Control Scheme for 300,000 Tons of Synthesis Ammonia Plant
I.Overview
Advanced DCS distributed control system is usedfor the instrument system with an annual output of 300,000 tons of synthesisammonia, and the energy-efficient complex control system of water / carbonratio, hydrogen / nitrogen ratio, boiler drum three impulses, compressoranti-surge systems and other sets of large-scale synthesis ammonia plants are realizedwith its powerful feedback control function, logic sequence control functionand various computing functions. In combination with the rich experience inlarge-scale synthesis ammonia production plant, safe and reliable interlockalarm intelligent logical sequence control strategy is realized. Key controlschemes are as follows:
Water/ Carbon Ratio Control System:
The system is two-way cascade control system tocarry out control by taking load FL0215 setting as master controller, raw gasquantity FI0001 and steam quantity FI0002 as auxiliary adjustment and water-carbonratio FL0209 as object, with logical increase / decrease functions.
Main Features:
1. According to the results of total carbon (FL0201)analysis, to automatically adjust steam quantity so as to ensure thecarbon-water ratio to be controlled.
2. When the system increases or decreases load,to ensure that the steam is to first increase steam and then natural gas; tofirst decrease natural gas and then steam, so as to prevent catalysts frombeing charred.
3. Two-factor control,to prevent misuse.
Hydrogen-nitrogenRatio Control System:
The system takes synthesis gas hydrogen-nitrogenratio AIC411at the inlet of synthesis tower as primary adjustment,fresh air hydrogen- nitrogen ratio AIC410 into synthesis system as the first auxiliaryadjustment, air capacity FI0003 added to secondary furnace103-D as the second auxiliary adjustment, and two-level cascade system from feed-forwardsignal FX0204 of water- carbon ratio system is introduced.
Main Features:
1. To overcome large delay characteristics ofthe process.
2. To introduce feed-forward signal to ensureload changes and make air capacity increase slowly and decrease quickly, so asto ensure the excess temperature of secondary furnace.
3. Two-factor control, to prevent misuse.
Outlet Temperature Control System of Preliminary Furnace:
The adjustment system is a cascade controlsystem by taking outlet temperature TIC117 of preliminary furnace asprimary adjustment and fuel gas FIC063 as auxiliary adjustment. In order toovercome lagging, total carbon flow signal of fuel gas is introduced, fuel gaspressure PIC029 is introduced for selection control, so as to prevent lowpressure blow-off.
Three-impulseSteam Drum Level Control:
The control system is three-impulse adjustmentsystem by taking liquid level as primary adjustment and steam production andwater supply as reference in order to overcome the impact of false level. LIC01is the main adjustor, BFWC01 is auxiliary adjustor (its measured value is to subtractsteam quantity FI0033 from water supply), and switch SW0006 determines whetherthe system uses three-impulse adjustment or single-impulse adjustment.
101-JCompressor Anti-surge Control System:
Technology staff provides an anti-surge formulato the system according to the theory and actual situation, the anti-surgeamount is calculated with 101-J outlet pressure PI0048 according to formula,and linear processing is carried out for this amount, so as to make emptyingamount FIC004 not lower than a safe value at any time, and therefore the systemis moving anti-surge control system with variable limits.
The system is a cascade control system bytaking high-pressure steam pressure PIC018 as primaryadjustment, fuel gas amount FIC301 and combustion air amount FIC302 FIC301 asauxiliary adjustment, and it analyzes AIC302 to control air / fuel ratioaccording to oxygen content in flue gas.
Main functions are as follows:
Toensure high-pressure steam pressure PIC018 stable through controlling fuel gasamount FIC301 of auxiliary boiler.
Tofirst add gas and then fuel gas, and to first reduce gas and then fuel gas.
Whenfuel gas pressure is low, fuel gas pressure is controlled by PIC029, so as toprevent fire extinction.
Accordingto the actual situation, to carry out line processing for AIC302 output, so asto maintain proper oxygen content in flue gas when the load is low.