There are many examples in plants where operators need to follow a particular procedure to safely transition the plant to a new condition. In an ethylene plant, furnace startup, decoke and shutdown operations are classic examples where following strict operating procedures is critical to prevent unsafe conditions as well as promote efficient operations. Since furnaces are decoked every 30-60 days depending on feedstock and severity, transitions happen relatively frequently.
Quite often we find different shifts execute the transitions differently. Many times, written procedures are either out-of-date or incomplete. Operators are also dealing with many other distractions in a control room environment, so frequently transitions take longer than they should.
These situations lend themselves to automating standard operating procedures in the DCS using state-based control (SBC). State-based control refers to the concept of operating a process in a number of different “states”. The whitepaper, Change Your Board Operator to a Process Manager with State Based Control provides an excellent overview of SBC.
For example, an ethylene furnace may have states such as Shutdown, Startup, Operating, Decoke, Safe Park, and so on. Each state has its own set of alarm settings, interlocks, overrides, controller states and setpoints. And there are defined procedures and sequences to transition from one state to another, often with manual operator steps required.
Procedure automation starts with a Discovery Study where the team identifies SBC opportunities and understands which processes are most likely to benefit from automating. As part of the process, Emerson consultants and plant staff review best practices and capture the knowledge of the key shift operators.
This project team identifies any new instrumentation, valve automation, pump starts, etc. that will be needed. Not every step in the process needs to be automated. Transition procedures may have several Hold points which require operator confirmation that a manual step has been completed.
Documentation from the Discovery Study is translated into requirements and used to design the structure and functionality to be delivered. The plant’s distributed control system (DCS) is used to implement the sequences. Depending on the age of the system, additional software may be needed to handle the settings and sequence function logic needed for transitioning between different states.
Benefits for implementing SBC on ethylene furnaces can be substantial. Shaving just 4 hours off of a decoke cycle for a furnace with a 60-day run length is worth a full day of production over the year. And 4 hours is relatively conservative. Automating startup of a vinyl acetate monomer plant increased annual capacity by 1.5% and enabled a 60% drop in operator interactions.
In a past project for an ethylene furnace, the automated system was able to save 1/3 of the time from start-of-decoke to furnace on-line again. Additional production from this lost operating time was worth millions annually. For furnace-limited ethylene plants, this should be a no-brainer!
Visit the Ethylene Solutions section on Emerson.com for more on the technologies and solutions to drive improved business performance.
