Do we really need slot screening in the approach flow of paper machine in today's times? Think.

1. Rethinking Approach Flow Screening, March 2018
Rethinking Approach Flow Screening
Around 10 years back, we took a decision to remove centricleaners from approach flow of paper machine.
The results were great. Dramatic energy saving, a significant improvement in basis weight control, better
machine performance as the web breaks reduced by more than 75%, better steam control as well as
increase yield.
While studying the machine characteristics, we observed a pattern in machine direction basis weight profile.
In fact, the present system comprises of a fan pump, followed by a slot screen and then the headbox. The
reject of pressure screen is sent to secondary screen, and secondary screen accept back to fan pump inlet.
To reduce fiber loss, the reject screen reject valve is timer operated which opens just for 5 seconds after
every 7 minutes. A close analysis indicates that this results in nearly 0.5-0.7gsm reduction temporarily.
Earlier, we had timer settings of 10 seconds ON after every 15 minutes OFF position, but in that case the
sudden basis weight reduction was 1.0-1.2gsm.
At present on a typical 50 TPD machine, the fiber loss is there to the tune of 70-80kg/day. Keeping the reject
valve open continuously is obviously not possible. That would result in significantly high fiber loss. An
alternate could be thought to reduce the main pressure screen reject valve, but with too little valve opening,
there exists a risk of line clogging at valve, thus creating quality and other problems.
I recall long back paper mills were facing the similar problems with centricleaners. If mills tried to maintain
proper pressure, differential pressure in centricleaners, the head and level in headbox suffered; and
papermakers usually struggled to fine tune one at the cost of other. That time, many suppliers came up with
‘Double Dilution’ system, thus keeping centricleaners a little out of field. Later several mills considered
complete removal of centricleaners from approach flow. Today, our fiberlines have improved a lot in terms
of quality and reliability; most of us have state of the art stock preparation systems, and thus expect a clean
furnish in the machine chest itself; leaving no significant improvement potential in approach flow screening.
This way, the approach flow screen becomes just a security guard to protect the headbox and wire from any
(big) undesirable material. I was wondering just the electricity consumed by the approach screen itself costs
more than a wire every month. Not only this, the pulsations from screen, or other issues as indicated above
create basis weight control problems. Loss, though minor are also associated with this screen. In short, the
approach flow screen reminds me the idiom- ‘penny wise pound foolish’.
With this, the question appears, “Should we not start thinking of ‘getting rid’ of approach screen?” Please do
share your valuable comments on the issue.
D K Singhal
deveshksinghal@gmail.com