Press Felt Management System Improves Productivity, Quality

Pulp & Paper, Oct 2005 by Ricks, Rick

An alternative to hand-held instruments, a new measurement and control system directly measures felt water weight, permeability, and temperature

Managing the quality and performance of the felts in the press section can result in tremendous savings for the papermaker in the form of improved process efficiencies, reduced energy consumption, higher product quality, and reduced rejects. However, tools previously available to measure press felts were limited to unsafe, hand-held instruments. As a result, direct felt monitoring is typically performed infrequently and decisions on felt quality, shut downs for wash-ups, and damage are made indirectly or inferred.

Recently, Voith Paper Automation introduced a new scanning measurement and control system capable of withstanding the harsh environment of the press section. Using direct measurement of felt water weight, permeability, and temperature, it is possible to identify problematic felts, providing data for reducing downtime between wash-ups and extending felt lifetimes.

Installation on the top pickup felt of a NipcoFlex press on a 7.3 m-trim-width copy papers machine has shown that this scanning measurement and control system can also predict paper moisture streaks after the press section and before the size press. This allows the capability for press section optimization, reduced steam consumption, and enhanced line efficiency.

Real Time Scanning Measurement

The FeltView system provides real time, continuous measurement for up to four press felts. Each scanner carries a traversing, single-sided measurement head containing three sensors: microwave moisture, permeability, and infrared temperature (Figure 1). As the measurement head scans across the surface of the felt, consistent contact is maintained using pneumatically loaded ceramic guides, ensuring accurate and repeatable results. Scanners are constructed of stainless steel and are environmentally protected for continued reliability in the press section.

The FeltView scanner is optimally installed on the first top pickup felt after the showers and vacuum boxes. In this position, it provides an accurate picture of the felt condition after cleaning.

The single-sided microwave moisture sensor is well suited for the high water weight of a press felt. Changes in water weight are detected as changes in the felt's dielectric constant and are insensitive to felt construction, thickness, and imbedded minerals. The measuring element is imbedded in a contacting, wear-resistant ceramic disk that does not damage or tear the felt. Sensor response is linear over the entire measurement range of 200-1,800 g/m^sup 2^.

The permeability sensor measures the penetration of a water jet of fixed pressure (20 bars) into the surface of the felt. Permeability is a function of the pressure difference between the water supply, a flow constrictor, and the surface of the felt. Ongoing compensation is made for viscosity changes by measuring the temperature of the water supply. The contacting sensor components are also imbedded in an abrasion-resistant ceramic plate.

A non-contacting infrared temperature sensor provides fast cross machine (CD) and machine direction (MD) surface temperature. This can be used to monitor steam boxes, avoiding felt damage due of to overheating during grade or line speed changes.

Online Measurement Results

A series of tests were conducted to confirm the profile shape accuracy of the FeltView measurement system on a 7.3-m-wide copy papers machine operating at 1,300 mpm. The FeltView scanner was installed on a two-week old felt on the top pickup position of a NipcoFlex press, a single-nip shoe press with double felts. The rotational frequency of this felt is approximately 1.6 seconds.

Scanning water weight profiles were compared with those obtained using hand-held instrumentation. Since the handheld instrumentation is transported across the felt at walking speed, expectations were that any streaks detected would differ in width but not in magnitude.

Figure 2 shows the results of the tests on the felt entering the nip. There is excellent correlation between the water weight profiles obtained from the scanner and hand-held instruments. Average water weights compare favorably as well. For the handheld sensor, the average water weight is 850 g/m2 compared to 900 g/m^sup 2^ measured by the microwave moisture sensor.

Troubleshooting Moisture Profile Problems

The felt water weight profile can predict moisture features such as streaks, which occur downstream in the process. Figure 3 shows the FeltView water weight profile stacked on sheet profile measurements taken after the press section (with the Voith EnvironScan scanner) and before the SpeedFlo Jet size coater. These profiles have been overlaid without regard for the y-axis scaling. The water weight in the felt is approximately 900 g/m^sup 2^, while the moisture following the press section is 48.3% and before the size coater it is 1.9%.

The strong correlation between the moisture streaks on the felt and those on the sheet gives the papermaker a diagnostic tool for identifying and troubleshooting problematic felts. This increased process visibility provides further opportunity for enhancing machine efficiencies by improving runnability and reducing drying requirements after the pressing operation.