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Vibration can be very harmful for an assembly because it may cause high fatigue stress to the metal hose, which leads to the development of cracks and premature failure of the assembly. Therefore, to decrease harmful effects of vibration, it is important during assembly fabrication to take into consideration such factors as minimum hose length requirements, operating pressure and respective derating factors for a type of vibration or dynamic stress.

If vibration is present it is especially important to follow Assembly Installation Handling Precautions which include making sure that the hose bend and the main direction of movement lie in one plain and avoiding dangerous torsions.

It is important to calculate proper length of the assembly, keeping in mind that there are some requirements as to the minimum live length of the assembly used in an application involving vibration. While minimum hose length requirements will vary based on the type and intensity of vibration experiences, suggestions are found in the table below.

Note: As been already pointed out there are some reduction factors (for published Working Pressure) that should be considered with vibration and other dynamic stresses. These factors can be as low as 0.32 in case of strong vibrations with pulsating and unsteady flow through the hose. Please consult Penflex for details.

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Note: To print this bulletin on austenitic steels and cryogenic temperatures, please click here.

Since we discussed the maximum service temperatures of common austenitic steels in Engineering Bulletin #106, we’ll now look at how mechanical properties of austenitic steels are influenced by cryogenic temperatures and what types of stainless steel alloys are best suited for low temperature applications.

During World War II, experience with the brittle fracture of steel ships caused engineers to look  closely at what happens to metals in cold weather. They found that though many metals have good “room-temperature” characteristics, they do not necessarily maintain those characteristics at low temperatures.

For example, Ferritic (405, 409, 430), Martensitic (403, 410, 414, 416) and Duplex stainless steels (329, 2205) tend to become brittle as the temperature is reduced. Fracture can occur, sometimes with catastrophic results. While stretching or bulging may serve as an indicator of impending plastic failure, such signs are absent in the case of these metals. Therefore alloys for low-temperature service are those that retain suitable properties such as yield, tensile strength and ductility.

The austenitic stainless steels such as 304 and 316 retain these engineering properties at cryogenic temperatures and can be classified as ‘cryogenic steels.’ They are commonly used in arctic locations and in the handling and storage of liquid gases such as liquid nitrogen and liquid helium. Liquid helium is the coldest material known with a boiling point of -452°F (-269 °C).

The table below shows mechanical properties of stainless steels at low temperatures. Elongation is an indication of their good ductility. There is an increase in tensile and yield strengths as the temperature decreases as well.

Mechanical Properties of 304, 321 and 316 Stainless Steels at Cryogenic Temperatures.

Note: In designing a metal hose assembly for cryogenic service, care must be taken to ensure that the fittings and any accessory (guard/liner) materials are suitable for the intended operating temperatures as well.

If you have any questions or comments, please contact us.

Disclaimer: The info presented here has been compiled from sources believed to be reliable, including the American Society of Materials Specialty Handbook on Stainless Steels. No guarantee is implied or expressly stated here and the data given is intended as a guide only.

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When selecting a hose for a specific pressure application among various factors that must be taken into consideration, the operating temperature and dynamic stress (pressure spikes, vibration, frequent movements etc.) are the most important parameters to consider. The working pressures published in Penflex’s product catalog apply to operating conditions at 70°F and therefore at higher operating temperature, they  need to be reduced by applying temperature adjustment factor.

Because braid is the main component of the assembly that prevents the hose from elongation due to internal pressure, temperature adjustment factors should be based on the alloy used in the braid wire and are listed in the table below.

Temperature Adjustment Factor (based on braid alloy)

For example to calculate the Maximum Allowable Working Pressure (MAWP) for 3/4″ ID, 321 Stainless Steel Corrugated hose single-braided with 304L braid, which will be used at operating temperature of 800°F, the following calculations should be performed:

MAWP = Working Pressure at 70°F x Temp. Adjustment Factor = 792 PSI x 0.73 = 578 PSI

If the assembly will be subjected to dynamic stress such as fluctuations in pressure, then additional reduction factors should be applied to calculate MAWP. Some reduction factors for common pressure fluctuations (designated by NAHAD) are listed in the table below:

So, if our assembly (from the example above) will be subjected not only to high temperature environment, but also will be experiencing pulsating pressure fluctuations, then MAWP has to be reduced even further:

MAWP = 792 PSI x 0.73 = 578 PSI x 0.50 = 289 PSI

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When designing piping systems, it’s important to consider pressure loss, or drop, through the hose. This will allow you to assess its impact on the piping system’s operation.

It’s a difficult calculation—being based on flow rate (volume per time)—and one that depends on several variables including the type of media passing through the hose, the geometry of the hose and the application setup. The only way to truly determine pressure drop in a hose is to set a gauge at one end and another at the other, and to then take a reading.

However, if accurate readings of inlet and outlet pressure are not readily available, we can roughly estimate pressure drop in a corrugated hose based on what we would see in steel pipe under similar flow conditions. Due to its profile, pressure loss in corrugated hose is significantly higher than that in steel pipes. As a general rule, pressure drop in the turbulence zone of corrugated hose will be 150% higher than in steel pipe while pressure drop in the high velocity zone will be 450% higher.

Pressure Loss Scenario

For example, let’s say we need to calculate the pressure loss in 2” (ID) corrugated hoses that are 85 feet long and transferring water with flow rate of 1400 cubic feet per hour.

By using Penflex’s Pressure Loss calculator we find that 1400 ft³/hour corresponds to 175 gal/min.

To convert flow rate from Cubic Feet per Hour to Gallons per Minute, use the calculator below:

Then we plot the 175 gal/min on the X-axis of this chart until we “hit” the line for 2″ hose ID, then by going over horizontally to the Y-axis, we find that pressure loss per foot of hose will be about 3.7 psi.

Thus, the total pressure drop over the hose length will be 314.5 psi (3.7 psi x 85 feet).

Keep in mind that if you transfer gaseous substance through the hose then you need to find the ratio of the density of gas over the density of water and adjust the pressure drop respectively.

For example if you transfer natural gas (density = 0.050 lb/ft3) and knowing that water density is 62.4 lb/ft3 we can find out the pressure drop as the following: 3.7 psi x (0.050/62.4) = 0.0030 psi/ft or 0.255 psi for entire length of hose (85 x 0.003).

For calculations relevant to your application, you can download and print a blank chart.

Please contact us with any questions.

Note: To print this bulletin on corrugated metal hose, please click here. 

When a corrugated metal hose is considered for use in a system which is externally pressurized or under vacuum conditions, as the case may be with vacuum pumps,  the question as to how the hose will behave under “full vacuum” or “perfect vacuum” is often asked.

The definition of vacuum is used to describe any pressure that is lower than standard atmospheric pressure. The most widely accepted unit of vacuum measurement is the Torr (after an Italian scientist Torricelli). So one standard atmospheric pressure can be expressed, in the units more commonly used within our community, as the following:

1 atmosphere = 760 Torr = 14.7 PSI

According to Columbia Encyclopedia, “a perfect vacuum has never been obtained,” and therefore expressions “full vacuum” or “perfect vacuum” are used loosely to express conditions with near “0” pressure.

Take a look at the table below to compare different “vacuum conditions.”

As to the above mentioned question the answer is corrugated metal hose can be used under vacuum conditions and will not be overstressed under such condition, provided the hose section is adequately braced against buckling. The design approach is similar to that for internally pressurized system keeping in mind that external air pressure causes the hose to contract inwardly rather than expand axially. The proper design though, requires evaluation of the system as a whole not just one segment – such as corrugated hose – at a time.

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Note: To print this bulletin on calculating pitting resistance equivalent number (PREN), please click here. 

When a metal hose is considered in an application for transferring an aggressive media, like high chlorides, or when it fails while in service due to pitting corrosion or chloride stress corrosion cracking, it is essential to use a hose or replacement hose manufactured from the proper alloy to prevent future failure. Many times a failed hose is replaced with the same alloy and most likely will fail within the same time frame.

Stress corrosion cracking usually starts with pitting corrosion. The most common cause of pitting in stainless steel is acid chlorides. Chlorides react with chromium to form the very soluble chromium chloride (CrCl3), removing chromium from the passive layer, and leaving only the active iron. As the chromium dissolves, chlorides bore into the surface of stainless steel creating spherical, smooth wall pits which become stress concentrators.

To improve the pitting corrosion resistance of stainless steel such alloying elements like molybdenum (Mo) and/or nitrogen (N) are added. To help in the selection of an appropriate alloy for an application an equation called the pitting resistance equivalent number, or PREN, has been developed. PREN is a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels based on the chemical compositions of an alloy.

The most commonly used formula is as follows:

PREN = %Cr + 3.3(%Mo) + 16(%N)

The table below shows a comparison range of calculated PREN values for common alloys. The higher PREN for an alloy the better its resistance to pitting corrosion.

Note: A PREN of 32 is considered the minimum for seawater pitting resistance.

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Disclaimer: The info presented here has been compiled from sources believed to be reliable. No guarantee is implied or expressly stated here and the data given is intended as a guide only.

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Note: To print this bulletin on corrugated metal hose and flow velocity consideration, please click here. 

The flow velocity in corrugated metal hose should never exceed 150 ft./sec. for gas or 75 ft./sec. for liquids otherwise a resonant vibration can occur. Resonant vibration may cause very rapid failure of the assembly.

To figure out maximum allowable flow rate for Penflex 700 and P5 Series hoses, please refer to the table below:

Maximum Permissible Flow Rates

Where the flow velocity exceeds these rates, an interlocked metal hose liner or larger hose I.D. is recommended. We dive deeper into this topic here.

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Penflex corrugated hose is engineered to provide maximum service life when properly installed. Improper assembly installation, incorrect flexing or careless handling in an application will reduce the effective service life of the hose and cause premature failure of an assembly. The following assembly installation and handling precautions should be observed to achieve optimum performance from your corrugated hose assemblies.

• Avoid torque.
  Do not twist the hose assembly during installation when aligning the bolt holes in a flange or in making up pipe threads. The utilization of lap joint flanges or pipe unions will minimize this condition. It is recommended that two wrenches be used in making the union connection; one to prevent the hose from twisting and the other to tighten the coupling.
• In plane lateral offset installation.
  Prevent out-of-plane flexing in an installation. Always install the hose so that the flexing takes place in only one plane. This plane must be the plane in which the bending occurs.
• Avoid over bending.
  The repetitive bending of a hose to a radius smaller than the radius listed in the specification tables for corrugated hose will result in premature hose failure. Always provide sufficient length to prevent over bending and to eliminate strain on the hose.
• Avoid sharp bends.
  Utilize sound geometric configurations that avoid sharp bends, especially near the end fittings of the assembly.
• Provide support.
  When installing the assembly in a horizontal loop, provide support for the arms to prevent the hose from sagging.
• Do not extend or compress axially.
  A piping system which utilizes metal hose to absorb movement must be properly anchored and/or guided. Always support the piping to prevent excessive weight from compressing the hose and relaxing the braid tension.
• Handle with care.
  Avoid careless handling of the hose assembly. Always lift or carry metal hose to prevent abrasion damage particularly to braided corrugated hose. Store metal hose assemblies away from areas where it can be subjected to spillage, corrosive fumes or sprays, weld splatter, etc.

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Most common reasons for using stainless steels are their corrosion resistance and their high temperature properties. Therefore stainless steels can be found in applications where high temperature oxidation resistance is necessary and in applications where high temperature strength is required. The high chromium content which is so beneficial to the wet corrosion resistance of stainless steels is also highly beneficial to their high temperature strength.

Most austenitic steels, with chromium contents of at least 18%, can be used at temperatures up to 1500°F and Grade 310 (Cr content: up to 26%) even higher – up to 2000°F. Because of the problem of grain boundary carbide precipitation, discussed in Engineering Bulletin #103, prolonged exposure to the temperature in the 1100°F to 1400°F range should be avoided.

The table below shows the (approximate) maximum service temperatures of austenitic steels & other common materials.

Maximum Service Temperatures of Austenitic Steels & Other Common Materials in Dry Air

Note: the temperature ratings in the table are general guidelines and could change if corrosives are present, such as sulfur, carbon, etc. In some cases ASME codes will reduce temperature limits too.

*For applications where temperatures exceed 1000°F, read about H Grade alloys in Bulletin #137 or contact us.

Disclaimer: The info presented here has been compiled from sources believed to be reliable. No guarantee is implied or expressly stated here and the data given is intended as a guide only.

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Note: To print this bulletin on chloride/chlorine levels and stainless steel alloy selection, please click here. 

Pitting corrosion occurs when chloride levels exceed recommended levels.

The 304 and 304L (18-8 stainless steel alloys) have been utilized very successfully in fresh waters containing low levels of chloride ion of up to 100 ppm. This level of chloride is considered to be the limit for the 18-8 alloys, particularly if crevices are present. Higher levels of chloride might cause crevice corrosion and pitting. The 18-8 alloys are not recommended for exposure to marine environments which have much higher levels of chloride.

The resistance of the stabilized Alloys 321 to pitting and crevice corrosion in the presence of chloride ion is similar to that of Alloy 304 or 304L stainless steels because of similar chromium content. And therefore 100 ppm chloride in aqueous environments is considered to be the limit for the stabilized alloys, particularly if crevices are present.

For more severe conditions of higher chloride level, lower pH and/or higher temperatures, alloys with Mo (molybdenum), such as Alloy 316, should be considered. The Mo-bearing Alloy 316 and Alloy 316L may handle waters with up to about 2000 ppm of chloride.

Another factor to consider is the amount of free Chlorine (Cl2) (usually derived from sodium hypochlorite) which is added to water (well water, drinking water, swimming pool water, etc.) to kill bacteria. Cl2 (chlorine) is a very potent oxidizer (reason it kills bacteria) and therefore high levels of Chlorine may accelerate chloride corrosion of stainless steels. 304 and 304L, 321 SS may be used for “water applications” with up to 2 ppm chlorine, while 316 and 316L alloys may “take” up to 4 ppm.

If looking for information about Chlorine Transfer Hoses, please click here.

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Disclaimer: The info presented here has been compiled from sources believed to be reliable. No guarantee is implied or expressly stated here and the data given is intended as a guide only.

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