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

Consistency Meters - Papermachine Automation

Consistency Meters

Rotor designed consistency meters

The most common consistency meter found, the rotor design CM (Consistency meter) sits just outside of the stock flow stream. A deflector rotor pulls stock into the recess where the measurement device rotates at a constant speed. The stock gets thicker the torque on the motor to maintain that speed increases. The consistency can then be measured against the motor torque. This is known as a strain gauge. These types of measurement devices require a certain flow to function correctly and can measure consistencies of 1% - 10%.

unlike fixed blade consistency meters the rotor design is not affected by the variations in stock flows because the device creates its own flow from the deflection rotor onto the measurment device.

Fixed blade Consistency meters

Fixed blade CM work in the same way as the rotor design except the paddle is placed within the fiber flow stream. The fixed paddles moves with the fiber flow. The consistency increases in the pip, this in turn increases the force against the blade. The force is measured by the meter and calculated to a consistency. This is another instance of a strain gauge. 

The limitation with this type of measurement device is the stationary aspect. As stock flows past the paddle, fiber and rejects can stick/ build up reducing the accuracy of the device.
Variable flow within the pipe will alter the consistency measurment. A higher flow will add a higher force onto the consistency meter resulting in a higher measured consistency.

The major advantage of this type of meaurment is cost - usually customers purchase a fixed blade/ dynamic blade consistency measurment will the intention of replaying it with a better model.

Microwave Consistency meters

Microwave transmitters work on the principle that sending microwaves through water the waves travel at a certain speed. When fiber is introduces the microwaves move faster through the stream. The consistency can therefore be measured depending on the speed of the microwaves being sent and received by the meter. 

Using the calculation;   Velocity = C / sqrt(e)   

Where; C = Speed of light in a vacuum

     E = Dielectric constant of liquid (water)

These devices are more accurate than the rotor/ blade design. The microwave Transmitter also has no moving parts for the pulp to affect/ build up on like the blade transmitter.

The CM is not affected by the flow rate, colour, and brightness, like traditional microwave ovens they are highly affected by metals. these consistency meters are used within very clean pulp systems like the aproach flow because the likleyhood of metals entering the stream are very low. The microwave transmitter needs to be the same size as the pipe being used. Due to the expensive nature of these devices microwave transmitters are typically used on smaller pipe work or substituted for cheaper models. 

For more Info on Instrumentation within paper-making check out my other blog posts!

Level transmitters;
https://www.papermakingbible.co.uk/2018/04/process-level-indicators-papermachine.html

PID controllers;
https://www.papermakingbible.co.uk/2018/02/pid-control-loop-parameters.htm