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Posted by Boiler Guy on December 02, 2006 at 19:30:02:
In Reply to: Re: FD Fan Operation posted by Rick on December 01, 2006 at 11:49:59:
I believe that the steam flow transmitter has been introduced to provide a feed-forward to the draft. Increased steam flow is interpreted by the contoller as increased demand and will initiate a feed forward to provide the additional combustion air that will be needed. Not that I know this is what is being done at this specific site, just a control scheme I have heard of in the past.
: : I am currently a control room operator of 2 200,000 lb an hr, balanced draft, pulverized coal fired boilers.These boilers provide process steam for a facility that constantly has load swings(40 to 50k in a matter of minutes). FD fans were always controlled by the trim setting(usually 4%o2). This worked well for several years with the ability to maintain o2 consistently + or - 1/5% of setpoint. Controls were upgraded and now the FD fan control is dominated by steam flow(so much so that the o2 meter can be taken out of service without a noticeable difference in o2) The problem is the o2 now swings between 3 and 7% with spikes down(maybe once every few hours) to as low as 1%.I am interested if many other facilities operate these type boilers with this FD fan control setup.
: : Thank You
: I dont understand what you mean by the fan now being dominate by steam flow. Has something changed in the way the fan is controlled? It sounds to me like the new controls do not have a feedback signal from the oxygen transmitter. But more info is needed to comment properly.