Over time, impurities build up inside the boiler. If they were left, they would cause problems, such as water carry-over, and boiler damage. We can remove some of these deposits by operating the blowdown valve, which we do at the end of every trip. After approximately fifteen days in steam, though, we have to give the boiler a washout.
In the days of steam, washouts were often done using hot water, but we don't have that luxury. Therefore, we have to give the boiler a chance to cool down slowly over about a week. It is then drained via the blowdown valve, and the washout plugs and mudhole doors removed. To gain access to the plugs in the front tubeplate, the spark arrestor screens have to be taken out.
We want to remove all the sludge that accumulates on the foundation ring, and all the scale that builds up on the boiler plate and tubes. For this, we need the water to be at a higher pressure than we can get from a hydrant. Therefore, we have modified an old Coventry Climax fire pump, using the tender tank as a reservoir. This gives us a substantial jet of water at approximately 150 psi. We have various nozzles, all made of copper so as not to damage the threads of the washout plugholes.
We start out by cleaning the foundation ring, until the water runs clear. We then go to the two plugs at the front of the boiler barrel, before moving on to those at the top and sides of the firebox. At each plughole, we use one or more of the copper attachments to direct water in as many directions as we can. For those on top of the firebox, this includes the insides of the thermic syphons. Next come the plugs on the backhead in the cab, followed by the four in the smokebox. Finally, we return to the two at the front of the boiler barrel.
Our next job is to have a look inside the waterspace, to check the condition of the visible stays and boiler plate. We sometimes use an endoscope for this, but the old-fashioned paraffin flare works best.
Then comes the job of "boxing up". The threads of each washout plug are cleaned, and checked for damage. We use graphite grease as a lubricant when fitting them. The mudhole doors use a PTFE-based gasket that is renewed every time. Fitting them is a tricky job - the doors must be in just the right place, and tightened just the right amount.
Then we can re-fill the boiler. We do this via the blowdown valve, adding water treatment in the process.
Because of the possibility of mudhole doors and washout plugs leaking after being re-fitted, we always have a steam test to prove their integrity. If a mudhole door does fail, we have to let the boiler cool, drain it, re-fit the door, re-fill the boiler, and have another steam test. We always allow time for this in our schedule.