Flexible hose or tubing and fittings

By: David Marlowe 

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Because of the numerous advantages seamless steel tubing possesses over other tubing&#;apart from the added cost&#;seamless steel tubing is widely used in hydraulic systems. Seamless steel tubing is easier to bend than steel pipe, so it does not require the number of fittings and can be reused. High-volume applications are probably one of the only limited areas to the use of tubing.

Tubing size is measured on the outside diameter of the tube. All sizes are available in a variety of wall thicknesses. The application and use of tubing, (material, diameter and maximum pressure) will determine the wall thickness of the tubing used. Sizes range from 1&#;8 to 2 in. in fractional sizes.
Considering the application, internal pressures, weight, environmental conditions and whether you are going to use flared or flareless tubing are all considerations you must know to help you in material selection. Good quality tubing must be selected to ensure a good seal.

Tubing comes in a variety of materials, including:

• Seamless carbon steel, which is the most commonly used. It has a tensile strength to 47,000 psi.
• Stainless steel is used when external conditions require noncorrosive materials. It also has a tensile strength to 47,000 psi.
• Plastic can be used in low pressure applications.
• When selecting a material for your specific application, don&#;t focus only on the price tag. The manufacturer&#;s information is the most reliable and should be used when selecting the material based on allowable stress and working pressures.

Fittings
Tubing does not use threaded fittings. The fitting attachment is accomplished by either a component or another tube. The attachment is made by the following:

• Flared fittings are used primarily in low to medium pressure applications.
• SAE standard J533B covers specifications for both single and double flared (37° and 45° flare) tubing. Double flared tubing is used with thin walled tubing.
• SAE 37° angle is the standard angle used for hydraulic system tubing.
• SAE 45° angle is used with soft copper tubing for applications such as refrigeration, fuel lines, etc.
• The maximum wall thickness recommended by SAE standard should be checked prior to flaring a tube.
• Flareless Fittings, also commonly known as compression fittings, are used primarily in medium- to high-pressure applications.
• As system pressures increase, you require a thicker wall thickness. Higher pressures also affect the ability to use a flared type fitting.
  • Pressure capacity can be calculated using the following formula:
    • Pressure (P) = 2 x S x T / D,

Where
P &#; pipe internal pressure in psi
T &#; wall thickness
D &#; O.D. tube
S &#; stress on tube material in psi
Flareless type tubing fittings are available in
two common styles.

• Ferrule or Sleeve Type: The ferrule is used to seal against the tapered seat of the fitting and the tube. When tightening the nut a sealing pressure is felt on sealing surfaces causing the tube to deform
• O-ring type: Sealing is obtained by the installation of an internally mounted O-ring. The O-ring type uses a split ferrule to clamp the tube.
  It is important to remember to never mix from different manufacturers and always refer to the manufacturers specifications (torque recommendations) prior to installation.
• Welding fittings are used primarily in high pressure, vibration and fixed applications.
  Butt and sockets welds are both used to connect the fitting to the tube, so material type wall thickness and welding skills must be considered when using a welding fitting.

Flexible hose

• As stated previously, pipe and tubing are rigid fluid conductors and therefore not suitable when there is movement of one end of the conductor while the opposite end is fixed.

• The benefits of flexible hose over rigid pipe and tubing is that flexible hose is simple to route, withstands vibration and thermal expansion or contraction.
• Flexible hose consists of the following:
  • Inner tube
  • Reinforcement
  • Outer protective cover

The inner tube is made of either oil resistant synthetic rubber or thermoplastic material. Reinforcement amount and type is determined by the hose&#;s pressure rating. As the requirement of reinforcement increases from a single fiber/wire braid to multiple fiber/wire braids, the hose loses its flexibility. Outer protective cover is manufactured to be oil and weather resistant. Primarily, the outer protective cover is made of a synthetic rubber or thermoplastic material. The Society of Automotive Engineers (SAE) has set standards for hydraulic hose specifications. Most manufactures of hydraulic hose meets or exceeds SAEJ513 requirements.

Final note: Because the hose meets standards it is extremely important to carefully match the end fittings to the hose selected

Safety, Reliability, Mechanical Integrity&#;important buzz words in the production of H2SO4 and any chemical processing plant. This editorial aims to investigate how we can proactively recognize areas of potential risk when incorporating flex hoses into modern plant operations. We&#;ll explore strategies to minimize these risks during the design and planning stages.

It All Begins with The Reliable Handling of Molten Sulfur

Sulfur is the key raw material in the industrial production of sulfuric acid using the contact process.  Solid sulfur is placed into a heated vessel where it&#;s brought to the melting point (115°C) and converted into a liquid state. The molten sulfur is then stored in a heated tank to keep it in liquid form before being pumped to the combustion furnace.  The temperature of the molten sulfur is usually maintained between 130°C and 150°C. The pressure required to inject molten sulfur into the combustion chamber typically ranges from 2 to 10 bar (approximately 29 to 145 psi), depending on the specific design of the injection system, the viscosity of the molten sulfur, and the desired atomization quality.

Maintaining Temperature During Transfer

Heat traced flex hoses are commonly designed into the system to maintain the temperature during pumping to ensure it remains in a fluid state and at the proper viscosity.  Full penetration welds are imperative to contain the high pressure required to achieve the proper flow rate. These flex hoses are typically fabricated from 316SS and are thoroughly pressure tested and incorporate 100% radiography to ensure the integrity of the welds. This carefully controlled programmatic approach helps ensure the safe and efficient transport of molten sulfur from storage to the injectors.

Inherent Risk

Flex hoses, like all critical process equipment, while essential in almost all chemical production sites, can pose certain risks if not properly designed, manufactured, installed, and maintained. Some of these considerations include:

Improper Design

Risk: If a flex hose is not properly designed from detailed specifications that are documented and signed off by the involved parties, reliability is likely to be compromised.  

Impact: Failure modes can include any number of physical breakdowns as well as improper temperature and related viscosity troubles that can readily shut down the process.

Material Compatibility Issues

Risk: Not all flex hoses are compatible with all concentrations of sulfuric acid. Using the wrong type of hose material can lead to rapid degradation.

Impact: This increases the likelihood of hose failure, posing a significant safety hazard in a sulfuric acid production environment.

Improper Installation

Risk: Incorrect installation, such as improper bending radius, inadequate support, or incorrect fitting selection, can lead to excessive stress on the hose.

Impact: This can cause premature wear, kinking, or failure of the hose, increasing the risk of leaks or bursts.

Mechanical Failure

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Risk: Flex hoses are subject to mechanical stresses, such as vibration, pressure fluctuations, and thermal expansion. If a hose is not properly rated for these conditions, it can rupture or fail.

Impact: A sudden failure could cause a spill or release of materials, which could result in serious injuries or damage to surrounding equipment.

Aging and Fatigue

Risk: Over time, flex hoses can degrade due to continuous exposure to high temperatures, pressure, or corrosive substances.

Impact: If hoses are not regularly inspected and replaced as needed, they can become a weak point in the system, leading to potential catastrophic failures.

Fire Hazard

Risk: In certain situations, if a flex hose fails and releases volatile substances, it can potentially contribute to fire hazards, especially if there are potential ignition sources and/or other combustible materials nearby.

Impact: A fire in any chemical plant could have devastating consequences, including the release of toxic fumes.

Risk Mitigation Strategy Begins with a Robust Hose Program

Documented Design Criteria: Proper design of flex hoses can be an iterative process.  Required operating parameters should be documented and submitted to the Application Engineer(s) for their input on materials, configuration, and end connections.  These are then returned to the customer for consideration and final approval.   

Pressure and Temperature Ratings: Ensure that the hoses used are designed and rated for the specific pressures and temperatures encountered in the sulfuric acid production process.

Material Selection: Ensure that the flex hoses are made from materials specifically designed to handle sulfuric acid and its associated gases.

Robust and Reliable Production: The production of heat traced hose assemblies for critical process applications requires the use of tried and true techniques that have been proven over time.  Full penetration welds that are challenged with 100% radiography for their integrity are non-negotiable.

Pressure Testing Requirements: Flex hose assemblies should be safely pressure tested prior to shipment from the manufacturer according to predetermined specifications.

Proper Installation: Follow manufacturer guidelines for installation, including proper bending radius, support, and fitting selection.

Regular Inspections and Maintenance: Predetermine and implement a routine inspection and maintenance schedule to check for signs of wear, corrosion, or mechanical damage. 

Routine Replacement Schedule: Hoses should be tagged for identification and replacement schedule. First and foremost, of course safety is paramount in any operation.  Reliability and mechanical integrity are aimed at achieving this goal.  A safe operating environment means the operation has been carefully planned and every detail thought through in advance.

Adhering to a rigorous design and selection process for all your flex hose applications is crucial for ensuring the reliability, mechanical integrity, and safety of your process, ultimately leading to uninterrupted operations and production.

Branham Industrial has developed their hose design and production process over the last 50 years.   For more information contact John Czerwinski   (502) 649-

Sulfuric Acid Today full issue: https://h2so4today.com/-fw-2

For more information, please visit Industrial Flexible Hose.