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