As with many valves on a boiler, the feedwater valves experience start-up conditions that differ significantly from the normal running conditions. Not only are flowrates lower, but the pressure drop is usually much greater. This results in cavitation and flashing damage close to the seat, as well as poor control.
One solution used in existing plants is to install a sacrificial valve in parallel to the control valve just to be used during start up.
Mitech has developed a control valve with a special trim to handle both conditions. At the bottom of the retainer are a series of restrictions that eliminate cavitation by gradually dropping the pressure. This is the ZZ design. As the plug opens, the flow then passes through passageways with a smaller number of restrictions. In the top half of the retainer are single hole-restrictions that can handle the high flow low-pressure drop condition that the valve experiences in normal running mode. The new trim makes turndown ratios as high as 200:1 a possibility. The idea was originally designed for use on attemporator spraywater valves on an Eskom boiler. Ten of these have been in use now for over twelve months at Hendrina.
Mitech has just supplied three high-pressure valves to a pulp mill near Durban. The valves were 50, 80 and 100 mm in size and rated to ANSI 2500#.
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