A new possible approach to have better GSM control on paper machines.

1. Aiming Better GSM Controls: a Possible Approach
Long backmore than 15-17 years back, we were having some issues in paper machine,
and decided to remove centricleaners from approach flow. In addition to increase in fiber
yield, we observed that the GSM variation reduced drastically. Later, many mills tried the
same, and found it a good approach to not have centricleaners in approach flow.
Now, the stock from fan pulp goes to slot screen and to headbox. The reject from slot
screen goes to secondary screen, with its accept to fan pump suction. The secondary
screen rejects are drained by means of an ON-OFF valve which operates say 500 sec
OFF and 5 seconds ON. This way we ensure fiber loss is minimal and system runs well.
Now, you know that the pulp is very clean, so why to waste more and more reject from
reject screen?
So far everything is fine.
But, as the secondary screen reject valve opens for such short duration, this creates a
small imbalance in the flow rates; and hence GSM variation by approx. 1-1.5 gsm takes
place, of course, for a few seconds only. Not only this, this variation in machine direction
also results in CD profile variation for some time, and the CD profile keeps on fluctuating
slightly. Typical figures for say 42 GSM paper are +/- 0.6 to +/- 0.8 gsm. That means, a
sample form the roll might have say 41.2gsm (minimum) to 42.6gsm (Maximum).
For most of the mills, this is something more than enough. But, if, somehow, we are able to
get rid of this ON-OFF mechanism, further fine control of GSM is expected. A simple way
seems to throttle the valve open continuously and get rid of the On-Off valve itself. But, in
that case, might be you need to throttle the valve to just 1-2% only, which is not practical.
So, here is another idea-
After the main slot screen reject valve, a screw pump is placed, with a VFD driven motor.
The speed of the motor is so controlled as to reduce the reject flow to ensure fiber loss is
low, and also there is no possibility of contaminants going with rejects of that reject screen.
It is also possible in several cases, that the reject screen itself might be removed with
rejects going out from main slot screen rejects only.
Such a system will give following benefits-
1. No need of having a secondary screen, its feed pump, VFD etc.
2. Energy saving as reject screen is omitted.
3. Energy saving as fan pump throughput requirement is also reduced.
4. Reduced flow through main slot screen (As there is no recirculation now).
5. Much improved GSM control
6. Easier and rapid control of paper GSM
By:
D K Singhal
www.dksinghal.in
D K
Singhal
Digitally signed
by D K Singhal
Date: 2021.03.25
15:47:34 +05'30'