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Internal Rust Product

A first level of concern for any piping system is obviously due to low wall thickness in some areas which may present the opportunity for leaks to occur.  This is of great importance for dry fire sprinkler systems wherever computer equipment is involved, and especially so at critical facilities.  Although defined and designed as pre-action piping systems where water should not exist and is actually assumed to be dry, sufficient water typically remains within the piping after testing to leak on electronic equipment and the occupants below should a failure occur.  In fact, any dry or pre-action fire sprinkler system is “dry” in name only, and many sections can remain filled 25 % or more with water.

Even having the best and most effective design and installation, and having a grade which is otherwise rarely installed, no pre-action fire system will fully drain of water.  Once again, such systems are assumed to fully drain although they never will.  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.  Where supplemental drain valves have been installed, such as for U bends around beams, ductwork, or to the ends of main supply lines, we find those drains generally unused.

The second concern, although much more important in many regards, is the generation of less dense iron oxide rust product resulting from corroding steel pipe, and the potential for its build-up to reduce or completely stop water flow during a true fire emergency, as we know to have happened in many tragic events.  Unlike a leaking pipe, this threat is generally undefined, and remains hidden from view unless actively sought, until a failure occurs, or during a true fire emergency.

Recognizing this threat after a fire has broken out is the worst of all possible scenarios.

From the hundreds of investigations performed at 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.

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 can be addressed through the installation of isolation test valves.  Yet, questions have been raised to the utilization of the test valves as intended.  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 unfortunately contradictory to fire codes.

Although water is inevitably trapped within any dry or pre-action fire sprinkler system, bottom drains are typically installed at low points of systems in order to facilitate water removal.  Corrosion activity at those low points where auxiliary drains have been installed should be minimal, although our investigations have frequently identified high wall loss having no other explanation than to indicate that such drains are not being utilized.

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 undersized pipe and its misleading indication toward a higher corrosion condition.

To establish a standard, we return to a prior pre-action fire sprinkler 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.

Shown in the photo at left, 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 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 at right.

Any reasonable interpretation of this finding would be that such a volume of loose rust product multiplied by approximately 25 times its same length prior to the first sprinkler head would result in sufficient rust 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 ineffective.  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.

In terms of wall thickness, our ultrasonic measurement of this failed pipe section identified an average wall thickness of 0.137 in. and a minimum wall thickness of 0.109 in.  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 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 pre-action fire sprinkler pipe.

For galvanized steel pre-action fire sprinkler pipe, the most significant benefit is the substantially lesser volume of iron oxide rust product created, and therefore far less threat exists for galvanized steel pre-action fire sprinkler systems having the same degree or depth of corrosion and pitting activity.  Corrosion at a galvanized steel pipe section is typically in the form of isolated deep and aggressive pitting to produce a failure over a shorter period of time, whereas for carbon steel pipe, a much larger surface area is more evenly corroded to produce substantially greater rust product over a longer period.  Since the rate of corrosion for carbon steel pipe is lower in terms of pitting, it typically remains in service for a longer duration where the hidden threat then amplifies.  This is addressed in our rust calculations for galvanized steel pipe.

A second example of this threat is provided from an 18 in. section of 1 in. diameter 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 any internal rust.  Had renovations not been carried out, its condition would never had been discovered until a fire emergency.

With only a moderate wall loss and 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 branch pipe in its original condition and at right a standard fire sprinkler head in new condition.

Below left, a downward view into the body of the sprinkler head shows the copper plug sealing the 0.250 in. diameter orifice which opens during a fire event.  This orifice is approximately half the size of the 0.534 in. inside diameter of the threaded body itself.  In the below right photograph, we show the sprinkler head filled by just a ½ teaspoon of rust product from the near 1/4 cup of iron oxide released from this 18 in. pipe section; its full volume of rust is also shown.

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 pipe attachment, 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 indicates 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 its downstream fire sprinkler head, there would have been little to no water delivered to the fire below.

An argument which has been presented to us both to champion the benefits of galvanized steel pipe over carbon steel pipe, as well as to rationalize the generally advanced failure rates of galvanized steel pipe, is that galvanized steel pipe will produce a failure far in advance of where the rust volume generated by the same level of corrosion of carbon steel pipe could render the entire fire system useless.

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