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