Summary of the "Consistency Control Book" by G. Ostrost (1993)

Focus is on low consistency, but much of the material is still relevant.

2. Hand Sampling:

Five conditions for a sample tap and valve:

1. It must extend into the process line beyond the pipe wall, at least 1/3 of pipe diameter.
2. The tap inlet should be cut off parallel to the stock flow.
3. It should be located in a turbulent area.
4. Must be able to throttle the sample line without plugging it.
5. Valve must shut off tight.

• For calibration of a transmitter, 3 to 5 samples should be taken in rapid sequence.

• The entire volume should be processed.

• The range of a multiple sample test indicates its precision.

3. On-line measurement

• No moving parts.

• Not subject to damage.

• Not velocity sensitive.

• Can be removed on the run.

• Wide range of consistencies (?)

• Linear in response

But:

• It cannot differentiate between fibre and additives.

• Entrained air, dilution water, fibre type can all impact this type of sensor.

• Since a light-based sensor looks at a very small sample size, it must be representative. This may be difficult with MC stock.

• Shear based.

• Measure the apparent viscosity, and present it as consistency.

• Rotary sensors are more sensitive at lower consistencies.

• Rotary sensors are inherently more accurate because:

1. They have a larger sensing area.
2. They seem to be less affected by other variables than the blade is.

• Flow sensitivity: Since the measurement of apparent viscosity is flow sensitive, it must be accounted for in some way.

• external flow compensation.
• built-in compensation.
• avoid flow effects altogether, say with a recirculation line.

• Mechanical transmitters are temperature sensitive.

• Mechanical transmitters are non-linear.

• Mechanical transmitters are affected by vibration.

• Mechanical transmitters can be physically damaged.

• Many of the advantages of the light-based system are the same here.

• Requires the use of radioactive material.

• Measures all solids in the line.

• Is affected by air bubbles.

• Is affected by build-up on the pipe wall.

• Is heavy.

3. The Control System

• If the major upsets in the system are due to changes in consistency, then use a rotary control valve.

• If the major upsets are caused by stock flow changes, then use a V-ball (if chest agitation is very good).

• If upsets are due to both flow and consistency changes, then use an eccentric plug valve.

• PI only. No derivative!

1. To be stable under all normal operating conditions.
2. To hold setpoint as closely as possible.
3. To return to setpoint as rapidly as possible after an upset.

• Always tune for the lowest flow rate, at the "normal" (design) incoming consistency.

1. Adjust the process flow to the minimum.
2. Put controller in manual (consistency).
3. Make a step change in the dilution valve position, and time how long it takes to be seen on the transmitter. This is the dead time lag.
4. Time how long it takes the consistency to reach a new stable point. 63% of this is the system time constant. Ideally, it should not be more than two times the dead time lag. If it is excessively long, check the system damping.
5. Calculate the integral setting:



6. If a digital system is used, and the sample interval is less than 1/5 of the dead time, then the previous equation can be used. If it is more, then:



7. Adjust the proportional controller by trial and error to give the desired result.

3. Special Systems

• Flow is now constant.

• Since flow is maximized, dead time is minimized.

• System pressure is stabilized.

• Can now use a linear dilution valve (rotary control valve).

• The return line must be located away from the pump suction if it goes back to the chest.

• It if goes into the pump suction, a properly designed standpipe is necessary.

• Some recirculation, say 10%, must always be maintained (at the process maximum flow rate).

• To prevent cavitation of the recirculation valve at higher openings, install a manual balancing valve downstream to reduce the pressure drop at high flows. A manual v-orifice knife gate works well.

• The consistency transmitter can be installed in a bypass or sample line, but it must be a flow controlled line.

• A good way to add consistency control to an existing system.

• Can shut down and service consistency loop without shutting down production.

• Requires much less energy than a full recirculation system.

• Very difficult to make work properly.

• Avoid if possible!

• If a pump cannot work, a mixer might.

• Dead time lag can be reduced compared to a pump because of reduced turbulence.

• The dilution water pressure will be greater than for a pump suction.

• Often a batch process.

• Subject to large changes in fibre.

• Consistency control is rough at best.

• Do not use nozzles at the bottom of the header.

• Do use openings in the top of the header with no nozzles.

• The conveyor housing must extend beyond the end of the thickener.

• A dam should be installed between the end of the conveyor screw and the discharge.

• Make the screw less efficient as a conveyor by increasing the clearances or adding notches (this will improve its mixing efficiency).

• Heavy stock breaks off into the dilute zone in chunks.

• A common system in use is the "double dilution control". The consistency transmitter sends a signal to the dilution valve on the pump suction. This valve output (position) then is used as a signal input for the dilution in the Hi-D tower bottom. Unfortunately, it rarely works well.

• A "Gap Action" controller can duplicate manual control by allowing the pump dilution valve to float within some pre-determined range. When it goes out of this range, then the tower dilution valve would move.

• A third option turns the tower dilution into a flow control loop that is ratioed to the stock flow. It makes sense if changes in flow are large, and it also solves the problem of variable tower head and dilution header pressure variation. This works best if the feed to the high density tower is a uniform consistency.

• Recirculation has some benefits in a Hi-D tower set-up (minimizes and stabilizes dead time lag, eliminates transmitter flow sensitivity, reduces magnitude of consistency upsets), but it does not eliminate the non-linear effects of flow change.

• Recirculation lines in this application usually plug.

• In addition to the issues facing consistency control on a Hi-D tower, a blow tank is hotter, a higher pH, and can have trash in the stock.

• Sometimes the amp measurement of the agitator is used to infer consistency. This can work, but any change in efficiency of the agitator will be read as a consistency change.

3. Transmitter Calibration

• Simple

• Not recommended because the trial and error method is not efficient. The calibration is not repeatable, at any point other than the calibration point, the error is unknown.

• More time consuming, but much more useful.

• Curves for different stocks are developed by the manufacturer. Good possibilities for compensation.

3. New Concepts

• Typically, a consistency controller calls for a valve position on the dilution, when what it really wants is flow. Use a flowmeter, and determine the required flow from the consistency measurement. This will compensate for changes in header pressure and chest level. The characteristics of the dilution valve are now isolated from the consistency loop.

• One problem still remains – non-linearity with changing process flow (the loop gain still increases as the process flow decreases).

• This problem can be alleviated by modifying the requirement for dilution flow by the process flow. When the process flow changes, the dilution flow will change immediately, not waiting for the consistency to change. A linear dilution valve should be used, or a modified linear valve for improved rangeability. An equal percent valve should not be used here.

• Do not use flow compensation and recirculation, as they are trying to accomplish the same thing.

• Use a double dilution system with both dilution valve in cascaded flow loops.

• Both dilution flow meters must give a linear signal, but the valves should have a modified linear characteristic to give sufficient rangeability.

• The dilution valve should be sized to be 80% open when the flow, tower head and incoming consistency are all at a maximum.