Ash handling improved at a pinch21 February 2004
Fitting sleeved pinch valves to the ash handling systems is improving efficiency and reducing maintenance costs in two solid fuel fired power stations in the USA.
Dealing with ash is a major issue for solid fuel firing power stations, whether coal or waste burning types.The material is highly corrosive and valves can be prone to leakage and operational failure.
Fly ash, the lighter material originating high in the combustion chamber, and bottom ash, which is heavier and remains as a residue of the combusted material, are handled in different ways, but both present big problems for the valves in the system. Since a solid fuel power plant will typically have 20 to 40 valves in the ash train, this can be a significant issue. Two plants in the USA have been able to show significant gains by replacing existing valves of several types with pinch valves made by Larox Flowsys.
In the first plant, fly ash at temperatures as high as 200°C is handled by a positive pressure system sending the ash to hoppers. The plant burns several types of coal as well as biomedical and domestic waste. When low sulphur coal is used, the fly ash is a mixture of silica, calcium, alumina, magnesium, iron and other minerals, and is saleable for use in concrete mixtures for construction. When high sulphur coal or waste is being burned, the ash is not suitable for resale.
Formerly the plant used knife gate valves as switching valves to send the ash to the correct hopper, enabling saleable ash to be collected with a minimum of contamination. These valves were prone to packing leaks and jamming (‘freezing’), and needed regular replacement as a result of seat wear.
The replacement PVE pinch valves have a sleeved construction that closes tightly even if coarse particles remain in the closing device. The owners have found that since the ash only contacts the polymer sleeve inside the valve, leakage and jamming have been eliminated, reducing maintenance. Service life has also been increased, and as the only significant wear is to the valve sleeve, replacement is straightforward and economic.
To handle bottom ash, normally mixed with water to form a slurry that is pumped to bottom ash hoppers, the plant uses 200 mm valves sited immediately below the hoppers. The slurry, at temperatures between 60 and 120°C, is pumped to the valves at a pressure of 280 psig. The valves are cycled 3-4 times a day.
Again, there were problems with the plug valves used to control the slurry, namely leakage through the valve stem packing, and a tendency to freeze in position and not respond to commands from the plc system owing to accumulation of material behind the seats and plug cavity area. Replacement with Larox PVE valves has solved both problems.
In the second plant, bottom ash is handled by adding water from the main boiler and pumping the resultant slurry to settling ponds, or to hoppers for landfill. The solids concentration of the slurry is usually quite low, between 2-10%, but can surge to 50%. High performance butterfly valves were used, but leakage again occurred through the stem packing, and valve seats eroded in 4-6 months.
Four Larox PVE valves were installed initially, and the plant has now replaced almost 30 of the previous valves with PVEs. Leakage has been eliminated, and the sleeved construction ensures that the valve is 100% open or 100% closed with no freezing or jamming. Service life has been extended, and maintenance costs markedly reduced.