The aim of piping stress analysis is to ensure safety against failure that could result from loading conditions both external and internal – that are expected to occur in the lifespan of a piping system. Verifying the structural integrity against various load conditions however needs to be undertaken with economic considerations and there are several factors that impact different types of piping systems – making the stress analysis more complex. Piping Systems are mainly classified into three main categories and some sub-categories:
- Hot Systems
- Cold Systems
- Cryogenic Systems
- Underground Systems
The main reason for this type of distinction is that hot lines and cryogenic lines have to undergo ‘Flexibility analysis’ to regulate thermal forces, stresses and displacements. Cryogenic Systems and plants use multiple cryogen transfer lines of many sizes and complexities to transfer fluids from plants to various applications. These transfer lines comprise many critical sections like tee section, elbows, other components etc. and they need to be designed for multiple constraints like line layout, support locations and space constraints. Moreover, the cryogenic lines are subjected many types of single and multiple load combinations like seismic load, sustained loads, occasional loads, operational loads etc.
Thus, the flexibility requirement analysis of Cryogenic systems is somewhat complicated and there are many challenges. Here’s taking a look at some of the Challenges in Piping Design that Engineers face when working with cryogenic system:
1) Cost Of Project: Expansion joints such as bellows enable carrying the pressure thrust and provide ample flexibility. But often clients are reluctant to include bellows in the design for flexibility because they are expensive and raise the overall cost of the project. However, the use of bellows is sometimes imperative and clients and project managers need to come to an agreement.
2) Calculating Thrust Load: Cryogenic systems use vacuum jacketed lines that need special consideration. Jacketed piping is used to convey viscous process fluids in cryogenic lines. However, you can’t use expansion bellows for vacuum jacketed lines and instead need to include welded field joints. Typically, vacuum jacketed systems use an inner and outer pipe. The inner pipe carries the cryogenic liquid with multiple layers of high insulation. Jacketed piping systems require special stress analysis and you will need to check for sustained and expansion stress check, including checking of buckling load. This has to be a manual calculation and you can’t rely on software alone to provide you the accurate thrust loads.
Field joints are required to create vacuum sleeves, to maintain the strength of the welded joints otherwise they might break. Calculating the interference between core and jacketed lines is important so that they do not lash.
3) Balance Between Flexibility And Efficiency: Cryogenic lines require innovative and intelligent design to maintain the balance between process efficiency and flexibility. Piping design engineer’s input and experience plays a crucial role in calculating the various strengths, loads and capacity of such a system.
4) Selecting The Appropriate Codes And Standards: The piping code for Cryogenic lines is B31.3 as it is included in the Process piping code requirements. However, there are special considerations with cryogenic lines for material selection, ductility, fabrication and testing. B31.3 can’t be used blindly and engineers consult with clients and take into consideration the scope, geography, type of plant to choose the applicability. Cryogenic piping “contracts” between anchor points when at operating temperature. This must be considered carefully during Piping Stress Analysis.
Cryogenic piping lines are a special case where the operating temperature and installed ambient temperatures are so different. There should be a degree of rigor that is relevant for the severity of operations and potential hazards and therefore accurate flexibility analysis is very important.
Our team at Rishabh Engineering is well-versed in Design and Stress Analysis of Cryogenic Piping Systems. Find out more about our recent projects, processes and expertise. Contact Us Now! or Call 1-877-RISHABH (1-877-747-4224).