System – “Dry” And Pre-Action Fire Sprinkler

• Overview

Dry and pre-action fire sprinkler systems are quite similar in piping design, and differ primarily by the way in which they are activated.  Their design developed out of a concern of having large volumes of water above critical computer equipment in data centers, and where the malfunction or accidental impact to a fire sprinkler head would produce massive water related damage.  The other application is primarily for dry systems at outdoor parking garages, loading docks, and in building space where the fire system is subject to freezing temperatures.

Virtually all banks, financial institutions, data centers, and the world’s largest movers of digital data have pre-action fire sprinkler systems designed to not only provide effective fire protection, but to absolutely positively never discharge water unless a true fire emergency exists.  The activation of multiple heat or smoke sensors provides further assurance that the system will only discharge in response to a true fire emergency.

Contrary to the understanding of those relying upon such systems, however, corrosion activity for dry and pre-action fire sprinkler systems is substantially greater than any wet system.  Counter balancing the benefit of preventing an accidental release of water through a sprinkler head, almost all dry and pre-action fire sprinkler systems will fail long before their exact same equivalent wet fire sprinkler system.

• Common problems

The most fundamental issue related to the advanced failure of such systems is the fact that they are never actually dry.  After decades of evidence to the contrary, they continue to be termed “dry,” but are dry in name only.  Property owners and operators will commonly express surprise and ask how a dry piping system could possibly corrode.  Yet part of the answer is quite simple.

Once filled with water for testing, a substantial amount of water remains within the system to create an aggressive corrosion condition.  Partially water filled pipe has a covering of air and oxygen to fuel corrosion to a far greater degree than fully water filled pipe – a condition well recognized by all corrosion authorities.  Replace that water and oxygen on a regular basis during regular testing the system and the process starts all over.  Such fire protection systems, while promoted, discussed, designed, installed, operated, and assumed to be dry and free of water as a safeguard to corrosion and accidental water release, are actually not even close to reaching that goal.

At left, a 4 in. schedule 40 “dry” fire sprinkler main at approximately 50 ft. from its zone discharge valve.  The water level which existed before it failed and was removed from service is evidenced at the approximate 5 o’clock and 7 o’clock positions.  Above the water line corrosion activity is minimal.  A poor quality ERW weld seam is shown at 1 o’clock.

Rather than an anomaly as viewed by most within the industry, almost any dry or pre-action fire system should be assumed to contain some volume of water capable of producing a similar result.  The fact that we have measured heavy deterioration at dry fire sprinkler pipe immediately above grooved assembly clamps at heavily inclined parking garage ramps has proven that not even this steep grade is sufficient to drain away the water.

Today, dry and pre-action fire sprinkler systems are known to fail in under 10 years, with the survival of any such system for more than 25 years highly unlikely.  Due to today’s lower pipe quality and other factors, we have identified near new pre-action fire sprinkler systems approaching the end of their reliable service life after only 5 years of operation, and have had to advise clients to the probability that their replacement will only provide equal amount of time before the system again fails.

While water released onto critical electrical equipment is always the primary concern to facility managers and building engineers, a far greater threat actually exists related to the volume of rust which may exist hidden within the lines.  As we have written about extensively, dry and pre-action fire systems can generate huge volumes of rust.

In the photo below left, a standard sprinkler head having an outside thread of 1/2 NPT has an orifice diameter of just 0.375 in.  At center, the volume of rust removed from an 18 in. section of only moderately corroded 1 in. branch piping.  At right, it takes only 1/2 teaspoon or 0.9 oz. of this same rust product to totally fill the body of the head.  With hundreds of feet of larger main line of typically 4 in. diameter, then reducing through distribution lines and branch piping until the first sprinkler head, the probability of this fire protection functioning becomes zero.

• Greatest threats

There are a large number of variables in the design and operation of a dry or pre-action fire sprinkler system, each of which negatively impacts its ultimate service life.  The combination of certain variables, such as thinwall pipe, galvanized steel pipe, lower quality pipe, and inadequate draining typically combine to accelerate failure beyond those issues occurring individually.

Some of the many issues contributing to the advanced failure of dry and pre-action fire systems are:

• • • • Pipe material

The transition by the fire protection industry from standard schedule 40 pipe decades ago to now thinwall schedule 10 pipe for wet systems has also become the standard for dry and pre-action fire system design.  Although considered unacceptable by many fire protection engineers, the installation of thinwall schedule 10 pipe is common where cost is the primary concern.

Thinwall schedule 10 pipe has approximately half the wall thickness of standard schedule 40 pipe.  For a common sized 4 in. main, this means a wall thickness of 0.237 in. for schedule 40 pipe as opposed to 0.120 in. for schedule 10.  This use of thinwall pipe has gone even one step further to allow ultra thinwall schedule 7 having an even lower beginning wall thickness of 0.098 in.  A further negative factor in the move toward thinner pipe materials is the far lesser quality of new steel and galvanized steel pipe resulting in much higher corrosion rates.

With thinwall schedule 10 pipe now permitted for threaded branch lines, advanced failure problems really should not be unexpected, yet always produce a surprised response.  Having as little as 0.039 in. or less wall thickness remaining after threading at a 2 in. line, a dimension just slightly more than a credit card, longevity only exists as long as the corrosion of metal is assumed to not exist.

Warning stamps printed on thinwall pipe warn to the potential for a thread failure upon assembly, as shown at left, clearly try to shed liability, yet no one seems to recognize that such thin pipe has just begun day 1 of what is expected to be a long length of service.

• • • • Construction

Most fire protection pipe installed today is grooved clamped; that groove either rolled or swaged, as shown at left, or cut into the outer pipe surface.  Where thinwall schedule 10 and schedule 7 pipe is installed, it must be roll grooved due to its low wall thickness.  Less common any longer are fire systems constructed entirely of threaded pipe, with only smaller diameter distribution and branch lines being the only examples of threaded pipe likely found.  Yet pipe construction plays an important role in the deterioration of such fire systems due to its more common rolled groove, and given the restriction of the internal roll to water flow.

As an example, a perfectly straight and level section of 2” schedule 10 pipe, roll grooved at both ends and 21 ft. long will contain over ½ cup of water between its roll grooves.  That is water which will never drain away, but above which remains abundant air and oxygen to drive the corrosion process.

• • • • Carbon steel vs galvanized steel

Recognition to the fact that dry and pre-action fire sprinkler systems suffer dramatically higher corrosion losses quickly led to the adoption of galvanized steel as a solution to the problem.  Had galvanized steel remained at the same high quality available 60 years ago, this transition would have likely provided a simple solution.  However, the quality of galvanized steel has dramatically declined, and seems to continue to deteriorate.  This has led to even more advanced failures for some dry and pre-action fire sprinkler systems given its method of deterioration to galvanized steel, which is for the development of more random but aggressive deep pitting.

When carbon steel pipe corrodes, the entire internal surface at and below the waterline is impacted.  For galvanized steel pipe, no wall loss occurs as long as the zinc protective finish remains.  Once the zinc protective finish fails, then all corrosion activity focuses at this localized area which now has a different galvanic potential between the remaining zinc finish and underlying carbon steel pipe.

Today, the debate over the use carbon steel or galvanized steel pipe in a dry or pre-action fire sprinkler system continues.  The move years ago from carbon steel to galvanized steel in order to reduce corrosion activity has clearly failed, as most within the fire protection industry have come to recognize.  This has now led to a return to carbon steel pipe although in some aspects it presents a potentially greater threat.

Of the many differences between both piping materials, the single issue of internal rust product stands out beyond debate.  For carbon steel pipe, substantial rust product is generated as corrosion occurs, and it is not unusual to identify larger supply mains covered 50% and even above the side branch take-offs with large volumes of rust.  For one dry fire system as we show at left, there is no possible chance of it to function during an actual fire emergency.  Ironically, this threat increases with the use of heavier schedule 40 pipe since corrosion can occur over a longer period of time before a leak occurs – thereby generating far greater rust deposits.

Galvanized steel pipe, on the other hand, suffers more random and far deeper pitting activity, and does not produce the volume of rust product capable of interfering with water delivery prior to its more premature failure.  Once again, the installation of heavier schedule 40 pipe, having the intent to provide a longer service life, in turn increases the time period during which corrosion and rust product can accumulate.

• • • • Corrosion mechanism

There are major differences between the corrosion mechanism of carbon steel to galvanized steel pipe as well as even greater differences based upon the corrosion product which is produced.

For carbon steel pipe, significantly greater corrosion occurs to a wider surface area.  For dry and pre-action fire systems, a significant deterioration at below the waterline means the production of a large volume of iron oxide deposits.  Depending upon the level of deterioration, pipe schedule, and age, sufficient rust deposits can be created – then having the capability of migrating downstream to a true fire emergency where they can clog the fire sprinkler head orifice.

For galvanized steel pipe, an entirely different corrosion mechanism exists.  The function of the zinc protective finish prevents corrosion from occurring, and is always a zinc finish remains intact, no wall loss will occur.  This is not the case for dry and pre-action fire sprinkler systems, however, and today’s galvanized steel pipe often begins to fail within the first few years.  Ultrasonic testing of any galvanized steel pipe showing a variance in wall thickness defines that the zinc protective finish is failing; in turn causing a loss to the underlying carbon steel.

The failure of this thin coating of zinc then allows all potential corrosion forces to focus on that area whereby produces deep pitting often compared to that of a drill bit.  Unlike the deterioration of carbon steel pipe where a wide surface area is under corrosion attack, galvanized steel pipe typically suffers a random but far more aggressive attack leading to an often premature failure.

• • • • Pipe quality

In general, pipe quality has suffered greatly over the past 25 years. This statement relates to carbon steel, galvanized steel, copper, ductile iron, and even stainless steel pipe, and regardless of where is manufactured.  Not only is pipe often manufactured to at or near its minimum permissible ASTM limit, its vulnerability to corrosion of all forms is significantly higher.

No greater evidence of such deterioration exists than for today’s galvanized steel pipe, which we have documented to provide under 10 years of reliable service for dry and pre-action fire systems, and to less than five years for domestic water service.  In one dry fire system investigation where advanced failure had occurred after only five years of service, ultrasonic testing identified extreme corrosion rates that would of never been possible at older pipe.  In fact, simple visual observation to a section of left behind 2 in. galvanized steel pipe showed the zinc protective finish so thin and poorly adhered, that it flaked off at each rolled groove – both inside and outside. Yet, with hundreds if not thousands of rolled grooves formed and installed by various pipe fitters on the project, no indication to the problem was raised by anyone involved.

Today, the incredibly poor quality and higher corrosion vulnerability of all pipe can be viewed as one of the greatest threats to any piping system, and one for which little to no corrective measures exist.  The fact that the pipe bundled on the floor for installation is of obvious poor quality in terms of coating failure, non-uniformity, or incomplete weld seam does not mean it will be rejected by anyone.

• • • • Auxiliary drains

Dry and pre-action fire sprinkler systems are expected to drain fully back to the main valve, which is an impossibility for most installations.  Whether designed with an incline or not, most dry and pre-action fire systems are installed perfectly level, and have no incline at all to move water to the drain.  Even if pitched as specified, the incline is never enough for water to pass over the internal ridge created as result of rolling the groove.  Simple calculations show substantial water being trapped between each end of every horizontal pipe section for rolled grooved pipe.

Cutting the groove eliminates this internal ridge, but also removes a substantial amount of pipe wall similar to a thread cut.  Although this loss of wall thickness has been considered an acceptable trade-off to better water draining, our investigations of such cut-grooved systems have shown no appreciable benefit in terms of corrosion loss – leading back to the observation that the incline is inadequate.

Many auxiliary drains installed at U bends and other navigations of the pipe around equipment, beams, ductwork, and other obstacles are often unused.  The installation of a bottom valve, pluged on its outlet and painted over, always provides evidence of having never been used.  For any inspection of a dry or pre-action fire system, the presence of supplemental drains should never be misassumed to have ever been opened.

Of course, “bottom” and auxiliary drains installed at the 3 o’clock or 9 o’clock positions will quickly provide the answer to why that fire piping failed so soon.

• • • • System design

A surprising number of fire protection systems, although designed according to well-established codes, still have vulnerabilities which have either not been addressed, or which have simply been ignored.  Others have new and unexpected vulnerabilities built into them.

Recommendations by third-party consultants to weld together galvanized steel fire lines in order to improve the reliability over threaded or clamped fittings serving over data centers actually introduce a new vulnerability.  Instead of removing a minimal threat from a coupling or thread, welding burns away the zinc protective coating to instead immediately initiate an aggressive pitting activity throughout the heat affected zone.

Compounding already high corrosion activity common to such systems, the installation and acceptance of foreign produced pipe, ultra thinwall schedule 7 pipe, and threaded schedule 10 pipe all ultimately contribute to less than adequate service life

• • • • Trim pipe

As often occurs in the mechanical world, the more complex the design, the greater the number of opportunities for some minor issue to result in its systemwide failure.  For all fire sprinkler systems, internal rust product is one such threat, and has been documented to disable fire systems when most needed.

The small diameter trim pipe controlling dry and pre-action fire system valves are another single source point of failure capable of preventing the operation of any such system.  Typically constructed using galvanized steel pipe, what is not considered is that the same corrosion activity occurring to the main lines also occurs at this smallest diameter pipe as well.

Over time, corrosion product can then constrict or clog the pipe – thereby disabling the function of the valve itself and rendering the system as worthless.

• • • • Prefabrication and assembly

All fire protection systems are made up of multiple take-off connections serving distribution and branch lines.  Smaller lines are typically constructed of threaded pipe whereas larger mains are clamped.  When constructed of galvanized steel pipe, multiple ports are required at the mains which may be fabricated in the field, but which we find more commonly prefabricated at the shop and then delivered to the jobsite ready to install.

A potentially fatal flaw exists, however, when the carbon steel ports to a main galvanized steel line are welded into place at already galvanized steel pipe.  During the process of welding the ports, whether done in the field or at the ship, the internal and external zinc protective finishes are totally burned away to reveal its underlying and unprotected carbon steel.  Of course, the entire purpose of installing a galvanized steel fire protection system is to benefit from the higher corrosion resistance of its internal zinc protective finish.  Except where installed into humid outdoor environments, oceanside locations, or where area conditions are corrosive, the fact that the outside surface of the pipe is galvanized is meaningless.

Once cutting into the pipe and welding on side or top ports, the original benefit of the zinc protection vanishes.  Not only does the exposed cut areas of the main now lack any galvanized protection, but the 2,800 °F plus heat produced by welding process completely burns away all internal galvanizing from the pipe in a much wider “heat affected zone.”

To counter this destruction of the zinc protective finish, which occurs to both the inside and outside surfaces of the pipe, welding slag and other impurities are wire brushed away from the outside surface only and a “cold galvanizing”coating or compound is sprayed on to protect the pipe.  (Grey or silver spray paint is also commonly used, although it does not provide equal protection.)  Obviously, the same surface preparation and cold galvanizing cannot be performed to the inside surface of the pipe, an impossibility for any diameter pipe.

This very obvious issue remains widely ignored by the fire industry, however, and the piping system is still assumed to be one of all galvanized steel when clearly it is not.  Instead, hundreds of sites exist where the pipe is not only unprotected carbon steel, but water draining back of the branch lines to this specific area then further accelerates its deterioration.  We typically focus our ultrasonic investigations at tie-ins from branch or distribution lines to such welded ports, with the typical finding of substantially higher corrosion activity.

Proper fabrication, as is performed for railings, light poles, and other galvanized steel products, is to weld on the necessary ports, remove the slag and welding debris and then run the modified pipe through the full surface preparation and galvanizing process of degreasing, caustic cleaning, pickling, rinsing, and fluxing.  Finally, the fire pipe is hot dipped in a vat of molten zinc covering inside and out equally.

It should also be noted that a galvanized steel fire protection system should be constructed of all galvanized steel rather than just the pipe itself.  This means the use of galvanized steel elbows and tees, rather than carbon steel fittings.  Larger mains should also be installed using galvanized steel clamps.

In the photograph at left, standard silver spray paint was used to cover over the carbon steel fittings installed to this galvanized steel pre-action fire sprinkler system.  Silver overspray also made itself onto the truss.  Proven by their actions, the installers obviously understood the importance of an all galvanized steel system and the potential for higher corrosion at each thread yet used cheaper carbon steel fittings instead.  They also likely knew that the specification called for galvanized steel fittings.

With so many negative factors to one piping system, each potentially contributing to a higher corrosion condition, the advanced failures we commonly document at carbon steel and galvanized steel dry and pre-action fire sprinkler systems have become no surprise.

• Testing Focus

Although many within the fire protection industry view pipe corrosion as an anomaly only caused by MIC or some out of the ordinary condition, our investigations of such systems have proven the problem as widespread.

A close look at any dry or pre-action fire sprinkler system is always warranted.  Low points absent a bottom drain should be addressed, as should areas of negative incline.  A frequently running compressed air jockey pump at least indicates an air leak if above the water line, or of more concern – a possible water leak.  Internal inspections should be more than looking inside two or three removed elbows with a flashlight, and instead be performed using a motorized robot platform capable of navigating across at least 100 ft. of main line at a time.  A pan and tilt camera head will allow views into the branch ports.

Areas of interest in any UT investigation of a dry or pre-action fire sprinkler system include:

• • • Inlet wet fire line
    • Horizontal lines
    • The lower half of all horizontal lines
    • Galvanized steel pipe
    • Cut-grooved pipe
    • Mains closest to the main valve
    • Pipe negatively inclined
    • Any threaded schedule 10 pipe
    • Low level pipe absent drains
    • Any schedule 7 pipe
    • Areas at or below branch tee connections

Risers present the least of problems since they will drain fully, and often provide evidence of the pipe’s original wall thickness for reference.  Ultrasonic testing after 5 years of operation will often show some beginning deterioration at the horizontal mains closest to the valve.  Any significant wall loss for carbon steel piping systems should be understood to contain potentially large volumes of iron oxide rust product capable of rendering the system worthless.  For sure, the installation of any thinwall pipe prompts an immediate concern, and pipe stamped with a promotional title citing its higher flow capacity should be understood in terms of how that higher flow rate was achieved.

Corrview maintains a large photo gallery specifically related to dry fire, pre-action, and trim piping in addition to other corrosion related issues. To visit a gallery, please click the system you would like to see:

Fire Sprinkler – Dry and Pre-Action

Trim Pipe

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