Getting flexible with fire protection

by FM Media
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Invented before the turn of the 20th century, flexible metal hoses have been utilised in a wide range of industries and applications, but only recently have they experienced significant growth.

The pioneering inspiration behind the original concept of a metal hose – providing flexible, leak-tight, reliable and corrosion-resistant conveyance of fluid is as relevant today as it was over 100 years ago, particularly when considering the life safety and property protection aspects that modern fire sprinkler systems are expected to provide.

Flexible hoses were used to connect fire sprinklers to their supply lines in lieu of traditional threaded steel pipe. Since then, the inherent mechanical benefits, together with the ease and speed of installation, have driven a significant surge in the popularity and use of these products. This has resulted in the adoption, specification and installation of flexible sprinkler hoses in fire sprinkler systems globally.

Why should I use flexible sprinkler hoses? I’ve always used hard-pipe
The adoption rate of flexible sprinkler hoses over traditional hard-pipe arm-overs has steadily increased over the past several years and will continue as the installation and performance benefits continue to be fully realised.

While the reasons for the rapid increase in popularity have focused on the labour savings and improved project completion rates, there are also several critical performance and safety benefits flexible sprinkler hoses provide over traditional hard pipe.

Foundations settle and ceilings can shift and sag over time. This presents a problem specifically with respect to the sprinkler position in relation to the ceiling surface. By code, hard piping is required to be anchored to the building structure separate from the ceiling structure.

Due to this decoupling between the pipe and ceiling, the sprinkler elevation relative to the finished ceiling surface cannot always be guaranteed to remain constant. Under an extreme ‘sagging’ condition of the ceiling surface, a sprinkler can become excessively recessed above the ceiling surface over time, potentially to an installed condition considered to be outside of its agency listing or approval.

[quote style=’1′ cite=”]While the reasons for the rapid increase in popularity have focused on the labour savings and improved project completion rates, there are also several critical performance and safety benefits flexible sprinkler hoses provide over traditional hard pipe.[/quote]

What this really means is that in the event of a fire, the sprinkler’s operational response time and spray pattern characteristics would be severely impacted, thereby allowing the fire to grow unchecked in the affected area. In contrast, a flexible sprinkler hose is required to be securely attached with an anchoring bracket to the ceiling’s structural elements, such as suspended ‘T-grid’ ceiling channels or wood/metal joists or studs.

This positive bracket attachment to the ceiling structure ensures the elevation of the sprinkler remains consistent over time. The inherent adjustability of the flexible hose accommodates the expansion, contraction and settlement of the ceiling structure, preserving the designed operational response and water distribution characteristics of the sprinkler.

Additionally, due to their inherent properties and required installation designs, flexible sprinkler hoses also provide a greater level of performance in seismic events than hard-pipe.

Different design elements: all hoses are not created equal
Flexible sprinkler hoses currently available on the market today have vastly different design constructions. Most are manufactured from an austenitic grade of stainless steel; however, some designs are ‘braided’ hoses, while others are ‘non-braided’.

Additionally, subsets of each of these may have ‘wide pitch’ or ‘narrow pitch’ convolutions – again, each having some very important trade-offs affecting the key performance characteristics of a hose, including minimum bend radius, flexibility and pressure resistance.

Braided sprinkler hoses are designed with an external stainless steel wire woven sleeve providing the necessary strength to withstand internal forces from system pressures, while the corrugated tubing underneath is able to remain relatively unaffected and flexible.

The non-braided hose’s convolutions are farther apart (wide pitch), thereby reducing flexibility and increasing stiffness. As this type must simultaneously maintain pressure resistance while attempting to provide flexibility, albeit significantly diminished, the non-braided type falls short of fully reconciling these two critical features.

Comparatively, braided hoses can withstand higher pressures than non-braided hoses without permanent elongation or deformation. Braided hoses typically are also capable of achieving a smaller bend radius, permitting a greater number of bends, resulting in the ability to accommodate installations in tighter, lower clearance locations. The external braiding also provides vibration attenuation properties in a high-flow condition and an added level of external abrasion protection.

Wide pitch hose, either braided or non-braided, is more difficult to bend due to its inherently stiff cross-section and is more susceptible to ‘kinking’ upon installation. ‘Kinking’, defined as a reduction in cross-sectional area due to inducing a bend beyond its limits, can occur during installation, which results in a deleterious effect on hydraulic friction-loss and corrosion resistance. In contrast, ‘narrow pitch’ hose provides greater flexibility due to the increased number of convolutions per unit length, resulting in less effort to bend, and in many cases, eliminating the possibility of ‘kinking’.

While non-braided hose types continue to be used and are accepted, the clear answer is that ‘narrow pitch’ braided hose types provide a higher level of performance and reliability. Due to their superior flexibility, higher pressure ratings, vibration attenuation properties and safer net-installed-condition, these ‘narrow pitch’ braided hose types have quickly become the standard within the fire sprinkler industry.

Regulatory testing and approvals
Over the past decade, industry standards governing the performance and manufacturing requirements of flexible sprinkler hoses and their anchoring brackets were set, with an average of 16 distinct performance tests between them.

The requirements include accelerated corrosion tests, high pressure activation and flow tests, long-term heat ageing tests, hydrostatic pressure tests including water hammer, fatigue flexibility tests, low temperature tests and an extensive series of vibration tests, among others.

During initial type testing of a design, each model of flexible sprinkler hose and its anchoring bracket is subjected to a series of controlled vibrations at varying frequencies and displacements. Connected to a branch-pipe and anchored to simulated ceiling grid structures, the flexible sprinkler hose assembly is subjected to 90 hours of vibration cycles in each three-dimensional axis from 18 to 37 Hertz and at a maximum displacement of about 0.3 of a centimetre.

Following the vibrational fatigue condition, the hose assembly must pass hydrostatic pressure tests to multiple times the rated working pressure without leakage. Additionally, approved flexible sprinkler hoses are subjected to cyclical fatigue tests, ‘U-bending’ a hose at its minimum allowable bend radius upwards of 50,000 times, and again it must pass hydrostatic pressure tests without leakage at several times its rated working pressure.

[quote style=’1′ cite=”]Today, there are a wide range of flexible sprinkler hose models, coupled with an equally large selection of brackets, fittings and other accessories, available for a broad range of building and construction types, system designs and water delivery requirements.[/quote]

To meet the requirements established by regulatory agencies, all manufacturers are required to conduct factory pressure tests on 100 percent of their flexible sprinkler hose production. These tests are intended to ensure leak-tight performance at a pressure of up to twice its rating. Also, random samples are required to be taken at periodic intervals to perform additional required quality checks, such as hydrostatic burst testing, elongation under hydrostatic pressure and dimensional checks.

Similar to any listed or approved fire protection product, regulatory agencies conduct periodic audits at each manufacturer’s production facility. Agency auditors will check records of the mandatory production testing, compare product drawings with actual production parts, verify the listing or approval mark is applied correctly, and conduct an overall survey of the manufacturing process.

All of this activity is designed to ensure flexible sprinkler hoses and anchoring brackets are consistently produced to the manufacturer’s specifications and continuously meet the performance levels defined by the agency’s standards.

Growing range meets requirements
Today, there are a wide range of flexible sprinkler hose models, coupled with an equally large selection of brackets, fittings and other accessories, available for a broad range of building and construction types, system designs and water delivery requirements.

Flexible sprinkler hoses not only install faster than hard-pipe but, more importantly, they are able to accommodate ceiling shifts and sagging over time, ensuring sprinklers remain at their original installed elevation. When manufactured under robust quality systems, subject to periodic audits, and carrying global product approvals, flexible sprinkler hoses are able to help ensure sprinkler systems remain reliably in service and ready to provide their rated fire suppression capability in any critical event.

As building owners, insurance underwriters, regulatory agencies and authorities having jurisdiction all require an increasingly high level of performance, safety and reliability of a fire sprinkler system throughout its life, these products have become an easy choice in meeting all of these requirements.

The author, Jack Carbone, is a senior fire protection engineer at Victaulic, a developer and producer of mechanical pipe joining systems. This article also appears in the April/May edition of Facility Management.

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