Design Points of Cryogenic Pipelines

Abstract: In petrochemical enterprises, cryogenic pipelines are frequently used. Generally speaking, pipelines conveying materials below +5°C are referred to as cryogenic pipelines. However, in practice, low-carbon steel can be normally used within the range of +5°C to -20°C, as the steel is mainly in a ductile state under these conditions. When the temperature drops below -20°C, carbon steel pipes gradually transition to a primarily brittle state, thus having certain limitations on their use. Therefore, pipes below or at -20°C are classified as cryogenic pipelines. This article mainly summarizes key points of cryogenic pipeline design through aspects such as piping design and pipe rack installation, and identifies issues that designers should pay attention to when designing pipelines under cryogenic conditions based on engineering practice.

Keywords: Cryogenic Pipeline, Piping Design, Cold Insulation

Classification Code: TB658
Document Code: A
Article Number: 1674-098X(2015)05(a)-0083-01

The design of cryogenic fluid pipelines in oil and gas field surface engineering and chemical process facilities has become increasingly common. For example, refrigeration units in auxiliary systems, cryogenic liquefied CO2, and cryogenic liquefied LNG. The design of cryogenic pipelines mainly considers two aspects: “cryogenic brittleness” and cold structure design, as well as a series of design requirements due to the need for cold insulation. “Cryogenic brittleness” requires designers to reasonably select steels with high “impact toughness,” while preventing brittle cracking and fracture from piping design and pipe system fabrication. Cold structure design and a series of design requirements due to the need for cold insulation are also very important, directly relating to energy consumption and construction, operation, and maintenance of equipment and pipelines.

1. Piping Design

Firstly, sufficient flexibility must be considered in the entire pipeline during the design of cryogenic pipelines, and full use of the natural compensation of the pipeline should be made. Expansion joints are used when the design temperature is very low and natural compensation cannot be achieved.

When arranging cryogenic pipelines, care should be taken to avoid vibration from pumps, compressors, and exhaust pipes, and it is necessary to prevent the entire pipeline from vibrating. If there are mechanical sources of vibration during the design of the pipeline, vibration reduction measures should be promptly implemented. In addition, elastic components, such as bellows expansion joints, should be installed near the source of vibration to isolate the vibration source.

When safety valves or blowdowns, vents are installed on cryogenic pipelines made of low-alloy steel or low-carbon steel, attention should be paid to whether the cryogenic liquid medium immediately vaporizes after discharge. If immediate vaporization occurs, significant heat absorption will occur, causing frost or even ice formation, which can lower the pipeline temperature to its lowest point. Therefore, when installing such branches, materials such as austenitic stainless steel should be used within the range prone to icing, using flange connections for different material branches.

Since the stress is the greatest at the bend of a cryogenic pipe, the bend is where brittle fracture is most likely to occur; therefore, hangers should not be welded here.

Generally, near bends or tees on cryogenic pipelines, flanges should not be directly welded to avoid damaging the cold insulation layer on the main pipeline when removing bolts. An additional length (short pipe) should be extended before welding the flange. The same requirement applies when welding instrument nipples on the main pipeline. For butt weld flanges, only ensure that one end of the flange leaves enough space for bolt removal.

When arranging pipelines, consider the movement of the pipeline and leave sufficient clearance between the pipeline and adjacent pipelines, equipment, and beams; sufficient clearance should be left for flanges and valves with thick cold insulation layers; the minimum installation dimensions for blowoffs and drains should be determined according to the thickness of the cold insulation layer.

When cold-insulated pipelines penetrate floors, note the distance between the pipeline and the beam, increasing the reserved hole size to avoid beams during piping; for vertical equipment with dense rising pipelines, specify the spacing for the installation of supports; appropriate spacing for cold-insulated pipelines should be given.

2. Installation of Pipe Racks

When designing supports for cold-insulated cryogenic pipelines, measures should be taken to prevent the formation of “cold bridges.”

(1) When horizontally laying cryogenic pipelines, wood blocks or hard cold insulating material blocks are generally placed under the pipeline to prevent loss of coldness.
(2) When vertically laying cryogenic pipelines, if supports are installed on cryogenic equipment, wood blocks or hard cold insulating material blocks should be placed both on the equipment and pipeline.
(3) The supporting weight of the rack should include the weight of the cold insulation structure of the pipeline.
(4) Due to the relatively lower strength of cryogenic pipeline materials, the spacing between supports for cryogenic pipelines should be smaller than that for general steel pipelines.
(5) When cold-insulated pipelines do not have supports, the elevation of the bottom of the pipeline depends on the thickness of the cold insulation on the support or sleeper. At locations where the diameter of the pipeline decreases, the thickness of the cold insulation also decreases. To maintain the elevation of the bottom of the pipeline, the thickness of the cold insulation block should not change.
(6) The thickness of the cold insulation block should be approximately 5 mm thicker than the cold insulation layer to prevent damage to the cold insulation layer due to contact between the pipe and the beam caused by pipe displacement.
(7) Cryogenic pipelines are prone to rusting, so sliding plates should be used on the sliding surfaces of the racks.
(8) Since the stress is the greatest at the bend of a cryogenic pipe, the bend is where brittle fracture is most likely to occur; thus, pipeline supports should not be welded at bends.

3. Installation of Process Valves

(1) When the fluid is a liquid, cryogenic valves should be installed with the stem upward, taking into account the longer length of the cryogenic valve stem.
(2) For cryogenic gate valves with pressure relief holes, the direction of the pressure relief hole should be marked on the valve cover, ensuring that the marking is not covered by the cold insulation layer.

4. Installation of Instrument Components

(1) Thermocouple bushings are generally flanged nipples with a length of about 4 times the thickness of the cold insulation layer. Thermocouple bushings should not be installed on vertical pipelines in cryogenic pipelines, considering that ice may form on the base when the cold insulation layer is eventually damaged.
(2) On cryogenic equipment, instrument readings often require warm columns. When designing, consider the space required for the installation of warm columns and the space needed for the cold insulation layer of the instrument.
(3) Orifice plate installation. The cold insulation structure of an orifice plate in a cryogenic pipeline is a block-type structure filled with granular material, requiring a large space. The threaded valves connected to the orifice plate flange must have enough clear distance from the outer side of the cold insulation layer of the orifice plate flange, at least 76 mm. Therefore, the minimum length of the threaded pipe should be the distance plus the thickness of the cold insulation layer of the orifice plate flange required to remove and operate the valve.
(4) Control valve installation. Control valves in cryogenic pipelines come with warm columns mounted on the bonnet, increasing the distance from the valve center to the top. As a result, the total height of the control valve assembly is higher.

5. Measures to Prevent Cold Shrinkage

(1) Attention should be paid to the properties of the piping material. Austenitic stainless steel used in cryogenic pipelines typically has a higher linear expansion coefficient than carbon steel. Therefore, the displacement of austenitic stainless steel is quite large. Due to the thin walls of many stainless steel pipes, their strength is lower, so consideration should be given to the setting of supports for these pipes.

(2) Several measures to reduce the forces acting on the pipeline due to contraction:
① Use self-expandable piping systems.
② Use expansion loops.
③ Use expansion joints.
④ Pay attention to displacements.

Before taking measures to absorb displacements, calculations and clearances should be performed at every point on the pipeline to prevent supports and beams from being pushed over.

6. Other Considerations

6.1 Selection of Cold Insulation Materials

Cold insulation materials should be selected to protect cryogenic fluids from the ambient environment temperature. Small defects within the cold insulation layer, such as cracks or fissures, can significantly impact the cryogenic fluid inside the pipeline. If cold insulation is inadequate, water vapor from the external environment can penetrate the cold insulation layer, causing condensation and subsequent icing. Continuous growth of ice particles will damage the cold insulation material, affecting its insulation performance. Therefore, materials with low moisture permeability should be considered for cold insulation, and another type of cold insulation material that is waterproof should be used on the outer surface.

6.2 Design of Operating Platforms and Passages

(1) Unlike high-temperature units, many items in cryogenic units require frequent inspections. Special attention should be paid to the leakage of liquids and damage to the cold insulation layer to address any issues promptly.
(2) In small independent structures, such as operating platforms and passages, temporary assemblies using threaded connections and welding are often special.

Conclusion

In summary, key considerations in the design of cryogenic pipelines include installation space, support setting, installation of instrument components, selection of cold insulation materials, attention to design parameters of cryogenic pipelines, requirements of cryogenic processes on pipelines, selection of cryogenic pipeline materials, requirements of cold insulation structures, requirements for installation, operation, and maintenance, as well as requirements for pipeline welding.