Real time nip profiling now possible, Solutions! Online Exclusives, August 2003

REAL TIME NIP PROFILING NOW POSSIBLE

Wiping out nip load variation is critical for papermakers—but most existing methods for evaluating nips have some limitations. So what’s the solution for accurate and complete nip measurement?

Papermakers use nipped rolls to remove water, consolidate the fibers, apply coatings and finish the sheet, and precision papermaking requires uniform nip profiles. While process variations can generate uneven wear patterns, most nip load variations can be attributed to initial load variations: uneven loading end-to-end, over crowning (under loading), over loading (under crowning), improper crown profile, improper dubbing, roll misalignment, faulty control systems, worn mechanical elements and mismatched roll sets.

Traditionally, papermakers evaluate roll nips by measuring the static nip contact width via carbon papers, micro-encapsulated films, embossed foils or similar methods. Static impressions are most useful in identifying and addressing initial load variations. Crown or load corrections can be easily calculated once the diameters of the mating rolls are known. An examination of the shape of the impression will help determine necessary corrections in alignment, dubbing, and crown profile. While simple in application, static nip width has long been the benchmark by which all nip measuring systems are measured.

These static nip impression methods, although cost effective and accurate, require dedicated mill nip impression programs and properly trained personnel. Also, since nip impression papers measure the widest width encountered, significant care must be taken to prevent localized overloads due to one end of the nip contacting before the other or improper crown adjustments with variable crown rolls. The push in recent years for increased machine efficiencies as well as reductions in maintenance personnel has reduced the time and “in-house” skill available for properly calibrating nip roller assemblies. As a result, most mill nip impression programs are “catch-as-catch-can”. This factor may actually reduce machine efficiencies and profits due to accelerated wear, increased cross machine sheet variations and, in extreme cases, through the untimely loss of felts or roll covers.

New methods bridge the gap
To address these issues, over the past few years suppliers have introduced new methods for measuring nip impressions. Among these are Pressurex tactile pressure sensing film (“TPSF”) and electronic “profiler” systems such as E-Nip. Both products are from Sensor Products Inc., East Hanover, New Jersey, USA.

Pressurex TPSF is primarily used in dynamic, rolling nips at operating temperatures with process variables in place (saturated wet felts, suction roll vacuum, etc.) The resulting nip impression yields high resolution cross machine nip pressure profiles, accurate in relative pressure variations. The TPSF is most useful in applications where static nip impressions made using traditional methods do not represent operating conditions (yankee pressure rolls), where nip widths are sufficiently narrow to make nip width readings impractical (calenders), and in diagnosing process variations over time (thermal crowning, wear patterns). Some highly skilled technicians have demonstrated similar accuracy to static nip impressions in diagnosing initial load variations and crown corrections. (Figure 1 shows a nip impression generated with Pressurex TPSF.)

Electronic profilers are the latest additions to the toolbox for measuring nip profiles. These profilers typically consist of an array of pressure sensors placed in the nip and electronically connected to a computer displaying pressure changes in real time. First generation systems offer a great advantage: they can identify the capabilities of measuring static impressions in real time while varying nip loads, thus reducing the need for time consuming multiple static nips. However, current pressure sensor technology produces systems affected by temperature, hysteresis and compression set, especially with repeated use, resulting in marginal accuracy and repeatability.

When measuring pressure the question arises, “What is the actual pressure being measured?” As usually happens in the case of nip measurements, pressure varies from zero at the entrance of the nip to some maximum near the center and back to zero at the exit of the nip. Include the effects of nipping two vented rolls (e.g.: a drilled roll mating a grooved roll) and one can imagine the multitude of pressure variations within the nip area of contact!

A different solution
Senor Products has developed its E-Nip electronic profiler to measure nip width in lieu of nip pressure. E-Nip relies on patented sensor technology that uses breakthrough technology to measure real time nip width to an accuracy of 0.01” (0.25mm). These sensors are placed in the nip and connected to a laptop computer via cabling and a process hub (Figure 2). The E-nip software operates the system and displays the resulting nip width in real-time.

E-nip sensors use a resistive substrate to measure the maximum points of nip width contact in the loaded nip. By measuring the maximum points of contact, the pressure variations due to drill or groove patterns are negated. Standard E-nip sensors are designed to measure drill diameters and groove widths up to 3/16” (4.75mm).

In measuring nip width, it is important that the measurement is normal to the nip load plane. Slight deviations in the angle of measurement result in reduced accuracy. E-nip sensors are unique in design and function in that each sensor actually consists of two individual sensing areas. While each sensing area operates independently, by comparing the difference between the two readings, the angle of deviation from the normal to the nip load plane is measured to an accuracy of one degree. This deviation is displayed on the computer screen and the width reading for the individual sensor automatically adjusted.

Measuring nip width, regardless of roll venting patterns, to an accuracy of 0.01” and correcting any sensor mis-alignment is irrelevant if this information is distorted when transmitted to the user. Unfortunately paper machine rooms are, electrically speaking, extremely noisy environments.

Sensor Products addressed this issue early in the E-Nip’s design, when technicians found that most paper machine power sources contained sufficient noise to negatively affect the accuracy of the system. In lieu of using costly and bulky power conditioners, the E-nip process hub and sensors are powered via self-contained, low cost C-cell batteries. To further preserve the accuracy of the system, the analog sensor signal is converted to digital as close to the measuring point as possible and transmitted to the process hub by shielded cables.

By far the most difficult nip load conditions to evaluate are multi-nip presses and calenders. For example, in the case of a Tri-Nip press, changes in load or crown in the third press nip affect both the first and second press nips. Thus E-nip is designed to analyze up to three nips simultaneously. This powerful feature allows papermakers to accurately see the results of changes in load in multi-nip presses on a single real-time graph.

Ease of operation
As mentioned earlier, E-nip consists of an array of width measuring sensors, a processing hub, connecting cables and operating software for a customer supplied laptop computer. In operation, the computer and process hub are normally located on the tending side of the machine with the sensor array located in the nip to be analyzed (Figure 3). For easy deployment, the sensors are typically mounted on a reusable plastic strip with the sensors spaced as necessary. The computer, process hub and sensor array can all be assembled prior use, reducing unnecessary machine downtime. When ready, the sensor array is spread across the machine and secured to one of the rolls to be checked.

The E-nip software has been designed for simplicity and accuracy. All functions necessary to operate the system are accessed through the computer and many have been automated for ease-of-use. For example, as with any high accuracy measurement system, the components of the system should be verified prior to use and wear components should be calibrated. With E-nip, verification and calibration before the system is activated for use can be done by simply clicking the computer screen button labeled “calibration”. This procedure is usually performed just prior to loading the rolls and takes approximately 30 seconds. Should all systems check OK, E-nip will be ready for use. Should the system detect a fault such as a sensor out of calibration or the batteries need replacing, the error will be displayed graphically on the computer screen as well as a report and possible remedy.

Once calibrated, the nip is ready for loading. Several loading procedures can be used, but usually the rolls are loaded at less than operating pressure. Thus as load pressure is increased, the resulting nip width can be viewed on the computer in real time. Once operating pressure is achieved and sufficient time is permitted for the roll elements to completely deflect, the graph displays the resulting nip width (Figure 4). Should a load correction be necessary to obtain a “flat” nip profile, nip width changes as a function of load changes are displayed in real time. The resultant measurements can also be used to generate changes in crown amount, crown shape or crown profile.

While impression papers provide low cost and high accuracy for measuring nip width, they are labor intensive and require extensive machine downtime to analyze “problem” nips. Although they produce accurate relative data (when areas are compared to each other on the same impression) useful in diagnosing nip variation due to process instabilities, tactile indicating sensor films fall short in providing the accuracy necessary for the average engineer to recommend crown or load changes. While providing real-time data, electronic pressure sensors (like pressure indicating films) suffer in accuracy and repeatability. Electronic profilers can offer a solution. The E-Nip analysis system combines the accuracy of measuring nip width with the speed and flexibility of real-time electronics in a system that is easy to operate.

Information on Pressurex TPSF may be found at www.sensorprod.com/pressurex. For more information on E-Nip, please go to www.sensorprod.com/enip. Sensor Products, Inc., 188 Rte 10 West, Suite 307, East Hanover, NJ 07936-2108; +1/973.884.1755; email: sensor1@sensorprod.com