19 Apr Vast volumes & values
The storage and warehousing of goods and materials has developed over time along with the evolution of mankind. Peter Stephenson, Assistant Director, BuroHappold examines the fire safety challenges associated with automated storage facilities.
Automated storage and retrieval system (ASRS or AS/RS) commonly consist of a variety of computer-controlled systems for automatically placing and retrieving loads from defined storage locations. Automated storage and retrieval systems are typically used in applications where:
- There is a very high volume of loads being moved into and out of storage
- Storage density is important because of space constraints
- No value is added in this process (no processing, only storage and transport)
- Accuracy is critical because of potential expensive damages to the load
An AS/RS can be used with standard and nonstandard loads and are also used in libraries allowing for greater safety when consulting and retrieving books. Providing dense storage that maximises floor space, automated storage and retrieval systems (AS/RS) integrate automated hardware and software for accurate picking and replenishment. These systems automatically locate and deliver the required inventory to a conveyor system, manual outfeed, or an ergonomic operator station. This translates to a reduction in labour, floor space and inventory levels, while increasing accuracy and productivity in comparison to manual storage methods. Typical storage applications include order picking, tooling, consolidation, work-in-process, and buffering in ambient, cold, freezer, or clean-room environments.
Within the UAE Fire & Life Safety Code 2017, Occupancies and Types of Buildings recognises Robotic or Mechanical Storage which by code definition is `a warehouse or storage structure that uses computer-controlled machines to store and retrieve goods, in multi-level storage racks with no floors’.
Whether we are looking at a new Automated Warehouse development or at an existing Warehouse/Storage facility it is important to understand the fire risks and storage hazards likely to be encountered during its operation. Because of the fundamental role risk assessments play as a starting point for developing designs and safety management systems, they must be conducted systematically. A systematic approach will help satisfy the local codes, guidance and regulations and ensure that nothing which could present a risk is inadvertently omitted. This can be relatively easily achieved by a straightforward progression through a number of logical steps.
There are a number of different methodologies that can be used to achieve a systematic approach to risk assessment. In essence the fundamental themes are:
- Identify the hazards
- Decide who might be harmed and how
- Evaluate the risks (in terms of likelihood and severity) and decide whether the existing precautions are adequate or whether more should be done
- Record the significant findings
- Review the assessment and revise if necessary.
There are a number of methods for evaluating risk and these can be applied for any particular risk and should consider a number of factors, such as the complexity of the activities carried out and the type and nature of the workplace. For many of the day-to-day risks that people in the workplace are exposed to, including fire, a simple qualitative assessment will suffice, for more complex risks a quantitative or semi-quantitative assessment may be needed, for example;
- Qualitative analysis – describes the quality of risk using words.
- Quantitative analysis – quantifies the risk with numerical data.
- Semi-quantitative analysis – uses numbers to quantify qualitative data.
The key to effective safety management, once the risks have been identified, is to establish and implement a control strategy. The control measures that are implemented to secure the safety of all those at work or who may be affected by the work or work processes should reflect the local requirements, as a minimum standard, and any technological advances that have been made.
A large automated warehouse facility may not present a significant potential for loss of life in fire due to low occupancy levels, however there could be significant risks to the environment and Civil Defence responders to a large fire incident. Consideration should also be given to protecting the assets and items stored in the facility, many of which may be high value items.
Fire engineered solutions
When considering warehouse fire safety, it is easy to follow prescriptive fire codes and general Health & Safety regulations. While compliance is a good starting point (and is obviously mandatory), there is more to warehouse fire safety than compliance.
During the development of a fire strategy for a warehouse/storage facility it is common practice to follow relevant local guidance contained within fire codes. Due to the size and variety of storage facilities compliance with code can often be difficult to achieve and an alternative performance-based fire engineered solution is often developed in consultation with the Authority Having Jurisdiction (AHJ). A fire engineering design basis should embrace the requirements of all relevant stakeholders and for a warehouse/storage facility this can include owners, developers, facility managers, insurers & loss adjusters, Civil Defence etc. All of these stakeholders will have different requirements and objectives but all of these must be considered within the fire strategy.
Fire detection, prevention and suppression systems
Compliance is only intended to provide a certain minimum level of safety. When working with a fire protection engineer you will likely want to incorporate additional safety measures. When designing a new system it is also recommended to investigate installing a design capable of handling a higher hazard classification. Should your product line or storage needs change in the future it can be very expensive to change your existing system to accommodate the higher hazard classification. There are also a lot of operational issues that relate to fire safety that should be considered. Like everything else in your operation, the level of fire safety will be greatly impacted by the procedures and training provided to the employees.
A warehouse environment presents many challenges for fire detection with varying fuel loads, ignition sources, large quantities and vertical arrangement of fuel loads making prediction of fire scenarios very difficult. The environment can be dirty, affecting the performance of traditional detection systems, generating false and potentially missed fire alarms. Some warehouse environments operate with open doors much of the time with the potential of creating a false activation.
Smoke dilution due to ceiling height, stratification, natural and mechanical ventilation can lead to delayed detection and false alarms where traditional detection is used.
Warehousing facilities often suffer false alarms due to the interruption of beam signals during movement of stock.
With all fire scenarios, fire types, doors status etc. aspiration systems will alert faster, giving an early warning of fire at an incipient stage, this allows time for investigation, manual intervention and, if necessary, suppression with a portable extinguisher. Early detection and multiple levels of alarm allow reduction in the cost of false/nuisance alarms, should the smoke event in fact be benign. Most importantly, it also allows reduced risks of a threat developing into a large fire, the risks to life safety and the consequential losses to assets, premises and business continuity.
Maintenance of traditional systems is more costly due to access equipment requirements, cleaning and replacement of detectors and labour time to carry out these tasks.
Aspirating fire detection systems are by far, more reliable and faster detecting than either beam or point detection systems and will stand up better to the dusty and dirty warehouse environment better. False alarms will almost certainly be a thing of the past, and maintenance will be far less costly.
For warehouses incorporating clean-rooms fire codes will generally recommend that the clean-room is contained within a fire separated area, normally to a minimum standard of 1-hour fire rating, however as indicated earlier these rooms can cover large areas, therefore the correct selection of fire systems is important. Early detection of a fire should not be underestimated – this can be achieved in the incipient stages of a fire developing by using an approved air sampling/aspirating system. Through the development of a cause & effect protocol this early detection can alert an in-house fire team to investigate the pre-alarm or initiate the operation of a fire protection system, for example charging a dry pipe sprinkler system through pre-action control.
Sprinkler protection with spacing and discharge density as per UAE F&LS Code and NFPA 13 has been a popular choice for clean-room protection, however there has been a fear relating to damage associated with water, heat, steam and combustion products as wet systems are reliant on thermal operation. Certain fire extinguishing agents used on fires can result in damage associated with the suppression agent(s), and in many cases the secondary damage resulting from the suppression agent can exceed the damage from the fire itself. Water systems (sprinklers and mists) will leave an electrical conducting media (water) which could lead to short circuits and water damage to electrical components. Dry powders and foam agents (particularly portable extinguishers) will also deposit residue on components which will require shutdown and extensive clean-up.
The use of `clean agents’ leave no corrosive or abrasive residues after their use which limits the problem of secondary (non-fire) damage associated with the use of extinguishing agents highlighted above. The clean agents are also electrically non-conductive, and can therefore be employed for the protection of electronic equipment. The use of new generation fire protection systems controlling the oxygen levels (Hypoxic Ventilation Solution) within the area of protection potentially have a part to play in cleanroom protection. By reducing the ratio of Oxygen to Nitrogen (normally 1:4) to a point where nitrogen begins to overwhelm the oxygen to a level where the non-reactive particles begin to limit the ability of materials to sustain a fire (ratio 1:6). The benefits of this type of system also include continuously filtered air, no potential for water based damage and the management of oxidation and humidity levels to reduce degradation of sensitive materials and items in long term storage. The CAP9 system (www.cap9.com) manufactured in the UAE with UL and Dubai Civil Defence approvals provides a viable system for cleanroom applications.
Sprinkler systems are engineered to cover a specific commodity classification in a specific storage configuration. Changes such as introducing a new product line, using a different packaging material, or changing from wood pallets to plastic pallets can increase your hazard classification and render your sprinkler system inadequate to control a fire. Also, changing the size of pallets or the way product is stacked in racking can infringe on flu space requirements, reducing the ability of the sprinkler system to control a fire. It’s also a common misconception that sprinkler systems are designed to extinguish fires. Although they can be designed to extinguish fires, systems designed to meet minimum code requirements are only expected to help control the spread of the fire until the fire department arrives to extinguish it. The fact is, every year buildings with inadequate sprinkler systems burn to the ground.
So how do we determine the level of fire protection a warehouse has/needs? Interpreting fire codes can get very complicated and evaluating your system’s engineering is not a do-it-yourself project. Balancing safety issues with operational issues is rarely a simple task. An overly cautious fire protection design may result in significant loss of storage capacity, high costs, or create ongoing maintenance issues (such as those related to in-rack sprinklers Versus High-Drop/Density sprinklers) without necessarily reducing your exposure to hazards. While an under designed system could mean loss of life and property.
Warehouse-type facilities with rack storage present a complex fire protection problem. As indicated in the fire test data included in NFPA 13 and NFPA 30, large-scale fire tests have demonstrated that many of these fires can become very large and difficult to control within two minutes or less.
Determining the correct automatic sprinkler system solution can be a challenge. The increasing popularity of large storage warehouses that have rack storage heights from 40 feet to over 100 feet is increasing the level of complexity for the automatic sprinkler system design. Currently, very little full-scale fire testing with automatic sprinklers has been done with rack storage in excess of 40 feet, which limits the automatic sprinkler choices and protection schemes that are available for these applications. This article will review the current NFPA 13 requirements as well as concepts on possible future sprinkler protection based upon today’s known and commonly accepted practices, science and technology.
The complexity of protecting storage occupancies with sprinkler systems seems to originate with the large number of sprinklers and categories available. For those designing this type of sprinkler system, it is important to understand that the science and math involved is universal and obvious to those trained in the field. Sprinkler hydraulics, sprinkler spacing, response times, fire control and fire suppression are subfields of sprinkler technology that use the same mathematics and science that have been applied for over 100 years. There are six significant characteristics in the manufacture of automatic sprinklers that can be changed in the manufacturing process to achieve different performance. These are:
- Thermal sensitivity
- Temperature rating
- Orifice size
- Installation orientation
- Water distribution characteristics
- Special service conditions
The science of thermal sensitivity is well documented. Transfer of heat governs all aspects of fire, from ignition through final extinguishment. Heat is transferred by one or more of three mechanisms: conduction, convection, or radiation. Convective heat transfer (heated air from a fire rising to the ceiling) is the primary means by which a sprinkler is activated.
The heated air rises in a plume to the ceiling. When the plume hits the ceiling, it produces a ceiling gas jet. The heat responsive elements of the sprinklers within the jet are then heated by conduction of the heat from the air. When the heat responsive element reaches its operating temperature, the sprinkler will activate.
Conclusion
As the boundaries for the construction of warehouses with higher volumes including rack storage of commodities over 40 feet continues to grow, the fire protection industry must develop a variety of new sprinkler system solutions and alternative suppression systems based on the perceived fire risks. We are fortunate that much of the way has been paved for us by the efforts of many professionals who have undertaken detailed research. Selecting the most appropriate fire protection system to be used in storage applications can be a challenging activity. Designers should not allow themselves to become distracted by the terminology and language of the installation standards.
The designer should be confident that by analysing the risks from first principles will ensure that an optimum and safe design will be achieved.