15 Jul Keeping cool under extreme conditions
Mechanical pipe connections stand up to direct fire to ensure the water supply they carry to critical fire protection systems is never compromised. For the daily users of mechanical piping systems, what may not be so evident is the ability of these compact pipe connections to withstand direct fire exposure under some of the industry’s most severe service conditions. Most noteworthy and the focus of this article, is the fire resistance capability these products will provide under some of the most extreme fire conditions writes Len Swantek, Director – Global Regulatory Compliance, Victaulic.
For more than 100 years, Victaulic mechanical grooved pipe couplings have been the product of choice across many fluid transport industries and applications based on their ability to provide self-restraining leak tight pipe connections that also provide flexibility, vibration attenuation, system and thermal movement accommodation as well as seismic dampening. However, these same products also have additional hidden benefits that often go unnoticed. Most noteworthy is the fire resistance capability these products will provide under some of the most extreme fire conditions. Whether it be in an office building; commercial warehouse; chemical plant; fuel refinery; offshore platform; or in a ship’s engine room, direct exposure to fire in no way disrupts the ability of these products to provide their full rated pressure and joint sealing performance.
The housing is the first line of defense
The coupling’s housings are cast of durable ductile iron in accordance with ASTM A-536 and of material grade 65-45-12. These structural elements of the joint are capable of withstanding direct fire exposure to temperatures at or near 871°C (1600˚F) with no adverse effects to their physical or metallurgical properties. This first line of defense completely surrounds and shields the internal gasket; protecting it from direct exposure to flame and intense heat. The cross-sectional geometry of each housing design is carefully analysed for section thicknesses and material placement to ensure a proper balance between heat absorption and dissipation. Other housing materials like cast steel and stainless steel provide similar protection for the internal gasket while again utilising their thermal properties to protect from extreme temperatures internally. However, with prolonged exposure, the coupling housings will eventually transfer a portion of the heat to the internal gasket surface. The elastomeric seal then becomes the second line of defense as part of a highly-engineered pipe connection.
Gasket properties ensure the highest degree of sealing performance
To ensure a consistent level of fire performance, the gasket compounds are engineered to provide a unique balance between thermal resistance and pressure responsiveness. The objective here is to maintain the highest degree of sealing force at the maximum temperature limits. This is where the gasket material and its properties play an important role.
There are several key factors that determine the gasket’s resistance to extreme heat:
1) Elastomer formulation
2) Curing effect on material properties
3) Design attributes and seal geometry
The gasket compound utilises ingredients with high thermal decomposition properties to ensure minimum material loss during fire. The compound is also designed to minimise changes in critical properties such as hardness; tensile strength; elongation; compression set and compressive stress relaxation under fire conditions.
Thermal stability of the material is based on many factors such as polymer type, curatives, antioxidants and other additives used in the formulation, as well as the resulting crosslink density established during the curing process. Each material ingredient is studied individually and as a complex compound using a series of experiments to achieve the highest heat resistance characteristics. The curing process is also optimised to achieve the maximum state of cure with desired material properties uniformly throughout the entire gasket cross-section. Both the elastomer compound and manufacturing process are highly controlled to ensure the heat resistance of the cured gasket is maintained in its finished shape and geometry.
In parallel with the material study, the design attributes are carefully developed to align with the specific material properties under different stress conditions. The volume of the gasket cross-section is optimised to accommodate both the thermal expansion of the seal as well as the pipe surface with which it will be in contact under extreme temperature conditions. The stress distribution across the entire seal geometry is studied to prevent the gasket from breaking down or becoming deformed, while ensuring there is always sufficient sealing force uniformly applied over the full circumferential surface. Combined, these characteristics are critical in maintaining a pressure responsive seal under the most extreme heat conditions and are individually analysed for each specific diametral size and tolerances over the entire product range.
Product qualification to codes & standards
It is vitally important to validate the seal design and material composition under real-world fire conditions. Conducting multiple internal fire simulations is both complex and time-consuming, however, this is the only true measure of how the joint will perform in a specific end-use application or industry. The test configuration, burn time, fuel type and volume and its heat release rate greatly impact each test and how the complete product assembly will perform. These trials also provide valuable data that correlates to the test conditions that will be present during the external qualification process conducted by the certifying agencies. These authorities have conducted extensive research and countless fire experiments over many years and have established very detailed criteria to which the product manufacturers must comply. While the manufacturer may specify the desired certification objective, the existing governing standards will apply and may include additional fire simulations that represent a specific hazard or a unique protection scenario required by the property insurer.
In order to qualify the complete mechanical joint in accordance with local and regional fire codes and certifying agency test standards, the coupling and gasket are assembled onto steel pipe ends and then placed over a fire source or within a fire test chamber as required by the specific governing standard. The assembled pipe joint is then subjected to intense fire endurance tests under the direction of the individual agencies who have jurisdictional authority in the local market or region.
For example, fire exposure qualification tests are conducted by VdS Schadenverhütung GmbH in Cologne, Germany, as part of the certification requirements for mechanical joints used in fire protection systems in commercial buildings. The laboratory exposes the assembled mechanical joint to a methanol fire, where the test joint is completely engulfed in a concentrated fire and must withstand a direct flame temperature at or near 800˚C (1470˚F) for up to 15 minutes with only trace amounts of water inside the assembly.
The purpose of this test is to simulate a scenario where the water supply to the fire system has somehow been compromised and the piping system has only a fraction of its required water volume. The fire duration coincides with an average response time for the firefighters to arrive on scene.
During this test, the heat transfer characteristics described earlier are working to deliver these extreme temperatures to the gasket surface inside the assembled coupling.
It is very important to note here, that there is no internal pressure to aid the gasket in maintaining an energised seal against the pipe surface in this test. The gasket must rely solely on its own physical properties to resist the temperature and maintain its overall shape, geometry and its compressive stress relaxation characteristics throughout the burn period. This fire test is immediately followed by a hydrostatic pressure test at the product’s maximum rated working pressure while the test assembly is monitored for sign of leakage.
Grooved mechanical pipe joints used in the maritime industry for onboard applications must also endure a similar fire exposure test for 30 minutes at 800˚C (1470˚F) with no leakage allowed. The test is conducted in accordance with requirements set forth by the International Association of Classification Societies (IACS), which also include fire test protocols established in ISO Standards ISO-19921 and ISO-19922.
Thermocouples are placed at various locations around the test assembly to monitor surface temperatures throughout the test period. The energy release rate during this fire test can drive the intense heat through the ductile iron coupling housing, resulting in very similar exposure temperatures at the gasket surface. This dry heat characteristic can be particularly damaging if the material properties are not maintained throughout the duration of the test.
Following the fire exposure conditions, the mechanical joint is then hydrostatically pressure tested to 1.5 times the rated working pressure and again with no allowance for leaks. This test represents one of the most demanding fire performance criteria for mechanical pipe connections across a wide range of heavy industries.
Other similar fire exposure tests including API-607, which is more commonly used to qualify fire-safe valves for the petrochemical industry and UL-852, which sets forth the qualification criteria for metallic fire sprinkler pipe, are used routinely to qualify new seal materials and coupling designs that are intended for high hazard applications where the inherent risk of fire is always present. Such standards can involve direct flame temperatures exceeding 871°C (1600˚F) as part of the qualification requirements for components used in dry pipe fire protection systems.
Victaulic mechanical grooved end pipe connections continue to demonstrate their superior performance capabilities in the laboratory and in real-world systems applications. As the world’s leading innovator of mechanical piping systems solutions, we remain dedicated to the advancement of our materials and manufacturing technologies to the collective fire safety community world-wide.
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