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WHY IS IT NEEDED? As most of us would know, two of the things which frequently cause problems in steam boilers are corrosion and scale. If not properly controlled, these can greatly reduce the life of a boiler, cause poor steaming, or even cause sudden premature failure. Iron (and many common steels) will corrode if in contact with water and air, which is just what we have in a boiler. This corrosion can be prevented by excluding the water or the air, by adding chemicals, which will prevent corrosion, or a combination of both. Also, normal tap water contains dissolved materials, which are left behind as solids when the water is boiled off. On hot surfaces such as boiler tubes these materials can form hard scale, which will reduce heat transfer, and can cause overheating and tube failure. Melbourne has fairly pure water, but it is still necessary to take preventive measures. WHAT IS IT? Boiler treatment is the use of chemicals to prevent corrosion and scale formation as much as possible. Proper boiler management will greatly reduce the opportunities for corrosion and scale formation to occur, and will allow the treatment chemicals to act properly. The chemical treatment supplied by DVR is "Tanblend" liquid chemical treatment. Tanblend is a combination of caustic soda (sodium hydroxide) and tannins. Tanblend reduces corrosion by forming a protective film on the interior surfaces of the boiler, and prevents scale by keeping any solids in suspension, so they are removed during blowdown. It also stops oxygen corrosion by absorbing the oxygen, which is dissolved in the water. The suppliers of Tanblend recommend the alkalinity is kept between a pH of 10.8 and 12.0 (see footnote) and the concentration of tannins about 200 parts per million (approximately a weak tea colour). HOW DO WE APPLY IT? Industrial boiler systems frequently de-aerate the feed water, and add treatment chemicals to the feedwater by metering pumps, so that steady conditions are maintained in the boilers. They are usually partially blown down regularly to keep dissolved and suspended solids at acceptable levels. For full scale locomotives, treatment is usually added to the tender tank at regular intervals, and build up of dissolved and suspended matter is controlled by regular blowdown These methods are not practical at DVR, where the normal procedure is to fill the boiler with tap water before lighting up, and to completely empty the boiler at the end of the day's operations. This makes addition of treatment chemicals simple to manage, by adding a single dose at the start of the day. Because we run only for a fairly short time before emptying the boiler, buildup of excess dissolved solids, and the priming problems that this can cause, are not likely to be a concern. However with this method there are two problems we need to be aware of. One, we are adding dissolved air with the tap water, and two, we must be sure the empty boiler is protected from corrosion. To counter the dissolved oxygen we must add sufficient Tanblend while filling the boiler, and make sure that the amount of Tanblend remains sufficient during the day's running. (Effectively excluding air). If a boiler is not blown down during running, and the water supply is neutral, the amount of caustic soda, and hence the alkalinity, will remain about the same, as all that is being removed from the boiler is pure water in the form of steam. The amount of tannin will decrease as it reacts with oxygen and dissolved solids in the feedwater. To be sure the empty boiler is protected from corrosion, we must still have sufficient Tanblend at the end of running for it to be able to leave a protective layer in the boiler. We should also keep enough heat in the boiler after blowdown so that the last traces of water will dry out. It is also essential that all openings such as blowdown valves and filling openings are left open for air to circulate. (Exclusion of water). RECOMMENDED PROCEDURE Step 1 - Buy a medicine measure from a Pharmacy, about 65 cents. (No more talk of "capfuls" please). For a nominal 50 litre boiler, use 35 to 40 ml of Tanblend concentrate (SEE SAFETY PRECAUTIONS BELOW). For smaller boilers, reduce the amount proportionately. Add some water to the boiler (not to the stage of showing in the glass), then add the Tanblend, preferably mixed in 1 or 2 litres of water. Complete the boiler filling. The water in the glass should be a moderately dark tea colour. Although this is a higher concentration then required, I have found it is enough to last through the day, and this concentration has not caused any priming problems. During running the colour in the glass may fade because of condensation, but will return after a gauge glass test. The water should still show a weak tea colour at the end of the day. Adding Tanblend to the feed water is not recommended, as it can leave a residue, which may cause problems with our small injectors. There should be no need for any additions of Tanblend during a normal day's running. If you are running again soon and do not empty and refill the boiler, you may need to add some more Tanblend directly to the boiler before lighting up again. About ? to 1/2 the normal dose should be enough. As mentioned above, blowing down during the day should not be necessary, but it is a good idea to open the blowdown valve(s) for about 1 second every couple of hours to clear any sediment from the valve and pipe. When blowing down at the end of running, first open the blowdown valve(s) fully to clear any sediment, then partly close until all water is discharged. Then fully open the valve(s) to discharge the steam as quickly as possible, leaving the boiler hot. The final discharge should consist of dry steam, with no dribbles of water. (Some operators like to leave a few hot coals on the grate to assist drying the boiler). Individual locomotives may require some adjustment of the amount of Tanblend added. Until sure of the amount, it is a good idea to collect some water at the end of the day during blowdown and check both colour and alkalinity. SAFETY PRECAUTIONS Tanblend concentrate is strongly alkaline (caustic). Avoid direct contact with skin and especially avoid splashing in eyes. If splashed in eyes flush immediately with copious amounts of water and seek first aid at once. If on hands, wash thoroughly with soap and water. Rinse all containers and measures, which have been used for the concentrate. HAPPY STEAMING AND A LONG LIFE TO YOUR BOILER! Footnote:
Acknowledgment: Thanks to the many experienced steam men at DVR who have generously shared their knowledge with this very new operator. This article is an attempt to put it together for other members new to steam. John Pisani August 2002 |
