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Interior Rust Deposits

Although rarely used, the most critical piping for any building property or plant operation is unquestionably at the fire sprinkler system.  Corrosion problems and induced failures at cooling tower condenser water, chill water, steam, or other HVAC and plumbing piping may produce a loss of service, inconvenience, property damage, shutdown, and even millions of dollars in monetary losses, but the failure of a fire sprinkler line always threatens the loss of human life.

Often considered trouble free, corrosion related failures at fire sprinkler lines have greatly increased over the past two decades – raising not only operating and repair costs, but the threat to building inhabitants as well.

• No Corrosion Protection Provided

Unlike most HVAC and industrial cooling applications, chemical corrosion protection is rarely provided to the steel pipe which carries fire protection water.  This is because traditionally, corrosion problems have not been a significant concern.  It is also due to the virtual impossibility of providing effective chemical protection to a system that is essentially comprised of hundreds of one way dead end lines.

Up until 15-20 years ago, it was almost unheard of for fire sprinkler pipe to fail, or to learn of sprinkler pipe in need of replacement due to corrosion effects.  This prior lack of concern has changed in recent years due to a major change in materials – thinner gauge pipe, more corrosion susceptible steel, weld seamed pipe, and undersized pipe.  Add to that a higher frequency of drain downs, testing, and modifications to fire protection systems which introduce fresh and oxygenated water to drive the corrosion reaction forward.

Corrosion rates which once could be expected to range below 0.5 mils per year (MPY) for fire sprinkler lines two decades ago, are now often measured at 5 mils per year and above.

• Numerous Initiating Sources

Many different factors can lead to a corrosion problem at a fire sprinkler line.  Absent or improper chemical clean-out prior to beginning service may leave rust, metal filings, mill scale, varnishes, iron oxide particulates, microbiological material and foreign matter behind to produce severe problems years later.  The failure to disinfect the pipe of micro-organisms, combined with nutrients from any remaining organic and particulate debris, can easily lead to a MIC condition.

Performing the specified actions for most new fire system construction is nearly impossible due to the vast quantity of dead ends, low points and absent drains.  Furthermore, a fire sprinkler system bringing in new, fresh and oxygenated water on a regular basis is virtually guaranteed to initiate severe corrosion problems.  A frequently running jockey pump or make-up water meter, or pipe which is cold and sweating, are two sure signs of a leak or other water loss problem.

• Wall Loss Equals Rust Production

It is a correlation often unrecognized, but the presence of rust deposits at the internal pipe surface defines that wall loss has occurred.  Likewise, an identified wall loss from an ultrasonic examination, defines that iron oxide deposits have been produced.  Both conditions are directly proportional, and one condition cannot exist without the other.

Exactly how much particulate debris remains within the system is generally dependent upon the piping application and any corrective measures applied.  An open tower or condenser water line, for example, will produce the same volume of deposits for the same MPY corrosion rate as a closed chill pipe of the same size.  However, a substantial volume is washed out of the system during blowdown, filtration, and through regular maintenance.

For open systems, the presence of rust in the tower pans, strainers, and chillers, etc. is often the first sign of a corrosion problem and will prompt further investigation.  Closed circulating systems typically hold their deposits unless regularly flushed, or unless side stream filtration is provided.  Rarely opened for visual inspection, a loss of heat transfer efficiency is often the first clue that an internal deposit problem exists for closed HVAC piping.

Fire sprinkler systems provide a flow of water in only one direction to multiple dead end branch lines, and lack the benefit of circulation to move either cleaners or debris laden water to a drain or into a filter for disposal.  All iron oxide deposits, therefore, are usually held captive within the piping.  While standard flushing of a fire sprinkler system might show a limited benefit in removing some loose rust material over a limited range of piping, it will not remove those heavier deposits under which the highest corrosion and pitting activity always exists.

High pressure cleaning or water jetting is the only method that will remove the deposits typically associated with a corrosion fouled fire protection system.  Its use, however, is not normally employed even where heavy internal deposits have been found.

• The True Meaning Of MPY Corrosion Loss

Minimizing the actual threat of many corrosion problems is the misconception of exactly what mils per year (MPY) means in terms of wall loss.  Different authorities may provide recommended acceptable wall loss estimates in MPY, but the true impact of that pipe loss is rarely understood or appreciated in real world terms.

A corrosion rate of 5 MPY is obviously worse that a 2 MPY rate, but to what degree in terms of pipe service life and volume of deposits produced?  A low corrosion rate of 1 MPY at a 10 in. fire sprinkler main, for example, while it would be viewed as acceptable by most authorities, actually translates to an annual physical loss of 11 lbs. of steel for every 100 linear feet of pipe.

At 10 MPY, approximately 107 lbs. of metal is lost.  And that is per year.  Multiplied by the number of years in service and its overall length, and the true magnitude of pipe corrosion takes on much greater significance than when reported as simply 1, 2, or 5 mils per year.  The below table illustrates just how much steel is lost at various corrosion rates and for various pipe sizes, and is applicable for any piping system.

• Deposits The True Threat

But while even a 5 MPY loss of metal can be tolerated by many piping systems for an extended period of time before resulting in a leak, it is the deposits created, and their eventual deposition and effect, that will inevitably produce far more serious and long term secondary problems.

Steel, when corroded back into iron oxide, produces a significantly greater volume of less dense material by a factor of approximately 12 times.  Such deposits, in turn, produce substantial loss of heat transfer efficiency, constricted flow, and under deposit pitting and wall loss.  At a low corrosion rate of 1 MPY for an office building having 40 floors of 24 in. chill water piping, 242 lbs. of steel will be lost for each year of service at just the risers alone.

In its less dense form of iron oxide, however, this same steel will exist having a volume of 5.92 cubic feet.  See the below table for rust volumes produced at various corrosion rates and pipe sizes.

• Key Factors For Failure

It is necessary to recognize that all carbon steel pipe will corrode to some degree.  Even when chemically protected, pipe corrosion can only be minimized, never stopped.  With the application of chemical corrosion inhibitors generally not feasible for fire sprinkler service, the rate of its inevitable deterioration becomes generally dependent upon certain fixed parameters.

Of first importance is the amount of fresh water entering the pipe – with highest corrosion rates consistently found where water flow is the greatest, such as near the inlet.

A second critical factor is pipe schedule.  At any given corrosion rate, the service life of a pipe before failure is directly dependent upon its initial wall thickness.  For this reason alone, far more sprinkler failures occur today due to the common use of thin wall schedule 10 and schedule 7 pipe.  Schedule 10 offers savings on material, time, and installation costs, but at the trade-off of severely reduced service life.

Whereas extra strong schedule 80 would have been typically installed 80 years ago for fire service, lighter schedule 40 has been used since around the mid 1960’s.  Over the past 25 years, this thin wall schedule 40 fire pipe has been widely replaced with thinner schedule 10 – leaving very little available pipe wall to corrode before reaching minimum acceptable thickness limits and inevitable failure.  Now, schedule 7 pipe is showing up in failure investigations – and amazingly even for threaded lines!

Under the clearly greater overall corrosion threat which exists today, little exists to corrode before this schedule 10 pipe will reach minimum acceptable standards.  For higher pressure applications having a higher minimum acceptable thickness limit, schedule 10 pipe will provide service only assuming that virtually no corrosion will take place – a known impossibility.

• Design vs. Reality

The design of a fire system will take into account the smoothness of the pipe interior surface as a factor related to water flow.  In fact, certain piping manufacturers stress the smoothness and therefor low resistance and internal flow coefficient of their product which in turn may allow a fire protection engineer to specify smaller diameter pipe offering the same water flow as a larger but more flow resistant material.  Fire sprinkler design programs take this into account as well as the flow reduction at every elbow and tee.

Fast forward a decade or more to the same fire system and those flow calculations are no longer valid, though presumed so.  Corrosion activity, still far below the point of producing a leak or failure, will have likely grown at the pipe surface to such a degree as to dramatically reduce flow rate characteristics – yet never considered as part of any follow-up inspection.  Corrosion activity at elbows can be especially limiting to water flow.

Traditionally, highest corrosion activity is present at water storage tanks and especially at dual purpose domestic cold water / fire reserve tanks which have unlimited oxygen to promote corrosion.  The severe constriction of such piping, and in an area of least water pressure to push the water forward, is a high priority concern greatly overlooked.  Clearly, water flow will never be provided to the original design specifications.  Internal pipe conditions at fire tank outlet piping as well as standard runs are provided for systems of about 30+ years of service.  Similar to worse pipe constriction has been documented at dry fire steel piping systems in under 10 years.

30 – 40 Years Of Service

Main Fire Service Lines	Fire Tank Outlets

• Threat Varies According To Application

The ultimate impact of internal deposits, similar to wall loss itself, is greatly dependent upon the piping system involved.  Deposits produced at an open piping system will be observed eventually and likely addressed, while a closed system will instead conceal is problem until a heat transfer loss, leak, or some other operating problem is realized.

Corrosion at a fire sprinkler system is often totally concealed from view, and may remain unrecognized for years, or until a fire occurs.  No external signs or indicators normally exist to suggest a corrosion problem prior to a leak occurring – at which time the major damage, often irrevocable, has already taken place.

Ultrasound, which is unquestionably the most cost-effective nondestructive technology available to detect a corrosion problem in pipe, is rarely used as a preventative tool in evaluating fire sprinkler systems.  Visual examination methods are obviously effective dependent upon the quantity of piping areas inspected and the potential to contain a problem.

In most cases, the concern raised due to a leak at fire sprinkler pipe is more directed to the potential for water damage or cost of replacement, rather than whether the pipe will provide the necessary water flow during a fire emergency.  And yet the latter, by far, presents the greatest threat.

• Threat Greatest To Dry Fire Systems

In fact, years of corrosion activity can easily produce thousands of pounds of debris capable of being dislodged from the shock of a fire pump kicking in, and then forced downstream into the critical actuating valves, and ultimately – the sprinkler heads.  At that point, all the fire fighting equipment, command and controls, sensing, planning, inspection, and emergency training suddenly becomes worthless if water cannot be supplied to the source of the fire.

The potential for such a catastrophe is easily demonstrated.  A 25% wall loss at an 8 in. schedule 10 sprinkler main, for example, is still not likely to produce any notice in the form of a leak or failure.  Yet, that same 25% loss of steel from new pipe which weighed 17 lbs. per linear foot, also means that 4.25 lbs. of steel per linear foot has now been removed from the pipe, and placed into its interior in the form of less dense iron oxide particulates.

Deposits Exceed Branch Connections

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.

For a 600 ft. main sprinkler feed, it is easy to estimate that 2,500 lbs. of rust product would now exists in some proportion of hardened deposits or tubercles attached to the pipe’s interior wall, and the rest as loose sediment and mud along the bottom.  This material accumulates with time, ultimately to the point where the pipe wall finally fails and brings attention to the problem, or to when a fire occurs.

In a very possible worse case scenario, this loose rust and mud will be dislodged by the shocking action of the fire pump starting up in response to a fire call.  With perhaps thousands of pounds of loose material suddenly rushing downstream toward the fire’s location, the potential to block closed any control or pre-action valves, reducers, tees, small diameter distribution lines, or fire sprinkler heads is tremendous.

Such an actual event, whereby the fire sprinkler lines have been found totally clogged with rust and mud in a fire emergency, has actually occurred in previous instances – leaving those involved without the fire protection they thought existed.

• Further Examples

CorrView International, LLC offers a series of Photo Galleries taken from past ultrasonic piping investigations, which address the above and additional corrosion conditions.  A review of the different types of corrosion is often helpful in initially determining the likely corrosion cause.

In many cases, however, a combination of conditions will exist within the same piping system.  View our extended Photo Galleries of different corrosion types and failure conditions, including fire protection.

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