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Mar-2013

Choose the right hose: a practical guide to hose selection

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.

Patrick Werrlein
Swagelok Company
Viewed : 3372
Article Summary
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 (see Figure 1):
• Core tube material and construction
• Reinforcement layers
• Covers
• 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.

Core tube material and construction
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 catalogs and your sales and service representative:
• Is the material chemically compatible with the system media? Will it corrode or deteriorate over time?
• Can it tolerate the temperature range of the system media?
• 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.
• 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 us 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 Table 1.

Metal cores (commonly 316L stainless steel) are a good choice for general needs. They are usually rated for temperatures between -325˚F and 850˚F (-200˚C and 454˚C), 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 -65˚F to 500˚F (-53˚C to 315˚C). Silicone became the material of choice for sanitary applications because of its flexibility. However, that advantage has disappeared with advancements in fluoropolymer hose construction (see below).

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 -65˚F to 450˚F (-53˚C to 230˚C).

 Fluoropolymer cores are the most chemically inert cores available. They are non-ageing, non-stick, 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. 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.
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