO TEMPERATURE CLASS
	SELECTION ACCORDING TO APPARATUS SUBGROUP
	SELECTION ACCORDING TO ENVIRONMENTAL CONDITIONS
	COMPLETION OF RECORDS
	MARKING OF APPARATUS
	INSTALLATION
	ELECTRIC LIGHTING
	LIGHTNING PROTECTION
INSPECTION, TESTING AND COMMISSIONING
	COMPETENCY AND TRAINING
	BASIS AND FREQUENCY OF INSPECTION
	FORMS OF INSPECTION
	INSPECTION FINDINGS AND FAULT CATEGORIES
	ISOLATION
	MAINTENANCE OF INTRINSICALLY SAFE SYSTEMS
	ALTERATIONS AND REPAIRS TO APPARATUS AND SYSTEMS
	INSULATION TESTING
	FASTENINGS AND TOOLS
	PORTABLE APPARATUS
	PERMITS TO WORK AND GAS FREE CERTIFICATES
	EQUIPMENT REPAIR
REFERENCES
	THAMES WATER PUBLICATIONS
	HEALTH AND SAFETY EXECUTIVE (HSE) PUBLICATIONS
	BRITISH STANDARDS
	MISCELLANEOUS PUBLICATIONS
                        
Document Text Contents
Page 1

THAMES WATER UTILITIES LIMITED
Capital Delivery
Rose Kiln Court,
Rose Kiln Lane
Reading
Berkshire RG2 0HP


DOCUMENT REF:

ISSUE DATE:

TWUL/E04
ISSUE 4.0
MAY 2009




STANDARD PRACTICE DOCUMENT E04

ZONING OF HAZARDOUS AREAS

ISSUE 4.0

































Date
Issued

May 2009

Manual No. Uncontrolled Copy





Page 1 of 167

Page 2

DOCUMENT REF: TWUL/E04
SECTION 0.0 ISSUE 4.0

0.1 CONFIDENTIALITY

Title to and copyright in this document vest in Thames Water Utilities Limited (TWUL).

This document is CONFIDENTIAL to the recipient and members of this organisation
with a need to see it. All issued paper copies must be returned when the recipient has
no further use for it, and if he/she is an employee, all copies (electronic or otherwise)
shall be surrendered on leaving the Company.

Reproduction of the whole or part thereof shall not be made without the express
permission of the Head of Capital Delivery.



Page 2 of 167

Page 83

DOCUMENT REF: TWUL/E04
SECTION 7.0 ISSUE 4.0

Detailed advice on piping for use with LPG can be found in UKLPG Code of Practice
No. 22 - LPG Piping, System Design and Installation (42).

Since small bore pipework is more prone to accidental damage and possible release to
atmosphere, pipes smaller than 12 mm should be avoided. When this is not possible,
e.g. with instrument systems, then additional upstream isolating valves must be
provided.

Condensate removal facilities provided on sludge gas pipework should be by means of
automatic drains, siphon chamber pumps or self-closing manually operated valves, as
appropriate. All low points in the pipe runs should be given special attention with regard
to condensate removal.

If the drain is not kept capped off it is possible under abnormal conditions for gas to leak
past the seal of the closed valve, and in the case of automatic drains for the water seal
to be lost.

Figure 7.11. Zoning for Condensate Drains on Gas Pipework in Chambers



It is considered that gas leakage from a condensate drain into a chamber is expected if
the drain operates frequently and leads to a primary grade of release, therefore a Zone
1 area is assumed within the chamber. A Zone 2 will then extend above the top of the
chamber for a distance of ‘x’ metres due to gas leakage through the condensate valve
(a 1% leak area has been used for a typical condensate valve size). The distance ‘x’
can be extrapolated from the table below for different gas pressures.

Table 7.8 Zone 1 Distances for Gas Leakage from Condensate Drains

Low Pressure Gas (100 mbar) ‘x’ = 0.4 metres

Medium Pressure (1 bar) ‘x’ = 0.8 metres

Medium Pressure (2 bar) ‘x’ = 0.9 metres

Note: Dispersion formulae used to determine zone extents are detailed in Appendix A3.3.

Page 83 of 167

Page 84

DOCUMENT REF: TWUL/E04
SECTION 7.0 ISSUE 4.0

Figure 7.12. Zoning for Condensate Drains on Gas Pipework in Freely
Ventilated Areas Above Ground.



Gas leakage from a condensate drain in the open air is expected if the drain is operated
frequently and leads to a primary grade of release, therefore a Zone 1 will exist,
extending around the drain valve a radius of ‘x’ metres. The distance ‘x’ can be
extrapolated from Table 7.6 above.

Wherever practicable drains should not be installed in plant rooms, covered ducts, etc.
If drains are installed in a plant room, the drain piping must be led to an easily visible
point outside the building. The end of the pipe should be zoned as described above.
None of the resulting zones should impinge on any opening back into the plant room.

7.14.3

7.14.4

Condensate Drains
Condensate drains should be led outside covered ducts as described above for plant
rooms.

Where gas mains and associated equipment are buried no zones exist externally.
However where fittings, etc., are located in chambers or above ground then the
classifications appropriate to the gas pressures and degree of ventilation in those
locations should be used.

Site drains must be water sealed, e.g. using a ‘U’ bend, to ensure that any gas releases
are not transmitted to other parts of the site via them acting as open ducts, thereby
requiring the classification of facilities some distance from the potential source of
release.

Low Pressure Gas Mains Installed in the Open Air
These are gas mains operating up to 500 mbarg on any of the gases mentioned
identified in Section 7.14.1.

Internally, the gas main should be classified as Zone 1. Devices inserted into the gas
flow must be rated for Zone 1 if integral electrical parts are in wetted contact with the
gas. If integral electrical parts could only abnormally come into wetted contact with the
gas (e.g. failure of: diaphragm, seal, protective tube, etc.) the devices must be rated

Page 84 of 167

Page 166

DOCUMENT REF: TWUL/E04
APPENDIX A3 ISSUE 4.0

PRV size = 150mm

PRV set pressure = 25mbar

PRV overpressure = 2mbar

Release temperature = 35°C

Molecular weight of Biogas = 25.8

Wind speed = 1m/s (high level release)

Also assume turbulent ideal mixing of gas release.

For a primary release from seal of PRV at low pressure (due to small amount passing
due to seal leakage), R1 and assuming only 0.1% of the total flow released under high
pressure relieving conditions will leak:

Volume released Qv = 0.001 x 9.09x10-1 m3/s (flow taken from manufacturers
data)

Qv = 9.09 x 10-4 m3/s

Mass released Qg = 8.25
273350821.0

1009.9 4

8.9 x 10-4 kg/s

Therefore Radius R1 = X = 2.10.17.68.25
1109.842300

55.04



R1 = 0.5m

For secondary release under abnormal high pressure operating conditions, assuming
the following conditions:

Digester gas production = 150 m3/hr

Number of PRV’s = 1

Ventilation factor (F) = 1

Using the above data Qv is obtained.

Volume released Qv =
3600
150





Qv = 4.2 x 10-2 m3/s

Mass released Qg =
8.25

)27335(0821.0
102.4 2

4.1 x 10-2 kg/s

Therefore radius R2 = X =
55.02

0.17.68.25
1101.442300

x 1.2

R2 = 4.4m rounded up to 5m

IGE/SR/25 Hazardous Area Classification of Natural Gas Installations provides further
useful guidance on zoning distances for pressure relief valve vents with tables of
dispersion distance based on relief vent diameter and release flowrate.

Page 166 of 167

Page 167

DOCUMENT REF: TWUL/E04
APPENDIX A3 ISSUE 4.0

A3.4 FLARE MINIMUM SEPARATION DISTANCE

Thermal radiation levels are measured in kilowatts per square metre and permissible
radiation levels (K) for different conditions are shown in the table below.

Table A3.2 Recommended Design Total Radiation

Permitted Design Level (K) kW m2 Conditions

15.77 Heat intensity on structures in areas where operators are
not likely to be performing duties and where shelter from
radiant heat is available (e.g. behind equipment)

9.46 Value of K at design flare release at any location to which
people have access (e.g. at grade below the flare or a
service platform of nearby tower); exposure should be
limited to a few seconds, sufficient for escape only

6.31 Heat intensity In areas where emergency actions lasting
up to 1 minute may be required by personnel without
shielding but with appropriate clothing

4.73 Heat intensity in areas where emergency actions lasting
several minutes may be required by personnel without
shielding but with appropriate clothing

1.58 Value of K at any location where personnel with
appropriate clothing may be continuously exposed



In most cases, equipment can safely tolerate higher degrees of heat intensity than
those defined for personnel. However, items vulnerable to overheating, flammable
vapours or electrical equipment may be susceptible to heat radiation due to resulting
high surface temperatures.

Minimum separation distance is calculated using the formula:

K
FQ

D
4



Where:

D = minimum distance from the midpoint of the flame to object being considered (m)

= fraction of heat intensity transmitted (taken as being 1.0)

F = fraction of heat radiated (0.2 for methane)

Q = heat release (kW)

K = allowable radiation (kW m2)

A K value of 6.31 has been assumed for calculating the fenced off exclusion zone, a K
value of 1.58 is assumed for calculating the minimum separation distance between the
flare and any flammable material, hazardous area, or site boundary.

D1 = minimum separation distance for open flares and incorporates an allowance for
flame length as distance is taken from mid point of flame for open flares (i.e. ½ flame
length is added to calculated distance D1). Flame lengths for open flares are derived
from API RP 520 – Guide for Pressure-Relieving and Depressuring Systems.

D2 = minimum separation distance for enclosed ground flares uses value for D only as
flame is shielded.

Page 167 of 167

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