Analysis of the reasons for the unexpected decrease in steam flow rate
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Our plant originally had a set of equipment in which the tower bottom vessel served as a thermosyphon reboiler, connected to a condensate tank; the condensate was automatically controlled via pneumatic regulation to be transferred to the preheater for heat exchange before being discharged. Later, during technical upgrades, a plate heat exchanger was added after the preheater to facilitate heat exchange with another tower. However, due to production constraints, the system has never operated at full capacity. It is now necessary to operate at full capacity or beyond to verify the production capacity, but the amount of steam available is not sufficient to reach the previous maximum level. Moreover, the liquid level in the condensate tank is kept low, as it used to be; it could previously be controlled at 50% level. Now, the amount of steam is inversely proportional to the liquid level in the condensate tank – the higher the liquid level, the less steam is produced (there is also a control system for the steam). The condensate level can also be controlled freely, but if it is too high, the steam flow will be lower. Currently, we are facing the issue that the steam output cannot be increased; the load is only at 85%. We seek expert assistance to analyze the reasons behind this.
Increasing the load must be preceded by temperature control. The steam flow rate and liquid level are theoretically inversely proportional. Try adding forced reflux at the bottom of the tower, and adjust the condensate valve so that its setting isn’t too high. With less steam to be condensed, the flow rate will be lower.
Firstly, the issue here is that your tower bottom steam has a control system; is this control system automatically regulated based on the pressure and temperature inside the tower? When the temperature and pressure inside the tower do not change much, it is definitely not possible to significantly increase the amount of steam. Also, your condensate tank features automatic level control; any uncondensed steam in the tank remains in the upper 50% of the tank volume and returns to the bottom of the tower to exchange heat. If you open the steam valve wider, it seems that there should be more condensate water, but in reality the tower doesn’t require that much heat; therefore, the amount of water formed through heat exchange and condensation does not increase in proportion. However, the drop in liquid level needs to be taken into consideration; check whether there is any internal leakage in the pneumatic control valve of the drain tank. The drop in level may be caused by internal leakage as pressure increases. Could the series connection of the preheater for heat exchange and the plate heat exchanger lead to pressure buildup in the drainage system?
:(The main reason is that the line for the discharge of your condensate is too long. The driving force for steam to enter the reboiler comes from two factors: your steam pressure and the efficiency of heat exchange (that is, the efficiency with which steam is utilized). The resistance, on the other hand, comes from the discharge of condensate and the valves used. If the valve openings are the same, then it’s likely that there is a problem with the discharge of condensate. . . :$I’m guessing it myself; since I haven’t seen the actual item, this is the only thing I can do
According to what you say, the liquid level in the condensate tank affects the amount of steam produced; therefore, the liquid level in your condensate tank is a bit too high. At certain levels, the liquid can reach the tubes inside the tank, thereby reducing the heat exchange area. It is recommended to lower the liquid level to below the bottom plate of the reboiler, within the allowable range. It is also recommended to increase the liquid level in the tower bottom and raise the circulation rate of the thermosyphon reboiler to see if this can increase the steam volume.