Praxair has responded to the US Chemical Safety Board\\$quot;s (CSB) findings that extremely hot weather was partly to blame for last year\\$quot;s fire at Praxair facility in the US.

The gas company told gasworld that it does not agree with the board\\$quot;s finding that high ambient temperatures contributed to the cause of the fire, which occurred at the company\\$quot;s St Louis facility on June 24 last year.

The company spokesman Nigel Muir said: "Praxair announced on September 29 last year that it believed a mechanical failure of a pressure relief valve on one or more gas cylinder containing propylene gas was the cause of the fire. Praxair provided data and information to the St Louis Fire Department, which also concluded that the cause of the fire was a mechanical failure of a pressure relief valve."

Praxair does not manufacture cylinders or valves.

Mr Muir continued: "During the CSB investigation, Praxair provided the investigators with all the data and findings from its investigation. It was also noted that in July last year, the manufacturer of the specific valve issued an inspection notification to the industrial gases industry in the US. In August, it issued a recall notification. The CSB makes no reference to this in its findings."

The CSB investigators on the other hand claim that the accident occurred on a hot summer day with a high temperature of 97 degrees F in St. Louis at Praxair, cylinders were stored in the open on asphalt, which radiated heat from the direct sunlight, raising the temperatures and pressure of the gas inside the cylinders.

CSB statement read: "At approximately 3:20pm, a propylene cylinder pressure relief valve began venting. CSB investigators believe static electricity, created by escaping vapour and liquid, most likely ignited the leaking propylene.

"The investigation determined that the pressure relief set points, specified in industry standards, are too low for propylene and may allow the gas to begin venting during hot weather - well below the pressures that could damage the cylinders. Not only are the specified set points too low for propylene, the CSB found some valves begin releasing gas even before the pressure reaches the set point. Each time a pressure relief valve opens, its performance deteriorates - making it more likely to vent gas at too low a pressure in the future."

CSB lead investigator Robert Hall continued: "The key lesson learned in our investigation is that the combination of high ambient temperatures and relief valves that open at too low a pressure increase the risk of catastrophic fires at these facilities."

Cylinders safe up to 150 degrees Fahrenheit
The pressure set point on all propylene valves is required by law to be 405psi. This set point is recommended by the Compressed Gas Association and is approved by the US department of transport, which oversees the industry.

Praxair informed the CSB that third-party controlled tests had been conducted on propylene cylinders\\$quot; external temperature and internal vapour pressure. In full, half full and quarter full cylinders, it was found that the internal propylene cylinder pressure remains well below the current set point of 405psi until the outside cylinder temperature exceeds 150 degrees F.

Muir continued: "On the date of the fire, the recorded high temperature in St Louis, according to the CSB, was 97 degrees F. Based on the known vapour pressure curve for propylene, the internal pressure of a propylene cylinder would be less than 200psi at that temperature.

"It should also be noted that the industrial gases industry has safely operated its business for decades in ambient temperatures of 97 degrees which are considered normal summertime temperatures in many parts of the United States. Even higher summer temperatures are regularly encountered in such areas as Texas, Arizona, Nevada, etc."

According to Praxair the only way that the propylene vapour release, which is identified by the CSB as the cause of the fire, could have occurred was through the mechanical failure of a safety relief valve when the internal pressure was less then half the set point for the pressure relief valve. The ambient temperature on June 24 was simply not hot enough for the internal cylinder pressure to approach 405 psi.