Protection layers plant design

The epidermis is a designation of location rather than type of cells. The epidermal layer of vertebrates, which is the outer layer of skin, is usually made up of stratified epithelium with an outer layer of dead cells and an inner layer of growing and dividing cells. The invertebrate epidermis is normally one cell thick and often forms a protective cuticle (Hale et al., 1995). The epidermis of plants is a one-ceU thick tissue that surrounds young roots, stems, and leaves. The epidermal cells (not epithelium) of stems and leaves secrete a cuticle (a protective layer of protein or lipids). 

SIS This is the first automatic protection layer to BPCS and second overall layer of protection. It is desired that this shall be independent of BPCS. Even if these are combined it is necessary to ensure that single failure does not take toll of safety. SIS may stop part of plant operation and/or diverts some flow safely, etc. It may have separate set of instrumentation to detect and take safety action in the event of instrument/system failure. It has to be more aggressive than BPCS for safety functions. Under SIS, there will be several interlocks and protections to save the system and in many places like off shore design, ESD is considered as last resort or emergency plan achievable through PEs. 

Design steps for a safe system. ALARP, as low as reasonably practicable BPCS, basic plant control system DCS, distributed control system IPL, independent protection layer PLC, programmable logic controller SIS, safety instrumentation system. 

The use of the event tree method is most appropriate when there are many safeguards and protective layers between the initiating event and the final outcome. In the nuclear industry, the scenario of greatest interest is loss of cooling to the reactor core. Were this event to occur and the subsequent safeguards not to work properly, then the reactor could melt down. Obviously, such a scenario is very serious, so the engineers responsible for designing nuclear power plants incorporate many layers of safety, thus... 

In general, the safety of a process relies on multiple layers of protection. The first layer of protection is the process design features. Subsequent layers include control systems, interlocks, safety shutdown systems, protective systems, alarms, and emergency response plans. Inherent safety is a part of all layers of protection however, it is especially directed toward process design features. The best approach to prevent accidents is to add process design features to prevent hazardous situations. An inherently safer plant is more tolerant of operator errors and abnormal conditions. 

The design of most process plants relies on redundant safety features or layers of protection, such that multiple layers must fail before a serious incident occurs. Barrier analysis ) (also called Hazard-Barrier-Target Analysis, HBTA) can assist the identification of causal factors by identifying which safety feature(s) failed to function as desired and allowed the sequence of events to occur. These safety features or barriers are anything that is used to protect a system or person from a hazard including both physical and administrative layers of protection. The concepts of the hazard-barrier-target theory of incident causation are encompassed in this tool. (See Chapter 3.)... 

The secondary reformer in an ammonia plant is a carbon steel vessel with a dual layer refractory lining. Internal temperatures reach -2,000°F (1,090°C) from burning as a result of air added through a burner at the top of the vessel to the feed gas (hydrogen, carbon monoxide, carbon dioxide, and steam). The burner is a refractory-lined device that is subject to failure if not carefully designed. Quench steam generators have refractory-lined inlet channels and tube sheets. Tubes are often made of carbon steel because the heat transfer from the steam on the outside of the tube is markedly better than that from the synthesis gas inside the tube. As a result, the metal temperature closely approaches the temperature of the steam. The inlet ends of the tubes are protected from the inlet gas by ferrules, usually made of type 310 (UNS S31000) SS with insulation between the ferrule and the tube. The tube material should be selected... 

Clean Air Act. Enacted in 1970 and substantially amended in 1977 and 1990, the Clean Air Act is the longest and most complex of all U.S. environmental statutes. It regulates everything from industrial plants and refineries (so-called stationary sources ) to consumer products (ranging from paints to underarm deodorants) to engines and fuels. It also mandates programs designed to reduce acid rain, to protect the stratospheric ozone layer, and to control emissions of 188 compounds listed as hazardous air pollutants (HAPs). 

Proper designing of the alarm system, as the layer of protection in hazardous plant, will contribute to reducing the hmnan error probability (HEP), what will result in decreasing the risk of potential accidents. The human and organizational factors should be also care fully considered in designing and operating of protections to eliminate or reduce probability of latent and active failures (Kosmowski 2007). 

When utility lines are connected to process lines or equipment, there is a danger of backflow of process material into the utility system and a danger of overpressurization of one system by the other. All connections of this kind require careful study during design and hazard analysis. Many plants review similar connections as a group and develop standard details. This standardization can include formal requirements for the number of layers of protection for each group. 

ANSI/ISA-84.00.01-2004-1 does not require physical separation. However, many owners/operators do use physical separation and diverse logic solvers. It is paramount to remember the primary purpose of many SIS. The SIS is designed to restore the plant to a safe state when the process moves out of the normal operating envelope for a number of causes, including a failure or erroneous operation of the control functions. The need for the SIS to act as a layer of protection in the event of control function failure has resulted in many standards and practices recommending that SIF and control functions be implemented in physically separate and diverse equipment. 

No specific reliability/availability targets are set against each of these categories or classes. There is however a maximum limit set for software-based systems of 10 " PFD. More generally the reliability/availability targets are set in the Plant Safety Design Base and can be set either quantitatively or qualitatively. There is a preference for quantitative plus basic requirements on layers and types of protection. 

Defence in depth is one of the most important principles, since it underlies the safety technology employed in nuclear power plants. All safety activities, whether organisational, behavioural or equipment-related, are subject to layers of overlapping provisions which are designed to ensure that if a failure should occur it would be compensated for or corrected without causing harm to individuals or the public at large. This idea of multiple levels of protection is the central feature of defence in depth, and it is repeatedly used in the specific safety principles applied in nuclear power plants. Two (related) principles of defence in depth are defined accident prevention and accident mitigation [1]. 

The physical protection system is an integral part of the plant layout of AHWR. The plant is divided into a nuclear island and an administrative island. The plant layout is designed with a dual-layered security arrangement to provide enhanced physical protection to the nuclear island. The nuclear island is isolated from the administrative facilities by double-wire fencing with an additional security arrangement. The double fencing also provides for electronic surveillance. Independent roads for patrolling by security personnel are also provided. The plant layout is shown in Fig. 15.4. 

In modern industrial plants, process safety relies on the principle of multiple layers of protection (AIChE, 1993, 2001 ISA, 1996). A typical configuration is shown in Figure 10.1. Each layer of protection consists of a grouping of equipment and/or human actions. The layers of protection are shown in the order of activation that occurs as a plant incident develops, with the most effective layers used first. The basic concept is that an incident should be handled at the lowest possible layer. In the interior of the diagram, the process design itself provides the first level of protection. The next two layers consist... 

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