Overpressure blast

The hazard posed can be limited by maintaining a zone free of people and property around a storage area of explosive material. The minimum radius of the zone depends on the type and quantity of explosive, the extent and type of barrica ding, and the magnitude of loss that would be encountered if an explosive incident occurred. The maximum distance to which hazardous explosive effects propagate depends on the blast overpressure created, which as a first approximation is a function of the cube root of the explosive weight, W. This is termed the quantity distance and is defined as... 

TABLE 26-6 Blast Overpressure Effects on Vulnerable Refinery Parts... 

Design control room to withstand blast overpressure... 

Vapor Cloud Explosion (VCE) Explosive oxidation of a vapor cloud in a non-confined space (not in vessels, buildings, etc.). The flame speed may accelerate to high velocities and produce significant blast overpressure. Vapor cloud explosions in plant areas with dense equipment layouts may show acceleration in flame speed and intensification of blast. 

This accident has the potential to seriously injure 50 people because of blast overpressure and thermal radiation effects. 

A deflagration can best be described as a combustion mode in which the propagation rate is dominated by both molecular and turbulent transport processes. In the absence of turbulence (i.e., under laminar or near-laminar conditions), flame speeds for normal hydrocarbons are in the order of 5 to 30 meters per second. Such speeds are too low to produce any significant blast overpressure. Thus, under near-laminar-flow conditions, the vapor cloud will merely bum, and the event would simply be described as a large fiash fire. Therefore, turbulence is always present in vapor cloud explosions. Research tests have shown that turbulence will significantly enhance the combustion rate in defiagrations. 

Sachs-scaled side-on blast overpressure (-) ambient pressure (Pa)... 

The semiempirical theory underlying this equation can be extended to describe blast overpressure decay. If acoustic behavior is assumed, results can be framed in the following expression for blast overpressure as a function of distance from the blast center. 

For /f < 2, the basic method gives too high a value for blast overpressure. In such cases, use the refined method, described in Section 6.3.3.2., to obtain a more accurate pressure estimate. 

Blast overpressures calculated by the TNT-equivalency method are in reasonable agreement with the overpressures deduced from observed damage (Sadee et al. 1976/1977). This is to be expected, because the Flixborough incident is one of the major vapor cloud explosion incidents on which the TNT-equivalency value of... 

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. 

Structural element Failure Approximate incident blast overpressure (psi)... 

A quantitative review of the effects of thermal radiation, flash fires, and blast overpressure on people and buildings surrounding the site. 

Explosion consequences in terms of overpressure and other effects may be evaluated by appropriate methods such as those described in Reference 5 and Appendix A. In evaluating the consequences of potential explosions, all these methods account for the energy of the explosion, the location of the explosion source, and attenuation of explosion effects with distance from the explosion source. From such an evaluation, maximum blast parameters can be determined at all locations of interest. Evaluation results can be graphically expressed by plotting contours of equal blast overpressure on a site plan of the facility, as shown in Figure 4.4. 

Based on this empirical information and that given in Table 3.5, a simple and conservative relationship between qualitative descriptions of explosion consequences and blast overpressure may be developed. Table 5.3 presents an example adapted from Appendix B of Reference 5. 

Turbulence is required for the flame front to accelerate to the speeds required for a VCE otherwise, a flash fire will result. This turbulence is typically formed by the interaction between the flame front and obstacles such as process structures or equipment. Turbulence also results from material released explosively or via pressure jets. The blast effects produced by VCEs can vary greatly and are strongly dependent on flame speed. In most cases, the mode of flame propagation is deflagration. Under extraordinary conditions, a detonation with more severe blast effects might occur. In the absence of turbulence, under laminar or near-laminar conditions, flame speeds are too low to produce significant blast overpressure. In such a case, the cloud will merely bum as a flash fire. 

It should be noted that consequence screening is performed without regard to the likelihood of an event s occurring. As a result, consequence screening does not determine risk. Furthermore, the consequence evaluation performed may not represent a detailed evaluation of consequences to the process plant. Instead, it is an approximation of expected consequences, given an estimate of potential blast overpressure and anticipated response of representative building types. The user should not mistake this evaluation for a detailed consequence assessment. 

The total inventory of flammable material that could be released was determined, and the TNT equivalence method (from Reference 5) was applied. Using this information, an incident side-on overpressure of 3 psi at 150 ft (0.21 bar at 45 m) was calculated. On this basis, it was determined that the building could sustain the maximum anticipated blast overpressure, and no further evaluation was needed. 

Table 5.3 may be used in conjunction with the estimate of blast overpressure contours discussed previously to conduct a qualitative site assessment for the design and siting of buildings in process plants. 

Quantitative consequence evaluation requires determination of the blast overpressure and other explosion or fire effects that can impact a process plant building and a detailed analysis of the building s response. 

Each chart has a series of curves. Each curve corresponds to the pane dimension shown to the right of the curve. Adjacent to the pane dimension is the value of B (peak blast overpressure capacity) corresponding to T = 1,000 msec. The posted value of B is intended to reduce errors when interpolating between curves. 

The blast load is modeled as a triangular-shaped overpressure time curve. The blast overpressure rises instantaneously to the peak overpressure, B, then decays linearly with a blast pressure duration, T. The pressure is uniformly distributed over the surface of the plate and is applied perpendicular to the pane. 

Figures 2 through 9 are design charts for ultraviolet stabilized polycarbonate under blast load. Charts are provided for pane thicknesses of 1/4, 3/8, 1/2, and 1 inch for pane areas up to 25 ft at pane aspect ratios (pane length to width ratios) of 1.00, 1.50, 2.00 and 4.00. The charts relate the peak experienced blast overpressure capacity, B, for convenient pane dimensions across the spectrum of encountered blast durations. Depending on the orientation of the window to the charge, the blast overpressure may either be incident or reflected. The pane dimensions (measured across the span from the gasket centerline) peak blast capacity at 1000 msec, B, static frame design pressure, r, and the required bite are printed to the right...

Whenever the operation to be performed involves the potential to cause the initiation of the propellant, explosive or pyrotechnic (PEP) component(s) of a munition item, the APE is either operated by remote control, with the operator behind a protection wall or barrier, or it is enclosed in a protective barricade or operational shield. Barricades or operational shields are designed to protect personnel and assets from the effects of blast overpressures, thermal effects or fireball, and fragments result from the initiation of PEP components, such as the fuze, primer, propelling charge, burster, etc. 

---