What are flexible hoses?
A flexible hose, in the fire protection industry, is also known as a flexible drop. It is used to connect sprinkler heads to their supply lines in lieu of traditional rigid pipes.
Flexible hose Rigid pipe drop
This assembly consists of a threaded nipple or captured coupling for connection to the branch line, a flexible hose, a straight or 90° sprinkler reducer, and a bracket. All of these are tested and approved together as part of a “system” by the different laboratories.
Invented before the turn of the 20th century, flexible metal hoses have been utilized in a wide range of industries and applications. But only recently have they experienced significant growth in the fire protection industry. Fire protection engineers in Japan sought to cancel out the adverse effects of seismic activity on structures
What makes the flexible hose so popular?
Since available, the inherent mechanical benefits combined with the ease and speed of installation, have driven a significant surge in the popularity and use of these products. This product also gives engineering confidence when adequately installed. This has resulted in the adoption, specification, and installation of flexible hoses globally except where the labor cost is very low.
When discussing with suppliers, they list the following advantages:
Reduced labor cost: It can significantly reduce the time of installation. A single return bend on branch piping can be composed of at least five pieces. Most of the time these pieces must be measured before, prefabrication on site (i.,e. cut and threaded). A flexible hose does not require the magnitude of assembly nor fixed measurements to secure an installation. This is efficient material handling with no power machine or cutting/threading/gluing of pipe
Faster project completion: People that work in construction know that in the phasing of a project, branch lines and main pipes must be installed before the secondary structure is (i.e., the structure of the false ceiling). However, the sprinkler heads must be positioned once the ceiling structure is installed. Therefore, beyond project completion, it saves time issues. Think about a high-rise building where every task is synchronized.
Typical installation of rigid drop pipes with a secondary sandwich Time saved during a project.
panels roof
While the reasons for the rapid increase in popularity have focused on the labor savings and improved project completion rates mentioned, there are also performance and safety benefits for the use of flexible hoses:
The site’s lifetime: Ceilings can shift and sag over time. The same issue can occur with ceilings made of sandwich panels. When additional loads are placed on or above the ceiling, such as equipment for inspection, testing, and maintenance, it can lower over time. This presents an issue with the sprinkler head position and the obstructions. Indeed, a rigid drop is required to be anchored to the building structure separate from the ceiling structure, directly or through the branch line. If the ceiling shifts, sags, or is lower, the sprinkler head can be misaligned, and the water pattern can be obstructed. However, the flexible hoses allow the sprinkler to move with the ceiling eliminating the water distribution concerns of the traditional movement.
Repositioning: When there are ceiling modifications, the use of a flexible hose will allow easy relocation of sprinklers within a short distance. This repositioning does not require cutting or modification of the existing pipes and fittings providing additional flexibility in the lifetime of the installation. In addition, some manufacturers now propose brackets where the central gate enables for vertical adjustment of the sprinkler head after the ceiling is installed. This is especially recommended for a hard-lid ceiling.
Engineering confidence: o A rigid drop pipe is generally made of five threaded connections. Pipe threads are not the most reliable connection for fluids as the thread itself provides a leak path. Pipe threads are deformed when tightened, and any subsequent movement, either loosening or tightening, increases the potential for leaks. As flexible hoses are tested after being fabricated, they bring engineering confidence from this perspective. o Approval: UL developed a specific standard for flexible hoses as well as FM, LPCB, and VDS (there are more than 10 performance tests available on the market). There are some requirements for testing flexible hoses for automatic sprinkler systems, which adds engineering confidence to the product. The requirements include 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. Once the product is approved or listed, there are still additional requirements during the manufacturing process. The manufacturers 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. As it is often said by laboratories, you get what you test, i.e., you need to install and design as it has been tested.
Different design elements: are all hoses created equal?
Flexible hoses that are currently available on the market have different design characteristics. They are manufactured with stainless steel; however, some are braided while others are not, some are approved for a large range of activities and some are not, etc. The corrugated technology: helical or annular
Corrugations are undulations and can be formed in a circumferential pattern (i.e., annular) or run in a spiral pattern (i.e. helical). While it seems to be a fabrication choice, the type of corrugation greatly influences the mechanical and hydraulic characteristics of a flexible hose. With a helical corrugated hose, the distance between each corrugation is larger than an annular corrugated hose. Though helical corrugation dramatically reduces the flexibility of the hose making it almost rigid and easy to tear or kink. Annular corrugations are 100% non kinkable and nor tearable.
Braided or non-braided
Braided flexible hoses are designed to offer tighter bend radii and increased number of allowable bends, delivering better performance. It helps to guide the flexible hose and make their installation easier and quicker
Un-braided hose Braided hose
Braided flexible hoses are preferred as they offer a higher performance in the lifetime and are more difficult to be installed improperly. In general, braided hoses can withstand higher pressures than non-braided hoses without permanent elongation or deformation.
Annular braided hoses bring the following advantages:
Have a smaller bend radius, permitting a greater number of bends, resulting in the ability to accommodate installations where there is much equipment.
Can be bent at the thread unlike helical which must not be bent 6cm on each end or it will kink. Making annular hose the best choice for tight spaces with the ability to work around pre-existing piping/duct work
Create fewer friction losses at high flow (for example when the first sprinkler head is activated).
Occupancies and roof type
There are various occupancies from light to extra hazard, and the design limits such as density or water delivery are different. A few flexible hoses are listed for storage applications. Some hoses are listed for the use of large K-factor (K200, i.e., K14)
Flexible hoses can work with wet, dry, or preaction sprinkler systems.
The choice of the flexible hose and the right bracket will depend on the construction type (suspended ceiling, sandwich panel, hard-lid, open or cloud ceilings, etc.). Additionally, there are different applications related to the type of bracket as special ones can be made for cleanroom ceilings (cleanroom ceiling manufacturers are numerous flexible connector manufacturers approve their systems for each ceiling manufacturer), and/or ducts. For all these construction types, it makes sense to install flexible hoses.
The adequate flexible hose and its bracket must be carefully chosen regarding the above-mentioned topics.
The limits of the approval
The approval standard may offer some products to specify their own specific requirements, such as the number of bends, the bend radius, etc. We know that a flexible hose with a low number of allowed bends is prone to being over-bent and wrongly installed. Does it mean we should prohibit the use of flexible hoses that don’t give enough engineering confidence? The question is tough but is real regarding the very large number of issues we observe on site. From Elen Risk Consulting perspective, annular braided hoses should be preferred.
What should we pay attention to?
Today, there is a wide range of flexible sprinkler hose models, coupled with a large selection of brackets, fittings, and other accessories.
While we have mentioned above that braided annular corrugated hoses should be preferred, there are many items to pay attention to. Here is a list of the main issues that Engineers observe.
Main issues encountered in the design phase: hydraulic calculation
Friction loss – The length: The most common deviation is the lack of adequate reported friction loss for the flexible hose. The friction loss is roughly estimated to be 5 to 20 times higher than a rigid pipe of the same length. For an extra hazard application, with a length of 2 m (6 ft.), it may represent a friction loss of 1.5 bar (21 PSI), which is a lot.
Friction loss – the internal diameter: Flexible hoses are made of different diameters from DN20 (3/4”) to DN25 (1”) and different corrugation. While a DN20 ID hose can be made of both helical and annular, the helical will have better hydraulics than the annular for the way the internal corrugations are described above. Helical corrugated hoses can be approved as a DN25 (1”) as manufacturers are allowed by the laboratories to do so. In reality, helical hoses have a max ID of DN22 (7/8”), or they would be so stiff that they would never pass the fatigue test. Annular corrugated ones will have a true DN25 (1”). This difference in ID makes the DN25 (1”) annular corrugated hose to have in some cases 50% less friction loss than helical. This is also something that is wrongly reported in the hydraulic calculations. This small change of diameter is of higher importance as it can increase the friction loss dramatically. Main issues encountered on the field
3. Minimum bend radius. The minimum radius is expressed in cm (inches) and is specified by the manufacturer in the approval. Therefore, there is no standard information. Misunderstanding and mistakes are regularly performed on-site regarding this minimum bend radius. The smallest radius on the market is 5 cm (2”).
Minimum band radius not respected. The above picture shows 2 bends.
4. Kink effect: When minimum bend radii aren’t respected, kinks are common with helical corrugated hoses. If a change in direction is made immediately after the reducer or adapter, it can also lead to the hose to kink. This is the worst case that is encountered on the field because it will strictly reduce the water available to the sprinkler head.
Un-braided hoses - Inadequate installation – both hoses are kinked
5. Number of bends: Some flexible hoses can be approved for a very limited number of bends (sometimes only one). This information is reported in the datasheet and is often not applied by the book on site.
6. The Residual torque issue: Some flexible hoses are called weld-by-weld flexibles which means both the branch line adaptor and the sprinkler reducer are welded to the hose. When a sprinkler system is in-stalled with pendent or recessed heads, the installers make sure that the frame arms of the sprinkler head are aligned with the branch lines. With a weld-by-weld flexible drop, there is a chance that the frame arm alignment was achieved by twisting the end fitting. Since this is welded construction, the hose does not allow for that torque to be relieved. This causes an undesirable stress on the weld that has not been tested for, which creates a greater opportunity for failure. The only way to tell is to loosen the set screw on the center bracket – the flexible will then spring back into the non-torque position.
7. Bracket technology: There are different bracket solutions for many applications. There are different benefits to using bracket technology. Some bracket technology allows the adjustment of the sprinkler after the hard lid ceiling has been installed. The bracket also allows easier movement when working around other building trades.
8. Additional hanger: According to NFPA, as soon as a flexible hose exceeds 1.8 m (6 ft.), an additional hanger shall be attached to the structure to ensure that the maximum unsupportable length is not exceeded.
9. Training: No additional training is required for the operators. Some specificities need to be taken into consideration, as a flexible hose is a technological mechanical product. Using the manufacturer’s submittals and installation instructions can ensure the flex hose is designed and installed properly.
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