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Multiple Threats

The most critical piping system for any building property or plant operation is unquestionably the fire sprinkler service.  Often considered trouble free, corrosion related failures at fire sprinkler lines have dramatically increased over the past decade – raising not only operating and repair costs, but the threat to building inhabitants as well.

By definition, fire sprinkler service always exists under a corrosion threat since carbon steel black pipe is traditionally used, and its water is almost never chemically treated.  This raises the importance of recognizing and addressing other corrosion influencing factors in order to maximize its useful service life.

CorrView International’s decades of involvement in ultrasonically testing fire sprinkler systems has provided us with an extensive database of hard, factual wall loss and corrosion rate data.  It has also allowed us to define very clear relationships between the design and operation of fire sprinkler systems, and resulting corrosion conditions.

Prior investigations have identified 75 year old sprinkler pipe in virtually new condition, and yet nearly new 4-5 year old installations in need of replacement.  Clearly evident, a much higher incidence of corrosion related failures exist at newer fire sprinkler installations.  Ultrasonic testing at hundreds of building properties has revealed wildly varying corrosion rates and estimates of remaining service life – which can be often correlated to sprinkler pipe location, material, pipe origin, age, thickness schedule, and most importantly, how often the pipe is drained and re-filled.  Each factor impacts fire sprinkler pipe significantly.

• Use Of Thin Wall Pipe

One major reason for such problems is the more common use of thinner schedule 10 pipe, which offers savings on both material and installation costs.  Whereas extra strong schedule 80 may have been installed 60-70 years ago, lighter schedule 40 has been used since around the 1970’s.

Over the past 20 years, this lite wall schedule 40 pipe has been often replaced with even thinner schedule 10 – leaving very little available pipe material to corrode before reaching minimum acceptable thickness limits and the inevitable failure.  For higher pressure applications, schedule 10 pipe will provide acceptable service only assuming that virtually no corrosion will take place – a known impossibility for steel pipe containing water.

Where 8 in. schedule 80 A 53 pipe having a wall thickness of 0.500 in. would have been installed back in 1935, it is not unusual to find 0.188 in. thick schedule 10 installed today.  This pipe offers less than half the wall thickness of schedule 80, as is shown in the below relative comparison, and will typically provide 10-20 years of service and little more, depending upon other factors related to the operation of the system.  Smaller diameter pipe has an inherently thinner wall, and combined with any corrosion condition, can lead to a failure in as little as 5 years or less.

A default minimum acceptable wall thickness of 0.100 in. exists for most steel fire sprinkler service, or higher depending upon pipe diameter, pressure, and construction.  For 8 in. schedule 10 steel pipe, therefore, only 0.088 in. or less remains available to corrode before reaching those minimum acceptable safe limits.  At a moderate corrosion rate of 5 MPY, the pipe will reach its minimum acceptable thickness limit in roughly 17 years, whereas should a high loss MIC condition develop having a 20 MPY rate, far less service life would exist.

• Water Flow Devastating

In most of our ultrasonic testing investigations, we can demonstrate that the advanced failure of a fire sprinkler system is directly related to water flow.  Where the pipe is filled, hydrostatically tested, and then left stationary, a certain amount of corrosion takes places, the oxygen level is depleted, the lines remain stagnant, and further corrosion virtually ceases.  This is the most reasonable explanation for older piping systems of the 1930’s found in virtually new condition today.

Where fire service piping is frequently drained or the system often extended or modified, fresh new water enters to greatly increase corrosion and pitting activity.  In the worst of examples, a constant flow of water may be present to create what is essentially an open and chemically untreated condenser water system.  This will occur where small and unaddressed leaks exist to constantly bring in fresh oxygenated water.

A frequently running jockey pump, or cold sweaty fire sprinkler pipe are two sure signs of a leak somewhere within the system.  But while a leak within a building property is likely to be addressed immediately, underground or other outdoor water loss problems often continue for years un-noticed.

In such cases, the greatest wall loss is typically found at the main line closest to the water inlet due to the greater flow of water, and due to turbulence from the city main, which introduces fresh oxygenated water.  Lowest corrosion activity, on the other hand, is typically found at the smallest diameter sprinkler branch lines – this due to their existing in an essentially dead ended and stagnant condition.

One sure tell tale sign of a water influx into fire sprinkler pipe is a cold temperature at the pipe surface, as a stagnant pipe should exist at ambient temperatures.  The rusted exterior of unpainted steel pipe, in the absence of high area humidity and especially along the bottom, is another sure sign.  Given enough cold water flow, typically un-insulated fire sprinkler pipe will actually sweat moisture condensation in the same way as an un-insulated domestic cold water, chill water, or any other cold pipe surface.  Tracking down and repairing the leak is the required response, although we have found insulating the fire pipe used as the easier alternative – ultimately leading to total system failure.

The below photographs show the type of evidence which often exists showing an active cold water flow at fire system piping.  Similar such evidence should always be investigated further and corrected.

Exterior Surface Rust This fire sprinkler pipe showed some mild general corrosion along its outer surface, and a much greater amount of rust build-up at its very bottom. Sufficient moisture had condensed at the pipe to drip onto the below steam condensate line, discoloring the fiberglass insulation.	Moisture Condensation Another example where continued water flow through the fire sprinkler main pipe produced excess moisture condensation at the surface. Sufficient moisture built up on the surface to drip on the sheetrock wall below and cause discoloration.

Two Separate Concerns This fire sprinkler pipe indicated a water flow condition due to the presence of condensed water droplets along its exterior.  In addition, the use of thin wall schedule 10 provides little available pipe wall to corrode before reaching minimum thickness limits. While either condition will limit sprinkler pipe life, both combined virtually guarantee limited service.	Cold Water Flow Another example of where a potential but hidden problem can be revealed by some close observation. Under no conditions would any stagnant piping system at room temperature condense moisture from the air.  Yet in this and other examples we have seen, the presence of water droplets at fire pipe sprinkler pipe may not be realized for its underlying threat.

Perhaps the strongest recommendation we can offer is to minimize the draining and filling of any fire sprinkler system.  This is difficult for modern office buildings, however, where renovations and changes in tenants is frequent – prompting piping changes and frequent draining and filling.  Regular testing of the fire sprinkler system, in itself and required by code, may also play a role in its deterioration.

• Pipe Quality

The quality of the steel pipe is an important factor in fire sprinkler system corrosion as well as for any piping application.  While it is almost impossible to judge the quality of a material source without first performing a metallurgical or salt spray corrosion study (an unlikely event), accepting pipe from only known and reliable sources is always a worthwhile recommendation.  Seamed pipe should always be avoided in favor of seamless stock, although commonly used in fire protection systems today.

Whenever a corrosion problem is identified at one pipe size but not at another, and given the absence of some other factor such as additions to a specific fire zone or frequent drain downs, the possibility that pipe from different steel mills, and perhaps different countries, should not be excluded.  The larger the installation, and the larger the pipe size, the more likely different sources of material were used.  Larger size projects always favor lower cost pipe.

• Piping Source

A significant volume of foreign produced pipe is found in fire protection systems.  Some may provide decades of excellent service, but often it does not.  Problems relating to quality, weld seam integrity, and corrosion resistance have been widespread enough to prompt many fire protection engineers to exclude foreign pipe products entirely.  Such specifications, however, are not always followed.  Pipe having no ASTM stamp should always be assumed of foreign origin and further investigated – especially if domestic product is specified.

Pipe identification stamps are required under Section 24 of the ASTM code.  This stamp is typically stenciled onto the pipe in ink or paint which can be easily removed by solvent – a common event where obscuring the origin of the pipe is desired.  In contrast, American manufacturers are proud of their products.

Orienting the pipe away from view is another common practice, as we document below.

A past ultrasonic testing investigation found multiple examples of pipe failure within 2 years of installation and severe pitting activity throughout.

A design specification only allowed U.S. produced pipe.  Yet the pipe had no markings of any kind.  Further investigation at areas of easily accessible pipe showed evidence that the ASTM stamps may have been removed.

28 ft. Above A closer inspection at a main distribution header located 28 ft. above the building walkway and immediately under the roof identified all pipe stamps facing up and away from view. At the left tee connection, each pipe section was stamped at both ends as schedule 10 originating from Thailand, and with all such stamps turned directly upward so as not to be visible from 28 ft. below.	A Closer View One such stamped end is shown where the Thailand stamps could not be seen at floor level. An informal survey of approximately 20 individual sections of upper dry fire supply pipe anchored from the roof beams showed all 20 with their Thailand stamp facing straight up – an unlikely random event in fact statistically impossible.

• Dry Piping Systems

Dry pipe or pre-action fire sprinkler systems are common wherever freezing is a concern, where accidental water damage may occur, and for other engineering and design reasons.  With them, they bring new and typically unseen problems in the area of corrosion activity.

Since dry fire systems are “dry” in name only, such fire sprinkler piping typically presents one of the most corrosion susceptible environments for steel pipe due to the moisture which inevitably remains within the system.  A “dry” fire sprinkler system may be drained after hydrostatic testing or normal flow testing, but a significant volume of water always stays behind to produce a moisture and oxygen saturated environment far more aggressive against steel pipe than a completely water filled system.

Pitch of the pipe is required under certain circumstances, but rarely followed.  Even when installed correctly, the pitch is inadequate to drains the pipe sufficiently to remove the moisture threat.  For this reason, corrosion at the lower areas of piping often exceeds the more elevated areas.

To combat this problem, galvanized steel pipe is often employed in dry systems, and will generally provide longer service life strictly depending upon the quality of the pipe and strength of the galvanizing finish.  This benefit provided by galvanized pipe is actually double edged.  Once one or more areas of the galvanized finish fail, a much greater degree of corrosive energy is focused on those select areas to produce extremely high corrosion rates which can easily exceed 20 MPY, and produce advanced failure.

Corrosion Activity At Top This 15 year old carbon steel dry fire sprinkler pipe offers extremely little corrosion activity at the top half or 12 o’clock area. Ultrasonic testing showed wall thickness values at or very near new schedule 40 pipe specifications, and visual inspection shows the pipe in like new condition.	Corrosion Activity At Bottom The bottom of the very same section of pipe shows dramatically different results.  Pipe corrosion is heavy, resulting in tuberculation of the entire bottom and lower sides. Here, ultrasonic testing shows severe wall loss and low thickness measurements below minimum acceptable standards.

Corrosion Dividing Line This photo well illustrates the separation between virtually new pipe above the water line, and significant deterioration below the water line. Ultrasonic testing proved the upper areas of the pipe in like new condition and suitable for decades of further service, while the bottom of the pipe was found in imminent danger of failure.  Some of this particular section of pipe had already been removed due to pinhole leaks.

Cut Groove Failure As is often the case, multiple problems often exist simultaneously to greatly advance a corrosion related failure. In the other three photos of this set representing investigations into the dry fire piping systems, testing found wide variance in condition based upon the pipe orientation. This problem becomes more critical, however, given that the pipe groove is cut, and therefore substantial pipe wall removed upon installation.

While a corrosive potential would be acting against the entire interior surface of a carbon steel pipe, much of that same corrosive energy in a “dry” galvanized system is now focused at a very limited surface area where the galvanizing finish has been compromised.  The result is typically pinholes through the pipe wall as if produced by a drill bit, at the same time the overwhelming majority of pipe wall remains at or near factory specification.  Under such conditions, galvanized pipe will actually fail sooner than standard carbon steel, where corrosive attack is distributed over a larger surface area.

• Microbiologically Influenced Corrosion

MIC has the potential to virtually destroy an entire fire sprinkler system in just a few years given corrosion rates which can exceed 50 MPY.  Microbiologically influenced corrosion is often suggested or mistaken for the more common condition of simple under deposit corrosion, which can produce a similar result.

Identifying an MIC condition requires a thorough metallurgical and microbiological examination of a current pipe sample.  What causes MIC and how it can be prevented are less known.  Since it is biological, a through cleaning and sterilization of the pipe when first installed is mandatory, although virtually impossible to complete due to the dead end configuration of a typical fire system.  Minimizing water flow through the system once placed into service is another easy recommendation to offer, since the greater the amount of water means the greater the potential microbiological source.

MIC presents a greater threat to victaulic or clamped joint piping systems due to the end to end gap which exists for microbiological growth to accumulate and flourish.  The threat of MIC is so severe that once it is firmly established, most corrosion authorities consider it impossible to correct in any piping system.  A cut groove further compounds the problem.

• Design And Layout

A close mechanical review of a failed fire protection system can often identify the potential cause.  For wet systems installed at large warehouse facilities, upper level supply pipe installed parallel to a graded roof will produce trapped air pockets.  A wet fire system then, in effect, becomes a dry fire system subject to the same severe corrosion and pitting at and below the water line.

Fire systems requiring fire booster pumps will often have small leaks at the pump packing or seals.  This is insignificant in terms of water loss, but severely damaging to the inlet piping due to a constant influx of fresh oxygenated water.  Insufficient top vents are as common to wet fire systems as insufficient drains are to dry fire systems.

• An Even Greater Threat

But while a leak at a fire sprinkler line presents the obvious problems, a further and often unrecognized threat exists in the form of the iron oxide deposits created as a result of the corrosion process.  Such deposits can easily add up to thousands of pounds of movable rust debris capable of being dislodged from the shock of a 150 HP fire pump kicking on, and moving the rust product downstream into the critical control and actuating valves, and ultimately – the sprinkler heads.

6 Year Old Dry Parking Garage In this “inclined” and “dry” fire sprinkler system virtually no corrosion loss was identified at the top of any pipe section. At the bottom, severe wall loss was measured suggesting the production of large iron oxide deposits and prompting further exploration. Removing various fittings identified loose rust deposits in most examples exceeding the top of the lateral branch tees.  No water could possibly flow into the branch lines during a fire emergency.

14 Year Old Dry Attic Pipe Ultrasonic testing of this attic level dry fire pipe showed severe wall loss along the bottom to approximately 50% of its original schedule 10 wall thickness. This suggested high interior deposits likely existing. Further investigation found sections of 4 in. main pipe which had been removed due to failure and left behind. No concern had been previously raised for fire pipe so constricted that water flow would have been impossible.

20 Year Old Dry Attic Pipe Failure at one dry fire pipe section prompted cutting out various examples for visual examination – showing severe internal constriction. Ultrasonic investigation identified severe wall loss along the bottom of the piping to suggest that a large volume of rust deposits exist. No water could possibly flow through this or other sections of pipe similarly deterioration and constricted.

Nursing Home Dry Attic Pipe Severe wall loss identified through ultrasonic investigation suggested high interior deposits at this small nursing facility. To confirm this finding, multiple sections of pipe from different areas of concern were removed to show rust lining the interior. Sections of pipe were tapped onto the sidewalk in front of the facility to reveal sufficient loose rust to likely prevent the fire system from functioning.

A pipe testing report showing a 40% average loss of wall thickness over 500 ft. of a 10 in. schedule 10 fire sprinkler main would clearly explain the leaks and operating problems experienced.  Yet, that same 40% loss of steel from new pipe which weighed 21 lbs. per linear foot, also means that 8.4 lbs. of steel per linear foot has now been removed from the pipe and placed into its interior in the form of less dense but of greater volume iron oxide particulates.

For this 500 ft. fire sprinkler run, over 4,000 lbs. of steel can be assumed to exist inside the pipe in the form of both hardened tubercular deposits and loose iron oxide mud – some of which will be easily re-suspended during a fire emergency.

Overall, new concerns exist for fire sprinkler systems which seem to not have existed decades ago.  This demands much greater planning in their construction and maintenance, and some form of corrosion monitoring to detect problems before they expand beyond repair.

© Copyright 2024 – William P. Duncan, CorrView International, LLC