Found this info online, courtesy Mr. Hassan Hakeem/Asim Riaz who have done extensive research on the said subject.
(by Hassan Hakeem on Monday, January 9, 2012 at 12:15am
Annexure-B CNG Vehicle Accidents: Issues, Causes and Recommendations)
Page 1 of 5
Last month 44 people died on accidents of CNG converted public sector transport vehicles. I was deputed to make technical reports of some accidents to ascertain reasons for these accidents and to recommend safety measures for future. Before discussing the technical aspects of the problem in hand, it is important to mention two things right at the outset which relate to inadequate awareness and incorrect perceptions about the CNG cylinders. It was surprising that electronic and print media persisted in erroneous reporting that the accidents occurred due to bursting/explosion of the CNG cylinders. Add to that, some of the Government departments were also relating these accidents to non-compliance of Hydrostatic testing of CNG cylinders. Both are absolutely wrong impressions. This incorrect reporting has continued till date which has developed serious apprehensions about CNG cylinders in general public. In the context of the foregoing following points are elaborated for clear and correct understanding of the problem:
a) Contrary to the media reporting no CNG cylinder having ever exploded in any accident of the CNG converted public service vehicles in the history of Pakistan CNG Industry. Approved CNG cylinders having manufacturing standard NZS 5454 should never burst or explode and so was the case in recent accidents that claimed 44 lives. Few CNG cylinder burst accidents that happened in the past were all in private vehicles not in the public service vehicles.
b) Non-compliance of Hydrostatic testing of CNG cylinders after the due date was not the cause of any cylinder accident that ever happened in Pakistan. There is no correlation between CNG accidents and non-compliance of Hydrostatic testing requirement of CNG cylinders; hence relating these accidents with such testing of CNG cylinders is totally incorrect.
c) CNG Production and Marketing Rules, 1992 do not cover all the causes of CNG converted vehicle accidents. These rules require updating.
On the spot examination of vehicles and their damaged CNG kits after the accidents have helped in establishing some causes of due to which the vehicles caught fire. In addition, undersigned has exercised all due diligence possible to avoid such accidents to happen in future. Visits to different Road side CNG kit conversion facility to observe their working, doing some practical experiments in HDIP CNG kit conversion facility, reading technical manuals and relevant international standards have further helped in finding the causes and identifying possible solution to prevent the causes of such accidents.
Annexure-B
CNG Vehicle Accidents: Issues, Causes and Recommendations
Page 2 of 5
A. CAUSES OF ACCIDENTS IN THE CNG CONVERTED VEHICLES/NGVs:
The fire accidents in the CNG converted vehicles were caused by the leakage of gas after collisions of vehicle and in some cases even without any collision. The leakage of gas and resultant catching of fire is attributable to following factors:
1. Leakage of gas from high pressure piping due to:
a. Breaking of high pressure piping;
b. Dislodging of high pressure fitting;
c. Damaging of high pressure pipe due to impact, mechanical wear and tear and constant vibration; all these can occur without a collision of the vehicle.
2. Detaching of CNG cylinder.
3. Sparking of air-gas mixture created in the passenger cabin can be due to cigarettes, lighter, match box, electrical instruments etc carried by the passengers.
4. After the collision the unwanted ignition could be caused:
a. Short circuiting of battery and electrical wiring; b. Contact of fuel with hot surfaces i.e. engine, exhaust manifold, radiator etc; c. By spark created when Kinetic Energy of motion is converted into heat energy.
5. Bursting of unapproved cylinders, welded cylinders and cylinders that are not manufactured for CNG use.
6. Bursting of unapproved valve.
7. Leakage of gas from the cylinder valve.
8. Leakage of gas from the filling valve (If Non Return Valve (NRV) is not installed).
Annexure-B
CNG Vehicle Accidents: Issues, Causes and Recommendations
Page 3 of 5
B. RECOMMENDATIONS TO PREVENT THE ACCIDENTS IN CNG VEHICLES/NGVs:
The safety recommendations to prevent the aforementioned causes of the CNG converted vehicle/NGVs are given below:
1. CNG CYLINDERS:
a. Integrity:
Only approved seamless (Type I) CNG cylinder having manufacturing standard NZS 5454 must be used in CNG converted vehicles or NGVs.
b. Positioning of CNG Cylinder:
i. In low-floor buses CNG cylinders must be installed at the roof (over carriage). In high floor buses CNG cylinders can be installed on the roof or beneath the Bus, with extra protection.
ii. In vans (Hiace, Mazda etc) CNG cylinder may be placed a) on the over carriage if it is strong enough to hold the weight of cylinders and the center of gravity of the vehicle is not disturbed b) beneath the vehicle c) at the end (behind last seat) of the passenger cabin. If the cylinders are installed behind the last seat it is recommended to isolate the passenger cabin from this portion.
iii. It is not recommended to place CNG cylinders beneath the passenger seats.
c. Fastening of CNG Cylinders
i. The CNG Cylinder fastening should be in accordance with ISO 15501-2. Copy of the standard is attached.
ii. The bracket holding CNG cylinder should be designed strong enough to withstand road accidents.
d. General Guidelines
i. Hydrostatic testing of the cylinders should be done after 5 years interval according to CNG Safety Rules, 1992, even though some experts and the undersigned have reservations on safety of these tests. It can take the CNG cylinder to plastic stage, after that it can fail during service. The cylinder is also unnecessarily subjected to 1.5 times its working pressure in hydrostatic testing.
ii. A Qualified pressure vessel inspector can declare the CNG cylinder fit for further use with the help of visual and ultrasonic inspection tools without Hydrostatic testing.
iii. Ensure that CNG cylinder have corrosion protection i.e. properly painted
Annexure-B
CNG Vehicle Accidents: Issues, Causes and Recommendations
Page 4 of 5
iv. CNG cylinders increase the weight of the vehicle. As a result it affects the structure, center of gravity, driving security and fuel consumption rate. Therefore, the public service CNG Vehicle driver should have one week training course on CNG system.
2. HIGH PRESSURE PIPE AND FITTING
a. Only approved (imported) high pressure piping/fitting in the CNG vehicle; because the breaking and dislodging of high pressure pipe and fitting are the main causes of fire in CNG vehicle after the accident.
b. The high pressure pipe should not pass from the passenger cabin in any public service vehicle. The pipe routing should either pass over roof or beneath the vehicle.
c. Corrugated pipe should be used to protect high pressure pipe from mechanical damage. Plastic fitting should be provided on the holes made on the vehicle body so that the high pressure piping remain stable and do not have a metal to metal contact with the body of the vehicle.
d. High pressure piping should be protected from vibration. Corrugated pipe fitting should be used to on the holes of the vehicle in order to prevent the vibrations.
e. A manual shut off valve should be provided in the high pressure line in the vehicle where multiple CNG cylinders are used.
3. CNG CYLINDER (3 WAY) VALVE AND ACCESSORIES
a. Only approved CNG cylinder valve must be used.
b. A Non Return Valve (NRV) i.e. Check valve must be installed between cylinder and the filling valve.
c. Gas Solenoid Valve should be installed on the 3-way cylinder valve in order to stop leakage of gas from high pressure line which is going to the CNG pressure regulator, in case of accident. It will also depressurize the CNG kit when the vehicle is switched off.
d. It is also recommended to integrate the Gas Solenoid Valve with the Accelerometer as it will work as an impact sensor. The Gas Solenoid Valve will stop the flow of gas once Accelerometer detects a collision. The Accelerometers are widely used in automotive industry in the air bag protection systems.
e. Excess Flow Valve (EFV) manufactured in accordance with ISO 15500-14: 2002 (copy attached) specifications should be installed in all the public service commercial vehicle.
f. Either Thermal Safety or Pressure Relief Device must be installed in the CNG cylinder valve.
g. Cylinder valve should be air tight cover and vented out of the Vehicle.
Annexure-B
CNG Vehicle Accidents: Issues, Causes and Recommendations
Page 5 of 5
4. GENERAL
a. Self activating fire extinguisher should be provided in the public service vehicles at an appropriate place.
b. The electrical wiring/connection of the vehicle should be of standard quality and must be properly installed so that it will not readily catch fire in case of accident.
c. Public awareness campaign about the CNG technology must be launched through electronic and print media.
d. CNG is a serious business i.e. a 50 WLC CNG cylinder at 200 bars and 1500 WLC CNG compressor blow-down/buffer pressure vessel at 5-10 bars stores millions of joules of energy in the form of pressure (copy of calculation attached). There have been verbal complaints by the licensees and technical staff of the CNG stations that some departments are sending 3rd party Inspection team have engineers who poses no knowledge and have no experience in CNG technology. For this reason there must be criteria of written test and practical assessment along with a minimum relevant experience requirement to become a 3rd party Inspection engineer for OGRA and C.I.E. Similar requirements must be made mandatory for C.I.E and OGRA professionals who are checking these reports.
e. Corruption in inspections of CNG stations and Vehicles must also be checked.
Prepared by:
Engr. Asim Riaz (Research Engineer)
Hydrocarbon Development Institute of Pakistan,
Ministry of Petroleum & Natural Resources,
Islamabad, Pakistan.