Behind the CNG blasts: A steel pipe and a series of unfortunate events

Dealers say oxygen and carbon dioxide cylinders are being installed which cannot withstand the gas pressure.

Saad Hasan December 29, 2011


A one-inch diameter stainless steel pipe has been the cause of 84 deaths and 120 injuries in recent months. 

This pipe in the CNG (Compressed Natural Gas) cylinder carries gas (chemically known as methane) from the cylinder to the engine. However, despite frequent accidents, it remains a mystery what the exact reason behind the instantaneous combustion of CNG-run vehicles is. While some vehicles have ignited after a head-on collision, others simply caught fire on the road.

The one thing experts are sure of though is that when methane leaks from the cylinder it can disintegrate anything within minutes - be it human flesh, wood or metal.

“The pipe should be able to withstand pressure up to 3,000 pounds per square inch (psi),” said Habibur Rehman, General Manager of Hydrocarbon Development Institute of Pakistan (HDIP).

The gas pressure is estimated to be that of 300 adult men hitting an inch of a human skull with their fists together. “So when the pipe breaks, the vehicle immediately turns into a ball of fire,” Rehman said.

After the government started raising prices of petrol and diesel, thousands of Hiace vans, pickups, buses and coaches started using CNG.

This surge in the use of CNG has inflated the price of authorised gas kits and cylinders, flooding the market with poor-quality material.

Azhar Khan, a CNG kit dealer said oxygen and carbon dioxide cylinders were being installed in vehicles instead of CNG cylinders. “Sher Shah has become the largest market for these cylinders and auxiliary equipment,” he said.

Ordinary oxygen and CO2 cylinders cannot take more than 1800 psi of pressure, he said. “It’s really no rocket science to imagine what happens when you fill ordinary tanks with CNG.”

Customising buses

At the 4K bus stop in New Karachi, about two dozen buses have been altered to use CNG instead of diesel.

The mechanics might not have engineering degrees, but they have learned to modify the 40-year old Bradford buses.

“We change the head of the engine. Atomizers, which spray diesel over the pistons, are replaced with plugs. The top part of the pistons need to be filled up with melted iron,” said Muhammad Taufeeq, who customises buses. The plugs are needed to create the spark to ignite gas and create motion.

“It’s easy to do this. You would know it too if you would have spent your life working on buses,” he said.

From the two cylinders fitted in the chassis of the bus, a thin iron wire criss-crosses up to the seat of the driver where it connects with the CNG kit. The wire remains exposed to any sharp object which could rupture it from underneath.

Diesel engines do not need a carburettor- the device that blends air with fuel. “We are using a used Nissan 6-cylinder carburettor. It’s available everywhere with scrap dealers,” said Taufeeq.

A cigarette, matchstick or a short circuit is enough to ignite the cylinder. There are 3.2 million CNG vehicles and almost 35 percent of them use poor quality cylinders, experts say.

Most of the accidents involve vans where cylinders were placed underneath the backseat. Dealers say they never advise such installations, but roadside mechanics are more than ready to do the job.

Precautionary measures

According to the law, every CNG car needs to get its equipment checked from HDIP, but like many other laws, this too is usually ignored.

However, these days there a rush of people coming to get their cars checked. “We are getting 70 vehicles every day. People are scared since there have been many deaths in only three months,” Rehman said.

If HDIP detects any fault in the cylinder, it destroys it immediately. “It’s better to destroy faulty cylinders than to sit on a bomb.”

Published in The Express Tribune, December 30th, 2011.


A. Khan | 9 years ago | Reply

Good information but why is HDIP destroying cylinders when its been stated earlier in article that problem is to do with the pipe leading from cylinder to engine ? Furthermore, apparently what HDIP is doing has this has nothing to do with using cylinders for CNG that are actually meant for oxygen. How about also checking how many cycles of compression and decompression these have been through ? The failure rate can be determined through that as well.

Timour Pasha | 9 years ago | Reply

Great report full of good information. Good to see journalists following up on these human interest stories and gathering facts which are useful for the general public.

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