Hose Selection 101

The correct hose can improve your customer's process.

By Patrick Kirk

Selecting the correct hose for a particular application is one key element that is often overlooked. Many hose failures are attributed to a mismatch of hose and application. Armed with a little knowledge, it is not difficult to select the best hose option for your customer's specialty gas application.

Permeation is the term used to describe how gas molecules pass through the pores of a hose. When gases escape out of a hose, this is called “effusion.” If permeation levels are high, ambient gases can enter into the gas stream through “diffusion,” thereby adding contaminants. To varying degrees, rubber, PTFE and ETFE hoses all permeate gases. The smaller the gas molecule, the higher the rate of permeation.

“All metal” hoses utilize a corrugated inner core that features “zero permeation.” One might conclude that “all metal” hoses are the final answer; however, cost, velocity issues and the application will dictate the best choice. PTFE, PTFE hybrids such as “post sintered,” and ETFE thermoplastic hoses maintain pressure capacity while dramatically reducing permeation. Their flexibility and durability are much more appealing than “all metal” hoses when outfitting a high capacity fill plant. It is important to evaluate the requirements of each application and then select the hose that best fits. Selecting the correct hose can maximize gas purity, reduce the risk of contamination and improve a customer's process.

There are several choices to consider when selecting a hose. The sidebar contains hose materials of construction and a summary of benefits and/or disadvantages.

Dynamic Stress and Velocity
Pressure flow characteristics are important factors when using an all metal hose. Dynamic stress is an important consideration when selecting an all metal hose. The hose must not be bent on more than one plane at a time or restricted (over-bent) in any way.

Corrugated hoses are not designed to handle high velocity flow rates. Confirm the velocity rate of each application, and make certain that the flow rate is below the recommended threshold for a given size hose. If the velocity exceeds this threshold, it will establish a resonant frequency that will prematurely crack the hose.

Braiding – The stainless steel braid encases the inner core and gives the hose its strength to hold pressure. A high pressure hose can have from one (3,000 psig) to four braids (5,000 psig), depending on your pressure requirements.

Armor Casing – Armor casing can easily be installed over the braided hose. It will prevent hose kinking and whipping in the event of a hose burst, and also protects the exterior of the hose.

Fittings – Swivel fittings, hand tights, 90-degree elbows and brass heat sinks for adiabatic compression are a few of the more popular fittings.

Selecting the Best Hose
In the specialty gases industry, there are several choices when considering a hose. Here are some basic things that one should take into consideration.

Pressure – Make certain that the average working pressure of the hose is well above the required maximum pressure of the application.

Molecular Weight – Hydrogen and helium have the smallest molecular weight, and therefore present a higher risk of permeation loss.

Permeation – If permeation is a concern, consider using an all metal hose. Make certain that the flow rate is below the suggested threshold level. Other options would be an ETFE or PTFE post sintered type hose. These hoses will reduce effusion by approximately 75 percent.

Purity – Hoses that effuse gases can also diffuse gases. Diffusion can introduce contaminants that could affect the high purity gas stream. If this is a concern, consider using an all metal hose.

Static State – If a gas is used in a static state, there will be times when the gas is not being used (off hours, weekend). In this case, gases trapped inside the hose could effuse out. The amount lost is minimal, but over time this loss can add up.

Fill Manifolds – Hoses used on fill manifolds are typically cycled (attached and disconnected from the cylinder) several times a day. The hose is exposed to continual flexing, and then disconnected (hanging open to the air). As a result, permeation on a fill manifold is of minimal concern. PTFE hoses work well in this application.

Laser Cutting Systems – Using the wrong hose to supply the laser can be detrimental to the operating efficiency of the laser system and contribute contamination, resulting in increased downtime. Ultimately this will affect the quality of cut.

Hose Maintenance
Factors to consider for hose maintenance are cycle rates, ambient conditions, and overall handling and care of the system. If a hose is cycled more than 12 times per day, it has a life expectancy of approximately 18 months.

Always inspect and leak check hoses on a regular basis. It is important to look for kinks in the hose or breaks in the braid, severe bends, discolorations or any other abnormalities. If any of these symptoms are found, the hose should be removed and replaced.