Controlling Processes Via Fieldbus Increasing Reliability and Reducing Costs

Controlling Processes via Fieldbus Increasing reliability and reducing costs

By John Rezabek

proprietary digital bus and smart instruments can limit chemical Aprocess design flexibility and inflate expansion, modification and life- cycle costs. FOUNDATION fieldbus pro- vides a wide selection of optimum or best-in-class field devices. Fieldbus also offers more features than do “smart” instruments and the digital communications of older distributed control system (DCS) technologies. To automate the first large-scale manufacture of 1,4-butanediol (BDO), BP Chemicals, Lima, Ohio, selected a powerful field-based control architecture based on FOUNDATION fieldbus. The new plant produces BDO directly from butane in a process jointly developed by BP Chemicals and Lurgi Oel-Gas Chemie, Frankfurt, Germany.

Reliability challenges A bank of fieldbus control and I/O modules, shown here, offers fully distributed logic. With a traditional DCS, reliability is maximized through redundancy in studied the failures possible with vari- good enough in the BDO plant because input/output (I/O), wiring, power sup- ous devices. A fieldbus control valve, for many vessels have short residence times plies, controllers and other areas. A DCS example, has few failure modes: 1) the or rapid changeover. Loss of variable concentrates too much logic in the con- fieldbus cable is severed, shorted or valve control requires a heroic effort on trollers. Dozens or even hundreds of opened, 2) the fieldbus card’s power the part of operators to avoid an upset loops are solved by a single processor.* conditioner fails or 3) the valve fails. or a shutdown. Depending on where the Pre-DCS panels of single-loop con- The last mode is irrelevant because the failure of the fieldbus segment occurs, trollers actually made for a more dis- problem and solution are identical in cascade control also will proceed nor- tributed architecture. both fieldbus and DCS architectures. mally if the cascade masters and slaves Centralizing DCS logic in a rack room System reliability can be maximized are distributed to the field as well. and fanning out hundreds of cables to 4- by taking the PID logic out of the BP’s failure analysis also indicated 20mA field devices creates vulnerabilities process controller and distributing it to that a highly logic-distributed, fieldbus- in the control system. Without redun- the fieldbus valve controller. If the connected, essentially nonredundant dancy, a single failure can bring down the transmitter is on the same segment, or automation system could provide relia- entire control system. FOUNDATION field- fieldbus H1 network, the entire loop is bility equivalent to a highly fault-tolerant bus creates a path for achieving high reli- solved in the field. If the valve controller DCS. BP then had to ensure high relia- ability with fewer components and less and the transmitter are powered exter- bility was established in the field. complexity, cost and redundancy. nally, the failure of host components In 1998 and 1999, when designing the causes loss of view only. BDO system, the most worrisome ele- When redundancy is With fully distributed logic, the ments of the field-based automation sys- redundant process also proceeds under full variable tem were the H1 segment power During BP Chemicals’ failure-modes- control, which often is superior to pro- conditioners and the physical segment and-effects analysis, plant engineers ceeding “as last instructed.” This is not wiring. BP had no reason to believe H1

*Controller outputs are computed based on inputs using the control algorithms, e.g., proportional-integral-derivative (PID).

Reprinted from Chemical Processing, August 2001 SPOTLIGHT: I NSTRUMENTATION & CONTROL

card failures would be epidemic, but knew fieldbus pigtail cord landing every pair. Defining Fieldbus the card was the only component of the Rigorous wiring was employed for two automation system that had not experi- reasons. First, the connectors would Fieldbus, Fieldbus Foundation, FOUNDATION enced several years of operating service in help prevent inadvertent shorting dur- fieldbus … these similar-sounding terms large numbers of installations. At the ing maintenance. Second, they would have confused many end users. Some time, the automation system did not offer reinforce the idea that fieldbus should users incorrectly regard fieldbus and redundant H1 cards. However, BP deter- not be handled casually. Since the intro- device networks as one and the same. For mined that an operator could handle a duction of short-circuit protectors for the record, however, fieldbus is an all-dig- temporary loss of view, diagnose the failed fieldbus spurs, BP has reverted to tradi- ital, serial two-way communications sys- card and replace it in an hour or less. tional terminations on all new devices. tem that interconnects measurement and BP conservatively placed all cascade control equipment such as sensors, actu- Spreading risk masters in the automation system’s scal- ators and controllers. It serves as a local BP saw no need to establish redundant able controllers instead of on the field- area network (LAN) for instruments used segment pairs. Single-pair fieldbus bus segments. The master’s process in process control and manufacturing home-run cables lie in overhead cable variable (PV) resides on a separate seg- automation applications and has the trays in pipe racks. Drops to junction ment, so if one segment fails, BP would built-in capability to distribute the control boxes — and from junction boxes to not lose both primary and slave PV indi- application across the network. devices — are in rigid conduit. A radial cations. If the company were to design Demand for fieldbus technology is topography for the 71 segments was the BDO plant again, it would probably reportedly being driven by the desire of selected, with an average of five or six place both masters and slaves on the manufacturing end users to move away devices per segment. Some critical seg- same segment because fieldbus H1 com- from older, centralized plant control strate- ments have as few as two devices. munications now are proving durable gies to distributed control in the field. Radial topography and low segment and reliable. Also, with cascade control However, the desire for the more complete loading were other conscious choices to removed from the process controller, information and additional process diag- ensure reliability. BP classified every reliability would improve because data nostics associated with field-based control field device by one of three levels of crit- interchange between cascaded devices architecture is a major influence as well. icality. The most critical — plus those would be closer to the process. The Fieldbus Foundation is an organiza- requiring the fastest response — reside The only 4-20mA in the new BDO tion dedicated to a single international, on the least-loaded segments. process is for large compressor blowoff interoperable fieldbus standard. Estab- After gaining experience with the valve actuators, large double-acting lished in September 1994 by a merger of new automation system, BP felt com- sliding-stem valve positioners and local WorldFIP North America and the Inter- fortable switching to a hybrid multidrop PV indicators at control valve bypasses. operable Systems Project (ISP), the foun- fieldbus topology for a process revision dation is a not-for-profit operation that underway. Instead of running new con- Single process consists of nearly 120 suppliers and end duit to the top of a reactor, the company control platform users of process control and manufactur- stubbed into an existing instrument, A programmable logic controller (PLC) ing automation products. added a junction box, made the box the was not selected to handle the BDO The foundation’s technology, FOUNDATION termination of the fieldbus home run process’s 400 nonemergency shutdown fieldbus, stands out from other fieldbus ini- and then wired to the new instruments. (ESD) discrete points, avoiding a third tiatives because it is designed to support The fastest macrocycle available in platform beyond the process automa- mission-critical applications where the the fieldbus implementation is 250 tion and the ESD systems. The discrete proper transfer of data is essential. Unlike microseconds (mS). The most demand- points are hardwired to process con- proprietary network protocols, FOUNDATION ing in terms of response time are field- troller I/O modules. fieldbus is an open interoperable fieldbus bus devices in several compressor All five of the automation’s dual- that is based on the International antisurge controls. A large number of redundant process controllers are located Standards Organization’s (ISO) Open Sys- function blocks must be solved; there- in a single cabinet in an equipment room tem Interconnect seven-layer communica- fore, these controls run at a 500-mS adjacent to the control room. Because the tions model. The FOUNDATION fieldbus specifi- cycle and do not follow the surge curve BDO plant is less than 500 meters long, cation is compatible with the officially as closely as they might with a very end-to-end, operators were not moti- sanctioned SP50 standards project of The high-speed processor. vated to distribute controllers to the field. Instrumentation, Systems and Automation The controllers and the control console’s Society (ISA) and the International Electro- Landing fieldbus leads seven sets of dual-monitor PC operator technical Committee (IEC). The BP fieldbus installation included a stations, as well as the plant’s engineering To learn more about Fieldbus Foun- hermetically sealed manual disconnect and application workstations, are net- dation activities, visit www.fieldbus.org. switch in the spur to every device, plus a worked on IEEE 802.3 Ethernet. CP Staff weatherproof circular connector on the An RS232C serial link allows the

Reprinted from Chemical Processing, August 2001 SPOTLIGHT: I NSTRUMENTATION & CONTROL

ESD system to publish data to the tus and the PID block adjusts its mode Many operator benefits automation system via Modbus. Unfor- appropriately, perhaps switching to man- BDO plant operators now can see con- tunately, too much BDO project engi- ual mode. Or if an impulse line becomes trol valve positions on their displays. neering time was spent mapping the plugged, the transmitter changes signal Every faceplate and valve icon indicate ESD system to the automation system. quality status from “good noncascade” to true position with every scan, which is Future projects requiring an ESD will “uncertain.” This flags the operator that extremely helpful when balancing employ an OLE for process control he might have a plugged line. flows during startup. Visible valve (OPC)/Ethernet link. Mode shedding is transparent and motion also informs the operator of permits graceful degradation of control. the presence of a poorly tuned or slug- Easy configuration Graceful degradation is of great value in gish valve. In a joint effort, BP Chemicals and an the BDO plant because it reduces Most instrument boxes incorporate engineering consultant configured the process upsets caused by instrument heaters to stave off cold weather prob- automation system in International Elec- malfunctions. The PID block also lems. Instrument case temperatures are brought back to a special display so operators can watch for impending freezeups or for devices being heated with steam hose. Transmitters in vapor service are kept warmer than others, and the difference can be monitored. All Coriolis mass flowmeter readings include calculated density, which informs operators which material is flowing and its concentration. Specific gravity is especially helpful in determin- ing the demarcation between process water and an acid, for example. BP was able to commission fieldbus devices extremely quickly because the devices reported themselves to the automation system as soon as their wires were landed, or fastened to their permanent wiring terminals. Infor- mation typically stored in a device data sheet became available immedi- ately. All valves were calibrated in place, and all instruments relied on the A termination of a fieldbus spur on a digital valve controller takes the PID logic out of the process manufacturers’ certified calibrations. controller. A calibration trailer was not necessary during construction, saving money trotechnical Commission (IEC) 61131.3 knows if the valve is limited — at one of and time. Function Block Diagram (FBD) and the extremes of its mechanical travel — Sequential Function Chart (SFC) lan- and cascade masters know if their slave’s Future directions guages. Pre-engineered function blocks valve are limited. No special program- BP plans to make extensive use of the such as PID include all necessary logic. ming is required for antireset windup or information available from FOUNDATION With fieldbus function blocks, control bumpless transfer. fieldbus devices to develop expert-sys- engineers need not program status bits to Beyond diagnostics, BP currently is tems operator advisories. The company perform certain functions — they are not taking advantage of the data that can probably will program into logic the ready right out of the box. The PID block, be extracted from fieldbus devices. How- combined intelligence and experience for example, looks at both the measured ever, one node on the automation net- of its engineers, operators and mainte- variable and its status. Fieldbus has more work is a process data server built into the nance personnel to speed troubleshoot- than 80 different designations, allowing automation system. A server-to-server ing and problem-solving and provide the programmers to customize controller link allows a larger historic data server on the best solutions. actions to particular PID inputs. the site’s information technology (IT) If, for example, the status is “good network to upload process data from the Rezabek is a controls specialist for BP noncascade” and an electronics problem automation’s server so customers can Chemicals Inc., Lima, Ohio. Contact him arises, the fieldbus device changes its sta- view recent process information. at (419) 225-3712.

D350848X012 / 00802-0100-2113/ 8K AQ / 10/01 Reprinted from Chemical Processing, August 2001