Quality, Consequences and the Construction Industrial Complex (part 65) – All IMHO:
Confusion and a lack of consensus!
The purpose of staircase pressurization systems is to provide protection to building occupants and fire fighters. They are vital life safety and fire fighting systems but there are fire fighters in many jurisdictions that have anecdotally, suggested they do not trust these systems, why?
IMHO it is because staircase pressurization systems are not always well designed, installed commissioned and importantly, maintained. From a failure analysis perspective, there are several “Single Points of Failure” such as the static pressure sensor, pressure relief dampers, the VFD or the duty standby fan change over control that must work on the day of emergency yet depend on maintenance and periodic testing by the facilities management teams.
The objectives of pressurization systems are simple, but the problems associated with them are many and complex.
In the design process there are a number of key issues that have to be taken into account. The most fundamental issue, is estimation of air leakage from the stair core.
The staircase pressurization system design methodology consists of:
- Assess the usage and layout of the building, the area to be pressurized and the class of system required.
- Assess the leakage paths (through doors, lifts, vents).
- Calculate the required total volume flow rates, including temperature factors.
- Calculate the required volume flow rates through open doors (ground floor escape and fire floors).
- Calculate / estimate stack effect (high rise buildings)
- Calculate the area of pressure relief dampers.
- Calculate the area of air release ventilation.
- Calculate pressure differential from stairwell and building at bottom to top of stairwell and building.
There are two requirements to maintain within a staircase pressurization system:
- Pressure difference for a closed door condition.
- Velocity for an open door condition.
Staircase pressurization systems need careful design to work as required.
High quality installation is an absolute requirement assuming a compliant design. The installation is multidisciplinary i.e. construction, mechanical, electrical and controls, and therefore it is vital that someone “owns it”.
The main contractor should, in my opinion lead this installation process, because the quality of the core construction, including doors, penetrations and coordination has a massive impact on the amount of air leakage when the stair core is pressurized. Quality matters because it is very difficult to make changes to components such as ducts, fan sizes, power supplies and relief dampers after the initial design then installation process completes and problems emerge.
The commissioning process cannot take place until the building is fundamentally complete. Any significant problems identified during commissioning that cannot be resolved by fine tuning the equipment already installed will to lead to increased costs and delay building occupancy.
It should also be noted that commissioning can be highly sensitive to the wind and temperature conditions on the day of testing, particularly if the building is high rise. An experienced commissioning professional is highly recommended to test these systems.
Key criteria that must be confirmed with the design team and Authority Having Jurisdiction (AHJ) prior to commissioning are:
- Relevant design and commissioning codes, regulations and guides
- Systems classification (if UK)
- Number of doors open at any one time in different test modes of operation
- Design air velocities through doors in different test modes of operation
- Design PD across closed doors & adjacent areas
- Design maximum door opening force
- Design fan flow rates and pressures
- System/equipment, failure/auto-changeover scenarios
Note: the system should be tested and demonstrated to the AHJ on stand-by power.
Watch points during commissioning are:
- Stack effect.
- In cold weather conditions, modulating the fan based on the lowest value will lead to, too high pressure differences high in the staircase, due to the stack effect.
- Response time of the system. When exit doors open and close, a modulating system is constantly ramping the fan up and down. This could lead to unacceptably high opening forces to open exit doors.
Design, installation and commissioning is only a fraction of the staircase pressurization system life cycle. The big issue is the maintenance and periodic testing of the system to ensure it works on the day it is needed.
Different countries and AHJ have different requirements that must be respected. However, in my opinion, the staircase pressurization system should be functionally tested each time the stand-by generators are tested.
The importance of testing this life safety system can be summarized as:
- Dormancy – equipment and systems that lie dormant for large periods of time often fail when required
- Criticality – it is a life safety system after all
- Owner obligations – meeting statutory requirements of AHJ
- Liability – Owners have a duty of care and legal responsibility to keep building users safe
From an OPR and commissionability perspective it is critical to highlight the importance of maintenance testing and formalize a periodic testing and report regime post occupancy.
I have included a table below with a summary of my findings to date. I would be grateful for any corrections, comments or additions so I can consolidate my final reference document.
Many thanks in advance for any comments received after reading this long post.
#40 GIT – The must do test that delivers working buildings ( https://www.linkedin.com/pulse/git-must-do-test-delivers-working-buildings-adam-muggleton?trk=mp-author-card )