Being the best
How to achieve best-in-class plant safety

The latest aids to plant safety include high-performance safety instrumented systems for automation and process control, and faster, smaller analytical devices to improve hazard monitoring, as this year’s Achema exhibition has shown.

Safety in chemical plants and refineries is generally good, but there is always room for improvement. Management and an understanding of human factors will always be key factors in operating a safe plant, so comprehensive, company-wide strategies are essential. But new technologies and refinements of existing technologies can play a big part, too. Technology-based examples include safety instrumented systems and alarm management systems to back up process control systems; smart valves and digital positioners to improve control performance; new analytical devices to improve hazard monitoring; wireless data transmission that makes monitoring simpler and cheaper; and ways to reduce the risk of dust explosions.

A key part of any process plant automation scheme is the safety instrumented system (SIS). Acting independently of the normal process control system, the SIS is passive most of the time. It is only called upon to render the plant safe when something goes seriously wrong. An SIS typically consists of sensors, a controller, and final control elements such as valves and relays. When the sensors report a process upset, the controller opens a cooling water valve, vents a reactor or turns off a machine. Safety-related systems are governed by standards from the International Electrotechnical Commission. IEC 61508 covers the design and manufacture of safety systems, while IEC 61511 addresses implementation and operation. The need to keep the functions of the SIS independent of the main process control system has traditionally meant that the two systems are physically separate and distinct, but this has changed in recent years. Companies including Siemens, Emerson Process Management, ABB, Honeywell Process Solutions and Yokogawa have all launched products that integrate the SIS into their respective control systems. Process automation systems that now incorporate SISs include Emerson’s PlantWeb process automation architecture, ABB’s Industrial IT Extended Automation System 800xA, ProSafe-RS from Yokogawa, and Honeywell’s Experion Safety Manager.
Control valves are critical components in most process plants. To monitor control valve wear and ensure that maintenance is carried out in a timely fashion, many process operators now choose digital positioners, for precise positioning with respect to the setpoint, and smart digital interfaces that allow valves to be interfaced to a supervisory control system.
Using algorithms and digital feedback sensors, smart valve assemblies can monitor stiction (the random, jerky motion of the valve shaft or stem that results from high friction in a control valve), reduce wear, and allow operators to plan their maintenance activities more effectively. According to Dresser Masoneilan, today’s smart valve assemblies can also transmit important valve-related information, such as the status of air supply, actuator pressure and valve plug position, to the control system, and announce to a supervisory system that the valve is deviating from its setpoint, that valve actuation is blocked, or that the positioner is struggling to maintain its position. Other leading valve vendors include Asahi/America, Fisher Rosemount, Invensys Flow Control, APV, Swagelok, Stahl-Armaturen Persta, and Samson.
Alarms are a key aspect of plant safety, alerting operators to abnormal situations at an early stage so that they can take action to correct the situation. In many modern plants, however, the number of alarms in use has proliferated—often beyond the point of efficiency—as processes and equipment have become more complex. False, hypersensitive or redundant alarms become a nuisance to operators, who may eventually ignore them or turn them off. According to safety experts at Honeywell, to improve the effectiveness of any plant-wide alarm strategy, operators must establish and analyze alarm performance benchmarks, assess current system performance, and justify all existing alarms to weed out unnecessary or redundant ones. Invensys Process Systems, Yokogawa, APV, Fluid Components International and Omega Engineering also offer expertise and services related to the management of complex, plant-wide alarm systems.
Improved monitoring devices
Workers in chemical plants and refineries face risks associated with exposure to toxic chemicals, flammable gases, and oxygen deficiency in confined spaces. Their primary line of defense against such hazards is the use of fixed-point and portable gas-detection devices, typically based on catalytic-bead, metal oxide semiconductor, infrared, or electro-chemical sensors. Gas detectors are becoming ever smaller, yet more sophisticated. Many are designed to operate maintenance-free in remote locations or confined spaces. Many can monitor many gases concurrently, with sensitivity in the ppm range. Device manufacturers include Sierra Monitor, General Monitors, Ametek Process Instruments, Marathon Sensors, Emerson Process Management, Draeger Safety, Endress+Hauser, MSA Instruments, Siemens, Zellweger Analytics, Sira Technology, Thermo Electron, Industrial Scientific, GFG Instrumentation and Extech Instruments.
Wireless technology
A typical large chemical plant or refinery has thousands of field sensors. The wiring for these is expensive—in fact, the wiring to a sensor in a remote location may cost many times as much as sensor itself.
Wireless sensors, which use radio-frequency signals instead of cabling, can save time and money in both installation and maintenance. Wireless devices are also useful in a portable role, for instance for leakage surveys, because they can be moved easily from one part of the plant to another. Not everyone is convinced that wireless technology is sufficiently robust or secure to be used for critical industrial applications, but there is a consensus that wireless instruments are well-suited to environmental and safety-related monitoring. This is especially true of sources that are remote or inaccessible, and those that produce intermittent emissions.
Vendors of wireless monitoring devices include Accutech, Phoenix Contact, Transcat, Siemens Energy & Automation, Ember, Honeywell Industrial Measurement and Control, Elpro Technologies and others.
Preventing dust explosions
Awareness of the hazards of dust explosions has increased in the last 20 years, but they remain a significant problem wherever products are handled in powder form—which means most plants. According to the US Chemical Safety and Hazard Investigation Board there have been 197 dust explosions in the U.S. since 1980, resulting in more than 109 fatalities, 592 injuries and considerable loss of property. The USA has codes of practice and some state-based programs to mitigate dust hazards, but no comprehensive federal laws on this topic. The European Union is ahead in this area, with directives on explosion protection covering the quality of specific products as well as dust explosion safety in general. The EU member states implement these directives as they see fit; in Germany, for instance, dust explosion safety standards are led by the Federal Institute for Materials Research and Testing (BAM) and the Federal Institute for Physics and Technology (PTB). Data on the flammability of particular dusts is available from databases including Dechema’s Chemsafe. The increased demands on air pollution control have resulted in the increasing use of extraction systems to remove dust from air—which in turn can create a dust explosion risk. Companies such as Ruwac, United Air Specialists, Donaldson Torit, Scientific Dust Collectors, and Flex-Kleen all offer dust collection systems that are said to have improved explosion prevention or protection capabilities built into them. Meanwhile, pressure-relief valves and rupture disks to mitigate the damage from dust explosions is available from companies including Fike, Oseco, Fenwal Protection Systems, Rembe, Continental Disc and BS&B Pressure Safety Management. Comprehensive plant safety requires a firm companywide commitment—a commitment to invest properly in the best tools available, a commitment to train and support workers in the proper techniques, and a commitment to regularly, rigorously review ongoing safety performance.