Overpressure scenarios

Creating a strong system that constitutes a passive design requires a complete knowledge and characterization of the potential overpressure scenarios. This requires knowledge of the chemistry outside the design conditions to evaluate effects of loss of utilities and the loss of control systems. 

In this chapter, we will define what is considered a potential overpressure scenario in process systems and where the safety relief valves (SRVs) are needed. 

Active control This involves selecting correct relief systems for the process by analysing different possible overpressure scenarios. 

Develop the different possible overpressure scenarios for a specific pressure-containing vessel. 

Evaluate each piece of equipment in a process for potential overpressure scenario (see also Sections 2.3 and 13.4). 

Once installed and as the process changes, it might be wise to take a dose look at the original selection of SRVs and evaluate whether this selection, made for different process criteria, is still valid. Then, organize compliance audits of pressure relief systems and particularly on individual valves and other devices and re-evaluate the potential overpressure scenarios described earlier in this book. The typical current standard internal audits might not catch the piece of equipment that, for instance, does not have a valve but should. ... 

This was identified as the number of deficiencies in which a piece of equipment had more than one potential overpressure scenario. 

Any situation not covered by the rest described herein, but covered by the 16 overpressure scenarios in API RP 521 Section 3.1. 

Overpressure Scenarios Applicable requirements requirements Comments... 

FMEA in Table 3.7 illustrates an overpressure scenario of the pump by the spurious closing of on-off valve at its discharge. This can generate unacceptable risk for the pump, furnace and people around. A SIF of SIL2 is at least necessary to mitigate this risk when high pressure is detected at the pump discharge. 

Safety relief valve. The plant safety relief valve used in an overpressure scenario is designed not to reseat until primary system pressure has fallen below the set pressure for activation of the passive ADS. 

A High Integrity Pressure Protection System (HIPPS) is a particular type of SIS that prevents overpressure scenarios. It shuts off the source of high pressure before the safe upper limit is reached. It is a barrier between high- and low-... 

Operational Failures A number of scenarios of various operational failures may result in the generation of overpressure conditions ... 

Figure 1 shows part of a solvent phase polypropylene plant. The plant consists of three process lines, denoted A, B, and C. During a risk assessment review, a scenario was identified that involved a release of reactor contents from a location near the west end of the A line. Estimates are needed of the blast overpressures that would occur if the resulting cloud of vapor, mist, and power ignites. 

Each scenario was modeled to determine the explosion overpressure at the three buildings under review. The resultant overpressures, and corresponding vulnerability estimates, for the most severe scenario in each process area are tabulated below. 

It is worth noting that blast capacity of a polycarbonate pane is sensitive to the duration of the blast load. Because of this, the typical short overpressure duration testing of polycarbonate with small close-in charges with frame set-ups that permit a rapid pressure clearing time may give an unconservative estimate of blast capacity in many real world threat scenarios. 

Consequence analysis Once hazards and specific incident scenarios through which those hazards might impact people, the environment, or property have been identified, methods exist for analyzing their consequences (size of vapor cloud, blast damage radius, overpressure expected, etc.). This is independent of frequency or probability. 

Cooling water failure The loss of cooling water is one of the more commonly encountered causes of overpressurization. Different scenarios should be considered for this event, depending on whether the failure affects a single piece of equipment (or process unit) or is plantwide. 

Internal Detonations or Explosions An internal detonation or explosion may occur due to several scenarios. Air leakage into the system may cause a combustible mixture to form, undesired chemical reactions may occur, and extremely rapid vapor expansion may occur. These almost instantaneous events have to be carefully protected against as many overpressure devices do not react quickly enough to prevent the vessel from rupturing. 

Another area of confusion might involve the definition of capacity and how the term is used in ASME and API. Relieving rates are determined from what can go wrong scenarios and, if allowed to go unchecked, would overpressure the vessel. 

Establish an appropriate design basis for each vessel that needs protection based on the different overpressure cases described in Section 2.3. Choosing the correct design basis requires assessing alternative scenarios to find the credible worst-case scenario. 

Overpressure accidents can not only damage equipment but also cause injury or even death to plant personnel. In order to reduce the potential number of incidents or accidents, it is the job of the process engineer to analyse the process design and to determine the what can go wrong scenarios and either find a way to design out of them or provide protection against catastrophic failure in the event an accident does occur, that is, install an SRV and/or rupture disc. 

Figure 8.34. Hypothetical porosity-depth curves for various diagenetic situations. A. "Normal" burial curve of chalk sequences. B. A typical curve stemming from porosity changes brought about by dissolution and cementation processes during meteoric zone diagenesis. C. A scenario for late stage porosity development brought about by a dissolution event related to hydrocarbon maturation and destruction. D. Porosity preservation resulting from conditions of overpressuring. (After Choquette and James, 1987.)...

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