PLC+HMI and DCS

It was my first project as Control System Engineer at a System Integrator Company. System Integrator is responsible for integration between vendors. Furthermore, as a control system engineer, one doesn’t have to bind or represent the manufacturer or vendor.

The fruitage of working in a system integrator company is the ability to program both PLC and HMI for various brands.

Working to revamp obsolete PLC to the latest and different products of PLC includes converting the PLC program from the ladder to a function block diagram. The PLC is located remotely offshore at gathering/ manifold platforms with a swamp vicinity. It monitors surface facilities and is able to generate production shutdown. PLC uses the Modicon quantum 651 series.

All data will be sent to the control room (onshore) via a telemetry system. DCS system deploys 2 systems i.e., TDC3000 and Experion PKS Honeywell. The obsolete PLC was connected to TDC3000. Although PLC has been replaced with the latest product the connection to TDC3000 is still maintained except for 1 RTU to be reassigned to Experion PKS based. Hereafter this specific RTU is called RTU No. 1 . For reference below is the simplified architecture (Schematic 1)

DCS reassignments for RTU No.1 are due to existing LAN interconnection being available near the manifold platforms.

 RTU No.1 will be connected to another existing RTU via an ethernet switch. The existing RTU is using controller C200 Honeywell and is connected to DCS Experion PKS via radio TDMA.

We didn’t find major issues during this RTU revamping. The only thing is to ensure the Modbus address arrangement is the same as the existing one.

Additional work occurs for RTU no.1 to create a display since the relocation of DCS assignments

In order to maintain the project pace, it was decided to copy it from a similar display, existing RTU, on Experion PKS.

After changing the entire tag name and comparing the Modbus address on the PLC. The next step was generating runtime on the Experion. I discovered many message errors during the runtime. In the beginning, I was confused either the client.

After looking at all the message errors, I realize the source problem was caused by incompatibility between HMI web display scripting and the function block diagram on Modicon Quantum PLC. The existing display is designed by a control builder integrated into Experion PKS engineering tools.

So it required modifying all HMI Web Display scripts related to animation, such as faceplate, all dynamic shapes, etc, in order to be compatible with the functionality of function block diagram Modicon Quantum PLC. Although this was not easy, at the end it was finished without the attendance Honeywell vendor.

Due to this problem, I understand between the difference between PLC+HMI and DCS.  

Besides the term system architecture refers to DCS where more than one PLC is connected to the control station. DCS could also refer to software capability. DCS provides integrated software PLC+HMI up to SCADA.

Example of software with these types is Experion PKS, Delta V, and Centum VP.

The advantages of DCS software are as follows:

1. Reduced the effort required to design and implement HMI.
2. It eliminates the complexity configuration between PLC and HMI.

However, it consumes more memory and processor capacity.

DCS is suitable for applications with much-advanced control, high diagnostics, etc. Therefore it is suited for complex and fixed process plants such as refineries, petrochemical, and power plant.

However, based on my experiences I found that DCS is not suited well for continuous expansion facilities such as offshore platforms.

An offshore platform continues to explore new wells which continues to add I/O and display.

I have experience with DCS with saturated memory and capacity processors on offshore platforms, although it is far from the maximum capacity of I/O or remote I/O.

Unless it is a separate system between fixed surface facilities and a group of continue facilities such as flow lines and wellhead platforms.

For example, DeltaV/Experion as the DCS while continuing expansion surface facilities use PLC.

The more you have experience the more you could decide when to use an application based on DCS or PLC + HMI.