Project: Biopharmaceutical filtration automation
July 20, 2005
The design process for this project follows a bottom up strategy from field devices to recipes in this order.
2. Control modules and interlocks
3. Equipment modules
The skid vendor has provided P&IDs and electrical diagrams. From these drawings, we have entered I/O information into our project database. One record in this database includes tag, description, P&ID drawing, field panel, I/O type, controller node, I/O slot, card channel, alarm area, and a few other fields that provide wiring details.
Managing the I/O information is an area where we must interface with other project disciplines. Because we keep our own database, we must be informed of changes to plant equipment design with impact to the instrumentation. This requires that we always be vigilant and monitor project communications or updates to documentation. For a small project such as this, it is not difficult to do. For a recent large project, we would receive I/O changes on a monthly basis even while software was being tested and validated.
On our project, we have a skid that feeds a tank that feeds another skid. Different PLC controllers control the two skids. The question then is: To which of the two controllers should the tank I/O be wired? The correct method is to wire the I/O to the PLC that contains the device control for that I/O. This may require that the tank I/O be split between the two PLCs, since the tank inputs are controlled by the upstream PLC and the outputs are controlled by the downstream PLC. This is important for a few reasons. First, if the control for a valve is in one controller and the I/O in another, the device requires a network connection for simple device operation. Should the network connection break between the two panels, the device can no longer be operated. A single controller should always be designed to manage basic device level control without requiring anything outside the controller and its I/O subsystem. Another reason is the value of modularity in the equipment. Wiring designs based upon geographic location often result in a system that is very difficult to maintain because a single field panel may include wiring for a variety of equipment. Although it may be more expensive to wire a point where it functionally belongs rather than wiring it to the nearest panel, this extra cost is more than offset by the cost of managing a bad I/O mapping for the life of the plant.
Separating the I/O cards based upon equipment boundaries will also simplify copying the wiring design. For example, the first 4 I/O cards (AI, AO, DI, DO) may be set aside for the skid I/O and the next 4 cards set aside for the tank I/O. This is not the most efficient use of the PLC hardware, but is much easier to manage and maintain over the long run and much easier to copy should a new tank or new skid be added to the plant.
The next post will cover the design of control modules.