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Pressure venting is an essential adjunct to many types of gaseous fire protection systems, but incorrectly designed venting can have serious consequences. Clark Priestly looks at the issues involved, paying particular attention to common pitfalls and misconceptions.
The basic principle behind pressure venting for fire extinguishing systems is very simply stated: the vent must be sized to suit the weakest part of the structure. Failure to observe this basic rule can lead to dire consequences, including structural damage such as cracked or moved walls, and ceilings or windows blown out in the event of an extinguishant discharge. Almost invariably this leads to loss of extinguishant, which means that the fire is not extinguished or it re-strikes. In such situations, the fire will continue to burn until it is extinguished by the fire brigade. This is not simply a theoretical problem – there are cases of this on record.
Before delving too deeply into pressure venting arrangements, however, let’s take a step back and consider which types of extinguishing system need it Those that involve inert gas always need venting, and those that involve CO2 will need venting if they are used in sealed areas such as vaults. Systems that use gaseous chemical agents sometimes need venting, but usually only when they are used in inherently weak structures.
An important point to note about chemical agents is that they knock down and cool the fire very quickly, which can result in the pressure of the protected area being temporarily below that of the surroundings. As a result, venting systems for chemical agents need to be able to cope with both positive and negative pressures. In addition, if a pressure relief vent is fitted, the negative pressure phase results in a significantly greater quantity of air being drawn into the protected area, which dilutes the effective concentration of the extinguishant. This dilution needs to be taken into account in the design of the installation and will require the use of a compensatory quantity of extinguishant.
Free vent area
In those applications where venting is required, the first step is to decide on the size of vent. This is not, however, as straightforward as it might at first appear. Calculations of vent size are based on the ‘free vent area’. This is not the same as the physical area of the vent as there are many issues that restrict the flow through an aperture, thereby reducing its effective area.
Unfortunately, the concept of the free vent area is not well known to HVAC (heating, ventilation and air-conditioning) suppliers. So care must be taken in a fire related application when using a vent system from such suppliers, as typically some will over-estimate the available free vent area of their vents. Conversely, the calculation of the free vent area in many fire protection system hydraulic calculations includes a worst case allowance for the development of the so-called ‘vena contracta’ effect (the narrowest point of the flow). This can actually lead to an overestimate of the free vent area, if the conditions to create such an effect do not exist, which may not matter from an operational or safety point of view, but can increase costs unnecessarily.
The next stage in designing a venting system is to decide on the type of vent. Four major ones are used: gravity flap; high efficiency gravity flap; electrically powered; and pneumatically powered. All of these types have specific benefits and drawbacks. It’s important to understand that, while every one of them can be the best choice for certain applications, none of them is the best choice for every application. What can be said is that for every application at least one type of vent will offer an efficient and cost effective solution.
Misplaced optimism
Gravity flaps are the least expensive and are usually adapted from HVAC applications rather than being designed for fire protection. They are an effective, low cost choice in many cases. Factors which have to be considered, however, are that they are not airtight, which means that they will chatter when exposed to external wind loads, and their free vent area varies with the pressure differential across them.
Many manufacturers of gravity flaps are often over-optimistic when it comes to computing their free vent area. Tests carried out by Siemens in conjunction with a supplier have demonstrated that the blades will never fully open – no matter how great the pressure differential – and that the degree by which the blades open is proportional to inlet pressure, among other factors. Consequently, the maximum theoretical free vent area cannot be achieved and at lower inlet pressures (i.e. room strengths) typically less than 600 Pa, the achievable free vent area is significantly less. Gravity flaps sourced from experienced fire prevention companies take this into account and quote realistic figures for the free vent area.
High efficiency
High efficiency gravity flaps have been developed specifically for fire protection and incorporate special features that allow wider opening than conventional ones. A smaller vent size can, therefore, be used to achieve the same result, often making high efficiency flaps a more cost-effective option than the nominally cheaper standard ones. They are a very useful option but, like standard gravity flaps, they are not airtight and will chatter when subject to external wind loads.
Electrically operated vents achieve very high efficiency, making excellent use of the ‘hole’ size. The free vent area figures quoted for them are invariably reliable as once open, there is no resistance to flow from the vent device itself. In addition, they are essentially draught proof, and they are unaffected by external conditions.
An important consideration often overlooked with electric vents is that they are slow to open and fast to close. This means that they need to be set up very carefully to ensure that vents are always fully open before extinguishant discharge is initiated – errors in operating sequence can mean that there is effectively no pressure relief at all. The trade-off for the high efficiency of electrically operated vents is that they cost more than gravity flaps.
Pneumatically powered vents, like their electrically operated counterparts, achieve very high efficiency, allowing the minimum physical size of vent to be used, and their claimed free vent area figures can be relied upon. Since they are operated by the fire protection system pressure they respond instantaneously with no possibility of incorrect timing. Most types are draught-proof.
Pneumatically powered
Pneumatically powered vents are an excellent option for critical applications, but they are more expensive than simpler types of vent, and they need to incorporate facilities for testing. Testing usually involves the use of an external pneumatic source and measures are needed to ensure that the test point on the vent is kept closed, except when it is in use. These requirements should be taken into account when considering the cost of maintenance and servicing. Pneumatically powered vents can also feature a degree of fail safe operation, such that positive room pressure will still open the vent.
Whatever type of pressure vent is selected, it is likely to be used with accessories and these can have a significant effect on performance. Working from inside the protected area to the outside, the first accessory may be an inner grille. This prevents accidental contact with the vent and is essential with powered types. Most inner grilles are highly efficient and have a negligible effect on the free vent area. If the vent is not exposed to weather, a similar high efficiency grille can also be used on the outside.
If there is exposure to weather, a weather protection louvre will be needed to prevent the ingress of rain. Such louvres do, however, reduce the efficiency of the vent by as much as 60%, so their effect must be taken into account when calculating the required free vent area.
The free vent area is calculated on the pressure being vented to the outside atmosphere but occasionally, pressure relief systems cannot be so arranged and ducting must be used. The ducting creates a pressure drop that has to be compensated by up-sizing the pressure relief installation. The calculations involved are quite complex, and are probably best left to specialist companies which have proven experience in carrying out this type of work.
Cascade
Another issue that is sometimes encountered is cascade venting, where one room is vented into another. In this case it’s important to remember that the second room, and so on, will be flooded with the same volume of gas as the first, so the second and any subsequent room must also be fitted with a vent – unless its volume is at least typically around seventy times greater than that of the protected area. The vent for the secondary room(s) must be designed using the same principles that have already been discussed, taking into account room strength, volume and the quantity of gas that will flood into it.
It is sometimes suggested that existing room leakage should be considered as making up part of the free vent area required for pressure venting. While superficially attractive in that it appears to save money, this approach has many drawbacks. For example, the actual source of the leakage is usually unknown so smoke, combustion products and extinguishant may be forced into undesirable areas. In addition, it is very difficult to assess changes in the equivalent leakage area (ELA) of the room, and there is no control over the retention of this leakage – the room may be re-sealed or the feature providing the leak path may be removed or altered at a later date. Furthermore, the room integrity test gives a worst case prediction for extinguishant retention (over estimating the ‘hole’), so is inappropriate for pressure relief and thus this practice is fundamentally flawed. A much better approach, which is always adopted by Siemens, is to ignore the ELA value when designing pressure relief venting, to seal the room in the best way practical, and then to fit a vent that, by itself, provides the correct free vent area.
Conclusion
The correct design of a pressure venting system to complement extinguishing installations that use gaseous agents is, in many ways, as important as the design of the installation itself. There are many options available, so it’s almost guaranteed that there will be a convenient and affordable solution to meet every requirement. Nevertheless as we’ve seen in this article, there are many factors that need to be considered in choosing and designing a venting system. There’s a lot to be said, therefore, for seeking advice from an organisation that’s experienced in this field. After all, fire suppression systems are all about preventing damage, not
causing it!
YOU WERE ONLY SUPPOSED TO BLOW THE DOORS OFF!
An example of how important it is to provide the correct pressure venting for gaseous extinguishing systems is starkly demonstrated with an installation in Strasbourg in France. The inert gas system which was protecting a computer room in the building had a pressure relief vent fitted. However, it is understood the vent was either too small, or the risk was not assessed properly, resulting in the design of the vent being unsuitable.
It is thought that an overheating wire started to smoulder and an alarm sounded. But as there was no one in the building at the time, the alarm was not detected. So a small fire developed which caused an automatic discharge of the extinguishant, leading to a pressure build up,which literally blew out the wall. Thankfully no one was injured, nor in fact was there any damage to the hardware in the room. It was just the wall that took the brunt.!
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Pressure venting is an essential adjunct to many types of gaseous fire protection systems, but incorrectly designed venting can have serious consequences. Clark Priestly looks at the issues involved, paying particular attention to common pitfalls and misconceptions.
The basic principle behind pressure venting for fire extinguishing systems is very simply stated: the vent must be sized to suit the weakest part of the structure. Failure to observe this basic rule can lead to dire consequences, including structural damage such as cracked or moved walls, and ceilings or windows blown out in the event of an extinguishant discharge. Almost invariably this leads to loss of extinguishant, which means that the fire is not extinguished or it re-strikes. In such situations, the fire will continue to burn until it is extinguished by the fire brigade. This is not simply a theoretical problem – there are cases of this on record.
Before delving too deeply into pressure venting arrangements, however, let’s take a step back and consider which types of extinguishing system need it Those that involve inert gas always need venting, and those that involve CO2 will need venting if they are used in sealed areas such as vaults. Systems that use gaseous chemical agents sometimes need venting, but usually only when they are used in inherently weak structures.
An important point to note about chemical agents is that they knock down and cool the fire very quickly, which can result in the pressure of the protected area being temporarily below that of the surroundings. As a result, venting systems for chemical agents need to be able to cope with both positive and negative pressures. In addition, if a pressure relief vent is fitted, the negative pressure phase results in a significantly greater quantity of air being drawn into the protected area, which dilutes the effective concentration of the extinguishant. This dilution needs to be taken into account in the design of the installation and will require the use of a compensatory quantity of extinguishant.
Free vent area
In those applications where venting is required, the first step is to decide on the size of vent. This is not, however, as straightforward as it might at first appear. Calculations of vent size are based on the ‘free vent area’. This is not the same as the physical area of the vent as there are many issues that restrict the flow through an aperture, thereby reducing its effective area.
Unfortunately, the concept of the free vent area is not well known to HVAC (heating, ventilation and air-conditioning) suppliers. So care must be taken in a fire related application when using a vent system from such suppliers, as typically some will over-estimate the available free vent area of their vents. Conversely, the calculation of the free vent area in many fire protection system hydraulic calculations includes a worst case allowance for the development of the so-called ‘vena contracta’ effect (the narrowest point of the flow). This can actually lead to an overestimate of the free vent area, if the conditions to create such an effect do not exist, which may not matter from an operational or safety point of view, but can increase costs unnecessarily.
The next stage in designing a venting system is to decide on the type of vent. Four major ones are used: gravity flap; high efficiency gravity flap; electrically powered; and pneumatically powered. All of these types have specific benefits and drawbacks. It’s important to understand that, while every one of them can be the best choice for certain applications, none of them is the best choice for every application. What can be said is that for every application at least one type of vent will offer an efficient and cost effective solution.
Misplaced optimism
Gravity flaps are the least expensive and are usually adapted from HVAC applications rather than being designed for fire protection. They are an effective, low cost choice in many cases. Factors which have to be considered, however, are that they are not airtight, which means that they will chatter when exposed to external wind loads, and their free vent area varies with the pressure differential across them.
Many manufacturers of gravity flaps are often over-optimistic when it comes to computing their free vent area. Tests carried out by Siemens in conjunction with a supplier have demonstrated that the blades will never fully open – no matter how great the pressure differential – and that the degree by which the blades open is proportional to inlet pressure, among other factors. Consequently, the maximum theoretical free vent area cannot be achieved and at lower inlet pressures (i.e. room strengths) typically less than 600 Pa, the achievable free vent area is significantly less. Gravity flaps sourced from experienced fire prevention companies take this into account and quote realistic figures for the free vent area.
High efficiency
High efficiency gravity flaps have been developed specifically for fire protection and incorporate special features that allow wider opening than conventional ones. A smaller vent size can, therefore, be used to achieve the same result, often making high efficiency flaps a more cost-effective option than the nominally cheaper standard ones. They are a very useful option but, like standard gravity flaps, they are not airtight and will chatter when subject to external wind loads.
Electrically operated vents achieve very high efficiency, making excellent use of the ‘hole’ size. The free vent area figures quoted for them are invariably reliable as once open, there is no resistance to flow from the vent device itself. In addition, they are essentially draught proof, and they are unaffected by external conditions.
An important consideration often overlooked with electric vents is that they are slow to open and fast to close. This means that they need to be set up very carefully to ensure that vents are always fully open before extinguishant discharge is initiated – errors in operating sequence can mean that there is effectively no pressure relief at all. The trade-off for the high efficiency of electrically operated vents is that they cost more than gravity flaps.
Pneumatically powered vents, like their electrically operated counterparts, achieve very high efficiency, allowing the minimum physical size of vent to be used, and their claimed free vent area figures can be relied upon. Since they are operated by the fire protection system pressure they respond instantaneously with no possibility of incorrect timing. Most types are draught-proof.
Pneumatically powered
Pneumatically powered vents are an excellent option for critical applications, but they are more expensive than simpler types of vent, and they need to incorporate facilities for testing. Testing usually involves the use of an external pneumatic source and measures are needed to ensure that the test point on the vent is kept closed, except when it is in use. These requirements should be taken into account when considering the cost of maintenance and servicing. Pneumatically powered vents can also feature a degree of fail safe operation, such that positive room pressure will still open the vent.
Whatever type of pressure vent is selected, it is likely to be used with accessories and these can have a significant effect on performance. Working from inside the protected area to the outside, the first accessory may be an inner grille. This prevents accidental contact with the vent and is essential with powered types. Most inner grilles are highly efficient and have a negligible effect on the free vent area. If the vent is not exposed to weather, a similar high efficiency grille can also be used on the outside.
If there is exposure to weather, a weather protection louvre will be needed to prevent the ingress of rain. Such louvres do, however, reduce the efficiency of the vent by as much as 60%, so their effect must be taken into account when calculating the required free vent area.
The free vent area is calculated on the pressure being vented to the outside atmosphere but occasionally, pressure relief systems cannot be so arranged and ducting must be used. The ducting creates a pressure drop that has to be compensated by up-sizing the pressure relief installation. The calculations involved are quite complex, and are probably best left to specialist companies which have proven experience in carrying out this type of work.
Cascade
Another issue that is sometimes encountered is cascade venting, where one room is vented into another. In this case it’s important to remember that the second room, and so on, will be flooded with the same volume of gas as the first, so the second and any subsequent room must also be fitted with a vent – unless its volume is at least typically around seventy times greater than that of the protected area. The vent for the secondary room(s) must be designed using the same principles that have already been discussed, taking into account room strength, volume and the quantity of gas that will flood into it.
It is sometimes suggested that existing room leakage should be considered as making up part of the free vent area required for pressure venting. While superficially attractive in that it appears to save money, this approach has many drawbacks. For example, the actual source of the leakage is usually unknown so smoke, combustion products and extinguishant may be forced into undesirable areas. In addition, it is very difficult to assess changes in the equivalent leakage area (ELA) of the room, and there is no control over the retention of this leakage – the room may be re-sealed or the feature providing the leak path may be removed or altered at a later date. Furthermore, the room integrity test gives a worst case prediction for extinguishant retention (over estimating the ‘hole’), so is inappropriate for pressure relief and thus this practice is fundamentally flawed. A much better approach, which is always adopted by Siemens, is to ignore the ELA value when designing pressure relief venting, to seal the room in the best way practical, and then to fit a vent that, by itself, provides the correct free vent area.
Conclusion
The correct design of a pressure venting system to complement extinguishing installations that use gaseous agents is, in many ways, as important as the design of the installation itself. There are many options available, so it’s almost guaranteed that there will be a convenient and affordable solution to meet every requirement. Nevertheless as we’ve seen in this article, there are many factors that need to be considered in choosing and designing a venting system. There’s a lot to be said, therefore, for seeking advice from an organisation that’s experienced in this field. After all, fire suppression systems are all about preventing damage, not
causing it!
YOU WERE ONLY SUPPOSED TO BLOW THE DOORS OFF!
An example of how important it is to provide the correct pressure venting for gaseous extinguishing systems is starkly demonstrated with an installation in Strasbourg in France. The inert gas system which was protecting a computer room in the building had a pressure relief vent fitted. However, it is understood the vent was either too small, or the risk was not assessed properly, resulting in the design of the vent being unsuitable.
It is thought that an overheating wire started to smoulder and an alarm sounded. But as there was no one in the building at the time, the alarm was not detected. So a small fire developed which caused an automatic discharge of the extinguishant, leading to a pressure build up,which literally blew out the wall. Thankfully no one was injured, nor in fact was there any damage to the hardware in the room. It was just the wall that took the brunt.!