Thursday, July 10, 2008

Valero Sunray Refinery Explosion 2007

The Chemical Safety and Hazard Investigation Board, CSB, released yesterday their findings on the causes of the explosion and fire at Valero's oil refinery in Sunray, Texas that occurred in February, 2007. They concluded that the cold weather caused accumulated water to freeze and split a pipe elbow. The split allowed high pressure propane to escape when the ice thawed, which then ignited. Sunray is in the far north of the Texas Panhandle, where the temperature drops below freezing for many weeks every winter.

The pipe elbow was isolated from other piping by block valves, and had no flow through that elbow. However, one of the valves leaked, allowing propane and water to enter the elbow. The accumulated water, being much heavier than propane, settled to the bottom of the elbow and eventually froze. The photo on the CSB website shows the elbow connects a horizontal run of pipe with a vertical run of pipe above. Thus, liquid leaking from a valve in the vertical run, above the elbow, would run down by gravity into the elbow and settle near the nearly-horizontal portion of the elbow.

The CSB report criticizes Valero for not recognizing that this isolated elbow was a dead-leg, or segment of pipe with no flow.

However, it should be pointed out that this refinery regularly experiences freezing weather, and has done so for many decades. The refinery personnel are aware of the hazards from freezing weather, especially the formation of ice. They have successfully managed many winters without splitting pipes that caused a fire. In this instance, though, they overlooked a potential hazard.

Existing standards in the industry require a double-block and bleed system, or a blind flange to prevent leakage. The double-block and bleed uses two block valves in series, with a small spool piece of pipe in between. The spool piece has a drain valve, or bleed valve, open to the atmosphere. Any leakage from either block valve will drain into the spool piece, and run out through the bleed valve. This design is not suitable for propane systems because propane vaporizes into the atmosphere and is highly flammable and explosive. The double-block and bleed design is more suited to heavier hydrocarbons.

The blind flange would have been appropriate. In this design, the piping must be isolated, drained, purged, then the elbow removed. Two blind flanges are installed on the ends of the piping that had connected to the elbow. This provides a positive seal and ensures that no material leaks.

Freezing water is not the only potential hazard that can split piping. Engineers are aware that many liquids can expand due to an increase in temperature. The thermal expansion is an extremely strong force and can easily split pipes or leak at flanges. Therefore, operators take special care to ensure that such liquids are not blocked in without a pressure relief device.

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