condition also possibly being the result of a plugged screen in the present system to avoid actual full plugging of a screen the controller may be set to alert the operator when there is only a small drop in the temperature of hot liquor and or small drop in the flow rate of the hot liquor passing through the stub pipe so that the operator may take remedial action immediately to remedy the plugging of the screen this combination of a reduction in the anticipated flow rate through a stub pipe as sensed by the flowmeter which also sends to the controller a signal representative of such reduced flow to the controller with the sensed reduction in temperature of the flowing hot liquor provides a novel improved concept for monitoring the operability of each individual screen thus the signal from the flowmeter provides the controller with a signal which compliments the signal to the controller from the temperature sensor in like manner if the temperature within the stub pipe is within a range recognized by the controller as acceptable but the flow rate of hot liquor through a given stub pipe increases above a standard value set in the controller such conditions may indicate that more.