An important decision when considering a distributed control system (DCS) migration is whether to migrate during online or offline process conditions. However, it is important to understand that for many companies offline migration is not an option at all, because the plant cannot be offline for the required period of time. This can for example be due to financial reasons or supplying agreements that the company must adhere to.
Offline DCS migration
An offline DCS migration requires a plant shutdown. The length of the migration is determined by the amount of (critical) loops (I/O) that are to be migrated. When a company decides to migrate offline, it is often combined with other activities that can only be done during offline process conditions. Think of maintenance and replacing machine parts for example. However, when time needed to migrate the total amount of loops exceed the time that is needed for other activities, the migration becomes even more costly than it already is. The costs of plant downtime depends on the type of industry, the size of the company, production rate, et cetera. These costs can run into millions.
Even though migrating offline is costly, it is a very safe way of migrating. The migration process control is maximized, and potential machine failures during the migration are eliminated. However, constantly developing technologies allow for maximized control and safety for DCS migrations during online process conditions, without necessarily having to deal with high costs.
Online DCS migration
When a plant shutdown is not an option, or the offline migration costs are simply too high, an online DCS migration is a better option. Several factors play an important role during online DCS migrations:
Safety
An online DCS migration requires continuous control of the process, available fail-safe and step-back options and a guided process to maximize safety during the online migration.
Efficiency
The migration should be a quick and easy process, with limited preparation time and downtime is eliminated.
One solution
In order to improve the efficiency and reliability of the migration process, there should be one solution for all (critical) loops.
Clear and structured process
To eliminate human failures, the migration process should be clear and structured, leaving no space for flaws during the process.
Online migration methodology
There are several methods to facilitate an online DCS migration. The most common methods are discussed consecutively.
Process bypassing
Using this method the process redundancy and bypass valves are used to block out segments. The blocked out segments can then be migrated.
- It is not always possible
- No control during the migration process
- No output match
- Time consuming
Manual valve control
Using bypass valves to create fixed process conditions.
- Offset control only by experience, which could lead to safety issues
- No output match
- Time consuming
- Expensive
Mechanical blocking
Using this method the process valves are mechanically fixed.
- No offset control
- No output match
- Time consuming
- Expensive
HART manual mode
HART is used to create a stand-alone instrument.
- Requires HART manual mode support
- Not always possible
- External power required
Electrical loop take-over
Using this method the loops are taken-over and controlled by an external tool during online process conditions. The loops can then be migrated.
- Offset control
- Fail-safe mode
- Output match
- No mechanical work
- Limited preparation time
IST-203 online migration tool
This online migration tool takes over and simulates 0(4)-20 mA loops during active process conditions. This allows the user to perform an online control system migration without a plant shutdown. It increases efficiency and control and eliminates downtime. The tool guides the user through a simple step-by-step loop take-over process and therefore minimizes human errors and risks. More information about the IST-203 online migration tool can be found here.
