Safety margins

To lessen the risk of pumping sludges or slurries into a unit, the practice is to leave a safety margin of 50 cm (heel) below the outlet nozzle or install a strainer on the pump suction line. The deposits accumulate with time and the tanks are periodically emptied and cleaned. 

State-of-the-Ahi Control. Computer control using feed-forward capabiUty can save 2—20% of a unit s utiUties by reducing the margin of safety (5). Unless the discipline of a controller forces the reduction of the safety margin, operators typically opt for increased safety. Operators are probably correct to do so when a proper set of analy2ers and controllers has not been provided and maintained. 

The vapor pressure may be dependent on the amount of the dissolved, not the entrained, air in the Hquid. This issue is important to appHcations of cooling-water double-suction pumps (58,59). Because of the unknowns, a safety margin is always recommended for use to minimise the effects of cavitation. 

Chemical Toxicity. Radiopharmaceuticals are subject to the same requirements for safety as are other pharmaceuticals, and are tested for chemical toxicity in much the same manner. It is generally understood, however, that patients are likely to receive relatively few doses of any given radiopharmaceutical so that the effects of long-term chronic exposure to the compound rarely need be assessed. Safety margins, that is, the ratio of the adininistered dose to the lowest dose that produces an observable effect, are usually on the order of 100 or more. 

The LOC depends upon the combustible material and the type of inert gas used. It decreases with increased temperature and pressure. A distinction has to be made between the determined LOC value and the concentration which results by subtracting a safety margin. 

There is still a substantial safety margin up to the ultimate tensile strength, which amounts to 60 to 90 percent, depending on the steel (Kirby, Siwek, Treventing Failures of Equipment Subject to Explosions, Chemical Engineering, Jtine 23, 1986). 

The definition of NPSHr may change in the future. A pump is in a definite state of cavitation with the 3% total head loss definition. Many pump users want a more explicit definition of NPSHr, and higher NPSHa safety margins to avoid inadequate NPSHa and cavitation altogether. 

Remember from Chapter 2, the NPSHa formula is NPSHa = Ha + Hs - Hvp - Hf - Hi. If you want to raise the NPSHa, it will be necessary to increase the elements (Ha, Hs) that add energy to the fluid, or decrease the elements (Hvp, Hf, Hi) that rob e- rgy from the fluid. Also remember that the NPSHr reading, printed on a pumn c -ve, currently represents a point where the pump is already suffering a 2. ss in function due to cavitation. Some people in the industry are calling for a more precise definition of NPSHr, and higher safety margins on NPSHa. 

In summary, for a eomponent/eharaeteristie it is possible to define an area of aeeeptable design on a graph of oeeurrenee versus severity. The aeeeptability of the design ean be enhaneed somewhat by the addition of inspeetion and test operations. The requirements of proeess eapability may be relaxed to a degree, as the eonditional probability of failure reduees, but this should be subjeet to a generous safety margin. 

The approaeh taken by Carter (1986, 1997) to determine the reliability when multiple load applieations are experieneed (equation 4.34) is first to present a Safety Margin, SM, a non-dimensional quantity to indieate the separation of the stress and strength distributions as given by ... 

Figure 4.30 Relative shape of loading stress and strength distributions for various loading roughnesses and arbitrary safety margin...

Figure 4.31 Failure probability (per application of load) versus safety margin for various loading roughness values (adapted from Carter, 1997)...

Again, a preferred value for the aetual seetion width would be Z) = 110 mm. In a more simplified way than that presented in Seetion 4.8.4, we have separated the failure of the pin from the eon-rod by approximately 3 + 3 strength standard deviations. The aetual separation ean be modelled for the distribution of the shear foree in the pin and tensile foree in the eon-rod, as illustrated in Figure 4.69. The safety margin, SM, is ealeulated to be 4.61, or defined another way, the reliability R = 0.999998 whieh is adequate for the applieation to avoid overdesign of the eon-rod. 

If auto-ignitions occur, then the design does not have sufficient safety margin between the auto-ignition delay time for the fuel and the residence time of the fuel in the pre-mix duct. Auto-ignition delay times for fuels do exist, but a literature search will reveal that there is considerable variability for a given fuel. Reasons for auto-ignition could be classified as follows ... 

The what-if analysis stimulates a PrHA team to ask What-if Through questions, the team generates a table of possible accidents, their consequences, safety margins, and mitigation. The accidents are not ranked or evaluated. 

The Seismic Safety Margins Research Program developed a computer code called SMACS (Seismic Methodology Analysis Chain with Statistics) for calculating the seismic responses of structures, systems, and components. This code links the seismic input as ensembles of acceleration time histories with the calculations of the soil-structure interactions, the responses of major structures, and the responses of subsystems. Since uses a multi-support approach to perform the time-history response calculations for piping subsystems, the correlations between component responses can be handled explicitly. SMACS is an example of the codes that are available for calculating seismic response for PSA purposes. 

The first step-in plant-system and accident-sequence analysis is the identification of earthquake-induced initiating events. This is done by reviewing the internal analysis initiating events to identify initiating events relevant to seismic risk. For example. Table 5,1 -5 shows the initiating events that were used in the Seismic Safety Margins Research Program for a PWR plant (Smith et al., 1981)... 

A flatter flux profile provides wider safety margins than previous designs. 

Adjustable speed motor driven feedwater pumps and high-capacity control rod drive pumps with backup power improve the safety margin by improving correct operator response to non-routine events. 

There is some concern that PSA will be used to erode safety margins. 

In risk characterization, step four, the human exposure situation is compared to the toxicity data from animal studies, and often a safety -margin approach is utilized. The safety margin is based on a knowledge of uncertainties and individual variation in sensitivity of animals and humans to the effects of chemical compounds. Usually one assumes that humans are more sensitive than experimental animals to the effects of chemicals. For this reason, a safety margin is often used. This margin contains two factors, differences in biotransformation within a species (human), usually 10, and differences in the sensitivity between species (e.g., rat vs. human), usually also 10. The safety factor which takes into consideration interindividual differences within the human population predominately indicates differences in biotransformation, but sensitivity to effects of chemicals is also taken into consideration (e.g., safety faaor of 4 for biotransformation and 2.5 for sensitivity 4 x 2.5 = 10). For example, if the lowest dose that does not cause any toxicity to rodents, rats, or mice, i.e., the no-ob-servable-adverse-effect level (NOAEL) is 100 mg/kg, this dose is divided by the safety factor of 100. The safe dose level for humans would be then 1 mg/kg. Occasionally, a NOAEL is not found, and one has to use the lowest-observable-adverse-effect level (LOAEL) in safety assessment. In this situation, often an additional un-... 

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