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Steam venting is history at Norampac Trenton

The sight and sound of vented steam outside a paper mill are sure signs of wasted energy inside. But, if that clearly visible problem can be solved, a good return can be achieved at today’s energy prices. With the steam venting problem put to rest by a paper machine steam and condensate system rebuild, the environment outside the Norampac mill in Trenton, Ont., is a little quieter and that formerly wasted steam is now being put to good, efficient use in the dryers.

The rebuild project, completed in early 2011 by Kadant Canada, has generated a significant return on investment by reducing the steam energy cost per tonne by 16%. Considering the mill’s steam costs, that results in a saving of more than $800,000 per year on the 180,000 tonne/year PM2 corrugating medium machine. Better cross-direction moisture uniformity and reduced sheet break downtime add to the return.

The venting of steam from the PM2 drying section was a chronic problem, says Gérald (Gerry) Nolin, production manager. “Since I have been at the mill it seems we have always been venting. When dryer amps rose (indicating a dryer flooding condition) the operators had to open one or more steam vent valves to get proper condensate removal. We have four manually-operated vent valves – one for each steam section – and it seemed that one was open all the time,” he explains.

Part of the problem was that the old cascading thermocompressor steam and condensate system was unstable and hard for operators to control. “It was not a well-designed system; the valves seemed to be fighting each other,” adds Nolin.

So mill staff approached Kadant Canada for a proposal to eliminate the steam venting problem, improve energy efficiency in the dryers, stabilize the drying system, and improve paper quality. It was expected the moisture profiles could be improved by uniform condensate removal and heat transfer to the sheet.

Before making a firm proposal, Kadant engineers performed a thorough audit of the existing steam and condensate system, including measurements of dryer surface temperatures and condensing rates, as well as an evaluation of drying and condensate handling deficiencies, control procedures, and bottlenecks. Control valve sizing, tank sizing, piping design, and instrumentation were included in the audit. Pocket ventilation was also reviewed.

Some of the dryers had been updated to stationary syphons on a case-by-case basis but most were equipped with original rotary syphons. Computer models were used to predict the steam and condensate system performance if the system were redesigned with matching stationary syphons and individually-controlled re-circulating thermocompressors for each of the four sections.

The delivered scope of supply by Kadant included;

• Stationary syphons and PTX steam joints for all dryers. The stationary syphons provide maximum condensate removal and operate with lower blowthrough steam at much lower differential pressures. This is because the stationary syphons do not have to overcome the rotational forces of the dryer shell.

• Full width Turbulator® bars for all dryers. Flush with inside of the dryer shell, the bars provide turbulence in the condensate layer, thereby improving the uniformity of heat transfer. Since the heat transfer is even across the shell, moisture profiles can be improved.
• Converting the older cascading-thermocompressor steam system to a re-circulating thermocompressor design with individual steam section control. The new thermocompressors with a lower motive pressure (220 psig) were matched to the lower blowthrough steam and differential pressure requirements of the new syphons.
• Regrouping of the steam sections to match the drive sections.
• Automated vent valves.
• Updating and recalibrating transmitters with new, more accurately engineered impulse lines.
• New steam and condensate piping, including stress analysis.

The entire project was completed within a very tight one-and-a-half month time window.

Since project completion Nolin has been checking the external steam vents for any signs of the previous problems and has found none. So the major objective of the project has been conclusively achieved. Actually, a small amount (less than 500 lb/hr) of steam venting is required to bleed non-condensable gases from the system, but that is a far cry from the 15,000 lb/hr previously wasted. With little venting, better heat transfer, and more efficient drying, the consumption of steam per tonne has been reduced substantially, by about 16%, as shown in Fig. 1.

With this significant energy saving, the successful project has caught the attention of Union Gas, the local gas utility, who has committed to a partial funding of the project.

The original venting problem was initiated by high dryer amperage loads which indicated a flooded dryer condition. Dryer amperage data shown in Fig. 2 indicate that this problem has been reduced since the project completion. The drive stability has had a positive effect on dryer-related breaks and lost time due to sheet re-threading on the hot dryer cans. As well, the dryer sections are now matched to the drive section, thereby improving draw control.

Nolin estimates the lost time due to draw problems, high dryer amps, and dryer pickouts has been reduced by more than 13%, even though the number of breaks was reduced by a modest 2.8%.

“The dryer section is much more stable and easier to control,” he says. In fact, the differential pressure required to drain condensate through the new syphons is now much lower. “They are 3 to 5 psig compared to 20 psig, if you could trust the measurements before,” quips Nolin. The pressure transmitters are now accurately measuring the correct pressure, as that was one of the issues to address with the project.

The operators can now control the first section to a lower pressure to get a gradual increase in the drying rate in the sheet and not trap moisture at the beginning of the drying cycle, therefore making drying less efficient. The drying is now more evenly distributed throughout the dryer section. Nolin believes the improved stability of the dryers has contributed to cleaner dryers in some sections. “There is less sticky buildup and they seem to clean themselves,” he says.

Projects aimed at energy efficiency can also have some side benefits, and that has been the case at the Trenton mill. Quality improvement was one of the original goals which has been realized thanks to the even CD heat transfer provided by the Turbulator bars. The 2-sigma CD variation from the QCS system has been reduced by 19%. Energy efficiency and quality improvement; that’s a win-win. PPC