Using pH & conductivity measurement for stock prep and wet end control: as paper mills have begun closing their water systems, the need for conductivity monitoring has increased. Careful sensor selection and placement are critical to accurate control.

Wet end chemistry is complicated and delicate. The complex fiber and chemical additive mixture in the furnish makes chemical monitoring a must for most paper mills.

Most chemical additives only work in a narrow range. Solution pH also affects the surface charge of the fiber, and bonding is dependent on this charge. Fibers and fines have a slight negative charge and attract positively changed particles or chemicals. As the pH drops, this negative charge increases. In stock preparation, pH will generally not be constant throughout the different stages. Often the optimal pH at one stage is different from another.

Fiber solubility, drainage optimization on the machine wire, and fiber and fines retention in the sheet are also affected by pH. Improper pH control can lead to deposits as chemicals interact in undesired ways. Also, improper pH control of broke and recycled fiber sources can lead to foaming problems and microbiological growth in the white water.

As paper mills have begun closing their water systems (reusing water to achieve a near zero effluent mill), the need for conductivity monitoring has increased. Paper mill water systems are similar to small boilers in that water is captured and reused, but there are still vapor losses. The salts and other chemicals do not vaporize-they concentrate. An increase in conductivity indicates an increase in total dissolved solids (TDS) concentration. If conductivity goes too high, salts will come out of solution, producing scaling and corrosion on the paper machine. This can damage the machine wire and felts and produce efficiency losses, deposits in the paper, and other quality defects. High conductivity/TDS lowers fiber and filler binding and damages the performance of many chemical additives.

Many machine breaks are accompanied by an increase in white water and headbox conductivity. When conductivity increases, either the white water system needs dilution with fresh water or additional chemicals must be used to control the increase in salts. Perhaps a regular blowdown cycle, such as those used with boilers and cooling towers, should be instituted.

PH SENSORS

Paper mills run continuously and lines cannot be shut down for sensor removal. For stocklines, ball valve insertion sensors--such as ABB's TB557--are advised because they can be inserted and removed online (see Figure 1). These sensors should have a flat glass electrode and flat surface porous Teflon reference. Surprisingly, sensor protrusions designed to protect the glass actually promote fouling and change consistency around the sensor. This adversely effects pH readings. Instead, mills should use "flat and flush" sensor designs in the wet end.

[FIGURE 1 OMITTED]

It is often wiser to make paper machine measurements off-line, especially if there are frequent grade changes on the machine. This allows easier access to the sensor for cleaning and calibration. The optimum installation is a 1 to 1-1/2 inch sample line off the headbox overflow that goes to a high volume sample chamber with flow "rushing" at the submersed sensor tip. Flow around the sensor tip must be high enough to keep fiber from accumulating. If fiber builds up around the tip, the pH reading will change. Low velocity chambers with slow moving upflow feed lines can cause problems.

CONDUCTIVITY SENSORS

Conductivity should only be measured directly in the main process lines. Four-electrode, ball valve insertion sensors with flush faces, such as the ABB TB461 (see photo), are advised for pulp stock above 0.25% consistency. The sensor surface does not disturb stock flow, hence consistency, in the measurement area. The four-electrode design compensates for both polarization of the solution and build-up of scales and other foulants, thus eliminating other sources of error.

Sensors with protrusions, such as Toroidal (electrodeless) conductivity sensors, cause disruption of stock flow around the measurement area, making for a noisy measurement. However, Toroidal sensors are advised for whitewater, tray and similar low Fiber applications.

WHAT AND WHERE TO MEASURE

There are four critical measurement points for both pH control and conductivity monitoring: the headbox, whitewater, broke system and machine chest. If recycled fiber is added to the furnish, monitor pH and conductivity of this source too.

For best results pH should be measured in other locations as well. For example, pH should be measured and controlled at every incoming flow point, such as dilution and wash waters, the refiner inlet, stuff box, and thin or sweetened stock.

Successful pH and conductivity monitoring depends not only on the sensor choice but also on sensor placement and installation. Here are some tips:

1. Use only sensors with flush surfaces.

2. Flat glass pH sensors must be mounted at least 5[degrees] above horizontal so the liquid in the glass electrode is always in contact with the glass interior.

3. Pulp slurries must be travelling horizontally or vertically for accurate measurement, never in a downflow line.

4. Sensors must be located in areas free from heavy vibration and at least 10 pipe diameters away from pumps. Areas with fast or large pressure variations must be avoided. Sensors should not be located in or after elbows. Flow need not be laminar, but heavy turbulence will cause measurement instability and harm sensors.

5. Sensors must be inserted 1/2 inch to 3/4 inch (12-18 mm) into the process.

6. Control lag must be minimized. For pH, the sensor should be located within 5-20 seconds after an addition point, making sure that sufficient mixing has taken place. Sample lines-especially those ending in measurement chambers or troughs-can significantly increase measurement lag.

7. Feedback control loops for pH are preferred to feedforward. Any feedforward pH control loop must have a feedback pH signal to optimize control. Flow must always be included in the pH control loop algorithm.

8. Toroidal conductivity sensors cannot be used in solutions containing more than 0.25% fiber. Toroidal sensors can be used off the machine in whitewater and tray measurements as long as fiber build-up around the sensor can be eliminated.

9. Five to 10 seconds of instrument damping is advised on all conductivity measurements in pulp stock.

Don Spriggs is senior applications engineer for ABB Instrumentation Division, Carson City, Nevada, USA. Contact him at don spriggs@ us.abb.com for a copy of the ABB Technical Paper "The Effect of Consistency on pH Measurement in the Paper Mill."