Check your sprinklers

This Issue This is a part of the Fire feature

By - , Build 132

Recent events have highlighted concerns around the reliability of the water sources essential for fire sprinkler systems in our buildings. What can be done?

NEW ZEALAND HAS A HIGH RATE of fire sprinkler systems in buildings compared to other developed countries, with an enviable record of reliability and performance and a proven process of design, certification, installation, maintenance and on-going inspections.

Fire sprinkler systems need a reliable water supply at a suitable pressure and flow to be effective. Most systems rely on the municipal mains as their primary supply of water, and the rest rely on in situ tank supplies. However, these sources are not always reliable.

Post-earthquake disruptions

At the extreme end, events in Canterbury highlighted the disruption to fire-fighting water supplies that seismic events can cause.

Water tanks damaged

One example was the damage to water tanks in some parts of Canterbury following the initial September 2010 earthquake. The water tanks were either the primary or secondary water supply for sprinkler systems. In some instances, tanks were damaged to the point where the water was completely lost, particularly with more modern timber stave and steel tanks. Older style concrete water tanks generally performed well.

Water lost to CBD

In the February 2011 aftershock, there was significant disruption to the water reticulation system, and most, if not all, sprinkler systems in the CBD lost their water supply source and would have been totally ineffective. Christchurch was lucky to have no significant post-earthquake fires – internationally, these are common.

Many old and new systems are reliant on a single town mains supply, even though this is ineffective when widespread network disruptions occur. It was permissible to rely on another town mains connection as the secondary water supply source prior to the 2007 version of the fire sprinkler standard NZS 4541.

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Passive fire ratings and sprinklers

Water supply reliability after earthquakes also has implications for other aspects of fire safety in buildings. A particular issue is the dispensation in the supporting – formerly compliance – documents for the protection from fire clauses of the Building Code, where passive fire protection ratings can be reduced by 50% when there are sprinklers in a building.

For example, in a multi-storey office building – Risk Group WB, refer Acceptable Solution C/AS5 – the life rating of 60 minutes and the property rating of 120 minutes can be halved if sprinklers are present. This regulatory provision does not actually take account of fire breaking out after an earthquake.

Christchurch highlights risk

In the Christchurch CBD where water supplies from the town mains were knocked out, the remaining – passive – fire protection systems such as fire-rated walls around stairwells were damaged by the seismic movement. In some cases, the means of escape were unusable and occupants were trapped in multi-storey buildings.

In addition, a post-earthquake fire is likely to grow more rapidly and be more severe and of longer duration, and the Fire Service may not be able to intervene. Thankfully, this wasn’t the case, but if fires had broken out in CBD buildings, the consequences could have been dire.

The implication for the Building Code is a possible rethink on the interaction between fire safety and seismic events.

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Mains pressure is being reduced

The other issue around water supply for fire protection is of a more general and on-going nature – reduced water pressure in town mains.

In many situations, increased demand from new subdivisions has reduced pressure in the mains.

Also, in recent years, some water supply authorities have consciously reduced the pressure in the reticulation network. There are various reasons for this, including:

  • being environmentally friendly by reducing water usage/consumption
  • addressing water shortages in drought-affected areas
  • reducing water losses from network leaks
  • reducing wear and tear on ageing infrastructure.

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Sprinklers need minimum water pressures

In some cases, the unintended consequence has been that town mains water supply to existing sprinkler systems is below the design pressure and flow for the system to be effective if activated by a fire.

Generally, a safety margin of 10% of water pressure is built into the design of sprinkler systems, but where significant reductions in the delivered pressure of the town mains supply occur, the sprinkler system is unlikely to be effective in controlling a growing fire.

In some cases, there have been pressure reductions around the country of 25–50% or more. There have also been instances where the supply to industrial areas has been turned off out of working hours. This poses a challenge to providing reliable and effective sprinkler systems.

Water supply authorities have no obligation to meet a minimum supply pressure, which means that there is no specific threshold for the design of new sprinklers systems.

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Approach needs to change

Traditional design practice, which is applicable to new systems, takes actual mains pressure readings at the location of the premises and designs accordingly.

The potential for pressure reduction means that this approach carries obvious risk. Without a guaranteed minimum pressure to suit the sprinkler system design, one possible solution, for both existing and new systems, could be to include booster pumps and/or tanks, but this has financial ramifications for building owners.

Another option is to place more reliance on independent water supplies, but this also has economic implications.

The best solution, by far, is effective consultation and communication so that all stakeholders are informed and have sufficient time to plan for and implement changes that minimise the impact and do not compromise fire safety in the built environment.

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