Designing Cryogenic Pipe Supports
When designing cryogenic pipe supports for, typically, LNG pipe systems a number of factors need to be addressed such as insulation type and thickness, cold shoe type and clamping
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Factors influencing the Design of Cryogenic Pipe Supports
When designing or specifying cryogenic pipe supports for a cryogenic piping system, the following issues should be addressed:
- Behavior when the ultra-low temperature piping system is operated for the first time
- Operation of ultra-low temperature piping system in normal operation
- Characteristics of ultra-low temperature piping system and pipe support
- Very cold pipe support requirements
- Optimizing cryogenic pipe support
- Comparison with other types of cryogenic pipe supports
- Confirmation of ultra-low temperature pipe support
LNG has a very low boiling point, below -162C, so cryogenic pipe supports, must have very good insulation, durability, and stability in pipe supports such as shoes, stops, anchors, etc. The problems that occur with cryogenic piping systems are: Material embrittlement, icing around / between cryotube holders, Pipe insulation and steel structure, large displacement (due to heat) Expansion and contraction), rapid phase change due to large heat flow (Large Delta T) and the small latent heat of the liquid involved. Very high reliability is required when designing a cryogenic pipe support system.
As a rule, the support should be designed to meet all structural requirements and dynamic operating conditions that may expose the pipeline. Support system must be provided and controlled according to requirements pipe configuration, movement due to thermal expansion, pipe contraction and connecting equipment. In addition to it Accurate and economical choice of pipe support for cryogenic pipelines Systems usually show varying degrees of difficulty, some of which are relatively minor, and others are of more important nature. Excellent pipe support design and layout means that you can avoid having a lot of problems with pipe supports with proper care at the planning stage.
Features of Cryogenic Piping Systems
Heat continues to enter the pipe through insulation and supporting members. This heat causes the contents of the liquid to boil. Because of this, there should be a heat leak minimization. The thermal efficiency of the pipeline should be carefully considered given as it impairs heat input to the system which usually leads to product loss. So it must be necessary to fully understand the cryogenic piping system.
Materials used in ultra-low temperature piping systems require the consideration of materials that have the appropriate mechanical and physical characteristics. Process fluids, manufacturability, cost, and regulatory compliance of standards such as: As ASMEB 31.3. Certain materials have a tendency to be brittle with the possibility of failure that does not normally occur at low temperatures. Hence, it is necessary to use ferroalloys.
The most commonly encountered iron alloys in cryogenic piping applications are usually classified as ferritic or austenitic. Most austenitic alloy steels are used at low temperatures. The piping is often AISI 300 type chrome nickel stainless steel, or stainless 304, 304L, 316 and 316L. Of the 300 series stainless alloys, the composition of 304 is the most common.
Most pipe for cryogenic liquid services is insulated. The line is not insulated usally because of: (1) its use is very rare and quick. (2) That it is a temporary installation; or (3) cold loss is not important.
The types of insulation used for cryogenic piping include (1) foam such as polyurethane; (2) Powder insulation materials such as pearlite; or (3) Vacuum insulation piping. To keep the insulation system effective moisture proofing of the system must be implemented to prevent the ingress of moisture from the air freezing against the pipe and cryogenic lines. When this happens, the ice that forms deteriorates or destroys the insulation system.
The cryogenic liquid has the boiling point of oxygen (-297 or -0183) Oxygen from the air can condense and accumulate in the insulation space. In this situation, the insulation system should not be flammable because of the presence of solid oxygen.
If cold insulation is required, the entire system should be completely insulated. All plumbing components, tubes / hoses from the insulation equipment, drains, nozzles and supporting members as well as the protruding metal parts.
Insulation Guide of HD Foam and Steel
Insulation Guides (often referred to as Cold Saddles made from HD Foam and Alloy Steel
Insulation Guide of HD Foam and Composite Shell
Cold Saddle Insulation Guide with HD Foam and corrosion resistant Composite Shell
Flexibility Analysis of Cryogenic Piping Systems
Cryogenic Piping Stress Analysis
Cryogenic Piping Stress Analysis for determining the types of Cryogenic Pipe Supports required.
Pipeline flexibility analysis, aka Piping Stress Analysis, is an important design consideration. A big difference between ambient temperature and extremely low temperatures will incur heat shrinkage. In addition, this analysis of pipeline flexibility should be considered must occur prior to designing the cryogenic pipe support. If the amount of pipe movement exceeds capacity, additional expansion loops need to be designed to reduced movement.
This should be taken into account in the flexibility analysis of cryogenic piping. Temperature range and other high temperature conditions can occur during thawing, or cooling. And extremely low temperature piping should be designed accordingly.
The analysis method used is the same as the conventional analysis method for pipe systems. The only difference is that the plumbing is that the cryoservice members contract instead of swelling as in high temperature applications. But analysts can calculate the resulting contraction, so the analytical method will be the same as that used in traditional plumbing systems.
Safe design usually requires a flexibility analysis of the exposed cryogenic piping system (see ASME B31.3)
Requirements for Cryogenic Pipe Support Systems
Consideration of cryogenic temperatures when choosing the material for the cryogenic pipe support is crucial. In addition to heat loss and increase, problems occur when operating at low temperatures, atmospheric condensation should be considered and such lines are usually insulated with a material with an outer cover or seal called vapor barrier. This barrier prevents the insulation from absorbing moisture. Therefore, for this reason, loads are not allowed to penetrate the insulation. Most low temperature insulation has a low compression ratio and must provide shielding to protect the outside of the pipeline insulation. Such a shield should match the outer diameter of the insulator and cover 180deg of the arc.
For cryogenic piping systems, the pipe support should be on the outside of the insulation and it must be ductile at low temperatures to withstand stress as well as having a relatively low thermal conductivity. And the vapor barrier should not be disturbed in the bargain. Therefore, the cryogenic pipe support must be met by the following minimum requirements.
Pipe Supports are said to be lighter than wooden blocks.
Resistant to corrosion
Resistant to outdoor weather conditions
Physically strong against compression and bending and shear forces
The carrier should be suitable for economic mass production.
Low water absorption – conferring resistance to cracking from ice accumulation during operation.
Heat and Flame resistant
High density cradle support with molded high density foam bonded with a stainless-steel shield are ideal. The excess density foam layers will be staggered and, collectively with the metallic jacketing, should mimic the layering and design of the existing materials already surrounding the pipe. All Joints of the insulation will fit tightly together and be staggered with as few voids as feasible so one can keep away ice from forming.
Cold Shoe Support with Staggered Foam Layers
Cold Shoe with Metal Shielding
Cryogenic Pipe Support Components
180 Deg Cold Shoe Cradle
The cold shoe cradle is made of high density polyurethane foam for strength and conductivity
A Complete Cold Shoe
A Complete Cold Shoe with HD PUF Cradles, moisture barrier and steels clamps
As reviewed above, reliability is required in the area of cryogenic pipe support systems such as those servicing LNG. Conventionally, wood insulators had been used for piping in those plants. Moreover, those substances contain problems of availability and unstable quality. In addition, this material is often heavy and expensive, with transportion delays. Therefore, this type of wood block can’t meet the necessities cited above. So we must locate and promote a more suitable one. Urethane block manufactured from excessive density polyurethane foam which has low thermal conductivity is a more favorable cryogenic pipe support among numerous varieties of substances of similar qualities. They had been already used, and nicely obtained in numerous plants.
Also known as cold saddle pipe supports. These are cold shoes consisting of 180deg layers of urethane foam and a steel clamp or shell surrounding it. Cold shoe pipe supports of this variety include: Slides, Guides, Hangers, Trunnions etc. They are all effective at resisting the formation of ice.
Cradles are typically made of dense polyurethane foam which shall have a unique structure. And every insulated cryogenic pipe helps shall have installed a vapor barrier. Easy assembly and completing polyurethane cradles to the pipe is additionally beneficial. Pipe support strength will be primarily based on the following:
a. Polyurethane foam shall fulfill the flame necessities of UL94. The minimal percent of weight retention of the PUF when examined in accordance with ASTM D3014 will be 75%.
b. Average density of foam cradle will be proven through dividing the weight of the cradle by it`s volume. Average density will be inside five percent of the exact density, for 224kg/m3 and for 320kg/m3 foam cradles. Average density for 160kg/m3 will be inside -0% and +10%. c. Minimum cost for the closing compressive energy for samples taken from the center i.e., within the center 60% of the thickness for all densities will be inside 10% of the required values.
d. The thermal conductivity of the polyurethane foam at -160, according with ASTM C177, will be inside +/-five% of the standard values. Samples will be taken from the within the center 60% of thickness, where practical.
For the cradle, the finishing touch should be a protective coating. The shoe clamps and/or bearing plates should be hot dipped galvanized. The bearing plate will have its bearing bonded in the fabricator’s shop and the entire cryogenic pipe support is then attached to the pipe in the field. These procedures are suitable from pipe sized 1/2in to 72in.
The standard bearing plate should be made of normal carbon steel (ASTM A36 or equivalent). Strapping should be accomplished with steel bands of the pusher style seal. Slide plates for cryogenic supports are an excellent means to allow shoe movement in any lateral or axial direction.
Adhesives, Protective Coatings and Seals
The adhesive will be implemented to a thickness of 0.015inch (0.38mm) while Foster`s 81-eighty four is used. Sufficient adhesive will be used to fill any gaps or voids within the surfaces to be bound. The bond adhesive will be allowed to cure over a single day at room temperature. If the adhesive advocated through the foam producer is apart from the required one, the substituted adhesive and thickness have to be well examined previous to being used. All surfaces of the polyurethane which calls for adhesive bonding or shielding coating of seal shall offer the necessary anchor profile. Any smooth surfaces have to be eliminated previous to the addition of adhesive or shielding coating.
Slide Plate Pair
Slide Plate pair consisting of a stainless steel upper plate with a 2B finish and a PTFE lower plate