Engineering - Virtues
Optimum heat exchanger cleaning cycles As supplied for publication to World Refining, July-August, 2002 Heat exchanger equipment retains its important role in today's highly integrated, modern refinery. Low refinery economic margins mandate closer attention to thermal energy recovery systems that utilise the latest condition monitoring techniques to advise on optimal inspection cycles; taking into account constraining throughput and product-quality considerations. Agreements to reduce greenhouse gas emissions made at the Rio Earth Summit in 1992 and Kyoto in 1997 also provide an ever-increasing environmental incentive to encourage efficient use of energy. At the large European refineries, site-wide maintenance cost savings of over $1 million per year are realistic for a 25,000-t/day refinery, in addition to the savings made in refinery fuel consumption and throughput losses. Whilst many studies highlight the large potential benefits of fouling mitigation, a more recent study suggests that specific fouling of crude distillation units costs the US around $1300 million per year. It is therefore surprising to find that the level of monitoring and the availability of timely advice for cleaning of fouled crude pre-heat trains remain relatively low. The combination of improved online measurement and powerful computers with the ability to handle large data sets, provides the refinery engineer with the ability to quantify and predict heat exchanger fouling and offer advice on cost-optimal maintenance. Quantitatively justified planning of heat exchanger cleaning produces considerable savings, particularly for large, complex refineries where shutdown engineers may deal with 200 to 300 exchangers. This paper highlights the essential functionality required of software for providing effective and cost-optimal maintenance advice for typical refinery heat exchangers. Essential elements The maintenance and inspection of process plants are costly, as they are labor intensive and reduce production levels. However, failure to undertake them at the appropriate level leads to reduced operating efficiency and, ultimately, to unplanned stoppages caused by equipment outages. These two effects have to be balanced against each other, taking into account the associated economics and risks, in order to identify the optimum time to perform maintenance. The efficiency of heat exchangers is a key economic driver in many processes, particularly in refining. Typically, the important exchangers are subject to fouling over time, which reduces their ability to transfer heat. This affects product-quality and throughput targets. At some point in time an exchanger has to be cleaned. The issue is to identify the optimum clean-out time - too early results in unnecessary maintenance costs and loss of production, too late results in excessive throughput loss and quality costs. Accurate determination of and advice on optimum heat exchanger cleaning cycles requires consideration of the following features (their significance is explained later using typical data): access to basic heat-exchanger design information automatic data cleaning facilities to identify and remove data abnormalities automatic reconciliation of temperature and/or flow measurements to guarantee proper calculation of the heat balances visualisation of data cleaning and reconciliation effects sufficient historic process data to calculate overall heat transfer coefficient (OHTC) values automatic compensation for any significant flow effects an accurate, tuned model of OHTC deterioration with time, with predictive capability ability to incorporate economic costs mixed integer optimisation solver.