Gravity Flow

Using gravity can be economical in some circumstances. It is also very reliable since it's pretty much constant. The place where gravity flow is most often used on a paper machine is the stuff box. The stuff box provides a constant head of pressure to the basis weight valve which meters in the appropriate amount of stock.

There are other places where gravity can be used to provide pressure. In order to utilize gravity one must utilize the Bernoulli equation. One of the few equations I learned in school that I was able to use in industry. Paper machines are constructed in such a way that there are various levels that can be used to provide pressure, from the basement to the mezzanine floor. If you go into enough paper mills you will also learn the term mezzaline, which is the proper way to say mezzanine in some mills. There is usually another floor above the mezzanine which is the deculator floor or some other floor. Not all paper machines have a deculator, but they could have a floor to provide access to dryer hood vents. Of course there is also the roof. I would not recommend putting anything on the roof unless it's relatively small.

I was able to utilize gravity flow in a project that required less fluctuation in a dye flow. The dye had a relatively small flow and the pump supplying the dye had been oversized. I was given a small budget of $20,000. The reason the flow was fluctuating was due to the small orifice size on the control valves and the oversized pump. The valves worked better when they were more open in the 50% range. I calculated the pressure I needed to open the valves in this range for the necessary flow. I was able to salvage a stainless tank and set it on the mezzanine floor, the control valves were in the basement. I used the pump to fill the tank to a level setpoint. I realized that the head pressure would change when the level in the tank changed, but it was within the operating range of the control valves.

The end result was a much happier stock tender. He didn't have to worry about keeping an eye on the dye flow. The valves were stroked regularly during breaks to ensure there was no build up at the orifice. In addition to the pressure reliability, the system did not go down if the pump went down. The tank could be filled manually with totes if necessary which added more reliability to the system. So when working with a limited budget and needing a reliable pressure source, look to gravity. The pressure is only limited by how high the tank or line can go.

Acid

This post is somewhat grade specific. Acid is used in some mills to control pH. In multiply linerboard it can also be used to improve drainage. However, too much will damage the fiber and lead to reduced strength. The reward of improved drainage may allow for the machine to be sped up, resulting in increased production.

Using acid on the base sheet is generally not an issue since there is plenty of flow to dilute the acid. On a secondary or top sheet controlling the amount of acid can be an issue due to the low acid addition rates. The acid addition should be controlled with a pH setpoint. The control response should be fairly quick. Some bump tests should be performed to determine how the control should be set up. This should be done with the assistance of the controls engineer and E&I personel. To improve the control, the pressure going to the control valve can be reduced to allow the valve to operate at a larger opening range.

Acid is relatively cheap. Increasing acid usage can not only increase production, but also reduce alum usage. This should not be a high priority project but is definitely worth looking at.

Ropes

Ropes are used to thread the paper machine. The rope run goes through all the dryer sections and are setup so that the tail, a narrow sheet, goes through the dryers properly and reaches the reel drum. Paper machine ropes, or carrier rope, is made from nylon and other synthetic materials which are abrasion resistant.

Ropes are very dangerous. When I first started working on a paper machine the first thing the machine manager told me was if I ever heard a rope break, run. If a rope breaks and catches anyone's limb it would surely catch and pull the person through the rope run which would more than likely kill the person. This is why it is important to correctly splice the ropes. This is where the ends of the rope come around to close a loop. The splice should be overlapping at least 18 inches. Glue should also be applied to the splice for extra strength. If a rope begins to fray it should be carefully monitored. If the fray worsens, then a decision should be made to replace the rope. If a rope breaks it can not only result in a break, but also take out other ropes. If the rope is in the first section and it breaks and takes out all the other ropes down the machine this can lead to several hours of downtime. Some rope runs are fairly complicated and take some time to re route. It is much simpler to replace a rope by tying a new rope to the old rope than to have to manually guide the new rope by itself through the rope run.

It's better to be safe than sorry, so do not take unnecessary risk with the ropes, you could get burned.