Friday 24 September 2010

Coker drum safety for crude oil refineries

Coker Drum Safety System prevents unsafe actions by coker drum operators.
The SIS-TECH Coker Drum Safety System (CDSS) automatically inhibits the opening of valves under unsafe process conditions. In a crude oil refinery, this prevents the release of hydrocarbons and/or hydrogen sulfide to the atmosphere, or introduction of hydrocarbons to an open drum. The CDSS logic solver makes sure that an in-service drum is not opened to the atmosphere, or that an open drum is not placed in service.

Understanding the Problem
OSHA (US Department of Occupational Safety & Health) says, in Hazards of Delayed Coker Unit (DCU) Operations, “The batch portion of DCU drum switching and coke cutting operations creates unique hazards, resulting in frequent and serious accidents.” The specific hazardous operating states are listed as coker drum switching, coker drum removal, and coke cutting (hydroblasting) operations.

Each DCU has at least two coker drums, and an operator is responsible for manually lining up the coker drums and valves. Although operator procedures and training emphasize the importance of correct line-up and the hazards of deviating from approved procedures, accidents often occur. Failure to open or close valves in the correct sequence can result in the release of hot hydrocarbons and/or hydrogen sulfide gas, and can also cause a fire.

Improper line-up of coker drums has led to several serious accidents, including fatalities. Because valve line-up is manually executed, operator error is one of the leading causes of these accidents.

Total Automation Solutions (TAS), recently installed a CDSS system at a major refinery. “The use of the CDSS in conjunction with the TAS Coker Interlock System eliminated the need to install a safety programmable logic controller (PLC),” said J.R. Thompson, senior vice president at TAS. “The installation was simple and straightforward, requiring only that we clearly defined the conditions under which certain valves could be opened or closed.”

Thompson has been working with refineries and coking systems for years, and he appreciates the safety and simplicity of the CDSS. “Without a CDSS, we would have to engineer, design and program a separate safety PLC system. This is a significantly more costly solution, and requires that the refinery have staff available to support the safety PLC.”
Monitoring Operator Actions
The SIS-TECH CDSS is a safety instrumented system independent of the TAS Coker Interlock System and other plant control systems. The CDSS monitors process conditions to prevent operations from opening the wrong valve or performing operations on the wrong drum. The CDSS provides a totally independent logic solver coupled with independent process sensors to provide a stand-alone layer of protection for the coker drum.

For each Coker Drum, the CDSS monitors three process variables: overhead pressure, overhead temperature and inlet temperature. It verifies safe conditions before allowing the TAS Coker Interlock System operator to open the vent, drain, top/bottom head, blowdown, overhead to fractionator, or inlet feed valves.

The CDSS logic solver monitors process variables to detect unsafe conditions. It then removes the permissive to operate the affected valves and sounds an alarm, thus preventing the valve from opening at the wrong time.

The CDSS is suitable for use up to SIL-3. The number of inputs and voting architecture (1oo1. 1oo2, 2oo2 or 2oo3) can be adjusted for each process variable to meet any SIL or reliability requirement. The CDSS is rated for -30 to 75C operation, meets Class I Div II requirements, and can be installed in harsh refinery environment near the equipment under control.

Field installation and implementation of the CDSS logic solver costs less than half that of a safety PLC. In addition, on-going support costs are much lower as the CDSS is much simpler to maintain than a safety PLC.

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