The retention of fibre and fillers on the forming section of the (wire/fabric) is calculated from the whitewater solids fraction and thinstock consistency fractions.
An optical (polarised) sensor is enclosed within our WSU2 package along with switching valves to sequentially measure whitewater solids and thinstock consistency on the machine.
After power-up the system normally samples whitewater for 10 minutes and then switches to thinstock for 60 seconds, before switching back to whitewater. On switching from one sample point to another the measurement is held for 30 seconds to stabilise the measurement. The thinstock sample measurement is then averaged and stored in a register with a further average to smooth the measurement between each 10 minute reading. In some applications the timing of the thinstock sampling interval can be changed from 15% (std.) to 50% (equal timing).
A retention sensor can also take a second whitewater sample point where Sample A (master) is sampled first after power-up and then switched to Sample B after 2½ minutes. 15 seconds settling time is allowed between each whitewater sample. It is important that the sample quality (entrained air) and the process temperature is similar.
If a second thinstock sample is required a separate 2 valve (3-way) switch-over is installed outside of the cabinet. It is important that a thinstock sample flows continuously and that the water back-flush cannot reach the machine flowbox to cause a pressure disturbance.
On power-up the system performs a back-flush to prime the sample system prior to measurement. If a sample is not present the unit will repeat the back-flush sequence twice before shutting down. The unit will try to re-start each hour until a sample is available.
Once the unit is operating the sample passes through in-line de-aeration vessels to remove a majority of the entrained air from the sample. The sample is then pressurised to 2 bar.g to minimise the measurement noise (light refraction) seen from any air that may remain.
The sample passes through the sensor and then out of the throttle valve into the drain system. The sample drain is normally returned to the process, either directly back into the wire-pit or into an excess water tank.
A system back-flush is normally performed each four hours in a retention system since the thinstock has a cleaning action on the sensor. Some applications may require cleaning intervals of 1 hour in order to keep the process piping clear.
