Jan-2009

Delayed coking: valves make the difference

Traditional manual methods of coke drum unheading are labour intensive, costly to maintain, expensive to repair and carry inherent risk for personnel on the unheading deck.

David Anderson, DeltaValve

Article Summary

The world’s first coke drum bottom unheading valve was a first step in what has become a new standard in safety and reliability in coke drum unheading. Improving safety, maximising throughput and profitability, and minimising long-term maintenance and indirect operating costs are difficult tasks for refineries today. Through innovative design, valves are put into processes where traditionally they have never been used before; a good example is coke drum unheading valves. Also proving successful are design efforts to address specific operational challenges of older valve technologies, such as the improvements that have been made in isolation valve technology. These successful efforts have resulted in refinery valves that are safer, faster and more reliable than ever before.

Safety of plant personnel and equipment
The process of coke drum unheading (or opening) within oil refining has traditionally been one of the most dangerous activities in the industry. Such operations have resulted in many accidents and some fatalities over the years. In a joint bulletin entitled, Hazards of Delayed Coker Unit (DCU) Operations, released in August 2003, the Occupational Safety and Health Administration and the Environmental Protection Agency’s Chemical Emergency Preparedness and Prevention Office stated: “Unlike other petroleum refinery operations, the DCU is a semi-batch operation, involving both batch and continuous stages. The batch stage of the operation (drum switching and coke cutting) presents unique hazards and is responsible for most of the serious accidents attributed to DCUs.”

The bulletin advised: “Consider equipment upgrades to further control the hazards associated with geysers and release of hot tar balls and undrained hot water during drum head removal, such as installing protective shrouds and auto- mating both top and bottom head removal operations to keep workers away from these unprotected areas.” Such advice is given because the most common injuries to plant personnel working on delayed coker structures are burns caused by steam, hot water or coke. In addition, many have suffered long-term effects from repetitive stress injuries caused from working pneumatic hammers and other impact equipment over extended periods.

As a result of extreme temperatures required by the delayed coking process, an incident in the coker often causes damage to surrounding equipment. Among the types of equipment affected are instrumentation, piping, valves, pumps, electrical wiring, insulation, hydraulic and air tubing, and others. This equipment is expensive to repair or replace, and the downtime associated with repairing or replacing the damaged equipment can be significant. In difficult economic times, refineries simply cannot afford this unscheduled downtime.

Innovation
Traditional methods of coke drum unheading include manual or semi-automated unheading systems, which are labour intensive, costly to maintain, expensive to repair and include inherent risk for personnel on the unheading deck. Until recently, the concept of using a valve for coke drum unheading had never been realised, held back by the severe nature of the coking and unheading process.

However, one US valve manufacturer success- fully designed and installed the world’s first coke drum bottom unheading valve in 2001. This event was the first step in what was to become a new worldwide standard in safety and reliability in coke drum unheading.

Critical criteria
When valves are to be used in coke drum unheading, design is critical. Above everything else, the valve must be both reliable and safe, and in coke drum unheading a valve is not safe if it is not reliable — the two principles are inseparable. Secondary considerations are little to no maintenance between turnarounds (which are typically every five to seven years), a self- cleaning capability, the ability to remain fully sealed in the event of a power failure, and the ability to quickly close the bottom of the coke drum in the event of coke drum fallout or fire.

Key considerations
The most effective way to keep coke drum personnel safe during the unheading process is to completely remove those people from the unheading deck and control the unheading process from a remote location. By introducing valve technology into coke drum unheading this remoteness becomes a reality. This is because personnel are no longer needed on the unheading deck to unbolt the unheading flange or disconnect the feed line. They also are not required to stand on the unheading deck with an open and exposed drum waiting for the discharge chute to raise to be connected to the drum. Valve technology makes simply pushing a button from a remote location to unhead the coke drum a reality.

As for reliability, if an unheading system is unreliable, personnel must go back on the unheading deck to resolve issues, which is completely counter to remote operation or the possibility of no-one on the unheading deck. If a breakdown occurs when the coke drum is not active, having personnel on the deck might not be a problem. However, if a breakdown occurs during a coking cycle with an energised drum or during the unheading procedure, personnel close to the drum may be exposed to significant hazards.

Many variables contribute to the design of a reliable valve, including the selection of materials, simplicity of design, diagnostics, cleaning and ease of repair capabilities. If incorrect materials are specified for valves in the delayed coker, extremely dangerous situations can arise. Standards in the industry dictate that materials that come into contact with the process should be Chrome Moly at minimum. This requirement also applies to any portion of an unheading valve that comes into contact with the process.

Of equal importance in correctly selecting valve materials is simplicity of design. The fewer mechanical or moving parts in an automated unheading valve, the lower the risk of failure. A single gate and a single actuator is the ideal design configuration for this application.