| Crack Down on Heat Exchanger Fouling | | | | When it comes to maintenance, refineries - |
| | | | like most of industry - tend to compete on |
| Written by Mike Watson, Technical and | | | | the same basis - a 21-day shutdown is decreed |
| Managing Director, T.T.I.L. | | | | because it's been done that way for maybe the |
| | | | last 20 years. The same cleaning methods are |
| Heat exchangers are the unsung heroes of many | | | | generally used slavishly, with high-pressure |
| industrial processes and as such they tend to | | | | water as the cleaning medium. |
| be taken for granted - nobody likes paying | | | | |
| for what is often seen to be unnecessary | | | | Most companies look at their heat exchangers |
| maintenance. Heat exchangers provide duty for | | | | in isolation and simply try to extend their |
| so long, that when they start to drop in | | | | run-time, instead of having them designed or |
| efficiency, it's usually a gradual process | | | | re-designed so they can be cleaned more |
| that goes largely unnoticed - until their | | | | regularly, but faster and better. BP's |
| performance has deteriorated sufficiently to | | | | Coryton refinery, for instance, managed to |
| be a problem. Then it really is a problem - | | | | reduce cleaning time on one shell-and-tube |
| and one requiring urgent attention. | | | | heat exchanger from three days to three hours |
| | | | by applying a different approach to cleaning |
| What aggravates the situation is the heat | | | | it. |
| exchanger that has never been cleaned | | | | |
| properly, coupled with the commercial need to | | | | If a plant is optimised for cleaning, almost |
| keep it on-line. When the decision is made to | | | | full production can be maintained throughout |
| carry out cleaning, often nobody knows what | | | | the cleaning process. Relatively minor |
| the performance of the exchanger is meant to | | | | mechanical changes, such as adding isolating |
| be, either because the drawings have been | | | | valves to heat exchangers, means that each |
| lost, or no record of any improvement was | | | | exchanger, or bank of exchangers, can be |
| made after the original cleaning. | | | | taken down and cleaned while the others |
| | | | remain on-line. A redesign of the exchanger |
| When the exchanger finally is opened up to | | | | so that a header can be removed, means it can |
| ascertain the extent of the fouling, it's not | | | | then be cleaned with a different system to |
| surprising to find it is so severe that | | | | the standard high-pressure water jetting, in |
| cleaning takes a lot longer than planned. Any | | | | a few hours instead of several days. |
| benefit that might have been gained by a | | | | |
| quick traditional clean is offset by the | | | | At Dow Corning's silicone plant in Barry, |
| extended cleaning duration and costs - and, | | | | south Wales, a tubular boiler and fire tube |
| of course, lost production. | | | | in the Energy Recovery Unit (ERU) required |
| | | | the removal of a 5mm layer of deposit in as |
| If that sounds like a nightmare scenario, | | | | short a time as possible to minimise lost |
| bear in mind that this is the sort of | | | | production. Another obstacle was that the |
| situation specialist cleaning companies | | | | unit, which carries waste gases, takes 48 |
| encounter every week. Cleaning is often | | | | hours to cool and prepare - even with the |
| carried out without any firm knowledge of how | | | | introduction of a chilled nitrogen purge - |
| much of an improvement the cleaning will give | | | | before personnel can enter to clean it |
| and how long its effects will last. Having to | | | | manually. |
| make 'finger in the wind' predictions clearly | | | | |
| is not a satisfactory way to plan | | | | The solution involved developing a bespoke |
| maintenance. | | | | remote de-scaler, which was inserted through |
| | | | a small 50cm man-way. Once inside, the |
| One of the most popular and widely-employed | | | | de-scaler expanded to fit the hot fire tube, |
| heat exchanger configurations in industry, is | | | | while reaching the full length of the carbon |
| the straight or hairpin shell-and-tube | | | | steel tube. With cooling time and man entry |
| exchanger. With hundreds or thousands of | | | | eliminated, the shutdown was reduced from |
| small-bore tubes bundled together, the extent | | | | five days to three and there was a noticeable |
| of quite modest scaling can involve major | | | | improvement in performance of the ERU when it |
| work to return the exchanger to anything near | | | | came back on line. |
| its commissioned performance. If the outside | | | | |
| of the bundle is heavily scaled as well, the | | | | Improved cleaning cycles also mean the rate |
| cleaning challenge rises by an order of | | | | of future fouling build-up is reduced, which |
| magnitude. | | | | in turn reduces the risk of tubes corroding |
| | | | as a result of the exchanger being open to |
| There is potential to bring about a | | | | the atmosphere longer for cleaning. |
| significant improvement in heat exchanger | | | | |
| accessibility and 'cleanability', by working | | | | Heat exchange surfaces therefore remain |
| more closely with the people who design heat | | | | smoother and provide better heat transfer. If |
| exchangers and fabricate industrial plants. | | | | and when the exchanger does foul up, it's |
| | | | easier to clean next time around, using |
| Better design would lead to improved cleaning | | | | whichever system is preferred. This would |
| - where improved means faster, cleaner and | | | | represent a change of practice to what has |
| safer, possibly in-situ or even on-line and | | | | been the norm since the 1980s, for instance, |
| with better waste containment. It would then | | | | when what was then Mobil in the UK was one of |
| be easier and quicker to clean exchangers | | | | the first refineries to decide that it would |
| back to bare metal to return them to duty and | | | | extend run-times by abandoning the annual |
| their design performance faster. | | | | clean and only clean every two years. |
| | | | |
| Plants are generally specified and ordered on | | | | Today, typical service intervals have become |
| the basis of throughput, not accessibility | | | | stretched to three and even four years in |
| and ease-of-cleaning. Suppliers are happy to | | | | some cases, but the apparent operational |
| comply with this and therefore tend to design | | | | savings are actually a false economy. |
| heat exchangers with 30-40% excess capacity | | | | Shareholders are indeed happy, because they |
| to ensure that they can continue to provide | | | | are getting longer run times, while competing |
| duty, even when quite extensively fouled. | | | | refineries have little choice but to play the |
| Heat exchangers the world over are currently | | | | same game or lose millions during more |
| designed and installed with a view to using | | | | frequent shutdowns. Four years down the line, |
| one of three systems for cleaning: chemical, | | | | however, the plant will have to come down for |
| pressure jetting and/or mechanical and this | | | | major cleaning and maintenance and it will |
| approach has remained unchanged for over 50 | | | | experience a far higher capital replacement |
| years. | | | | cost than ever before. |
| | | | |