Overpressure Causes

Remembering the basic principles of flameproof enclosures, two main characteristics of such enclosures shall be proved by type testing they shall withstand the internal overpressure caused by the ignition of a combustible gas-air mixture inside, and they shall prevent flame transmission to the environmental hazardous atmosphere. So, such a test cycle is divided into three parts ... 

The explosion of combustible gas is a very severe problem to keep the reactor safety. The problem can be considered as overpressure caused by blast to the safety-related components. There are three principal countermeasures against explosion, that is, i) place a distance between the reactor and the hydrogen production system enough to mitigate the overpressure within an allowable range, ii) limit the leak amount of combustible gas, and iii) protect blast with barriers such as wall, bank and so on. As for the hydrogen production system connected to HTTR, the countcnncasure-ii) was mainly considered. 

Once a set of premises is available for each failure, the list of possible overpressure causes can be narrowed down. A possible cause can be eliminated from the list if it is certain that its relief requirement is lower than or identical to the relief requirement of another source. For instance, when coliunn pressure is controlled by manipulating cooling water to the condenser, failure of the pressure controller may have identical consequences to coolant failure. In this case, failure of the pressure controller can be eliminated from the list. Another example is a column whose heat-input control valve and all feed control valves fail shut, while cooling is likely to continue normally during an instrument air failure in this case, the relief load is likely to be small (if any) upon instrument air failure, and this cause can be eliminated from the list. 

Once the long-distance natural gas transportation pipeline leaks, in case of immediate ignition, it will lead to jet fire or explosion, while in case of delayed ignition, it will cause vapor cloud explosion or flash fire. The main damage form of the leak is thermal radiation produced by jet fire and overpressure caused by vapor cloud explosion. 

A third negative characteristic is the absence of a complete pressure containment building around the nuclear part of the plant which would resist the overpressure caused by possible accidents. The majority of the world s reactors are contained in this manner in order to prevent the release of radioactive substances to the outside even in serious accidents. Figure Al-1 shows that a large part of the building which contained the Chernobyl reactor was very similar to a light factory shed. 

Moreover, a specific study to generalize the protection of the primary circuit against overpressurization caused by inadvertent operation of the high pressure injection pumps (TQ n3) and high pressure boron injection pumps (TQ n4) is underway. 

Serious and damaging consequences may result from accidents caused by fires and explosions. Therefore, the incineration system and its off-gas treatment system should be designed to withstand the effects of the overpressure caused by an explosion, and provided with a suitably located pressure relief mechanism. Furthermore, the following measures shall be instituted to minimize the potential for explosions or fires ... 

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. 

A blast incident overpressure of 35 kN/m" (5 psi) is often used to define the region beyond which the damage caused will be minor and not lead to significant involvement of plant and equipment beyond the 35 kN/m" boundary. 

A common cause of a BLE T] in plants of the hydrocarbon-chemical industry is exposure to fire. With an external fire below the liquid level in a vessel, the heat of vaporization provides a heat sink, as with a teakettle evolved vapors exit tnrough the relief valve. But if the flame impinges on the vessel above the liquid level, the metal will weaken and may cause the vessel to rupture suddenly, even with the relief valve open. The explosive energy for a BLE T] comes from superheat. This energy is at a maximum at the superheat hmit temperature. (SLT is the maximum temperature to which a hquid can be heated before homogeneous nucleation occurs with explosive vaporization of the hquid and accompanying overpressure.) The SLT... 

Vapor Pressure The pressure exerted by a vapor above its own liquid. The higher the vapor pressure, the easier it is for a liquid to evaporate and fill the work area with vapors which can cause health or fire hazards. Venting Emergency flow of vessel contents out of a vessel. The pressure is controlled or reduced by venting, thus avoiding a failure of the vessel by overpressurization. The emergency flow can be one-phase or multi-phase, each of which results in different flow characteristics. 

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. 

On initial start-up and shut-down the heat exchanger can be subjected to damaging thermal shock, overpressure or hydraulic hammer. This can lead to leaky tube-to-tubesheet joints, damaged expansion joints or packing glands because of excessive axial thermal, expansion of the tubes or shell. Excessive shell side flowrates during the "shake down can cause tube vibrations and catastrophic failure. 

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. 

The first step in the design of protechon against overpressure is to consider ah contingencies which may cause overpressure, and to evaluate them in terms of the pressures generated and/or the rates at which fluids must be relieved. 

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... 

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