Mechanical Loading Arms
Loading arms are flexible piping systems used to transfer chemicals to railcars, trucks, tankers,
ships, and barges. If the loading arm is going to be used for transportation systems such as trucks or railcars, they will either be top-loading or bottom-loading. For ships and barges, marine loading arms (also known as mechanical loading arms – MLA) are used.
These loading arms have swivels joints, which is what enables the piping system to be flexible. The swivel joints typically have, a split flange design to make it easy to swap product seals without having to disassemble the entire swivel joint. The swivel joints are usually made of stainless steel, while the mechanical seals used are polymeric or elastomeric.
In many designs you will find a nitrogen drying system included. This consists of a cavity located between two the product seals into which nitrogen is injected. The purpose of the nitrogen is to prevent the product from being contaminated by moisture from the atmosphere.
For more information on Cryogenic seals check out the following posts from the Advanced EMC Technologies Blog:
- 6 Things You Should Know about Cryogenic Seals
- Five Energizers for PTFE Spring-Energized Seals
- Six Kinds of Polymer Jacket Materials for Spring-Energized Seals
Cryogenic Chemical Transfer
When natural gas is cooled to -162°C, it becomes a liquid and is much easier to transport and handle, except for the fact that this involves cryogenic temperatures. That means that cryogenic seals must be used in loading arms to transfer liquid natural gas (LNG), as well as other liquid media such as liquefied petroleum gas (LPG, which is usually at -48°C), liquid nitrogen (-196°C), liquid helium (-269°C), liquid hydrogen (-253°C), cryogenic ethylene, or liquid oxygen(-183°C).
The seal is part of the swivel joint. A cryogenic seal is a special type of mechanical seal used for media at temperatures below freezing. Care must be taken to address dimensional changes that can occur after installation. There are, of course, safety issues working with media at extremely low temperatures. In addition, lubrication becomes a major problem, which in turn can lead to issues with slip-stick. Lubrication or moisture on the faces of the seal can also freeze, and resulting in the seal shattering or the equipment locking up.
Most polymer cryogenic seals used in loading arms are made from a form of PTFE (Polytetrafluoroethylene), such as low-temperature PTFE, PCTFE, TFM; and have a metallic energizer. You might also see UHMW PE (Ultra-high-molecular-weight polyethylene) used as the sealing material. PTFE is often a first choice because it is compatible with a wide range of chemicals, has an extremely low coefficient of friction, and is thermally stable.
Cryogenic seals for loading arms are a key component in the loading arm system. Applicable polymeric such as energized PTFE, PCTFE, TFM and UHMW PE materials can be counted on for low friction, dimensional stability, and chemical compatibility.
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