Total Plant Integration at Merisol's Greens Bayou Plant
High Speed Ethernet-based fieldbus architecture restores single-loop integrity
By Henry Marks and Jimmy Keys
Merisol's Greens Bayou plant, a major cresylic acid production facility located near Houston, Texas, has been in operation since the mid-1940s. The facility includes process units which upgrade and refine phenol, cresols and xylenols. These products find uses in resins, solvents, antioxidants, functional fluids, cosmetics, disinfectants, agricultural chemicals and many other chemical intermediates.
Recently, the Greens Bayou facility undertook a total plant integration project aimed at modernizing its process automation systems and improving plantwide data access. The work involved migrating a host of disparate controls and instrumentation into an integrated control system providing full access to process information and displays from any plant location.
A basic justification for the project was to provide sufficient and accurate historical data to permit unit performance evaluation. This, in turn, would enable Merisol to realize its objectives of financial savings and related benefits.
Over the years, the Greens Bayou plant had a series of process automation upgrades, and thus was equipped with a hodgepodge of control and instrumentation equipment. This included pneumatic and conventional analog instruments, aging distributed control systems (DCSs), numerous programmable logic controllers (PLCs), and other special function subsystems.
The plant operated from three control rooms with the human-machine interface (HMI) consisting of conventional panelboards and DCS operator stations. Additionally, boilers were operated from conventional panels in the utilities building.
In the late 1990s, Merisol decided to migrate Greens Bayou's various instruments and controls into a single, fully integrated, plantwide control system. In particular, the company was seeking a total plant architecture integrating displays and navigation and allowing users at any HMI station, in any part of the plant, to have full access to the control system.
Merisol's controls integration began with engineering and design work for Phase One of the project, which focused on the seven units making up the refinery's Crude Unit. Early decisions included: installation of a total plant, fiber optic, high-speed Ethernet control network connecting existing PLCs and all locations with HMI stations and control system subsets; and integration of navigation and unit displays based on a common approach and dynamic subpictures.
Merisol also decided to integrate its plant information database with an application to accumulate historical information for all process variables, calculated variables, and manual input data at a five-second granularity (excluding manual inputs). Additionally, the control database server would act as the interface to the plant network, providing users with access to daily management and operating reports.
At the time Merisol was developing the basic engineering and funding for the total plant integration project, engineers believed additional PLCs with remote I/O were the logical solution for replacing Greens Bayou's aging DCS systems and other disparate controls. Factors such as the sequential control requirements of the initial units supported this view.
Enter FOUNDATION Fieldbus
Despite its plans for an expanded use of PLCs, Merisol decided that an enterprise automation system based on FOUNDATION fieldbus technology would better support total plant integration, while co-existing with control systems already operating throughout the facility. Unlike PLCs, fieldbus provides robust diagnostics allowing for asset management on field devices and enabling a plantwide preventative maintenance program. Additionally, equipment costs associated with a fieldbus installation would be at least 10 percent lower than with PLCs.
Key to Merisol's choice of FOUNDATION fieldbus was the release of specifications for the High Speed Ethernet (HSE) development. The HSE protocol offered all of the characteristics essential for plantwide integration, including plug-and-play equipment, redundancy with double tolerance (bus and equipment), scalability, and "bridging" to support device interoperability with another bus technologies.
HSE, together with the FOUNDATION H1 (31.25 kbit/s) fieldbus, provided a complete, complementary fieldbus solution: H1 fieldbus for continuous control, and HSE (100 Mbit/s) for high-performance control applications and plant information integration. Within this architecture, HSE functions a high-speed Ethernet backbone for device, subsystem and enterprise integration. It also supports high-performance control applications using the same open and interoperable function blocks as devices on the H1 network (Fig. 1).
For Merisol, an important feature of HSE was its ability to provide the common interface needed for plantwide data servers to access control information. HSE servers deliver the information to “clients” across the enterprise, and software application packages and workstations use the clients for functions such as operator display, configuration and maintenance, plant optimization and supervisory control, enterprise resource planning, and building of databases or “warehouses” for access by business systems.
In addition, Merisol was attracted to FOUNDATION fieldbus' ease of implementation. The company was seeking a migration strategy allowing fieldbus instrumentation to be implemented throughout the operating units "one bite at a time," thus eliminating the need for a large-scale shutdown. Because of the simplicity of fieldbus wiring and configuration, plant personnel could replace conventional instruments with new fieldbus devices with minimal training.
Merisol also believed it was important to adopt fieldbus at the top side of the product lifecycle. In this way, the company would realize long-term benefits from its early experiences with this new technology, while keeping pace with the latest developments by downloading new firmware as it becomes available.
Most importantly, Merisol regarded FOUNDATION fieldbus as a natural migration from 4-20 mA technology which would restore single-loop integrity to the plant. Fieldbus allows almost all regulatory and advanced control functions to be performed in field devices such as transmitters and smart valve positioners/controllers. With control in the field, the loss of a single device only affects a fraction of the overall control capabilities.
HSE Bridge Holds The Key
Despite the growing availability of FOUNDATION fieldbus-based control equipment, Merisol found only one supplier (Smar International, Houston, Texas) providing an enterprise automation system fully implementing the FOUNDATION HSE protocol. At the time, Smar also offered the only HSE linking device that had passed the Fieldbus Foundation's device registration process.
In the open, distributed fieldbus architecture, the HSE linking device holds the key to integrating H1 fieldbus segments into the HSE backbone. The linking device functions as a "bridge" between HSE and H1 channels, interconnecting field devices to each other and to other hosts, and is an essential component for integrating system communication with direct I/O access and advanced control applications (Fig. 2).
Total Plant Architecture
Working with Smar as its main automation supplier, Merisol developed a new, FOUNDATION fieldbus-based control architecture providing seamless integration of control equipment, field networks, databases, HMI displays and other subsystems at the Greens Bayou refinery.
As part of the control system architecture, six multi-segment bridges are connected to the HSE backbone at judicious locations throughout the plant. The bridges not only support communications between H1 fieldbus segments, but distribute field device information accessible from any HMI station via an OPC (Ole for Process Control) server. This approach permits the co-existence of different systems by allowing displays for each unit to incorporate tags from PLCs, HMI applications, and fieldbus devices connected to any segment (Fig. 3).
The control system's tree topology supports 12 field devices per H1 segment, for a total capacity of 48 devices per bridge. The bridges are connected via armored cable to junction boxes which, in turn, are wired point-to-point to field devices on each segment. Power supply redundancy at the termination of each segment ensures devices remain powered even in the event that power is lost to the bridge.The open, distributed nature of the fieldbus architecture enables Merisol to implement more than 95 percent of regulatory control, and most advanced control functions, in field devices. In doing so, the plant will eliminate the complexities of the DCS environment and return to the reliability and robust performance of single-loop control. Personnel can also implement standardized control strategies on a loop-by-loop basis.
Phase One of the integration project has involved the installation of nearly 175 fieldbus devices, resulting in 732 tags in the HMI, for the plant's initial unit. This equipment was supplied by seven different control and instrumentation vendors, and ranges from HSE linking devices with back-up power supply and Syscon configuration and operation software, to an assortment of fieldbus-based pressure, flow and temperature transmitters; Vortex transmitters; flowmeters; 4-20mA to fieldbus transducers; and smart valve positioners.
Phase Two of the project will address the distillation train in Greens Bayou's Finished Products Unit, and is expected to include more than 600 fieldbus devices and over 800 tags.
As with any new technology, Merisol faced numerous hurdles during the implementation of a FOUNDATION fieldbus control system. Indeed, the Greens Bayou project was one of the first major installations of a complete HSE system. Many of the challenges were anticipated by plant management, others were not. Nevertheless, the company's total plant integration project demonstrated how end users and suppliers, working in close cooperation, can overcome technology issues in order to produce successful results.
The difficulties encountered, and subsequently resolved, by Merisol and its automation suppliers mostly involved configuration, installation and commissioning of the new control system and field instruments. In many cases, the interoperability of the multi-vendor system architecture was put to the test. Examples include: data transfer between devices on a common bridge, communication failures upon connection of devices from various suppliers, variations in tag value readings, devices disappearing from the live list, missing function block documentation, transmitter link failures, difficulty of communication between devices with different description revision level, etc.
Interoperability testing for the fieldbus system proved to be a critical task, and was aided by the staff of the Lee College Center for Digital and Fieldbus Technological Education in nearby Baytown, Texas. Working together, engineers from Merisol, Smar and Lee College were able to identify the nature of various device communications problems and implement effective measures to ensure the required interoperability between different suppliers' controls and field instruments.
The aforementioned observations should not be taken as a criticism of the technology in general, or any product in particular. Rather, they illustrate the "real world" experiences of a process plant seeking a total change in its automation strategies through an innovative, ground-breaking control system architecture. In the end, technical challenges were overcome as a result of supplier commitment to leading, negotiating and affecting solutions that ensured complete interoperability.
Merisol attained significant operational improvements and business benefits from the total plant integration project. These include:
Total plant integration at the Greens Bayou plant has proven to be a challenging, but worthwhile, undertaking. By employing FOUNDATION fieldbus HSE technology, Merisol has achieved a modern, fully integrated plant architecture assuring its personnel of access to critical data and displays across the enterprise. Furthermore, by replacing its outdated, centralized controls with a fieldbus-based system that restores single-loop integrity, Merisol has taken a major step towards ensuring superior plant performance for years to come.
About the Authors
Henry Marks, Marks and Associates
Since joining Honeywell in Montreal, Canada, in July 1947, Henry Marks has been closely involved in all aspects of instrumentation and automation. This has included domestic and worldwide sales and marketing, as well as product management in a wide range of industries. Of note has been product management of TDC2000, TDC3000, and the third generation of DCS. Based on his early interest in FOUNDATION fieldbus, the fourth generation of industrial automation, Henry has been responsible for detailed engineering for the first and second phases of Merisol's Greens Bayou project. He also served as project manager for implementation of the first phase. Henry Marks can be reached at email@example.com
Jimmy Keys, Merisol
Jimmy Keys has had a long term involvement in the operation and control of chemical manufacturing. He joined the Merisol Co. in March 1979, becoming responsible for pneumatic, DCS, PLC and other specialized systems at the Greens Bayou facility. With the advent of HSE, he became interested in FOUNDATION fieldbus and its implementation at Merisol, being responsible for all aspects of installation, networking, configuration, interoperability testing and loop integrity. Jimmy can be reached at firstname.lastname@example.org