Validation of the Powell Emergency Tank Car Actuator As a SIL2 LOPA IPL for in a Chlorine or Ahcl Release Scenario

Catastrophic releases of dense toxic compressed gases such as chlorine, anhydrous HCl, and hydrogen sulfide have occurred in the past and been attributed to unload system failures. These releases involving liquid release and vaporization affect wide areas with potentially lethal impact requiring defense in depth. The several approaches including unload buildings with scrubbers will be discussed.

the Powell actuator system will be reviewed including its design and SIL (safety integrity level)

The Powell actuator is designed to attach directly to the tank car terminal valve and provide remote shutoff in the event of instrumented toxic detection or manual initiation.

This IPL (independent layer of protection) is capable of terminating a leak caused by failure of the screwed valve connector (known as the gander peck or unloading adapter) and by unload hose failure.

Commonly a SIL2 capable gas analyzer is paired with a totally independent logic solver that activates the actuator mounted on the tank car valve. The actuator must be placed on the tack car terminal valve after it is opened for use and so a human element in the system is introduced.
A function test is performed at the beginning of each unload. A highly detailed operating procedure is developed and trained annually and with new personnel including a written test.

We will show that SIL calculations indicate a SIL2 allocation is possible.

Along with other credits to be discussed, even a target event frequency of one in a million years can be achieved by LOPA analysis. Other credits such as shelter in place (with hypothetical training exercises) are discussed.

in summary, the Powell system is independent, capable, maintainable, and auditable.