Design protection systems

Normal loss prevention practices are to design protection systems for the worst case fire event that can occur at a facility (within the limits of probabilities). To interpret this literally would mean in some cases that an oil or gas facility is completely on fire or totally destroyed by an explosion. Practical, economic, and historical review considerations indicate this rationale should be redefined as the Worst Case Credible Event (WCCE) or as referenced in the insurance industry, the Probable Maximum Loss (PML), which could occur at the facility. 

It should be noted that corrosion inhibitors and protection systems ate generally designed for specific conditions, and the effectiveness of the inhibitor can change with conditions. 

Stanley Grossel, President, Process Safety Design, Inc. Fellow, American Institute of Chemical Engineers Member, American Chemical Society Member, The Combustion Institute Member, Explosion Protection Systems Committee of NFPA (Section 26, Process Safety)... 

Where hazardous conditions can develop within a process, a protective system of some type must be provided. Sometimes these are in the form of process hardware such as pressure rehef devices. However, sometimes logic must be provided for the specific purpose of taking the process to a state where the hazardous condition cannot exist. The term safety interlock. system is normally used to designate such logic. 

Depending upon the design criteria of the installed suppression system, an unsuppressed explosion overpressure of around 7 to 10 bar is reduced to a suppressed reduced explosion overpressure which lies in the range of Fred,max = 0-2 to 1 bar. Thus, vessels need to be explosion resistant for an overpressure of maximum 1 bar (ISO Standard 6184/4, Explosion Protection Systems Paii 4 Determination of Efficacy of Explosion Suppression Systems, Geneva, 1985). 

Cathodic protection can be useful, although its ability to protect tube interiors is generally limited to the first 4 to 6 in. of tube length. Such systems, however, must be properly designed and maintained to be effective. Corrosion can be intensified if the polarity of the cathodic protection system is inadvertently reversed. 

Provide damper mechanical position stop to prevent complete closure of damper Eliminate flammables or combustibles Provide inert atmosphere Design ventilation system to keep flammable concentration below lower flammable limit Install on-line flammable gas detection system that activates an inerting system Provide automatic sprinkler protection Use deflagration vents... 

The philosophy to assume the impedance of the source of supply (generator or a transformer) as the impedance of the faulty circuit may be far from reality and may give a very high fault current. In actual operation, the fault intensity may be far less, as every device and component connected in the circuit will tend to add to the effective impedance of the faulty circuit and limit the magnitude of the fault current. Figure 13.15 also subscribes to this theory. But it is customary to design the systems for the worst fault conditions which, in all likelihood, may not arise, and decide the protective scheme and the current settings of the protective relays for the minimum possible fault current. 

Series compensation would mean a low value of Zq and hence a higher system fault level. This needs be kept in mind while designing the system and selecting the switching devices or deciding on the protective scheme or its fault setting. 

The required number, n, of anodes can be calculated using Eq. (17-2) from the current requirement, together with the maximum current output 1 of the anodes. The arrangement of the anodes is dealt with in Section 17.3.2.2. Galvanic protection systems are usually designed to give protection for 2-4 years. After this period, a maximum of up to 80% of the anodes should be consumed. 

Table 12.14 Fire characteristics determining protection system design...

Secondary containment systems are best described as passive protective systems. They do not eliminate or prevent a spill or leak, but they can significantly moderate the impact without the need for any active device. Also, containment systems can be defeated by manual or active design features. For example, a dike may have a drain valve to remove rain water, and the valve could be left open. A door in a containment building could be left open. 

The third line includes fixed fire protection systems (sprinklers, water sprays, or deluge systems, monitor guns, etc.), dikes, designed drainage systems, and other systems that control or iiiitigaic ha. ardous releases. 

Process definition and design criteria Process and equipment design Company memory (management information) Documentation of risk management decisions Protective systems Normal and upset conditions Chemical and occupational health hazards... 

This chapter introduces the basic items of design and specification for the principal systems and components of an electrical industrial installation. Electrical supply systems are discussed with regard to interface with the supply authorities and the characteristics. Salient features of switchgear, transformers, protection systems, power factor correction, motor control equipment and standby supplies are identified and discussed together with reference to the relevant codes of practice and standards. The equipment and systems described are appropriate to industrial plant installations operating at typically 11 kV with supply capacities of around 20MVA. 

Where appropriate, designs should facilitate the application of adequate corrosion-protection systems that can be readily maintained. This can be achieved by attention to the geometry of the initial and any retrofitted design and methods of construction, and by making provision for good inspectability and accessibility. 

Gooch, T. G. and Gregory, E. N., Br. Corros. J., Suppl. issue, Design of Protective Systems for Structural Steelwork, 48 (1968)... 

By contrast a cathodic protection system based on sacrificial anodes is designed from the outset to achieve the required protection potential. If this is not achieved in practice there is no control function that can be exercised to improve the situation. Some remodelling of the system will be required. Moreover, the currents from each current source (the sacrificial anodes) is modest so that field gradients in the environment are not significant. It is at once clear that potential measurements are less significant in this case and instant-off measurements are neither necessary nor possible. 

Recommended Practice Design, Installation, Operation and Maintenance of Internal Cathodic Protection Systems in Oil Treating Vessels, RP-05-75, NACE, Houston (1975)... 

Before a satisfactory cathodic protection system using sacrificial anodes can be designed, the following information has to be available or decided upon ... 

It is important that the correct current density requirement is assigned for design purposes. If too high a value is used the structure may be waste-fully overprotected, whereas a value too small will mean that the protection system will underprotect and not achieve its design life. 

System Life Cathodic protection systems may be designed with a life of between 1 and 40 years. The greater the time of protection, the greater the mass of anode material that is required. 

Obviously, the total weight of the anode material must equal or be greater than the total weight, IF, calculated above. Similarly each anode must be of sufficient size to supply current for the design life of the cathodic protection system. The anodes must also deliver sufficient current to meet the requirements of the structure at the beginning and end of the system life. That is, if current demand increases (as a result of coating breakdown, for example) the output from the anodes should meet the current demands of the structure. 

The latter part of this chapter has dealt with the design considerations for a sacrificial anode cathodic protection system. It has outlined the important parameters and how each contributes to the overall design. This is only an introduction and guide to the basic principles cathodic protection design using sacrificial anodes and should be viewed as such. In practice the design of these systems can be complex and can require experienced personnel. 

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