Leak Sizes

Leak size determination at a refinery heat exchanger... 

The combined result of two such determinations yielded a leak size figure of 8.8% of the feed flow (with a relative standard deviation of less than 5%). This figure could sufficiently explain the product quality problems experienced, whose alternative explanation in turn was catalyst poisoning. 

The leak size can be reduced by using double mechanical seals or a mechanical seal and a throttle bush, the space between the two being vented to a safe place. Major leaks may still occur, however, due to collapse of the bearing or seal. LFG pumps should therefore be fitted with emergency isolation valves (see Section 7.2.1), particularly if the temperature is low or the inventory that can leak out is high. 

Crawley and Grant (1997) have developed a risk assessment tool for new offshore installations. They have examined typical leak frequencies of equipment items and the ignition probability of these leaks in four pressure bands. With this information it was possible to define leak size and frequency for any piece of equipment and the ignited leak frequency. In off-shore installations gas separation vessels were found to have ten times higher ignited event frequency than oil pumps. 

In theory, there are an infinite number of leak sizes, ranging from a tiny pinhole to a full rupture of piping or equipment. It is clearly impractical to investigate them all. Thus, some practical guidance is necessary in selecting leak sizes that will allow a reasonable range of fire scenarios to be evaluated. 

Fire hazard calculation techniques for combustible and flammable liquids and gases range from the basic rule-of-thumb to the sophisticated, including computer modeling techniques. A relatively simplistic approach is adopted for this FHA framework in recognition of the uncertainty of other inputs to the FHA (e.g., leak sizes, orientations, ignition delays, and total volume of discharge). 

Leak rate, leak size, mass flow... 

Under the assumption of a reformer tube rupture creating a leak size of 80 cm with process gas to escape into the containment over 20 s before the stream is automatically cutoff, an estimated 3 % of flie containment volume is occupied by the process gas. Furthermore, pessimistically assuming that a stoichiometric mixture is formed and explodes, results in an overpressure of 46 kPa within 3 s which is expected to cause no damage to the containment [10]. 

Typical leak size is usually divided into small, medium, large and extra-large level. In this simulation, in order to determine the consequences of the accident in the ordinary and extreme cases, main consideration is taken into pipeline medium aperture leak and large aperture leak. According to the criteria recommended by DNV (TNO 1997, TNO 1992, TNO 1988, TNO 1999), this paper determines the representation of aperture 100 mm (large aperture) and 1200 mm (complete rupture). 

In all the events irrespective of a leak size no increase in pressure in the expansion tank up to the alarm setting has been observed and the operational power plant safety has remained within the limits. 

The second step characteristics comprises the initial and boundary conditions for assessing the consequences of the event sequence for employees and the population at large (Example leak cross section 10 cm, elevation of release point 10 m, pressure difference 500 kPa). It makes sense to assign the different sequences to categories (e.g. small leaks, medium leaks, large leaks, fires, explosions etc.), each of them representing several event sequences by one set of initial and boundary conditions. This set must lead to the most severe consequences of all the event sequences covered by the category and hence be conservative. The boundary conditions usually are stochastic, i.e. at most the probability of occurrence, for example for the above mentioned leak, may be indicated. It is normally not equal to 1, as supposed in the deterministic approach. Other leak sizes and locations are, of course, possible. 

The methods of calculation employed are those which are used as well for deterministic analyses. The difference is that stochastic boundary conditions, which are closer to reality, are used for the calculations. For example, instead of a fixed leak size a whole spectrum of leak sizes is treated with pertinent expected frequencies of occurrence being assigned to the different leak sizes. Instead of calculating the dispersion of a toxic substance based on a specific weather situation, different possible weather situations with their corresponding probabilities of occurrence are accounted for. This is reasonable, since the instant in time of the accident and the weather condition, which then prevails, are not known beforehand. 

The leak sizes are the starting point for accident consequence calculations. If the results of the latter are combined with the corresponding expected frequencies of leak occurrence we obtain an estimate of the risk. 

Given the difficulty of determining leak sizes and the frequencies of their occurrence these are usually fixed in safety reports (deterministic approach). An important role is played by the leak before break criterion, which implies that a leak of stable size formed before large area leaks in vessels or full cross section ruptures of pipes occur. However, the applicability of this criterion is subject to numerous prerequisites being fulfilled. Details can be found in [7]. 

In Eqs. (10.1)-(10.3) Dl denotes the diameter of the leak in mm (a circular leak geometry is assumed) and DN is the nominal diameter (occasionally called nominal bore) of the pipe (approximately equal to the internal diameter in mm). Equation (10.3) is the only one to estabhsh a relationship between leak size and its expected annual frequency of occurrence h. The latter refers to a length of 1 m and must therefore be multiplied by the length of the pipe under consideration. Equation (10.3) is based on evaluations for steel pipes in the process and petrochemical industries. 

Only the total failure of the pipeline is considered. It is regarded as dominating aU leak sizes. 

The masses of materials potentially involved in accidents are derived from accident reports. The main bases were the ARIP data bank of the U.S. Environmental Protection Agency [7] and the ZEMA data bank of the German Environmental Protection Agency [12]. The released quantities are regarded as random variables and represented by probability distributions. Thus discussions on assumptions of leak sizes, of pressure differences between the interior of vessels and their exterior as well as of durations of releases become superfluous. Table 12.3 gives examples for mean values of releases from process plants and storages. 

A pipe break need not be assumed if a successful qualification for leak before break, for break preclusion or for low probability of failure has been performed for the piping under consideration, resulting in a sufficiently low frequency of the occurrence of a spontaneous break In general, a fracture mechanics analysis should be performed to calculate the leak size. In lieu of such an analysis, a subcritical crack corresponding to a leak size of 10% of the flow cross-section should be postulated The leak detection system should be shown to have a sensitivity that is adequate to detect the minimum leakage from a crack that is just subcritical. 

FIGURE 8.1. Steam loss rates at varying leak sizes and steam pressure. (From Kenney (1984), reprinted with permission by Elsevier.)... 

This conditional modifier is intended to represent such factors. However, there is insufficient information available at present to know which of the above factors, if any, are relevant to the probability of explosion. Nor is it clear whether commonly used generic probabilities of explosion (typically derived from onshore and offshore process data and applied to a wide range of leak sizes with some or no relationship to leak size) can be applied to the type of event considered in this report. 

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