Almost every branch of modern industry, whether it is oil, gas, mining, or agriculture, needs hoses to pump various liquids. Depending on the type of product being pumped, specific hoses and fittings are selected to convey the material. Although almost any type of hose can easily convey a chemically non-aggressive liquids like water, pumping aggressive gaseous, media such as Ammonia and Acetylene, or cryogenic liquids with extremely low temperatures, requires a completely different solution. Cryogenic applications, in particular require specialized hoses; one of the most common types of cryogenic hose are metal hoses.
By Jurij Artiukhov, General Manager, PipeTime, LLC
Metal Cryogenic Hose Compositions
General Structure
Metal hoses are composed of a stainless steel corrugation that is braided with a protective mesh-type sheath made of stainless steel wire. This method of construction ensures the hose’s performance when it is under the influence of various loads or experiencing pressure pulsations that occur during transportation through the flexible element (corrugation) of the working medium. The braid must fit snugly around the hose to avoid ripples, and can be single-layer, two-layer, and three-layer, depending on the margin of safety (working pressure).
Before commissioning, cryogenic metal hoses are tested for tightness by mass spectrometric method using a helium leak detector. The working pressure depends on the number of layers of metal braid, as well as on the diameter of the hose.
Typically, metal hoses are manufactured with a diameter that ranges from 6mm to 300mm. As a rule, the larger the diameter of the hose, the lower the working pressure. For example, the working pressure on small diameters can reach up to 400 bar. The length of the cryogenic metal hose varies depending on the place of connection, but the maximum length, as a rule, reaches 10m.
Materials
Stainless steel was chosen as the material of construction due to its wear resistance, its strength, and high corrosion resistance. The use of stainless steel also allows metal hoses to have a working temperature range between -200˚C to -600˚C. Operation of this type of hoses is possible with the following working media: Nitrogen (N2), Ammonia, Argon (Ar), Acetylene (C2H2), Hydrogen (H2), Helium (He), Oxygen (O2), Propane, Welding gases, Carbon dioxide, cryogenic liquids (liquid nitrogen, liquid argon, liquid oxygen, liquid carbon dioxide, liquid helium), and pure gases.
If the media being conveyed must remain at a specific temperature, a hose equipped with a special screen-vacuum thermal insulation is typically used. The composition of this hose generally consists of several layers of special heat insulating materials. The system used to maintain the temperature follows the principle of ‘thermos’; a cryogenic product is pumped through the inner hose, while the air is pumped out of the cavity of the outer hose, to create a vacuum in which the heat-insulating materials are placed (the thinnest aluminum foil + glass paper). This design ensures that the cryogenic products maintain their low temperatures.
The use of this design simultaneously increases the productivity of the system, as product losses are minimized due to the almost complete reduction in heat gain. In these cases, the temperature of the working environment is often as low as -270˚C.
Transportation and Connections
If the distance between two connection points is short, then the transported product dose not remain in the line for a long period of time and the transported product absorbs little heat.
As only a small part of the substance evaporates during the flow, the rest of the product retains its original thermodynamic state. Thus, screen-vacuum insulation of cryogenic pipelines is an innovative solution in the field of cryogenic transmission lines and provides significant savings.
To connect cryogenic metal hoses to each other, or to connect them to external equipment, flange connections and fittings that meet the European Industrial Gases Association (EIGA) or Asia Industrial Gases Association (AIGA) standards can be used.
EIGA connections are typically used as end fittings for cryogenic metal hoses that range in sizes form Du-40 – Du-50 mm. These have a nipple that can transition from the standard diameter Du- 60 mm to a diameter of 50 mm and are welded to the end of the metal hose. It is possible to connect EIGA nut nipples to the metal hose using NPT-1.5″ or 2″ taper pipe thread. The counterpart of the EIGA nut is a fitting that is attached to the cryogenic plant. The connection is sealed by squeezing a fluoroplastic ring when the union nut is tightened.
The union nut and mating fitting have special ‘key’ structural elements that do not allow accidental confusion of the pumped cryogenic liquids.
AIGA connections are designed for hermetic connection of flexible metal hoses, with a conditional passage of 40, 65 mm, to equipment that produces, stores, and uses liquid nitrogen. This type of connection consists of a nipple connected to a metal hose by welding or NPT thread, a union brass nut on the nipple, and a threaded fitting attached to the equipment. Sealing occurs when the brass nut is tightened and the nipple tooth cuts into the copper gasket in the union groove.
Although every country has its own connections designed for pumping cryogenic liquids – in Russia it is the ROT connection- most hoses connections can be completed using adapters from one standard size to another, or mating flange connections.
Additional Components
Cryogenic metal hoses can be equipped with the following additional elements:
- Compensators used in the case of connecting two sections that are at unequal levels or to dampen the vibration of operating equipment,
- Supports and connecting elements;
- Protective membrane devices;
- Sections of cryogenic equipment (pipelines)
Rubber and Composite Hose Compositions
Rubber Hoses
While it is more common to use metal, rubber hoses can also be used for pumping certain cryogenic liquids, such as carbon dioxide (CO2). Rubber hoses used in cryogenic applications are made of a special CO2 compatible elastomer and are reinforced inside with stainless steel braid.
The available hose diameters range from 1″ to 2″ with a maximum working pressure of up to 30 bar; working temperature ranges from -54˚C to +100˚C. Carbon dioxide rubber hoses are equipped with fittings with increased corrosion resistance.
Composite Hoses
One of the more modern solutions for pumping cryogenic liquids is the use of composite hoses, which has a number of advantages over metal and rubber hoses. Cryogenic composite hoses are immune to icing and have been specially designed to withstand temperatures down to -200˚C and pressures up to 25 bars. They have been developed based on multi-layer composition that consists of: polyamide fabrics and films, and polyester films reinforced with internal and external stainless steel wire spirals. These hoses are manufactured in accordance with EN 13766 – for liquefied gas- and EN 2010 standards – for LNG.
One of the main advantages of composite hoses is the lightness of the linear hose meter. For example, a hose with a diameter of 2” weighs roughly 2.5 kg. The bending radius is another advantage as it can reach up to 180 mm and maintain this flexibility at -200˚C.
The hoses are also suitable for a number of fully refrigerated liquids. These include: LPG, propane and butane, down to -105°C, liquid ethane at temperature and liquid ethylene, and LNG, liquid methane and liquid nitrogen service at temperature -200°C. The main consumers of composite hoses are oil and gas companies, petrochemical plants, bunkering companies.
Cryogenic Couplings
When pumping cryogenic liquids, leakage or spillage is unacceptable. Breakaway couplings and dry-break couplings are therefore used when conveying liquefied gas or LNG, to ensure that the media is not lost.
Breakaway Coupling
A breakaway coupling consists of two flange connections that have valves installed within them. When the flanges are closed, the valves are open and flow of the media is completely free; when the flange connections are opened, the valves instantly seal the opening. The use of breakaway couplings therefore mitigates the risk of leaks if the maximum load occurs along the axis and the bolts break off.
Parameters of the breakaway coupling for cryogenic liquids:
- Production material – stainless steel
- Operating pressure up to 25 bar
- Connection: Female NPT thread, EN1092 (DIN), ANSI flanges
- Operating temperature up to -200˚C
- Diameters: 1″ to 6″
- Material of sealing elements: PTFE
Dry Quick Couplings
Dry quick couplings, while very similar to breakaway couplings, are used to connect cryogenic hoses to various devices and containers. This type of connection is primarily used for: vapor recovery lines, container discharge, fuel bunkering, loading/unloading of tank trucks, rail tankers, bunkering, and tank vessels.
Final Thoughts
As cryogenic hoses are an important element in the transport of hazardous cryogenic liquids, the selection of material and fittings should be based on their ability to withstand low temperatures, as well as the chemical aggressiveness of the pumped medium. It is therefore important to understand which types of hoses are most suitable to convey the various medias. To ensure that none of the media is lost during the transfer process, leak tests should also be conducted prior to commissioning.
About the Author
Jurij Artiukhov works as the General Manager at PipeTime, LLC. He previously worked as a Sales Manager for MTT Company and Project Manager for SOL SP Company. He holds a graduate degree from the University of Foreign Affairs, Economy and Law.