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Jan-2002

Revamping conceptual process design

With the desirability of getting the maximum use out of existing equipment when revamps are planned a rigorous approach to CPD is required to avoid scope growth

Tony Barletta and Gary Martin
Process Consulting Services

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Article Summary

Conceptual process design (CPD) largely determines crude unit revamp costs. This stage of engineering should identify all significant changes influencing total installed cost. Otherwise large scope growth can occur as more engineering work is done (Figure 1). The conceptual designer’s goal is to maximise the use of the existing equipment, which will minimise investment costs. First and foremost is the need for a thorough test run that gathers the necessary field data to allow an experienced revamp engineer to select a reliable minimum-cost flow scheme.

Finding the process flow scheme that circumvents major unit bottlenecks without compromising operability or reliability is the key to minimising costs. Rote solutions that simply make the existing equipment larger, or parallel undersized equipment, almost always result in unnecessary capital expenditure.

If done properly, CPD will avoid scope growth that inevitably results from office-based software solutions done solely in the engineering, procurement and construction (EP&C) company’s office. Comprehensive field-measured data are needed to determine real equipment operation, as opposed to presumed or office-based calculations of performance.

Scope growth occurs because high cost changes are not identified until late in front-end engineering design (FEED) or detailed engineering, because only a cursory amount of engineering was performed in the conceptual design stage.  A properly executed test run permits a higher level of detail engineering to be performed in the conceptual design stage. Hence, more unit modifications (scope) are identified during the conceptual design stage.

Often, when scope growth increases revamp costs, project management activities include scope rationalisation. This generally means that whole systems or major pieces of equipment must be eliminated to reduce costs. Once the process flow scheme is set, rarely can major equipment be eliminated without operability or reliability consequences.

With the trend towards seven- to eight-year turnaround intervals, lost profits due to poor reliability or unstable operation result. Poor reliability and operability have increased the frequency of unscheduled shutdowns to correct revamp design errors. With a more thorough approach to CPD, scope growth can be minimised without compromising unit reliability or operability.

Scope growth
Revamp scope growth is common as engineering proceeds from CPD through detailed engineering. It occurs because many companies approach the conceptual design (or feasibility level) work by focusing solely on office-based computer modelling, calculations, and equipment specifications sheets. This approach nearly always fails to determine actual equipment performance and does not provide sufficient information to identify all the changes that contribute to revamp costs.

It is not uncommon to incur scope growth of 30–70 per cent as engineering proceeds. Selecting the wrong process flow scheme because insufficient engineering had been done to identify real costs, and then reducing scope has been the root cause of several crude unit revamp failures. For example, a common scope growth area is unforeseen crude charge, pumparound, and product pump/piping system modifications. Often, hydraulics are not evaluated in sufficient detail until FEED or detailed engineering.

In several instances, it was stated by the EP&C company that these details would be looked at “in the next stage of engineering”. During detailed engineering the EP&C company determined that crude charge system hydraulics and other major equipment modifications were needed to meet the future operating pressures.

CPD work activities need to provide all the necessary information for an experienced revamp engineer to avoid major scope growth in subsequent engineering. Small changes can be dealt with by contingency, but large changes ultimately result in scope growth or scope rationalisation. Unfortunately, scope rationalisation is often done quickly without reviewing the reliability and operability consequences. A more comprehensive CPD approach that focuses on the activities required to understand current performance and how to circumvent major bottlenecks is needed to avoid scope growth.

Competing objectives
Revamps need to meet processing objectives, control capital investment, and produce a reliable and operable design. An experienced conceptual designer balances these competing objectives. The perfect design that is never built due to high capital cost will not increase the refiners’ profitability. While a low cost design that is not reliable or is difficult to operate can cause an unscheduled outage that results in millions of dollars of profit loss.
CPD needs to include the following steps:
— A comprehensive test run
— Defining and quantifying all major cost bottlenecks
— Evaluating process flow scheme alternatives to select least-cost flow scheme
— Equipmenting lists/cost estimate.

Traditionally, project feasibility studies start with a battery of computer simulations performed in an EP&C company’s office with little data except that available from the equipment specification sheets, process information system, routine laboratory analysis, and/or crude assays.

This approach rarely will find the real unit limits because there is insufficient information to accurately determine current performance. Without knowing the actual process and equipment performance, it is impossible to sensibly direct revamp capital investment at the conceptual stage.


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