Thermo Structural Analysis of Cryogenic Fluid Circuit – Mechanical project
The Cryogenic piping circuit under discussion is designed to handle the Liquid Hydrogen and is one of the important piping networks present between the heat exchange and the ground delivery terminal of Cryogenic upper Stage in Geo Synchronous Launch Vehicle of Indian Space Research
Organization at Sriharikota. It consists of many flow components like vacuum jacketed control valves, check valves, instrumentation with associated piping elements like expansion joints/loops and various support elements (anchors, Guides, Vstops etc). This Paper mainly discusses about the thermal stresses
induced in the piping circuit when liquid Hydrogen flows through it and how these stresses can be reduced by incorporating various expansion loops/joints with optimal placement of supports. Towards this, a Process & Instrumentation diagram drawn in AutoCAD version and associated design features have been taken as an input data.
Considering the same, a smart P&ID has been generated using Autodesk P&ID software. By considering the given space constraints, a 3D piping layout is developed using Autodesk Plant 3D software. The total piping circuit has been segmented into 5 parts and corresponding nodal isometric drawings were generated. Subsequently the individual isometric segments have been modelled in piping analysis software – “AutoPIPE” and the corresponding loading & boundary conditions are imposed. The analyzed results were evaluated and necessary modifications in flexible elements, supports, component data and nodal distances are carried out in an iterative manner towards achieving the overall code compliance.
Cryogenics is defined as that branch of physics which deals with the production of very low temperatures . In a more operational way, it is also defined as the science and technology of temperatures below 123 K. As the critical temperature of a cryogenic fluid is less than ambient temperature, it cannot be liquefied by the application of pressure alone at or above ambient temperature. Helium, Hydrogen, Neon, Nitrogen, Fluorine, Argon, Oxygen, Methane and Krypton are some of the cryogenic fluids.
Cryogenic engineering deals with the low temperature techniques, processes, design & development of storage equipment and associated transfer circuits. Since the normal boiling point of cryogenic liquids is far less than ambient temperature, these liquids cannot be stored in uninsulated tanks unlike the other liquids. Because of the heat in-leak, this demands some special constructional aspects while design of storage & handling equipment. This includes
double wall construction (vacuum jacket) of the vessel/piping, insulation & well designed suspension system. In addition to the above, proper selection of metallic & non-metallic materials plays a major role in design of the cryogenic systems.
Flexibility is another important factor that needs tobe addressed in design of the cryogenic piping circuits. This is achieved by incorporating flexible metallic bellows or expansion loops, as the case may be. In case of complex fluid circuits involving number of branches, bends and associated flow components, estimation of thermal contractions, support reactions and piping stresses involves a greater analytical computations. By using the hand calculations it may not be possible to predict the above in an accurate & effective manner within the given time frame. In addition to the above, establishing the code compliance for the entire piping system is a complex task.
STORAGE AND TRANSFER OF CRYOGENICPROPELLENTS
After a cryogenic fluid has been liquefied and purified to the desired level, it may be required to store or transport as the case may be. The thermal performance of the storage vessels can range from ordinary to very high depending upon the type of insulation employed and the nature of suspension system. Accordingly the type of insulation will vary between normal foam insulation to MultiLayer Insulation (MLI) and suspension system can be of nonmetallic to metallic construction based on the desired thermal performance. There are six most commonly used insulations.
These are listed in order of increasing performance and generally in order of increasing cost. The specific insulation to be used for a particular application is determined through a compromise among cost, ease of application, weight, ruggedness and the required thermal performance of the insulation .
The various insulations used are:
a. Expanded foam insulations
b. Gas-filled powders and fibrous insulations
c. Vacuum insulation
d. Evacuated powder & fibrous insulations
e. Opacified powder insulations
f. Multilayer insulations
This project deals with the cryogenic pipelines which are of vacuum jacketed construction and provided with multilayer insulation. Multilayer insulations consist of alternative layers of a highly reflecting material such as aluminium foil, copper foil, or aluminized mylar and a low conductivity spacer, such as fiberglass mat or paper, glass fabric or nylon net. The reflecting layers may also be separated by crinkling or embossing the sheets so that they
touch only at a few discrete points and a spacer is not required.
For the cryogenic piping design austenitic steels with FCC crystal structure are selected. These constitute the largest stainless family in terms number of alloys and usage. The austenitic steels are non-magnetic and cannot be hardened by heat treatment. They possess excellent ductility, formability and toughness even at cryogenic temperatures. In addition, they can be substantially hardened by cold work. Nickel is the chief element used to stabilize austenite. Carbon and nitrogen are also used because they are readily soluble in the FCC structure. A wide range of corrosion resistance can be achieved by balancing the ferrite forming elements (such as chromium and molybdenum) and austenite-forming elements.
The material being used is austenitic stainless steel of 304L grade and its composition is Carbon <0.035%, Chromium 18- 20%, Nickel 8-12%, Manganese < 2%, Nitrogen < 0.1%, Iron 66-74% and small quantities of silicon, sulphur and phosphorous.
Factors for selection of material:
Characteristics to be considered in selecting the proper type of stainless steel for a specific application include:
a. Corrosion resistance.
b. Resistance to oxidation and sulfidation.
c. Strength and ductility at ambient and service temperatures.
d. Suitability for intended fabrication techniques.
e. Suitability for intended cleaning procedures.
f. Stability of properties in service.
h. Resistance to abrasion, erosion, galling, and sizing.
i. Surface finish.
j. Physical property characteristics.
k. Sharpness or retention of cutting edge.
Design of pipeline considering the following inputs:
a. Medium: Liquid Hydrogen
b. Flow rate : 2.2 Kg/s
c. Density : 70.2 Kg/m3
d. Design Pressure: 13.75 bar.
e. Limiting velocity: 7 m/s.
f. Material of construction: SS304 L
g. The allowable stress for the pipe material: 115 MPa.
h. Corrosion allowance : 0
Advantages of an Expansion Joint over a Pipe Loop
An expansion joint and a pipe loop are two methods employed to safely absorb thermal expansion or contraction in piping systems due to thermal temperature changes. During a design consideration, where an expansion joint or a pipe loop can be utilized, the major advantages of using an bellow/expansion joint are as follows:
1. Space is inadequate for a pipe loop with sufficient flexibility.
2. A minimum pressure drop throughout the pipe line is required and the absence of flow turbulence from the elbows and piping is required by process flow conditions.
3. The fluid is abrasive and flows at a very high velocity.
4. There is no adequate support structure to support the size, shape and weight of a pipe loop.
5. The pipe loop is impractical and in an application of low pressure or large diameter.
6. Construction schedule does not allow for the man-hours required to install the pipe loop and the piping loop support structure.
7. In most cases it is more economical to use an expansion joint instead of pipe loops.
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