Case Study: The impacts of Microbial Influenced Corrosion (MIC) in wet sprinkler piping
12 January 2016
Author: David McGreal, senior mechanical engineer, Eli Lilly
Will your wet sprinkler system function correctly in the event of a fire? Does the periodic inspection and regular flow testing through the inspector test valves prove that everything is OK? What is the condition of the inside of the sprinkler piping and what condition is it in?
In this particular case study, the Microbially Influenced Corrosion (MIC) manifested itself as a pin-hole leak in a wet sprinkler system in a facility – the sprinkler piping in question had been in service for approximately five years.
The sprinkler system serving the facility is a wet system with the ranges following the slope of the roof (which slopes at approximately 5º) and ending in a screwed pipe blank at the upper end of each range. The ranges on each side of the building are fed by two horizontal headers running the full length of the building on either side of the apex of the roof (see Fig. 1).
The pin-hole leak occurred on one of the DN50 sloped ranges approximately one metre from the upper end of the range and on the ‘bottom dead centre’ position of the pipe.
A coupon approximately 600mm long was cut from the pipe that leaked and the coupon was centred on the pin-hole site. This coupon was further split length-ways and visually inspected. Fig. 2 shows the pipe coupon when cut open – the segment with the heavier amount of debris (to the bottom of the image) being the lower half of the pipe. Note the ‘yellow staining’ visible through the debris.
The following image (Fig. 3) shows the lower half of the pipe coupon having had the loosely adhering debris washed away leaving yellow ‘tubercles’ adhering to the pipe wall.
Fig. 4 shows a close up of the ‘tubercles’ located directly over the site of the pin-hole leak.
Fig. 5 shows the pipe coupon having removed the ‘tubercles’ seen in Fig. 4 and clearly shows the level of corrosion present – both to the seam weld of the pipe and also the localised pitting corrosion under the ‘tubercle’ that was approximately 1.2 – 1.5mm deep whereas other areas of the pipe still have the mill scale present on the pipe bore indicating that there was no generalised corrosion of the pipe.
In total, four sprinkler ranges were removed as part of the leak investigation and these were split length-ways (as with the original pipe coupon) and immediately a corrosion pattern emerged.
Note: the ‘tide mark’ denoting the air-water interface in the sprinkler ranges was visible in each of the four ranges that were cut open and occurred approximately one metre from the blanked end of the range. The severity of the MIC was highest at the air-water interface.
Fig. 6 shows the levels of corrosion seen in the sections of upwards sloping ranges (from the supply headers) noting as before the yellow/brown ‘tubercles’ at various locations but concentrated mostly at the air-water interface.
Causes of MIC
The mechanism of MIC starts by the formation of a bio-film on the pipe bore, often at the site of an imperfection or flaw in the bore to which bacteria and micro-organism adhere. Over time other micro-organisms develop a more complex bacteria colony with the original ‘pioneer’ bacteria – usually a mix of aerobic and anaerobic species and forming a bio-film over the colony. Over time the bio-film also traps corrosion particles, dead cells and the byproducts of the bacterial action turning the bio-film a yellow/brown colour and causing it to harden resulting in the ‘tubercle’ shown in the Fig. 4
As a result, the local chemistry under the ‘tubercle’ at the metal surface is altered eg. ion concentration, pH, O2 concentration, corrosion potential etc. as compared with the water in the rest of the system and causes accelerated localised corrosion at the metal interface leading to MIC. Some of the main bacteria types in a ‘tuberlce’ include – sulphate reducing bacteria, acid-forming bacteria, iron-oxidising bacteria, slime-forming bacteria, sulphur oxidising bacteria and nitrate reducing bacteria. The most commonly implicated bacteria types in MIC are acid-forming and sulphate reducing bacteria.
Many of these species are anaerobic but some, the acid-forming bacteria for example, can survive in either an aerobic or an anaerobic environment.
The leading contributory factors to MIC are thought to be:
- The presence of sufficient quantities of bacteria in the fire water or in the sprinkler piping at installation (carry-over from cutting fluids);
- The presence of a food source for the bacteria, typically oils, greases, cutting fluids present in the sprinkler piping that was not removed by flushing following the initial installation;
- Imperfections in the pipe bore allowing the pioneer bacteria to form colonies that lead to the yellow ‘tubercles’ seen in the images above;
- Stagnant water especially at dead-ends of sprinkler piping can provide ideal environments for MIC. This can be particularly acute where there is an air-water interface in a stagnant branch line or range.
Apart entirely from the severe localised pitting corrosion observed under the ‘tubercles’ (and in this case severe to the point it caused a pin-hole leak) the bacterial action and associated corrosion generates deposits and debris that adhere to the pipe bore as well as becoming lodged in sprinkler heads and ultimately cause obstructions in the system that could impair the correct operation of the sprinkler system in a fire scenario.
How to combat MIC
Fig. 7 shows a schematic view of the design change adopted to allow the upward sloping ranges to be fully vented of air. Each range was connected to a common high-level vent pipe (approximate arrangement shown in red in Fig. 7) and run the length of the building and routed externally to a vent valve. This allowed the fire water to fully flood the ranges and the vent lines themselves up to the isolation valves thus eliminating the air-water interface in the existing installation and with it the potential to have MIC occurring at that point.
- Approved automatic air vents should be installed in automatic sprinkler systems to ensure the system is fully vented of air. Such air vents should be located to facilitate inspection and maintenance. Consideration will also have to be given to the fact the air vents can cause ‘nuisance leaks’ if debris gets lodged in the valve seat;
- Ensure that sprinkler piping and fittings are flushed with clean potable water to remove any residual cutting fluid, oil or greases left over from fabrication and installation and thereby minimise – or eliminate – a ready food source for pioneer bacteria that are present in the fire water;
- The sprinkler piping and fittings should be inspected to ensure the bores are free of damage and surface defects that encourage the formation of bacteria colonies and to repair any defects found;
- Consider the addition of an inhibitor to the fire water when filling the system for the first time – this inhibitor being a dual acting chemical to protect against ‘normal’ pipe corrosion due to oxygen enrichment etc but also containing biocides to prevent MIC. The concentration of inhibitor in the system will have to be monitored and the system will have to be dosed to allow for inhibitor depletion when flow testing the system using the inspector test valves as prescribed by facility insurers and/or design codes for the automatic sprinkler system.