The right hose keeps your process performing safely and cost effectively. The wrong hose could undermine your process, put personnel at risk, and compromise your bottom line – sometimes without you being aware of it.
For example, an improperly chosen hose may kink. This permanent buckling of the hose disrupts the system media flow and creates a rupture threat. But because kinked hoses are not easily detected, they are in operation throughout industry.
Despite its importance, hose selection is often treated as an afterthought. Proper hose selection starts with an understanding of the four main parts of a hose:
1. Core tube material and construction
2. Reinforcement layers
3. Covers
4. End connections
Selection requires making choices in those areas while paying attention to the variables in your application. Temperature, pressure and flow requirements, as well as requirements ranging from chemical compatibility to drainability, are some of the specifications that will dictate your choice.
Hoses cost more than their purchase price. Selecting wisely also requires the consideration of hose longevity, maintenance and replacement costs, and other cost-of-ownership factors. The steps discussed below will help you arrive at the right hose for your application.
Hose selection requires making choices in the four main areas of a hose – core tube material and construction, reinforcement layers, covers and end connections.
Core tube material
When selecting a hose, the place to start is the core tube, which is the hose's innermost layer, the one that comes into contact with the system media.
Here are some basic questions to answer when selecting the core tube material. You can address these with the help of product catalogues and your sales and service representative.
1. Is the material chemically compatible with the system media? Will it corrode or deteriorate over time?
2. Can it tolerate the temperature range of the system media?
3. Will the material prevent or limit permeation and absorption? All materials, even metals, are subject to permeation and absorption, so this question is one of degree. Permeation occurs when media passes through a material, whereas absorption is when media absorbs into and becomes part of a material. Depending on your application, permeation and/or absorption may not be an issue.
4. Will the core material stand up to the cleaning practices for your system, both in terms of temperature, pressure, and material compatibility with any solvents and cleaning agents employed?
First, let's review the materials that core tubes are made of; then, we will review some choices for core tube wall construction.
For a summary, refer to the Selection Guideline Overview table below:
|
Core Tube Material |
Summary |
Wall Construction |
|---|---|---|
|
Metal |
· Good general purpose hose · Widest temperature range · Resists permeation and absorption · Not compatible with highly caustic or acidic system media · Not well suited for operations with repetitive movements |
· Available in convoluted core only, typically annular, to be used when flexibility is a priority |
|
Fluoropolymer (PTFE, PFA, FEP) |
· Good chemical compatibility · Advancements have made it very flexible · Good cleanability · Low absorption · Available with static dissipation · Highly permeable |
· Available in smooth-bore core, for maximum flow control and drainability · Also available in convoluted core, typically helical, to be used when flexibility is a priority · Smooth-bore cores can kink, particularly in larger sizes, but reinforcement layers limit kinking · Helical is better than annular for flow control |
|
Silicone |
· Very flexible · Limited chemical compatibility · Highly absorptive · Poor cleanability |
· Available in smooth-bore core only |
|
Thermoplastic (Nylon) |
· Economical, general purpose hose · Good for hydraulic fluids · Limited temperature range |
· Available in smooth-bore core only |
|
Rubber |
· Economical, general purpose hose · Durable · Available in large sizes (above 2 inches) · Limited temperature and pressure range |
· Available in smooth-bore core only |
Metal cores (commonly 316L stainless steel) are a good choice for general needs. They are usually rated for temperatures between -200C and 454C, which makes them an especially good choice – sometimes the only choice – for system media at extreme temperatures.
A metal core is also a good choice when there is little allowance for permeation or absorption. With the advent of fluoropolymers, metal is usually not chosen for highly caustic or acidic media because of issues with corrosion.
Historically, silicone has been a common choice for sanitary applications.
A typical temperature range for silicone is from -53C to 315C. Silicone became the material of choice for sanitary applications because of its flexibility. However, that advantage has disappeared with advancements in fluoropolymer hose construction.
Silicone, which is incompatible with common solvents, has limited chemical compatibility overall. In addition, it is absorptive, which can lead to contamination. If a fluid is absorbed into the walls of the core tube, it may remain there for a period of time before leaching out, at which point it may contaminate the media currently in the system.
With silicone, removing the absorbed fluid is usually not possible.
Steam cleaning, one of the most common sterilisation methods for silicone, may not remove it, and the high temperatures may cause premature failure. The hose will become brittle and break down.
In place of silicone, fluoropolymer cores are becoming the material of choice for sanitary applications. PTFE, PFA, and FEP are three common fluoropolymers, with a typical temperature range from -53C to 230C.
Fluoropolymer cores are the most chemically inert cores available. They are non-ageing, nonstick, easy to clean, and can withstand repetitive steam cleaning. Like metal, fluoropolymers also have a low absorption rate.
In addition, advancements in the use of reinforcement layers have allowed fluoropolymer cores to overcome their stiffness and gain flexibility comparable to that of silicone. New technology has enabled a bonding technology that allows a fibreglass braid to be added as a layer for increased flexibility.
The bonding technology uses no glue. The glue-free process means there is no potential for glue absorbing into the core walls and later contaminating the process. PTFE cores comply with FDA regulation 21CFR Part 177.1550, USP <88> Class VI, and 3-A.
In convoluted cores, the tube walls are folded in a pattern to increase the hose's flexibility. The two types of convoluted cores are helical, found primarily in fluoropolymer cores, and annular, typical in metal cores.
One drawback of fluoropolymers is that they are highly permeable. If your application cannot tolerate permeation, then you can specify a less permeable core material, such as metal.
With many fluoropolymer hoses, you can specify a carbon black filled core if your process requires static dissipation. Carbon allows the charge to travel to the end connections and exit. Static dissipative cores are important because fluid can generate electrostatic as it passes through the hose. Static sparking can damage hose and pose a safety hazard.
Thermoplastic (nylon) hoses, which can contain high pressures, are often chosen for hydraulic applications. They are available in sizes up to 1 inch and have a typical temperature range from -40C to 93C.
Rubber hoses are economical general purpose hoses with a typical temperature range like that of thermoplastic. They are only for low-pressure uses. An advantage of rubber hoses is their ability to be crushed without permanent damage. They are also made in sizes above 50.8 mm (2 inches). The other hoses described typically range in size from 3.17 mm (1/8 inch) or 6.35 mm (1/4 inch) to 50.8 mm (2 inches).
Before you make your final choice in core tube material, you must first understand core tube wall construction. You must decide whether the core tube wall should be smooth or have convolutions, which allow it to bend like a flexible straw.
Smooth-bore cores, built with a smooth inner tube wall, are a good choice when precise flow control and drainability are priorities.
Your application's requirement for hose bendability will guide your decision, as will pressure, flow and drainability needs. In a smooth-bore core, the tube's inner wall is smooth. All core materials except for metal are offered in smooth bore.
Choose smooth bore if precise flow control is a priority. With smooth bore, precise flow control is possible because there are no irregularities in the wall to cause interruptions. Smooth bore also promotes drainability. The primary disadvantage is kinking, particularly in larger diameters. Reinforcement layers, discussed later, help solve kinking.
In the other wall construction choice, convoluted, the walls of the tube are folded in a pattern that increases the hose's ability to bend without kinking. Metal and fluoropolymer cores are offered with convolutions. This construction is chosen when flexibility is the priority.
Convoluted cores come in two types – helical and annular. The helical design, found primarily in fluoropolymer cores, is a single convolution that spirals down the length of the hose. Sometimes you must choose a convoluted core because you need flexibility, but flow maintenance and drainability are also important. Select a helical design, because it promotes better flow downstream than annular convolutions.
Annular design, typical in metal cores, is a series of connected rings. Annular metal cores come with deep convolutions for maximum flexibility. Despite their flexibility, convoluted metal hoses are not well suited to operations where they are moving in a repetitive pattern, because the movements can cause metal fatigue and breaking.
Reinforcement layers
Reinforcement layers are your next consideration. In most cases, the core tube is reinforced by a flexible, stainless steel woven braid, which is layered on top of the core tube. Proper reinforcement layers improve pressure containment and flexibility in the hose. To compare pressure ratings among hoses, you can consult product catalogues.
Pictured alongside: Minimum bend radius measures how far a hose can bend before kinking. Although not as easy to measure as minimum bend radius, force-to-bend is an equally critical factor in your selection.
Comparing flexibility is trickier. To do so, you need to understand bend radius. All hoses have a minimum bend radius, which measures how far a hose can bend before kinking. Measurements for minimum bend radius are standard in industrial hose literature.
However, there's more to flexibility than bend radius – and many people in the industry do not understand this. Also consider the force required to bend an unpressurised hose. A hose with a good bend radius is not much help to you if the force required to bend it is too great. Is the hose so stiff that an operator can't bend it or has difficulty installing it?
Will it slow down or break a machine in a dynamic operation? Force-to-bend is just as important as bend radius but not as easy to measure, and guidelines don't exist across hose manufacturers. You may ask your sales and service representative to show you hose samples that you can test with your own hands.
Cover selection
Next, you'll need to decide if your hose should have a cover – and, if so, what kind. A cover is an outer layer that protects underlying layers, personnel and surrounding equipment. Covers come in materials such as silicone and rubber and are integral to the hose.
The most common cover for general purposes is made of silicone. Silicone covers help prevent fraying of the braids in stainless steel reinforcement layers, which can happen from abrasion. Silicone covers can provide enhanced burn protection for operators who grab or bump hoses that are carrying very hot fluid. They provide insulation as well, helping to maintain process temperature.
Silicone covers are a particularly good choice for sanitary applications. Their smoothness makes them easy to wash down. And by covering the stainless steel reinforcement layer, they eliminate bacteria buildup in the braid's crevices.
You'll also find covers for specialty applications. For maximum burn protection, consider a fire jacket, a fibreglass cover coated in silicone rubber. Keep in mind, however, that fire jackets connect loosely to the hose and can snag and rip. Another cover type, bend restrictors, help prevent the hose from being bent beyond its bend radius.
On the downside, covers add cost, restrict flexibility, and make the hose larger, a concern for routing and angling. In most applications, the goal in cover selection is to achieve the smallest diameter and not decrease the flexibility of the hose.
End connections
End connections, usually made of metal, are where most leaks occur. The performance of the hose assembly you purchase depends largely on the hose manufacturer's ability to attach end connections, so choose a reputable manufacturer.
For metal hoses, there are a variety of end connection choices. The connections are welded, which completely and permanently seals the product.
For fluoropolymer hoses, you will choose between swaging or crimping. Swaging puts pressure on the hose itself, while crimping squeezes the end connection. While both methods are widely accepted, crimping has a slight advantage in that it is less likely to damage the hose, because the pressure is exerted in a carefully controlled manner.
Pictured alongside: A common cover for specialty applications is the fire jacket, a fibreglass cover coated in silicone rubber that offers maximum burn protection.
Many chemical applications require that fluoropolymer-wetted surface end connections be used.
The industry has devised some creative solutions. One is called "flare-through," because the core tube is flared such that it covers the entire inner surface of the metal end connection.
The chief benefits of the flare-through approach are that there is no step or drop between the core tube and fitting, ensuring smooth flow, and the result is an all fluoropolymer-wetted surface. However, flare-through is costly and fragile and is not recommended for high-temperature applications.
Another common solution is "encapsulation," because the stainless steel end connection is entirely encapsulated in fluoropolymer, inside and out. Advantages are cost and availability. Disadvantages are reduced orifice size, raising the possibility of reduced flow and entrapment.
[Patrick Werrlein is Product Manager, Hose Products for Swagelok Company.]