The unknown problem with sprinkler systems
An explosion in a sprinkler control room. Flame tongue bursting out from a sprinkler pipe when drilling a hole in the pipe. Pipes are bursting apart at the joints.
These are recent examples of dramatic incidents caused by the chemical process of forming flammable hydrogen gas in galvanised sprinkler systems.
Systems designed to limit or stop a fire were suddenly reported actually being the cause of hazardous situations. This has until recently been an unknown problem in the sprinkler industry, and investigations were initiated to bring clarity to these incidents.
Galvanisation is a common process to protect steel from corrosion in atmospheric conditions and ensure a long life span. A thin layer of metallic Zinc is added to the mild steel construction. The hot dip galvanisation protects steel from corrosion by providing a thick, tough, metallurgical bonded zinc envelope, which completely covers the steel surface and seals it from the corrosive action of its environment.
The benefits of using this technology for sprinkler pipes were seen as obvious, being suitable for installing outdoors and in harsh environments to prevent the steel from corrosion. It also allows mild steel quality and thinner piping. Press-fitting installations and lighter pipes could then decrease the installation time on site and system weight compared to the more traditional threaded couplings.
These benefits were also seen as favorable for indoor use in wet sprinkler systems. The pros of this technology used in wet sprinkler systems however have proven unfavourable after all. The investigations following the losses described earlier have shown formation of hydrogen gas as being the root cause. The chemical reaction creating hydrogen in a reaction between oxygen, zinc and iron is not new knowledge. Under certain conditions, these materials will react, creating hydrogen gas.
Not familiar in the sprinkler piping industry
This knowledge was not familiar in the sprinkler piping industry where the galvanised, closed wet sprinkler systems are filled with stationary water. Filling up wet sprinkler systems with water, there will always be some airpockets left above the water level. Dissolved oxygen in the systems will react with the metals causing a corrosion process until the oxygen is consumed, a condition called dead water.
In traditional blacksteel piping, a protective layer of Magnetite will protect the steel from further corrosion until adding fresh oxygen when replacing the water. When galvanised pipes are pressurised with water, the dissolved oxygen will react with the zinc surface, forming zinc hydroxide, a pulverised corrosion product with poor protection characteristics.
As with all corrosion products, the zinc hydroxide can accumulate in the lower parts of the pipes, blocking pending sprinkler heads preventing water from reaching the seat of the fire or reducing the water discharge. When all the dissolved oxygen is consumed, a cathodic reaction will cause increased production of hydrogen gas, which gathers in the air-pockets above the water level in the pipes. This can be revealed by reading abnormal pressure increase at the pressure gauges.
Hydrogen gas has a wide range of flammability limits, between 4% and 75%. Obviously, accumulation of considerable amounts of the flammable hydrogen gas can be hazardous when exposed for sparks, i.e., by drilling holes or cutting pipes. The excess pressure itself has also shown to be substantial in some systems where the design criteria allows a maximum pressure of 12 bar (EN 12845).
Galvanised pipes are used in press-fitting installations. A well-known loss driver in such
systems is pipes bursting apart due to inadequate or faulty workmanship when pressing the fittings together. Another theory, however one that is not verified for some of these losses, is that accumulation of hydrogen gas has caused an increase of the static pressure beyond the design criteria of the systems, followed by bursts in the joints.
Given the right circumstances, the Zinc will be consumed by corrosion during a relatively short period of time, 2-3 years. After this, the pipes will be considered normal, however thinwalled.
Some mitigation actions can be made
Galvanised sprinkler systems are commonly used in Norway. Preferably, the systems should be replaced by another steel quality. However, some mitigating actions can also be made in existing systems;
- Weekly monitoring and logging the static pressure above the sprinkler valve. Dramatic increase could indicate accumulation of hydrogen gas. This periodic monitoring is not more
- than described in the maintenance process in the European sprinkler standard EN 12845.
- Replacing air with nitrogen has proven to be successful in preventing the corrosion process and will increase the life span, but will have limited effect on the formation of Hydrogen.
- Personnel maintaining the systems and especially when mechanically working with the pipes, needs to be aware of the problem.
- Different material quality should not be mixed in the sprinkler systems; this might increase the corrosion and the cathodic process.
- Flushing the systems before pressurising. However, this is not recommended for systems in use.
- Venting of excess pressure has proven to be effective.
- Adjusting the pH of the water. The zinc corrosion rate has been shown to be the lowest at around pH 10, however, not above 11.5, which may cause etching of human skin.