Tech Talks

Retrofitting an Existing Control System

Over the last few years, we have encountered many projects that involve removing an obsolete or incompatible controller and replacing it with a new one.

This is done for any number of reasons. It is difficult to find parts for controllers that are no longer supported by the manufacturer. An old controller may not have the ability to send data to the plant's SCADA system. Or, maybe the controller is an obscure brand, and the plant engineers and technicians don't have access to the program and they don't care to spend the money on the software.

Usually, our clients want the new controller to operate exactly like the old one.

Whatever the reason for replacing a control system, there are 3 major steps that we take to ensure the success of the change out.

Phase 1: Determine how the existing system functions.

Before we can begin retrofitting a controller, we must first fully understand how the existing system works. This requires a certain amount of documentation and knowledge, including:

  • Drawings such as P&IDs, mechanical, process or electrical

  • An understanding of any hardwired safety circuits

  • Identifying the existing I/O on the controller and the function of each device connected to the I/O

  • Capturing HMI screens and determining what each active element (such as a pushbutton) does

  • A list of all setpoints and alarms that have been programmed into the existing system

  • An understanding of all data that may be sent to the SCADA system

  • A clear and concise Functional Specification that explains the details of how the system functions and the expected product throughput of the system. If written properly, this document can also be integrated into an Operator's Manual.

  • A Control Narrative that verbally describes the sequence of operation of the machine or system

  • A list of applicable personnel, environmental and machine safety standards that apply

In most cases, all of this information is not readily accessible. That is not the fault of the owner, as he may never have received that information in the first place.

In any event, if any of the items listed above are not available, we will perform an audit of the existing system so that we have the required information.

This serves a dual purpose. If the owner, does not have that information, he is usually very happy to get it. Also, it gives us the information we need to proceed to the next phase.

Sometimes, this first phase is completed in a couple of days. Other times, it may take a few weeks or even months. We have been involved in projects where there was very little documentation, resulting in us interviewing the machine operators to find out how the system worked.

Phase 2: Create new drawings, write the control program and generate the necessary HMI and SCADA screens

Of course, if we are installing a new controller new drawings will be required.

We will sit down with the owner and go over the new drawings, the new program, the screens, alarms, SCADA data collection and safety concerns. We want to capture any issues that may present a problem in the final phase.

During this phase, our primary goal is to make sure that the system operates as defined in the Functional Specification. However, we certainly understand that the owner might want to make enhancements to increase productivity, decrease downtime or otherwise improve performance. We are more than willing to accommodate any changes to the original operation.

Phase 3: Retrofitting and Start-Up

We work with the owner to determine who will actually remove the old controller and install the new one. Perhaps the owner wants his personnel to perform the work, or maybe he would prefer that the work be done by contractors.

As physical situations vary, sometimes a new controller can be installed in an existing enclosure. Other times, a new enclosure is required. In either case, we will provide drawings and define a step-by-step procedure for installing the new controller.

With the owner, we will determine how much downtime is available to replace the existing controller with a new one. We understand that production demands must be met and we will accommodate those demands. This may require making provisions to operate the equipment manually during the controller changeout. In large systems, it probably would be best to divide the systems into sub-systems, so that the entire system is upgraded a section at a time.

We will define the acceptance criteria for the project. Typically, this is as simple as going through the Functional Specification and confirming that the system operates as stated. However, there may be other variables, and we want to make sure the client's needs are met.

After we have a clear plan for the start-up, we will monitor and assist in the physical retrofit, load the new program and test the system to the defined acceptance criteria.

Above all, it is our goal to make sure our clients are satisfied.