Valve Condition Monitoring simplifies evaporator operation
June 1, 2001 By Pulp & Paper Canada
Iggesunds Bruk in northern Sweden is known for its high quality solid bleached board, with annual capacity around 310 000 tonnes per year (t/y). The mill traces its roots back to 1685, although the mo…
Iggesunds Bruk in northern Sweden is known for its high quality solid bleached board, with annual capacity around 310 000 tonnes per year (t/y). The mill traces its roots back to 1685, although the modern era of paperboard production began in 1963 with the installation of BM 1. A second board machine was added in 1971.
In 1999, the mill undertook a major rebuild of the evaporator lines, with the installation of a new six-effect, 210-t/d Kvaerner plant able to produce thick black liquor at 74% solids for firing into the recovery boiler. Then, in mid-2000, as part of a continuing program of preventative maintenance in the mill, a new Neles Field Browser system was installed to monitor the condition of the control valves in the new evaporator line. The experience with this monitoring system has been very positive so far. This article will describe the evaporator project, as well as the valve condition monitoring system, in more detail.
Before the new line was added, the evaporating operation at Iggesund had been a bottleneck in the pulp production lines, meaning that the evaporators could not handle all the black liquor being produced if the pulp lines ran at full speed. So a higher capacity evaporator line was needed. As a side benefit, it was clear that increased use of black liquor in the recovery boiler would reduce oil consumption at the mill and thereby cut environmental emissions, which is an important consideration.
In 1998 the decision was taken to invest approximately SEK 127 million (US$16 million at the time) in the new No. 4 evaporator line. The project was run under the direction of two Iggesund project engineers, Lennart Olsson and Bo Skogkvist. As part of the permit requirements of the project, the mill was placed under very tight constraints with regard to noise. As Lennart Olsson said, “We were forced to keep the total noise level under 80 db, which is certainly not an easy task when we are talking about a massive industrial process. So it was necessary to go through every individual component in the system to see where we could cut noise. We formed a working group amongst ourselves and the major suppliers on the project, Kvaerner and Neles Automation (now Metso Automation), to look at each and every possible source of noise.”
Olsson continued, “With the assistance of Neles ‘Nelprof’ valve sizing and selection software, as well as Kvaerner’s process simulation system, we were able to pinpoint potential sources of noise in a very efficient manner. It isn’t often one gets the chance to look at an entirely new process unit in the operation and go through all the simulations in advance. But the No. 4 evaporator line gave us such an opportunity to look at the total function and I think we all found it very educational.”
To monitor the amount of incoming process steam in the new evaporator line, two Nelflow control valves were installed in conjunction with a differential pressure transmitter. Thus, the control valves are acting as the flow meter, since the flow can be calculated based on the differential pressure across the valves and the valve flow coefficients, Cv. So instead of traditional methods like Pitot tube and orifice plate, which can be both more expensive and prone to clogging, the control valve provides good measurement of the steam flow into the line which is important for the total heat balance.
The new line started up very well in late September 1999, with the start-up taking just seven hours. The project engineers as well as the production managers were all pleased with the rapid and smooth start-up, as well as the continuing smooth operation of the line. As a stroke of luck for the mill, both the pulp price and the price of oil went up at the same time just after the No. 4 line started.
As part of the installation, essentially all of the control valves were equipped with Neles ND 800 ‘smart’ digital valve controllers. In September 2000, after the new evaporator line had been running for about one year, the mill started up the new Neles Field Browser predictive maintenance system. The smart digital controllers have a built-in diagnostic function for on-line monitoring. This allowed Iggesund to use such a system, which would not be possible with analog valve controllers.
This system was of interest because of the mill’s ongoing program to be more proactive, rather than reactive, in maintenance. The old, traditional method of maintenance had been simply to wait until a valve broke down or the operators noticed something going wrong. In recent years, the mill has moved to a more systematic program of valve inspection, with the instrument men doing rounds to check the valve condition. But this is also somewhat limited by the fact that the process must be shut down when the tests are performed meaning the results reflect a ‘dry’ test, rather than the state under true operating conditions.
The Field Browser system includes two PC workstations, with one situated in the mill engineering office and the other sitting in the electrical board room next to the evaporator line in the mill. The 34 control valves are each connected to the system via a Hart multi-plexer which systematically moves from valve to valve on a regular schedule to check the condition. This polling of the smart valves is carried out continuously.
The parameters that are most closely followed at Iggesund are the deviation from the internal set point in the valve controller and the actuator load on the valve. If the deviation from set point is too great, in this case greater than 2%, a warning is triggered and shown on the PC monitor. An E-mail can also automatically be sent to the valve engineer. As soon as the valve engineer receives a warning, the valve in question is entered into the maintenance scheduling system and the valve will end up on the priority list for attention during the next maintenance shutdown.
The mill did have to make some modifications in the deviations bands that had been pre-set before installation. They had been quite wide, at 5 and 10%, meaning a yellow warning was issued at 5% deviation from internal set point and red alarm at 10% deviation. But they were not sensitive enough so Iggesund tightened the band down to 2 and 5% and now feels that the system is at the correct sensitivity to give proper control. The system consists of four colours indicating the operating condition: green for all OK, yellow for a warning that there is small deviation, red for an alarm for a large deviation and white for communication lost.
Advanced diagnostic tool
In addition to the continuous real-time condition monitoring function that the system provides, it also is set up to automatically maintain an historical database of the individual valve data, so that the history of the valve can easily be called up. This can be very useful when trying to analyze the performance of the valve or see if there are other problems.
To date, the experience with the system has been very good. Since it has been installed, there have not been any process breakdowns that were due to a valve failure or malfunction. The entire process is running very smoothly and we are rather confident that the predictive condition monitoring of the valves is playing an important role in that.
Previously, if a valve was acting up, a temporary fix might be used, such as manually reducing the gain in the PID-controller to dampen the valve action. When the valve has been repaired, the gain in the PID-controller is restored. With the new system, the mill knows far in advance that the condition may be deteriorating and it can act to correct it at a much earlier stage.
In conclusion, it can be said with confidence that the installation and operation of the valve condition monitoring system has been very successful. Due to Iggesund’s satisfaction with this first system on the new evaporator line, it is now planning to put a similar but larger system on the board machines. The valve monitoring system on BM 1 will include 120 valves while the system on BM 2 will encompass 125 valves. Installation is expected to
take place during 2001.
Peder Hagglund, Tommy Nielsen, Iggesunds Bruk, Sweden.
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