All of the GWR's working steam locomotives undergo a routine boiler washout after a prescribed number of days in steam. SR Merchant Navy pacific no. 35005 Canadian Pacific, on loan from the Mid-Hants Railway for the 2006 season, is no exception and its boiler was washed out during October 2006.

This is a procedure typically carried out after 21 days in steam and to a strict procedure laid down in a document called HS(G)29, published by the ORR - or Office of Rail Regulation (which took over from the Health & Safety Executive's HM Railway Inspectorate in April this year). This comprehensive, 40-page document details the practices for maintenance and safe operation of steam locomotive boilers.

You can see a copy by visiting the Office of Rail Regulation website: http://www.rail-reg.gov.uk/upload/pdf/RSP6-locoblrs.pdf. This PDF document includes diagrams of boilers and photographs of the main fittings on a locomotive footplate. Note that GWSR plc accepts no responsibility for the content or availability of external sites.

In simple terms, the procedure involves the aptly named washout plugs and mudhole doors being systematically removed and marked to ensure they are returned to the same positions from which they came. Water at high pressure (over 70 psi) is then methodically hosed through the washout plugs to thoroughly rinse out the inside of the boiler. This cleans sediment from the inside surfaces - the boiler barrel, tubes, stays, thermic siphons, tube plates, the water space between inner and outer firebox and the foundation ring.

While the washout is being undertaken the washout plugs, fusible plugs and mudhole doors are cleaned and inspected and if there is any doubt about any of these components they are replaced.

The fusible plugs, which are a vital safety feature, are replaced at predetermined intervals in any case. Screwed into the firebox crown (depending on the type of boiler there are between two and six of them), they have a bore filled with lead. If the boiler water level falls so low that they are uncovered, the lead melts. This admits steam and water into the firebox, dousing the fire and relieving boiler pressure and thus reducing the risk of expensive damage to the firebox crown. Other boiler fittings are checked and cleaned as well, including the water gauge frames, gauge glasses and their rubber seals.

The lowest mudholes are immediately above the foundation ring. The doors comprise an oval flanged plate which fits inside a similarly dimensioned oval opening. At every washout, a new gasket is fitted to the each door.

Fresh water is used for the washout, pumped from the milk tank normally parked in the siding adjacent to the car park at Toddington station. The oil tanker next to it contains treated water which is used to refill the boiler after the washout is complete. The water treatment chemicals prevent solids in the water supply - such as limescale - from adhering to and damaging the inside surfaces of the boiler. You only have to look inside your domestic kettle to see how these solids form as water is boiled! The chemicals also help to prevent corrosion of the steel and copper surfaces inside the boiler.

After the washout, the boiler is steam-tested and carefully examined for signs of water or steam leaks before the locomotive is passed back into traffic.

We thought you might be interested to see the process in the following photographs.