Apr-2000

Development tools to improve plant design

New tools are available today to improve LNG process, plant safety and overall efficiency. At the same time, advancing computer technology is enabling engineering development to improve plant designs and reduce costs

Charles A Durr, David A Coyle, Don K Hill, Scott A Ray & Julio Rios
Kellogg Brown & Root (Now KBR Technology)

Article Summary

The constant drive to improve plant safety and efficiency while reducing capital cost has been helped in recent years by the addition of new engineering development tools, such as Pinch Technology – used to compare and contrast process technologies and to optimise designs – and Shipping Simulations, used to optimise the entire LNG chain and avoid over-investing in any segment.

Depressurisation Programs model the depressurisation of complex systems, permitting proper material selection (safe and economical) and optimum relief system design consistent with new codes and regulations.

Dynamic Simulation Programs simulate the startup, shutdown, and a variety of trip scenarios of complex equipment such as single shaft gas turbines. The need for this type of analysis increases with the larger, more powerful, and complex equipment and systems.

Reliability Analysis Programs and Equipment Failure Rate databases permit cost optimisation and aid in decision making during design and when comparing different processes.

Three Dimensional Graphic Systems permit interference analysis, constructability analysis, layout and plot plan opimisations, etc. thereby reducing costs and schedules.

Vapour Dispersion Models define exclusion zones requirements, which can be used for initial site selection screening, plot plan layout, etc. to ensure safe plant operation, while Computational Fluid Dynamics allow analysis of real fluid flow and heat transfer such as hot air (for example, gas turbine exhaust and the outlet of air fin coolers) recirculation.

The capital investment necessary to produce an LNG facility frequently amounts to several billion US dollars. Owners and operators of these facilities want to ensure, prior to construction of a plant, that latest industry standards for safety and efficiency can easily be met, while maintaining or reducing the necessary capital investment. Over the past decade we have seen the emergence of many useful engineering design tools. These modern computer-based programs allow the engineering and construction contractors, as well as the owners, to evaluate many scenarios in an attempt to ensure the safety and reliability of the plant.

Software-driven technologies may be applied to the design of an LNG facility to ensure a safe layout for construction and operation. Using the successes of the past, proven reliable and efficient equipment may be chosen, and constructed of the most cost-effective materials consistent with the latest codes and regulations. Through the evaluation of the startup, shutdown and various trip scenarios, via dynamic simulation, proper operational procedures can be developed. And in the event of an incident, an inherently safe design, as well as thoroughly analysed safety systems, will be in place.

Various software programs and developers are mentioned in this article. The following is not intended to be all-inclusive, but only a sampling of the available tools.

Pinch technology
During the late 1970s and early 1980s, when energy prices were soaring to new heights, a great interest in applying the second law of thermodynamics to energy intensive processes emerged. Exergy analysis, an application of the second law concept of energy availability, was applied to cryogenic processes for process scheme selection and improvement. It was recognised that minimisation of plant capital and operating cost was an inherent need in baseload LNG plants.

One of the earliest studies, published in 1980, highlighted two liquefaction process schemes that were compared; the single pressure mixed refrigerant process and the propane-precooled mixed refrigerant process. In this case, the exergy loss is higher for the single pressure mixed refrigerant.

The purpose of an exergy analysis is to determine the locations where the greatest irreversibilities occur. Moderate success was achieved but, when this type of analysis was attempted on a larger, more complex flowsheet, it was generally found to be tedious because of the traditional iterative approach. Today, exergy analysis is used primarily for discovering and eliminating loss of efficiencies, such as thermal, mixing, and momentum losses.

Pinch Technology enables the optimal balance of energy consumption and capital costs. It also helps to design the utility system that best fits the process. Actual utility consumption can then be compared to the minimum utility consumption, and process improvements can be identified. Working with an experienced group of application engineers using Pinch Technology, the process design engineer can more clearly track the global energy flows and interrelationship in the process, and modify the process to reduce the energy consumption.

In the late 1980s and early 1990s, pinch studies were applied to LNG processes and greater efficiencies were achieved. Significant improvements were made to the warm end of the processes by further cooling of the feed gas before it entered the precooler exchanger, and by additional compressor stages to minimise temperature approach at inter/after coolers. At the cold end, further efficiency improvements were obtained by tightening the temperature approach at the expense of additional exchanger surface area.

Pinch Technology is no longer used as a technique for energy savings alone. The technology is now considered as a major process synthesis and analysis tool and utilised very effectively on front end designs. The technology leads to effective plant designs where capital and operating cost tradeoff are well understood, and costly inefficiencies are avoided.