Harmful piping systems are required to be designed to a code and obtain a complete stress analysis in order to gain statutory approval. Typical systems are those in chemical plants, power stations, oil and gas applications, on-board piping systems (FPSOs) and general utilities, such as water. In these instances, manual calculations become difficult, time consuming and expensive.

CAESAR II is the industry standard software for stress analysis of piping systems in the power, process and related industries. Other software, such as CADWorx Plant, can be used to create outputs such as Layout Drawings, Stress Isometrics and Fabrication Isometrics.

Hot and/or safety critical piping systems present a unique problem to the mechanical engineer or analyst. In addition to requiring strategically placed supports to carry their own weight, these systems often experience great thermal strain which must be absorbed by the piping, supports and attached equipment.

Analysis of the piping model in CAESAR II calculates these loads, displacements, and stresses. Our expertise and experience allows us to interpret the analysis results and check for a compliant system.

This can involve

• assessment of stress levels against design code allowable limits
• comparison of actual versus permitted loads on equipment such as pumps, heat exchangers, turbines, compressors
• checks of permissible levels of displacement to ensure that spatial envelopes are maintained or to ensure that pipes do not move off supports
• spring selections where variable or constant effort supports are required
• checks for excitation frequencies from a modal analysis
• other acceptance criteria such as load limitations on steelwork, limiting local stress on vessels (using WRC 107/297, PD 5500 Appendix G, or FEA techniques)

Loads can be assessed against standards such as API 610, API 617, API 661, API 560, NEMA SM23 and the HEI standard.

PV Elite software and 30 years of employee experience in pressure vessel design make an excellent combination to provide our customers with a first class service.

We are able to perform analysis and design on most horizontal and vertical tower configurations to check or ensure compliance with ASME VIII, PD 5500 and EN 13445 design codes.

Our calculations can include the following static loading components

• the effects of internal and external pressure
nozzle and flange calculations
• base ring design
• stiffening rings
• centre of gravity and weight data
• saddles and skirts,
• legs and lugs
• rigging analysis

Live load conditions such as wind and earthquake can be analysed. Unique to the software, PV Elite includes a true modal analysis and response spectrum technique. This results in savings on material and costs compared to a traditional static technique.

• 7 wind profile codes in addition to a user defined option
• 10 seismic design codes

Nozzles will be assessed against the respective requirements of the design codes in our standard analysis.

We are also able to offer a local stress analysis service (at structural discontinuities) to assess the effects of external loads from piping on nozzles, heads, and shells.

Local stress analysis can take the form of comparisons with established empirical methods such as WRC 107/297 or PD 5500 Annex G, or a more accurate finite element analysis (FEA).

This study involved the static analysis of boiler feed water and large diameter steam lines within a Sugar Refining Plant.

The piping systems are situated in Saudi Arabia and subject to static loadings of weight, pressure, temperature and applied displacements.

The piping systems need to comply with the allowable stress requirements of the ASME B31.3 piping design code and in addition, the restrictions imposed by suppliers on forces and moments applied at equipment connections.

Fern modelled the piping systems using CAESAR II software, and applied Operating, Sustained and Expansion Load Cases to ensure compliance with the design code.

On some systems, re-design was carried out in order to reduce thermal expansion loads on equipment nozzles.

The customer also required that certain nozzle connections were checked to ensure that 'local stresses' within the nozzle and surrounding vessel area did not exceed permissible limits. Fern used NozzlePRO to perform a Finite Element Analysis and assess results in accordance with the ASME VIII Division 2 design code.

The customer required a design service and, after a number of design iterations, Fern were able to supply a report showing a code compliant piping system. To achieve this, a number of design modifications were required.

A comprehensive report was produced constituting a summary, introduction, references, boundary conditions, assumptions, load cases, design methodology, results summary, conclusions and recommendations.

The supports locations, loads and high stress locations were depicted on a number of stress isometrics.

Trevor Endres from USC stated

Fern were asked to perform Static and Dynamic Stress analysis of steam piping associated with Power Generation on board an FPSO (Floating Production Storage and Offloading) platform to be moored in West Africa. The piping systems would be subject to various operating and environmental loads in addition to emergency conditions resulting from a steam turbine fault. The piping systems need to comply with the allowable stress requirements of the ASME B31.1 Power Piping design code and in addition, the restrictions imposed by the steam turbine and boiler manufacturers on forces and moments applied at equipment connections.

The system was modelled in CAESAR II Version 5.10 and supported with a combination of rigid type restraints, variable spring hangers, lateral restraints and guides.

The Static loadings applied catered for the effects of gravity, vessel motions ('g' loads), temperature changes, internal and external pressures and wind. 79 load cases were generated in order to ensure that all necessary combinations of wind and 'g' load directions were accommodated when combined with weight, pressure and temperature conditions. Normal(everyday) wind velocity loading was applied as a Sustained condition and 1000 year storm wind velocity loading was applied as an Occasional condition.

In order to provide fluid force loads resulting from the sudden arresting of high velocity steam (due to sudden valve closure), a transient fluid flow analysis was carried out using valve and steam velocity data.

This established:

• The pressure drop through the system (to derive downstream pressures) - steady state conditions
• The unbalanced forces on each system pipe leg. These are produced from a pressure wave resulting from the simultaneous closure of turbine stop valves for a given turbine - transient conditions

Unbalanced forces with respect to time were then plotted for a given location along each pipe leg, each plot being the graphical representation of a 'loading' file to be used for a Time History based Mechanical Dynamic Analysis of the system.

The unbalanced forces with respect to time produced from the fluid flow analysis were used as input to the piping Dynamics analysis and the system was subjected to the shock loading induced by the steam pressure wave for a time period of 2.5 seconds.

A comprehensive report was produced constituting an introduction and background, executive summary, references, boundary conditions, assumptions, load cases, design methodology, results summary, conclusions and recommendations.

The supports locations, loads and high stress locations were depicted on a number of stress isometrics.

Paul Charlton, Managing Director at PDL stated

"Fern proved to be an excellent partner with first class technical capabilities. They maintained regular communication during the contract, produced a comprehensive report, and most importantly, provided value for money".

Fern used CAESAR II to model the flare header and associated skid piping.

More than 40 static load cases were applied to cater for the various combinations of wind loading and 'G' forces in order to simulate pitch and yaw accelerations. Finally, the PSV closing times where converted to an equivalent response spectrum and a dynamic analysis performed to assess the effects of the sudden closure.

Stress isometrics depicted support locations, loads and high stress locations.

This was comprehensive, comprising:

• summary
• introduction
• references
• boundary conditions
• assumptions
• load cases
• design methodology
• results summary
• conclusions
• recommendations

To provide

• Full installation for all users
• Configuration of servers, networks and data stores
• Customisation of databases to deliver ease of use, productivity, consistency and compatibility

 

All this was structured to work within the CADWorx database schema, with CADWorx Internet Publisher and with the customers existing Document Management System.

The users benefited from full training on the product and its customisation.

Full testing was done by Fern before supplying and commissioning on site.

Final testing was undertaken by the customer prior to live integration into their plant operations.

Minor enhancements not previously considered have since been implemented but primarily the product and service delivered everything asked of it.

A very successful project.