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Generally Speaking

News and Information about General Air Products Fire Protection and Industrial Equipment

  • What do Beef Jerky and Corrosion Mitigation for Dry Pipe and Pre-Action Sprinkler Systems Have In Common?

    Dry Air for Corrosion Mitigation – it’s being used all around you!

    Dry air is a proven technology for corrosion mitigation in many industries, including fire sprinklers. You might be surprised to learn that your bag of beef jerky is utilizing the same principle that we apply to your sprinkler system with our dry air generators.

    The picture below shows the small bags of desiccant that you find in all sorts of products you buy, sneakers, medicine, and many types of food. They often say “Desiccant: Do not Eat” - for real, don’t eat it. These packets of desiccant are designed to soak up the moisture in the package so that the air inside remains dry, protecting the product from growing mold, from discoloration or from corrosion.

    Dry Air by means of desiccant is used in a variety of industries, including fire sprinklers, to inhibit corrosion and mold.

    A dry air generator, like our Dry Air Pac®, contains several pounds of desiccant (don’t eat ours either) that the air from our compressor travels through before it goes into the system. When the compressed air meant for pressurizing your dry pipe sprinkler system travels through the desiccant beds nearly every bit of moisture is removed. The air, upon exiting the compressor / dryer unit, is then dry to anywhere between -40F and -70F PDP (pressure dew point). Note that PDP is a moisture measurement not a temperature rating.

    The effect this has on the moisture inside your dry or pre-action sprinkler system is twofold. First, as the air being used to pressurize the system is dry that means that there is not a steady contribution of moist air as there is when a standard air compressor is being used.

    Second, the air pressurizing the system is so dry that as it sits in and ultimately leaks out of your system it causes any residual moisture inside your sprinkler piping to be removed. Any environment kept at a low dew point or low relative humidity will interrupt the corrosion process. Put another way, the dry air inside the pipe evaporates the moisture sitting on the inside walls of the pipe or in shallow pools along the length of the pipe.

    The Corrosion Triangle: For corrosion in fire sprinkler systems to occur three things must be present: a metal, water and oxygen. Dry air generators remove the water and nitrogen generators remove the oxygen. The Corrosion Triangle: For corrosion to occur three things must be present: a metal, water and oxygen. Dry air generators remove the water and nitrogen generators remove the oxygen.

    While nitrogen eliminates the air part of the corrosion triangle, dry air reduces and eliminates the water part of the equation – thereby protecting your dry pipe or pre-action sprinkler system against corrosion. So next time you see one of these little desiccant packages that you found at the bottom of a bag of beef jerky remember that dry air is at work! … go ahead, you can eat the jerky.

    To learn more about the benefits of dry air as a corrosion mitigation technology comment here or contact us today!

  • Why We Never Recommend Nitrogen for Freezer Room / Cold Storage Sprinkler Systems

    Dry Air vs. Nitrogen for Cold Storage or Freezer Room Sprinkler Systems

    At General Air Products, we are often asked, “Should I Use Nitrogen or Dry Air in freezer rooms and cold storage sprinkler systems?” Our answer – as a manufacturer of nitrogen generators, dry air generators and air compressors - is dry air. Every time.

    There are three technologies available for filling dry pipe and pre-action sprinkler systems – standard compressed air, dry air generators – such as the Dry Air Pac® - and nitrogen generators. General Air Products is the only company in the fire sprinkler industry that manufactures all three of these types of equipment. This puts us in a unique position to speak to the pros and cons of each technology given a specific application. So, the first thing to take away here is that all of these products – compressors, dry air generators and nitrogen generators – have positives and negatives when applied to a dry pipe or pre-action system in any type of building.

    Choosing Your Cold Storage and Freezer Room Air Supply

    Let’s start with the easy one – never use standard compressed air in a freezer room application. When you compress air you also draw in water vapor from the humid ambient air – this vapor collects as liquid water in the tank and some of it continues downstream as vapor. Any water or water vapor sent downstream from the fire protection air compressor in a freezer room system will freeze at the length of pipe that is at the interface of the freezing and ambient area.

    Ice Plug in a Freezer Room Fire Sprinkler System

    (Note that pulling air from the freezer room and then through the compressor does not change this as any air pulled through an air compressor will be heated up in the process.  As this water vapor freezes it will not take long to turn into a sizable ice plug. For this reason, standard compressed air should never be used in a freezer room or cooler.)

    (ALSO NOTE:  A poorly maintained dry air generator (Dry Air Pac®) or a poorly maintained nitrogen generator will cause ice plugs to form in freezer rooms, just like an standard air compressor!)

    So, we are left with a choice between dry air or nitrogen.

    Where is the Corrosion in the First Place?

    Pipe Samples from a Freezer Room Sprinkler System governed by a Dry Air Pac show no signs of corrosion Above: Pipe samples taken from a freezer room sprinkler system governed by a Dry Air Pac for 13 years look like they just rolled off the production line.

    Though both dry air and nitrogen have been proven to mitigate corrosion in dry systems, the thing to remember here is that freezer rooms and cold storage fire sprinkler systems are not prone to corrosion. For corrosion to take place, liquid water in the pipe is required. With both dry air and nitrogen generators, we’re allowing virtually no moisture into the system, and the lack of frequent hydrostatic flow testing leaves us with minimal liquid water ever being introduced.

    Typically, a hydrostatic flow test is done at the commissioning of a dry pipe or pre-action system and then performed annually. This is not the case with most cold storage systems. While the initial hydrostatic test is required, allowances are made for freezer and cold storage sprinkler systems that this test does not have to be performed by running water through the entire system every year – to do so would require the freezer to be turned off each time. Cold storage facility managers will avoid this at all costs.

    If we are not introducing liquid water to the system and the piping system sits in an environment where liquid water cannot exist, then there is little to no cause for concern with corrosion.

    Dry air generators are dramatically less expensive than nitrogen generators in systems larger than 1000 gallons in capacity. If there is little cause for corrosion in a sprinkler system then why should you opt for the more expensive option of nitrogen? This point alone makes dry air the optimal choice, though there are several more to consider. General Air Products still recommends a dry air generator even in the few instances where a nitrogen generator is less expensive – this has to do with the performance of each unit type over time.

    Addressing Air Leaks in a Freezer Room

    One of the biggest issues with nitrogen generation is the lack of supervisory gas flow. In the chart below you can see how dramatically air flow from the compressor is reduced when it goes through a nitrogen membrane versus going through the dry end of a dry air generator like the Dry Air Pac®, or when it is not filtered at all as in a fire protection air compressor. This is a big problem with nitrogen generators in fire sprinkler systems as a whole, but it is of paramount importance in freezer room applications.

    Freezer Room Sprinkler Systems - Air Supply Comparison Nitrogen Generators struggle to keep up with system leak rates due to their inherently lower flow (CFM).

    What the diagram tells us is that a fire protection air compressor has the best supervisory gas flow and can keep up with extreme leak rates. This is because air compressors and dry air generators are sized for 30-minute fill requirements whereas a nitrogen generator that is generating 98% nitrogen is only sized for system maintenance. There has been no real discussion on the leakage rates of dry air pre-action systems over that past many years – air compressors can keep up with them. The reason we need to discuss them now is that nitrogen generators cannot.

    Air leaks in dry and pre-action sprinkler systems will always grow over time. While maintaining 98% nitrogen in a sprinkler system will severely limit the amount of corrosion that takes place, it will not slow all of the other causes of air leaks in sprinkler systems.

    Nitrogen Generators on Fire Sprinkler Systems Have Lower CFM due to the filtration required by multiple prefilters as well as the nitrogen membrane. Nitrogen Generators: the air compressor must feed air through multiple pre-filters and eventually the nitrogen membrane itself - which, along with it's high level of filtration, acts as a bottleneck which strictly reduces air flow to the sprinkler system, in order to produce 98% purity nitrogen.

    Keeping all of this in mind, let’s apply a nitrogen generator to a freezer room…

    Upon install in a newly commissioned sprinkler system a properly sized nitrogen generator should keep up with system demands for pressure (at 98% nitrogen). Time passes, forklifts shake the shelving that contains the in-rack sprinkler system causing permanent leak points to form at threads and couplings. Gaskets dry rot, valves don’t seat properly and all the other day-to-day things occur that cause leaks.

    The nitrogen generator, due to its inherently reduced flow, is running more and more each day to keep up with the pressure loss. Before long, it cannot keep up, and a run time or low-pressure alarm is triggered.

    In most cases, when a nitrogen generator can no longer keep up with the demands of the system you can turn it to air mode and then hunt down all of the air leaks so that you can get it back into nitrogen mode again. A costly and painstaking process in any system, but in a freezer room or cold storage warehouse sprinkler system, it is nearly an impossible one without shutting down the freezer room.

    Ask your service manager about the techniques used to find leaks – spraying soapy water, introducing mint scent – then ask about using those techniques in a freezing environment.

    Referring back to the diagram, you can see the difference between the flow rate of a nitrogen generator and that of a dry air generator – and it is substantial – meaning the need to find and address these leaks will be substantially less with a dry air generator. It can keep up.

    General Air Products’ Dry Air Pac® has been dutifully serving as the air supply in freezer room and cold storage fire sprinkler systems for over 25 years and the issues mentioned here are why it works where other technologies run into costly struggles and freezer room down-time. In addition, dry air is almost always a lower cost solution than a nitrogen generator.

    As mentioned in the beginning of the article, all of these technologies have pros and cons. Standard compressed air is the least expensive but does contribute to corrosion to varying degrees. Nitrogen generators reduce corrosion best where there are large amounts of standing water in the fire sprinkler system, and dry air generators are best applied where there are small amounts of standing water, such as in freezer rooms. Both nitrogen and dry air generators are maintenance intensive – filters must be changed or the units will not perform as designed. In the case of our Dry Air Pac®, one additional step is necessary – the desiccant needs to be changed every 3 years. Dry air can also serve more systems than a nitrogen generator – regardless, we recommend no more than 3 systems per air supply device, not matter which you choose.

    It is for all of these reasons that we definitively tell our customers that they should use a dry air generator on their cold storage or freezer room dry pipe sprinkler systems. Every time.

    If you want to know more about how dry air mitigates corrosion, how and why dry systems grow leaks despite corrosion protection, why you should not put more than 3 systems on one air supply or if you have any other questions about air supply technologies for dry pipe and pre-action fire protection systems please do not hesitate to contact us!

  • Dry Compressed Air as a Supervisory Gas in Dry Pipe Fire Suppression Systems

    Please continue reading below, or download the PDF version of this 3rd party white paper.


    L. A. Krebs, Ph.D.
    Anderson Materials Evaluation, Inc.
    9051 Red Branch Road, Suite C
    Columbia, MD 21045


    Dry pipe fire suppression systems are, frequently, not entirely dry.  Water may become trapped in the system during hydrostatic testing, or after a triggering event.  Moisture may enter and accumulate in the system through the use of ordinary compressed air as the supervisory gas.  Trapped and accumulated water in an ordinary compressed air system promotes corrosion in the wet areas.  Interior corrosion may be general and/or localized (e.g. pitting) from the inside out.  General corrosion with associated voluminous corrosion can lead to blockages.  Pitting often leads to pinholes through the tube wall.  Dry compressed air can suppress both general and localized corrosion in dry pipe systems, first by preventing the accumulation of moisture over time, and second by aiding in evaporation of bodies of standing trapped water.

    Corrosion and Dry Pipe Systems

    Dry pipe fire suppression systems ideally operate with a fully dry interior until a fire triggers the system to respond.  Actual systems in service, however, often contain some trapped moisture.[1]  Conditions may range from condensed moisture on interior surfaces to “ponds” of water trapped at unintended low points without proper drainage.  Moisture can enter a system through several routes, for example water can become trapped after hydrostatic testing or may accumulate more slowly through the use of untreated compressed air.  In the latter case the rate of moisture accumulation will depend in part on atmospheric humidity levels.

    Trapped moisture inside the pipes of a fire suppression system can promote corrosion and result in premature failure.[2],[3]  Interior corrosion may be general and/or localized (e.g. pitting) from the inside out.  General corrosion with associated voluminous corrosion can lead to blockages.  Pitting often leads to pinholes through the tube wall.  Corrosion is an electrochemical process that requires a specific set of conditions.1,[4]  If a necessary condition is disrupted the process cannot proceed.  The necessary conditions for an electrochemical reaction that results in pitting corrosion, for example, are illustrated in the schematic of Figure 1.  These include:

    • an anode (an area that will oxidize or corrode),
    • a cathode (an area that will support a reduction reaction to consume electrons released by the oxidation reaction),
    • a path for electrons to travel from the anode to the cathode (typically the body of the metal pipe itself),
    • and a medium that allows ionic conduction between the anode and cathode (accumulated water with a corrosion-promoting agent, dissolved oxygen in this example).

    The combination of the above conditions allows corrosion-related electrochemical reactions to go forward.  If any one of these necessary conditions can be eliminated corrosion can be halted.  Aqueous corrosion reactions other than those shown in Figure 1 require the same set of general conditions to proceed.

    The use of untreated compressed air as the supervisory gas introduces moisture into the pipe interiors.  The amount of moisture that may be carried into a system by normal compressed air will depend on the flow rate and on atmospheric humidity, which is dependent on external environmental factors.  Many instances of premature failure due to corrosion associated with trapped moisture have been reported (references).  An example of a carbon steel pipe removed from a dry pipe system after experiencing pinhole leaks is shown in Figure 2.  In this case standing water in the bottom of the tube led to accelerated general corrosion of the interior surface as well as pitting attack from the inside out.

    pitting in steel shows the conditions needed for aqueous corrosion in dry pipe sprinkler systems to proceed Figure 1. This schematic representation of pitting in steel shows the conditions need-ed for aqueous corrosion to proceed (here assuming aerated water of neutral or alka-line pH). The Fe(OH) 2 reaction is one of several common oxidation reactions that oc-cur in the formation of rust, a complex mixture of iron-based oxides and hydroxides.

    Water is critical for the progress of corrosion reactions in the dry pipe interior environment.  If standing water and/or condensate layers on pipe walls can be minimized or eliminated corrosion reactions will likewise be slowed or halted.  Without corrosion, failure events such as the one depicted in Figure 2 can be avoided and the expected service life of such systems may be greatly extended.

    A carbon steel pipe removed from a dry pipe system after having developed pinholes due to corrosion. Figure 2. A carbon steel pipe removed from a dry pipe system after having developed pinholes is shown. Top Row: Upstream (left) and downstream (right) views show evidence of standing water in the bottom of the tube – corrosion products and wall thinning are obvious. Middle Row: Exterior (left) and interior (right) views of an ex-ample pinhole from the bottom of the tube are shown. Bottom Row: Upstream (left) and downstream (right) cross section surfaces of the sample containing the pinhole were subjected to fine grinding to better show the loss of wall thickness and the pit-ting at the bottom of the tube. – from AME 2009 archival report.

    Methods exist to dry the air provided by an air compressor, reducing trapped moisture and thereby slowing or preventing corrosion.  The benefits of introducing dry air into the system are twofold.  First, less moisture is carried in with a dry supervisory gas.  Second, dry air has the capability to absorb moisture through evaporation, reducing that amount of trapped moisture that may be present.  Since supervisory gas flows through a system (via inherent pipe leakage or through a purge/vent device) rather than remaining stagnant, the dry air is constantly replenished.  Consequently, trapped moisture can continually be absorbed, ideally until it is entirely removed.

    The rate of flow for supervisory gases and the rate of moisture evaporation by dry compressed air are both affected by multiple and sometimes joint factors, and so will differ from system to system under a range of normal service conditions.  The National Fire Protection Association (NFPA) provides standards and codes that identify acceptable flow rates for supervisory gases.  Specifically, NFPA 13[5] allows a new system to have a leak rate of 1.5 psi in 24 hours (maximum), and NFPA 25[6] allows an existing system to have a leak rate of 3 psi in 2 hours (maximum) which works out to 36 psi in 24 hours.  In practice, leaks within working systems can sometimes elevate flow rates to even higher levels while still allowing the overall system to function.  Assuming a range of possible flow rates, certain generalizations regarding the behavior of a system using dry compressed air as the supervisory gas can be made.

    The evaporation rate of water, whether present as moisture condensed on a surface or as standing ponds within a dry pipe system, is related to the pressure dew point of the supervisory gas.  Generally, the dew point is the temperature at which moisture present in the vapor phase can condense onto a surface, forming “dew,” and is a function of the actual amount of moisture vapor present at atmospheric pressure.  The lower the dew point, the less vapor-phase moisture is present (the air is drier).  The pressure dew point (PDP) of a supervisory gas is a function of both the degree of moisture saturation and the pressure.  For untreated compressed air the PDP is higher than the atmospheric dew point because the air is under greater than atmospheric pressure.  As a result, the use of untreated compressed air can lead to condensation inside a pipe even if non-condensing conditions exist outside.  The use of dry compressed air with extremely low PDP avoids this condition.  This concept is understood and exploited outside the fire protection industry as well.  Clean dry compressed air is used in a variety of industries (i.e. food and beverage, pharmaceutical, semi-conductor and electronics, chemical) for a number of reasons including but not limited to reduced contamination, reduced moisture intrusion, and reduced corrosion of delivery systems and, in some cases, items to be protected in storage.

    When the PDP of the supervisory gas is low little or no vapor-phase moisture is carried into the system, and evaporation of trapped moisture is favored.  If we assume for a moment that the supervisory gas is stagnant (no flow) and all other relevant conditions remain constant, eventually an equilibrium would be achieved in which the rate of moisture condensation out of the gas would equal the rate of evaporation into the gas and no additional drying of the pipe would occur.  However, when very dry low PDP air flows continuously through the system equilibrium cannot occur and evaporation continues.  If moisture is present primarily as condensation on the pipe walls evaporation should proceed relatively quickly.

    In cases involving a pond of trapped standing water the situation is more complex.  The evaporation rate will depend in part on the surface area of the pond, which will change as evaporation proceeds.  Larger surface areas support more rapid evaporation, smaller surface areas slower evaporation, regardless of the total volume of trapped water.  The time to complete dryness will depend in part on the evaporation rate and in part on the total volume.  Complete elimination of a trapped water pond using dry compressed air as a supervisory gas is possible but may take considerable time.  It is also important to note that, like ordinary compressed air, dry compressed air will supply oxygen to the pond. Dissolved oxygen can participate in corrosion reactions such as those shown in the example provided in Figure 1 as long as moisture is present.

    An example of a galvanized tube removed from a dry pipe system after approximately four years of service is shown in Figure 3.  This tube, and nine others, comprised a set of samples from different galvanizing batches installed in one system and exposed to dried compressed air as the supervisory gas in service.  All were found to have some loss of the galvanized (zinc) layer due to corrosion, but no corrosion of the substrate steel was found.

    A galvanized pipe removed from service after exposure to dry compressed air showing no signs of corrosion. Figure 3. A galvanized pipe removed from service after exposure to dry compressed air is shown. The interior surface was generally smooth and fairly shiny with some surface foreign material and spotting. Slight haziness of the zinc surface suggested mild general corrosion of the galvanized layer. Example cross section images show nearly full thickness of the zinc and intermetallic layers around part of the inner di-ameter (left). In some localized spots the zinc layer was partially consumed (right), but intermetallic layers remained and no substrate corrosion was noted. Original magnification 100x. – from AME 2018 archival report.


    It is not uncommon for dry pipe fire suppression systems to have some form of interior moisture, whether trapped during testing or accumulated over time.  This wetness is a crucial component for the corrosion cycle, both general and localized.  General corrosion with associated voluminous corrosion can lead to blockages.  Pitting often leads to pinholes through the tube wall.

    The above discussion described how dry compressed air can suppress both general and localized corrosion in dry pipe systems.  Interior corrosion is controlled first by preventing the accumulation of moisture over time, and second by aiding in evaporation of bodies of standing trapped water.


    [1] P. Su and D. B. Fuller, “Corrosion and Corrosion Mitigation in Fire Protection Systems,” FM Global technical report, 2nd Edition, July 2014, p34

    [2] O.J. Van Der Schijff, “MIC in Fire Sprinkler Systems Field Observations and Data”, CORROSION/2008, paper no. 08508, (Houston, TX: NACE International, 2008).

    [3] P. Su, et al, “Corrosion of Sprinkler Piping Under Compressed Nitrogen and Air Supervision”, CORROSION/2015, paper no. 5548, (Houston, TX: NACE International, 2015).

    [4] Jones, D.A., “Principles and Prevention of Corrosion”, Macmillan Publishing Company, New York, NY, 1992

    [5] “NFPA 13: Standard for the Installation of Sprinkler Systems,” National Fire Protection Association, Quincy, MA, current edition 2019

    [6] “NFPA 25: Standard for the Inspection, Testing, and Maintenance of Water-Based Fire Protection Systems,” National Fire Protection Association, Quincy, MA, current edition 2017

  • Homes for Our Troops & AFSA Partner to Bring Residential Fire Sprinklers to Our Vets

    You may have heard of the amazing work done around the country by Homes for Our Troops – a Massachusetts-based non-profit which builds and donates specially adapted custom homes for severely injured post-9/11 veterans. As of November 2018, the organization had built more than 250 homes across 42 states.

    Homes for Our Troops Partners with the American Fire Sprinkler Association and General Air Products to bring residential fire sprinklers to the homes of our vets.

    With the rise in popularity and regulation surrounding residential fire sprinkler systems, the American Fire Sprinkler Association began partnering with HFOT in 2017 to use their network of connections within the fire sprinkler industry to bring added safety and security to these homes.

    As a long-time AFSA member, General Air Products was invited to participate with the AFSA and HFOT in donating a Pump & Tank System to the home of Purple Heart recipient Marine Sergeant Joshua Bouchard. In an article published by Sprinkler Age highlighting the build, our own Ray Fremont Jr. expressed our excitement to be a part of the process: “We are proud to contribute to this amazing cause. Many of our family, friends, and employees are military veterans so partnering with the HFOT to bring life-saving fire sprinklers to the homes of veterans across the country is an honor and privilege.”

    We highly encourage everyone to visit Homes for Our Troops at hfotusa.org to learn more about their mission and consider making a donation.

    About Homes for Our Troops: Homes For Our Troops (HFOT) is a privately funded 501(c) (3) nonprofit organization that builds and donates specially adapted custom homes nationwide for severely injured post – 9/11 Veterans, to enable them to rebuild their lives. Most of these Veterans have sustained injuries including multiple limb amputations, partial or full paralysis, and/or severe traumatic brain injury (TBI). These homes restore some of the freedom and independence our Veterans sacrificed while defending our country, and enable them to focus on their family, recovery, and rebuilding their lives. Since its inception in 2004, nearly 90 cents out of every dollar spent has gone directly to our program services for Veterans. HFOT builds these homes where the Veteran chooses to live, and continues its relationship with the Veterans after home delivery to assist them with rebuilding their lives.

    About the American Fire Sprinkler Association: The American Fire Sprinkler Association, (AFSA) is a non-profit, international association representing merit shop fire sprinkler contractors, dedicated to the educational advancement of its members and promotion of the use of automatic fire sprinkler systems.

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