Helene Casellas, Product Manager, Rosemount Flow, Emerson Process Management
March 4, 2013
Selecting and installing the right technology for effective flow measurement in mill processes reduces variability enabling improvements in plant performance and productivity.
Mill operators are constantly looking for ways to improve plant performance and productivity against a background of high energy costs and environmental sustainability. In a highly competitive market it is increasingly important to maximise production by closely monitoring production processes to ensure that they are operating at maximum efficiency. By deploying advanced measurement and control capabilities, process variability and unscheduled downtime can be reduced and maintenance costs minimized.
Many processes in the pulp and paper industry rely on the stable and accurate measurement of flow. However, the industry poses special challenges for instrumentation, including noisy stock flows, aggressive chemicals, abrasive materials, and high process temperatures. Selecting and installing the right technology for individual applications is therefore critical for effective flow measurement in the mill environment.
Magnetic flow meters are widely used
Magnetic flow meters can be used to measure volume flow of almost every conductive liquid and are accepted as the primary flow technology used in the mill. Even aggressive or corrosive fluids can be measured since the flow tube is lined with materials such as polyurethane, Teflon or a hard rubber.
Magnetic flow meters are accurate, obstruction-less (so fiber does not build up in the meter leading to plugging) and can be manufactured from a range of materials to match process needs. With no moving parts that would require regular maintenance, they provide a cost-effective solution for applications across a wide range of pipe sizes up to 36 in. (90 cm).
Manufacturers are continually improving the accuracy and performance of their products and the latest generation magnetic flow meters feature diagnostics, which provide the user with a comprehensive package of instrument and process information. These help to reduce commissioning time, improve process control and increase long term reliability.
For example, Emerson's E-Series flowmeter includes diagnostics for ground and wiring fault detection and high process noise detection, combined with a meter self-verification feature, which will confirm the health of the entire flowmeter. In addition, the recent introduction of wireless adaptors enable existing and new devices to be connected wirelessly to IEC 62591 (WirelessHART) networks providing easy access to both measurement and diagnostic information.
When selecting a magnetic flow meter for mill applications, there are a number of factors to be considered. These include the risk of process noise which impacts the quality of the flow measurement and affects process control. A further consideration is the use of aggressive chemicals, which impact meter life and process uptime. In some circumstances, permeation occurs which reduces meter life and process uptime. Permeation is the molecular diffusion of a fluid or vapour through the liner over time which shows up as "blisters" or "bubbles". This is a particular problem at high process temperatures and pressures.
Noisy stock flow is caused by fiber impinging on the magnetic flow meter electrodes resulting in unstable control and excess variability. In applications like pulp stock slurries, the properties of the process fluid can lead to an unstable output from the flow meter. If the flow meter output is driving a valve, this will cause excessive travel in the valve's position as it tries to keep the flow rate steady at a set point. This results in increased valve wear and a greater need for maintenance.
Adding damping may make the output look stable, but the flow meter's response time to actual process changes is compromised, making it slower to respond to actual process changes.
The end result of excessive damping can be a very inconsistent, out of control process. While the flow meter output and valve travel may be indicating steady process flow conditions, they may just be hiding what is really happening which could be an increase in process variability.
To overcome this problem, instead of increasing the damping, some flow meters allow the user to change the characteristics of the device, for example by using a higher coil drive frequency. In many noisy applications, this has the effect of providing a stable output and maintaining a fast response time.
Selecting the right materials
The harsh chemicals used in the pulping process, such as caustic and high concentration bleach chemicals, combined with high process temperatures and abrasion due to pulp stock solids, provide unique challenges for magnetic flow meters. Selecting the best liner and electrode materials is critical to extending operational life and minimizing the issues associated with meter failure.
Teflon is the most commonly used liner material in the mill due its good resistance to chemicals and abrasion. Electrode materials can be matched to provide corrosion resistance for the specific chemicals. Beyond basic material selection, the mill can take additional steps to significantly extend meter life in the most challenging applications.
The most aggressive applications in the mill may be black and white liquor, where the combination of high temperatures and harsh chemicals can drive permeation in Teflon liners. As there is a trend within mills to increase production and efficiency by increasing temperature, an increase in the incidence of permeation is being observed.
Permeation rates can be affected by temperature gradients where the heat moves from the hot process fluid towards the ambient surroundings, driving fluid permeation. Resistance to permeation can be increased by selecting the correct Teflon resin, manufacturing the liner in a manner that increases resistance, and increasing its thickness. Most flow meter manufacturers offer a choice of liner materials and can advise on compatibility with different media.
The importance of ground wiring
To ensure a magnetic flow meter performs accurately, it is important that it is correctly grounded. Improper grounding and wiring is the number one cause of magnetic flow meter issues. This can occur in new installations where the magnetic flow meter is not properly referenced to the process and in existing installations where corrosion, for example caused by an aggressive environment, results in deterioration of the electrical ground wire. Poor grounding allows electrical noise to be picked up by the sensor electrodes and consequently affects the signal to noise ratio and the stability of the transmitter output.
One way to resolve this issue is to use a flow meter with a built-in ground/wiring fault diagnostic as found in Emerson's Rosemount E-Series Magmeter. This works by specifically looking at the signal amplitude at frequencies of 50 Hz and 60 Hz which are the common AC cycle frequencies found throughout the world. If the amplitude of the signal at 50 Hz or 60 Hz exceeds 5 mV, the diagnostic alert will activate indicating that the ground and wiring of the installation should be carefully examined. The diagnostics can be accessed through the meter's Local Operator Interface, downloaded to a portable device, or viewed using asset management software which allows trouble shooting from a central control room.
Certain applications require regular calibration of flow meters and traditionally this means that the measuring device has to be removed from the line to be calibrated. During this time a spare meter is installed so that the process can continue to operate while the original meter is being verified. This process often incurs third party costs and results in lost production.
As an alternative, there are now magnetic flow meters that have the ability to carry out a self-verification test - alerting the operator when the calibration drifts outside set parameters.
Magnetic flow meters are widely accepted as the primary flow technology used in the mill, however problems such as noisy stock flows, aggressive chemicals, abrasive materials, and high process temperatures means that selecting and installing the right technology for individual applications is critical.
Powerful diagnostics in the latest generation products and features such as meter self-verification make magnetic flow meters easy to install and use. The accurate and reliable data enabled helps users better manage their production processes by increasing availability, reducing maintenance and operating costs, and improving product quality.
A good example of where a pulp and paper manufacturer has benefitted from correctly selecting and installing advanced magmeter technology is at Norske Skog in Styria, Austria.
Norske Skog had installed a magnetic flow meter in the basis weight flow line at its production plant. The mill was experiencing instability in the output from the flow meter which meant it was unable to reliably run its basis weight control loop in automatic mode. Even with the damping applied in both the meter and the control system, there was still too much noise to control the speed of the basis weight pump effectively.
In addition, the heavy damping meant that the system was very slow to respond to changes to the process. For example, after pipe cleaning operations with a caustic soda solution, it would take up to 20 minutes for the meter to respond to a change in flow rate. The manufacturer of the flow meter was unable to provide a solution to combat the noise issue.
To compensate for the unstable signal from the basis weight flow meter and ensure the final paper product met quality specifications, the basis weight set point had to be set higher than the desired target point.
Following discussions with Emerson Process Management, Norske Skog installed a 16-in. Rosemount E-Series magnetic flow meter with the high process noise diagnostic on a trial basis. After installation, the high process noise diagnostic indicated a low signal to noise ratio at 5 Hertz. By changing the coil drive frequency to 37 Hertz, the noise was significantly reduced and within an hour of operation, there was enough confidence in the stability of the signal that automatic control of the loop was restored.
By reducing the measurement variation, the customer is now able to more accurately control the process and this has resulted in reduced raw material usage and cull, helping to increase paper production by 1.5%.
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