Causes of overpressure

Pressure-relief-device requirements are defined in Subsec. A. Set point and maximum pressure during relief are defined according to the service, the cause of overpressure, and the number of relief devices. Safety, safety relief, relief valves, rupture disk, breaking pin, and rules on tolerances for the reheving point are given. 

Plants, situations, and causes of overpressure tend to be dissimilar enough to discourage preparation of generalized calculation procedures for the rate of discharge. In lieu of a set procedure most of these problems can be solved satisfactorily by conservative simplification and analysis. It should be noted also that, by general assumption, two unrelated emergency conditions will not occur simultaneously. 

The first three causes of overpressure on our list are more amenable to generalization than the others and will be discussed. 

This section discusses the principal causes of overpressure in refinery equipment and describes design procedures for minimizing the effects of these causes. Overpressure is the result of an unbalance or disruption of the normal flows of material and energy that cause material or energy, or both, to build up in some part of the system. Analysis of the causes and magnitudes of overpressure is, therefore a special and complex study of material and energy balances in a process system. 

Although efforts have been made to cover all major circumstances, the reader is cautioned not to consider the conditions described as the only causes of overpressure. Any circumstance that reasonably constitutes a hazard under the prevailing conditions for a system should be considered in the design. 

Fire as a Cause of Overpressure - Equipment in a plant area handling flammable fluids is subject to potential exposure to external fire, which may lead to overpressure resulting from vaporization of contained liquids. This hazard may exist even in items of equipment containing nonflammable materials. 

Now let us consider utility failure as a cause of overpressure. Failure of the utility supphes (e.g., electric power, cooling water, steam, instrument air or instrument power, or fuel) to refinery plant facihties wiU in many instances result in emergency conditions with potential for overpressuring equipment. Although utility supply systems are designed for reliability by the appropriate selection of multiple generation and distribution systems, spare equipment, backup systems, etc., the possibility of failure still remains. Possible failure mechanisms of each utility must, therefore, be examined and evaluated to determine the associated requirements for overpressure protection. The basic rules for these considerations are as follows ... 

In some cases, the loss of utility supply is not a direct cause of overpressure, but... 

Figure 7-14, Operational Check Sheet [25], lists 16 possible causes of overpressure in a process system. There are many others, and each system should be reviewed for its peculiarities. System evaluation is the heart of a realisdc, safe and yet economJcal overpressure protection installation on any single equipment or any group of equipment. Solving formulas with the wrong basis and/or data can be disastrous. The following should be reviewed ...

Cause of Overpressure Capacity Requirement Split tube(s) 1482 Lb.Hr. 

Rupture disks are used for the same purpose as safety valves and, in addition, serve to relieve internal explosions in many applications. If the pressure rise can be anticipated, then the volume change corresponding to this change can be calculated by simple gas laws, and the capacity of the disk at the relieving pressure is knowm. The system must be examined and the possible causes of overpressure and their respective relief capacities identified before a reliable size can be determined. See Figure 7-14. 

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. 

Because these causes of overpressures are considered random and infrequent, the pressure relief capability has to be automatic and constantly available. 

The first step in designing a pressure-relief system is to evaluate the possible causes of overpressure so as to determine the rate of pressure accumulation associated with each and hence estimate the relief load (the flow rate that must be discharged through the relief device). The API Recommended Practice (RP) 521 suggests the following causes ... 

In summary, the new technique developed at BP uses a proprietary transformation that relates velocity directly to effective stress, temperature and gross lithology, takes account of the major causes of overpressure in clastic basins (namely, undercompaction, clay dehydration and diagenesis, buoyancy and charging of fluids in dipping, permeable beds), and predicts effective stress directly, which is the most fundamental quantity for pressure prediction. 

The pore-pressure profiles for the wells belonging to hydraulic compartments II and III show a rapid increase versus depth for the lowermost Cretaceous-Upper Jurassic interval (Figs. 4 and 14 red trend line). This pressure increase versus depth is higher than lithostatic, and is taken to suggest that the primary cause of overpressure for this interval is related to the pulse of increased maturation and pressure build-up within the Spekk Formation source rock caused by increased maturation and kerogen transformation to liquid hydrocarbons, again primarily in response to the Pliocene to Recent subsidence. 

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