The positive pressure phase of an explosion is in the order of a few hundred milliseconds.
Thus, the effects of an explosion are realised immediately. Conversely, the time for the effects of a fire to be realised, even upon personnel, is of at least an order greater than that for explosions. Thus the early detection of a leak is of greater significance for explosion hazards than for fire hazards.
It follows that leak-detection and associated alarms, are the only means of providing adequate warning to personnel. Fire-detection is of no value in this context. However, fire-detection is still of importance when fire hazards are considered.
It may be that very rapid response fire-detection systems, such as those that utilise flame-detection, could detect an explosion before they are damaged by the explosion blast or drag effects. In addition, rapid response fire detection may also be of benefit with respect to the initiation of explosion mitigation systems (such as deluge on gas detection). As stated above, this would be of no significance when explosion hazards are considered. However, it may be important for the management of fire-hazards to detect any fire that may follow an explosion.
4.3.1.1 Automatic release detection systems and alarms Two generic types of release detectors are available:
Type 1) The detection of an accumulation of flammable gas from the release, or Type 2) The direct detection of a release based on its acoustic signature.
Type 1) detectors may be point detectors e.g. pellistors, or beam detectors. In each case, there is a requirement to assess the minimum flammable cloud size that should be detected.
There is little guidance available to assist in this. One approach may be to consider both the thermal effects and the blast effects on a flammable cloud. The thermal effects may be assessed by treating the combustion of the flammable cloud as a fireball. The minimum cloud size, intended to be detected, may then be based on an acceptable probability of persons in the area surviving these thermal and blast effects. A range of figures for the probability of survival of detection systems following an initial blast load would be between 0.5 and 0.9. The lower end of the range has been used often in QRAs submitted to the Health & Safety Executive but a survival probability of 0.9 if other protective steps have been considered.
Type 2) detectors appear to have an advantage over type 1) detectors in as much as they detect a leak directly. However, it is important that the sensitivity of such detectors is adequate; the advantage of leak detection is its sensitivity; there are definite benefits to detect leaks at as low rates as possible, this can be used to alert the operator even if a shut-down is not initiated at this stage of the incident. In addition, there is no clear evidence as to whether or not such acoustic detector will operate adequately when two-phase leaks occur. Thus it is suggested that the ideal leak detection system should employ both types of detectors. The detection of a leak should be annunciated on an installation-wide basis. All installation personnel, including visitors, should have received clear instruction as to the correct action to be taken on the receipt of such an alarm.
The minimum cloud size to be detected should also consider the potential escalation. This assessment should be conservative due to the diversity of potential escalation paths and the analysts’ inability to assess them all.
4.3.1.2 Automatic fire-detection systems and alarms
Only leak detection can provide adequate warning to personnel of a potential explosion hazard. Fire detection is of very limited value in the event of an explosion already having occurred.
Several fire detection device-types are commonly available, they include:
• UV and IR flame-detectors
• Rate of temperature rise detectors
• Smoke detectors
• CCTV with or without embedded flame imaging software
The flame-detectors will have the most rapid response of the above if they are in the vicinity of the fire. However, very early smoke detector (VESDA) systems can respond rapidly based on an arrangement where the detection system samples smoke at very low concentrations, this has often been used in sensitive enclosed areas, computer systems have a long history of
It is advisable that total reliance is not placed on flame-detectors alone. Conditions arise where fires are obscured by smoke and indeed, smoke generation is the major hazard to personnel.
IR detectors may also be obscured by water, for example, triggering deluge on gas detection may impair subsequent fire detection, or the IR detectors may fail to register fire escalation to adjoining process systems.
The fire-detection system should include a mix of each type of detectors and be appropriate to the mix of release/fire/explosion hazards considered in the escalation path.
The comments concerning alarms made for leak detection are equally pertinent for fire detection.
4.3.1.3 Reducing the available inventory of fuel
This is of equal importance to the management for both fire and explosion hazards. Obviously, where explosion hazards are concerned, any beneficial effects will arise if the fuel inventory is partially or completely depleted before an ignition takes place. This contrasts with the situation where fire hazards are concerned, where the beneficial effects will continue to operate after an ignition takes place. However, these beneficial effects on the fire-hazard will only ensue if the automatic isolation and blow-down and flare systems are not damaged by any prior explosion, to such an extent as to prevent their correct operation. Isolation and blow-down valves should, wherever possible, fail to a “safe” condition; generally, this means that isolation valves “fail closed” and blowdown valves “fail open” although there can be extenuating circumstances for this rule.
Especially for large high pressure valves, the actuators can be significantly large pieces of equipment, and their destruction in an accident can compromise the “fail safe” mode; such valve actuators should be protected against blast and drag-effect damage as much as reasonably practicable, but it must be accepted that there will be practical limitations on the extent to which this can be achieved.
This emphasises the importance of early detection of leaks and the automatic initiation of the ESD and blow-down systems upon the detection of a leak.
Whereas, ESD and blow-down systems do provide benefit in the management of fire and explosion hazards, it is important to realise that these systems alone cannot be relied upon to prevent subsequent failures of equipment or structural elements due to the effects of a fire.
Blow-down systems are almost universally designed to API RP520 [4.2]. This requires that the internal pressure be reduced by 50 % or to 100 psig (22 barg); whichever is the lower; within fifteen minutes. However, even if this is achieved, a significant fire could still be ongoing which could cause failures, especially in congested areas.
4.3.2 Significance of area ventilation
4.3.2.1 On explosions
The equilibrium size of a flammable cloud, produced by the accumulation of flammable gas from a leak will be inter-alia, a function of the area-ventilation rate. The severity of an explosion following the ignition of such a flammable cloud, will be, inter-alia, a function of the mass of flammable gas in the cloud. Thus it follows that an increased rate of area ventilation
4.3.2.2 On fires
The influence of area ventilation rate on fires is less apparent than its influence on explosions.
The reported research [4.3] indicates that the suppression of pool fires by water/foam deluge systems is aided by increased wind-speed i.e. an increased ventilation rate. However, the possible distortion of the water spray pattern by the wind could mitigate this.
Where mechanically ventilated, enclosed areas are concerned; the internal wind-speeds are unlikely to be sufficient to distort water-spray patterns. Note that for enclosed areas, the rate of mechanical ventilation will usually be of the order of 12 air changes per hour. Research [4.4]
has indicated that for naturally ventilated areas ventilation rates in the order of a few hundred air changes per hour are achievable.
It is possible to shut down ventilation systems to provide ventilation control of a fire, but it should be noted that in order to prevent smoke and combustion products migrating along ducts if the HVAC is not shutdown and isolated, (which could lead to fire or fire effects spreading to other areas) that common practice is to shut down and isolate HVAC systems in all but the most critical areas (such Temporary Refuges) on confirmed detection of fire.
4.3.2.3 Maximisation of ventilation rates
For existing installations, there are constraints to the options for increasing the existing ventilation rates. For mechanically ventilated areas, major refits are required for fans and ducting sizes, for naturally ventilated areas, the ventilation rate may be increased by the removal of any louvered wind walls.
For ‘new builds’ the influence of the rate of ventilation on both fire and explosion hazards should be addressed in the design. For naturally ventilated areas a conflict may arise between protecting the temporary refuge against smoke ingress and the maximisation of area ventilation. It is conventional wisdom that wherever possible, the temporary refuge should be up-wind of the prevailing wind direction. There will normally be bulkheads between the temporary refuge and drilling and process areas. This means that the open sides of these areas will be at a right angle to the prevailing wind direction limiting natural ventilation.
4.3.2.4 Other factors influencing ventilation rate Equipment layout
The equipment layout within an area will have an influence on the area ventilation rate. The most efficient layout to maximise the ventilation rate will be the same as that to minimise the explosion hazard. Layout guidance can be found in Section 3.2.6.4 of this guidance.
For existing installations, it must be accepted that little can be done to change the existing layout of equipment. For ‘new builds’ cognisance should be taken of the recommendations given in the FLACS explosion handbook.
Influence of release rate
The release rate of gas can itself have a significant influence on the rate of natural ventilation.
Published research [4.3] provides some, albeit limited, data on this effect. Analysis of these data indicates that:
• If the leak is counter-flowing to the ventilation flow direction, the ventilation rate will be reduced.
• If the leak is co-flowing with the ventilation flow direction, the ventilation rate will be increased.
• If the leak is crosswind to the ventilation flow direction, the ventilation rate is effectively unchanged.
The following correlations are suggested as representing a conservative estimate of these effects.
For leak direction counter to ventilation flow direction;
( )
′ = 1 22.4−
V V x ...Equation 4-1 Where,
V is the reduced ventilation rate (m3 s-1) V' is the original ventilation rate (m3 s-1) x is the gas release rate (m3 s-1)
For leak direction co-flowing with the ventilation flow direction;
( )
′ = 1 12.45+
V V x ... Equation 4-2 Where,
V is the increased ventilation rate (m3 s-1) V' is the original ventilation rate (m3 s-1) x is the gas release rate (m3 s-1) Influence of water deluge
Research [4.3] has indicated that the presence of water deluge reduces the ventilation rate in naturally ventilated areas. The data on this are very limited and applies only to deluge rates in the order of 24 l min-1 m-2.
It is suggested that a conservative estimate of this effect would be to reduce the area ventilation rate by 30 % when the area deluge system is operating.