Pressure screen monitoring apparatus and method

A screening system includes a pressure screen which receives a liquid/solid suspension and filters the suspension to provide an accept portion and a reject portion. The pressure screen includes a housing which encloses a chamber containing a screen basket through which a portion of the liquid/solid suspension passes from a first side of the basket to a second side to form the accepts. A pressure sensor monitors the pressure differential between the first side and the second side and provides a sensed differential pressure signal indicative thereof. The system also senses the accept flow rate and provides a signal indicative thereof to a controller. The controller sets a differential pressure threshold value as a function of the accept flow rate signal and monitors the sensed differential pressure signal. If the sensed differential pressure signal exceeds the threshold value, the controller automatically initiates corrective action in order to reduce the differential pressure across the screen.

DESCRIPTION 
TECHNICAL FIELD OF THE INVENTION 
This invention relates generally to pressure screens used to separate 
larger particles from liquid/solid suspensions, and is concerned in 
particular with an improved system and method for monitoring the 
performance of such devices. 
BACKGROUND OF THE INVENTION 
Screening systems are widely used in manufacturing processes to separate 
larger particles from a liquid/solid suspension. For example, applications 
for screening systems include processing chemical pulp, mechanical 
groundwood, bleached or unbleached kraft, old corrugated containers, mixed 
news, drink stock, waste paper or any other type of fiber. 
The system typically receives several thousand gallons per minute of 
liquid/solid suspension, and separates the larger particles using a 
pressure screen which typically provides an accept flow (smaller 
particles) and a reject flow (larger particles). An example of a pressure 
screen is the Model 400 available from Voith Sulzer, the assignee of the 
present invention. 
The system typically includes an automated controller which controls the 
overall system operation, monitors the system for component failures/flow 
anomalies, and automatically initiates corrective action. To detect an 
obstruction across the pressure screen (typically caused by accumulated 
particles which are too big to pass through the screen), the controller 
monitors pressure across the screen. If the pressure exceeds a fixed, 
predetermined differential pressure threshold value, the controller 
initiates the necessary corrective action in order to remove the blockage. 
In many prior art systems, the controller simply sounds an alarm when the 
pressure across the screen exceeds the threshold value, and system 
operating personnel manually take the necessary corrective action. 
Significantly, in both automatic and manual systems, initiating corrective 
action involves interfering with the conventional flow processing, and 
therefore reduces the processing efficiency of the system. 
The predetermined differential pressure threshold value is often selected 
during the system design of the screening system and remains fixed. 
Selecting this threshold value has involved a balancing of several 
factors. If the value is set too low, the controller initiates unnecessary 
corrective action. In contrast, if the value is set too high, an excessive 
amount of the liquid/solid suspension will be regularly routed to the 
reject flow. 
A problem with both automatic and manual systems is that the threshold 
value remains fixed. Specifically, what might be a "high" differential 
pressure for one accept flow rate or system operating condition, might 
well be "normal" for another accept flow rate or operating condition. 
Therefore, there is a need for an improved system and method for monitoring 
a pressure screen for flow disturbances. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a system and method for 
detecting flow disturbances within a pressure screen. 
Briefly, according to the present invention, a screening system includes a 
pressure screen which receives a liquid/solid suspension and filters the 
suspension to provide an accept portion and a reject portion. The pressure 
screen includes a housing which encloses a chamber containing a screen 
basket through which a portion of the liquid/solid suspension passes from 
a first side of the basket to a second side to form the accepts. A 
pressure sensor monitors the pressure differential between the first side 
and the second side and provides a sensed differential pressure signal 
indicative thereof. The system also senses the accept flow rate and 
provides a signal indicative thereof to a controller. The controller sets 
a differential pressure threshold value as a function of the accept flow 
rate and monitors the sensed differential pressure signal. If the sensed 
differential pressure signal exceeds the threshold value, the controller 
generates a high differential pressure condition signal. 
Upon detecting that the pressure differential across the screen basket 
exceeds the adjustable differential pressure threshold value, the 
controller may automatically initiate corrective action in order to reduce 
the pressure across the screen. For example, the controller may command 
the system to reduce the accept flow rate, increase the reject flow rate, 
or completely shutoff the accept flow. Each of these actions is designed 
to remove accumulated particles which are blocking flow through the screen 
basket. However, in some cases the controller may simply trigger an alarm. 
Advantageously, by dynamically adjusting (i.e., scheduling) the 
differential pressure threshold value as a function of accept flow rate, 
the number of unnecessary corrective actions which the system initiates is 
reduced. Therefore, the processing cycle of the system has less 
unnecessary flow interruptions, and hence, provides improved system 
screening efficiency. 
These and other objects, features and advantages of the present invention 
will become more apparent in light of the following detailed description 
of preferred embodiments thereof, as illustrated in the accompanying 
drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a pressure screening system 10 which receives a 
liquid/solid suspension through a conduit 12 and a dilute (e.g., water) 
through a conduit 14. The liquid/solid suspension and the dilute flow are 
mixed in a conduit 20 which delivers the mixture to a pressure screen 22. 
As known, the pressure screen 22 includes a housing 24 which forms a 
cylindrical chamber sized to receive a screen basket, illustrated by 
broken lines 25. The screen basket 25 includes a cylindrical sidewall 
having a plurality of slots, holes or other geometric shapes sized to form 
a barrier that retains particles which are larger than the size of the 
openings. Liquid and smaller solids (hereinafter referred to as the 
"accepts") which pass through the openings in the cylindrical sidewall of 
the screen basket 25 exit the pressure screen 22 at an outlet 28, and 
enter a conduit 30. The particles which are too large (hereinafter 
referred to as the "rejects") to pass through the screen basket 25 migrate 
through the suspension to the bottom of the basket and exit the pressure 
screen 22 via conduit 32. The pressure screen includes foils 34, 35 which 
rotate within the screen basket to remove the particles which are too 
large to pass through the basket. As the foils rotate they create positive 
and negative pressure pulses as they pass the openings in the sidewall of 
the screen basket. One of ordinary skill would appreciate that rather than 
foils, other pressure pulse generating devices such as bumps, lobes, etc. 
may be used. 
The system 10 also includes a differential pressure sensor 40 (e.g., strain 
gauge) disposed to monitor the pressure across the screen basket 25 via 
sense lines 41, 43, and provide a signal indicative thereof on a line 42 
to a controller 44. In general, many different types of pressure sensors 
may be used, including capacitive, differential transformer, force balance 
or piezoelectric. 
The controller 44 preferably includes a microprocessor (not shown) which 
executes programmable software routines to control the system 10 and 
monitor the system for any failures. The system 10 also includes a flow 
rate transducer 46 which measures the flow rate of the accepts in the 
conduit 30 and provides a signal indicative thereof on a line 48 to the 
controller 44. Specifically, the transducer 46 includes a venturi sensing 
element 47 and a differential pressure sensor 49 which senses the pressure 
drop across the venturi 47. The controller 44 computes the flow through 
the conduit 30 as function of the square root of the pressure drop across 
the venturi. The controller also receives a reject flow rate signal on a 
line 50 from a reject flow sensor 52 (e.g., a magnetic flow sensor) 
disposed to measure the reject flow in the conduit 32. One of ordinary 
skill will recognize that many different types of flow meters/sensors can 
be used including an interference mass flow meter, a thermistor flow 
meter, a magnetic flow meter, a mass flow meter or a rotameter. 
The controller 44 provides an accept flow control signal on a line 56 to an 
accept flow control proportional valve 58 disposed to receive accept flow 
from the conduit 30 and provide a variable output accept flow in conduit 
60. The controller 44 also provides a reject flow control signal on a line 
62 to a reject flow control proportional valve 64 which controls the 
rejects flow. The valves 58, 64 are proportional valves which allows the 
controller 44 to modulate the flow area of each valve to control the 
accept and reject flow. 
According to the present invention, the controller 44 includes pressure 
monitoring control logic 65. FIG. 2 illustrates a flow chart illustration 
of the processing steps performed by the monitoring control logic 65. 
Referring now to FIGS. 1 and 2, upon entering the control logic 65, step 
66 is performed to read the signal on the line 42 indicative of the 
pressure across the screen basket 25. Step 68 is then performed to 
determine the accept flow rate in the conduit 30. The controller 
calculates the flow rate by reading the pressure signal on the line 48 and 
calculating the flow rate as a function of the square root of the pressure 
drop. Step 70 is then performed to calculate a screen differential 
pressure threshold value as a function of the accept flow rate. This step 
can be performed using a look-up table or an equation which characterizes 
the relationship between the screen differential pressure threshold value 
and accept flow rate. To insure the system 10 is operating correctly, the 
controller then performs step 72 to compare the differential pressure 
signal on the line 42 against the screen differential pressure threshold 
value to determine if the pressure across the screen basket exceeds the 
threshold value. If it does, the controller initiates corrective action in 
step 74 to remove the blockage across the screen basket which reduces the 
pressure across the screen basket. The controller executes the pressure 
monitoring control logic 65 on a regular basis (e.g., every five seconds) 
in order to detect flow disturbances. 
The corrective action taken in step 74 includes reducing the accept flow 
rate, increasing the reject flow rate, or completely shutting off the 
accept flow. The controller achieves these corrective actions by issuing 
signals on the lines 56, 62 to proportional control valves 58, 64, 
respectively. Each of these corrective actions is designed to reduce the 
amount of accumulated particles which are blocking flow through the screen 
basket 25. Advantageously, scheduling the screen differential pressure 
threshold value as a function of the accept flow rate reduces the number 
of unnecessary corrective actions which the system 10 initiates. 
FIG. 3 illustrates a cross sectional view of a portion of the cylindrical 
sidewall of the screen basket 25 and the foil 35. The screen basket 
includes a plurality of openings 100 sized to allow an accept portion of 
the liquid/solid suspension to pass from a first side 102 through the 
screen basket to a second side 104. The screen basket includes a first 
orifice 106 and a second orifice 108 (each about 1/8" in diameter). The 
differential pressure sensor 40 senses the pressure across the pressure 
screen 25 via the first and second orifices 106, 108. 
To prevent large particles from blocking the orifices 106, 108, the system 
includes valves 110, 112 which each provide a small flow of a flushing 
liquid (e.g., water) through flowing sense lines 114, 116. The valves 110, 
112 provide the flow through the sense lines 114, 116 at a pressure 
sufficient to ensure that the flushing liquid exits the orifices 106, 108 
at all nominal system operating conditions. 
FIG. 4 illustrates a cross sectional view of a portion of the cylindrical 
side wall of a screen basket 119 having a first non-flowing orifice 120 
covered by a flexible diaphragm 122, and a second non-flowing orifice 124 
covered by flexible diaphragm 126. The orifices 120, 124 are located on 
opposite sides of the screen basket, and the differential pressure sensor 
40 is connected to the orifices 120, 124 via nonflowing lines 128, 130. 
In general, and as shown in FIGS. 3 and 4, it is preferred that the 
differential pressure is sensed directly across the screen basket. 
However, it is contemplated that the pressure may be monitored at 
different locations to detect pressure anomalies indicative of accumulated 
particles blocking flow through the screen basket. In addition, it is 
contemplated that the accept flow may be determined by using a separate 
accept flow rate sense line (not shown). This line would be relatively 
small with respect to the accept conduit and include a flow sensor. The 
flow in the main accept flow line (i.e., conduit 30) would then be easily 
computed since the flow in the main accept flow line would be proportional 
to the flow measured in the accept flow rate sense line. 
In addition, while the controller 44 preferably includes a microprocessor, 
an alternative, less expensive controller may include dedicated electronic 
circuitry to perform the pressure monitoring control logic of the present 
invention. The present invention is also not limited to the use of 
differential pressure sensors. One of ordinary skill will recognize that 
absolute pressure sensors may be used. 
Although the present invention has been shown and described with respect to 
several preferred embodiments thereof, various changes, omissions and 
additions to the form and detail thereof, may be made therein, without 
departing from the spirit and scope of the invention.