Process control plant safety

As of the early 1990s, annual worker fatalities ran about 9 per 100,000 employees annual lost-time disabling injuries ran about 4,000 per 100,000 employees (1). Property losses increased fourfold from the 1970s (2). The trends in fatalities and property losses can probably be ascribed to the increasing complexity and productivity of the highly automated chemical plants, where personnel are isolated from processes. Whereas exposure to health and safety hazards maybe reduced, the ability of experienced operating personnel to sense process problems and to correct these problems frequently is decreased. Another aspect of process management which has tended to increase hazards is the effort to reduce the formation of wastes and undesired by-products. This effort requires dose approach to temperature and pressure limits, at which points loss of control can be catastrophic (see Process control). Process and plant safety issues have been discussed (3—8). 

VDI/VDE-Richtlinien (Nov 2000). Safeguarding of industrial process plants by means of process control engineeringusing safety-related programmable electronic systems. 

A limited number of companies possess the basic knowledge and the experience which makes them able to prepare the basic engineering information-process flow diagrams with heat and mass balances, equipment specifications, specification of instrumentation and process control including safety precautions, operating manuals etc. -which is required for the design of an ammonia plant. These companies have also developed their own proprietary design of critical equipment, most often the ammonia synthesis converter and the primary reformer. 

The four process control parameters are temperature, pressure, flow, and level. Modem process level detection systems are varied and ubiquitous in modem chemical plants there are thousands of processes requiring Hquid level indication and Hquid level control. From accumulators to wet wells, the need for level devices is based on the need for plant efficiency, safety, quaUty control, and data logging. Unfortunately, no single level measurement technology works rehably on all chemical plant appHcations. This fact has spawned a broad selection of level indication and control device technologies, each of which operates successfully on specific appHcations. 

The pilot plant must also be carehiUy designed so that its control and safety systems are "fad-safe" and any unexpected equipment or utdity fadure brings the unit into a safe and de-energized condition. Unexpected or rapid process changes, if they can herald or lead to dangerous conditions (eg, mnaway exothermic reaction), should be continuously monitored by appropriate instmmentation and suitable automatic action provided (1,55—67). 

As microprocessor-based controls displaced hardwired electronic and pneumatic controls, the impac t on plant safety has definitely been positive. When automated procedures replace manual procedures for routine operations, the probability of human errors leading to hazardous situations is lowered. The enhanced capability for presenting information to the process operators in a timely manner and in the most meaningful form increases the operator s awareness of the current conditions in the process. Process operators are expected to exercise due diligence in the supervision of the process, and timely recognition of an abnormal situation reduces the likelihood that the situation will progress to the hazardous state. Figure 8-88 depicts the layers of safety protection in a typical chemical jdant. 

Although microprocessor-based process controls enhance plant safety, their primaiy objective is efficient process operation. Manual... 

Control of Crushers Lower-grade raw materials, higher energy costs, larger-scale operations, and more complex, capital-intensive plants make automatic control of size-reduction equipment more important (Suominen, 21st International Symposium—Applications of Computers and Operations Research in the Mineral Industry, 1011-1018). Benefits are increased productivity, process stability and safety, improved recoveiy of mineral values, and reduced costs [Horst and Enochs, Engineering Mining J., 181(6), 69-171 (1980)]. 

It was not nndl the 1950s that detonation flame arresters made of crimped metal ribbon elements were developed and began to be used more freqnendy (Binks 1999). The major impetus for die use of crimped metal ribbon detonation flame arresters in the US was the enactment of clean air legislation (Clean Air Act of 1990) which inadvertently created a safety problem by requiring reductions in volatile organic compound (VOC) emissions. To do this, manifolded vent systems (vapor collection systems) were increasingly installed in many chemical process industry plants which captured VOC vapors and transported them to suitable recovery, recycle, or destruction systems. This emission control requirement has led to the introdnction of ignition risks, for example, from a flare or via spontaneous combustion of an activated carbon adsorber bed. Multiple... 

Some questions could arise concerning process control and especially concerning the problem of deviation detection. Solutions can be found by using software able to monitor and supervise the process in order to guarantee production quahty and safety of the plant and operators [38]. 

For accidents affecting process plant buildings, the potential for serious or fatal injury to building occupants is the foremost concern. Additionally, in cases where buildings house critical controls or equipment, proper design and siting may also help reduce indirect safety impacts (e.g., due to loss of process control), as well as business interruption costs and property loss from such events. 

Scale-up can also have a significant effect on the basic process control system and safety systems in a reactive process. In particular, a larger process will likely require more temperature sensors at different locations in the process to be able to rapidly detect the onset of out-of-control situations. Consideration should be given to the impact of higher-temperature gradients in plant-scale equipment compared to a laboratory or pilot plant reactor (Hendershot 2002). 

Process control plays an important role in how a plant process upset can be controlled and subsequent emergency actions executed. Without adequate and reliable process controls, an unexpected process occurrence cannot be monitored, controlled and eliminated. Process controls can range from simple manual actions to computer logic controllers, remote from the required action point, with supplemental instrumentation feedback systems. These systems should be designed such as to minimize the need to activate secondary safety devices. The process principles, margins allowed, reliability and the means of process control are mechanisms of inherent safety that will influence the risk level at a facility. 

Process hazard reviews, in pilot plant safety, 18 733 Process hazards analysis of, 21 861 control of, 21 861-863 preventing, 21 832-846 Process industries, weighing in, 26 248 Processing... 

The American Institute of Chemical Engineers (AlChE) has helped chemical plants, petrochemical plants, and refineries address the issues of process safety and loss control for over 30 years. Through its ties with process designers, plant constructors, facility operators, safety professionals, and academia, the AlChE has enhanced communication and fostered improvement in the high safety standards of the industry. AlChE s publications and symposia have become an information resource for the chemical engineering profession on the causes of incidents and means of prevention. 

---