by Jackie Johnson Jackie Johnson No Comments

Guide to Cryogenic Seals for Marine Loading Arms

cryogenic seals for marine loading arms

A marine loading arm is a flexible, mechanical arm that assists loading or unloading ships. Typically, they transport petroleum and other chemicals between vessels and containers at the docks. 

Marine loading arms are an alternative to using direct hookups. Like direct connections, you must completely drain the loading arms before breaking off the links by using high-pressure air to blow out traces or stripping the line using a pump.

Due to what these loading arms carry, they can operate at cryogenic temperatures. Choosing the appropriate seals for this use is essential to ensure the safety of operators and machines alike. Let’s look further into cryogenic seals for marine loading arms.

Why Use Cryogenic Temperatures

Some liquids are too volatile to transport naturally. That is why they are cryogenically cooled into their liquid form. Cooling the air to cryogenic temperatures requires a process of compression, cooling, and expansion.

Moving cryogenic liquids instead of gas is safer and less likely to explode or cause a fire in the event of an accident. However, as these liquids are at sub-zero temperatures, you should use protective equipment when handling them.

There are many types of gasses transported using cryogenic temperatures. The most common use of marine loading arms to load onto ships include liquified petroleum gas, natural gas, liquid oxygen, liquid nitrogen, liquid hydrogen, and liquid helium. The table below shows the temperatures at which these gasses are transported.

Gas Temperature °C Temperature °F
Liquified Petroleum Gas -48°C -54.4°F
Natural Gas -162°C -259.6°F
Liquid Oxygen -182°C -295.6
Liquid Nitrogen -196°C -320.8°F
Liquid Hydrogen -253°C -423.4°F
Liquid Helium -269°C -452.2°F

Cryogenic Seal For Marine Loading Arms Design Consideration

The most common cryogenic loading arm seals are a polymer material with a metallic energizer. These materials include

  • PTFE
  • PCTFE
  • TFM
  • UHMW PE

PTFE is often the first choice because it is compatible with a wide range of chemicals, has an extremely low coefficient of friction, and is thermally stable. Another valuable material for cryogenic seals is Torlon® Polyamide-imide. Torlon PAI is rigid even at cryogenic temperatures. 

These materials have excellent chemical compatibility, low friction, dry-running, and good dimensional stability. Dimensional changes can be accounted for using a spring-energized seal or sizing the seal by accounting for the plastic’s coefficient of thermal expansion. 

Cryogenic seals made with PTFE and its variants offer a high strength-to-weight ratio, excellent durability, and self-lubricating properties.

What Cryogenic Seals Materials to Avoid

Traditional compression seals are not a viable choice for cryogenic use. Natural rubber, silicone, Buna-N, fluorocarbon, and ethylene-propylene can handle sub-zero temperatures. However, they cannot correctly seal at cryogenic temperatures. Temperatures below -32°C (-25.6°F) cause the rubber to become brittle.

If you use an inappropriate seal, it will eventually fail. Upon failure, the hazardous liquids flowing through the marine loading arm will escape and can be life-threatening. Some dangers include explosion, fire, asphyxiation, or frostbite.

In addition, there will be dimensional changes between when the seal is installed and when it experiences cryogenic operating conditions. You must ensure that the chosen polymer or elastomer doesn’t become brittle at the cryogenic temperatures involved.

Cryogenic Seal Maintenance Considerations

Periodically, cryogenic seals will require maintenance and replacement. Some things to ensure a longer seal life include understanding conditions, knowing what the seal can withstand, and knowing what to look for when it comes to wearing and lubricating.

Understand Conditions 

The conditions in which your marine loading arm works will affect the seals. Temperature, movement, and pressure will eventually cause the seal to wear out and increase leak rates.  If you know and understand the exact conditions where the seals will work, you can pick the suitable material for longer-lasting usage.

Knowing What the Seal Can Withstand

All seals have a limit to what they can withstand. Cryogenic seals can withstand temperatures from -269°C (452.2°F) to 148°C (300°F). They typically resist chemicals, natural gas, petroleum, and liquid nitrogen. They can also withstand high-pressure conditions.

Know What to Look for When It Comes to Wear

All seals wear out. Eventually, cryogenic seals are not excluded. Seals begin to wear on the seal face, causing a leak. You should inspect seals regularly for signs of distress, such as chips and grooves. If there are any indications of wear, then you should replace the seal immediately.

Lubricate

The cryogenic fluids themselves usually make for poor lubricators.  Any added lubricants or even moisture can freeze onto the face of the seal, causing the seal to shatter or, worse yet, the system to lock up and experience catastrophic damage.  However, not using lubrication can result in issues like slip-stick vibration.  

Lubricating cryogenic seals is virtually impossible. As a result, using unfilled polymer materials or a modified material may be the only option.

Best Cryogenic Seals for Marine Loading Arms

Choosing the best cryogenic seals for marine loading arms will depend on what you are transporting. Most cryogenic seals will work in marine loading arms, but some materials work better than others. The most common materials are PTFE, PCTFE, TFM, and UHMW PE.

Advanced EMC offers a wide array of cryogenic seals. If you are interested in purchasing cryogenic seals, contact us today!

FAQ

How do you seal liquid nitrogen?

Sealing liquid nitrogen requires either silicone or PTFE seals. If the seal comes into contact with the liquid nitrogen, PTFE seals are the better choice as this material can handle cold flow without causing creep.

What is the purpose of marine loading arms?

Marine loading arms load or unload vessels carrying petroleum products. They are made of several sections of pipes connected by quick-connect fittings and swivel joints. Cryogenic seals are used between the fittings and joints when transporting liquid nitrogen, liquid petroleum, or any other liquid stored at cryogenic temperatures.

Cryogenic Seals for Low Temperature Situations
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Meeting the Challenges of Cryogenic Seals

The term “cryogenic” typically refers to temperatures that are below freezing, extending to absolute zero (-460° F / -273° C). At these extreme temperatures, cryogenic seal design and specification becomes especially challenging–but far from impossible.

Where Cryogenic Seals Are Used

In general, cryogenic temperatures are often required for food preservation, transporting gases, cryosurgery, and cryoelectronics. Such low temperatures are needed in connection with some types of food storage, cooling superconductors, recycling, and storing blood and tissue samples. In fact, there are many different critical applications that require seals that can handle media stored at cryogenic temperatures, such as loading arms for transporting LNG, infrared telescopes, dilution refrigerator units, and sealing the liquid helium used formagnetic resonance imaging. 

Reliability

The first priority for a cryogenic seal is that it be reliable. While any seal failure is a problem, cryogenic seal failures can be especially dangerous. Aside from obvious issues such as environmental impact and toxicity, there can be an explosion that results from the rapid expansion of the cryogenic materials as well as potential asphyxiation and the effects of sudden exposure to extreme cold on employees and equipment.

Fluids Near Their Boiling Point

If fluid being stored at cryogenic temperatures is near its boiling point, then there is a strong possibility that it could flash into a gas if there is a sudden change in either pressure or temperature. This, in turn, would lead to a catastrophic (and possibly very dangerous) seal failure. 

Dimensional Stability

The installation temperature and service temperature of a cryogenic seal are going to be vastly different, which means that dimensional changes due to temperature are unavoidable. When designing or specifying a cryogenic seal, it is very important to account for these dimensional changes and adjust critical size and clearance parameters accordingly. In fact, it is vital to remember that the seal material be chosen for its properties at the expected operating temperature, not at room temperature.

Elasticity

Another challenge in sealing cryogenic fluids lies in the elasticity of the seal material itself. At extremely low temperatures, both elastomers and metals can reach a temperature where they transition from being an elastic material to a brittle one. This is another example of why cryogenic seals need to be selected based on their material properties — including stiffness and strength — at the cryogenic temperature of the application.

Lubrication

Lubrication is another major problem for cryogenic seals: at such extremely low temperatures, traditional lubricants simply will not function or will freeze and the fluids being sealed within cannot provide lubrication. If there is any moisture present in the lubrication, this can lead to freezing and a shattered seal. Cryogenic applications require seal materials that are dry running and/or self-lubricating, such as PTFE or TFM.

Meeting the Challenges

For many cryogenic sealing applications, spring-energized seals work extremely well. For seal jacket materials, polymers are an excellent choice and available options include PTFE, modified PTFE (also referred to as TFM), and UHMW PE. These materials are ….

  • Corrosion-resistant
  • Self-lubricating
  • Durable
  • Posses excellent thermal insulation properties
  • Retain elasticity and strength at low temperatures
  • Exhibit good wear properties
  • Are compatible with a wide variety of chemicals

Not that in some cases, especially those where flash boiling is an issue, polymer labyrinth seals may be a better choice than a spring-energized seals.

Conclusion

While there are a host of challenges involved in designing effective seals for cryogenic applications, there are effective solutions available. These include polymer spring-energized seals and labyrinth seals.