Process Level Indicators - Papermachine Automation

Level Indicators

Load cells

A load cell is a transducer which emits an electrical signal when a load is applied. The electrical signal is proportional to the force being applied. Strain gauge load cells work in a similar way to the paddle consistency meters. The electrical signal can be calibrated to measure the weight/ fill capacity of a tank or hopper. Some starch mixing tanks, for example, use load cells to weigh the amount of starch for each batch.

The downside to load cells are, they have to be mounted in such a way that all of the weight/ load is on the load cell. The load cell can become deformed/ inaccurate if the transmitter is overloaded and as such fail-safes have to be fitted to prevent overloads. After a time the load cell loses its accuracy due to constant deformation of the transducer. Compared with the ultrasonic level transmitter the operating range is low.

Ultrasonic level transmitter

An ultrasonic transmitter uses ultra-sonic radio waves to determine the fill level in a tank. The transmitter sends out a signal. The signal hits the top of the liquid in the tank, reflects back to the transmitter where the time taken is calculated. The time is compared with the time calibrated when the tank was empty. A percentage can be calculated and this is the level of the tank. For example

If the tank, when empty took 5 mili seconds to send and receive the signal, at 50% fill volume the transmitter would record a 2.5 mili second delay.

Ultrasonic transmitters have a high rate of accuracy and tend to maintain their calibration over time. The transmitter can work on lower ranges than load cell transmitters. 

The disadvantage of ultrasonic transmitters can be the substance you're trying to measure, for example in a starch silo lots of dust is produced when filling. this dust can obstruct the ultrasonic transmitter giving a false or erratic reading

Guided wave radar

fundamentals of guided wave radar level measurement come directly from Time Domain Reflectometry (TDR), a technology that has been employed for decades to find breaks in underground cables and in-wall cable installations in large buildings. TDR instruments launch low amplitude, high-frequency pulses onto the transmission line, cable, or waveguide under test, and then sequentially sample the reflected signal amplitudes.

Guided wave Transmitters work on a similar principle as ultrasonic transmitters. the only difference is how the wave is emitted. the radar wave is sent through a "cable probe" that spans the height of the tank for example. the radar wave travels down the cable and reflects back. the transmitter uses time of flight principle to calculate a level. 
the pulses traveling through the probe are disturbed by the liquid or dry powder medium and reflects the signal back early giving a level reading.

Hydrostatic Transmitters

Hydrostatic transmitters are similar construction to pressure transmitters. the transmitter design is the same the application and calculations behind the transmitter vary. Placed at the 0 level of a tank or silo the hydrostatic transmitter will measure a hydrostatic head or "total head pressure" which is basically the pressure exerted by the water column in the tank. 

So, the filling height is calculated from the distance of the medium surface to the measuring point by the pressure measurement. The weight force of the liquid column, thus the hydrostatic pressure, however, is not only directly proportional to the filling height but also varies with the specific gravity of the medium and the force of gravity.

PID Control loop parameters - Papermachine Automation

Control loop tuning

PID Control

PID control refers to the control system that alters the process outputs to bring the measured valued closer to the setpoint value.

The computer uses an error value (the difference between the setpoint and the measured value) to base the calculations on. PID is comprised of 3 parts, the proportionality value, the integral action and the derivate action. The majority of control systems tend to use PI control because of the limitations of the D action.

The Proportional Action

The proportional term produces an output that is proportional to the error value. The proportional response to the error value can be multiplied by the proportional gain constant (Value P on the control system).

A high proportional gain constant (Pk) relates to a larger change in the output to correct the error value. If the proportional gain constant value is too high it can make the control loop unstable. Likewise, if the value is too low the control action can be too small and results in a small output when responding to a higher input. This can lead to a less sensitive controller.

The integral action

The contribution to the PID control from the integral term is proportional to both the magnitude (highest value) of the error and the duration of the error. Plainly speaking the Integral value affects the time it takes for the proportional value to meet the set point by reducing the error value to 0. The controller works by adjusting the repeats per minute value. The bigger the integral action the quicker the proportional value meets the set point. By increasing the repeats per minute value the faster/ bigger the integral action is.

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The derivative action

The derivative action looks at the slope of the error value and tried to predict the measured value. This allows you to have higher P and me values while also keeping the system stable. The value of the derivative action describes how far in the future it should look so 20 would mean it looks 20 seconds into the future.

The issue with using the derivative action is if there is noise on the measured value (small spikes) this confuses the algorithm used and increases the effect of the Derivative value leading to an unstable control system. Most control systems do away with the Derivative action for this reason. The derivative function looks at the “steepness” of the curve, having noise on the MV leads to high steepness but no actual chance this is why the derivative action breaks down.

The PID control display page can be accessed through the face plates on the DCs system but cannot be altered. Fine tuning of the PID controls can be difficult. Fine tuning of the controls should dampen the oscillations created by the output so that the MV matches that of the SP quickly and stabilizes.

For more information of the devices that can be used with PID controllers within the papermaking industry take a look at this; Process Level Control - Paper machine Automation

Hand Sheet Making - Paper Testing

Hand sheet making

Hand sheets are created for a variety of uses; to determine the composition of stock/ pulp in a tank, to look at the cleanliness of papermaking stock, to test the strength properties of the pulp etc. There are a variety of ways to calculate the hand sheet weight and volume of stock needed to form a specific hand sheet.

Consistency method

Hand sheets are made to a grammage of 120gsm for testing to achieve this weight the stock volume needed is calculated. The hand sheet weight should be about 2.4g. (this depends on the diameter of the final hand sheet) Firstly the consistency of the stock sample is calculated. Once this is known a simple calculation can be made to work out the stock volume.

  

There will be a tolerance error using this calculation as the consistency of the stock sample can be inaccurate due to pouring the sample/ collecting the sample. A more accurate method can be used.

A good consistency measurement example can be found here; Technical Papermaking Consistency

Test sheet Method

Before making a hand sheet a test sheet is created to determine the amount of stock need to make a specific weight hand sheet. This method is ideal if the consistency is unknown.

First the stock needs to be diluted to around 0.3% consistency in a bucket. Using the same calculation above a volume of stock is calculated for the test sheet. The test sheet is formed and dried. Once the sheet has been dried the test sheet is weighed and recorded.

The weight is used in another calculation to determine how much extra or less stock is needed to achieve the desired weight. This step removes the errors made when creating the 0.3% solution.

 

This calculation will give you the new volume needed to make the hand sheet.

Forming Hand sheets for physical test of pulp

Using the test method as described by Tappi T205 hand sheets can be made from pulp ready for physical testing.

The stock samples are taken. The first step is to identify the amount of stock needed to make a standard 80GSM hand sheet. This can be achieved in two ways, either by conducting a consistency test on the pulp sample, which takes time and not always accurate.

The other method requires making a test sheet before the hand sheets.

Creating a hand sheet

With the hand sheet cylinder down (open) the hand valve is opened to allow water to pass through the wire, gently rubbing the surface allows any remaining fibres to be removed. The wire is now clean.

The cylinder is locked back in place. The valve is opened slightly to allow some water to fill up, next add 500ml of 0.3% stock solution. The water is filled up to 350mm from the wire to the inscribed line.

Insert the perforated stirrer 5 times for about 6 seconds trying not to spin the stirrer and remove the perforated disk from the liquid. Carefully remove the stirrer on the last up movement and wait for 5 seconds allowing entrapped air to leave and the fibres to settle. Fully open the drain and let the water drain under the vacuum from the water leg. The hand sheet will now form on the wire.

Blotting paper is placed on the formed hand sheet as well as a couch plate on top; the couch roll is applied with no extra force apart from the rolls weight. The couch roll is rolled five times across the plate. With the movement likened to opening a book, the disk and blotting paper is removed and placed onto the drying rings or hot plate.

Cleanliness

Creating hand sheet can be a good indication on the cleanliness of the stock; they can be used especially when troubleshooting quality problems on the machine. For example if a screen was passing/ the slots or holes were damaged you can see an increased level of contaminates in the hand sheet of the accepted stock line. It is a good idea to have a base line hand sheet of the process to be used as a comparison when trouble shooting appearance quality issues. 

Paper Machine Water Chemistry - Technical Papermaking

Paper Machine Water Chemistry

Process water is the name given to the back water on the paper machine, water that is reused within the papermaking process.

The backwater is a mixture of chemicals and fines that were not used the first time around (one pass retention). The back water can have detrimental effect ts to the process if the parameters of the water fall outside of specific ranges/ conditions. For example if the conductivity of the back water became too low or outside of normal operating conditions it would mean the retention on the wire is reduced.

Low conductivity (-18mV) shows there is a high volume of anionic trash within the system which when mixes with the cationic starch or polymer, hydrocol will bond with the trash rather than the fibres leading to poor formation on the wire, pick outs and deposits forming around the machinery and a reduction in run ability. 

pH of process water

pH can be described as the single most important aspect of wet end chemistry, this is because most/ all aspects of the chemistry relies on in some aspect the pH.

When the pH of the water increases (becomes more alkaline), the surface charge of the fibres also increases. This will affect the attraction of the retention aids and other cat-ionic substances to the fibres. The most undesired effect would be the substantial increase of bacteria in the system.

Another effect of high pH ius fiber swelling, fiber swelling is useful during refining because the higher surface area and increased flexability of the fiber leads to high de-fibrilation (versus cutting of the fiber). Caustic acid is added to create this effect.

fiber swelling is also used within de-inking plants. swelling of the fibers pre floatation allows the inks and binder to split and break off when the fiber swells this aids in the  bleaching and deinking process leading to high brightness of the finished pulp.

pH can affect quite strongly the dissolving ability of wood components and to changes in the dissolved substances. Increase in pH improves wood components dissolving ability in the water system and thus the amount of anionic particles dissolved and colloidal substances.

Because of the undesirable fractions within water, the water needs to be cleaned at some stage with different kind of methods like for example with disc-filtering and or chemicals. The wet-end of the paper machine contains the highest amount of water and its chemistry has to be controlled by a variety of chemicals like, retention chemicals, fixatives, de-foaming agents and biocides.

A decrease of the pH value leads to deposits on the machine as the precipitation of non-wood materials increases.  A fine balance has to be made; typically a pH of 7 is achieved within the water loop.

Machine Issues caused by change in pH
Increase in pH	Decrease in pH
Level of bacteria in water increases	Deposit precipitation increases – deposits on the machines
Higher amount of anionic trash in the system
Surface charge of fibres increase

Bugs/ Bacteria - Biocides

Paper machines run well when the operating conditions remain constant. Water plays a large part in paper production and any slight disturbances in the water can cause negative effects on the machine.

Bugs, bacteria thrive in water and damp conditions. Bacteria build up within the water system will cause a variety of issues. For example; Slime build up, lowering PH within the water (acidification) and upsetting the chemical balance within the water loops.

Biocides are used to control the bacterial problem. The main goal of the biocides is to limit the growth of sessile bacteria, i.e. those that are attached to surfaces. These are the bacteria that tend to build up, cause slime deposits and holes, hurt productivity, and hurt product quality.

Pulp Grinders - Mechanical Pulp Production

Types of grinders

Chain grinders

Chain grinders are the most common type of grinder, the chamber between the vertical chains houses the logs ready to be processed. The logs are continuously driven by the chains onto the surface of the revolving pulp stone. The chains apply a force on the log keeping them against the surface of the stone. Due to the surface profile fibers are torn out of the wooden compound. The logs are stacked horizontally due to the orientation of the fibers (minimizes fiber damage - increases fiber length)

Water showers are used to keep the stone clean and dilute the stock suspension. Water temp is usually higher than 80 deg. using a low water temperature can cause the stone surface to crack (due to temperature difference). Using hot water allows the stone to maintain the high temperatures. these high temperatures affect the lignin of the wood softening the material which in turn means the fibers can be separated with less damage

The basin collects the pulp washed off by the water showers. typically this produces a low consistency pulp, a thickening stage is added after to bring the consistency to a more manageable level. 

Pocket grinder

The pocket grinder is a technological advancement of the chain grinder above. processing the logs within a sealed "pocket" allows the sealed area to operate at a higher atmospheric pressure than the pocket grinder.

At higher pressures (approx: 5 bar) the boiling effect of water is affected. the higher temperature of the water will soften the lignin within the logs, longer fibers can be created with less damage done to the fibers. 

Revolving pulp stone is surrounded with a metal housing and 4 pockets, feeding of the pocket grinder was done manually. The debarked logs are pushed against the revolving surface of the pulp stone by hydraulically driven pistons. The pulp is collected underneath the pulp stone.  

 

The entire process of feeding the logs to the groundwood outlet is done under pressure. (up to 5 bar housing pressure, shower water temp up to app 120 deg) pressure affects the boiling temp of water. 


For more info on Mechanical ground wood follow this link
Stone Ground Wood - Mechanical Pulp Production