Keep Vendors in Check

The quality of the products purchased from anyone should be checked periodically. This keeps the suppliers and vendors honest. It shouldn't be that way but it is. I'll give you an example of a very big headache that occurred due to a supplier/vendor.

I was working on a fine paper machine and shortly after an outage a mysterious diagonal curl started appearing in the sheet. The thing with diagonal curl is that it's easy to diagnose but difficult to cure. The curl had not been there prior to the outage so it must have come from something that was changed out during the outage. The problem is that there are lots of the things changed out during an outage. So after a week or so of investigating the cause of the curl still couldn't be identified. The problem was that the curl was moving. It wasn't always in the same position. So whatever was causing the curl had to be a moving part. On a paper machine everything moves. So the next step was to send in the corporate engineers. The were as puzzled as the rest of us. Another week passes and still no answer. Next some paper machine experts are brought in, they are the ones that built most of this particular machine. They also supplied some parts that were routinely replaced. By this time it was about time for another outage. The paper machine is taken apart and things are be changed out as normal. Then an old machine tender is looking around at some of the headbox parts. He picks up the lexan sheet and notices something. He says, "these sheets feel flimsier than normal." He tells the machine manager who then tells the paper machine experts. They experts say no it can't be the lexan sheets, but for argument sakes lets put in a new set. Note that they supply the lexan sheets. So the machine is started back up and the diagonal curl is still there. A few days goes by and the machine tender keeps saying he thinks its the lexan sheets, there's something wrong with them. Finally the machine manager finds the old lexan sheets which had a few pieces broken off and says to put them back in. This takes about an hour and a half or so. The machine starts back up and the curl is gone. Hmmm.... The paper machine experts call back to their home office and are told that some changes to the lexan sheet production process had been made but that shouldn't have affected the sheets. After hearing of this change the mill manager makes a call and lets just say that they went back to the old way of making lexan sheets.

So trust your operators when they say something isn't right. And just because they say they're experts it doesn't mean they're always right.

Refining

Refining in the paper making processing refines the fiber. Sounds easy enough right? It's a little more complicated than that. Refining the fibers means cutting or brushing the fibers. Too much cutting and the fibers produce too many fines which is lost during the drainage process. So we want to keep the fibers long enough to provide the strength properties we are looking for while also cutting the fibers to a length that will produce an aesthetically pleasing sheet. Too long of fibers will produce knotting and blobs in the sheet, not very uniform. The brushing part of the refining process fibrillates the fiber to produce more surface area for bonding. The more bonding that takes place the stronger the sheet. Imagine a fiber starting as a long stick. After refining the stick may be cut in half or thirds. It also will have the outside bark stripped off and look like its been chewed up a bit. That's basically what refining does.

There are different types of refiners from cones to cylinders to discs. The most common seem to be the disc models. Pulp is pumped into the refiner through a set of discs. One disc is stationary while the other spins. The plates on the disc have bars which differ depending on the refiner model and the desired results. So plates cause more cutting while others provide more brushing. As the bars from the opposing plates cross the squeeze and cut the fibers. Operators adjust the load or amount of pressure on the plates to attain a desired freeness which is the measurement used to gauge refining. The freeness is usually tested by a lab person or a utility hand.

Different plate patterns are available for different purposes, as well as different fiber species. Hardwood plates are designed for more brushing since the fibers are already relative short. Hardwood refiners are also generally set in parallel since they usually only require one pass through the main refiners. Softwood fibers on the other hand are generally set in parallel since the fibers are longer. This gives the operator more flexibility in controlling the freeness.

This is a very brief overview of refining. There's much more to it. So if there is anything in particular you would like to know, just let me know.

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.

Troubleshooting Tip: Holes and Busted Edges

Holes and busted edges are probably the leading cause of breaks on a paper machine. The good news is the cause is usually easy to find. Most machines have hole detectors and busted edge detectors, so the problems are automatically diagnosed. Machines that don't have these detectors must rely on dry end operators to detect the defects. Busted edges are generally easy to spot since the edges are exposed and are easily examined while on the reel. Holes on the other hand are hidden inside the reel and must be detected when the reel is being unwound at the winder. More often the holes are detected when the winder has a break which can be quite messy.

So once the issue is found, how do you find the cause of the problem? For busted edges the usual suspect is the trim squirt. The trim squirts are high pressure nozzles that are used to trim the edges off the sheet as it goes to the press section. The nozzles can get partially plugged reducing the focus of the nozzle. The nozzles can also be misaligned. Due to the position of the nozzles some buildup can form on the nozzles or nozzle apparatus. Other causes of busted edges are light edges, slice bleeds, and edge deckles. Check the weight profile to see if the edges are light and adjust accordingly. Slice bleeds can be adjusted to bring the edges up and edge deckles can be reset.

Holes can be caused by a number of things. They can be caused by material falling into the sheet, such as part of a tail that wasn't cleaned up during the previous break or a leaking shower that drips onto the sheet. The can also be caused by flaws in machine clothing or flaws in rolls on the machine. Issues with pressure screens can also cause holes. To determine the cause of the hole first examine the hole. If the hole is tear drop shaped it is likely caused by material falling into the sheet. Also see if there is some foreign material embedded in the sheet. If the hole is irregular in shape and does not appear to be caused by falling material see if there is a pattern. Does the hole occur only in one position, is there a consistent distance between holes? These are the questions that should be asked. If there is a consistency in distance, measure the distance and determine if the hole is from a roll defect or machine clothing defect. The culprit should be easily found from there unless there are multiple rolls or clothing runs that have the same circumference. If there is no pattern to the holes and it has been determined that there is no debris falling into the sheet, then further investigation is required. Stock samples should be taken at various points in the approach system to see if there are any upsets. The headbox should also be checked to see if there is anything inside that could be causing the holes.

Holes and busted edges are bad, but the causes are generally easy to find. If the cause can't be found immediately patches can be placed on holes until the next outage. However, if the problem must be solved be sure to have a checklist of areas that must be checked. Also coordinate with the maintenance crews to determine what work can be done during the downtime to shorten or delay the next scheduled downtime.

Broke Management

Broke management sounds like what I did with my finances while I was in college. In reality it's the non postconsumer waste that is put back into the sheet. So it's kinda recycled, but not really. For reference, fiber that comes straight from wood is called virgin fiber. Back to the broke, every paper machine produces broke. Broke comes from the trim off the machine, that's the edges that are trimmed off as the sheet goes into the press section. The trim goes into the couch pit which then goes to some kind of broke thickening process to increase the consistency for ease of storage. Another source of broke is when there is a break on the machine. A sheet break is when the sheet tears or breaks and the sheet is generally put into the couch pit while the machine is being rethreaded. Since a break can last anywhere from 10 minutes to over an hour, a considerable amount of broke can be built up. The last source of broke is rejected paper. Reject paper is repulped using some kind of hydrapulper or the like. It's like a big blender.

Okay, so now we have all this broke, what do we do with it? As I said earlier it gets thickened and sent to some storage tank. From there it is sent back to the sheet. It's a little more complicated than that, but we'll keep it simple for now. Depending on the grade of paper being made the broke can be anywhere from 5% to 15%. The benefits of broke is it closes the sheet. It makes it less porous. The bad part is that it can over do it and seal the sheet which is detrimental to drainage. If that occurs more broke will be made.

So to properly manage broke levels, operators should add as much as they can since it has to be used. Dry end operators should also keep an eye on the broke level so that they don't make too much from reject rolls. This is especially important going into an outage. An outage is maintenance downtime that can last anywhere from 12 hours to forever. Maybe not forever, but sometimes it seems like it. There are times when broke storage tanks must be cleaned out or inspected, during these times broke should be reduced to the lowest levels possible. In mills where there are multiple machines the broke may be sent to the other machines if the grades being made are similar. Issues occur when the broke has chemicals such as fluorescent dye present.

So if broke is managed properly money will be saved. After all it is still fiber. It may not be virgin but it's still good.

Addition Points

The addition points of various chemicals should be analyzed from time to time to see if there are better places to add the chemicals. For example, new defoamers may be available that are less detrimental to sheet properties, but have a lower retention time. Thus, the defoamer should be added closer to the are where foam is an issue. Any chemical can be moved to improve efficiency. Optical brightening agents (OBA) or fluorescent whitening agents (FWA) may be added in more places to increase whiteness. Here's a hint look to add OBA's and FWA's further back in the system, think way back. For acid paper look at places where acid can be added to improve drainage. Too much acid will damage fiber and possibly machine clothing. Alum is another chemical that can be moved around. I was able to reduce alum usage by simply moving the addition point. Moving addition points can be easy. Setup some temporary lines and run some simple trials.

Starch

Starch is added at the wet end and at the size press. The wet end starch is cationic, has a positive charge, and bonds to the anionic pulp fibers. The wet end starch strengthens the web formed by the fibers. Wet end starches generally have solids of about 6%.

Starch is used at the size press to further strengthen the sheet. It is also used as a substrate for other chemicals. There are some important aspects of starch used at the size press that may be different from that of the wet end. The cooking of the size press starch is dependent on the product that is being cooked. The starch can be made from potatoes, corn, or tapioca. Solids and pH are other variables that also must be controlled.

In my experience wet end starch was fairly issue free. I don't recall ever having a major issue with the wet end starch. There were times when supply was an issue due to the boom in corn use for ethanol. At the size press there were problems from time to time. If not cooked properly the starch can be burned and have a brown appearance. This can affect the shade of the sheet. Increasing solids was also an issue due to the increase in viscosity as solids increased. Other issues that occurred from time to time was foaming in the starch run tanks. Defoamer was added in small amounts to help control this. Too much defoamer would hurt the sizing, reduce Cobb and HST test. Addition of fan nozzle showers in the run tanks also helped. The problems occurred during runs of special grades, such as grease resistent paper.

Breaker Stack

The breaker stack is located in the dryer section of the paper machine. Not all paper machines have a breaker stack. It is usually located after the third or fourth section of the dryers. The purpose of the breaker stack is to provide some calendering action to improve the smoothness of the sheet. The loading of the breaker stacks should be carefully monitored from the front to the back. Uneven loading can cause caliper, thickness, issues. The amount of loading capacity is dependent on the roll cover material. There should be specifications for a particular roll cover. Operators should be aware of the roll covers and be careful not to overload the stack. Overloading can lead to the destruction of the roll cover which can be catastrophic. It could lead to losing dryer felts and ropes, essentially a wreck. The stacks should automatically unload during a break to protect the stack. However, if a wad of paper comes through it could damage the rolls even when the stack is unloaded.

Calcium Carbonate

Ok, so since I mentioned it briefly in my last post I figured I'd go ahead and talk about it some more. Calcium carbonate is naturally formed as limestone. It can also be manufactured through a precipitation process. The addition of calcium carbonate offsets the amount of wood fiber necessary to produce a certain weight of paper. The calcium carbonates also improve some properties of the paper depending on the shape and characteristics of the particles.

Precipitated calcium carbonate (PCC) is manufactured so the size and shape of the particle can be controlled. Some particles are scalenohedral in shape, they look like spiky balls. These particles can bulk up the sheet. They make the sheet thicker to some extent. There are also prismatic particles. These particles are rice shaped. They are used to improve the appearance of the sheet and porosity. It also strengthens the sheet with improved bonding. PCC is generally added at the wetend of the paper machine. PCC levels should be looked at from time to time on different grades. A good process engineer should look at opportunities to increase the amount of PCC added. To justify the addition compare the cost of the increase in PCC with the reduction in fiber. Also look at the reduction in refiner load due to the reduction in fiber usage.

Ground calcium carbonate (GCC) is naturally and as the name states is ground. The shape of the particle is rhombehedral. The particles are less uniform than PCC. They also differ much more in size from smallest particle to largest particle than PCC. GCC can be added at the wetend, but is generally added at the size press. GCC, aside from offsetting fiber, also improves sheet appearance and print qualities.

The addition of PCC should be looked at as a low hanging fruit. Running trials at higher rates if carefully planned run little risk of reject paper. Increasing the addition rates can push paper ash levels above 20%. So if you haven't looked at your calcium carbonate addition rates lately, it might be time to take a look. There's also new technology and process innovations being made that can help paper machines achieve higher addition rates. So also talk to your local vendor, they can be very helpful at times.

Fiber

There are mainly two types of fiber used in making paper, softwood and hardwood. Softwoods are conifers, such as pines. Hardwoods are broad-leafed trees like oaks. Softwood fibers are in general twice as long as hardwood fibers. The blend of fibers used for a particular type of paper is dependent on its end-use and economics. In most cases hardwood fiber is more expensive, mainly due to the longer period of time it takes for a hardwood tree to mature as compared to a softwood. However, depending on past land management there are cases where hardwoods are cheaper than softwoods. In any case the best mix is the least expensive mix that will produce the desired results. For grades that require more strength more softwood fiber is required. The longer the fiber the stronger the paper. If a sheet needs to be smoother and have better formation, a more uniform appearance, then more hardwood should be used. For economic purposes fillers are also added to reach the desired result. For example, for an inkjet paper the most important aspects of the sheet are appearance, printability, and curl. For this type of sheet a higher percentage of hardwood fiber would be required as opposed to say a bag grade. Fillers can be added to make a smoother sheet and improve print quality. Calcium carbonate can be added to offset fiber costs and still provide the required result. The more calcium carbonate that can be added, the more savings can be accumulated. There are two types of calcium carbonates, precipitated calcium carbonate and ground calcium carbonate. More on that in a later post. This one is supposed to be about fiber. Another chemical that can be added to improve print quality is a sizing agent. This is added at the size press, but this chemical can be expensive so less is best. Anyhow, back to fiber, there are ways to make softwood fibers mimic hardwood fibers. By increasing the refiner load on softwood fibers this will cut the fibers which makes them closer to the length of hardwood fibers. However, doing this will also increase energy cost and may not be worth the offset of softwood fiber replacing the hardwood fiber. I apologize for this post I feel like I'm rambling a bit. Just remember that for strength add softwood, for looks add hardwood. There's more to it than that actually. I think I'll leave it at that for now. Just don't seem to be real focused right now for some reason. Maybe I need a five hour energy shot or something.

Paper Basics

This is a general overview of the papermaking process. I won't go into great detail in any area, but if you would like more information on something let me know and I'll go into more in a later post.

There are four main areas in a paper mill: powerhouse, wood yard, pulp mill, and paper machine. Additional areas areas are roll fishing and converting.

The powerhouse is where the power is made, pretty obvious huh? Besides providing the mill with power the powerhouse also provides a key component in making paper, steam. Without steam no paper making would be taking place. It is necessary for the pulp mill and the paper machine. The powerhouse utilizes byproducts from the paper making process, as well as alternative fuels to create energy. The alternative fuels are dependent on what the powerhouse is permitted to burn. Some places use tire chips or chicken litter. I've also known some places to burn marijuana that had been confiscated by local law enforcement. Anyhow, the process of making energy is somewhat complicated. Basically fuel is burned to produce steam. The steam is then fed to turbine generators which make electricity.

The wood yard is where the wood comes in, again pretty obvious. Wood can be delivered in different ways. There are log trucks, train cars, and boats. It's too expensive to bring in by air, but they would if they could. The wood is delivered as long timbers, short timbers, or wood chips. If the wood is delivered as timbers, it must be debarked. This is done with a debarking drum. Once the timbers are debarked, they are then chipped with a chipper. The bark is sent to the powerhouse to burn as fuel. The wood is chipped into uniform chips. There is an optimum size for chips to efficiently make pulp. The chips are piled into chip piles. They are separated by wood type, hard wood or soft wood. That's pretty much the wood yard.

The pulp mill gets the chips from the wood yard and turns it into pulp. The chips are cooked in digesters to form pulp. The cooking process breaks down the chemicals, lignon, that hold the fibers together. After getting cooked or digested, the pulp is bleached if necessary. The pulp is then screened for quality, before being sent to the washer. The washing process gets rid of any residual chemicals and reclaims some of the chemicals used in the cooking process. After the pulp is washed, it is then screened a second time. Then it is sent to storage. From the storage tank it can be sent to a pulp dryer or to the paper machine. The pulp dryer dries the pulp in sheets or bales which can be sold or saved for use later.

On to the paper machine, this is where rubber hits the road so to speak. The pulp from the pulp mill comes over and is refined. The refining process fibrillates and cuts the fibers. The fibrillation referst the removal of the primary fiber wall and loosening of the internal structure. This makes the fiber more flexible and allows the fiber to absorb more water. Refiners are setup in series or parallel depending on the fiber being processed and the paper being made. From there the pulp is sent to a stock chest. Then the different kinds of stock and chemicals are mixed in a blend chest depending on the grade of paper being made. Note, not all machines have a blend chest. From there the mix is sent to the machine chest, which is essentially a holding tank or surge tank or large are in the pipe. Then it goes to a stuff box. The stuff box is important because it provides a constant pressure head for the basis weight valve which is between the stuff box and the fan pump. The fan pump pulls water from the wire pit, which is where all the water drains off from the table or forming section and the stock mixture from the stuff box and pumps it all to pressure screens. The pressure screens ensure the quality of the stock. Note, there is also some equipment in between to remove entrained air, I can discuss this in a future post. The accepts from the screens go to the headbox which delivers the stock to the forming section. The forming section can be a Fourdrinier which is flat or a gap former which is sandwiched between two fabrics. I can discuss the difference in a later post. The forming section is where the sheet is initially formed. As the sheet goes across the forming section water is drained off. From there it goes into a press section where the sheet is pressed to remove more water. After the press section is the primary dryer section where the sheet is dried. Then depending on the machine there can be breaker stacks or size presses in between. Again these can be discussed in a future post. At the end of the dryers is a calender stack. The calender stack smooths the sheet for a uniform thickness. Then the paper is rolled onto a reel. The jumbo reel is then cut into rolls with a winder. From there the paper is processed for shipping or sent to the converting area. The converting area converts the reels into reems of paper or notebooks for consumber use.

That was my brief overview of the process of making paper. There are many items I can discuss in further detail, but I'm not ready to write a book quite yet. The moral of the story is the things we take for granted often take alot of effort to make. So the next time you jot down a note or throw away a paper cup, think about what it took to make that simple convenience.

Metrics

Developing key metrics for any process or project is very important. The right metrics ensure that you are on the right path and that the right goals have been set. For example, if a dye salesman comes in and says he has a new fluorescent dye that is much more efficient and that it can reduce current fluorescent dye usage one might say the key metric is the fluorescent dye feed rate. Although this may be important it is the wrong metric. If the new dye costs more than the old dye then what have you really done? The machine manager would say that a trial wouldn't be worth it because there may be no savings gained and it could also upset the current process. The key metric for this project should be the savings, which is the key metric in many cases. There may be other times when a new chemical or technology can speed up a machine which increases the overall machine efficiency. This also improves the bottom line since it would ultimately increase the amount of paper out the door. Another key metric that is overlooked at times is safety. These are the three key metrics most often used to justify a trial. Quality is important but if there are no customer complaints then most of the time it is not a reason for change. So basically I'm saying make sure you know what you want, and make sure you know how to convince others that you need what you want.

Introduction

Hello everyone. I'm starting this blog to share some of my experiences as a paper maker. I have several years of experience in the industry. I'd like to share my practical experience, as well as technical experience. Some topics may be quite simple, but I assure you there is a lesson worth reading about somewhere in each entry. If there as a specific topic you are interested in please let me know and I will do my best to blog about it in future entries. I'll be candid in any recommendations or replies. I'll also try my best to keep the blog current as best I can. A few entries a week is my goal. So thank you for reading and enjoy.