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.

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. 

Fourdrinier Retention Mechanisms - Technical Papermaking

Mechanisms of retention/ drainage

Retention aids traditionally based on Alum, Alum neutralizes the charge of the paper making furnishes and was seen a "fix all" regarding wet end chemistry problems. 

Modern Retention aids based on PEI single polymers used "bridging" as the dominant mechanism for retention. the first type of polymers were of high molecular weight which brougfht fibers and filler together and formed "bridges". Nowa days new micro particle systems follow a complex flocculation system to improve wire retention.

What is of interest are the flocculation properties of polymers (retention aids) because many components of the stock furnish (sludge, fillers, fines etc) are too small to be mechanically retained on the wire and need to be bound to the larger fibers through flocculation. The ideal scenario would be to restrict fiber to fiber flocculation and encourage the smaller particles and additives to flock to the fibers. This would give the best retention and dewatering of the sheet.

Bellow is a table that describes the elements of papermaking that will affect the retention on a fourdrinier machine, they can be catagorised into, Pulp conditions, Wire conditions and the additives added to the stock/ furnish

Stock Factors	Conditions of Wire	Additives
pH	Sheet grammage	Types and amounts of fillers
Consistency	Sheet formation	Shape and density of mineral particles
Temperature	Fabric characteristics	Types and amounts of other additives
Fiber characteristics	Type of dewatering elements	Order of addition
Degree of system closure	Machine speed	Ionic balance
	Shake (if used) – old technique used on high quality paper machines	Level of anionic trash

Improving retention on the wire has many benifits, primarly cost. Papermachines producing News print have a retention of about 50%, increasing this retention to 55% for example will reduce the amount of primary stock needed to an extra 45%. for example at a retention of 50% producing 25t/hr throughput the mass of stock through the headbox needs to be 50t/hr, by increasing the retention to 55% the mass through the head box is reduced to 45T/hr