Use of a Tank Car Terminal Valve Emergency Shutoff As an SIS in Control of Toxic Gas Releases

July 14, 2001, at the ATOFINA Chemicals, Inc., (ATOFINA) plant in Riverview, Michigan, a pipe attached to a fitting on the unloading line of a railroad pressure tank car containing methyl mercaptan fractured and separated resulting in the release of 147,350 pounds of methyl mercaptan, a liquified poisonous and flammable gas. The break occurred at the connection at the tank car terminal valve and prior to the flex hose isolation automatic valves that had closed in response to vapor detection. The incident resulted in multiple fatalities, fire loss, and evacuation of 2000 nearby residents..

In response to similar scenarios, a terminal isolation actuator was developed by the Powell Valve Company designed to fit over the tank car valve and shut it off in the event of toxic or flammable gas release.

The alternatives this approach are commonly enclosed offloading sheds equipped with scrubbers; at considerable capital cost and complexity.

When such a release is analyzed by LOPA (layers of protection analysis), this IPL (independent layer of protection) must possess a SIL2 or SIL3 reliability in order to close the scenario to remove gap to desired target risk for a potentially off site, multiple potential casualty event.

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 and turns out to be the limiting element of this approach (as the reliability elements of the full instrumented loop are additive)
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.

The reliability of the human element can be estimated by the required training and feedback by CCPS guidelines. Proof in service feedback from operations is necessary and may take considerable experience to tie this down.

Powell does have feedback from customers that can help as back up in LOPA sets.

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. It is currently used in Cl2, aHCl, and compressed flammable gas service.