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Multiple Threats

Any time water is introduced into steel piping it will corrode that pipe to varying levels and produce a potentially substantial volume of iron oxide rust product as a result.  This rust product in turn causes various negative impacts to building piping systems ranging from reduced heat transfer efficiency, to accelerated pump seal damage, to severe under deposit pitting resulting in system failure.

For a recirculating system, rust typically migrates around until it settles into low flow or dead-end areas, cooling tower basins and sumps, heat exchangers, condenser heads, or is captured by strainers and water filtration units.  Normal maintenance performed on most HVAC piping systems will reveal evidence of a corrosion problem, which if acted upon appropriately and in a timely manner, will minimize the damage caused.

Fire systems, however, are static, and any rust product created through corrosion remains trapped within the system where it is hidden from view.  For fire systems, there is no advance notice of a corrosion problem unless one of six events occur:

• • • A water leak occurs prompting further investigation
    • Renovation or modification to the system reveals internal rust deposits
    • Ultrasonic inspection identifies a moderate to large wall loss
    • A 5 year visual inspection identifies internal rust deposits
    • A removed fire sprinkler head is found filled with rust product
    • The fire sprinkler system fails to deliver water during a true fire emergency

Of the above potential scenarios, the failure of a fire system to deliver water to the source of the fire is obviously the greatest threat, although frequently the least of anyone’s concern.

Ironically, dry and pre-action fire sprinkler systems are installed over the most critical environments specifically due to the concern for an unintentional sprinkler head release, pipe leak, or some other failure issue.  By their design, however, both systems exist under significantly higher corrosion threats whereby the entirely opposite result is produced.

Of the various types of water based fire protection systems, standpipe systems exhibit the lowest corrosion levels and the least threat.  Wet fire sprinklers are generally more vulnerable to corrosion at their inlet line where a greater amount of fresh oxygenated water exists, prior to any booster pump, and at any top level branch lines which may be air bound.  Otherwise, long service life can be expected well exceeding 50 years.

Dry and pre-action systems are similar in their piping layout, and suffer greatly from the fact that they are never “dry.”  They can be named dry systems, viewed as such, expected to drain fully, and believed to hold no water over critical computer and electronic equipment, but the fact remains that all such systems contain some water in all horizontal lines.  If they are filled even once, some volume of water will remain due to the virtual impossibility of removing it completely.  Of the many misconceptions that exist regarding fire protection systems, believing that a dry fire system is actually “dry” tops the list.  Its remaining water, covered with abundant air and oxygen, then initiates an accelerated and often a very aggressive corrosion attack against the pipe.

Even having the best and most effective design and installation, and incorporating a grade which for most such systems never exists, no pre-action fire system will fully drain of water.  Some designs have their one and only drain at the pre-action discharge valve, making it virtually impossible for water to drain back along as much as 300 ft. or more through perfectly level pipe.

With water inevitably trapped within any dry or pre-action fire sprinkler system, bottom drains are typically installed at low points of the system in order to facilitate its removal.  Corrosion activity at those low points where auxiliary drains have been installed should be minimal, although our ultrasonic investigations frequently identify high wall loss having no other explanation than to indicate that such drains have not being utilized.  This is a problem found in almost all properties investigated, with the results illustrating the fact that drain valves installed for the purpose of removing water from the system and therefore reducing corrosion activity are often ignored.  In many examples, evidence exists in the form of unbroken paint at threaded plugs that such valves have never been utilized for their intended purpose – not even once.

Bottom drains, of course, are rendered somewhat ineffective due to the raised internal groove impressed into the pipe for coupling purposes.  This raised groove has been shown to produce an internal dam to water movement for which even a steep incline or slope to the pipe will not permit water to fully drain.  High pipe wall deterioration above a rolled groove has been documented under even the most dramatic incline of a parking garage having dry fire pipe installed parallel to the parking ramps.

In the two pipe sections shown below, taken off a commercial pre-action fire sprinkler system, the internal raised groove is clearly shown along with an obvious water line. Also shown is the corrosion product along the bottom of the pipe produced as a result of water having been trapped by this raised groove.  Deep pitting along the bottom of the left side example has produced the characteristic form of failure common to most pre-action systems in the form of a pinhole leak, which can be seen.

Below we show two further examples of “dry” fire sprinkler systems having failed, with both having much higher water levels at approximately 50% their volume.  Our Internet site, offers extensive photographic evidence to this common issue.

The movement toward galvanized steel pipe in the interest to avoid this problem has produced no benefit in terms of system longevity, and in fact has been well documented to fail sooner in most examples.  A very significant benefit of galvanized steel use, however, is that it produces significantly lesser iron oxide rust deposits in comparison to carbon steel.  This is well illustrated by the above left photo showing a pinhole failure at a section of galvanized steel pipe which has produced insignificant rust deposit due the very aggressive yet localized mode of failure for galvanized steel pipe.

In contrast, we show another 4 in. pre-action main of carbon steel at left.  Its removal from service due to failure exposes a large volume of rust deposits lining 25% or more of its entire length.

Quite obviously, even though both examples of pipe failed after a similar length of time in service, and produced a panic due to a water release, the greater threat is from the much greater volume of rust deposits from the steel pipe migrating downstream where it would most likely clog the fire sprinkler heads.

Stated earlier, although the primary concern of most facility managers and engineers is to a leak occurring at a dry or pre-action fire system over critical electrical and computer related equipment, the far greater threat is due to the production of internal rust deposits and the potential for those deposits to restrict or even stop water flow during a true fire emergency.

The potential for rust to render an entire fire sprinkler system useless has been well documented, and CorrView has been involved in multiple such investigations where either ultrasonic testing identified high wall loss revealing massive internal deposits when disassembled, or as a result of an actual building fire where the system failed to function.  Fortunately, tragedies resulting from such events have been relatively few.

The potential of such rust interfering with and even stopping water flow should be obvious as well as a primary concern to anyone.

A dramatic illustration of this rust threat is provided not by a fire system, but at an operating 24 in. diameter condenser water riser.  Shown at left in one of our more spectacular photographs, rust product accumulating within the small 1 in. threaded pipe nipple between the brass valve and threadolet created an aggressive corrosion condition – ultimately corroding completely through the 1 in. steel pipe and allowing the brass valve to totally separate.  One can see the block wall uninterrupted.

Even though under more than 150 PSI of pressure, this small amount of rust inside the remaining nipple and threadolet, and with no barrier or restriction to its lateral movement, was enough to hold back an exceptional internal force and over 14,000 gallons of water to the roof 21 floors above.  Damages due to its release would have easily exceeded $1 million dollars had it not been identified during our ultrasonic assessment of the condenser water system and repaired.  Yet this small amount of rust potentially existed for years prior to its discovery.

In comparison, a typical fire sprinkler head has a substantially smaller inside diameter; requiring far less rust to produce the same result.  Lower pressures may also exist.

In the photograph at left, a thread leak at a fire sprinkler head to a dry fire system revealed its inside chamber totally packed with rust product driven downstream during test cycles.  Quite obviously, were this head to “open” in response to a fire occurring below, no water would have ever passed through its orifice to reach the fire.

Subsequent removal of additional sprinkler heads along the same fire zone identified the same condition.

• Internal Rust Product:

From the hundreds of CorrView investigations performed at dry and pre-action systems, the common denominator to all identified problems has been water remaining within the system after testing.  Although efforts are made to minimize this threat by design, introducing a grade to the lines to facilitate draining, and through the use of galvanized steel pipe, we consider the threat inherent to virtually all pre-action systems.

We have clearly and repeatedly stated that as long as water is introduced into the piping system even once, corrosion activity leading to its accelerated failure is almost inevitable.

For all such systems, the greatest negative impact is caused by the first flow or hydrostatic test, since substantial water always remains.  Minimizing the introduction of additional fresh water into the piping system during each test cycle is favored; an issue which many properties have sought to address through the installation of isolation test valves.  If the isolation valve is utilized, the flow valve can be tested for proper operation without introducing additional water into the fire distribution network.

If not, fresh oxygenated water is introduced on a regular basis which then amplifies the corrosion problem, with accelerated system failure being the usual result.  From a corrosion perspective, the only means to guarantee trouble free operation of any dry or pre-action fire system is to never test it with water, which is quite contradictory to fire codes.

One adaptation we have made to our pre-action fire sprinkler piping assessment reports is to attempt to quantify the volume of internal rust product present within any section of horizontally oriented pre-action fire sprinkler pipe based upon wall thickness measurements alone.

By modifying our field testing method to record the first 9 wall thickness measurements along the bottom and lower sides of the pipe, and then the last 3 measurements at the top, we can produce a more accurate representation of the deterioration caused by water remaining within the pipe.  We can also somewhat negate the inherent error caused by the now common manufacture of undersized pipe, and its misleading indication toward a higher corrosion condition.

At left, the bargraph of our 12 wall thickness measurements clearly defines a high wall loss along the bottom of this 4 in. schedule 40 carbon steel pipe by the first 9 low values; having a bottom loss of over 0.050 in.  It also defines that the pipe was likely manufactured undersized, and below its intended ASTM specification.

To establish a standard for our reports, we returned to a prior pre-action fire system investigation where we had the opportunity to ultrasonically document the wall thickness at a section of carbon steel pre-action fire sprinkler pipe which had failed, and where visual inspection of the saved pipe section revealed significant rust product build-up.

Provided by the lower left photo is an internal view of a 4 in. schedule 40 pipe section showing a large volume of rust occupying the bottom 20% of its volume.  This is substantially loose rust product which will be forced downstream once the pre-action valve opens in response to a fire, or during a full test cycle.  In order to better illustrate the significance of this finding and concern to facility managers and building engineers during our site visit, this approximate 4 ft. length of failed 4 in. fire pipe was turned on its end and tapped against the floor – dislodging a significant volume of loose iron oxide rust product, as shown below right.

More than 1 cup of rust product was released – itself capable of covering and completely preventing the movement of water through the typically small 0.250 in. sprinkler head orifice.  Even though the volume of rust product released was large, much more rust product still remained on the surface!

Any reasonable interpretation of this finding would be that such a volume of loose rust product presents a severe threat.  For this pre-action zone, approximately 125 ft. of the same 4 in. main line existed before reaching the first fire sprinkler head.  Taking this 1 cup volume of rust product originating from a 4 ft. length of the same pipe and extrapolating it over its entire 125 ft. run then suggests a potential volume of rust totaling 31 cups, or a volume of approximately 2 gallons of rust.

Quite obviously, such a volume of loose rust deposit would be sufficient to completely cover and clog the small 0.250 in. orifice of any fire sprinkler head, or multiple fire heads.  In fact, substantially less rust product would likely produce the same impact and render the fire sprinkler system totally worthless.

In terms of wall thickness, our ultrasonic measurement of this failed 4 in. pipe section having an initial wall thickness of 0.237 in. identified an average wall thickness of 0.137 in. and a minimum wall thickness of 0.109 in.  A wall thickness of 0.000 in. existed at the leak.  Such measurements now become very useful in judging the condition of other examples of pipe given that the volume of internal rust deposits of any in-service section of pipe will be directly proportional to the amount of pipe wall lost.

For the above example of pipe, our theoretical calculation based upon wall loss produces an estimate of 4.3 in.³ of rust per linear foot at this 4 in. diameter schedule 40 steel pipe representing an average wall loss of 42.2% of wall thickness along its bottom from between 4 o’clock and 8 o’clock.

Based upon this information, we have established a gradient of concern beginning at 0 rust — to over 6 in.³/ft. of rust, and present that theoretical calculation for every section of horizontal dry or pre-action fire sprinkler pipe tested in our ultrasonic investigations.  This allows us to provide a reasonable expectation or estimate of accumulated iron oxide rust deposits based upon ultrasonic testing results, and most importantly, provides an estimate to the potential for blockage at the most vulnerable sprinkler heads.

At left, a further analysis of the same 12 wall thickness measurements referenced above defines high pitting activity and a significant volume of iron oxide rust product generated.  With an estimate of 4.3 in.³ of rust product generated per each linear foot, we can estimate a severe blockage threat present.

In such cases, the removal of the suspected problem area is recommended in order to facilitate a robotic visual inspection of the entire line.

A second example of this threat is provided from an 18 in. section of 1 in. schedule 40 “dry” fire sprinkler branch pipe removed for renovation purposes.  The pipe was not leaking, had not failed, nor was it suspected by anyone of containing any internal rust.  Had renovations not been carried out, its condition would never had been discovered until it failure or a fire emergency occurred.  With only a moderate corrosion loss and high wall thickness still suggesting acceptable service life remaining, a significant 1.9 oz. (53.4 grams) of iron oxide rust product was found contained within this short length; having a volume of just slightly under 1/4 cup.

In order to dramatize the impact rust product has upon the operation of a fire sprinkler system, we used an actual fire sprinkler head for our informal experiment.  With a ½ in. thread, this brass body sprinkler head has an actual inside diameter of its orifice once activated at 0.250 in. or 1/4 in.

	At left we show the example of 1 in. schedule 40 branch pipe in its original condition.
	Next we show the volume of loose rust removed from this same 1 in. pipe section by moderately brushing its interior.
	At left, that same volume of rust almost fills a 1/4 measuring cup – an amazing volume of rust originating from just one 18 in. section of 1 in. branch piping.
	At left, a standard fire sprinkler head in new condition and having a ½ in. thread.
	Next, a downward view into the body of the sprinkler head shows the copper plug sealing the 0.250 in. or 1/4 in. diameter orifice which opens in response to fire.  This orifice is approximately half the size of the 0.534 in. inside diameter of the threaded body itself.
	With the 1/4 cup volume of rust removed from this branch pipe shown at right, it takes just ½ teaspoon of that rust product to completely fill the sprinkler head body or cavity. This ½ teaspoon volume inside the sprinkler head body represents rust having a weight of only 0.09 oz. or 2.6 grams.

In fact, calculations show that the 1/4 cup volume of rust removed exclusively from this one 18 in. section of 1 in. branch piping is capable of completely filling more than 20 fire sprinkler heads.

Of course many different variables exist such as water pressure, water velocity, rust composition and texture, rust particulate size, level of its attachment to the pipe wall, orientation, orifice size, the length of travel from the beginning of the system to the sprinkler heads themselves, system size, pipe size, system design and functioning, and many other criteria.  Nevertheless, this simple experiment clearly illustrates the potential for relatively small volumes of rust product to significantly impact water flow.

Similar to the 4 in. example beginning this discussion, we believe it is reasonable to conclude that were this same section of 1 in. branch pipe still in service, and were a fire to occur activating any downstream fire sprinkler head, there would have been little to no water delivered to the fire below.

• Conclusion

Dry and pre-action fire sprinkler systems are substantially more complicated than any other form of fire protection system, yet all rely on the delivery of a sufficient volume of water to extinguish the fire.  Extensive flow calculations are required by skilled fire protection engineers in their layout and design, and even take into account the surface texture of the pipe and the number of elbows or tees present in order to ensure proper water delivery.

Unconsidered in this advance planning is the ever present issue of corrosion, and the fact that its action to negate those flow calculations and slow the flow of water down begins on the very first day water is introduced.

Initially, this reduction in water flow is small and even insignificant.  As it develops further, it will produce an impact, and in its worst form as evidenced by the above two real life examples – potential tragedy of unknown proportion.

In virtually all our initial conversations relating to a leak at a fire sprinkler system, the one and only concern raised is to potential damage to equipment and operations.  While corrosion activity is the known cause to a leak event, its potential to render the entire fire protection system worthless is almost never a concern.

© Copyright 2023 – William P. Duncan, CorrView International, LLC