In the past, the design best practice for critical facility areas was providing a pre-action type sprinkler system. The type of pre-action system can vary with locality and should be always confirmed with the local fire department.
In general, a double-interlock pre-action system has been designed for most of the applications (with some exceptions — NYC approves only single interlock pre-action systems). This type of pre-action system requires two events to happen before the valve discharges water into the pipes — activation of a detection device (smoke or heat detector) and loss of pressure in the sprinkler piping due to opening of a sprinkler head. This arrangement prevents accidental discharge from a broken sprinkler head or false alarm from a detection device. Pipes are filled with compressed air, which maintains the clapper on the pre-action valve closed, thus no water is present in the pipes. Each pre-action system is provided with a permanent oil-less air compressor, which maintains required pressure in the system. Best practice for pre-action system design has been using galvanized steel piping, utilizing treaded, or welded connections.
As described above, everything sounds perfect — the facilities are protected and no water is present in the pipes to accidentally damage valuable equipment and destroy irreplaceable information. That is, unless or until visible damages (rusty spots at the bottom of the pipes) start showing after several years in exploitation. Fixing this damage by replacing the pipes (sometimes in live data centers) is an expensive and risky exercise.
What is the reason for the damaged piping in pre-action and dry systems? Why do the pipes rust if they are empty? The fire protection community started talking about microbiologically influenced corrosion (MIC) as the main reason for damaged pipes. Syska Hennessy Group’s clients were concerned, and as a result, we started looking for ways to protect the pre-actions systems from corrosion and guarantee the systems for the life of the facility.
WHAT IS THE REASON FOR CORROSION IN SPRINKLER PIPES?
The main cause of corrosion is the oxygen, trapped inside the dry and pre-action type sprinkler systems. It dissolves in the small amounts of water, introduced with the compressed air or trapped after the testing. Oxygen reacts with the metal and produces rust. This reaction, even for a very short time, can produce enough solids to significantly reduce the pipe size and thus make all hydraulic calculations obsolete.
As per NFPA 13/2010 Section 24.2 and NFPA 25/2011 Section 13.4, dry and pre-action systems shall be hydrostatically tested after initial installation and annually (with some exceptions for critical areas) after this. In addition, dry and pre-action systems shall be air pressure tested, which introduces fresh oxygen and moisture to the system. NFPA 13 requires all pre-action systems to be pitched back to the valve; however, in almost every installation, there are portions where water stays trapped inside the pipes, creating ideal environment for corrosion.
By removing the oxygen from the pipes, treating MIC becomes much easier, primarily because the system remains free of solids, which support the growth of the bacteria. Pre-action systems that are free of oxygen will not support the aerobic forms of microbial contaminants.
Per NFPA 25, all sprinkler systems shall be inspected for MIC every five years.
PREVENTING CORROSION IN DRY AND PRE-ACTION SPRINKLER SYSTEMS
The different available options for corrosion protection are providing dry compressed air to the systems, providing pipe-shield coating inside the sprinkler pipes, and pressurizing sprinkler pipes with nitrogen. These methods provide different level of protection and can be used separately or in combination.
Air dryer. Dry-type sprinkler systems are pressurized with compressed air, which can introduce a lot of moisture to the sprinkler piping. A compressed air dryer is a device for removing water vapor from compressed air. The process of air compression concentrates atmospheric contaminants, including water vapor. This raises the dewpoint of the compressed air relative to the atmospheric air and leads to condensation within pipes as the compressed air cools downstream of the compressor. Most condensed water has a pH in the acid range of 5.5. This acid water will greatly increase the rate of corrosion in sprinkler pipes.
There are various types of compressed air dryers. Applicable for fire protection are deliquescent dryers, which are simple, energy-free, and environmentally safe, and are available from 2 to 16,000 csfm.
- Easy to install and maintain.
- Remove the moisture only from the compressed air introduced to the system.
- Cannot eliminate the moisture trapped inside the sprinkler piping that was introduced during testing of the sprinkler system.
- Continuously add oxygen to the system.
More sophisticated air maintenance devices may include prepackaged units, consisting of air compressor, tanks with desiccant, and a control panel and testing kit prewired in a turnkey installation.
Sprinkler systems cannot be built with perfect pitch due to field conditions and required coordination with other engineering systems. Therefore residual water inside dry-type sprinkler systems, trapped there after the required testing, always will be present. Air dryers can address only small part of the big variety of factors, contributing to the corrosion inside sprinkler pipes.
GUARDING AGAINST MICROBES
MIC refers to the existence of microbes inside sprinkler piping. Generally MIC is not caused by a single organism, but by multiple microbes that are either aerobic (requiring oxygen as a catalyst) or anaerobic (requiring little or no oxygen). Unlike general corrosion, which affects the surface of the metal, these organisms are localized in their attack on pipe wall integrity, and therefore often result in a premature system failure of sprinkler pipe installations.
To address the issue with MIC, different manufacturers are offering shield coating pipe guards. These materials deliver corrosion protection without adverse affect to the environment. Some of these products follow.
- MIC shield coating piping, offered by Wheatland Tube Company, is available in all sizes of black steel sprinkler piping. The MIC shield coating, when initially applied to the inner wall of the pipe, first acts as a rinsing agent to clean the contact surface. MIC shield coating thereafter adheres to the pipe wall, serving as a protective coating, which guards against contamination by impeding the attachment of microbes to the pipe wall.
- Pipe-shield corrosion inhibitor works by forming a molecular bond on the pipe wall, which in turn provides a barrier for oxygen cell and bacteria-related corrosion. Usually, these materials are non-regulated, non-hazardous, and completely biodegradable. Some companies provide a color changing test for easier monitoring.
Some jurisdictions will require the installation of a reduced pressure zone (RPZ) backflow preventer on the fire sprinkler system if any chemical additive is used in the system.
USING NITROGEN GAS IN DRY AND PRE-ACTION SYSTEMS
Dry and pre-action sprinkler systems usually are pressurized with compressed air, which contains 78% nitrogen, 21% oxygen, and 1% other trace gases. By introducing nitrogen to the system and providing vents at the end, the concentration of nitrogen gradually increases and the oxygen is eliminated from the system through the vents. The nitrogen also acts as a drying agent, removing any trapped water from inside the pipes and leaving the system practically dry.
Nitrogen is effective solution to control oxygen corrosion because:
- Nitrogen is non-toxic; it is odorless and colorless and can be considered as a “green” gas.
- Nitrogen will not support or reinforce the corrosion of galvanized or black steel pipes.
- Pre-action systems charged with nitrogen will not support aerobic forms of microbial contaminants, which generally pose the greatest risk of creating slimes in fresh water systems.
- Nitrogen will not degrade the elastomeric seats found in valves and other system components that are used in dry and pre-action systems like the oxygen in the air does.
Nitrogen can be introduced to the pre-action sprinkler piping in two ways:
- Using a nitrogen maintenance device and pressurized nitrogen cylinders. Nitrogen cylinders provided by an approved source shall provide the nitrogen supply. Double interlock pre-action system shall only require between 10 and 26 psi (0.7 to 1.8 bar) supervisory pressure for proper setting of the low pressure pneumatic actuator in accordance with the manufacturer’s instructions. The nitrogen cylinder pressure shall be regulated and supervised through the use of a nitrogen regulating device and low-pressure trim kit. The low-pressure trim kit shall be included to monitor the regulated nitrogen supply pressure to provide a low-pressure supervisory alarm. This is a good solution for smaller pre-action and dry systems.
- Using a nitrogen generator system. The incorporation of a properly sized nitrogen generator system combined with venting can prevent further deterioration of dry and pre-action sprinkler systems. The generator provides very cost-effective continuous supply of nitrogen to the sprinkler system, eliminating the risk and the need to change out of nitrogen gas cylinders. nitrogen generator requires only compressed air supply to produce the nitrogen. There is no need to use desiccant dryers when membrane nitrogen generators are used, since the injected nitrogen is completely dehydrated. This system is very suitable for large applications with multiple pre-action and dry zones.
EVALUATING DIFFERENT CONFIGURATIONS
Below is a comparison between two options for filling a multiple-zone double interlock pre-action fire sprinkler system with nitrogen gas produced from a nitrogen generator for corrosion control. Both approaches will employ an initial “gas exchange phase” wherein the initial pipeline gas will be vented from the systems to achieve +97% nitrogen gas composition within the fire sprinkler zone piping. At a +97% nitrogen concentration, corrosion control will be attained. This example is from an actual data center design for one of our clients, where the sprinkler system was designed with 21 double interlock pre-action zones protecting a two-story building.
Example scenario for two pre-action riser room configurations per floor:
- Five double interlock pre-action fire sprinkle zones on one water delivery manifold per room;
- One common air supply line manifolded to all zones on the floor;
- Total volume of the five zones is 2,000 gallon (gal) (total 4,000 gal/floor — zones vary in volume);
- Operating pressure of 25 psi;
- Each zone equipped with an air maintenance device with a field-adjustable regulator.
Option #1. Operating sequence for continuous breathing, continuous zone gas sampling, no nitrogen storage tank:•
- Bypass nitrogen generator and use compressor to fill zones with air to meet NFPA 13 30-minute fill requirement. The compressor shall be sized to meet NFPA 13 requirements for most demanding pre-action zone on the manifolds.
- Perform high rate exchange venting to allow each pre-action zone to vent and breathe to exchange the initial air replacing it with nitrogen gas.
- Use the gas analyzers on each of the zones to verify 97% nitrogen concentration.
- Continue high rate exchange venting for one month to remove trapped moisture.
- Manually close riser room vents on each of the pre-action zones.
- Continue using the zone vents to provide slow rate exchange venting and moisture removal.
- Nitrogen generation system provides 97% nitrogen as needed.
- Monitor pipeline gas composition continuously.
Option #2. Operating sequence for no breathing, with nitrogen storage tank:
- Bypass nitrogen generator and use air compressor to fill zones with air to meet NFPA 13 fill requirement of 30 minutes. The compressor shall be sized to meet NFPA 13 requirements for most demanding pre-action zone on the manifolds.
- Allow vents located on each zone to vent and breathe to exchange the initial air and achieve 97% nitrogen concentration in all of the pre-action zones.
- Use single gas analyzer to verify gas composition in one of the zones.
- Continue venting for one to two weeks.
- Manually close vents that are located remotely on each of the pre-action zones.
- Nitrogen storage tank provides +97% nitrogen gas to compensate for any leakage in the zones.
After detailed evaluation and comparison of both options, we came to the conclusion that by continually venting the pre-action fire protection piping system with specially designed system vents, the exhausted gas can carry any residual water that is trapped in the piping out through the vents, resulting in a system that is completely dry. This will stop the corrosion and greatly decrease the possibility of MIC.
We recommended the option with continuous breathing for the system, which will continue to evacuate any residual moisture and oxygen during the entire life of the system. After taking into consideration the fact that the data center facility is Tier IV along with the owner’s requirements for 100% redundancy, we combined the two options and designed a system with continuing venting and storage tanks as a backup
We strongly recommend utilizing nitrogen generator systems for corrosion protection of dry and pre-action sprinkler systems in large mission critical facilities. This system may not be cost-effective for smaller pre-action systems, but for big data centers, it is a must. It saves money and trouble for the owner in the long run and provides peace of mind.
• Engineered Corrosion Solutions, www.ecscorrosion.com.
• Reliable Automatic Sprinkler Co, www.reliablesprinkler.com.
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