By Jonathan M. Rubes, P.Eng., Leber/Rubes
Blowing smoke: Commissioning smoke management systemsEngineering
Smoke management systems play several roles as a component of a building's fire protection and life safety system. Also referred to as smoke exhaust or smoke control systems, they help to provide tena...
Smoke management systems play several roles as a component of a building’s fire protection and life safety system. Also referred to as smoke exhaust or smoke control systems, they help to provide tenable conditions for people evacuating a building. Typically, they limit the spread of smoke between areas and keep the stair shafts free of smoke. They also provide smoke venting to aid firefighters.
Many people expect that the use of smoke management systems will increase as building codes change to be “objective-based” rather than being based on prescriptive rules. As this happens, it is likely that the performance of smoke management systems will come under greater scrutiny — especially considering that a number of people currently place little confidence in them.
The reliability of smoke management systems depends, of course, upon how well they perform according to the designers’ intention. And unless a system has been properly commissioned, its performance might not be known until it is too late.
The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Guideline 5, “Commissioning Smoke Management Systems,” defines a smoke management system as “an engineered system that includes all methods that can be used singly or in combination to modify smoke movement.” This definition puts our expectations of such systems into perspective. That is, they can only be expected to modify smoke movement.
Systems vary but typically include at least one of the following: stair pressurization, vestibule pressurization, floor area smoke exhaust, atrium smoke exhaust and supply air systems. Generally they are controlled automatically in conjunction with automatic fire detection, and have the means for manual override.
The operation of smoke management systems is made more complex and difficult because they tend to be interdependent with other building components such as mechanical, fire alarm, and automated building control systems.
Having them properly installed is another critical factor. Even in the case of good designs, there are many opportunities for things to go wrong, including during the commissioning process. However, commissioning is still the most effective way of confirming that a smoke management system will perform properly.
Orchestrating the parts
Commissioning a smoke management system means testing all the components as a single system. One cannot simply test the fire alarm system, mechanical fans and control systems independently. The ASHRAE Guideline defines a commissioning plan as “the overall document that outlines the organization, scheduling, allocation of resources, documentation, etc. pertaining to the overall commissioning process.” It further defines functional performance testing as “that full range of checks and tests carried out to determine if all components, subsystems, systems and interfaces between systems function in accordance with the contract documents. In this context ‘function’ includes all modes and sequences of control operation, all interlocks and conditional control responses, and all specified responses to emergency conditions.” The Guideline makes it clear that the system is a package deal.
Given that the key to effective smoke management is for the components to work as a single system, it makes sense that a single person should be ultimately responsible for the commissioning. It is not effective to have three independent people or organizations confirm that three systems work independently. The ASHRAE guideline defines a commissioning authority as “the qualified person, company, or agency that will plan and carry out the overall commissioning process…. The design professional, contractor, independent commissioning agency or owner may be the commissioning authority.”
While it is always important to commission a smoke management system, commissioning is critical where a performance approach to life safety has been used in order to confirm that the performance objective has been achieved.
Effective commissioning ideally begins at the design stage. At this point the system objectives and performance should be clearly identified so that they can be verified upon completion. This stage can include pass/fail criteria.
Once the system has been installed, there are ideally three elements of testing:
— Functional testing. Confirms that the system components work as intended, specifically, that the right fans and dampers operate when and how intended.
— Performance testing. Confirms the anticipated flow rates, applicable air velocities and pressure differentials are achieved.
— Demonstration testing. Provides a visual demonstration of air patterns created by the smoke management system using visible smoke.
In order to do functional testing, someone such as the mechanical or controls engineer must clearly identify how the systems and all associated equipment should operate. The ASHRAE Guideline defines such an event matrix as “a matrix of zone inputs and outputs that describe the totality of control events or sequences required for each piece of equipment or control device in response to inputs from each zone in a smoke management system.”
Presumably, the event matrix has been prepared at the design stage. Otherwise, how could the fire alarm or building control system manufacturer who is responsible for programming the system do it properly? Surprisingly, while consultants usually give the equipment manufacturers’ programmers operational information in some form, they often do not provide it in a format that lends itself to effective commissioning.
Each system should be tested in all modes of operation and all zone scenarios. Someone must also observe each system, including all individual interlocks, conditional control logic, control sequences, and simulation of any abnormal conditions for which there is a specified system or control response. This stage includes testing the systems under emergency power.
Ascertaining that the smoke management system is performing as intended may appear to be a straightforward task where the smoke venting system is simply designed to exhaust at a certain air change rate. The exhaust flow rate can be measured to confirm the expected flow rate is achieved. Even in this case, however, it is important to know where the measurements are taken. It is pointless, for example, to measure the exhaust rate at the discharge of the fan where the fan may be at the top of a shaft that has numerous control dampers and associated leakage. The exhaust rate must be measured at the location from which the exhaust is required. In a more complex system, air velocities may be critical across a zone interface, or a pressure differential may be critical across a door. If the performance criteria are not identified at the design stage, there may be substantial controversy at the testing stage as to whether the system has performed as intended.
Demonstration testing is generally performed with visible smoke. While there has been much criticism written about this tool in testing smoke management systems, visible smoke can be very useful if it is properly applied and its limitations are understood. While smoke bombs or theatrical smoke should not be used to determine the smoke clearing ability of exhaust fans, there is a place for such demonstrations in showing the air patterns created by mechanical air moving systems. Visible smoke testing has provided valuable lessons that have resulted in modifications to installed systems and improved the design of subsequent projects. Many system improvements would never have occurred without such testing.
An interesting example of the benefit of using theatrical smoke in testing a smoke management system occurred during the verification of a system designed to limit the infiltration of smoke into a stair enclosure. The enclosure was designed without doors at the stair opening. The system was designed to supply 100 per cent fresh air within the stair, with the air directed at the stair opening. The system was first functionally tested to ensure it operated
as intended. Then it was performance tested to confirm that it was supplying the anticipated quantity of air. The design indeed did supply a high velocity of air through the stair opening, sufficient to prevent smoke infiltrating into the stair. It was only when the system was tested using theatrical smoke, however, that observers saw how the air velocity through the stair opening was so great it was pushing the smoke across a corridor and into other parts of the building that would otherwise have been kept smoke free. In this case, the smoke provided a means to visualize the air patterns created by the smoke management system. Of course, this type of testing does not simulate a real fire condition, but it does help us understand the air patterns that are mechanically created, and it provides one last chance to confirm that all systems will work as intended.
The observations and lessons learned from testing and commissioning smoke management systems can be applied not only to protocols for commissioning, but also to the design of the systems as well. Probably the most important lessons designers can learn are:
— There is a need to prepare a clear statement of the objectives and intended performance of the system.
— Commissioning is critical to confirm that the systems perform to expectations.
— Commissioning and corrections to the systems take longer than anybody makes allowances for.
Finally, anyone involved in designing fire systems will discover quickly that they frequently have to make modifications, so above all they have to design them to be flexible.CCE
Jonathan Rubes, P.Eng. is a principal in Leber/Rubes Inc., fire protection consultants of Toronto.
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