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Ir Simon NGOSenior Network Manager
The Hong Kong and China Gas Co. Ltd.
The Risk of Flying Manhole Covers
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2
The Unlucky Mexican at the Wrong Place and the Wrong Time
• A methane gas explosion at a manhole
• The unlucky Mexican just walked on it
• Tossed him 25 ft into the air
• Hit on a overhead power cable
• Fell onto the roof of a passing bus
• The fortunate thing was … he was still alive
Monterrey, Mexico 13 Dec 2001
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3
But How about us in Hong Kong?
• OK in the past
• But
• A threatening phenomenon in recent years
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4
Telecom Manhole Blasting at Chung On Ston 12/01/2003
• 2 manholes, 8 covers• Gas leak due to road settlement• 4 injuries
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Telecom Manhole Blasting at Pokfield Rdon 13/02/2003
• 6 manholes, 14 covers
• Gas leak due to 3rd
party damage
• 2 injuries
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14
1312
1110
9
87
6
5
4
3
1
2
Telecom Manhole Blasting at Chik Wan StDue to Unidentified Gas on 17/04/2004
• 14 manholes, 30 covers
• No injury
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• All telecom manholes
• All “Multi-Manhole” Explosion
• More and more serious 2 6 14
Similarities of these Serious Incidents
(30 if 2Xn-1+2) ??
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Incidents related to other Flammable Gasesin Hong Kong
14/11/2003 Chemical 13/02/2003 Sewer Gas
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7-Manhole Explosion in JapanDue to Gasoline on 5/10/2001
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Telecom Manhole Explosion in SingaporeDue to Unknown Gas on 17/12/2004
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Manhole Explosion in Shenzhen, Chinaon 11/05/2005
The escaped LPG entered into the nearby ducts, manholes and cable troughs resulting in a number of cascaded explosionblowing up the covers
Damage to underground LPG pipe
A flying cover hit a passer-by and killed her instantly
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Power Cables could also Cause Manhole Explosion
Suspected Power cable fault created air pressure or evolution of combustible gas and led to explosion.
1/08/2005
Leighton Road
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Power Cables could also Cause Manhole Explosion
Suspected Power cable fault generated combustible gas and caused explosion after triggering by ignition sources.
20/11/2005
Prince Edward Road West
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September 19, 2005
September 6, 2005
Berkeley19 Sep 05
An underground connection
splice failure sent a
manhole cover into
the air.
Indianapolis, 6 Sep 05
underground cables went bad, ……… The underground pressure from the buildup forced the manhole cover into the air.
San Francisco, 19 Aug 05
An explosion in an underground utility chamber in downtown San Francisco sent manholes cover flying …
Pyrolysis - Formation of combustible gases when cable sheath melts at high temperature and anaerobic condition
Electrical Manhole Explosion is not UNCOMMON in the U.S.
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Why Bother?
• Hong Kong is just too congested.– too many people on the ground– too many underground ducts– too many manholes
• A single manhole explosion could injury a dozen.
AND
• The risk of multi-manhole explosion is too high!
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16
12
345
6
7
891011
1213
How Many Covers Can You Find?
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What Went Wrong?
Hostile & Irresponsible Type of Criticism →“No Leak, No Explosion!”
Reality:• Not only piped gas – LPG, Sewages, Illegal
disposed chemical, gas generated from electricity fault.
• 3rd party damage and external disturbances.
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What has the Gas Company done?
• Replaced all cast iron mains– Probably the only gas company > 30 years old had
no C.I. mains
• Reduced 3rd Party Damage– 80%↓ vs 1997
• Enhanced leakage survey– 1 per year 3 per year
ButIt is not possible to eliminate all possibilities of gas leak
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What Really Went Wrong?
• Congested Underground Facilities.
• Dramatic Increase of Utilities e.g. Liberalization of Telecom Market: 1 → 10+Hong Kong is unique !
• No orderly disposition rules.
• Increased Road Work Activities → damage to underground facilities such as fragile PVC ducts left unaware.
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BUT• No increase in pavement width (3.5m max. typically even in
new town)• No orderly disposition rules for U/G utilities
3.0M Terabits (roughly 3-4 fold a year)
NoneInternet Cyber Traffic Volume
1,943km (+52%)1,279kmLength of Roads
17 (+143%)7U/G Utility Undertakings
27,137TJ (+293%)6,907TJGas Consumption
152,313TJ (+168%)56,829TJElectricity Consumption
2.4M (+60%)1.5MPermanent Quarters
6.9M (+28%)5.4MPopulation
20041984Year
Characteristics of a Rapidly Developing Metropolitan City
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Characteristics of a Rapidly Developing Metropolitan City
Impact
Increasing Risk of 3rd Party
Damage
Increasing Congestion and
Interaction
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Spaghetti at Construction Sites
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Cracks or Holes at Ducts in HK
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5 Elements of TypicalFlying Manhole Cover Incident
5. Manhole Cover3. Manholes
2. Cracked Duct1. Flammable Gas
4. Ignition
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Means to Eliminate Flying Manhole Cover
• Flammable Gas– Various types of gas? 3rd party damage? External disturbances
and road settlement?
• Ignition– Unpredictable. Too many potential sources
• Enhance Integrity of Underground PVC Ducts– Remove cracks/holes on fragile ducts? Use stronger ducting,
e.g. PE? Too costly? How about those already installed?
Modify Manhole Design– Venting– Mechanical Restraint– Eliminate / Reduce Explosive Volume– Duct Sealing
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Venting – Perforated Manhole Cover
• Vent size – too small? too big?• Rainwater, trash or even ignition sources?• Hygiene• Huge cost for modification?
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Mechanical Restraint
Anchor for lock pin to fit
Combustible Gases
Mechanical Lock Metallic Chains Hinged Cover
• Long term integrity• Not eliminate problem – Explosion still occurs!• Huge cost for modification
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Testing of Mechanical Restraint Design
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Volume (Energy) Reduction
Sand Bags Water / Air Bags Foam Materials
• Hindrance to future maintenance• Long term integrity - Deflation of Bag
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Testing of Volume Reduction Design
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Duct Occupied with Cable
Reserved Duct
Duct Openings to be Sealed
Duct Sealing
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Sealing of Reserved Ducts by Blank Plug
Reserved Duct Manhole
Cover
Soil Plug(Wrapped with Denso Tape)
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Blank Plug with Denso Tape
Plugs for Sealing Reserved Duct
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Custom Design Removal Tool
Hook
Nut
Custom Design Tool for Easy Removal of Installed Plug
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Speedy Removal of Installed Plug
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2 Part Resin (Resin Pack 7A)
Sealing of Ducts Occupied by Cables
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Speedy Removal of Expanded Foam Seal
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Even if one seal slightly deteriorates,
it will not enter the next manhole
Double Safety
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Sample of Duct Sealing adopted by a company in the U.S. for street manholes
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Duct Sealing as the Simplest and Most Cost Effective Solution
- Totally eliminate the accumulation of hazardous gases inside manhole
- Easily applied to existing manholes
- Simple Installation
- Low Cost
- Already a standard for ducts entering building
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What Have Been Achieved To Date
• Duct Sealing for Building Entry promulgated in Building Rules and Office of Telecommunications Authority (OFTA) Codes.
• Highways’ recommendations to seal ducts.
• Most telecom companies agreed to seal at least reserved ducts of new manholes.
• Highways’ consultancy study to determine way forward.
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Gas Safety – How well are we?Public Reported Gas Escape (PRE)
per 10km Pipe Length from 1998 to 2004
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
1998 1999 2000 2001 2002 2003 2004Year
No.
of i
ncid
ent p
er 1
0 km
HK
Metropolitan City Din East Asia
Metropolitan European Country C
USMetropolitan City Bin US East Coast
Metropolitan City Ain US East Coast
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3rd Party Damage to Underground Gas Pipes
163172 195
164152
118107 115
96
59 54 48 46 4128
0
20
40
60
80
100
120
140
160
180
200
1990 1992 1994 1996 1998 2000 2002 2004
-75% vs
1997
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Typical Pressure inside Manhole during Gas Explosion
Pres
sure
(kPa
)
Time (sec)
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Normal state with ventilation opening
Pit
Polystyrene ball
Pit cover
Archimedes’ Principle
Upthrust of polystyrene ball blocks the entrance of rain water
Ventilation resumed as long as water evaporatedTop view of
Pit Cover
Side view
Pit cover
Rain water
Venting – Idea to Prevent Ingress of Water