10 I PROCESS INDUSTRY I quantified risk assessment I
Full pressure leaks This scenario category is intended to be consistent with QRA models that assume a leak through the defined hole, beginning at the normal operating pressure, until controlled by isolation and blowdown, with a probability of isolation/blowdown failure. This is subdivided as follows: ■■
Full leaks which are intended to be consistent with QRA models that assume a leak through the defined hole, beginning at the normal operating pressure, until controlled by ESD1 and blowdown, with a small probability of ESD/blowdown failure. These are subdivided as follows – ESD isolated leaks, which are defined as cases where the outflow quantity is comparable with that predicted for a leak at the operational pressure. – Late isolated leaks, presumed to be cases where there is no effective ESD of the leaking system, resulting in a greater outflow quantity. Late isolated leaks are defined as cases where the outflow is greater than predicted for a leak at the operational pressure controlled by the slowest credible ESD and no blowdown.
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Limited leaks, presumed to be cases where the outflow quantity is significantly less than from a leak at the operational pressure controlled by the quickest credible ESD (after 30 seconds) and blowdown (according to API) initiated 60 seconds later. This is presumed to be cases where there exist restrictions in the flow from the system inventory, as a result of local isolation valves initiated by human intervention or process safety systems other that ESD and blowdown.
Normally a quantitative risk assessment will assume that all leaks are full leaks because these have the potential of developing into serious events endangering personnel and critical safety functions. From these leak frequencies the analyst can use a standard event tree approach for the subsequent consequence assessment. This includes probabilities for ESD and BD failure.
Limited leaks may be of as much concern for personnel risk as full leaks in the period immediately following the start of the release, but they will have a shorter duration. Hence the potential for them developing into any major concern for other safety functions, such as structural integrity, evacuation means, escalation, etc. Any consequence calculations should be modelled as for ESD and late isolated leaks, but
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reflect that these events involve reduced release mass and durations.
Zero pressure leaks This scenario includes all leaks where the pressure inside the leaking equipment is virtually zero (0.01 barg or less). This may be because the equipment has a normal operating pressure of zero (e.g. open drains), or because the equipment has been depressurised for maintenance, but not de-inventoried. These leaks may typically be very small gas releases, short lasting oil spills, or liquid releases from atmospheric tanks. Most likely they represent a significantly reduced major accident risk potential relative to a pressurised release through the same hole size (although they do pose occupational safety issues) and the contribution to the overall risk level as predicted in QRA studies is considered negligible.
4.3 UNCERTAINTIES There are several significant uncertainties in fitting a curve to the available leak data. Some uncertainties are due to the way that leak data is reported to the HSE and the lack of data for larger events. Other sources of uncertainty include issues about an accurate population count and the accuracy with which the operator records the data. The sources of uncertainty are illustrated in Figure 3.
4.4 ALLOCATION OF LEAK EVENTS The method of allocating leak records in HCRD into the scenarios is as follows: ■■ Identify the zero pressure leak events in order to discount them from the analysis ■■ Estimate the initial release rate Qo from the hole, based on parameters recorded in HCRD, ■■ Estimate a range of plausible release quantities, REmin to REmax, based on typical ESD and blowdown response ■■ Compare the recorded release quantity in HCRD to the estimated release quantity range to determine the scenario. The scenario allocation criteria are (in order): Zero pressure leaks – actual pressure in HCRD < 0.01 barg. ■■ Limited leaks – recorded release quantity in HCRD < REmin·/ D ■■ ESD isolated leaks – recorded release quantity in HCRD in the range REmin / D to REmax· ■■ Late isolated leaks – recorded release quantity in HCRD > REmax· ■■
With the assumption that process shutdown (PSD) is the shutdown of a particular section rather than the whole platform the PSD system may have the same effect as far as QRA modelling of release rates is concerned.